Entire Visual Cortex Research Articles

Enhanced auditory responses in visual cortex of blind rats using intrinsic optical signal imaging

Functional maturation of the visual cortex is induced by visual experiences during critical periods. Blind animals and humans exhibit improved auditory abilities after losing their vision. Here we investigated the response of the visual cortex to white noise stimuli during the progression of photoreceptor degeneration in a rat model of blindness (Royal College of Surgeons [RCS] (rdy/rdy) rats). Optical coherence tomography of RCS (+/+) rats with normal visual function revealed normal photoreceptor cells, whereas 3-month-old RCS (rdy/rdy) rats demonstrated photoreceptor cell degeneration. Visual cortex responses (VCRs) to a single flash stimulus were negligible in 3-month-old photoreceptor-degenerated rats. However, VCRs with white noise stimuli were significantly increased in blind versus RCS rats (+/+). Slight changes in the intrinsic optical signals of the control rats were observed on the ventral side of the visual cortex. In contrast, responses were markedly increased throughout the visual cortex of RCS (rdy/rdy) rats. These results indicate that the visual cortex rapidly acquires auditory system function over the first 3 months of life and that the entire visual cortex, rather than just the portion close to the auditory cortex, responds to white noise.

Modulation of Visual Responses and Ocular Dominance by Contralateral Inhibitory Activation in the Mouse Visual Cortex.

Both hemispheres connect with each other by excitatory callosal projections, and whether inhibitory interneurons, usually believed to have local innervation, engage in transcallosal activity modulation is unknown. Here, we used optogenetics in combination with cell-type-specific channelrhodopsin-2 expression to activate different inhibitory neuron subpopulations in the visual cortex and recorded the response of the entire visual cortex using intrinsic signal optical imaging. We found that optogenetic stimulation of inhibitory neurons reduced spontaneous activity (increase in the reflection of illumination) in the binocular area of the contralateral hemisphere, although these stimulations had different local effects ipsilaterally. The activation of contralateral interneurons differentially affected both eye responses to visual stimuli and, thus, changed ocular dominance. Optogenetic silencing of excitatory neurons affects the ipsilateral eye response and ocular dominance in the contralateral cortex to a lesser extent. Our results revealed a transcallosal effect of interneuron activation in the mouse visual cortex.

A survey of neurophysiological differentiation across mouse visual brain areas and timescales.

Neurophysiological differentiation (ND), a measure of the number of distinct activity states that a neural population visits over a time interval, has been used as a correlate of meaningfulness or subjective perception of visual stimuli. ND has largely been studied in non-invasive human whole-brain recordings where spatial resolution is limited. However, it is likely that perception is supported by discrete neuronal populations rather than the whole brain. Therefore, here we use Neuropixels recordings from the mouse brain to characterize the ND metric across a wide range of temporal scales, within neural populations recorded at single-cell resolution in localized regions. Using the spiking activity of thousands of simultaneously recorded neurons spanning 6 visual cortical areas and the visual thalamus, we show that the ND of stimulus-evoked activity of the entire visual cortex is higher for naturalistic stimuli relative to artificial ones. This finding holds in most individual areas throughout the visual hierarchy. Moreover, for animals performing an image change detection task, ND of the entire visual cortex (though not individual areas) is higher for successful detection compared to failed trials, consistent with the assumed perception of the stimulus. Together, these results suggest that ND computed on cellular-level neural recordings is a useful tool highlighting cell populations that may be involved in subjective perception.

Deep Residual Network Predicts Cortical Representation and Organization of Visual Features for Rapid Categorization

The brain represents visual objects with topographic cortical patterns. To address how distributed visual representations enable object categorization, we established predictive encoding models based on a deep residual network, and trained them to predict cortical responses to natural movies. Using this predictive model, we mapped human cortical representations to 64,000 visual objects from 80 categories with high throughput and accuracy. Such representations covered both the ventral and dorsal pathways, reflected multiple levels of object features, and preserved semantic relationships between categories. In the entire visual cortex, object representations were organized into three clusters of categories: biological objects, non-biological objects, and background scenes. In a finer scale specific to each cluster, object representations revealed sub-clusters for further categorization. Such hierarchical clustering of category representations was mostly contributed by cortical representations of object features from middle to high levels. In summary, this study demonstrates a useful computational strategy to characterize the cortical organization and representations of visual features for rapid categorization.

One Picture Is Worth at Least a Million Neurons

How many neurons participate in the representation of a single visual image? Answering this question is critical for constraining biologically inspired models of object recognition, which vary greatly in their assumptions from few “grandmother cells” [1] to numerous neurons in widely distributed networks [2]. Functional imaging techniques, such as fMRI, provide an opportunity to explore this issue, since they allow the simultaneous detection of the entire neuronal population responding to each stimulus. Several studies [3–6] have shown that fMRI BOLD signal is approximately proportional to neuronal activity. However, since it provides an indirect measure of this activity, obtaining a realistic estimate of the number of activated neurons requires several intervening steps. Here, we used the extensive knowledge of primate V1 to yield a conservative estimate of the ratio between hemodynamic response and neuronal firing. This ratio was then used, in addition to several cautious assumptions, to assess the number of neurons responding to a single-object image in the entire visual cortex and particularly in object-related areas. Our results show that at least a million neurons in object-related cortex and about two hundred million neurons in the entire visual cortex are involved in the representation of a single-object image.

Uniformity, specificity and variability of corticocortical connectivity.

In many studies of the mammalian brain, subjective assessments of connectivity patterns and connection strengths have been used to subdivide the cortex into separate but linked areas and to make deductions about the flow of information through the cortical network. Here we describe the results of applying statistical analyses to quantitative corticocortical connection data, and the conclusions that can be drawn from such quantitative approaches. Injections of the tracer WGA-HRP were made into different visual areas either side of the middle suprasylvian sulcus (MSS) in 11 adult cats. Retrogradely labelled cells produced by these injections were counted in selected coronal sections taken at regularly spaced intervals (1 mm) through the entire visual cortex, and their cumulative sums and relative proportions in each of 16 recognized visual cortical areas were computed. The surface dimensions of these areas were measured in each cat, from contour lines made on enlarged drawings of the same sections. A total of 116,149 labelled neurons were assigned to all visual cortical areas in the 11 cats, with 5212 others excluded because of their uncertain location. The distribution of relative connection strengths, that is, the percentage of labelled cells per cortical area, was evaluated using non-parametric cluster analyses and Monte Carlo simulation, and relationships between connection strength and area size were examined by linear regression. The absolute size of each visual cortical area was uniform across individual cats, whereas the strengths of connections between the same area pairs were extremely variable for injections in different animals. The overall distribution of labelling strengths for corticocortical connections was continuous and monotonic, rather than inherently clustered, with the highest frequencies presented by the absent (zero density) and the very-low-density connections. These two categories could not, on analytical grounds, be separated from each other. Thus it seems that any subjective description of corticocortical connectivity strengths by ordinal classes (such as 'absent', 'weak', 'moderate' or 'strong') imposes a categorization on the data, rather than recognizes a structure inherent in the data themselves. Despite the great variability of connections, similarities in the distribution profiles for the relative strengths of labelled cells in all areas could be used to identify clusters of different injection sites in the MSS. This supported the conclusion that there are four connectionally distinct subdivisions of this cortex, corresponding to areas 21a, PMLS and AMLS (in the medial bank) and to area PLLS (in the lateral bank). Even for tracer deposits in the same cortical subdivision, however, the strength of connections projecting to the site from other cortical areas varied greatly across injection in different individual animals. We further demonstrated that, on average, the strength of connections originating from any given cortical area was positively and linearly correlated with the size of its surface dimensions. When analysed by specific injection site location, however, this relationship was shown to hold for the individual connections to the medial bank MSS areas, but not for connections leading to the lateral bank area. The data suggest that connectivity of the cat's visual cortex possesses a number of uniform global features, which are locally organized in such a way as to give each cortical area unique characteristics.

Cortical and subcortical influences on the nucleus of the optic tract of the opossum

In the present work we propose a new phylogenetic hypothesis for the role played by cortical and subcortical afferents to the nucleus of the optical tract, the main visual relay station of the horizontal optokinetic reflex in mammals. The hypothesis is supported by anatomical and physiological data obtained in the South American opossum ( Didelphis aurita) using the following experimental approaches: (i) single-unit recordings in the nucleus of the optic tract and simultaneous electrical stimulation of the contralateral nucleus of the optic tract; (ii) single-unit recordings in the nucleus of the optic tract and simultaneous electrical stimulation of the ipsilateral striate cortex; (iii) injection of cholera toxin subunit B into the striate cortex and subsequent immunohistochemical reaction to reveal the presence of the marker in the thalamus and mesencephalon; and (iv) single-unit recordings in the nucleus of the optic tract both before and after ablation of the ipsilateral visual cortex. The main results are: (i) there is a strong inhibitory reciprocal effect upon the nucleus of the optic tract following stimulation of its contralateral counterpart; (ii) electrophysiological and anatomical data imply that the visual cortex does not project directly to the nucleus of the optic tract. Rather, cortical terminals seem to target the nearby anterior and posterior pretectal nuclei and orthodromic latencies in the nucleus of the optic tract following stimulation of the visual cortex were twice as large as in the superior colicullus; and (iii) ablation of the entire visual cortex did not have any effect upon binocularity of cells in the nucleus of the optic tract. These results strengthen the model proposed here for the role of the interactions between the nuclei of the optic tract under optokinetic stimulation. The hypothesis in the present work is that the cortical influences upon the nucleus of the optical tract, in addition to the subcortical ones, appeared only recently in phylogenesis. In more primitive mammals, such as the opossum, subcortical interactions are thought to play a relatively important role. With the emergence of retinal specializations, such as the fovea, one might suppose that there followed the appearance of new ocular movements, such as the smooth pursuit and certain types of saccades, that came to join the pre-existent optokinetic reflex.

Activation PET scanning in pretreatment evaluation of patients with cerebral tumours or vascular lesions in or close to the sensorimotor cortex.

Treatment of tumours and vascular lesions in or close to eloquent cortex may inflict neurological deficits. Intra-operative mapping procedures have been used for many decades in efforts to minimize neurological sequelae. The possibility for non-invasive pre-operative brain mapping has emerged with the advent of positron emission tomography (PET). In this paper we report on 11 patients with a tumour or vascular lesion in or close to the sensorimotor (10 patients) or visual cortex (one patient). The patients were subjected to activation PET scanning by means of vibrotactile or visual stimulation. The results show that in most of the patients the important relation between the lesion and the sensorimotor cortex could be determined. The patient with a lesion in the occipital lobe had involvement of the entire visual cortex as judged by comparison with activated areas on the nonlesion side.

Distribution of neuropeptide Y immunoreactivity in human visual cortex and underlying white matter

Immunocytochemical techniques have been used to study neuropeptide Y (NPY) distribution in the human visual cortex (Brodman's areas 17, 18 and 19) NYP cell bodies belong mostly to inhibitory (multipolar and bitufted) but also to excitatory (bipolar and some pyramidal) neuronal types. Their distribution is similar in the three cortical areas studied: 20 to 40% of the NPY perikarya are located in the cortical gray matter, mostly in the deep layers, while the remaining 60 to 80% are located in the underlying white matter. Immunoreactive NPY processes form a rich network of intersecting fibers throughout the entire visual cortex. A superficial plexus (layers I and II) and a deep plexus (deep layer V and layer VI) of NPY fibers are present in areas 17, 18 and 19. In area 17, an additional well developed plexus is present in layers IVb and IVc. These plexuses receive branches from long parallel fibers arising from deep cortical layers or underlying white matter and terminating in superficial layers. Local or extrinsic NPY terminals wind around vessels in the cortex as well as in the white matter, and either penetrate them or form clusters of club endings on their walls. Our results suggest a role for NPY in human visual circuitry and in cortical blood flow regulation.

The effect of devascularization of the visual cortex on visual function in the rabbit

The role of the visual cortex in brightness and pattern vision was re-examined in the rabbit. Animals were trained on both a brightness and a horizontal-vertical striation discrimination. Bilateral removal of the dura mater overlying the entire visual cortex produced no impairments in either brightness or pattern vision. When the visual cortex was devascularized by removal of the pia mater bilaterally, total loss of pattern vision was found with no impairment of brightness discrimination ability. The significance of the finding that the effects of slowly developing degenerative lesions are the same as those of surgical ablation was discussed.

Selective collateral elimination in early postnatal development restricts cortical distribution of rat pyramidal tract neurones.

The pyramidal tract, comprising those axons which pass from the neocortex to the medulla and spinal cord, is among the most thoroughly studied projections of the mammalian cortex. Recent studies using anterograde axon tracing techniques have provided information concerning the time course of the growth of pyramidal tract fibres, yet much remains to be learned about its development. We have now begun to study the distribution of the neurones of origin of the pyramidal tract during the postnatal development of the rat neocortex using the recently introduced retrogradely transported fluorescent marker, True blue. During the first postnatal week, injections of True blue into the pyramidal decussation result inthe labelling of pyramidal tract neurones which are distributed virtually throughout the tangential extent of layer V of the neocortex, whereas after comparable injections during the fourth postnatal week the distribution of such cells is much more restricted and remains restricted into adult life. This developmental restriction is most dramatic in the occipital cortex: during the first postnatal week many pyramidal tract neurones are found throughout the visual cortex while none is seen in this area of the adult. When True blue is injected into the pyramidal decussation during the first postnatal week and the animals are allowed to survive until the fourth postnatal week, the distribution of pyramidal tract neurones is as widespread as in the immediate postnatal period and includes the entire visual cortex. This implies that many of the neurones in the occipital cortex initially send a collateral into the pyramidal tract which is later eliminated, although the neurones themselves persist. These findings, together with similar recent observations on the development of the callosal connections, indicate that the elimination of axon collaterals may be a general feature of the development of cortical projection systems, and that such transitory collaterals may traverse considerable distances.

Projections from the superior colliculus and the neocortex to the pulvinar nucleus in Galago.

We have studied the projections from the superior colliculus and the neocortex to the pulvinar nucleus in Galago senegalensis by using the retrograde transport of horseradish peroxidase (HRP). Injections of various parts of the pulvinar complex, both the inferior and superior divisions, both the tectorecipient zone and the nontectorecipient zone as defined by Glendenning et. al. ('75), produce labeled cells in the lower tier of stratum griseum superficiale. The distribution of labeled cells in the superior colliculus varies with the locus of the injection, indicating a retinotopic projection system from the entire superior colliculus to all sectors of the pulvinar complex. These experiments also provide an opportunity to study the distribution and laminar origin of neurons giving rise to cortical descending projections. The entire visual cortex projects onto the pulvinar complex. The cells or origin can be divided into two populations--one located in layer V and the other in layer VI. In seven of the nine cases reported, the layer V population is restricted entirely or mainly to the striate area. In the two exceptional cases, the layer V population is located in the adjacent extrastriate cortex, areas 18 and 19. The difference in the layer of origin of the cortical descending fibers reflects a difference in the layer of termination of the reciprocal ascending projection. These findings identify the entire visual field as primary visual cortex. The importance of this conclusion is underscored by the fact that the visual field comprises as much as one-half of the whole neocortex.

Genetic instructions and developmental plasticity in the kitten's visual cortex.

Two major properties of neurons in the kitten’s visual cortex, binocularity and orientation selectivity, are present when the eyes first open, and therefore can be established by genetic instructions alone. However, both of these attributes require visual experience for their maintenance or strengthening; and both can be rapidly modified by unusual kinds of experience. Alternating sequences of cells dominated by one eye, then the other, can be recorded during penetrations through the cortex in binocularly deprived kittens, typical of the ‘ocular dominance columns’ of the normal adult cat. However, if one eye is deprived by lid-suture, the entire visual cortex becomes strongly dominated by the open eye. Experiments in which each eye saw separately through a transparent neutral density filter or a translucent diffuser showed that this phenomenon is caused not by the reduction in retinal illumination, but by the abolition of contrast in the deprived eye. A study of the retrograde transport of horseradish peroxidase from the visual cortex to the principal laminae of the lateral geniculate nucleus suggested that monocular deprivation from early in life may lead to a gross reduction in the distribution of afferent fibres from the deprived laminae. Previous experiments have found that if a kitten is exposed only to contours of one orientation, its cortical neurons become modified in their distribution of preferred orientations. This phenomenon was re-confirmed in a new study using a rigorously objective method of analysis.