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.     

The contribution of sensory experience to the maturation of orientation selectivity in ferret visual cortex.

Sensory experience begins when neural circuits in the cerebral cortex are still immature; however, the contribution of experience to cortical maturation remains unclear. In the visual cortex, the selectivity of neurons for oriented stimuli at the time of eye opening is poor and increases dramatically after the onset of visual experience. Here we investigate whether visual experience has a significant role in the maturation of orientation selectivity and underlying cortical circuits using two forms of deprivation: dark rearing, which completely eliminates experience, and binocular lid suture, which alters the pattern of sensory driven activity. Orientation maps were present in dark-reared ferrets, but fully mature levels of tuning were never attained. In contrast, only rudimentary levels of orientation selectivity were observed in lid-sutured ferrets. Despite these differences, horizontal connections in both groups were less extensive and less clustered than normal, suggesting that long-range cortical processing is not essential for the expression of orientation selectivity, but may be needed for the full maturation of tuning. Thus, experience is beneficial or highly detrimental to cortical maturation, depending on the pattern of sensory driven activity.     

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.     

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.     

虚拟环境热触觉的显示装置与材质识别实验

基于半无限体模型,研究了热触觉感知的机理.设计了热触觉显示装置,通过控制半导体制冷器的温度变化来实现热触觉显示.该装置的控温范围为-10 ～60℃,温度分辨率和精度分别为0.02和0.1℃,升温或降温速率大于10℃/s.定制了柔性镍金属薄片式温度传感器,尺寸为8mm×20 mm ×0.6 mm,时间常数τ＜0.15 s...     

Receptive field dynamics in adult primary visual cortex.

The adult brain has a remarkable ability to adjust to changes in sensory input. Removal of afferent input to the somatosensory, auditory, motor or visual cortex results in a marked change of cortical topography. Changes in sensory activity can, over a period of months, alter receptive field size and cortical topography. Here we remove visual input by focal binocular retinal lesions and record from the same cortical sites before and within minutes after making the lesion and find immediate striking increases in receptive field size for cortical cells with receptive fields near the edge of the retinal scotoma. After a few months even the cortical areas that were initially silenced by the lesion recover visual activity, representing retinotopic loci surrounding the lesion. At the level of the lateral geniculate nucleus, which provides the visual input to the striate cortex, a large silent region remains. Furthermore, anatomical studies show that the spread of geniculocortical afferents is insufficient to account for the cortical recovery. The results indicate that the topographic reorganization within the cortex was largely due to synaptic changes intrinsic to the cortex, perhaps through the plexus of long-range horizontal connections.     

Columns for visual features of objects in monkey inferotemporal cortex.

At early stages of the mammalian visual cortex, neurons with similar stimulus selectivities are vertically arrayed through the thickness of the cortical sheet and clustered in patches or bands across the surface. This organization, referred to as a 'column', has been found with respect to one-dimensional stimulus parameters such as orientation of stimulus contours, eye dominance of visual inputs, and direction of stimulus motion. It is unclear, however, whether information with extremely high dimensions, such as visual shape, is organized in a similar columnar fashion or in a different manner in the brain. Here we report that the anterior inferotemporal area of the monkey cortex, the final station of the visual cortical stream crucial for object recognition, consists of columns, each containing cells responsive to similar visual features of objects.     

Functional specialisation in the visual cortex of the rhesus monkey.

Anatomical and functional studies of the visual cortex of the rhesus monkey have shown that it is made up of a multiplicity of distinct areas. These seem to be functionally specialised to analyse different features of the visual environment.     

Early experience of tactile stimulation influences organization of somatic sensory cortex

Visual experience is essential for the establishment of the cerebral cortical circuitry that allows normal binocular vision. For example, the pattern of right-eye, left-eye dominance columns is permanently altered by simply closing an eye of a young primate. A critical issue is whether environmental factors also influence the development of other cortical sensory areas. In the present experiments we manipulated the tactile experience of young rats by depriving them of the sensory information that is normally provided by their large facial whiskers. Electrophysiological analyses showed that simply trimming the whiskers from the day of birth results in pronounced abnormalities in the response properties of single neurons in the adult somatic sensory cortex. Thus functional plasticity in response to early experience appears to be a fundamental aspect of cortical development.     

Feature-based attention influences motion processing gain in macaque visual cortex.

Changes in neural responses based on spatial attention have been demonstrated in many areas of visual cortex, indicating that the neural correlate of attention is an enhanced response to stimuli at an attended location and reduced responses to stimuli elsewhere. Here we demonstrate non-spatial, feature-based attentional modulation of visual motion processing, and show that attention increases the gain of direction-selective neurons in visual cortical area MT without narrowing the direction-tuning curves. These findings place important constraints on the neural mechanisms of attention and we propose to unify the effects of spatial location, direction of motion and other features of the attended stimuli in a 'feature similarity gain model' of attention.     

Dark-rearing delays the loss of NMDA-receptor function in kitten visual cortex.

Some features of the visual cortex develop postnatally in mammals. For example, geniculocortical axons that initially overlap throughout cortical layer IV segregate postnatally into two sets of interleaved eye-specific bands. NMDA (N-methyl-D-aspartate) receptors are necessary for eye-specific axon-segregation in the frog tectum, and as NMDA receptors play a greater part in synaptic transmission in early life and decrease in function during the period of axon segregation, they may be involved in the segregation of geniculocortical axons: they are well placed to do so as they transduce retinally derived signals essential for segregation. Rearing animals in the dark in early life delays segregation and prolongs the critical period for plasticity. We now report that dark-rearing of kittens also delays the loss of NMDA receptor function in the visual cortex, supporting the view that they play an important part in neuronal development and plasticity.     

Vernier acuity of neurones in cat visual cortex

The ability of human observers to detect Vernier breaks of as little as 5 s arc has been termed hyperacuity as this distance is substantially less than the angular separation of the bars of the highest spatial frequency of grating (approximately 1 arc min) that can be detected. Although the visual cortex is a likely candidate for the location of detectors involved in this performance, it is not known whether there are cells sensitive enough to detect deviations from co-linearity that are small compared with their spatial resolution (defined in terms of the highest spatial frequency that the cell can detect). We report here the results of physiological experiments on single units in area 17 of the cat visual cortex in which we studied the effect of introducing a Vernier break into a bar stimulus moved across the receptive field of the cell at a constant velocity. Our results show that the responses of most simple and complex cells are significantly reduced by the introduction of a Vernier break that is substantially smaller than the spatial resolution of the cell. The most sensitive cells in our sample could discriminate Vernier offsets of 3-6 arc min with a reliability of approximately 70%. This was much smaller than their spatial resolution, which was in the range 25-30 arc min. We interpret these results in terms of mechanisms that could underly the orientation selectivity of cortical neurones and suggest how our results relate to human Vernier acuity.     

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.     

Topography of contextual modulations mediated by short-range interactions in primary visual cortex.

Neurons in primary visual cortex (V1) respond differently to a simple visual element presented in isolation from when it is embedded within a complex image. This difference, a specific modulation by surrounding elements in the image, is mediated by short- and long-range connections within V1 and by feedback from other areas. Here we study the role of short-range connections in this process, and relate it to the layout of local inhomogeneities in the cortical maps of orientation and space. By measuring correlation between neuron pairs located in optically imaged maps of V1 orientation columns we show that the strength of local connections between cells is a graded function of lateral separation across cortex, largely radially symmetrical and relatively independent of orientation preferences. We then show the contextual influence of flanking visual elements on neuronal responses varies systematically with a neuron's position within the cortical orientation map. The strength of this contextual influence on a neuron can be predicted from a model of local connections based on simple overlap with particular features of the orientation map. This indicates that local intracortical circuitry could endow neurons with a graded specialization for processing angular visual features such as corners and T junctions, and this specialization could have its own functional cortical map, linked with the orientation map.     

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.