Morphology and intracortical projections of functionally characterised neurones in the cat visual cortex

The neuronal structure and connectivity underlying receptive field organisation of cells in the cat visual cortex have been investigated. Intracellular recordings were made using a micropipette filled with a histochemical marker, which was injected into the cells after their receptive fields had been characterised. This allowed visualisation of the dendritic and axonal arborisations of functionally identified neurones.     

Iso-orientation domains in cat visual cortex are arranged in pinwheel-like patterns

The mammalian cortex is organized in a columnar fashion: neurons lying below each other from the pia to the white matter usually share many functional properties. Across the cortical surface, cells with similar response properties are also clustered together, forming elongated bands or patches. Some response properties, such as orientation preference in the visual cortex, change gradually across the cortical surface forming 'orientation maps'. To determine the precise layout of iso-orientation domains, knowledge of responses not only to one but to many stimulus orientations is essential. Therefore, the exact depiction of orientation maps has been hampered by technical difficulties and remained controversial for almost thirty years. Here we use in vivo optical imaging based on intrinsic signals to gather information on the responses of a piece of cortex to gratings in many different orientations. This complete set of responses then provides detailed information on the structure of the orientation map in a large patch of cortex from area 18 of the cat. We find that cortical regions that respond best to one orientation form highly ordered patches rather than elongated bands. These iso-orientation patches are organized around 'orientation centres', producing pinwheel-like patterns in which the orientation preference of cells is changing continuously across the cortex. We have also analysed our data for fast changes in orientation preference and find that these 'fractures' are limited to the orientation centres. The pinwheels and orientation centres are such a prominent organizational feature that it should be important to understand their development as well as their function in the processing of visual information.     

Ocular dominance plasticity in adult cat visual cortex after transplantation of cultured astrocytes

During a critical restricted period of postnatal development, the visual cortical circuitry is susceptible to modifications that are dependent on experience. If vision is restricted to only one eye during this period, the territories innervated by the deprived eye shrink considerably, whereas those innervated by the non-deprived eye expand, and the deprived eye loses the ability to influence almost all of the cells in the cortex. Thus, changes in ocular dominance are paralleled and possibly mediated by synapse elimination and axonal sprouting. Hypotheses about the mechanisms underlying ocular-dominance plasticity assume the activation of NMDA (N-methyl-D-aspartate) receptors and subsequent calcium influx as a trigger of synaptic modifications. In addition, plasticity relies on functional neuromodulatory afferents. On the basis of immunocytochemical studies, it was recently proposed that the presence of immature astrocytes is a prerequisite for visual cortical plasticity, and that the end of the critical period is causally linked to the maturation of astrocytes. Here we report, in support of this hypothesis, that resupplementation of the visual cortex of adult cats with astrocytes cultured from the visual cortex of newborn kittens reinduces ocular-dominance plasticity in adult animals.     

A comparison of inhibition in orientation and spatial frequency selectivity of cat visual cortex

Neurones in the visual cortex are highly selective for orientation and spatial frequency of visual stimuli. There is strong neurophysiological evidence that orientation selectivity is enhanced by inhibitory interconnections between columns in the cortex which have different orientation sensitivities, an idea which is supported by experiments using neuropharmacological manipulation or complex visual stimuli. It has also been proposed that selectivity for spatial frequency is mediated in part by a similar mechanism to that for orientation, although evidence for this is based on special use of visual stimuli, which hampers interpretation of the findings. We have therefore examined selectivity for both orientation and spatial frequency using a technique which allows direct inferences about inhibitory processes. Our method uses microiontophoresis of an excitatory amino acid to elevate maintained discharge of single neurones in the visual cortex. We then present visual stimuli both within and outside the range of orientations and spatial frequencies which cause a cell to respond with increased discharge. Our results show that orientations presented on either side of the responsive range usually produce clear suppression of maintained discharge. In marked contrast, spatial frequencies shown to either side of the responsive range have little or no effect on maintained activity. We conclude that there is an intracortical organization of inhibitory connections between cells tuned to different orientations but not different spatial frequencies.     

Oscillatory responses in cat visual cortex exhibit inter-columnar synchronization which reflects global stimulus properties

A fundamental step in visual pattern recognition is the establishment of relations between spatially separate features. Recently, we have shown that neurons in the cat visual cortex have oscillatory responses in the range 40-60 Hz (refs 1, 2) which occur in synchrony for cells in a functional column and are tightly correlated with a local oscillatory field potential. This led us to hypothesize that the synchronization of oscillatory responses of spatially distributed, feature selective cells might be a way to establish relations between features in different parts of the visual field. In support of this hypothesis, we demonstrate here that neurons in spatially separate columns can synchronize their oscillatory responses. The synchronization has, on average, no phase difference, depends on the spatial separation and the orientation preference of the cells and is influenced by global stimulus properties.     

Foci of orientation plasticity in visual cortex

Cortical areas are generally assumed to be uniform in their capacity for adaptive changes or plasticity. Here we demonstrate, however, that neurons in the cat striate cortex (V1) show pronounced adaptation-induced short-term plasticity of orientation tuning primarily at specific foci. V1 neurons are clustered according to their orientation preference in iso-orientation domains that converge at singularities or pinwheel centres. Although neurons in pinwheel centres have similar orientation tuning and responses to those in iso-orientation domains, we find that they differ markedly in their capacity for adaptive changes. Adaptation with an oriented drifting grating stimulus alters responses of neurons located at and near pinwheel centres to a broad range of orientations, causing repulsive shifts in orientation preference and changes in response magnitude. In contrast, neurons located in iso-orientation domains show minimal changes in their tuning properties after adaptation. The anisotropy of adaptation-induced orientation plasticity is probably mediated by inhomogeneities in local intracortical interactions that are overlaid on the map of orientation preference in V1.     

Induction of visual orientation modules in auditory cortex

Modules of neurons sharing a common property are a basic organizational feature of mammalian sensory cortex. Primary visual cortex (V1) is characterized by orientation modules--groups of cells that share a preferred stimulus orientation--which are organized into a highly ordered orientation map. Here we show that in ferrets in which retinal projections are routed into the auditory pathway, visually responsive neurons in 'rewired' primary auditory cortex are also organized into orientation modules. The orientation tuning of neurons within these modules is comparable to the tuning of cells in V1 but the orientation map is less orderly. Horizontal connections in rewired cortex are more patchy and periodic than connections in normal auditory cortex, but less so than connections in V1. These data show that afferent activity has a profound influence on diverse components of cortical circuitry, including thalamocortical and local intracortical connections, which are involved in the generation of orientation tuning, and long-range horizontal connections, which are important in creating an orientation map.     

Experience-dependent representation of visual categories in parietal cortex

Categorization is a process by which the brain assigns meaning to sensory stimuli. Through experience, we learn to group stimuli into categories, such as 'chair', 'table' and 'vehicle', which are critical for rapidly and appropriately selecting behavioural responses. Although much is known about the neural representation of simple visual stimulus features (for example, orientation, direction and colour), relatively little is known about how the brain learns and encodes the meaning of stimuli. We trained monkeys to classify 360 degrees of visual motion directions into two discrete categories, and compared neuronal activity in the lateral intraparietal (LIP) and middle temporal (MT) areas, two interconnected brain regions known to be involved in visual motion processing. Here we show that neurons in LIP--an area known to be centrally involved in visuo-spatial attention, motor planning and decision-making-robustly reflect the category of motion direction as a result of learning. The activity of LIP neurons encoded directions of motion according to their category membership, and that encoding shifted after the monkeys were retrained to group the same stimuli into two new categories. In contrast, neurons in area MT were strongly direction selective but carried little, if any, explicit category information. This indicates that LIP might be an important nexus for the transformation of visual direction selectivity to more abstract representations that encode the behavioural relevance, or meaning, of stimuli.     

Adaptive filtering enhances information transmission in visual cortex

Sensory neuroscience seeks to understand how the brain encodes natural environments. However, neural coding has largely been studied using simplified stimuli. In order to assess whether the brain's coding strategy depends on the stimulus ensemble, we apply a new information-theoretic method that allows unbiased calculation of neural filters (receptive fields) from responses to natural scenes or other complex signals with strong multipoint correlations. In the cat primary visual cortex we compare responses to natural inputs with those to noise inputs matched for luminance and contrast. We find that neural filters adaptively change with the input ensemble so as to increase the information carried by the neural response about the filtered stimulus. Adaptation affects the spatial frequency composition of the filter, enhancing sensitivity to under-represented frequencies in agreement with optimal encoding arguments. Adaptation occurs over 40 s to many minutes, longer than most previously reported forms of adaptation.     

一种视皮层非经典感受野的模型

根据视觉信息加工过程中视皮层细胞非经典感受野对中心感受野区域具有抑制作用的生物机制,提出了一种模拟该机制的模型.用Gabor能量作为感受野响应,以高斯差分函数为基础构造两个具有方向的半椭圆环来作为非经典感受野区域,设计中心感受野与周边抑制区域的相位差权重函数来模拟非经典感受野的抑制机制.实验结果与这种生理特性相一致.     

Grouping of image fragments in primary visual cortex.

In the visual world, objects are partially occluded by nearer objects, separating them into image fragments. However, the image fragments of the object can easily be grouped and organized together by the visual system. Psychophysical data and theoretical analysis indicate that such perceptual grouping might be mediated in the early stages of visual processing. Here I show that some orientation-selective cells in the primary visual cortex (V1) have response properties that can mediate the grouping of image fragments. These cells stopped responding to a stimulus bar when it was partly occluded by a small patch. The cells also did not respond when the patch had uncrossed disparity so that it appeared to be behind the bar. However, the cells began responding again when the patch had crossed disparity so that it appeared to be in front of the bar. These results indicate that cells as early as V1 have the computational power to make inferences about the nature of partially invisible forms seen behind occluding structures.     

Imaging cortical correlates of illusion in early visual cortex

Exploring visual illusions reveals fundamental principles of cortical processing. Illusory motion perception of non-moving stimuli was described almost a century ago by Gestalt psychologists. However, the underlying neuronal mechanisms remain unknown. To explore cortical mechanisms underlying the 'line-motion' illusion, we used real-time optical imaging, which is highly sensitive to subthreshold activity. We examined, in the visual cortex of the anaesthetized cat, responses to five stimuli: a stationary small square and a long bar; a moving square; a drawn-out bar; and the well-known line-motion illusion, a stationary square briefly preceding a long stationary bar presentation. Whereas flashing the bar alone evoked the expected localized, short latency and high amplitude activity patterns, presenting a square 60-100 ms before a bar induced the dynamic activity patterns resembling that of fast movement. The preceding square, even though physically non-moving, created gradually propagating subthreshold cortical activity that must contribute to illusory motion, because it was indistinguishable from cortical representations of real motion in this area. These findings demonstrate the effect of spatio-temporal patterns of subthreshold synaptic potentials on cortical processing and the shaping of perception.     

Involvement of visual cortex in tactile discrimination of orientation.

The primary sense modalities (vision, touch and so on) are generally thought of as distinct. However, visual imagery is implicated in the normal tactile perception of some object properties, such as orientation, shape and size. Furthermore, certain tactile tasks, such as discrimination of grating orientation and object recognition, are associated with activity in areas of visual cortex. Here we show that disrupting function of the occipital cortex using focal transcranial magnetic stimulation (TMS) interferes with the tactile discrimination of grating orientation. The specificity of this effect is illustrated by its time course and spatial restriction over the scalp, and by the failure of occipital TMS to affect either detection of an electrical stimulus applied to the fingerpad or tactile discrimination of grating texture. In contrast, TMS over the somatosensory cortex blocked discrimination of grating texture as well as orientation. We also report that, during tactile discrimination of grating orientation, an evoked potential is recorded over posterior scalp regions with a latency corresponding to the peak of the TMS interference effect (about 180 ms). The findings indicate that visual cortex is closely involved in tactile discrimination of orientation. To our knowledge, this is the first demonstration that visual cortical processing is necessary for normal tactile perception.     

Parallel colour-opponent pathways to primary visual cortex

The trichromatic primate retina parses the colour content of a visual scene into 'red/green' and 'blue/yellow' representations. Cortical circuits must combine the information encoded in these colour-opponent signals to reconstruct the full range of perceived colours. Red/green and blue/yellow inputs are relayed by the lateral geniculate nucleus (LGN) of thalamus to primary visual cortex (V1), so understanding how cortical circuits transform these signals requires understanding how LGN inputs to V1 are organized. Here we report direct recordings from LGN afferent axons in muscimol-inactivated V1. We found that blue/yellow afferents terminated exclusively in superficial cortical layers 3B and 4A, whereas red/green afferents were encountered only in deeper cortex, in lower layer 4C. We also describe a distinct cortical target for 'blue-OFF' cells, whose afferents terminated in layer 4A and seemed patchy in organization. The more common 'blue-ON' afferents were found in 4A as well as lower layer 2/3. Chromatic information is thus conveyed to V1 by parallel, anatomically segregated colour-opponent systems, to be combined at a later stage of the colour circuit.     

Attentional modulation in visual cortex depends on task timing

Paying attention to a stimulus selectively increases the ability to process it. For example, when subjects attend to a specific region of a visual scene, their sensitivity to changes at that location increases. A large number of studies describe the behavioural consequences and neurophysiological correlates of attending to spatial locations. There has, in contrast, been little study of the allocation of attention over time. Because subjects can anticipate predictable events with great temporal precision, it seems probable that they might dynamically shift their attention when performing a familiar perceptual task whose constraints changed over time. We trained monkeys to respond to a stimulus change where the probability of occurrence changed over time. Recording from area V4 of the visual cortex in these animals, we found that the modulation of neuronal responses changed according to the probability of the change occurring at that instant. Thus, we show that the attentional modulation of sensory neurons reflects a subject's anticipation of the timing of behaviourally relevant events.