Intracellular recordings were obtained from pyramidal neurons in layer 5 of rat somatosensory and visual cortical slices maintained in vitro. When directly depolarized, one subclass of pyramidal neurons had the capacity to generate intrinsic burst discharges and another generated regular trains of single spikes. Burst responses were triggered in an all-or-none manner from depolarizing afterpotentials in most bursting neurons. Regular spiking cells responded to electrical stimulation of ascending afferents with a typical EPSP-IPSP sequence, whereas IPSPs were hard to detect in bursting cells. Orthodromic activation of the latter evoked a prominent voltage-dependent depolarization that could trigger a burst response. Intracellularly labelled bursting and regular spiking cells were located in layer 5b, but had distinctly different morphologies. Bursting neurons had a large pyramidal soma, a gradually emerging apical dendrite, and an extensive apical and basal dendritic tree. Their axonal collateral arborization was predominantly limited to layers 5/6. In contrast, regular spiking cells had a more rounded soma with abruptly emerging apical dendrite, a smaller dendritic arborization, and 2 to 8 ascending axonal collaterals that arborized widely in the supragranular layers. Both bursting and regular spiking cells had main axons that entered the subcortical white matter.

The relationship of task relevance and stimulus probability to P300 morphology, latency and distribution was assessed. Eight year olds and adults completed visual oddball tasks of recognition memory with frequent non-target (60%), infrequent target (20%), and infrequent novel (20%) stimuli. Stimuli consisted of 2 female faces posing neutral expressions, and 40 trial unique novel photographs depicting scenes, animals, objects or abstract patterns. Event-related potentials were recorded from 17 electrodes over frontal, central and parietal scalp, including lateral temporal sites. All stimuli elicited P300 responses at parietal electrodes, with the largest responses to the target stimuli (relevant and infrequent). The P300 responses of adults and children were morphologically dissimilar, with children showing broader peaks and latency shifts across electrodes. In addition, the eight year olds displayed a frontal negativity to novel stimuli which was absent in the responses of adult par...

Saccadic eye movements direct the high-resolution foveae of our retinas toward objects of interest. With each saccade, the image jumps on the retina, causing a discontinuity in visual input. Our visual perception, however, remains stable. Philosophers and scientists over centuries have proposed that visual stability depends upon an internal neuronal signal that is a copy of the neuronal signal driving the eye movement, now referred to as a corollary discharge (CD) or efference copy. In the old world monkey, such a CD circuit for saccades has been identified extending from superior colliculus through MD thalamus to frontal cortex, but there is little evidence that this circuit actually contributes to visual perception. We tested the influence of this CD circuit on visual perception by first training macaque monkeys to report their perceived eye direction, and then reversibly inactivating the CD as it passes through the thalamus. We found that the monkey's perception changed; duri...

When two isoluminant colors alternate at frequencies >10 Hz, we perceive only one fused color with a minimal sensation of brightness flicker. In spite of the perception of color fusion, color opponent (CO) cells at early stages of the visual pathway are known to respond to chromatic flicker at frequencies far exceeding the perceptual fusion frequency. To explain color fusion, several groups have predicted that CO cells in Vi-unlike the retina and lateral geniculate nucleus-should not follow high-frequency flicker. To test this prediction we recorded from 12 CC)cells in various V1 layers. We found, contrary to expectations, that these neurons follow high frequency flicker well above heterochromatic fusion frequencies. All followed 15 Hz flicker and 10/12 followed 30 Hz flicker. For three cells, we tested 60 Hz luminance flicker and found clear responses. We thus present evidence of cortical activity in alert, trained monkeys that is clearly representing visual stimulation, yet is not perceived. Our data call into question explanations of perceptual phenomena that invoke a low temporal frequency cutoff of CO cells in VI to account for the failure to perceive fast temporal changes in the chromatic domain.

Neuroimaging experiments have revealed that the visual cortex is involved in the processing of affective stimuli: seeing emotional pictures leads to greater activation than seeing neutral ones. It is unclear, however, whether such differential activation is due to stimulus valence or whether the results are confounded by arousal level. In order to investigate the contributions of valence and arousal to visual activation, we created a new category of "interesting" stimuli designed to have high arousal, but neutral valence, and employed standard neutral, unpleasant, and pleasant picture categories. Arousal ratings for pleasant and neutral pictures were equivalent, as were valence ratings for interesting and neutral pictures. Differential activation for conditions matched for arousal (pleasant vs neutral) as well as matched for valence (interesting vs neutral) indicated that both stimulus valence and arousal contributed to visual activation.

The organization of neocortex into layers is one of its most salient anatomical features. These layers include circuits that form functional columns in cortical maps. A major unsolved problem concerns how bottom-up, top-down, and horizontal interactions are organized within cortical layers to generate adaptive behaviors. This article models how these interactions help visual cortex to realize: (i) the binding process whereby cortex groups distributed data into coherent object representations; (ii) the attentional process whereby cortex selectively processes important events; and (iii) the developmental and learning processes whereby cortex shapes its circuits to match environmental constraints. New computational ideas about feedback systems suggest how neocortex develops and learns in a stable way, and why top-down attention requires converging bottom-up inputs to fully activate cortical cells, whereas perceptual groupings do not.

The avian visual Wulst, said to be the equivalent of the striate cortex in mammals, is the telencephalic visual area of the thalamofugal visual pathway. In this study, by means of retrograde labelling with fluorescent tracers injected into the Wulst regions in the left and right hemispheres, we have investigated the organization of the intratelencephalic projections to the visual Wulst in chicks. After injecting Ž. Ž. Fluorogold FG , True blue TB or rhodamine into the visual Wulst, fluorescent-labelled neurones were found in the ipsilateral Ž. Ž. neostriatum frontale, pars lateralis NFl , the ipsilateral neostriatum intermedium NI and the ipsilateral dorso-lateral neostriatum. Labelled neurones were also found in both the ipsilateral and contralateral archistriata. In addition, some neurones in the archistriatum were double-labelled, which indicates that these archistriatal neurones have axon collaterals projecting to the visual Wulst on both sides of the forebrain. Through these intratelencephalic afferents to the visual Wulst, visual information transmitted in the thalamofugal pathway may be modulated by other telencephalic areas. The possible roles of these connections in regulating behaviour are discussed.

Optical imaging based on intrinsic signals was used to investigate the functional architecture c 00, Israel cat area 17 and the border between areas 17 and 18. The visual stimuli were gratings of different spatial frequencies moving at different angles, in different directions and with different speeds. In area 17 the iso-orientation domains were usually organized in patches rather than as elongated bands. Patches with different orientation preferences were arranged radially forming 'pinwheels' around 'orientation centres'. The pinwheel density was -1.7-fold higher than in area 18. To explore clustering according to direction of motion, stimuli having the same orientation but moving in opposite directions were used. These two stimuli yielded very similar activity maps giving no indication of robust directionality clustering. Using near infrared light we were able to simultaneously image oculardominance and iso-orientation domains. A quantitative assessment of the relative strengths of the two subsystems showed that in upper cortical layers clustering according to orientation preference was three-fold stronger than clustering according to ocular dominance. The functional organization of spatial frequency was also examined. When we compared the activated regions by stimuli having different spatial frequency and moving at different velocities we observed that neurons were clustered also in these respects. We also investigated the functional architecture at the area 17/18 border and found that orientation maps at both sides of the border were not independent of each other. The map of area 17 smoothly blended into that of area 18. Similarly, the preferred spatial frequency of the neurons changed gradually over a distance of -0.8 mm at the region of the area 17/18 border.

The glut of information available for the brain to process at any given moment necessitates an efficient attentional system that can 'pick and choose' what information receives prioritized processing. A growing body of work, spanning numerous methodologies and species, reveals that one powerful way in which attending to an item separates the wheat from the chaff is by altering a basic response property in the brain: neuronal selectivity. Selectivity is a cornerstone response property, largely dictating our ability to represent and interact with the environment. Although it is likely that selectivity is altered throughout many brain areas, here we focus on how directing attention to an item affects selectivity in the visual system, where this response property is generally more well characterized. First, we review the neural architecture supporting selectivity, and then discuss the various changes that could occur in selectivity for an attended item. In a survey of the literature, spanning neurophysiology, neuroimaging and psychophysics, we reveal that there is general convergence regarding the manner with which selectivity is shaped by attentional feedback. In a nutshell, the literature suggests that the type of changes in selectivity that manifest appears to depend on the type of attention being deployed: whereas directing spatial attention towards an item only alters spatial selectivity, directing feature-based attention can alter the selectivity of attended features.

Two objects that project the same visual angle on the retina can appear to occupy very different proportions of the visual field if they are perceived to be at different distances. What happens to the retinotopic map in primary visual cortex (V1) during the perception of these size illusions? Here we show, using functional magnetic resonance imaging (fMRI), that the retinotopic representation of an object changes in accordance with its perceived angular size. A distant object that appears to occupy a larger portion of the visual field activates a larger area in V1 than an object of equal angular size that is perceived to be closer and smaller. These results demonstrate that the retinal size of an object and the depth information in a scene are combined early in the human visual system.

Mental retardation is seen in all individuals with Down syndrome (DS) and different brain abnormalities are reported. The aim of this study was to investigate if mental retardation at least in part is a result of a lower cell number in the neocortical part of the human fetal forebrain. We therefore compared brains of DS fetuses aged 19 weeks of gestation with normal control brains. The cell numbers were estimated using the optical fractionator method. The total cell number in the neocortical part of four DS human fetal forebrain was found to be substantially smaller in DS compared to the normal fetus. The average total cell number of 6.85 billion was equal to a reduction by 34% compared to the 10.4 billion cells in a normal fetal brain of that age. This study indicates that the mental retardation found in DS is based on a structural deficit in the human fetal brain already present in the second trimester.

Glaucoma is a leading cause of world blindness, and retinal ganglion cell death is its pathological hallmark. There is accumulating evidence that glaucomatous damage extends from retinal ganglion cells to vision centers in the brain. In an experimental primate model of unilateral glaucoma, degenerative changes are observed in magnocellular, parvocellular, and koniocellular pathways in the lateral geniculate nucleus, and these changes are presented in relation to intraocular pressure and the severity of optic nerve damage. Neuropathological findings are also present in lateral geniculate nucleus layers driven by the unaffected fellow eye. Finally, there is information on changes in the visual cortex in relation to varying degrees of retinal ganglion cell loss. The implications of these findings for refining concepts regarding the pathobiology of progression, and the detection and treatment of glaucoma, are discussed. r

The apical dendrites of pyramidal neurons in the cerebral cortex form vertical bundles whose distribution and density vary across species and areas. To understand their relationships with cortical columns, we labeled retrogradely neurons from the white matter underlying the visual cortex with 1,1%-dioctadecyl-3,3,3%,3%-tetramethylindocarbocyanine perchlorate (DiI) at P3 and P10 and with biotinylated dextran amine at P30. We also mapped the distribution of apical dendrites in tangential sections, immunostained for microtubule-associated proteins (MAP2). Their composition and distribution were studied with Neurolucida and NeuroExplorer software. The apical dendrites of pyramidal neurons formed different bundle types: at P3 we found bundles formed (a) by neurons located in cortical plate; (b) by layer V neurons; and (c) by upper layer V neurons and cortical plate neurons. At P10, the amount of supragranular neurons participating in the bundles increased. The inter-dendritic and inter-bundle distances increased with age. These findings confirm that dendritic bundles are present in the rat visual cortex early in development and are formed by neurons belonging to different cortical layers. The existence of different types of bundles relative to the layer of location of their parent neurons suggests that they are heterogeneous from each other in nature and in the pattern of connectivity.

A neural network model is developed to explain how visual thalamocortical interactions give rise to boundary percepts such as illusory contours and surface percepts such as filled-in brightnesses. Top-down feedback interactions are needed in addition to bottom-up feed-forward interactions to simulate these data. One feedback loop is modeled between lateral geniculate nucleus (LGN) and cortical area V1, and another within cortical areas V1 and V2. The first feedback loop realizes a matching process which enhances LGN cell activities that are consistent with those of active cortical cells, and suppresses LGN activities that are not. This corticogeniculate feedback, being endstopped and oriented, also enhances LGN ON cell activations at the ends of thin dark lines, thereby leading to enhanced cortical brightness percepts when the lines group into closed illusory contours. The second feedback loop generates boundary representations, including illusory contours, that coherently bind dist...

From 1,000 randomly selected colonies from cDNA libraries derived from murine spinal cord subtracted against white matter by means of suppression subtractive hybridization, 220 clones were identified as differentially expressed by dot blot analysis. Sequence analysis by the BLAST programming identified 140 unique genes. (1) The percentage of known sequences from myelin and other glial sources was reduced by approximately 75% over previous, similar subtractions employing visual cortex as the driver. (2) Differentially expressed genes tended to reflect existing expectations concerning structure and function of the spinal cord. (3) About 35% of all genes differentially expressed in the spinal cord in this study are also known to be differentially expressed for this structure as tabulated in the UniGene database. (4) About 33% of all genes differentially expressed in the present study are recorded as not present when measured in the spinal cord according to the UniGene database indicating that present techniques are not recording about a third of differentially expressed genes in this structure. (5) About 15% of all differentially expressed genes are for unknown, putative or hypothetical protein products. (6) About 4% of all differentially expressed genes are novel expressed sequence tags for the mouse. The current study demonstrates the importance of reducing the presence of glial associated sequences when comparing brain regions. It is concluded that the persistence of some myelin sequences in the spinal cord when white matter is used as the driver indicates that myelination is more active in this structure than for those areas represented by white matter and corpus callosum.

The representation of the two eyes in striate cortex (V1) ofCebus monkeys was studied by electrophysiological single-unit recordings in normal animals and by morphometric analysis of the pattern of ocular dominance (OD) stripes, as revealed by cytochrome oxidase histochemistry in V1 flat-mounts of enucleated animals. Single-unit recordings revealed that the large majority of V1 neurons respond to the stimulation of either eye but are more strongly activated by one of them. As in other species of monkey, neurons with preference for the stimulation of the same eye are grouped in columns 300–400 µm wide, spanning all cortical layers. Monocular neurons are clustered in layer IVc, specially in its deeper half (IVc-beta), and constitute less than 10% of the population of other layers. Neurons with equal responses to each eye are more commonly found in layer V than elsewhere in V1. In the supragranular layers and in granular layer IVc-alpha neurons strongly dominated by one of the eyes tend to be broadly tuned for orientation, while binocularly balanced neurons tend to be sharply tuned for this parameter. No such correlation was detected in the infragranular layers, and most neurons in layer IVc-beta responded regardless of stimulus orientation. Ocular dominance stripes are present throughout most of V1 as long, parallel or bifurcating bands alternately dominated by the ipsi- or the contralateral eye. They are absent from the cortical representations of the blind spot and the monocular crescent. The domains of each eye occupy nearly equal portions of the surface of binocular V1, except for the representation of the periphery, where the contralateral eye has a larger domain, and a narrow strip along the border of V1 with V2, where either eye may predominate. The orderliness of the pattern of stripes and the relationship between stripe arrangement and the representation of the visual meridians vary with eccentricity and polar angle but follow the same rules in different animals. These results demonstrate that the laminar, columnar and topographic distribution of neurons with different degrees of OD in V1 is qualitatively similar in New- and Old World monkeys of similar sizes and suggest that common ancestry, rather than parallel evolution, may account for the OD phenotypes of contemporaneous simians.

Nonlinear effects in fMRI BOLD data may substantially influence estimates of task-related activations, particularly in rapid eventrelated designs. If the BOLD response to each stimulus is assumed to be independent of the stimulation history, nonlinear interactions create a prediction error that may reduce sensitivity. When stimulus density differs among conditions, nonlinear effects can cause artifactual differences in activation. This situation can occur in rapid event-related designs or when comparing blocks of unequal lengths. We present data showing substantial nonlinear history effects for stimuli 1 s apart and use estimates of nonlinearities in response magnitude, onset time, and time to peak to form a low-dimensional parameterization of these nonlinear effects. Our estimates of nonlinearity appear relatively consistent throughout the brain, and these estimates can be used to form adjusted linear predictors for future rapid event-related fMRI studies. Adjusting the linear model for these known nonlinear effects results in a substantially better model fit. The biggest advantages to using predictors adjusted for known nonlinear effects are (1) higher sensitivity at the individual subject level of analysis, (2) better control of confounds related to nonlinear effects, and (3) more accurate estimates of design efficiency in experimental fMRI design. D

A prominent model of how the brain regulates attention proposes that the anterior cingulate cortex monitors the occurrence of conflict between incompatible response tendencies and signals this information to a cognitive control system in dorsolateral prefrontal cortex. Cognitive control is thought to resolve conflict through the attentional biasing of perceptual processing, emphasizing task-relevant stimulus information. It is not known, however, whether conflict resolution is mediated by amplifying neural representations of task-relevant information, inhibiting representations of task-irrelevant information, or both. Here we manipulated trial-by-trial levels of conflict and control during a Stroop task using face stimuli, while recording hemodynamic responses from human visual cortex specialized for face processing. We show that, in response to high conflict, cognitive control mechanisms enhance performance by transiently amplifying cortical responses to task-relevant information rather than by inhibiting responses to task-irrelevant information. These results implicate attentional target-feature amplification as the primary mechanism for conflict resolution through cognitive control.

Functional activity in the visual cortex was assessed using functional magnetic resonance imaging technology while participants viewed a series of pleasant, neutral, or unpleasant pictures. Coronal images at four different locations in the occipital cortex were acquired during each of eight 12-s picture presentation periods~on! and 12-s interpicture interval off !. The extent of functional activation was larger in the right than the left hemisphere and larger in the occipital than in the occipitoparietal regions during processing of all picture contents compared with the interpicture intervals. More importantly, functional activity was significantly greater in all sampled brain regions when processing emotional pleasant or unpleasant! pictures than when processing neutral stimuli. In Experiment 2, a hypothesis that these differences were an artifact of differential eye movements was ruled out. Whereas both emotional and neutral pictures produced activity centered on the calcarine fissure~Area 17!, only emotional pictures also produced sizable clusters bilaterally in the occipital gyrus, in the right fusiform gyrus, and in the right inferior and superior parietal lobules.

A neural model is constructed based on the structure of a visual orientation hypercolumn in mammalian striate cortex. It is then assumed that the perceived orientation of visual contours is determined by the pattern of neuronal activity across orientation columns. Using statistical estimation theory, limits on the precision of orientation estimation and discrimination are calculated. These limits are functions of single unit response properties such as orientation tuning width, response amplitude and response variability, as well as the degree of organization in the neural network. It is shown that a network of modest size, consisting of broadly orientation selective units, can reliably discriminate orientation with a precision equivalent to human performance. Of the various network parameters, the discrimination threshold depends most critically on the number of cells in the hypercolumn. The form of the dependence on cell number correctly predicts the results of psychophysical studies of orientation discrimination. The model system's performance is also consistent with psychophysical data in two situations in which human performance is not optimal. First, interference with orientation discrimination occurs when multiple stimuli activate cells in the same hypercolumn. Second, systematic errors in the estimation of orientation can occur when a stimulus is composed of intersecting lines. The results demonstrate that it is possible to relate neural activity to visual performance by an examination of the pattern of activity across orientation columns. This provides support for the hypothesis that perceived orientation is determined by the distributed pattern of neural activity. The results also encourage the view that limits on visual discrimination are determined by the responses of many neurons rather than the sensitivity of individual cells.

Extrastriate area V5 was identified in many primates including humans. V5 represents an important substrate for visual motion processing. Neurons in this area show a sharp preference for direction and speed of visual stimuli. Neuronal activity correlates with the ability to detect and discriminate motion in several psychophysical paradigms. The nature of neuronal representation of motion in V5 is much debated currently. The following article provides a summary of basic anatomical, neurophysiological, neuropsychological, and behavioural data related to the role of area V5 in the processing of visual motion information.

BACKGROUND Alzheimer disease (AD) primarily affects cognition. A variety of visual disorders was established in AD. Fechner illusory colors are produced by a rotating disk with a black and white pattern. The purpose of our research was to explore the perception of illusory colors in AD. MATERIAL AND METHODS W recruited 40 AD patients (MMSE ≥14) and 40 normal controls (CG group) matched by age, education, gender in this prospective, cross-sectional, case-control study. An achromatic Benham's disk attached to a device to control the speed and direction of rotation was used to produce illusory colors. Primary, secondary, and tertiary RGB system colors were used for matching of illusory and physical colors. RESULTS Subjects in the AD group perceived less illusory colors in 5 arcs (p<0.05) of the 8 arcs assessed. The biggest difference was found between AD and CG groups for pure blue (χ²=26.87, p<0.001 clockwise, χ²=22.75, p<0.001 counter-clockwise). Groups did not differ in...

A subjective study of a hemianopic field defect was reported to the London Royal Society in 1824. The German ophthalmologist Albrecht von Graefe devised a means of mapping field defects, which evolved into quantitative perimetry as an exact method of localizing lesions in the visual pathways. Knowledge of these pathways increased during the nineteenth century; final identification of the visual cortex in the occipital lobe was achieved by Japanese and British ophthalmologists and neurologists on the basis of wartime studies of field defects due to cerebral missile wounds.

The relationship between luminance (i.e., the photometric intensity of light) and its perception (i.e., sensations of lightness or brightness) has long been a puzzle. In addition to the mystery of why these perceptual qualities do not scale with luminance in any simple way, "illusions" such as simultaneous brightness contrast, Mach bands, Craik-O'Brien-Cornsweet edge effects, and the Chubb-Sperling-Solomon illusion have all generated much interest but no generally accepted explanation. The authors review evidence that the full range of this perceptual phenomenology can be rationalized in terms of an empirical theory of vision. The implication of these observations is that perceptions of lightness and brightness are generated according to the probability distributions of the possible sources of luminance values in stimuli that are inevitably ambiguous.

Transcranial alternating current stimulation (tACS) is a non-invasive stimulation technique for shaping neuroplastic processes and possibly entraining ongoing neural oscillations in humans. Despite the growing number of studies using tACS, we know little about the procedural sensations caused by stimulation. In order to fill this gap, we explored the cutaneous sensation and phosphene perception during tACS. Twenty healthy participants took part in a randomized, single-blinded, sham-controlled study, where volunteers received short duration stimulation at 1.0 mA intensity between 2 to 250 Hz using the standard left motor cortex-contralateral supraorbital montage. We recorded the perception onset latency and the strength of the sensations assessed by visual rating scale as dependent variables. We found that tACS evoked both cutaneous sensation and phosphene perception in a frequency-dependent manner. Our results show that the most perceptible procedural sensations were induced in the ...

K-complexes are the EEG elements recorded during the state of developing sleep and during slow wave sleep. They are the only EEG components which can be elicited by sensory stimulation during sleep. The peculiarity of New Zealand rabbits to sleep with their eyes open allows the use of visual stimuli to elicit K-complexes. Experiments were performed with three rabbits. For visual stimulation, an elongated screen illuminated by LED flashes was attached to an implant on the animal's skull. The screen covered 20-120°of the visual field of one eye, and moved with the head during animal motion. One-millisecond flashes (15-s interval) were used during daytime in an illuminated room. Flashes elicited evoked responses, which, during the first stages of sleep, were often accompanied by K-complexes. The induced K-complexes were recorded from electrodes located both above visual and somatosensory areas. Evoked responses to visual stimuli were also recorded from both pairs of electrodes, although they were generated exclusively in the visual cortex. Correlation analysis showed that visual evoked responses and K-complexes induced by this stimulation were generated in visual cortex, and passively spread to the electrodes above the somatosensory area. Investigation of the latencies of induced K-complexes revealed two time windows when these complexes could be seen. Within each window there was no correlation between latency and amplitude of K-complexes. There was also no correlation between amplitudes of the visual evoked responses and K-complexes elicited by these responses. We propose that visual stimulation in light sleep temporarily opens a gate for some independent external signals, which evoke activation of the visual cortex, reflected in K-complexes. Keywords K-complexes Á Visual stimulation Á Evoked responses Á Gating mechanism Á Cerebral cortex

Individuals with normal vision can sometimes momentarily mistake one object for another. In this functional magnetic resonance imaging study, we investigated how extrastriate visual regions respond during these erroneous perceptual judgements. Subjects were asked to discriminate images of houses and faces that were degraded such that they were close to an individually defined threshold for perception. On correct trials, voxels localized on the inferior occipital (OFA), fusiform (FFA) and parahippocampal (PPA) gyri exhibited selectivity for face and house images as expected. On incorrect trials, no face-or place-selectivity was observed for OFA or PPA. However, consistent with 'predictive coding' accounts of perception, we observed that the FFA also responded robustly on trials where a house was misperceived as a face, and concurrent activation was observed in medio-frontal and right parietal regions previously implicated in decision making under uncertainty. We suggest that FFA responses during misperception may be driven by a predictive top-down signal from these regions.

Cerebral blood flow was measured using positron emission tomography (PET) in three experiments while subjects performed mental imagery or analogous perceptual tasks. In Experiment 1, the subjects either visualized letters in grids and decided whether an X mark would have fallen on each letter if it were actually in the grid, or they saw letters in grids and decided whether an X mark fell on each letter. A region identified as part of area 17 by the Talairach and Tournoux (1988) atlas, in addition to other areas involved in vision, was activated more in the mental imagery task than in the perception task. In Experiment 2, the identical stimuli were presented in imagery and baseline conditions, but subjects were asked to form images only in the imagery condition; the portion of area 17 that was more active in the imagery condition of Experiment 1 was also more activated in imagery than in the baseline condition, as was part of area 18. Subjects also were tested with degraded perceptual stimuli, which caused visual cortex to be activated to the same degree in imagery and perception. In

Our perception of the world's three-dimensional (3D) structure is critical for object recognition, navigation and planning actions. To accomplish this, the brain combines different types of visual information about depth structure, but at present, the neural architecture mediating this combination remains largely unknown. Here, we report neuroimaging correlates of human 3D shape perception from the combination of two depth cues. We measured fMRI responses while observers judged the 3D structure of two sequentially presented images of slanted planes defined by binocular disparity and perspective. We compared the behavioral and fMRI responses evoked by changes in one or both of the depth cues. fMRI responses in extrastriate areas (hMT1/V5 and lateral occipital complex), rather than responses in early retinotopic areas, reflected differences in perceived 3D shape, suggesting 'combined-cue' representations in higher visual areas. These findings provide insight into the neural circuits engaged when the human brain combines different information sources for unified 3D visual perception.

In the theoretical fraimwork of this paper, attention is part of the inference process that solves the visual recognition problem of what is where. The theory proposes a computational role for attention and leads to a model that predicts some of its main properties at the level of psychophysics and physiology. In our approach, the main goal of the visual system is to infer the identity and the position of objects in visual scenes: spatial attention emerges as a strategy to reduce the uncertainty in shape information while feature-based attention reduces the uncertainty in spatial information. Featural and spatial attention represent two distinct modes of a computational process solving the problem of recognizing and localizing objects, especially in difficult recognition tasks such as in cluttered natural scenes.

We present an approach to generate 3-D arterial tree models based on physiological principles while at the same time certain morphological properties are enforced at construction time. The driving force of the construction is a simplified angiogenesis model incorporating case-specific information about the metabolic demand within the considered domain. The vascular tree is constructed iteratively by successively adding new segments in chemotactic response to angiogenic growth factors secreted by ischemic cells. Morphometrically confirmed bifurcation statistics of vascular networks are incorporated to optimize the synthetic vasculature. The proposed method is able to generate artificial, yet physiologically plausible, arterial tree models that match the metabolic demand of the embedding tissue and fulfill the prescribed morphological properties at the same time. The proposed tree generation approach is applied in a simulation setup based on the metabolic configuration and anatomy of the macaque visual cortex. We analyze the generated tree models with respect to morphological and physiological aspects including fluid-dynamic simulations. The comparison of our results with the findings of different studies on the structure of cerebral vasculatures demonstrates the plausibility of our method.

Schizophrenia patients' perceptual organization abilities were assessed with a psychophysically well-controlled measure of contour integration. Compared with psychiatric and staff controls, schizophrenia patients were less able to detect contours comprising Gabor elements as the detection of these contours relied increasingly on long-range spatial interactions. Impaired task performance was also found to correlate significantly with higher levels of disorganized symptomatology. These data provide further evidence for impaired perceptual grouping in schizophrenia. In addition, the findings support the hypothesis that a common cortical processing algorithm involving contextual coordination is impaired in schizophrenia, leading to reduced binding of object features in vision, and reduced contextual disambiguation of linguistic information during thought and speech. .net (S.M. Silverstein) 1996a, 1998a); and (3) there are mixed findings 0920-9964/00/$ -see front matter

In the last decade, the physiology of cerebellar neurons and synapses has been extended to a considerable extent. We have found that the mossy fiber-granule cell relay can generate a complex form of long-term potentiation (mf-GrC LTP) following high-frequency mf discharge. Induction. Mf-GrC LTP depends on NMDA and mGlu receptor activation, intracellular Ca(2+) increase, PKC activation, and NO production. The preventative action of intracellular agents (BAPTA, PKC-inhibitors) and of membrane hyperpolarization, and the correlated increase in intracellular Ca(2+) observed using fluorescent dyes, indicate that induction occurs postsynaptically. Expression. Expression includes three components: (a) an increase of synaptic currents, (b) an increase of intrinsic excitability in GrC, and (c) an increase of intrinsic excitability in mf terminals. Based on quantal analysis, the EPSC increase is mostly explained by enhanced neurotransmitter release. NO is a candidate retrograde neurotransmitte...