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C D Gilbert

Publications and source records attributed to C D Gilbert.

At least 37 records · Page 2Linked to original sources
PubMed1997

Perceptual learning of spatial localization: specificity for orientation, position, and context.

Discrimination of simple visual attributes can improve significantly with practice. We have trained human observers to perform peripherally presented tasks involving the localization of short line segments and examined the specificity of the learning for the visual location, orientation, and geometric arrangement of the trained stimulus. Several weeks of training resulted in dramatic threshold reductions. The learning was specific for the orientation and location of the trained stimulus, indicating the involvement of the earliest cortical stages in the visual pathway where the orientation and location of stimuli are mapped with highest resolution. Furthermore, improvement was also specific for both the configuration of the trained stimulus and the attribute of the stimulus that was under scrutiny during training. This degree of specificity suggests that the learning cannot be achieved by cortical recruitment alone, as proposed in current models, but is likely to involve a refinement of lateral interactions within the cortex and possibly a gating of lower level changes by attentional mechanisms.

Cues↗
PubMed1996

Spatial integration and cortical dynamics.

Cells in adult primary visual cortex are capable of integrating information over much larger portions of the visual field than was originally thought. Moreover, their receptive field properties can be altered by the context within which local features are presented and by changes in visual experience. The substrate for both spatial integration and cortical plasticity is likely to be found in a plexus of long-range horizontal connections, formed by cortical pyramidal cells, which link cells within each cortical area over distances of 6-8 mm. The relationship between horizontal connections and cortical functional architecture suggests a role in visual segmentation and spatial integration. The distribution of lateral interactions within striate cortex was visualized with optical recording, and their functional consequences were explored by using comparable stimuli in human psychophysical experiments and in recordings from alert monkeys. They may represent the substrate for perceptual phenomena such as illusory contours, surface fill-in, and contour saliency. The dynamic nature of receptive field properties and cortical architecture has been seen over time scales ranging from seconds to months. One can induce a remapping of the topography of visual cortex by making focal binocular retinal lesions. Shorter-term plasticity of cortical receptive fields was observed following brief periods of visual stimulation. The mechanisms involved entailed, for the short-term changes, altering the effectiveness of existing cortical connections, and for the long-term changes, sprouting of axon collaterals and synaptogenesis. The mutability of cortical function implies a continual process of calibration and normalization of the perception of visual attributes that is dependent on sensory experience throughout adulthood and might further represent the mechanism of perceptual learning.

Animals↗
PubMed1996

Plasticity in visual perception and physiology.

Many factors influence our perception of local features. What we see is not strictly a reflection of the physical characteristics of a scene, but instead is highly dependent on the processes by which our brain attempts to interpret the scene. As a result, our percepts are shaped by the context within which local features are presented, by our previous visual experiences (operating over a wide range of time scales), and by our expectations of what is likely to be before us. The substrate for these influences is found in the lateral interactions operating within individual areas of the cerebral cortex and in the feedback from higher to lower order cortical areas.

Animals↗
PubMed1995

Long-range horizontal connections and their role in cortical reorganization revealed by optical recording of cat primary visual cortex.

The cortical 'point spread' (PS) is the area of cortex activated by a minimal visual stimulus. Here we use the PS to explore the functional role of lateral connectivity in normal cat primary visual cortex (V1) and its involvement in topographic reorganization of cortex following retinal lesions. We compared the distributions of PSs measured with optical recording, which reflects both spiking and subthreshold activity, with those measured with extracellular electrodes, which reveal spiking activity alone. The spiking PS represented only 5% of the area of activation shown in the optical PS, indicating that the remaining 95% was probably generated by subthreshold activation. The orientation dependence of the pattern of the subthreshold activation and its close match with orientation columns suggests that long-range horizontal connections radiating from the locus of spiking activity were responsible for the observed activation. The spike PS showed anisotropies and inhomogeneities that were related to the pattern of orientation columns and indicated distortions in the representation of visual space on the cortical surface. In the reorganized cortex the spike PS expanded, approximating the extent of the optical PS seen in normal cortex, and suggesting that reorganization was mediated by an unmasking of normally subthreshold activation to suprathreshold levels. The orientation map of the reorganized cortex showed a close match to that obtained before placing the lesion, despite the large shift in topography, supporting the idea that intrinsic horizontal connections were responsible for the remapping.

Action Potentials↗
PubMed1995

Interocular transfer of receptive field expansion in cat visual cortex.

Receptive fields in primary visual cortex have been shown to be capable of rapid expansion and contraction when exposed to an artificial scotoma, a masked segment of the visual field. To distinguish cortical from thalamic contributions to receptive field mutability, we tested interocular transfer of the effect in binocular cortical receptive fields, presenting the conditioning stimulus to the field in one eye and measuring size changes in the receptive field of the other eye. The expansion of the receptive fields in the non-conditioned eye was comparable to that in the conditioned eye. This result suggests that the expansion is due to mechanisms intrinsic to the cortex.

Animals↗
PubMed1995

Improvement in visual sensitivity by changes in local context: parallel studies in human observers and in V1 of alert monkeys.

To explore the role of primary visual cortex in contour integration, we measured the contextual sensitivity of human contrast thresholds and of superficial layer complex cells in monkey V1. An observer's contrast detection was 40% improved by a second suprathreshold bar; the effect was decreased as the two bars were separated along their axis of orientation, were displaced from colinearity, and had their relative orientation changed. Recordings from V1 showed that 42% of complex cells demonstrated facilitation for a second bar outside their classical receptive fields with a similar dependency on relative location and orientation. Both effects were eliminated by an orthogonal line between the two iso-oriented lines. Multiple randomly placed and oriented lines in the receptive field surround often caused a reduction in a cell's response to an optimally oriented stimulus, but this inhibition could be eliminated by changing the orientation of a few of these elements to colinearity with the centrally located target.

Animals↗
PubMed1995

Receptive field expansion in adult visual cortex is linked to dynamic changes in strength of cortical connections.

1. Receptive field (RF) sizes of neurons in adult primary visual cortex are dynamic, expanding and contracting in response to alternate stimulation outside and within the RF over periods ranging from seconds to minutes. The substrate for this dynamic expansion was shown to lie in cortex, as opposed to subcortical parts of the visual pathway. The present study was designed to examine changes in cortical connection strengths that could underlie this observed plasticity by measuring the changes in cross-correlation histograms between pairs of primary visual cortex neurons that are induced to dynamically change their RF sizes. 2. Visually driven neural activity was recorded from single units in the superficial layers of primary visual cortex in adult cats, with two independent electrodes separated by 0.1-5 mm at their tips, and cross-correlated on-line. The neurons were then conditioned by stimulation with an "artificial scotoma," a field of flashing random dots filling the region of visual space around a blank rectangle enclosing the RFs of the recorded neurons. The neuronal RFs were tested for expansion and their visually driven output again cross-correlated. After this, the neurons were stimulated vigorously through their RF centers to induce the field to collapse, and the visually driven output from the collapsed RFs was again cross-correlated. Cross-correlograms obtained before and after conditioning, and after RF collapse, were normalized by their flanks to control for changes in peak size due solely to fluctuations in spike rate. 3. A total of 37 pairs of neurons that showed distinct cross-correlogram peaks, and whose RF borders were clearly discernible both before and after conditioning, were used in the final analysis. Of these neuron pairs, conditioning led to a clear expansion of RF boundaries in 28 pairs, whereas in 9 pairs the RFs did not expand. RFs that did expand showed no significant shifts in their orientation preference, orientation selectivity, or ocularity. 4. When the RFs of a pair of neurons expanded with conditioning, the area of the associated flank-normalized cross-correlogram peaks also increased (by a factor ranging from 0.84 up to 3.5). Correlograms returned to their preconditioning values when RFs collapsed.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1995

Topographic reorganization in the striate cortex of the adult cat and monkey is cortically mediated.

In primary sensory and motor cortex of adult animals, alteration of input from the periphery leads to changes in cortical topography. These changes can be attributed to processes that are intrinsic to the cortex, or can be inherited from alterations occurring at stages of sensory processing that are antecedent to the primary sensory cortical areas. In the visual system, focal binocular retinal lesions initially silence an area of cortex that represents the region of retina destroyed, but over a period of months this area recovers visually driven activity. The retinotopic map in the recovered area is altered, shifting its representation to the portion of retina immediately surrounding the lesion. This effectively shrinks the representation of the lesioned area of retina, and expands the representation of the lesion surround. To determine the loci along the visual pathway at which the reorganization takes place, we compared the course of topographic alterations in the primary visual cortex and dorsal lateral geniculate nucleus (LGN) of cats and monkeys. At a time when the cortical reorganization is complete, the silent area of LGN persists, indicating that changes in cortical topography are due to alterations that are intrinsic to the cortex. To explore the participation of thalamocortical afferents in the reorganization, we injected a series of retrogradely transported fluorescent tracers into reorganized and surrounding cortex of each animal. Our results show that the thalamocortical arbors do not extend beyond their normal lateral territory and that this physical dimension is insufficient to account for the reorganization. We suggest that the long-range intrinsic horizontal connections are a likely source of visual input into the reorganized cortical area.

Animals↗
PubMed1994

Learning. Neuronal dynamics and perceptual learning.

Perceptual learning is accompanied by changes in the properties of individual neurons and in the functional cortical architecture; these are observed in a number of cortical areas, over short and long time scales.

Adult↗
PubMed1994

Axonal sprouting accompanies functional reorganization in adult cat striate cortex.

Removal of sensory input from a focal region of adult neocortex can lead to a large reorganization of cortical topography within the deprived area during subsequent months. Although this form of functional recovery is now well documented across several sensory systems, the underlying cellular mechanisms remain elusive. Weeks after binocular retinal lesions silence a corresponding portion of striate cortex in the adult cat, this cortex again becomes responsive, this time to retinal loci immediately outside the scotoma. Earlier findings showed a lack of reorganization in the lateral geniculate nucleus and an inadequate spread of geniculocortical afferents to account for the cortical reorganization, suggesting the involvement of intrinsic cortical connections. We investigated the possibility that intracortical axonal sprouting mediates long-term reorganization of cortical functional architecture. The anterograde label biocytin was used to compare the density of lateral projections into reorganized and non-deprived cortex. We report here that structural changes in the form of axonal sprouting of long-range laterally projecting neurons accompany topographic remodelling of the visual cortex.

Animals↗
PubMed1994

Circuitry, architecture and functional dynamics of visual cortex.

A fundamental understanding of the mechanisms of cortical processing requires an examination of the relationships of cortical circuitry, cortical functional architecture and receptive field properties. Ultimately, this kind of analysis can be used to explore the neurobiological basis of psychophysics and perception. At the outset our studies were intended to account for the then known receptive field properties of cortical cells in terms of their underlying circuitry but, surprisingly, a good part of the cortical circuit appeared to be in violation of the principles of cortical architecture. This led us to explore the possibility of new, more complex properties of cortical cells. It has become increasingly possible to relate the responsive specificity of cortical cells and the circuitry underlying this specificity to the perceptual capabilities of the visual system by performing analogous experiments on single cells and in human psychophysics.

Animals↗
PubMed1994

A quantitative measure for short-term cortical plasticity in human vision.

The human visual system is normally very good at determining the relative positions of objects in space, but under certain conditions contextual influences can cause significant errors in this process. We studied spatial localization around an artificial scotoma, a small mask that occludes part of the visual field while a dynamic pattern is shown over a surrounding region, and found that the ability to determine the position of short line segments was strongly biased toward the interior of the scotoma. We attribute this "shift" or misassignment of position to receptive field (RF) expansions within the artificial scotoma as seen in recent physiological studies. Furthermore, our findings show that this shift begins within 1 sec of stimulus presentation, suggesting that RFs are constantly altered by their local context and that these dynamics are a part of normal vision.

Humans↗
PubMed1993

Rapid dynamic changes in adult cerebral cortex.

While the plasticity of cortical connections has long been known to be a property of developing cortex, a new view of cortical function reveals a substantial degree of mutability of receptive field properties and cortical topography. The most recent studies show that the dynamic properties of cortex can be observed over a time scale of minutes.

Animals↗
PubMed1993

Long-term changes in synaptic strength along specific intrinsic pathways in the cat visual cortex.

1. The dense system of horizontal connections that arise and course within the striate cortex are thought to inform single cells about stimuli arising in disparate points in visual space and to modulate responses evoked from within the receptive field. To learn whether or not the strength of the horizontal connections could vary over the long term, and if such changes could affect the integration of vertical, interlaminar inputs, we have recorded intracellularly from the superficial layers in slices of the adult cat's visual cortex. 2. The monosynaptic EPSP evoked by stimulating horizontal fibres showed long-term facilitation in twelve of the twenty cells that were conditioned by repetitively pairing synaptic responses with depolarizing pulses of current; the maximum increase observed was 200%. Strong inhibition present in the postsynaptic response usually indicated that facilitation would not occur. 3. In instances where horizontal input evoked both mono- and polysynaptic EPSPs, both early and late events showed facilitation, with the most dramatic enhancement contributed by the polysynaptic components. 4. For the twenty-eight cells whose responses to stimulation of interlaminar as well as horizontal pathways were assessed, all were found to receive non-overlapping inputs from each source. Conditioning produced long-term changes in the strength of the interlaminar inputs. 5. Changes in synaptic strength were usually confined to the conditioned pathway, though in four out of twenty-six times we observed heterosynaptic facilitation of polysynaptic EPSPs. 6. The conditioning protocol led to lasting depression rather than facilitation in three out of eleven instances; the reduction was only observed in the multisynaptic components. 7. We suggest that the synaptic changes observed here may be related to certain dynamic changes in receptive field properties that have been characterized in vivo.

Animals↗
PubMed1992

Dynamic changes in receptive-field size in cat primary visual cortex.

Immediately after focal retinal lesions, receptive fields (RFs) in primary visual cortex expand considerably, even when the retinal damage is limited to the photoreceptor layer. The time course of these changes suggests that mere lack of stimulation in the vicinity of the RF accompanied by stimulation in the surrounding region causes the RF expansion. While recording from single cells in cat area 17, we simulated this pattern of stimulation with a pattern of moving lines in the visual field, masking out an area covering the RF of the recorded cell, thereby producing an "artificial scotoma." Over approximately 10 min this masking resulted in a 5-fold average expansion in RF area. Stimulating the RF center caused the field to collapse in size, returning to near its original extent; reconditioning with the masked stimulus led to RF reexpansion. Stimulation in the surrounding region was required for the RF expansion to occur--little expansion was seen during exposure to a blank screen. We propose that the expansion may account for visual illusions, such as perceptual fill-in of stabilized images and illusory contours and may constitute the prodrome of altered cortical topography after retinal lesions. These findings support the idea that even in adult animals RFs are dynamic, capable of being altered by the sensory context.

Animals↗