bluuebunny/crossref_metadata_2025 huggingface dataset:
The Covert Learning of Affective Valence Does Not Require Structures in Hippocampal System or Amygdala
Authors: Tranel Daniel , Damasio Antonio R.
Published: February 2008

Following bilateral damage to the entire medial temporal lobe and interconnected cortices in the anterior temporal and medial frontal regions, patient Boswell developed a severe learning defect for all types and levels of Factual knowledge, including faces. In the experiments described here, however, we demonstrate that Boswell can acquire a non conscious bond between entirely new persons and the affective valence they display. The finding is important on the followng accounts. First, Boswell's lesions guarantee that the entorhinal and perirhinal cortices, hippocampus, amygdala, and higher-order neocortices in the anterior temporal region are not required to support this form of covert learning. Second, this demonstration is possible only in a patient such as Boswell, because in individuals with normal or only partially impaired factual learning, fact memory will contaminate the performance.
Egocentric Spaw Representation in Early Vision
Authors: Pouget Alexandre , Fisher Stephen A. , Sejnowski Terrence J.
Published: February 2008

Recent physiological experiments have shown that the responses of many neurons in V1 and V3a are modulated by the direction of gaze. We have developed a neural network model of the hierarchy of maps in visual cortex to explore the hypothesis that visual features are encoded in egocentric (spatio-topic) coordinates at early stages of visual processing. Most psychophysical studies that have attempted to examine this question have concluded that features are represented in retinal coordinates, but the interpretation of these experiments does not preclude the type of retinospatiotopic representation that is embodied in our model. The model also explains why electrical stimulation experiments in visual cortex cannot distinguish between retinal and retinospatiotopic coordinates in the early stages of visual processing. Psychophysical predictions are made for testing the existence of retinospatiotopic representations.
Constraints on Plasticity in a Connectionist Model of the English Past Tense
Authors: Marchman Virginia A.
Published: February 2008

This paper investigates constraints on dissociation and plasticity in a connectionist model undergoing random “lesions” both prior to and during training. When networks were trained only on phonological encodings of stem-suhed pairs similar to English regular verbs (e.g., walk  walked), long-term deficits (i.e., “critical period” effects) were not observed, yet there were substantive short-term effects of injury. When training vocabulary reflected the English-like competition between regular (suffixed) and irregular verbs (e.g., go  went, hit  hit), the acquisition of regular verbs became increasingly susceptible to injury, while the irregulars were learned quickly and were relatively impervious to damage. Patterns of generalization to novel forms conflicts with the assumption that this behavioral dissociation is indicative of selective impairment of the learning and generalization of the past tense rule, while the associative lexical-based mechanism is left intact. Instead, we propose a view of network performance in which the regular-irregular dissociation derives from a general reduction in the ability to find a single-mechanism solution when resolving the competition between two classes of mappings. In light of other models in which “regular” and “irregular” forms compete (e.g., Patterson, Seidenberg, & McClelland, 1989), as well as patterns of performance in normal and disordered English speakers (e.g., Pinker, 1991), two general implications are discussed: (1) critical period effects need not derive from endog-enously determined maturational change, but instead may in part result from learning history in relation to characteristics of the language to be learned (i.e., entrenchment), and (2) selective dissociations can result from general damage in systems that are not modularized in terms of rule-based vs. associative mechanisms.
Event-Related Brain Potentials While Encountering Semantic and Syntactic Constraint Violations
Authors: Rösler Frank , Pütz Peter , Friederici Angela , Hahne Anja
Published: February 2008

The objective of the present study was to delineate brain-electrical correlates of semantic and syntactic integration processes during language comprehension. Twenty-eight subjects were engaged in a lexical decision task. The target item (a legal word or a pseudo word) was always preceded by a prime consisting of a sentence fragment that provided a particular context. With respect to the prime a word target could be either a correct completion, a violation of a semantic selection restriction, or a violation of a syntactic subcategorization rule. An N400-like wave was elicited by both types of deviations. Syntactic anomalies evoked a negative shift predominantly over the anterior scalp with a preponderance over the left hemisphere, while semantic anomalies were accompanied by a much more widespread negativity with the maximum over posterior temporal areas. The amplitude of the semantic vie lation effect was found to be related to the strength of the priming constraints. The topographic difference is consistent with the idea that syntactic and semantic aspects of comprehension are handled by different neural subsystems.
Visual Search for Feature and Conjunction Targets with an Attention Deficit
Authors: Arguin Martin , Joanette Yves , Cavanagh Patrick
Published: February 2008

Brain-damaged subjects who had previously been identified as suffering from a visual attention deficit for contralesional stimulation were tested on a series of visual search tasks. The experiments examined the hypothesis that the processing of single features is preattentive but that feature integration, necessary for the correct perception of conjunctions of features, requires attention (Treisman & Gelade, 1980 Treisman & Sato, 1990). Subjects searched for a feature target (orientation or color) or for a conjunction target (orientation and color) in unilateral displays in which the number of items presented was variable. Ocular fixation was controlled so that trials on which eye movements occurred were cancelled. While brain-damaged subjects with a visual attention disorder (VAD subjects) performed similarly to normal controls in feature search tasks, they showed a marked deficit in conjunction search. Specifically, VAD subjects exhibited an important reduction of their serial search rates for a conjunction target with contralesional displays. In support of Treisman's feature integration theory, a visual attention deficit leads to a marked impairment in feature integration whereas it does not appear to affect feature encoding.
The Role of the Anterior Attention System in Semantic Processing of both Foveal and Parafoveal Words
Authors: Fuentes Luis J. , Carmona Encarna , Agis Inmaculada F. , Catena Andrés
Published: February 2008

This research takes advantage of combined cognitive and anatomical studies to ask whether attention is necessary for high-level word processing to occur. In Experiment 1 we used a lexical decision task in which two prime words, one in the fovea and the other in the parafovea, appeared simultaneously for 150 msec, followed by a foveal target (word/nonword). Target words were semantically related either to the foveal or to the parafoveal word, or unrelated to them. In one block of trials subjects were also required to perform an auditory shadowing task. From PET studies we know that shadowing activates the anterior cingulate cortex, involved in selective attention. If the anterior attention system is always involved in semantic processing, shadowing should reduce semantic priming obtained from both foveal and parafoveal words. In contrast, if semantic priming by parafoveal words is independent of activation in that attention area, priming will not be affected by shadowing. Our results supported the latter hypothesis. A large priming effect arose from foveal primes, which was reduced by shadowing. For parafoveal primes a smaller priming effect arose, which was not affected by shadowing. In Experiment 2 prime words were masked. Semantic priming was reliable for both foveal and parafoveal words but there were then no differences between them. Most important, the size of priming was similar to that obtained from parafoveal words in Experiment 1. We conclude that the anterior attention system increases the potency of processing of consciously perceived stimuli, but there is a component of semantic priming that occurs without both focusing of attention and awareness, involving different cerebral areas to those involved in attention to language.
Coding of Spatial Information in the Somatosensory System: Evidence from Patients with Neglect following Parietal Lobe Damage
Authors: Moscovitch Morris , Behrmann Marlene
Published: February 2008

Unilateral parietal lobe damage, particularly in the right cerebral hemisphere, leads to neglect of stimuli on the contra lateral side. To determine the reference frame within which neglect operates in the somatosensory system, 11 patients with unilateral neglect were touched simultaneously on the left and right side of the wrist of one hand. The hand was tested in both the palm up and the palm down position. Patients neglected the stimuli on the side of space contra lateral to the lesion regardless of hand position. These results indicate that point-localization in the somatosensory system is accomplished with respect to a spatially defined frame-of-reference and not strictly with respect to somatotopically defined coordinates.
Responses of Anterior Superior Temporal Polysensory (STPa) Neurons to “Biological Motion” Stimuli
Authors: Oram M. W. , Perrett D. I.
Published: February 2008

Cells have been found in the superior temporal polysensory area (STPa) of the macaque temporal cortex that are selectively responsive to the sight of particular whole body movements (e.g., walking) under normal lighting. These cells typically discriminate the direction of walking and the view of the body (e.g., left profile walking left). We investigated the extent to which these cells are responsive under “biological motion” conditions where the form of the body is defined only by the movement of light patches attached to the points of limb articulation. One-third of the cells (25/72) selective for the form and motion of walking bodies showed sensitivity to the moving light displays. Seven of these cells showed only partial sensitivity to form from motion, in so far as the cells responded more to moving light displays than to moving controls but failed to discriminate body view. These seven cells exhibited directional selectivity. Eighteen cells showed statistical discrimination for both direction of movement and body view under biological motion conditions. Most of these cells showed reduced responses to the impoverished moving light stimuli compared to full light conditions. The 18 cells were thus sensitive to detailed form information (body view) from the pattern of articulating motion. Cellular processing of the global pattern of articulation was indicated by the observations that none of these cells were found sensitive to movement of individual limbs and that jumbling the pattern of moving limbs reduced response magnitude. A further 10 cells were tested for sensitivity to moving light displays of whole body actions other than walking. Of these cells 5/10 showed selectivity for form displayed by biological motion stimuli that paralleled the selectivity under normal lighting conditions. The cell responses thus provide direct evidence for neural mechanisms computing form from nonrigid motion. The selectivity of the cells was for body view, specific direction, and specific type of body motion presented by moving light displays and is not predicted by many current computational approaches to the extraction of form from motion.
Neurophysiological Manifestations of Phonological Processing: Latency Variation of a Negative ERP Component Timelocked to Phonological Mismatch
Authors: Praamstra Peter , Meyer Antje S. , Levelt Willem J. M.
Published: February 2008

Two experiments examined phonological priming effects on reaction times, error rates, and event-related brain potential (ERP) measures in an auditory lexical decision task. In Experiment 1 related prime-target pairs rhymed, and in Experiment 2 they alliterated (i.e., shared the consonantal onset and vowel). Event-related potentials were recorded in a delayed response task. Reaction times and error rates were obtained both for the delayed and an immediate response task. The behavioral data of Experiment 1 provided evidence for phonological facilitation of word, but not of nonword decisions. The brain potentials were more negative to unrelated than to rhyming word-word pairs between 450 and 700 rnsec after target onset. This negative enhancement was not present for word-nonword pairs. Thus, the ERP results match the behavioral data. The behavioral data of Experiment 2 provided no evidence for phonological Facilitation. However, between 250 and 450 msec after target onset, i.e., considerably earlier than in Experiment 1, brain potentials were more negative for unrelated than for alliterating Word-word and word-nonword pairs. It is argued that the ERP effects in the two experiments could be modulations of the same underlying component, possibly the N400. The difference in the timing of the effects is likely to be due to the fact that the shared segments in related stimulus pairs appeared in different word positions in the two experiments.
Monitoring the Visual World: Hemispheric Asymmetries and Subcortical Processes in Attention
Authors: Mangun G. R. , Luck S. J. , Plager R. , Loftus W. , Hillyard S. A. , Handy T. , Clark V. P. , Gazzaniga M. S.
Published: February 2008

Hemispheric specialization and subcortical processes in visual anention were investigated in callosotomy (split-brain) patients by measuring reaction times to lateralized stimuli in a spatial cuing paradigm. Cuing effects were obtained for targets presented to the right hemisphere (left visual hemifield) but not for those presented to the left hemisphere. These cuing effects were manifest as faster reaction times when the cue correctly indicated the location of the subsequent target (valid trials), as compared to trials in which the cue and target appeared in opposite hemifields (invalid trials). This pattern suggests that the right hemisphere allocated attention to cued locations in either visual hemifield, whereas the left hemisphere allocated attention predominantly to the right hemifield. This finding is consistent with a body of evidence from studies in patients with cortical lesions who display different attentional deficits for right versus left hemisphere damage. Because the present pattern occurs in patients whose cerebral hemispheres are separated at the cortical level, it suggests that right hemisphere attentional allocation to events in the ipsilateral visual half-field is mediated in part via intact subcortical systems.
Scene-Specific Memory for Objects: A Model of Episodic Memory Impairment in Monkeys with Fornix Transection
Authors: Gaffan David
Published: February 2008

A series of five experiments investigated the relationship between object memory and scene memory in normal and fornix-transected monkeys. An algorithm created formally defined background and objects on a large visual display; the disposition of some particular objects in particular places in a particular background constitutes a formally defined scene. The animals learned four types of discrimination problem: (1) object-in-place discrimination learning, in which the correct (rewarded) response was to a particular object that always occupied the same place in a particular unique background, (2) place discrimination learning, in which the correct response was to a particular place in a unique background, with no distinctive object at that place, (3) object discrimination learning in unique backgrounds, in which the correct response was to a particular object that could occupy one or the other of two possible places in a unique background, and (4) object discrimination learning in varying backgrounds, in which the correct response was to a particular object that could appear at any place in any background. The severest impairment produced by fornix transection was in object-in-place learning. Fornix transection did not impair object discrimination learning in varying backgrounds. The results from the other two types of learning task showed intermediate severity of impairment in the fornix-transected animals. The idea that fornix transection in the monkey impairs spatial memory but leaves object memory intact is thus shown to be an oversimplification. The impairments of object memory in the present experiments are analogous to the impairments of episodic memory seen in human amnesic patients.
Dissociation of Motor Hyperactivity and Spatial Memory Deficits by Selective Hippocampal Lesions in the Neonatal Rat
Authors: Praag Henriette van , Dreyfus Cheryl F. , Black Ira B.
Published: February 2008

Hippocampal lesions can disrupt the acquisition of new memories and tend to increase motor activity. Although hyperactivity may affect exploration, it is unclear how these performance variables contribute to the learning deficit and it is also not known which brain structures are involved. The present study provides evidence for a dissociation between activity and memo?. Following unilateral or bilateral electrolytic lesions of the hippocampus in neonatal rats, we assessed open field behavior and performance of discrete trials alternation in a T-maze. When tested 6 and 20 weeks postoperatively, rats subjected to bilateral lesions were hyperactive. Their performance in the discrete trials alternation task was impaired. In contrast, rats subjected to unilateral lesions did not display an increase in motor activity, but were still deficient in performance on the T-maze. To define whether these behavioral changes were accompanied by secondary changes in structures that project to the hippocampus, we studied the function of the septum and locus coeruleus after the lesions. Septal choline acetyltrans-ferase (CAT, the acetylcholine-synthesizing enzyme) activity was reduced and tyrosine hydroxylase (TH, the rate-limiting enzyme in catecholamine biosynthesis) activity in noradrenergic lc neurons was increased after both the unilateral and bilateral lesion. Therefore, these changes may contribute to the memory impairment but are not necessarily related to motor hyperactivity. We conclude that a deficit in spatial memory is not attributable to altered performance variables such as activity. Furthermore, spatial memory deficits in both the unilateral and bilateral lesion paradigms may be associated with changes in septal and lc function.
Temporal Integration in Sensorimotor Synchronization
Authors: Mates Jiří , Müller Ulrike , Radil Tomáš , Pöppel Ernst
Published: February 2008

The concept of a temporal integration process in the timing mechanisms in the brain, postulated on the basis of experimental observations from various paradigms (for a review see P$oUppel, 1978), has been explored in a sensorimotor synchronization task. Subjects synchronized their finger taps to sequences of auditory stimuli with interstimulus-onset intervals (ISIs) between 300 and 4800 msec in different trials. Each tonal sequence consisted of 110 stimuli; the tones had a frequency of 500 Hz and a duration of 100 msec. As observed previously, response onsets preceded onsets of the stimuli by some tens of milliseconcls for ISIs in the range from about 600 to 1800 msec. For ISIs longer than or equal to 2400 msec, the ability to time the response sequence in such a way that the response 5 were placed right ahead of the stimuli started to break clown, i.e., the task was fulfilled by reactions to the stimuli rather than by advanced responses. This observation can he understood within the general framework of a temporal integration puce 55 that is supposed to have a maximal capacity (integration interval) of approximately 3 sec. Only if successive stimuli fall within one integration period, can motor programs be initiated properly by a prior stimulus and thus lead to an appropriate synchronization between the stimulus sequence and corresponding motor acts.
Mechanisms of Spatial Attention: The Relation of Macrostructure to Microstructure in Parietal Neglect
Authors: Cohen Jonathan D. , Romero Richard D. , Servan-Schreiber David , Farah Martha J.
Published: February 2008

Parietal-damaged patients respond abnormally slowly to targets presented in the affected hemifield when preceded by cues in the intact hemifield. This inability to disengage attention from the ipsilesional field to reengage it in the contralesional field has been interpreted as evidence for a distinct “disengage” mechanism, localized in parietal cortex. We present a computational model that accounts for normal attentional effects by interactivity and competition among representations of different locations in space, without a dedicated “disengage” mechanism. We show that when the model is lesioned, it produces the “disengage deficit” shown by parietal-damaged patients. This suggests that the deficit observed in such patients can be understood as an emergent property of interactions among the remaining parts of the system, and need not imply the existence of a dedicated “disengage” mechanism in the normal brain.
Endogenously Generated and Visually Guided Saccades after Lesions of the Human Frontal Eye Fields
Authors: Henik Avishai , Rafal Robert , Rhodes Dell
Published: February 2008

Nine patients with chronic, unilateral lesions of the dorso-lateral prefrontal cortex including the frontal eye fields (FEF) made saccades toward contralesional and ipsilesional fields. The saccades were either voluntarily directed in response to arrows in the center of a visual display, or were reflexively summoned by a peripheral visual signal. Saccade latencies were compared to those made by seven neurologic control patients with chronic, unilateral lesions of dorsolateral prefrontal cortex sparing the FEF, and by 13 normal control subjects. In both the normal and neurologic control subjects, reflexive saccades had shorter Latencies than voluntary saccades. In the FEF lesion patients, voluntary saccades had longer latencies toward the contralesional field than toward the ipsilesional field. The opposite pattern was found for reflexive saccades: latencies of saccades to targets in the contralesional field were shorter than saccades summoned to ipsilesional targets. Reflexive saccades toward the ipsilesional field had abnormally prolonged latencies; they were comparable to the latencies observed for voluntary Saccades. The effect of FEF lesions on saccacles contrasted with those observed in a second experiment requiring a key press response: FEF lesion patients were slower in making key press responses to signals detected in the contralesional field. To assess covert attention and preparatory set the effects of precues providing advance information were measured in both saccade and key press experiments. Neither patient group showed any deficiency in using precues to shift attention or to prepare saccades.

The FEF facilitates the generation of voluntary saccatles and also inhibits reflexive saccades to exogenous signals. FEF lesions may disinhibit the ipsilesional midbrain which in turn may inhibit the opposite colliculus to slow reflexive saccades toward the ipsilesional field.
On Neural Codes and Perception
Authors: Newsome William T.
Published: February 2008

Bill Newsome is a professor in the Department of Neurobiology at the Stanford University School of Medicine. He received his B.S. in physics from Stetson University in 1974 and his Ph.D. in biology from Caltech in 1980. Following postdoctoral work at MH, he served on the faculty at SUNY Stony Brook before joining the Stanford faculty in 1988. His research has focused on the neural mechanisms underlying visual perception and visually guided behavior. Bill was a corecipient of the Rank Prize in optoelectronics in 1992, and received the Minerva Foundation's Golden Brain Award in the same year. This fall he received the Spencer Award, granted yearly by the College of Physicians and Surgeons at Columbia University for highly original contributions to research in neurobiology. In addition, he won the Kaiser Award for excellence in preclinical teaching granted annually by the Stanford School of Medicine.
Is There a Social Module? Language, Face Processing, and Theory of Mind in Individuals with Williams Syndrome
Authors: Karmiloff-Smith Annette , Klima Edward , Bellugi Ursula , Grant Julia , Baron-Cohen Simon
Published: February 2008

Many species can respond to the behavior of their conspecifics. Human children, and perhaps some nonhuman primates, also have the capacity to respond to the mental states of their conspecifics, i.e., they have a “theory of mind.” On the basis of previous research on the theory-of-mind impairment in people with autism, together with animal models of intentionality, Brothers and Ring (1992) postulated a broad cognitive module whose function is to build representations of other individuals. We evaluate the details of this hypothesis through a series of experiments on language, face processing, and theory of mind carried out with subjects with Williams syndrome, a rare genetic neurodevelopmental disorder resulting in an uneven lin-guisticocognitive profile. The results are discussed in terms of how the comparison of different phenotypes (e.g., Williams syndrome, Down syndrome, autism, and hydrocephaly with associated myelomeningocele) can contribute both to understanding the neuropsychology of social cognition and to current thinking about the purported modularity of the brain.
Word and Nonword Repetition Within- and Across-Modality: An Event-Related Potential Study
Authors: Rugg Michael D. , Doyle Michael C. , Wells Tony
Published: February 2008

The effects on event-related potentials (ERPs) of within- and across-modality repetition of words and nonwords were investigated. In Experiment 1, subjects detected occasional animal names embedded in a series of words. AU items were equally likely to be presented auditorily or visually. Some words were repetitions, either within- or across-modality, of words presented six items previously. Visual-visual repetition evoked a sustained positive shift, which onset around 250 msec and comprised two topographically and temporally distinct components. Auditory-visual repetition modulated only the later of these two components. For auditory EMS, within- and across-modality repetition evoked effects with similar onset latencies. The within-modality effect was initially the larger, but only at posterior sites. In Experiment 2, critical items were auditory and visual nonwords, and target items were auditory words and visual pseudohomophones. Visual-visual nonword repetition effects onset around 450 msec, and demonstrated a more anterior scalp distribution than those evoked by auditory-visual repetition. Visual-auditory repetition evoked only a small, late-onsetting effect, whereas auditory-auditory repetition evoked an effect that, at parietal sites only, was almost equivalent to that from the analogous condition of Experiment 1. These findings indicate that, as indexed by ERF's, repetition effects both within- and across-modality are influenced by lexical status. Possible parallels with the effects of word and nonword repetition on behavioral variables are discussed.
Spatial versus Object Working Memory: PET Investigations
Authors: Smith Edward E. , Jonides John , Koeppe Robert A. , Awh Edward , Schumacher Eric H. , Minoshima Satoshi
Published: February 2008

We used positron emission tomography (PET) to answer the following question: Is working memory a unitary storage system, or does it instead include different storage buffers for different kinds of information? In Experiment 1, PET measures were taken while subjects engaged in either a spatial-memory task (retain the position of three dots for 3 sec) or an object-memory task (retain the identity of two objects for 3 sec). The results manifested a striking double dissociation, as the spatial task activated only right-hemisphere regions, whereas the object task activated primarily left-hemisphere regions. The spatial (right-hemisphere) regions included occipital, parietal, and prefrontal areas, while the object (left-hemisphere) regions included inferotemporal and parietal areas. Experiment 2 was similar to Experiment 1 except that the stimuli and trial events were identical for the spatial and object tasks; whether spatial or object memory was required was manipulated by instructions. The PET results once more showed a double dissociation, as the spatial task activated primarily right-hemisphere regions (again including occipital, parietal and prefrontal areas), whereas the object task activated only left-hemisphere regions (again including inferotemporal and parietal areas). Experiment 3 was a strictly behavioral study, which produced another double dissociation. It used the same tasks as Experiment 2, and showed that a variation in spatial similarity affected performance in the spatial but not the object task, whereas a variation in shape similarity affected performance in the object but not the spatial task. Taken together, the results of the three experiments clearly imply that different working-memory buffers are used for storing spatial and object information.
PET Studies of Auditory and Phonological Processing: Effects of Stimulus Characteristics and Task Demands
Authors: Fiez J. A. , Raichle M. E. , Miezin F. M. , Petersen S. E. , Tallal P. , Katz W. F.
Published: February 2008

Positron emission tomography (PET) was used to investigate the functional anatomy of auditory and phonological processing. Stimulus sets were designed to determine areas of the brain significantly activated during speech and nonspeech acoustic processing for stimuli with or without rapidly changing acoustic cues. Performance of auditory target detection tasks using these stimulus sets produced increased activation in superior temporal, frontal opercular, and medial frontal (SMA) cortices, relative to a visual fixation control task. While the medial frontal and superior temporal changes are best explained by motor and sensory components of the task, respectively, the frontal opercular changes were dependent upon the task performed upon the auditory input (mere presentation of the stimuli did not result in significant activation). On the left, the frontal opercular increases were larger when subjects performed an auditory detection task upon stimuli that incorporated rapid temporal changes (words, syllables, and tone sequences) than steady-state vowels. A converging study involving performance of orthographic (ascending letter) and phonological (long vowel sound) word discrimination tasks supports anatomical and behavioral evidence suggesting the left frontal opercular region is important for certain types of auditory/temporal analysis, as well as high-level articulatory coding.

In addition to the activation increases associated with performance of auditory target detection tasks, decreases in activation were observed bilaterally along the intraparietal sulcus and superior parietal cortex, in the Rolandic sulcus, and the posterior cingulate; these decreases may reflect an attentional shift away from areas involved in the fixation task during the performance of a difficult auditory task.

These results demonstrate that focusing more closely on basic neural processing differences (such as temporal integration rates) may lead to a better understanding of the specific neural processes that underlie complex phonological tasks.
A Neural Network Simulation of Hallucinated “Voices” and Associated Speech Perception Impairments in Schizophrenic Patients
Authors: Hoffman Ralph E. , Rapaport Jill , Ameli Rezvan , McGlashan Thomas H. , Harcherik Diane , Servan-Schreiber David
Published: February 2008

The mechanism of hallucinated speech, a symptom commonly reported by schizophrenic patients, is unknown. The hypothesis that these hallucinations arise from pathologically altered working memory underlying speech perception was explored. A neural network computer simulation of contextually guided sequential word detection based on Elman (1990a,b) was studied. Pruning anatomic connections or reducing neuronal activation in working memory caused word “percepts” to emerge spontaneously (i.e., in the absence of external “speech inputs”), thereby providing a model of hallucinated speech. These simulations also demonstrated distinct patterns of word detection impairments when inputs were accompanied by varying levels of noise. In a parallel human study, the ability to shadow noisecontaminated, connected speech was assessed. Schizophrenic patients reporting hallucinated speech demonstrated a pattern of speech perception impairments similar to a simulated neural network with reduced anatomic connectivity and enhanced neuronal activation. Schizophrenic patients not reporting this symptom did not demonstrate these speech perception impairments. Neural network simulations and human empirical data, when considered together, suggested that the primary cause of hallucinated “voices” in schizophrenia is reduced neuroanatomic connectivity in verbal working memory.
Computerized Mappings of the Cerebral Cortex: A Multiresolution Flattening Method and a Surface-Based Coordinate System
Authors: Drury Heather A. , Van Essen David C. , Anderson Charles H. , Lee Chris W. , Coogan Thomas A. , Lewis James W.
Published: February 2008

We present a new method for generating two-dimensionnl maps of the cerebral cortex. Our computerized, two-stage flattening method takes as its input any well-defined representation of a surface within the three-dimensional cortex. The first stage rapidly converts this surface to a topologically correct two-dimensional map. without regard for the amount of distortion introduced. The second stage reduces distortions using a multiresolution strategy that makes gross shape changes on a coarsely sampled map and further shape refinements on progressively finer resolution maps. We demonstrate the utility of this approach by creating flat maps of the entire cerebral cortex in the macaque monkey and by displaying various types of experimental data on such maps. We also introduce a surface-based coordinate system that has advantages over conventional stereotaxic coordinates and is relevant to studies of cortical organization in humans as well as non-human primates. Together, these methods provide an improved basis for quantitative studies of individual variability in cortical organization.
Monkey and Human Face Perception: Inversion Effects for Human Faces But Not for Monkey Faces or Scenes
Authors: Wright Anthony A. , Roberts William A.
Published: February 2008

Three rhesus monkeys and two groups of 10 human subjects judged upright or inverted pictures as same or different. The pictures were black and white pairs of human faces, monkey faces, or scenes. The monkeys were trained with sets of 50 pictures and were tested with other sets of 36 pictures from each category. The groups of 10 human subjects were tested with the same pictures used to test monkeys. Both monkeys and humans showed large performance decrements to inverted human faces relative to upright human faces but neither species showed inversion effects for monkey faces or scenes. A second test with both monkeys and humans showed the same pattern of results with a different set of human-face pictures that varied more in sex (female as well as male), facial hair, eyeglasses, haircut, view angle, and background than those of the first test. The results indicate similar face-processing mechanisms in monkeys and humans despite experiential and evolutionary differences.
Effects of LTP on Response Selectivity of Simulated Cortical Neurons
Authors: Kilborn Karl , Lynch Gary , Granger Richard
Published: February 2008

We report here on specific ways in which synaptic long-term potentiation (LTP) affects the response selectivity of primary sensory cortical cells. LTP increases synaptic efficacy by incremental “steps,” up to a “ceiling” at which additional bursts of afferent stimulation cause no further potentiation. Endogenous and exogenous agents have been shown to modulate these two paramenters of LTP, raising the question of the functional implications associated with the sizes of steps and ceiling. We provide an analytical treatment of the effects of these two physiological LTP parameters on the behavior of simulated olfactory (piriform) cortex target cells in response to a range of inputs. A target cell's receptive field, i.e., the set of input patterns to which the cell responds, is broadened with potentiation of the cell's synapses, and is broadened more when the LTP step size is smaller, and when the LTP ceiling is higher. Moreover, the effects of step size and ceiling interact, and their joint relationship to receptive field breadth is nonlinear. Values of step size and ceiling are identified that balance the tradeoff between learning rate and receptive field breadth for particular sensory recognition tasks, and these model values are compared to corresponding known and inferred physiological values.
The Magnitude of the Fixation Offset Effect with Endogenously and Exogenously Controlled Saccades
Authors: Forbes Kelly , Klein Raymond M.
Published: February 2008

Two experiments examined saccadic reaction time (RT) in response to visual targets as a function of fixation offset condition (no offset; target simultaneous with offset and 200-msec offset-target SOA) in prosaccade and antisaccade tasks. The second experiment also included a condition in which saccades were made in response to verbal commands presented auditorally. To ensure that observers were equally prepared in each condition, auditory warning tones preceded target onset on every trial. The RT reduction associated with fixation offset (FOE, or gap effect) was identical with visual targets in the prosaccade task and in response to verbal signals, strongly implicating motor, rather than sensory, mechanisms in the FOE. The FOE in the antisaccade task was significant, but it was also significantly smaller than in the other tasks. We speculate that the reduced FOE in the antisaccade task may be due to the requirement to inhibit the superior colliculus when the target directed saccadic programs are, by instruction, erroneous.
On Knowing the Auxiliary of a Verb That Cannot Be Named: Evidence for the Independence of Grammatical and Phonological Aspects of Lexical Knowledge
Authors: Miozzo Michele , Caramazza Alfonso
Published: February 2008

We report the case of an Italian anomic subject who was invariably able to provide the auxiliary of verbs he failed to produce in oral naming tasks. This pattern of performance contrasts with that of another Italian-speaking patient documented by Miceli and Caramazza (1988) who showed a selective impairment in accessing syntactic features of words, but not their phonological forms. This double dissociation suggests that syntactic and phonological information in the lexicon are accessed independently and represented in distinct neural structures.
Hemispheric Organization of Visual Memories
Authors: Gratton Gabriele , Corballis Paul M. , Jain Shamini
Published: February 2008

This study investigates the hemispheric organization of visual memories. In five experiments, we examine the processes associated with the recognition of line patterns that are flashed laterally during a study phase. The first three experiments demonstrate a recognition advantage for patterns presented at test in the same (rather than in the opposite) hemifield in which they were presented during a previous study phase. This difference was obtained even when stimuli were presented in different locations within the same hemifield. Experiment shows that patterns presented centrally during the recognition phase elicit ERPs that are systematically more negative over the hemisphere contralateral to the side at which they were presented during the study phase. In Experiment 5, however, we found that subjects were unable to indicate the side of initial presentation of the patterns. The results suggest that the memory traces left by laterally presented stimuli are more easily accessible in the contralateral hemisphere, which suggests a contralateral organization of visual memories.
Residual Vision in Multiple Retinal Locations within a Scotoma: Implications for Blindsight
Authors: Kentridge R. W. , Heywood C. A. , Weiskrantz L.
Published: February 2008

There is an important new proposal that “blindsight”-the ability to detect and identify visual stimuli by forcedchoice guessing and in the absence of conscious awareness when they fall in blind regions of the visual field—is a function of residual “islands” of undamaged visual cortex. This stands in contrast to the widely accepted view that blindsight is exclusively a function of secondary visual pathways. According to the new view, residual vision in blindsight should be patchy. Thus, when apparently wide areas of residual vision in blindsight are found, these may be due to eye-movements that allow stimuli to pass over retinal locations corresponding to islands of sparing. We tested this hypothesis by examining the distribution of residual vision in blindsight when the effects of eye movements on the retinal location of stimuli were minimized. We report a series of experiments that examined twealternate forcedchoice discrimination in the blind field of the subject GY. Using a dual-Purkinje image eye-tracker we applied three methods of minimizing the effects of retinal slippage due to eye-movements on discrimination performance: fixation stability-dependent trials, software image stabilization, and post hoc rejection of trials in which saccadic eye-movements were detected. In the first experiment, GY's discrimination performance was significantly above chance in 8 of 15 locations tested. In the subsequent experiments the subject knew the location of the target in each block of trials, and this resulted in improvements to performance in a further three locations. Increasing the luminance of the stimulus display (while maintaining 95% target contrast), and increasing the temporal discriminability of the forced choice produced performance above chance in all but two of the locations tested. The consistent chance performance observed in two locations in the lower visual field nevertheless implies that GY's blindsight does not extend over the whole of his scotoma. Nevertheless, abolishing, or minimizing, the effects of eye-movements did not result in a loss of detection in all the widely separated regions tested, and we thus conclude that GY's blindsight cannot adequately be explained in terms of islands of spared vision. Islands may account for residual vision in scotomata in some patients, but cannot be a universal account of the phenomenon of blindsight.
Spatial Transformations in the Parietal Cortex Using Basis Functions
Authors: Pouget Alexandre , Sejnowski Terrence J.
Published: February 2008

Sensorimotor transformations are nonlinear mappings of sensory inputs to motor responses. We explore here the possibility that the responses of single neurons in the parietal cortex serve as basis functions for these transformations. Basis function decomposition is a general method for approximating nonlinear functions that is computationally efficient and well suited for adaptive modification. In particular, the responses of single parietal neurons can be approximated by the product of a Gaussian function of retinal location and a sigmoid function of eye position, called a gain field. A large set of such functions forms a basis set that can be used to perform an arbitrary motor response through a direct projection. We compare this hypothesis with other approaches that are commonly used to model population codes, such as computational maps and vectorial representations. Neither of these alternatives can fully account for the responses of parietal neurons, and they are computationally less efficient for nonlinear transformations. Basis functions also have the advantage of not depending on any coordinate system or reference frame. As a consequence, the position of an object can be represented in multiple reference frames simultaneously, a property consistent with the behavior of hemineglect patients with lesions in the parietal cortex.
Are the Same Attentional Mechanisms Used to Detect Visual Search Targets Defined by Color, Orientation, and Motion?
Authors: Girelli Massimo , Luck Steven J.
Published: February 2008

Motion information tends to be segregated from color and form information in the visual system, both perceptually and neuroanatomically, and it is therefore possible that different mechanisms of attention are used to select targets defined by these different feature types during visual search. To test this hypothesis, we recorded the N2pc component of the event-related potential waveform during visual search tasks with color, orientation, and motion targets. The N2pc component has previously been shown to reflect a specific attentional mechanism that is present for color and form targets, and we sought to determine whether this component would also be present for motion targets. The N2pc component was indeed observed for motion targets as well as color and orientation targets, consistent with the use of a common attentional mechanism across feature types. In addition, we found that motion singletons (i.e., individual items that moved in the opposite direction from the other items in the army) elicited an N2pc component even when they were task-irrelevant, indicating that motion discontinuities may produce an automatic orienting of attention.
A Neural Dissociation within Language: Evidence that the Mental Dictionary Is Part of Declarative Memory, and that Grammatical Rules Are Processed by the Procedural System
Authors: Ullman Michael T. , Corkin Suzanne , Coppola Marie , Hickok Gregory , Growdon John H. , Koroshetz Walter J. , Pinker Steven
Published: February 2008

Language comprises a lexicon for storing words and a grammar for generating rule-governed forms. Evidence is presented that the lexicon is part of a temporal-parietalhnedial-temporal “declarative memory” system and that granlmatical rules are processed by a frontamasal-ganglia “procedural” system. Patients produced past tenses of regular and novel verbs (looked and plagged), which require an -ed-suffixation rule, and irregular verbs (dug), which are retrieved from memory. Word-finding difficulties in posterior aphasia, and the general declarative memory impairment in Alzheimer's disease, led to more errors with irregular than regular and novel verbs. Grammatical difficulties in anterior aphasia, and the general impairment of procedures in Parkinson's disease, led to the opposite pattern. In contrast to the Parkinson's patients, who showed sup pressed motor activity and rule use, Huntington's disease patients showed excess motor activity and rule use, underscoring a role for the basal ganglia in grammatical processing.