bluuebunny/crossref_metadata_2025 huggingface dataset:
Responses of neurons in macaque MT to stochastic motion signals
Authors: Britten Kenneth H. , Shadlen Michael N. , Newsome William T. , Movshon J. Anthony
Published: June 2009

Dynamic random-dot stimuli have been widely used to explore central mechanisms of motion processing. We have measured the responses of neurons in area MT of the alert monkey while we varied the strength and direction of the motion signal in such displays. The strength of motion is controlled by the proportion of spatiotemporally correlated dots, which we term the correlation of the stimulus. For many MT cells, responses varied approximately linearly with stimulus correlation. When they occurred, nonlinearities were equally likely to be either positively or negatively accelerated. We also explored the relationship between response magnitude and response variance for these cells and found, in general agreement with other investigators, that this relationship conforms to a power law with an exponent slightly greater than 1. The variance of the cells' discharge is little influenced by the trial-to-trial fluctuations inherent in our stochastic display, and is therefore likely to be of neural origin. Linear responses to these stochastic motion stimuli are predicted by simple, low-level motion models incorporating sensors having relatively broad spatial and temporal frequency tuning.
Motion selectivity and the contrast-response function of simple cells in the visual cortex
Authors: Albrecht Duane G. , Geisler Wilson S.
Published: June 2009

The responses of simple cells were recorded from the visual cortex of cats, as a function of the position and contrast of counterphase and drifting grating patterns, to assess whether direction selectivity can be accounted for on the basis of linear summation. The expected responses to a counterphase grating, given a strictly linear model, would be the sum of the responses to the two drifting components. The measured responses were not consistent with the linear prediction. For example, nearly all cells showed two positions where the responses approached zero (i.e. two “null phase positions”); this was true, even for the most direction selective cells. However, the measured responses were consistent with the hypothesis that direction selectivity is a consequence of the linear spatiotemporal receptive-field structure, coupled with the nonlinearities revealed by the contrast-response function: contrast gain control, halfwave rectification, and expansive exponent. When arranged in a particular sequence, each of these linear and nonlinear mechanisms performs a useful function in a general model of simple cells. The linear spatiotemporal receptive field initiates stimulus selectivity (for direction, orientation, spatial frequency, etc.). The expansive response exponent enhances selectivity. The contrast-set gain control maintains selectivity (over a wide range of contrasts, in spite of the limited dynamic response range and steep slope of the contrast-response function). Rectification conserves metabolic energy.
Extraretinal representations in area V4 in the macaque monkey
Authors: Maunsell John H. R. , Sclar Gary , Nealey Tara A. , DePriest Derryl D.
Published: June 2009

Several neurophysiological studies have shown that the visual cerebral cortex of macaque monkeys performing delayed match-to-sample tasks contains individual neurons whose levels of activity depend on the sample the animal is required to remember. Haenny et al. (1988) reported that the activity of neurons in area V4 of monkeys performing an orientation matching task depends on the orientation for which the animal is searching. It was proposed that these neurons contribute to a representation of the orientation being sought.

We have further characterized these neurons by recording visual responses from individual neurons during multiple behavioral tasks. Animals were trained to perform an orientation match-to-sample task using either a visual or a tactile orientation sample. In a set of 89 neurons examined using both types of sample, 25% showed statistically significant effects of sample orientation regardless of whether the sample was visual or tactile. Most of these preferred the same sample orientation in both conditions. These results allow us to specify the nature of the information signaled by these neurons more precisely than has previously been possible.

For 193 units tested using one of the matching tasks, responses were also recorded while the animal performed a simple fixation task. In this task the animal was not required to attend to the visual stimuli that were presented. A few neurons that were responsive during the matching task were silent during fixation, but a comparable number was much more responsive during fixation. Across the whole population there was no systematic change in either responsivity or selectivity for orientation under the two conditions.
Reduction of ocular blood flow results in glial fibrillary acidic protein (GFAP) expression in rat retinal Müller cells
Authors: Osborne N. N. , Block F. , Sontag K. -H.
Published: June 2009

Reduction of blood flow to the rat retina was achieved by either clamping both carotid arteries briefly (24 min) or combining clamping of the carotid arteries with permanent occlusion of the vertebral arteries. Analysis of retinas 6 days after operations showed that GFAP immunoreactivity is expressed throughout the retinal Müller cells, although this was variable in retinas from animals where only the carotid arteries were clamped. GFAP immunoreactivity was not associated with retinal Müller cells from control animals and no obvious neuronal damage was observed in retinas from operated animals. These data suggest that Müller-cell GFAP expression may be used as an index to follow possible processes leading to an ischemic insult.
Spectral properties of short-wavelength (blue) cones in the turtle retina
Authors: Itzhaki Aviran , Malik Shoshana , Perlman Ido
Published: June 2009

Long- and medium-wavelength cones in the turtle retina participate in complex neural interactions. They are coupled via excitatory pathways to other cones and receive negative feedback inputs from luminosity-type horizontal cells. Little information has been collected on the S- (short-wavelength or blue) cones because they are scarce in the turtle retina and of smaller dimensions compared to the other cone types.

In this paper, flash sensitivity action spectra and photoresponses of seven turtle S-cones were measured in the dark-adapted state and during chromatic background illuminations. The desensitizing action of monochromatic background lights was not uniform across the visible spectrum. A red background was most effective in desensitizing the S-cones to long-wavelength stimuli while a blue background light produced its strongest action on the photoresponses elicited by short-wavelength stimuli. The effects of chromatic adaptation on the S-cone action spectrum and on the kinetics of the small-amplitude photoresponses suggested that the S-cones in the turtle retina were involved in complex neural interactions. These included excitatory inputs probably originating in neighboring L-cones and inhibitory long-wavelength inputs probably mediated by L-type horizontal cells.
Development of vernier acuity and grating acuity in normally reared monkeys
Authors: Kiorpes Lynne
Published: June 2009

The developmental time courses for vernier acuity and grating acuity were measured longitudinally in infant Macaca nemestrina monkeys. Behavioral measurements of vernier and grating acuity were made at regular intervals during development. Near birth, grating acuity is relatively more mature than vernier acuity. The proportional rate of vernier acuity development is faster than that for grating acuity. During the course of development, grating acuity improves approximately 15-fold whereas vernier acuity improves about 60-fold. Both visual functions approach adult levels at about the same age, around 40 weeks postnatally. Although grating acuity develops about four times faster in monkeys than in humans, vernier acuity development in monkeys and humans does not appear to reflect the same relationship. Adult levels of vernier acuity for the monkeys are about a factor of 2 poorer than are typically reported for humans. The differential development of vernier acuity and grating acuity does not necessarily reflect development at different levels of the visual system.
Subcortical connections of visual areas MST and FST in macaques
Authors: Boussaoud Driss , Desimone Robert , Ungerleider Leslie G.
Published: June 2009

To examine the subcorctical connections of the medial superior temporal and fundus of the superior temporal visual areas (MST and FST, respectively), we injected anterograde and retrograde tracers into 16 physiologically identified sites within the two areas in seven macaque monkeys. The subcortical connections of MST and FST were found to be very similar. Both areas were found to be reciprocally connected with the pulvinar, mainly with its medial subdivision, and with the claustrum. Nonreciprocal projections from both MST and FST were consistently found in the striatum (caudate and putamen), reticular nucleus of the thalamus, and the pontine nuclei. The labeled terminals in the pons were in the dorsolateral, lateral, dorsal, and peduncular nuclei. Additional nonreciprocal projections were found in one MST and one FST case to the nucleus of the optic tract, and, in one FST case, to the lateral terminal nucleus. Finally, three cases showed a nonreciprocal projection to FST from the basal forebrain. The subcortical structures containing label following MST and FST injections were largely the same as those labeled after injections of the middle temporal visual area (MT), but the label within each structure after MST and FST injections was more widespread than that from MT, overlapping the distribution of label that has been reported after injections of parietal visual areas. This finding is consistent with the known contributions of MST and FST to the functions of parietal cortex, such as eye-movement control.
Binocular depth perception following early experience with interocular torsional disparity
Authors: Shinkman Paul G. , Timney Brian , Isley Michael R.
Published: June 2009

The relationship between the behavioral and physiological consequences of rearing with optically induced cyclotropia was assessed. Beginning at the age of 4 weeks, kittens wore goggles that rotated the visual field in opposite directions in each eye for several hours each day over a period of several weeks. The amounts of interocular rotation were 0 deg (control), 16 deg, and 32 deg. Subsequently, they were tested to determine their monocular and binocular depth thresholds and, in some cases, visual acuity. In several kittens recordings were also made from the visual cortex. Binocular performance of all kittens in the 0-deg condition and three out of six kittens in the 16-deg condition was comparable to, although slightly lower than, that of normally reared kittens. In contrast, none of the 32-deg kittens showed any evidence of the binocular superiority that would suggest the presence of stereopsis. Extracellular unit recordings from the visual cortex confirmed our earlier results with goggle-reared kittens. In 16-deg kittens, the distribution of the cells' preferred interocular disparities (IOD) in receptive-field orientation showed a compensating shift so that the mean matched the experienced rotational disparity. In the 32-deg kittens, binocularity was greatly disrupted and there was no compensatory shift in the IOD distribution. Two 32-deg kittens were afforded 3 years of subsequent normal visual experience. Both the behavioral and the physiological findings were unaffected by normal visual exposure in adulthood. Control measurements of acuity indicated that any deficits in depth perception were not due to reduced spatial-resolution abilities. The data indicate that the kitten visual system is able to maintain functional binocularity sufficient to subserve a moderate level of stereoacuity with interocular rotations of up to at least 16 deg.
Visual thresholds in albino and pigmented rats
Authors: Tejada Pilar Herreros De , Green Daniel G. , Tedó Carmen Muñoz
Published: June 2009

Albino rats have recently been reported to have increment thresholds against dim backgrounds that are two log units higher than those of pigmented rats. We, on the other hand, have failed to confirm these differences using electroretinogram b waves and pupillary light reflexes. This paper reports on experiments using evoked potentials from cortex and colliculus and single-unit recordings from colliculus.

We recorded visual-evoked potentials from cortex and superior colliculus in the strains of albino (CD) and pigmented (Long-Evans) rats used in the earlier studies. Thresholds were determined on eight fully dark-adapted animals by extrapolating intensity-response curves to the point at which there was zero evoked potential. The average dark-adapted threshold for the visual-evoked cortical potential was —5.26 log cd/m2in pigmented and —5.80 log cd/m2 in albino animals. The average dark-adapted threshold for the superior colliculus evoked response was —5.54 log cd/m2 in pigmented and —5.84 log cd/m2 in albinos. The differences were not statistically significant. On the same apparatus, the average absolute threshold for three human observers was —5.3 log cd/m2, a value close to the rat dark-adapted thresholds. Thus, visual-evoked cortical potentials and superior collicular evoked potentials failed to confirm the report of higher dark-adapted thresholds for albinos. In addition, we find that single units in superior colliculus in the albino rat respond to very dim flashes.
Light has many meanings for cephalopods
Authors: Young J. Z.
Published: June 2009

The uses of light for cephalopods living at various depths are described. Aphakic apertures are shown in the eyes ofAmphitretusand bolitaenids. In cirrate octopods, the eye is an open cup without lens and the retinal rhabdoms are disorganized. The photosensitive vesicles of cephalopods are extraocular receptors present either in the mantle or on the head. In some mesopelagic forms, they serve to compare the downwelling light with that emitted by the animal's own photophores, thus allowing regulation of counterillumination. In bathypelagic species, the photosensitive vesicles are very large and may serve to ensure reproduction at great depths.

Some of the uses of the paired eyes in shallow water species are discussed. The mechanism for visual learning consists of a system for allowing many possible combinations of the output from numerous feature detectors. This begins with a set of columns in the optic lobes, followed by a tangential system. Outputs from the optic lobe lead to either attack or retreat: a third output leads to a memory system of four matrices allowing for interaction among the visual signals and between them and signals of taste or pain. These matrices allow conjunctive interaction between particular sets of signals and the setting up of memories ensuring appropriate responses. The matrices may be considered as analogous with those of the mammalian hippocampus. They include re-excitation among themselves and with the optic lobes. The tactile memory apparatus of the octopus has four similar lobes and also makes use of the four lobes of the visual system. These are therefore striking examples of adaptive networks allowing learned reactions by statistical selection among numerous channels. The anatomy, function, and generalizing powers of these networks emerged from Boycotts's early work, whose significance for computation can now be appreciated.
Cone bipolar cells and cone synapses in the primate retina
Authors: Boycott B. B. , Hopkins J. M.
Published: June 2009

Primate retinal bipolar cells synapsing with two adjacent cones (2C bipolars) are further described. Their synaptic contacts are either as the central (invaginating) component of the cone triads or as basal (flat) contacts on the membrane of the cone pedicle base. Correspondingly, their axons end either in the b (inner) half or in the a (outer) half of the inner plexiform layer. The shape and size of the axon terminals of 2C bipolars are indistinguishable from those of adjacent midget bipolars. Therefore 2C bipolars, like midget bipolars, probably synapse with midget ganglion cells. Two C bipolars have not been identified as connected to foveal cones. But they are not restricted to the retinal periphery, as has previously been supposed, since they occur, mixed with midget (single cone) bipolars, throughout all parts of the retina from about 2.5 mm to atleast 10.0 mm from the fovea. It is likely that 2C bipolars are a variant of the midget bipolars; and that they contact some members of the same population of cones, instead of the midgets. This paper briefly reviews, and raises some new, problems concerning our current understanding of the synaptic connectivity patterns of the midget, 2C, and diffuse cone bipolar cells.
Discrete reduction patterns of parvalbumin and calbindin D-28k immunoreactivity in the dorsal lateral geniculate nucleus and the striate cortex of adult macaque monkeys after monocular enucleation
Authors: Blümcke Ingmar , Weruaga Eduardo , Kasas Sandor , Hendrickson Anita E. , Celio Marco R.
Published: June 2009

We analyzed the immunohistochemical distribution of the two calcium-binding proteins, parvalbumin (PV) and calbindin D-28k (CB), in the primary visual cortex and lateral dorsal geniculate nucleus (dLGN) of monocularly enucleated macaque monkeys (Macaca fascicularis and Macaca nemestrind) in order to determine how the expression of PV and CB is affected by functional inactivity. The monkeys survived 1–17 weeks after monocular enucleation. The distribution pattern of each of the proteins was examined immunocytochemically using monoclonal antibodies and compared with that of the metabolic marker cytochrome oxidase (CO). We recorded manually the number of immunostained neurons and estimated the concentration of immunoreactive staining product using a computerized image-acquisition system. Our results indicate a decrease of approximately 30% in the labeling of PV-immunoreactive (ir) neuropil particularly in those layers of denervated ocular-dominance columns receiving the geniculocortical input. There was no change in the number of PV-ir neurons in any compartment irrespective of the enucleation interval. For CB-ir, we found a 20% decrease in the neuropil labeling in layer 2/3 of the denervated ocular-dominance columns. In addition, a subset of pyramidal CB-ir neurons in layers 2 and 4B, which are weakly stained in control animals, showed decreased labeling. In the dLGN of enucleated animals, PV-ir and CB-ir were decreased only in the neuropil of the denervated layers.

From these results, we conclude that cortical interneurons and geniculate projection neurons still express PV and CB in their cell bodies after disruption of the direct functional input from one eye. The only distinct decrease of PV and CB expression is seen in axon terminals from retinal ganglion cells in the dLGN, and in the axons and terminals of both geniculocortical projection cells and cortical interneurons in the cerebral cortex.
Platelet-derived growth factor (PDGF) receptors in the developing mouse optic pathway
Authors: Hutchins James B. , Zhang Xiaorong
Published: June 2009

The molecules which control the patterns of cell division, growth, and precise interconnections characteristic of the central nervous system still remain largely unidentified. The protein platelet-derived growth factor (PDGF) has been shown to mediate interactions among glial cellsin vitro.More recent evidence has indicated that PDGF may also be involved in controlling communication between neurons and glial cells and among neurons. The presence of receptors for PDGF on neurons of the developing nervous system is an essential piece of evidence in this chain of events. Ganglion cells are labeled with antibodies to PDGF receptor only during the period of active process outgrowth. These findings suggest that PDGF is used as a mediator of intercellular signaling during neuronal development.
Steady discharges of macaque retinal ganglion cells
Authors: Troy J. B. , Lee B. B.
Published: June 2009

Steady discharges were collected from ganglion cells of the magnocellular (MC) and parvocellular (PC) pathways of the macaque while their receptive fields were uniformly illuminated with a 4.7-deg steady yellow light of photopic illuminance. The mean rates, coefficients of variation, interval distributions, serial correlation coefficients, and power spectra of these discharges were determined. The results presented permit one to estimate the noise power in the discharges of macaque ganglion cells and hence determine how visual signals of different amplitudes will be affected by the noise resident in their discharges.

Although there was some small serial correlation in the discharges of both MC- and PC-pathway cells, their discharges can be considered to result from renewal processes with reasonable accuracy. As with the discharges of cat ganglion cells, macaque ganglion cell discharges can be considered to have approximately gamma-distributed intervals. Steady discharges of MC- and PC-pathway cells show considerable overlap in their statistics, although small but significant differences are present.
Lid-suture myopia in tree shrews with retinal ganglion cell blockade
Authors: Norton Thomas T. , Essinger John A. , McBrien Neville A.
Published: June 2009

To determine whether central communication of retinal signals is necessary for the development of an experimentally induced myopia, tree shrews were exposed to monocular deprivation (MD) while the action potentials of retinal cells in the deprived eye were blocked with intravitreally injected tetrodotoxin (TTX-MD animals). TTX injections (0.6 μ 3 μL) and MD began about 15 days after eye opening, at the start of the susceptible period for the development of lid-suture myopia. Six injections were given, one every second day to produce 12 days of MD and TTX-blockade. Control TTX animals (TTX-open) received TTX in one eye, but not MD, on the same injection schedule and were always found to be behaviorally unresponsive to visual stimuli through the injected eye indicating that TTX blocked central communication of action potentials. Other control animals received intravitreally injected saline in either an open eye (saline-open), or an MD eye (saline-MD). A sham-injected group (sham-inj-MD) received MD and all anesthetic and surgical manipulations except for penetration of the sclera. In all groups, one eye in each animal was an untreated control.

Two effects were found. All MD groups, including the TTX-MD animals, developed a significant vitreous chamber elongation in the deprived eye, indicating that an experimental myopia developed despite ganglion cell blockade. Thus, retinal mechanisms in tree shrew can detect the presence of a degraded visual image and produce an experimental myopia that does not depend on the receipt of visual messages by central neural structures. In addition, eyes in which the sclera was punctured had smaller vitreous chamber depths than comparable uninjected eyes, indicating that puncturing the sclera reduced the normal elongation. These data suggest that forces within the eye normally contribute to its expansion and may be resisted by the choroid and/or the sclera.
Half-squaring in responses of cat striate cells
Authors: Heeger David J.
Published: June 2009

Simple cells in striate cortex have been depicted as rectified linear operators, and complex cells have been depicted as energy mechanisms (constructed from the squared sums of linear operator outputs). This paper discusses two essential hypotheses of the linear/energy model: (1) that a cell's selectivity is due to an underlying (spatiotemporal and binocular) linear stage; and (2) that a cell's firing rate depends on the squared output of the underlying linear stage. This paper reviews physiological measurements of cat striate cell responses, and concludes that both of these hypotheses are supported by the data.
Direction selectivity of cells in the cat's striate cortex: Differences between bar and grating stimuli
Authors: Casanova C. , Nordmann J. P. , Ohzawa I. , Freeman R. D.
Published: June 2009

We have investigated the notion that directional responses of cells in the visual cortex depend on the type of stimulus used to drive the cell. Specifically, we have asked if sinusoidal gratings provide a different estimate of direction selectivity than bars that are brighter or darker than the background.

Using standard techniques, we recorded from 176 cells in the visual cortex of nine cats. For each cell, bright bars, dark bars, and sinusoidal gratings were presented in a randomly interleaved fashion. Complex cells exhibited around twice as many direction-selective as nondirection-selective responses. Estimates of direction selectivity were nearly identical for bright and dark bars and for gratings. For simple cells, a similar ratio of direction-selective to nondirection-selective responses was observed for gratings. However, a larger proportion of simple cells were classified as direction selective when bars were used for stimulation.

A simple cell that exhibited direction selectivity to a grating behaved in a similar manner when stimulated with bright or dark bars. However, in contrast to complex cells, some simple cells classed as directionally nonselective on the basis of their responses to gratings, displayed directionally selective behavior to bars. In addition, the preferred directions for dark and bright bars sometimes differed. These results demonstrate that the classification of a simple cell as directionally selective or nonselective can depend critically on the visual stimulus used.
3H-adenosine uptake selectively labels rod horizontal cells in goldfish retina
Authors: Studholme Keith M. , Yazulla Stephen
Published: June 2009

There are four types of horizontal cell in the goldfish retina, three cone- and one rod-type. The neurotransmitter of only one type, the H1 (cone) horizontal cell, has been identified as GABA. 3H-adenosine uptake was examined as a possible marker for the other classes of horizontal cell. Isolated goldfish retinae were incubated in 3H-adenosine (10–40 μCi) in HEPES-buffered saline for 30 min, then fixed, embedded in plastic, and processed for light-microscopic autoradiography (ARG). For double-label immuno/ARG studies, l-μm-thick sections were processed for GABA postembed immunocytochemistry, then for ARG. 3H-adenosine uptake was localized to cone photoreceptors, presumed precursor cells in the proximal outer nuclear layer, and to a single, continuous row of horizontal cell bodies in the inner nuclear layer. No uptake was localized to the region of horizontal cell axon terminals. 3H-adenosine uptake did not colocalize with GABA-IR in H1 horizontal cells, but it did colocalize with adenosine deaminase immunoreactivity. It is concluded that 3H-adenosine uptake selectively labels rod horizontal cells in the goldfish retina based on position and staining pattern, which are similar to rod horizontal cells stained by Golgi or HRP injection methods. The use of 3H-adenosine uptake may provide a useful tool to study other properties of rod horizontal cells (i.e. development) as well as provide clues as to the transmitter used by these interneurons.
Modulation of rod, but not cone, cGMP-gated photoreceptor channels by calcium-calmodulin
Authors: Haynes Lawrence W. , Stotz Stephanie C.
Published: June 2009

Inside-out patches containing cGMP-gated channels were excised from catfish rod or cone outer segments and held under voltage clamp. The net cGMP-dependent currents elicited by saturating and subsaturating concentrations of cGMP at ±30 mV were measured and the dependence of current upon cGMP concentration was determined. The apparent affinity of the channel for its ligand was estimated by fitting these data with the Hill equation. The concentration of cGMP required to give half the maximum current (K1/2) in rod and cone channels at +30 mV was ~28 μM and ~37 μM, respectively, and was weakly voltage dependent. Thus, cone channels have an intrinsically higherK1/2than rod channels. For both types of channel, the Hill coefficient was ~2.3. In the presence of calcium-calmodulin, the apparent affinity of the rod channel for cGMP decreased by about twofold, but the apparent affinity of the cone channels was unaffected. These results indicate that the open probability of the cone channel for its ligand cannot be modulated by calmodulin. This represents the first significant departure between rod and cone photoreceptors in mechanisms used by phototransduction and suggests that the β subunit of the cone channel must be different from that of the rod channel.
Immunological demonstration of Gq-protein inLimulusphotoreceptors
Authors: Dorlöchter Marlies , Klemeit Monika , Stieve Hennig
Published: June 2009

The phototransduction cascade in invertebrates involves the coupling of rhodopsin activation to the action of the enzyme phospholipase C. This step is performed by G-proteins. An antibody against the α-subunit of a mouse Gqtype G-protein recognized protein bands in Western blots of lateral eye and ventral nerve photoreceptors ofLimulus.The protein bands had an apparent molecular mass of about 42 kDa. The antibody also recognized protein bands of a similar molecular mass in immunoblots of brain and intestine tissue. Immunoreactivity was found in lateral eye frozen sections where it was confined to the rhabdom region. When the antibody was applied to ultrathin sections of ventral nerve photoreceptors, the highest density of labeling was found on the rhabdomeral microvilli, but gold particles were also scattered in the cytoplasm. We conclude that a G-protein of the type Gqparticipates in the phototransduction ofLimulus.
Comparison of contrast-normalization and threshold models of the responses of simple cells in cat striate cortex
Authors: Tolhurst D. J. , Heeger D. J.
Published: June 2009

In almost every study of the linearity of spatiotemporal summation in simple cells of the cat's visual cortex, there have been systematic mismatches between the experimental observations and the predictions of the linear theory. These mismatches have generally been explained by supposing that the initial spatiotemporal summation stage is strictly linear, but that the following output stage of the simple cell is subject to some contrast-dependent nonlinearity. Two main models of the output nonlinearity have been proposed: the threshold model (e.g. Tolhurst & Dean, 1987) and the contrast-normalization model (e.g. Heeger, 1992a, b). In this paper, the two models are fitted rigorously to a variety of previously published neurophysiological data, in order to determine whether one model is a better explanation of the data. We reexamine data on the interaction between two bar stimuli presented in different parts of the receptive field; on the relationship between the receptive-field map and the inverse Fourier transform of the spatial-frequency tuning curve; on the dependence of response amplitude and phase on the spatial phase of stationary gratings; on the relationships between the responses to moving and modulated gratings; and on the suppressive action of gratings moving in a neuron's nonpreferred direction. In many situations, the predictions of the two models are similar, but the contrast-normalization model usually fits the data slightly better than the threshold model, and it is easier to apply the equations of the normalization model. More importantly, the normalization model is naturally able to account very well for the details and subtlety of the results in experiments where the total contrast energy of the stimuli changes; some of these phenomena are completely beyond the scope of the threshold model. Rigorous application of the models' equations has revealed some situations where neither model fits quite well enough, and we must suppose, therefore, that there are some subtle nonlinearities still to be characterized.
Calretinin in the cat retina: Colocalizations with other calcium-binding proteins, GABA and glycine
Authors: Goebel Dennis J. , Pourcho Roberta G.
Published: June 2009

Immunocytochemical techniques were used to determine the distribution of the calcium-binding protein calretinin in the cat retina. Comparisons were made with parvalbumin and calbindin as well as with the inhibitory neurotransmitters GABA and glycine. Calretinin immunoreactivity was seen in horizontal cells and multiple subpopulations of amacrine and ganglion cells. Cone outer segments were also stained. Calbindin immunoreactivity was present in cone photoreceptors, horizontal cells, at least two subtypes of cone bipolar cell, numerous amacrine cells, and cells residing in the ganglion cell layer. Heavy staining for parvalbumin was found in both A- and B-type horizontal cells, distinct subpopulations of amacrine and ganglion cells, and a small population of cone photoreceptor cells. To confirm the identity of cone photoreceptors, comparisons were made with retinas stained for opsins specific for red/green or blue cones (Szé1 et al., 1986). The localization of parvalbumin corresponded with that of blue-type cones only whereas calretinin and calbindin staining showed the same distribution as both red/green and blue cones. Double-label immunofluorescence studies revealed colocalization of all three of the calcium-binding proteins in a number of neurons including horizontal cells and AII amacrine cells. To assess a possible transmitter-specific relationship for calretinin, double-label studies were carried out with GABA and glycine. However, the staining patterns for each of these inhibitory amino acids differed substantially from that of calretinin. The possibility remains that calretinin and other calcium-binding proteins may play a role in neurotransmission through interactions with receptors or second-messenger agents.
Quadrature subunits in directionally selective simple cells: Spatiotemporal interactions
Authors: Emerson Robert C.
Published: June 2009

I explore here whether linear mechanisms can explain directional selectivity (DS) in simple cells of the cat's striate cortex, a question suggested by a recent upswing in popularity of linear DS models. I chose a simple cell with a space-time inseparable receptive field (RF), i.e. one that shows gradually shifting latency across space, as the RF type most likely to depend on linear mechanisms of DS. However, measured responses of the cell to a moving bar were less modulated, and extended over a larger spatial region than predicted by two different popular “linear” models. They also were more DS in exhibiting a higher ratio of total spikes for the preferred direction. Each of the two models used for comparison has a single “branch” with a single spatiotemporally inseparable linear filter followed by a threshold, hence, a “1-branch” model. Nonlinear interactions between pairs of bars in a 2-bar linear superposition test of the cell also disagreed in time-course with those of the 1-branch models. The only model whose 1-bar and 2-bar predictions matched the measured cell (including a complete “4-branch” motion energy model that matches complex cells) has two branches that differ in phase by about 90 deg, i.e. in quadrature. Each branch has its own threshold that helps define the preceding spatiotemporal unit as a subunit even after the outputs of the two branches are summed. As subunit phases differ by only 90 deg, flashing bar responses of the 2-subunit model are similar to those of the 1-subunit model. Therefore, the number of subunits is hidden from view when testing with a conventional stationary bar. In summary, movement responses and nonlinear interactions between pairs of bars in the measured cell matched those of the 2-subunit model, while they disagreed with the popular 1-subunit model. Thus, multiple nonlinear subunits appear to be necessary for DS, even in simple cortical cells.
Quadrature subunits in directionally selective simple cells: Counterphase and drifting grating responses
Authors: Emerson Robert C. , Huang Morgan C.
Published: June 2009

Here we examine further the basis for directional selectivity (DS) in simple cells of the cat's striate cortex. We use a distinctly different input stimulus and different analysis of the output signal for the same type of space-time inseparable receptive field (RF) that was measured with flashing bars in the companion paper (Emerson, 1997). As in the companion paper, we have mimicked a popular “linear” model with a single “branch” that consists of a linear spatiotemporal filter followed by a soft threshold, which should mimic any simple cells that have a single subunit. Counterphase sinusoidal measurements of such a configuration always generate elliptically shaped polar plots of amplitude versus temporal phase, often pinched along the minor axis because of a high threshold. However, for many spatiotemporal frequencies, such polar phase plots, as measured in simple cells by others, show a consistent rotational phase skew. Here we apply counterphase analysis to the same 2-branch model as developed in the companion paper. We show that the model accounts for the skew as the summation of signals from linear filters separated spatially and temporally by approximately 90 deg (i.e. in spatiotemporal phase quadrature), each separated from the output stage by a soft-threshold nonlinearity. We also prove conclusively that such skew cannot be generated by a single-subunit configuration. This demonstration supports the proposed two-subunit structure for DS simple cells, such as in the example from the companion paper, which has strong linear contributions from its inseparable RF. The presence of at least two nonlinear subunits appears to be an obligatory concomitant of DS in all visual cortical cells. The primary function of these subunits may be to enhance the strength of responses to images moving in the preferred direction, as in complex cells. However, subunits may also aid in identifying the moving object through overcoming, at least partially, the phase-concealing properties of the neuron's threshold by generating a steady signal that effectively decreases the threshold for the preferred direction.
Single-neuron activity in the dorsomedial frontal cortex during smooth-pursuit eye movements to predictable target motion
Authors: Heinen S. J. , Liu M.
Published: June 2009

A region of dorsomedial frontal cortex (DMFC) has been implicated in planning and executing saccadic eye movements; hence it has been referred to as a supplementary eye field (SEF). Recently, activity related to executing smooth-pursuit eye movements has been recorded from the DMFC, and microstimulation here has been shown to evoke smooth eye movements. This report documents neuronal activity present in smooth-pursuit tasks where the predictability of target motion was manipulated. The activity of many neurons in the DMFC reached a peak when a predictable change in target motion occurred. Furthermore, the peak activity of some cells was systematically shifted by manipulating the duration of the target event, indicating that the network these neurons were in could learn the temporal characteristics of new target motion. Finally, the activity of most neurons tested was greater when target motion was predictable than when it was unpredictable. The results suggest that the DMFC participates in planning smooth-pursuit eye movements based on past stimulus history.
Spectral sensitivity of macaque monkeys measured with ERG flicker photometry
Authors: Jacobs Gerald H. , Deegan Jess F.
Published: June 2009

Macaque monkeys are widely used as a model species for investigations of the biology of human vision. Previous measurements suggest that the cone-based spectral sensitivity of these two primates is greatly similar, but perhaps not identical. We measured the photopic spectral sensitivity of 42 male macaque monkeys from two species (Macaca mulatto, M. fascicularis) using an objective index, electroretinogram flicker photometry. The variations among individuals and between the two species were very small and there was no evidence for any significant cone pigment polymorphism in this sample. There are small but systematic differences in spectral sensitivity between macaque monkeys and equivalently tested human subjects–the monkeys were slightly more sensitive to short wavelengths (<520 nm) and slightly less sensitive to wavelengths longer than this value. The results obtained from the curve fitting of standard photopigment absorption spectra to the spectral-sensitivity functions suggest that the difference between human and macaque monkey spectral sensitivity principally reflects differences in the relative proportions of the long- and middle-wavelength cones in the retinas of the two species.
Contributions of GABAAreceptors and GABACreceptors to acetylcholine release and directional selectivity in the rabbit retina
Authors: Massey Stephen C. , Linn David M. , Kittila Christopher A. , Mirza Wajid
Published: June 2009

GABA is a major inhibitory neurotransmitter in the mammalian retina and it acts at many different sitesviaa variety of postsynaptic receptors. These include GABAAreceptors and bicuculline-resistant GABACreceptors. The release of acetylcholine (ACh) is inhibited by GABA and strongly potentiated by GABA antagonists. In addition, GABA appears to mediate the null inhibition which is responsible for the mechanism of directional selectivity in certain ganglion cells. We have used these two well-known examples of GABA inhibition to compare three GABA antagonists and assess the contributions of GABAAand GABACreceptors. All three GABA antagonists stimulated ACh release by as much as ten-fold. By this measure, theED50sfor SR-95531, bicuculline, and picrotoxin were 0.8, 7.0, and 14 μM, respectively. Muscimol, a potent GABAAagonist, blocked the effects of SR-95531 and bicuculline, but not picrotoxin. This indicates that SR-95531 and bicuculline are competitive antagonists at the GABAAreceptor, while picrotoxin blocks GABAAresponses by acting at a different, nonreceptor site such as the chloride channel. In the presence of a saturating dose of SR-95531 to completely block GABAAreceptors, picrotoxin caused a further increase in the release of ACh. This indicates that picrotoxin potentiates ACh release by a mechanism in addition to the block of GABAAresponses, possibly by also blocking GABACreceptors, which have been associated with bipolar cells. All three GABA antagonists abolished directionally selective responses from ON/OFF directional-selective (DS) ganglion cells. In this system, theED50sfor SR-95531, bicuculline, and picrotoxin were 0.7 μM, 8 μM, and 94.6 μM, respectively. The results with SR-95531 and bicuculline indicate that GABAAreceptors mediate the inhibition responsible for directional selectivity. The addition of picrotoxin to a high dose of SR-95531 caused no further increase in firing rate. The comparatively high dose required for picrotoxin also suggests that GABACreceptors do not contribute to directional selectivity. This in turn suggests that feedforward GABAAinhibition, as opposed to feedback at bipolar terminals, is responsible for the null inhibition underlying directional selectivity.
Neuronal responses to edges defined by luminance vs. temporal texture in macaque area V1
Authors: Chaudhuri Avi , Albright Thomas D.
Published: June 2009

We examined the responsivity, orientation selectivity, and direction selectivity of a sample of neurons in cortical area V1 of the macaque using visual stimuli consisting of drifting oriented contours defined by each of two very different figural cues: luminance contrast and temporal texture. Comparisons of orientation and direction tuning elicited by the different cues were made in order to test the hypothesis that the neuronal representations of these parameters are form-cue invariant. The majority of the sampled cells responded to both stimulus types, although responses to temporal texture stimuli were generally weaker than those elicited by luminance-defined stimuli. Of those units exhibiting orientation selectivity when tested with the luminance-defined stimuli, more than half were also selective for the orientation of the temporal texture stimuli. There was close correspondence between the preferred orientations and tuning bandwidths revealed with the two stimulus types. Of those units exhibiting directional selectivity when tested with the luminance-defined stimuli, about two-thirds were also selective for the direction of the temporal texture stimuli. There was close correspondence between the preferred directions revealed with the two stimulus types, although bidirectional responses were somewhat more common when temporal texture stimuli were used. These results indicate that many V1 neurons encode orientation and direction of motion of retinal image features in a manner that is largely independent of whether the feature is defined by luminance or temporal texture contrast. These neurons may contribute to perceptual phenomena in which figural cue identity is disregarded.
Intracortical connections are not required for oscillatory activity in the visual cortex
Authors: Ghose Geoffrey M. , Freeman Ralph D.
Published: June 2009

Synchronized oscillatory discharge in the visual cortex has been proposed to underlie the linking of retinotopically disparate features into perceptually coherent objects. These proposals have largely relied on the premise that the oscillations arise from intracortical circuitry. However, strong oscillations within both the retina and the lateral geniculate nucleus (LGN) have been reported recently. To evaluate the possibility that cortical oscillations arise from peripheral pathways, we have developed two plausible models of single cell oscillatory discharge that specifically exclude intracortical networks. In the first model, cortical oscillatory discharge near 50 Hz in frequency arises from the integration of signals from strongly oscillatory cells within the LGN. The model also predicts the incidence of 50-Hz oscillatory cells within the cortex. Oscillatory discharge around 30 Hz is explained in a second model by the presence of intrinsically oscillatory cells within cortical layer 5. Both models generate spike trains whose power spectra and mean firing rates are in close agreement with experimental observations of simple and complex cells. Considered together, the two models can largely account for the nature and incidence of oscillatory discharge in the cat's visual cortex. The validity of these models is consistent with the possibility that oscillations are generated independently of intracortical interactions. Because these models rely on intrinsic stimulus-independent oscillators within the retina and cortex, the results further suggest that oscillatory activity within the cortex is not necessarily associated with the processing of high-order visual information.