BACKGROUND
during the last decade, the bat p. discolor has been used increasingly for psychophysical and neurophysiological studies of echolocation  <cit> .
p. discolor is medium-sized and forages for fruit, nectar, pollen and insects in a neotropical forest habitat.
its vocal emissions are brief , broadband multi-harmonic, downward frequency modulated  echolocation pulses with a frequency range of about  <dig> to  <dig> khz.
in contrast, the rich repertoire of communication calls used for species-specific social interaction covers the lower frequency range from  <dig> to  <dig> khz  <cit> .
the responses of cortical neurons to complex stimuli relevant for echolocation in p. discolor have been compared to the behavioral performance of the bat  <cit> .
therefore, it is important to gain knowledge of the detailed organization of the ac with respect to basic response properties.
the auditory cortex  of mammals is composed of distinct fields, which can be characterized by physiological and cytoarchitectural features and their specific thalamo-cortical connections .
the functional organization of the ac in bats has been extensively studied physiologically in several species .
among the best studied acs so far are those of the mustached bat p. parnellii and the horseshoe bat, rhinolophus rouxi, both belonging to the group of the so-called cf/fm bats whose echolocation pulses consist of a constant frequency  and a frequency modulated  component.
as common to all mammals studied so far, their acs contain a tonotopically organized primary auditory field  with the frequency gradient running from caudal to rostral.
however, in both cf/fm bats frequencies of the cf component of the calls are largely overrepresented in ai while frequencies of the fm component are only weakly represented  <cit> .
the ai is surrounded by cortical regions with neurons that show facilitated responses to specific spectral and temporal combinations of the cf and fm parts of the different harmonics of the echolocation pulses.
cf/fm bats are rather specialized echolocators in that they hunt almost exclusively insects on the wings, whereas other bat species display more varied feeding ecology  and very commonly use short downward fm echolocation pulses often with several harmonic components.
in these bats the functional specialization of the ac is often not so clearly apparent, but still cortical fields can be segregated based on neurophysiological criteria like best frequency  representation and response threshold  <cit> .
in the phyllostomid fm bat carollia perspicillata for example, two dorsal fields containing mainly neurons with bfs in the high-frequency range have been reported in addition to the tonotopically organized fields ai and aaf   <cit> .
in these high-frequency fields some neurons exhibited pulse-echo delay sensitivity as in cf/fm bats  <cit> , but without topographical organization.
except for a short autoradiographic labeling study  <cit>  the topography of the ac of p. discolor has not been studied.
in general, the ac of only one other phyllostomid fm bat, c. perspicillata, has been investigated, so far  <cit> .
therefore, the aim of the present study was to investigate neuroanatomical and neurophysiological properties of the ac of p. discolor in order to delineate its subdivisions.
RESULTS
auditory responses in the cortex of phyllostomus discolor
as shown in fig.
1a, units responding to acoustic stimuli are found at parieto-temporal portions of the neocortex of p. discolor.
the distribution of neurophysiological recording sites defines the functional location of the bat's ac.
external features roughly delineating the ac are the pseudocentral sulcus  <cit> , which is located at the rostro-dorsal border and the fissura rhinalis, which is located at the ventral border.
the auditory cortical area covers a rostro-caudal distance of about  <dig> μm and has a dorso-ventral extension of about  <dig> μm in the lateral view as shown in fig.
1a.
to obtain a realistic estimate of the cortical surface containing auditory units, the locations of the recorded units were projected on an unrolled and flattened surface projection along the medio-lateral coordinate .
in this projection, the lateral extension of the auditory responsive area is roughly  <dig> μm.
neuroanatomy
in the area responsive to acoustic stimuli four major fields are recognized based on cyto- and myeloarchitectural features and zinc staining pattern, i.e.
the anterior and posterior dorsal fields , an anterior ventral field  and a posterior ventral field .
the pvf maybe further subdivided into a dorsal and a ventral part  and a border zone , due to minor modifications of the neuroarchitectural characteristics.
the topographic position of these fields is depicted in fig.
1c.
reliable borders of cortical fields are indicated by solid lines.
dashed lines represent the more variable outlines of the ac itself and for pvf possible anatomical subdivisions that are not corroborated by neurophysiological data of the study.
frontal sections in fig.
<dig> give showcase characteristics at two rostro-caudal levels  to get a general idea of field differences.
total cortical thickness, relative thickness of the different layers, composition of cell types, cell density, content of myelinated fibers and zinc are considered as parameters for the distinction of the different fields.
cut-outs of frontal sections stained for cells , myelinated fibers  and for zinc  from the centers of the different cortical fields are mounted in fig.
<dig> for detailed comparison.
total cortical thickness varies between  <dig> μm  and  <dig> μm from dorso-rostral to ventro-caudal locations  in the ac.
despite the high density of granular elements in layer iii and iv of the dorsal fields, the cortical layers v and vi take more of the total thickness of the cortex.
therefore, they are addressed as parietal cortical fields.
the ventral fields are thought to belong to the temporal cortex, as layers iii and iv dominate layers v and vi in thickness although granular elements are sparse and the cell density is lower in layers iii/iv compared to the dorsal fields.
dorsal and ventral fields also differ with respect to layer i, which is generally thinner in the dorsal than in the ventral fields.
the myelinated horizontal fibers in the superficial part of layer i form a narrow dense band in the dorsal fields, but a broader one in the ventral fields, which is paralleled by a nearly zinc-free stripe in the ventral fields.
the most characteristic features used for delimitation of cortical fields are described in detail as follows.
adf has the greatest cortical width that seems to be caused by a doubled layer v: layer v of the non-auditory cortical area dorsal to the ac seems to continue beneath layer v of adf .
in avf, the rostral beginning coincides with the most caudal part of the claustrocortex without a sharp border.
the myelin content in avf is comparably low, whereas the stain for zinc is generally intense .
even the paler staining band corresponding to layer iv is relatively dark.
the posterior fields are more homogeneous.
cortical width in pdf is smaller than in adf and characteristic differences to the neighboring pvf are obvious at higher magnification .
in the zinc stain, layer iv is narrower and more heavily stained in pdf than in pvf, whereas layer v is thicker and of higher staining intensity.
pvf has the most conspicuous wide and pale zinc staining band in layer iv, encompassing deep layer iii.
layer v shows three subdivisions in the dorsal part , whereas in the ventral part  only two subdivisions are recognizable.
layer vb, the most intense staining part of layer v is strikingly darker in pvfv.
in pvfbz a gradual change of the characteristic features from pvf toward the ventrally adjacent perirhinal cortex takes place, e.g.
the layered organization fades as well in the cell stain as in the zinc stain, and so do the myelinated fibers.
layer iv appears as a pale band with the lowest staining intensity in the zinc stain and the staining intensity of layer iv varies characteristically between the fields.
it is faint in layer iv of pdf, comparably lighter in adf, relatively dark in avf and faintest in pvf.
layer iv  contains a high number of granular elements in the dorsal fields, whereas the neuronal somata in the ventral fields are larger and the cell density is lower.
layer iv is heavily myelinated in all fields except in the sparsely myelinated avf.
basic neuronal response properties
because it was not always possible to discriminate the activity of a single neuron, the term 'unit' will be used in the following to describe the activity of one neuron to clusters of three neurons recorded at a distinct recording site.
extracellular recordings were derived from a total of  <dig> units from both hemispheres of ten bats.
the number of units recorded in one bat ranged from  <dig>  to  <dig>  with an average of  <dig> units  per bat .
thus, on average two units were recorded per penetration.
recording depths of units derived from roughly orthogonal electrode penetrations  were in the range of  <dig> to  <dig> μm from the cortical surface .
the first row shows the number of electrode penetrations in the ac of each bat, the total number of recorded units in each of the ten experimental animals is shown in the second row.
best frequency and threshold
as shown in fig.
4a, bfs of units ranged from five to  <dig> khz .
seventy nine percent of the units  had bfs above  <dig> khz, i.e.
in the range of the dominant harmonics of the echolocation pulse of p. discolor.
the response threshold at bf was determined in  <dig> units and varied between zero and  <dig> db spl.
the frequency distribution of thresholds reaches its maximum around  <dig> db spl with roughly symmetric flanks to lower and higher thresholds .
latency and response duration
the first spike latency was measured in  <dig> cortical units and ranged between five and  <dig> ms with a bias toward short latencies .
median latency of all units was  <dig> ms .
units often exhibited onset responses to pure tones frequently followed by a sustained response component that generally exhibited a considerable amount of variability.
other response types e.g.
on-off responses were not observed in the ac of p. discolor.
the response duration to a  <dig> ms pure tone was determined in  <dig> units and ranged from three to  <dig> ms .
short phasic responses with durations below  <dig> ms  were found in  <dig> %  of units, medium response durations between  <dig> and  <dig> ms  were found in  <dig> %  of units and  <dig> % of units  had response durations longer than  <dig> ms .
thus, in  <dig> % of units the response duration exceeded the duration of the pure tone stimulus .
frequency response areas
q10db values covered a range between  <dig>  and  <dig> .
the distribution showed a peak around five , indicating that sharp frequency tuning is rare in cortical units of p. discolor.
the regression line in fig.
4h shows that q10db values roughly increased with increasing bfs.
q30db values covered the range between  <dig>  and  <dig> .
the difference between the medians of the q10db and q30db values suggests that in many units the sharpness of frequency tuning was only slightly decreasing with increasing stimulus level.
in  <dig> units, the fras could be classified into six different types .
most units  showed a v-shaped fra with equal share of monotonically  and non-monotonically  responses.
sixteen percent  of cortical units showed double- or multi-tuned fras two thirds  of which displayed monotonic  and one third  non-monotonic response behavior .
in  <dig> %  of multiple tuned units, threshold minima were roughly harmonically related.
in six percent  of units, the fras constituted closed areas in the frequency-intensity field with spike rates dropping to zero at all frequencies with increasing stimulus level .
the remaining units  featured complex-shaped fras .
binaural response properties
in  <dig> units the binaural response properties were measured and units were classified following their contra versus ipsilateral response characteristics .
ee responses were measured in  <dig> %  of units,  <dig> %  of units were inhibited by the ipsilateral ear  and  <dig> %  of units showed no reaction to ipsilateral stimulation .
four percent of units , could not be classified in one of the above classes and were thus named as "other".
representation of neuronal response properties in cortical fields
the local representation of selected response properties on the cortical surface was visualized using the voronoi tessellation method .
the tessellation field around the recording site has its limits at halfway distance to the next surrounding recording sites and its color displays the respective response strength of the unit at the center.
this forcibly entails large tessellation fields in areas in which recording density is lower or in border regions.
as a consequence, there is no proper meaning in the area of single tessellation fields, but rather the clustering or systematic trends of tessellation fields within the ac are important information.
response properties of cortical units were analyzed for the four major subfields.
possible anatomical subdivisions of pvf were not corroborated by the neurophysiological data, and thus, pvf was analyzed as a whole.
representation of best frequency and q10db
the organization of bfs within the ac of p. discolor is shown in fig.
6a.
units in the dorsal fields had a relatively restricted range of mainly high bfs, which covered frequencies in the range of the dominant harmonics of the echolocation pulse of this species.
most units in the adf had bfs above  <dig> khz, whereas units in the pdf had mainly bfs above  <dig> khz.
only at the most rostral positions of the adf units with low bfs were found.
no tonotopic arrangement of bfs was found in the dorsal fields.
in contrast, fields in the ventral part of the ac showed a tonotopic organization of bfs.
the frequency gradient in avf developed along the rostro-lateral to caudo-medial axis with low bfs at rostro-lateral positions, whereas in pvf the bfs decreased from rostral to caudal cortical positions.
thus, pvf and avf shared a common high-frequency border.
the rough direction of bf gradients in the ventral fields is schematically shown in fig.
6b.
as shown in fig.
6d, the median bf  in pdf was significantly higher than the median bfs of the other cortical fields .
the lowest median bf  was found in pvf.
q10db values of units were also not uniformly distributed on the cortical surface but showed a tendency to increase from anterior to posterior locations .
in both anterior fields, units showed broader frequency tuning with significantly lower q10db values  than found in units in the posterior fields .
representation of threshold, latency and response duration
the distribution of first spike latencies in the ac of p. discolor is shown in fig.
6g.
as a general trend, short first spike latencies were preferentially represented in units in the two anterior fields, whereas in units in the posterior fields also long latencies were found.
the median latency of pvf  was significantly longer  than the median latencies of adf , avf  and pdf .
the representation of response durations within the ac  showed the same trend as the representation of first spike latencies: short durations were characteristic for units in the anterior fields, whereas in the posterior fields units with long response durations were found, too.
the median response duration of units in adf  and avf  were significantly shorter  than the median response duration of units in pvf  but only slightly shorter than the median response duration of units in pdf .
in addition, the median response duration in pdf was significantly shorter  compared to pvf.
representation of fra type and binaural response properties
the representation of the different fra-types showed slight differences between anterior and posterior cortical fields .
most units of anterior fields had monotonic v-shaped or monotonic double tuned fras , whereas non-monotonic v-shaped and non-monotonic double tuned fras were mainly found in posterior fields .
the cortical representation of double tuned fras with harmonically related components did not show a specific clustering within certain subfields.
discussion
this study investigated the localization, neuroarchitecture and basic physiological response properties of the auditory cortex of the bat p. discolor.
the large area of neocortex responsive to acoustic stimulation comprises four anatomically distinguishable fields: a ventral part of the ac with an anterior and a posterior subdivision  and a dorsal part of the ac further divided into anterior and posterior subfields, adf and pdf.
the anatomical parcellation of the ac was consistent with the differentiated distribution of basic acoustical response properties of units in the cortical subfields.
location and extension of the ac of p. discolor as defined by neurophysiological and neuroarchitectural methods in this study roughly corroborated the findings from an earlier 2-dg study in the same species by esser  <cit> .
comparison with the auditory cortex of other bats
the distribution of bfs in the ac of p. discolor  showed an overrepresentation of frequencies in the spectral range of the bat's echolocation pulse .
this overrepresentation might be due to the fact that the dorsal fields are mainly containing units with bfs in the high-frequency range.
thus, a larger amount of cortical surface is devoted to units with high bfs as compared to units with low bfs.
in addition, more units were recorded from the dorsal fields and this bias might also be reflected in the distribution of bfs.
a similar overrepresentation of bfs in the spectral range of the echolocation pulses has been found in the ac of other microchiropteran bats e.g.
e. fuscus  <cit> , myotis lucifugus  <cit>  and c. perspicillata  <cit> .
in the cf/fm bats p. parnellii  <cit> , r. rouxi  <cit>  and r. ferrumequinum  <cit>  the overrepresentation of a narrow call-specific spectral range of the cf-part is reflected throughout the ascending auditory pathway  <cit>  and is based on specializations already implemented on the cochlear level  <cit> .
in cf/fm bats, a large part of the ac is involved in processing of specific combinations of the different harmonics in the echolocation pulse and echo.
in p. parnellii and slightly less distinctive in r. rouxi, several fields can be segregated by virtue of selective responses of neurons to different combinations of fm-parts or cf components of call and echo.
neurons are tuned to delays between the components and are topographically ordered  <cit> .
such a complex representation of pulse and echo delay has generally not been found in fm-bats so far, although pulse-echo delay sensitive neurons exist in the ac of e.g.
m. lucifugus  <cit>  and in the dorsal parts of the ac of the phyllostomid bat c. perspicillata  <cit> , which is closely related to p. discolor.
in the present study, only pure tones but no echolocation related stimuli  were used for acoustic stimulation .
consequently, no statements concerning the functional involvement of cortical subfields other than pure tone processing  can be made, so far.
besides that, the organization and parcellation of the ac of p. discolor is generally in accordance with that of c. perspicillata.
the dorsal fields in the ac of carollia were similarly characterized by neurons with bfs in the high-frequency range, and the ventral part of the ac was also composed of two tonotopically organized subfields, which shared a common high-frequency border  <cit> .
in contrast, the ac of other non-phyllostomid fm bats like m. lucifugus and e. fuscus showed differentiation into subfields to a lesser degree  <cit> .
in both species tonotopically organized fields with frequency gradients running in opposite directions were found, but no fields with characteristics similar to the dorsal cortical fields in p. discolor were described.
a high percentage  of units in the ac of p. discolor had first spike latencies below  <dig> ms. as other studies in fm-bats reported mean latencies measured at  <dig> to  <dig> db above threshold around  <dig> to  <dig> ms already on the level of the ic  <cit> , this seems to be unusually short.
however, short latencies in the ac of fm-bats were also reported  <cit> .
in addition, haplea et al.
showed that on the level of the ic latencies in e. fuscus still could be as short as about three milliseconds  <cit> .
although most cortical neurons in r. rouxi had latencies in the range between  <dig> to  <dig> ms, short latencies below  <dig> ms were found in a substantial  <dig> % of neurons  <cit> .
the bias toward short latencies below  <dig> ms in p. discolor might be explained by the fact that a large number of recordings in our study were derived from dorsal regions of the ac where short latencies are predominately represented.
in contrast to findings in other mammals where the ai has been described to have the shortest latencies  <cit> , the lowest median first spike latency in p. discolor was found in the adf, whereas in the pvf, which might be equivalent to the ai, the longest first spike latencies were found.
our findings in p. discolor are supported by the fact that in the ac of r. rouxi shortest latencies were also found in the dorsal fields  <cit> .
in r. rouxi, ventral and dorsal cortical fields are not sequentially connected but rather receive connections from different parts of the auditory thalamus via different projection pathways  <cit> .
in p. discolor, a similar connection pattern could explain the shorter latencies in the dorsal fields compared to the ventral fields.
however, this hypothesis needs to be proved by future tracer studies in this species.
influence of anesthesia and multi unit recordings on response patterns
a large proportion of units in the p. discolor ac showed onset-type responses.
it is known that the response pattern of cortical neurons is influenced by anesthetics.
for example, the use of barbiturates like pentobarbital that enhances gabaa-mediated inhibition biased the distribution of response types toward short onset responses in the rat auditory cortex  <cit> .
the drug midazolam used in our study, like other benzodiazepines, also enhances action of gaba on gabaa receptors.
thus, the high abundance of phasic onset-type responses in our study could be a consequence of the use of midazolam.
however, a high percentage of onset-type response patterns  were also found in neurons in the ac of awake r. rouxi  <cit>  and awake mustached bats  <cit> , indicating that even in the un-anesthetized bat phasic responses are common.
this minimizes the potential effect of anesthetics on response types of cortical units in the present study.
another point that could have influenced the response pattern is that recordings in our study were not always derived from single units .
seshagiri and delgutte  <cit>  showed that adjacent neurons recorded simultaneously with tetrodes in the inferior colliculus of cats could differ significantly in their temporal discharge pattern although bfs and threshold were highly correlated.
therefore, blurring of response pattern is possible in some cases in multi unit recordings, but markedly dependent on the number of contributing neurons with dissimilar response types.
as multi unit recordings in this study were generally derived from only up to three neurons , and most units showed phasic responses the pooling of spikes in multi units should not have severely effected the response pattern in most cases.
parcellation of the ac in p. discolor
one way of defining different fields of ac is by comparing the afferent connections with the auditory thalamus.
common to all mammals is a strong tonotopically organized projection from the ventral division of the medial geniculate body  to ai  <cit> .
in contrast to this, non-tonotopically organized non-primary cortical areas receive major projections from the dorsal division of the mgb   <cit> .
cf/fm bats like horseshoe bats  and p. parnellii  <cit>  follow this connectivity pattern.
in both animals, ai contains a tonotopic map where frequencies of the cf-components of the calls are overrepresented while frequencies of the fm-components are largely absent  <cit> .
in r. rouxi, neurons specialized for the processing of acoustic parameters relevant for echolocation  are mainly represented in dorsally located cortical areas that are targeted by projections from the mgbd  <cit> .
the view of a congeneric projection from mgbv to ai covering the entire frequency range has been challenged by recent findings in the gleaning bat antrozous pallidus.
in this bat, the low frequency portion of a coherent frequency representation in the ac is innervated by the mgbv, whereas the high-frequency part received major projections from the supra-geniculate nucleus, a part of mgbd  <cit> .
neurons in the high-frequency range were moreover of a distinct binaural input type  and preferred fm-sweeps, whereas neurons in the low frequency part were classified as ei and responded preferentially to noise.
two functionally distinct pathways evidently exist in this bat species: first, a low frequency pathway serving for passive prey location and second, a high-frequency pathway serving for obstacle avoidance during active echolocation  <cit> .
the approach that part of ai may not receive input from the mgbv challenges one of the traditional definitions for ai via connectivity that, however, does not constitute the only valid criterion.
the distribution of binaural response types in p. discolor resembles that in a. pallidus, whereas neuroarchitectonic and neurophysiological features argue for a cortical fields pattern of the ac in p. discolor similar to the arrangement in r. rouxi and other mammals.
tracer experiments were not included in the present study in p. discolor, and consequently data on thalamo-cortical connections are not available.
however, neuroarchitectonic features provide valuable tools for the definition of subdivisions in ac when combined with neurophysiological outcomes.
undoubtedly, the combination of all three approaches would yield optimal possibilities to classify cortical subdivisions.
the specific usefulness of different neurohistological stains to determine cortical fields will be discussed in the following: the staining for zinc proved to be specifically helpful, complementing staining for cells and myelinated fibers.
the zinc stain reveals horizontal bands in the neocortex that show characteristic variation in width and staining intensity among cortical regions, sharply defining the borders between cortical areas and their subdivisions, and are generally coincident with the limits defined in the nissl stained sections.
the banding patterns are stable among the species studied so far, allowing for an interspecies comparison .
the pvf shows several features in the zinc stain characterizing primary sensory regions in the mammalian neocortex: conspicuous pale zinc staining of the outer portion of layer i, the differentiation of layer v in sublayers and, most characteristic, a wide, light band corresponding to layer iv encompassing the lower portion of layer iii.
in pdf, most distinctive to the pvf is the zinc staining of layer iv, which is narrower and more heavily stained, whereas layer v is thicker and of higher staining intensity.
zinc staining in avf is generally intense.
these features of pdf and adf have been described to be characteristic for secondary sensory regions .
furthermore, the structure of adf seems unique in that it could be an amalgam of auditory and neighboring non-auditory, probably somatosensory areas.
mixed auditory-somatosensory fields neighboring the ac rostrally have been reported for several mammalian species .
the accordance of response properties of cortical units in avf and pvf with the anatomical partitioning of ventral auditory fields in the cortex is a further strong support, that the pvf might correspond to ai in p. discolor.
pvf is tonotopically organized with bfs increasing from caudal to rostral.
the avf shows also a tonotopic organization but in contrast to the pvf, bfs increase from rostro-ventral to caudo-dorsal locations.
this organization is in accordance to findings in other mammals where tonotopic gradients with roughly opposite directions have also been identified in the ai and the aaf .
because of the common high-frequency border of the avf and pvf, high frequencies are represented all the way along the medial border of the pvf down to the most ventral border of this field .
in addition, no clear tonotopic gradient can be seen in the pdf that could be a tonotopic continuation of the frequency representation in the ventrally located pvf.
thus, it is unlikely that the pdf just represents the high-frequency portion of a contiguous frequency representation starting with low frequencies in the pvf as assumed for the ac of a. pallidus  <cit> .
it is more likely that the pdf represents a non-tonotopically organized field like the dorsal fields found in other bats before .
however, also in c. perspicillata information about the thalamo-cortical connections is lacking.
in addition to bf, the distribution of several response properties like duration and first spike latency showed significant differences between the pdf and the pvf.
to further test the possibility that high-frequency units in the pdf and pvf belong to a contiguous frequency representation like in a. pallidus  <cit> , we additionally tested the differences of latency and response duration only for the high-frequency units  of both cortical fields.
latency and response duration for high-frequency units alone were still significantly different  between pdf and pvf.
these findings further strengthen our parcellation of the ac based on the anatomical findings.
CONCLUSIONS
in this study, four subfields were identified in the ac of the bat p. discolor.
these differed in their neuroanatomical attributes and in the response properties of their units to pure tone stimulation.
the neuroanatomical and neurophysiological properties of the tonotopically organized pvf reflected common characteristics of ai in the mammalian ac .
the avf located rostrally to the pvf showed a tonotopic bf gradient running roughly in opposite direction to the gradient of the pvf.
thus, the avf of p. discolor might represent the aaf described in other mammals.
the dorsal part of the ac of p. discolor was non-tonotopically organized with bfs mainly in the high-frequency range.
as most energy of the echolocation pulse of this bat species is contained in the frequency range between  <dig> to  <dig> khz, the dorsal region of the ac of p. discolor seems to be particularly important for echolocation.
with regard to the relative position of cortical fields and their basic properties of bf representation, the ac of p. discolor seems to follow the organizational pattern seen in other phyllostomid bats.
to further compare the parcellation of the ac of p. discolor with those of other bats and mammals, the specific connectivity between subdivisions of the ac and different divisions of the auditory thalamus of p. discolor must be investigated in future studies.

 during the last decade, the bat p. discolor has been used increasingly for psychophysical and neurophysiological studies of echolocation. p. discolor is medium-sized and forages for fruit, nectar, pollen and insects in a neotropical forest habitat. its vocal emissions are brief, broadband multi-harmonic, downward frequency modulated echolocation pulses with a frequency range of about to khz. in contrast, the rich repertoire of communication calls used for species-specific social interaction covers the lower frequency range from to khz. the responses of cortical neurons to complex stimuli relevant for echolocation in p. discolor have been compared to the behavioral performance of the bat. therefore, it is important to gain knowledge of the detailed organization of the ac with respect to basic response properties. the auditory cortex of mammals is composed of distinct fields, which can be characterized by physiological and cytoarchitectural features and their specific thalamo-cortical connections. the functional organization of the ac in bats has been extensively studied physiologically in several species. among the best studied acs so far are those of the mustached bat p. parnellii and the horseshoe bat, rhinolophus rouxi, both belonging to the group of the so-called cf/fm bats whose echolocation pulses consist of a constant frequency and a frequency modulated component. as common to all mammals studied so far, their acs contain a tonotopically organized primary auditory field with the frequency gradient running from caudal to rostral. however, in both cf/fm bats frequencies of the cf component of the calls are largely overrepresented in ai while frequencies of the fm component are only weakly represented. the ai is surrounded by cortical regions with neurons that show facilitated responses to specific spectral and temporal combinations of the cf and fm parts of the different harmonics of the echolocation pulses. cf/fm bats are rather specialized echolocators in that they hunt almost exclusively insects on the wings, whereas other bat species display more varied feeding ecology and very commonly use short downward fm echolocation pulses often with several harmonic components. in these bats the functional specialization of the ac is often not so clearly apparent, but still cortical fields can be segregated based on neurophysiological criteria like best frequency representation and response threshold. in the phyllostomid fm bat carollia perspicillata for example, two dorsal fields containing mainly neurons with bfs in the high-frequency range have been reported in addition to the tonotopically organized fields ai and aaf. in these high-frequency fields some neurons exhibited pulse-echo delay sensitivity as in cf/fm bats, but without topographical organization. except for a short autoradiographic labeling study the topography of the ac of p. discolor has not been studied. in general, the ac of only one other phyllostomid fm bat, c. perspicillata, has been investigated, so far. therefore, the aim of the present study was to investigate neuroanatomical and neurophysiological properties of the ac of p. discolor in order to delineate its subdivisions.  auditory responses in the cortex of phyllostomus discolor as shown in fig. 1a, units responding to acoustic stimuli are found at parieto-temporal portions of the neocortex of p. discolor. the distribution of neurophysiological recording sites defines the functional location of the bat's ac. external features roughly delineating the ac are the pseudocentral sulcus, which is located at the rostro-dorsal border and the fissura rhinalis, which is located at the ventral border. the auditory cortical area covers a rostro-caudal distance of about μm and has a dorso-ventral extension of about μm in the lateral view as shown in fig. 1a. to obtain a realistic estimate of the cortical surface containing auditory units, the locations of the recorded units were projected on an unrolled and flattened surface projection along the medio-lateral coordinate. in this projection, the lateral extension of the auditory responsive area is roughly μm. neuroanatomy in the area responsive to acoustic stimuli four major fields are recognized based on cyto- and myeloarchitectural features and zinc staining pattern, i.e. the anterior and posterior dorsal fields, an anterior ventral field and a posterior ventral field. the pvf maybe further subdivided into a dorsal and a ventral part and a border zone, due to minor modifications of the neuroarchitectural characteristics. the topographic position of these fields is depicted in fig. 1c. reliable borders of cortical fields are indicated by solid lines. dashed lines represent the more variable outlines of the ac itself and for pvf possible anatomical subdivisions that are not corroborated by neurophysiological data of the study. frontal sections in fig. give showcase characteristics at two rostro-caudal levels to get a general idea of field differences. total cortical thickness, relative thickness of the different layers, composition of cell types, cell density, content of myelinated fibers and zinc are considered as parameters for the distinction of the different fields. cut-outs of frontal sections stained for cells, myelinated fibers and for zinc from the centers of the different cortical fields are mounted in fig. for detailed comparison. total cortical thickness varies between μm and μm from dorso-rostral to ventro-caudal locations in the ac. despite the high density of granular elements in layer iii and iv of the dorsal fields, the cortical layers v and vi take more of the total thickness of the cortex. therefore, they are addressed as parietal cortical fields. the ventral fields are thought to belong to the temporal cortex, as layers iii and iv dominate layers v and vi in thickness although granular elements are sparse and the cell density is lower in layers iii/iv compared to the dorsal fields. dorsal and ventral fields also differ with respect to layer i, which is generally thinner in the dorsal than in the ventral fields. the myelinated horizontal fibers in the superficial part of layer i form a narrow dense band in the dorsal fields, but a broader one in the ventral fields, which is paralleled by a nearly zinc-free stripe in the ventral fields. the most characteristic features used for delimitation of cortical fields are described in detail as follows. adf has the greatest cortical width that seems to be caused by a doubled layer v: layer v of the non-auditory cortical area dorsal to the ac seems to continue beneath layer v of adf. in avf, the rostral beginning coincides with the most caudal part of the claustrocortex without a sharp border. the myelin content in avf is comparably low, whereas the stain for zinc is generally intense. even the paler staining band corresponding to layer iv is relatively dark. the posterior fields are more homogeneous. cortical width in pdf is smaller than in adf and characteristic differences to the neighboring pvf are obvious at higher magnification. in the zinc stain, layer iv is narrower and more heavily stained in pdf than in pvf, whereas layer v is thicker and of higher staining intensity. pvf has the most conspicuous wide and pale zinc staining band in layer iv, encompassing deep layer iii. layer v shows three subdivisions in the dorsal part, whereas in the ventral part only two subdivisions are recognizable. layer vb, the most intense staining part of layer v is strikingly darker in pvfv. in pvfbz a gradual change of the characteristic features from pvf toward the ventrally adjacent perirhinal cortex takes place, e.g. the layered organization fades as well in the cell stain as in the zinc stain, and so do the myelinated fibers. layer iv appears as a pale band with the lowest staining intensity in the zinc stain and the staining intensity of layer iv varies characteristically between the fields. it is faint in layer iv of pdf, comparably lighter in adf, relatively dark in avf and faintest in pvf. layer iv contains a high number of granular elements in the dorsal fields, whereas the neuronal somata in the ventral fields are larger and the cell density is lower. layer iv is heavily myelinated in all fields except in the sparsely myelinated avf. basic neuronal response properties because it was not always possible to discriminate the activity of a single neuron, the term'unit' will be used in the following to describe the activity of one neuron to clusters of three neurons recorded at a distinct recording site. extracellular recordings were derived from a total of units from both hemispheres of ten bats. the number of units recorded in one bat ranged from to with an average of units per bat. thus, on average two units were recorded per penetration. recording depths of units derived from roughly orthogonal electrode penetrations were in the range of to μm from the cortical surface. the first row shows the number of electrode penetrations in the ac of each bat, the total number of recorded units in each of the ten experimental animals is shown in the second row. best frequency and threshold as shown in fig. 4a, bfs of units ranged from five to khz. seventy nine percent of the units had bfs above khz, i.e. in the range of the dominant harmonics of the echolocation pulse of p. discolor. the response threshold at bf was determined in units and varied between zero and db spl. the frequency distribution of thresholds reaches its maximum around db spl with roughly symmetric flanks to lower and higher thresholds. latency and response duration the first spike latency was measured in cortical units and ranged between five and ms with a bias toward short latencies. median latency of all units was ms. units often exhibited onset responses to pure tones frequently followed by a sustained response component that generally exhibited a considerable amount of variability. other response types e.g. on-off responses were not observed in the ac of p. discolor. the response duration to a ms pure tone was determined in units and ranged from three to ms. short phasic responses with durations below ms were found in % of units, medium response durations between and ms were found in % of units and % of units had response durations longer than ms. thus, in % of units the response duration exceeded the duration of the pure tone stimulus. frequency response areas q10db values covered a range between and. the distribution showed a peak around five, indicating that sharp frequency tuning is rare in cortical units of p. discolor. the regression line in fig. 4h shows that q10db values roughly increased with increasing bfs. q30db values covered the range between and. the difference between the medians of the q10db and q30db values suggests that in many units the sharpness of frequency tuning was only slightly decreasing with increasing stimulus level. in units, the fras could be classified into six different types. most units showed a v-shaped fra with equal share of monotonically and non-monotonically responses. sixteen percent of cortical units showed double- or multi-tuned fras two thirds of which displayed monotonic and one third non-monotonic response behavior. in % of multiple tuned units, threshold minima were roughly harmonically related. in six percent of units, the fras constituted closed areas in the frequency-intensity field with spike rates dropping to zero at all frequencies with increasing stimulus level. the remaining units featured complex-shaped fras. binaural response properties in units the binaural response properties were measured and units were classified following their contra versus ipsilateral response characteristics. ee responses were measured in % of units, % of units were inhibited by the ipsilateral ear and % of units showed no reaction to ipsilateral stimulation. four percent of units, could not be classified in one of the above classes and were thus named as "other". representation of neuronal response properties in cortical fields the local representation of selected response properties on the cortical surface was visualized using the voronoi tessellation method. the tessellation field around the recording site has its limits at halfway distance to the next surrounding recording sites and its color displays the respective response strength of the unit at the center. this forcibly entails large tessellation fields in areas in which recording density is lower or in border regions. as a consequence, there is no proper meaning in the area of single tessellation fields, but rather the clustering or systematic trends of tessellation fields within the ac are important information. response properties of cortical units were analyzed for the four major subfields. possible anatomical subdivisions of pvf were not corroborated by the neurophysiological data, and thus, pvf was analyzed as a whole. representation of best frequency and q10db the organization of bfs within the ac of p. discolor is shown in fig. 6a. units in the dorsal fields had a relatively restricted range of mainly high bfs, which covered frequencies in the range of the dominant harmonics of the echolocation pulse of this species. most units in the adf had bfs above khz, whereas units in the pdf had mainly bfs above khz. only at the most rostral positions of the adf units with low bfs were found. no tonotopic arrangement of bfs was found in the dorsal fields. in contrast, fields in the ventral part of the ac showed a tonotopic organization of bfs. the frequency gradient in avf developed along the rostro-lateral to caudo-medial axis with low bfs at rostro-lateral positions, whereas in pvf the bfs decreased from rostral to caudal cortical positions. thus, pvf and avf shared a common high-frequency border. the rough direction of bf gradients in the ventral fields is schematically shown in fig. 6b. as shown in fig. 6d, the median bf in pdf was significantly higher than the median bfs of the other cortical fields. the lowest median bf was found in pvf. q10db values of units were also not uniformly distributed on the cortical surface but showed a tendency to increase from anterior to posterior locations. in both anterior fields, units showed broader frequency tuning with significantly lower q10db values than found in units in the posterior fields. representation of threshold, latency and response duration the distribution of first spike latencies in the ac of p. discolor is shown in fig. 6g. as a general trend, short first spike latencies were preferentially represented in units in the two anterior fields, whereas in units in the posterior fields also long latencies were found. the median latency of pvf was significantly longer than the median latencies of adf, avf and pdf. the representation of response durations within the ac showed the same trend as the representation of first spike latencies: short durations were characteristic for units in the anterior fields, whereas in the posterior fields units with long response durations were found, too. the median response duration of units in adf and avf were significantly shorter than the median response duration of units in pvf but only slightly shorter than the median response duration of units in pdf. in addition, the median response duration in pdf was significantly shorter compared to pvf. representation of fra type and binaural response properties the representation of the different fra-types showed slight differences between anterior and posterior cortical fields. most units of anterior fields had monotonic v-shaped or monotonic double tuned fras, whereas non-monotonic v-shaped and non-monotonic double tuned fras were mainly found in posterior fields. the cortical representation of double tuned fras with harmonically related components did not show a specific clustering within certain subfields. discussion this study investigated the localization, neuroarchitecture and basic physiological response properties of the auditory cortex of the bat p. discolor. the large area of neocortex responsive to acoustic stimulation comprises four anatomically distinguishable fields: a ventral part of the ac with an anterior and a posterior subdivision and a dorsal part of the ac further divided into anterior and posterior subfields, adf and pdf. the anatomical parcellation of the ac was consistent with the differentiated distribution of basic acoustical response properties of units in the cortical subfields. location and extension of the ac of p. discolor as defined by neurophysiological and neuroarchitectural methods in this study roughly corroborated the findings from an earlier 2-dg study in the same species by esser. comparison with the auditory cortex of other bats the distribution of bfs in the ac of p. discolor showed an overrepresentation of frequencies in the spectral range of the bat's echolocation pulse. this overrepresentation might be due to the fact that the dorsal fields are mainly containing units with bfs in the high-frequency range. thus, a larger amount of cortical surface is devoted to units with high bfs as compared to units with low bfs. in addition, more units were recorded from the dorsal fields and this bias might also be reflected in the distribution of bfs. a similar overrepresentation of bfs in the spectral range of the echolocation pulses has been found in the ac of other microchiropteran bats e.g. e. fuscus, myotis lucifugus and c. perspicillata. in the cf/fm bats p. parnellii, r. rouxi and r. ferrumequinum the overrepresentation of a narrow call-specific spectral range of the cf-part is reflected throughout the ascending auditory pathway and is based on specializations already implemented on the cochlear level. in cf/fm bats, a large part of the ac is involved in processing of specific combinations of the different harmonics in the echolocation pulse and echo. in p. parnellii and slightly less distinctive in r. rouxi, several fields can be segregated by virtue of selective responses of neurons to different combinations of fm-parts or cf components of call and echo. neurons are tuned to delays between the components and are topographically ordered. such a complex representation of pulse and echo delay has generally not been found in fm-bats so far, although pulse-echo delay sensitive neurons exist in the ac of e.g. m. lucifugus and in the dorsal parts of the ac of the phyllostomid bat c. perspicillata, which is closely related to p. discolor. in the present study, only pure tones but no echolocation related stimuli were used for acoustic stimulation. consequently, no statements concerning the functional involvement of cortical subfields other than pure tone processing can be made, so far. besides that, the organization and parcellation of the ac of p. discolor is generally in accordance with that of c. perspicillata. the dorsal fields in the ac of carollia were similarly characterized by neurons with bfs in the high-frequency range, and the ventral part of the ac was also composed of two tonotopically organized subfields, which shared a common high-frequency border. in contrast, the ac of other non-phyllostomid fm bats like m. lucifugus and e. fuscus showed differentiation into subfields to a lesser degree. in both species tonotopically organized fields with frequency gradients running in opposite directions were found, but no fields with characteristics similar to the dorsal cortical fields in p. discolor were described. a high percentage of units in the ac of p. discolor had first spike latencies below ms. as other studies in fm-bats reported mean latencies measured at to db above threshold around to ms already on the level of the ic, this seems to be unusually short. however, short latencies in the ac of fm-bats were also reported. in addition, haplea et al. showed that on the level of the ic latencies in e. fuscus still could be as short as about three milliseconds. although most cortical neurons in r. rouxi had latencies in the range between to ms, short latencies below ms were found in a substantial % of neurons. the bias toward short latencies below ms in p. discolor might be explained by the fact that a large number of recordings in our study were derived from dorsal regions of the ac where short latencies are predominately represented. in contrast to findings in other mammals where the ai has been described to have the shortest latencies, the lowest median first spike latency in p. discolor was found in the adf, whereas in the pvf, which might be equivalent to the ai, the longest first spike latencies were found. our findings in p. discolor are supported by the fact that in the ac of r. rouxi shortest latencies were also found in the dorsal fields. in r. rouxi, ventral and dorsal cortical fields are not sequentially connected but rather receive connections from different parts of the auditory thalamus via different projection pathways. in p. discolor, a similar connection pattern could explain the shorter latencies in the dorsal fields compared to the ventral fields. however, this hypothesis needs to be proved by future tracer studies in this species. influence of anesthesia and multi unit recordings on response patterns a large proportion of units in the p. discolor ac showed onset-type responses. it is known that the response pattern of cortical neurons is influenced by anesthetics. for example, the use of barbiturates like pentobarbital that enhances gabaa-mediated inhibition biased the distribution of response types toward short onset responses in the rat auditory cortex. the drug midazolam used in our study, like other benzodiazepines, also enhances action of gaba on gabaa receptors. thus, the high abundance of phasic onset-type responses in our study could be a consequence of the use of midazolam. however, a high percentage of onset-type response patterns were also found in neurons in the ac of awake r. rouxi and awake mustached bats, indicating that even in the un-anesthetized bat phasic responses are common. this minimizes the potential effect of anesthetics on response types of cortical units in the present study. another point that could have influenced the response pattern is that recordings in our study were not always derived from single units. seshagiri and delgutte showed that adjacent neurons recorded simultaneously with tetrodes in the inferior colliculus of cats could differ significantly in their temporal discharge pattern although bfs and threshold were highly correlated. therefore, blurring of response pattern is possible in some cases in multi unit recordings, but markedly dependent on the number of contributing neurons with dissimilar response types. as multi unit recordings in this study were generally derived from only up to three neurons, and most units showed phasic responses the pooling of spikes in multi units should not have severely effected the response pattern in most cases. parcellation of the ac in p. discolor one way of defining different fields of ac is by comparing the afferent connections with the auditory thalamus. common to all mammals is a strong tonotopically organized projection from the ventral division of the medial geniculate body to ai. in contrast to this, non-tonotopically organized non-primary cortical areas receive major projections from the dorsal division of the mgb. cf/fm bats like horseshoe bats and p. parnellii follow this connectivity pattern. in both animals, ai contains a tonotopic map where frequencies of the cf-components of the calls are overrepresented while frequencies of the fm-components are largely absent. in r. rouxi, neurons specialized for the processing of acoustic parameters relevant for echolocation are mainly represented in dorsally located cortical areas that are targeted by projections from the mgbd. the view of a congeneric projection from mgbv to ai covering the entire frequency range has been challenged by recent findings in the gleaning bat antrozous pallidus. in this bat, the low frequency portion of a coherent frequency representation in the ac is innervated by the mgbv, whereas the high-frequency part received major projections from the supra-geniculate nucleus, a part of mgbd. neurons in the high-frequency range were moreover of a distinct binaural input type and preferred fm-sweeps, whereas neurons in the low frequency part were classified as ei and responded preferentially to noise. two functionally distinct pathways evidently exist in this bat species: first, a low frequency pathway serving for passive prey location and second, a high-frequency pathway serving for obstacle avoidance during active echolocation. the approach that part of ai may not receive input from the mgbv challenges one of the traditional definitions for ai via connectivity that, however, does not constitute the only valid criterion. the distribution of binaural response types in p. discolor resembles that in a. pallidus, whereas neuroarchitectonic and neurophysiological features argue for a cortical fields pattern of the ac in p. discolor similar to the arrangement in r. rouxi and other mammals. tracer experiments were not included in the present study in p. discolor, and consequently data on thalamo-cortical connections are not available. however, neuroarchitectonic features provide valuable tools for the definition of subdivisions in ac when combined with neurophysiological outcomes. undoubtedly, the combination of all three approaches would yield optimal possibilities to classify cortical subdivisions. the specific usefulness of different neurohistological stains to determine cortical fields will be discussed in the following: the staining for zinc proved to be specifically helpful, complementing staining for cells and myelinated fibers. the zinc stain reveals horizontal bands in the neocortex that show characteristic variation in width and staining intensity among cortical regions, sharply defining the borders between cortical areas and their subdivisions, and are generally coincident with the limits defined in the nissl stained sections. the banding patterns are stable among the species studied so far, allowing for an interspecies comparison. the pvf shows several features in the zinc stain characterizing primary sensory regions in the mammalian neocortex: conspicuous pale zinc staining of the outer portion of layer i, the differentiation of layer v in sublayers and, most characteristic, a wide, light band corresponding to layer iv encompassing the lower portion of layer iii. in pdf, most distinctive to the pvf is the zinc staining of layer iv, which is narrower and more heavily stained, whereas layer v is thicker and of higher staining intensity. zinc staining in avf is generally intense. these features of pdf and adf have been described to be characteristic for secondary sensory regions. furthermore, the structure of adf seems unique in that it could be an amalgam of auditory and neighboring non-auditory, probably somatosensory areas. mixed auditory-somatosensory fields neighboring the ac rostrally have been reported for several mammalian species. the accordance of response properties of cortical units in avf and pvf with the anatomical partitioning of ventral auditory fields in the cortex is a further strong support, that the pvf might correspond to ai in p. discolor. pvf is tonotopically organized with bfs increasing from caudal to rostral. the avf shows also a tonotopic organization but in contrast to the pvf, bfs increase from rostro-ventral to caudo-dorsal locations. this organization is in accordance to findings in other mammals where tonotopic gradients with roughly opposite directions have also been identified in the ai and the aaf. because of the common high-frequency border of the avf and pvf, high frequencies are represented all the way along the medial border of the pvf down to the most ventral border of this field. in addition, no clear tonotopic gradient can be seen in the pdf that could be a tonotopic continuation of the frequency representation in the ventrally located pvf. thus, it is unlikely that the pdf just represents the high-frequency portion of a contiguous frequency representation starting with low frequencies in the pvf as assumed for the ac of a. pallidus. it is more likely that the pdf represents a non-tonotopically organized field like the dorsal fields found in other bats before. however, also in c. perspicillata information about the thalamo-cortical connections is lacking. in addition to bf, the distribution of several response properties like duration and first spike latency showed significant differences between the pdf and the pvf. to further test the possibility that high-frequency units in the pdf and pvf belong to a contiguous frequency representation like in a. pallidus, we additionally tested the differences of latency and response duration only for the high-frequency units of both cortical fields. latency and response duration for high-frequency units alone were still significantly different between pdf and pvf. these findings further strengthen our parcellation of the ac based on the anatomical findings.  in this study, four subfields were identified in the ac of the bat p. discolor. these differed in their neuroanatomical attributes and in the response properties of their units to pure tone stimulation. the neuroanatomical and neurophysiological properties of the tonotopically organized pvf reflected common characteristics of ai in the mammalian ac. the avf located rostrally to the pvf showed a tonotopic bf gradient running roughly in opposite direction to the gradient of the pvf. thus, the avf of p. discolor might represent the aaf described in other mammals. the dorsal part of the ac of p. discolor was non-tonotopically organized with bfs mainly in the high-frequency range. as most energy of the echolocation pulse of this bat species is contained in the frequency range between to khz, the dorsal region of the ac of p. discolor seems to be particularly important for echolocation. with regard to the relative position of cortical fields and their basic properties of bf representation, the ac of p. discolor seems to follow the organizational pattern seen in other phyllostomid bats. to further compare the parcellation of the ac of p. discolor with those of other bats and mammals, the specific connectivity between subdivisions of the ac and different divisions of the auditory thalamus of p. discolor must be investigated in future studies. 
the dorsal cortical fields were not tonotopically organized but contained neurons that were responsive to high frequencies only. responses of units to pure tone stimulation were recorded extracellularly. the auditory cortical area of p. discolor is located at parieto-temporal portions of the neocortex. cortical units were characterized and classified depending on their response properties such as best frequency, auditory threshold, first spike latency, response duration, width and shape of the frequency response area and binaural interactions. the representation of response properties within the different auditory cortical fields was analyzed in detail. it covers a rostro-caudal range of about μm and a medio-lateral distance of about μm on the flattened cortical surface. besides the primary auditory cortex, echolocating bats have cortical fields for the processing of temporal and spectral features of the echolocation pulses. based on neurophysiological and neuroanatomical criteria, the auditory cortex of p. discolor could be subdivided into anterior and posterior ventral fields and anterior and posterior dorsal fields. the mammalian auditory cortex can be subdivided into various fields characterized by neurophysiological and neuroarchitectural properties and by connections with different nuclei of the thalamus. this paper reports on location, neuroarchitecture and basic functional organization of the auditory cortex of the microchiropteran bat phyllostomus discolor. the auditory cortex of p. discolor resembles the auditory cortex of other phyllostomid bats in size and basic functional organization. the tonotopically organized posterior ventral field might represent the primary auditory cortex and the tonotopically organized anterior ventral field seems to be similar to the anterior auditory field of other mammals. the two ventral fields were distinguished by their tonotopic organization with opposing frequency gradients. as most energy of the echolocation pulse of p. discolor is contained in the high-frequency range, the non-tonotopically organized high-frequency dorsal region seems to be particularly important for echolocation. the auditory cortices of ten adult p. discolor were electrophysiologically mapped in detail.
 the mammalian auditory cortex can be subdivided into various fields characterized by neurophysiological and neuroarchitectural properties and by connections with different nuclei of the thalamus. besides the primary auditory cortex, echolocating bats have cortical fields for the processing of temporal and spectral features of the echolocation pulses. this paper reports on location, neuroarchitecture and basic functional organization of the auditory cortex of the microchiropteran bat phyllostomus discolor.  the auditory cortical area of p. discolor is located at parieto-temporal portions of the neocortex. it covers a rostro-caudal range of about μm and a medio-lateral distance of about μm on the flattened cortical surface. the auditory cortices of ten adult p. discolor were electrophysiologically mapped in detail. responses of units to pure tone stimulation were recorded extracellularly. cortical units were characterized and classified depending on their response properties such as best frequency, auditory threshold, first spike latency, response duration, width and shape of the frequency response area and binaural interactions. based on neurophysiological and neuroanatomical criteria, the auditory cortex of p. discolor could be subdivided into anterior and posterior ventral fields and anterior and posterior dorsal fields. the representation of response properties within the different auditory cortical fields was analyzed in detail. the two ventral fields were distinguished by their tonotopic organization with opposing frequency gradients. the dorsal cortical fields were not tonotopically organized but contained neurons that were responsive to high frequencies only.  the auditory cortex of p. discolor resembles the auditory cortex of other phyllostomid bats in size and basic functional organization. the tonotopically organized posterior ventral field might represent the primary auditory cortex and the tonotopically organized anterior ventral field seems to be similar to the anterior auditory field of other mammals. as most energy of the echolocation pulse of p. discolor is contained in the high-frequency range, the non-tonotopically organized high-frequency dorsal region seems to be particularly important for echolocation. 