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PubMed2001

Lightness constancy in primary visual cortex.

When the illumination of a visual scene changes, the quantity of light reflected from objects is altered. Despite this, the perceived lightness of the objects generally remains constant. This perceptual lightness constancy is thought to be important behaviorally for object recognition. Here we show that interactions from outside the classical receptive fields of neurons in primary visual cortex modulate neural responses in a way that makes them immune to changes in illumination, as is perception. This finding is consistent with the hypothesis that the responses of neurons in primary visual cortex carry information about surface lightness in addition to information about form. It also suggests that lightness constancy, which is sometimes thought to involve "higher-level" processes, is manifest at the first stage of visual cortical processing.

Animals↗
PubMed2002

Changes in visual cortex excitability in blind subjects as demonstrated by transcranial magnetic stimulation.

Any attempt to restore visual functions in blind subjects with pregeniculate lesions provokes the question of the extent to which deafferented visual cortex is still able to generate conscious visual experience. As a simple approach to assessing activation of the visual cortex, subjects can be asked to report conscious subjective light sensations (phosphenes) elicited by focal transcranial magnetic stimulation (TMS) over the occiput. We hypothesized that such induction of phosphenes can be used as an indicator of residual function of the visual cortex and studied 35 registered blind subjects after partial or complete long-term (>10 years) deafferentation of the visual cortex due to pregeniculate lesions. TMS was applied over the visual cortex in 10 blind subjects with some residual vision (visual acuity <20/400; Group 1), 15 blind subjects with very poor residual vision (only perception of movement or light; Group 2), 10 blind subjects without any residual vision (Group 3) and 10 healthy controls. A stimulation mapping procedure was performed on a 1 x 1 cm skull surface grid with 130 stimulation points overlying the occipital skull. We analysed the occurrence of phosphenes at each stimulation point with regard to frequency and location of phosphenes in the visual field. Previous experiments have shown that repetitive TMS reliably elicits brief flashes of white or coloured patches of light. Therefore, stimulation was performed with short trains of seven consecutive 15 Hz stimuli applied with an intensity of 1.3 times the motor threshold. Under such conditions, phosphenes occurred in 100% of subjects in Group 1, in 60% of Group 2 and in 20% of Group 3. Phosphene thresholds were normal, but the number of effective stimulation sites was significantly reduced in Groups 2 and 3. The results indicate that in blind subjects there is alteration in TMS-induced activation of the deafferented visual cortex or processes engaged in bringing the artificial cortex input to consciousness. The ability to elicit phosphenes is reduced in subjects with a high degree of visual deafferentation, especially in those without previous visual experience.

Adolescent↗
PubMed1998

Integration of surface information in primary visual cortex.

Ample evidence suggests that primary visual cortex is involved in the perception of form, and there is increasing evidence that it may also be important in the perception of surfaces. Perceptual qualities of surfaces, such as brightness, are based on extensive integration of information throughout the visual field. In primary visual cortex, we found that the responses of neurons to surfaces were also influenced by the intensity and organization of light in large portions of the visual field. Interactions with surrounding stimuli typically extended 10 to 20 degrees beyond a cell's receptive field the same spatial scale as perceptual interactions. Moreover, there were both facilitatory and inhibitory influences, just as there are additive and substractive perceptual interactions. Surprisingly, influences from outside the receptive field obtained with surface stimuli did not reliably correlate with influences recorded with gratings. These properties suggest that the underlying neuronal interactions may serve as the fundamental building blocks of surface perception.

Animals↗
PubMed1980

Retinotopic organization of striate and extrastriate visual cortex in the mouse.

Detailed retinotopic maps of primary visual cortex (area 17) and the extrastriate visual regions surrounding it (areas 18a and 18b) have been constructed for the C57BL/6J mouse using standard electrophysiological mapping techniques. Primary visual cortex (area 17), as defined cytoarchitectonically, contains one complete representation of the contralateral visual field, termed V1, in which azimuth and elevation lines are approximately orthogonal. The upper visual field is represented caudally and the nasal field laterally. Binocular cells are encountered in the cortical representation of the nasal 30--40 degrees of the visual field, and there is an expanded representation of the nasal field. Extrastriate visual cortex of the mouse, like that of other mammals, contains multiple representations of the visual field. The cytoarchitectonic region of cortex lateral and rostral to area 17, termed area 18a, contains at least two such representations. The more medial of these, which by convention we have called V2, is a narrow strip surrounding V1 on its lateral and rostral aspects; the vertical meridian lies along a portion of its common border with V1. The visual field representation in V2 is not a mirror image of that in V1; the representation of the horizontal meridian forms the lateral border of V2, and the visual field representation is split so that adjacent points on either side of the horizontal meridian are represented in nonadjacent parts of V2. The other visual field representation within area 18a, which we have termed V3, is a small but apparently complete representation that lies lateral to V2. The visual field representations medial to area 17 correspond to cytoarchitectonic area 18b. Area 18b contains two representations of the temporal visual field that we have labeled Vm-r and Vm-c, and contains little or no representation of the most nasal aspect of the field.

Animals↗
PubMed1998

Representation of spatial frequency and orientation in the visual cortex.

Knowledge of the response of the primary visual cortex to the various spatial frequencies and orientations in the visual scene should help us understand the principles by which the brain recognizes patterns. Current information about the cortical layout of spatial frequency response is still incomplete because of difficulties in recording and interpreting adequate data. Here, we report results from a study of the cat primary visual cortex in which we employed a new image-analysis method that allows improved separation of signal from noise and that we used to examine the neurooptical response of the primary visual cortex to drifting sine gratings over a range of orientations and spatial frequencies. We found that (i) the optical responses to all orientations and spatial frequencies were well approximated by weighted sums of only two pairs of basis pictures, one pair for orientation and a different pair for spatial frequency; (ii) the weightings of the two pictures in each pair were approximately in quadrature (1/4 cycle apart); and (iii) our spatial frequency data revealed a cortical map that continuously assigns different optimal spatial frequency responses to different cortical locations over the entire spatial frequency range.

Animals↗
PubMed2003

Visual cortex is rescued from the effects of dark rearing by overexpression of BDNF.

Visual deprivation such as dark rearing (DR) prolongs the critical period for ocular dominance plasticity and retards the maturation of gamma-aminobutyric acid (GABA)ergic inhibition in visual cortex. The molecular signals that mediate the effects of DR on the development of visual cortex are not well defined. To test the role of brain-derived neurotrophic factor (BDNF), we examined the effects of DR in transgenic mice in which BDNF expression in visual cortex was uncoupled from visual experience and remained elevated during DR. In dark-reared transgenic mice, visual acuity, receptive field size of visual cortical neurons, critical period for ocular dominance plasticity, and intracortical inhibition were indistinguishable from those observed in light-reared mice. Therefore, BDNF overexpression is sufficient for the development of aspects of visual cortex in the absence of visual experience. These results suggest that reduced BDNF expression contributes to retarded maturation of GABAergic inhibition and delayed development of visual cortex during visual deprivation.

Animals↗
PubMed1996

Feasibility of a visual prosthesis for the blind based on intracortical microstimulation of the visual cortex.

The feasibility of producing a visual prosthesis for the blind using intracortical microstimulation (ICMS) of the visual cortex was studied in a 42-year-old woman who had been totally blind for 22 years secondary to glaucoma. Thirty-eight microelectrodes were implanted in the right visual cortex, near the occipital pole, for a period of 4 months. Percepts reported as small spots of light, called phosphenes, were produced with 34 of the 38 implanted microelectrodes. Threshold currents for phosphene generation with trains of biphasic pulses were as low as 1.9 microA, and most of the microelectrodes had thresholds below 25 microA. Phosphene brightness could be modified with stimulus amplitude, frequency and pulse duration. Repeated stimulation over a period of minutes produced a gradual decrease in phosphene brightness. Phosphenes did not flicker. The apparent size of phosphenes ranged from a "pin-point' to a "nickel' (20 mm diameter coin) held at arm's length. Phosphene size usually decreased as stimulation current was increased but increased slightly as the train length (TL) was increased. At levels of stimulation near threshold, the phosphenes were often reported to have colours. As the stimulation level was increased, the phosphenes generally became white, greyish or yellowish. Individual phosphenes appeared at different distances from the subject. When two phosphenes were simultaneously generated, the apparent distances of the individual phosphenes sometimes changed to make them appear to be at about the same distance. When three or more phosphenes were simultaneously generated, they became coplanar. Except for rare occasions, phosphenes extinguished rapidly at the termination of the stimulation train. When stimulation TLs were increased beyond 1 s, phosphenes usually disappeared before the end of the train. The duration of phosphene perception could be increased by interrupting a long stimulation train with brief pauses in stimulation. Intracortical microelectrodes spaced 500 microns apart generated separate phosphenes, but microelectrodes spaced 250 microns typically did not. This two-point resolution was about five times closer than has typically been achieved with surface stimulation. With some individual microelectrodes, a second closely spaced phosphene was sometimes produced by increasing the stimulation current. Phosphenes moved with eye movements. When up to six phosphenes were simultaneously elicited, they all moved with the same relative orientation during eye movements. All phosphenes were located in the left hemi-field with the majority above the horizontal meridian. There was a clustering of most of the phosphenes within a relatively small area of visual space. The potentially greater microelectrode density and lower power requirements of ICMS compared with surface stimulation appears encouraging for a visual prosthesis. However, further studies with blind subjects are required to optimize stimulation parameters and test complex image recognition before the feasibility of a visual prosthesis based on ICMS can be established.

Adult↗
PubMed2001

Effect of the group I metabotropic glutamate agonist DHPG on the visual cortex.

Metabotropic glutamate receptors have a variety of effects in visual cortex that depend on the age of the animal, the layer of the cortex, and the group of the receptor. Here we describe these effects for group I receptors, using both in vivo and in vitro preparations. The metabotropic group I glutamate receptor agonist 3,5 dihydroxyphenylglycine (DHPG) potentiates the responses to N-methyl-D-aspartate (NMDA) and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) in slices of rat visual cortex. It also increases, initially, the visual response in the cat visual cortex. Both these effects are largest at 3-4 wk of age and decline to insignificance by 10 wk of age. Both are also largest in lower layers of cortex, which explains why the facilitatory effects found with the general metabotropic glutamate agonist 1S,3R aminocyclopentane-1,3-dicarboxylic acid (ACPD) are observed only in lower layers. Prolonged application of DHPG in the cat visual cortex, after the initial excitatory effect, produces depression. We also found that DHPG facilitates the NMDA response in fast-spiking cells, which are inhibitory, providing a partial explanation for this. Thus there are multiple effects of group I metabotropic glutamate receptors, which vary with layer and age in visual cortex.

Action Potentials↗
PubMed2003

Increased regional cerebral blood flow but normal distribution of GABAA receptor in the visual cortex of subjects with early-onset blindness.

Before the completion of visual development, visual deprivation impairs synaptic elimination in the visual cortex. The purpose of this study was to determine whether the distribution of central benzodiazepine receptor (BZR) is also altered in the visual cortex in subjects with early-onset blindness. Positron emission tomography was carried out with [(15)O]water and [(11)C]flumazenil on six blind subjects and seven sighted controls at rest. We found that the CBF was significantly higher in the visual cortex for the early-onset blind subjects than for the sighted control subjects. However, there was no significant difference in the BZR distribution in the visual cortex for the subject with early-onset blindness than for the sighted control subjects. These results demonstrated that early visual deprivation does not affect the distribution of GABA(A) receptors in the visual cortex with the sensitivity of our measurements. Synaptic elimination may be independent of visual experience in the GABAergic system of the human visual cortex during visual development.

Adult↗
PubMed1977

[Types of receptive fields of neurons in different laminae of the rabbit visual cortex].

Neurons of the primary rabbit visual cortex were classified into 7 large groups according to features of their receptive fields. The neurons with receptive fields were mostly revealed in the layers IV and VI, those with uniform directional receptive fields--in layer V, those with simple I-in layer VI, those with simple II--in layer II + III, and those with hypercomplex--in layer IV. The neurons with concentric receptive fields and those without responses to visual stimuli were equally distributed over the layers. The data obtained in rabbits, cats, and monkeys suggest that the tendency towards function stratification of primary visual cortex is successive in an evolutionary order of mammals.

Animals↗
PubMed2003

Emergent properties of layer 2/3 neurons reflect the collinear arrangement of horizontal connections in tree shrew visual cortex.

The superficial layers of primary visual cortex, unlike layer 4, have an extensive network of long-range horizontal connections linking sites of similar orientation preference. To identify possible functional consequences of this distinct anatomy, we compared the receptive field properties of layers 2/3 and 4 neurons in tree shrew primary visual cortex with electrophysiological recordings. We found that elongated receptive fields, strong orientation tuning, and length summation (properties predicted by the anatomy of the horizontal connections) are present in layer 2/3 neurons, but not in layer 4 neurons. We further characterized the summation fields of layer 2/3 neurons and found axis and orientation-specific facilitation that matched the distribution of horizontal connections. The functional signature of horizontal connections was also evident in the population response of layer 2/3 neurons; the intrinsic signal activation pattern elicited by an array of collinear Gabor elements was significantly stronger than that elicited by a noncollinear array. Furthermore, our results showed that this enhancement of population response was achieved without compromising spatial resolution along the collinear axis, providing stimulus-specific facilitation without filling in between stimuli. Taken together, these results suggest that horizontal connections play a significant role in shaping the visual responses of layer 2/3 neurons.

Animals↗
PubMed2002

Diffuse light increases metabolic activity in the lateral geniculate nucleus, visual cortex, and superior colliculus of the cone-dominated ground squirrel visual system.

Ground squirrels were monocularly exposed to either steady- or flashing-diffuse light for 45 min following an injection of 14C 2-deoxyglucose (2-DG). Autoradiographic analysis indicated greater metabolic activity in the lateral geniculate nucleus, visual cortex and superior colliculus (SC) of the hemisphere lying contralateral to and receiving input from the diffusely stimulated eye (covered by a white mask), than in the corresponding regions of the other hemisphere receiving input from the occluded eye (black mask). The diffuse light results for the cortex and colliculus of the diurnal ground squirrel are different from those for the nocturnal rat. In the rat visual cortex, there is no difference between metabolic activity under conditions of diffuse light (steady or flashing) and under darkness. In the rat SC, although flashing-diffuse light increases metabolic activity (as is the case for the squirrel), steady-diffuse light decreases it to a level below that which occurs in darkness. The cortex and colliculus differences in 2-DG response to diffuse light between the ground squirrel and rat were attributed to differences in the operations of their respective cone- and rod-dominated visual systems.

Animals↗
PubMed1998

Functional evaluation using magnetic resonance imaging of the visual cortex in patients with retrochiasmatic lesions.

OBJECT: The goal of this study was to evaluate the clinical potential of combining functional magnetic resonance (fMR) imaging with conventional morphological MR imaging and to assess its usefulness for objective evaluation of visual function as part of treatment planning in patients harboring space-occupying lesions involving the posterior afferent visual system. METHODS: It was hypothesized that regional activation of the visual cortex during visual stimulation would show an asymmetric response consistent with the well-known retinotopical organization of the human visual cortex. To test this hypothesis, the pattern of regional cortical activity detected by fMR imaging during binocular repetitive photic stimulation (10 Hz) was compared with the findings of conventional visual field testing. Functional mapping of the visual cortex was performed using a noninvasive blood oxygen level-dependent MR technique in 10 patients with intraaxial and two with extraaxial lesions. Experiments involving two of the patients were unsuccessful because of motion artifacts. In all the remaining patients functional activity was demonstrated in the primary visual area that corresponded to the anatomical location of the calcarine cortex. In nine patients, the identified patterns of activation in the visual cortex were consistent with the visual field deficits (seven homonymous hemianopsias, one homonymous central scotoma, and one inferior quadrantanopsia) and with the traditional teaching of retinotopical representation. Discordance between fMR imaging and perimetric findings was observed in one case. CONCLUSIONS: These results demonstrate that fMR imaging can be performed routinely and successfully in patients with visual abnormalities as part of a conventional neuroradiological evaluation. The technique provides essential information about the function-structure relationship specific to an individual patient and holds promise not only for diagnosis and therapy planning, but also for understanding the topography and functional specialization of the human visual cortex.

Adult↗
PubMed1983

Single unit studies in the visual cortex of rodents.

This paper deals with the visual topography in the rat and hamster visual cortex, and with receptive field maps and trigger features of visual units in rat visual cortex. The visuotopic organization of the primary visual cortex (area 17) and the extrastriate visual regions surrounding it (areas 18a and 18) was studied in gray rats and hamsters using standard microelectrode techniques. The results confirm and extend previous observations in the rat and reveal in hamster an organization more complex than previously reported. In the rat, apart from the representation of the contralateral visual field (VF) in area 17, in which the upper VF is represented caudally and the nasal VF laterally, there are additional representations of the VF in the extrastriate cortex. In lateral extrastriate cortex (area 18a) there are at least four such representations, namely latero-medial (LM), antero lateral (AL), latero intermediate (LI) and latero-lateral (LL). In LM (second visual area) the upper VF is represented caudally at the nasal FV, medially, being thus a mirror image of V1. In AL (third visual area) the upper VF is represented rostrally and the nasal VF, medially, being thus a mirror image of LM. In LI the upper VF is medial, and the nasal VF, lateral, being thus a mirror image of LM, or a reduced copy of V1. In LL, the upper VF is caudal and the nasal VF, medial, being thus a mirror image of LI, or a reduced copy of LM. In medial extrastriate cortex (area 18) there are two representations of the temporal VF, labelled antero-medial (AM) and postero medial (PM). In AM, the upper temporal VF is medial and the lower temporal VF, lateral, the extreme temporal field being rostral. The 30 degree azimuth provides the boundary between AM and PM. Thus, AM is organized as a counter clock wise rotation by 90 degrees of the V1 representation. In PM, the upper lower VF topography is like in AM, but the extreme temporal VF is caudal, being thus a mirror image of AM. Our preliminary study of visual topography is hamster shows an organization at least as elaborate as that of rat. Finally, we have compared RFs of striate vs extrastriate units. Our extrastriate units showed similar trigger features and field maps as in V1, but with larger RF sizes and a proportion of rapidly habituating cells. The percent distribution of field types, at least in LM and AM was similar to that in V1.

Animals↗
PubMed1989

A hexagonal orthogonal-oriented pyramid as a model of image representation in visual cortex.

Retinal ganglion cells represent the visual image with a spatial code, in which each cell conveys information about a small region in the image. In contrast, cells of primary visual cortex employ a hybrid space-frequency code in which each cell conveys information about a region that is local in space, spatial frequency, and orientation. Despite the presumable importance of this transformation, we lack any comprehensive notion of how it occurs. Here we describe a mathematical model for this transformation. The hexagonal orthogonal-oriented quadrature pyramid (HOP) transform, which operates on a hexagonal input lattice, employs basis functions that are orthogonal, self-similar, and localized in space, spatial frequency, orientation, and phase. The basis functions, which are generated from seven basic types through a recursive process, form an image code of the pyramid type. The seven basis functions, six bandpass and one low-pass, occupy a point and a hexagon of six nearest neighbors on a hexagonal sample lattice. The six bandpass basis functions consist of three with even symmetry, and three with odd symmetry. The three even kernels are rotations of 0, 60, and 120 degrees of a common kernel; likewise for the three odd kernels. At the lowest level, the inputs are image samples. At each higher level, the input lattice is provided by the low-pass coefficients computed at the previous level. At each level, the output is subsampled in such a way as to yield a new hexagonal lattice with a spacing square root 7 larger than the previous level, so that the number of coefficients is reduced by a factor of seven at each level. In the biological model, the input lattice is the retinal ganglion cell array. The resulting scheme provides a compact, efficient code of the image and generates receptive fields that resemble those of the primary visual cortex.

Computer Simulation↗
PubMed1982

Plasticity in the kitten's visual cortex: effects of the suppression of visual experience upon the orientational properties of visual cortical cells.

The orientation selectivity of visual cortical cells was tested in two groups of kittens. In one group the animals were reared normally for the first 4-6 weeks of life then kept in darkness. Those in the other group were dark-reared for the first 6 weeks then exposed to light for 6 h and returned to the dark. The properties of the receptive fields of visual cortical cells were examined in these kittens after periods of dark-rearing ranging from 3 days to 12 weeks. In both groups, the proportion of orientation selective cells was found to decrease with time spent in the dark. The critical period for orientation appeared to end at 10-12 weeks of age. Two populations of visual cells were distinguished functionally by their different behaviour during prolonged dark-rearing. Most of the cells which retained their orientation specificity longest during dark-rearing were tuned to horizontal or vertical orientations and more of them were monocular than in normal kittens. These functional characteristics resemble those exhibited by neurons of very young kittens. Changes in specificity observed during loss of selectivity are compared to those observed during early development. We suggest that the extent to which the orientation selectivity of a cell is plastic depends very largely upon the time, during the course of development, at which its selectivity was acquired.

Animals↗