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Results for “Primary Visual Cortex”

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At least 271 records · Page 15Linked to original sources
PubMed1998

Serotonin inhibits the induction of long-term potentiation in rat primary visual cortex.

1. The effect of serotonin (5-hydroxytryptamine; 5-HT) on the induction of long-term potentiation (LTP) in rat visual cortex was investigated by using slice preparations in vitro. 2. Bath application of 5-HT (0.1-10 microM) did not affect the baseline synaptic potentials evoked by single-pulse test stimulation, but inhibited the induction of LTP in a concentration-dependent manner. 3. The effect of 5-HT was blocked by the 5-HT1 receptor antagonist pindolol or the 5-HT2,7 receptor antagonist ritanserin, but not by the 5-HT3,4 receptor antagonist MDL72222. 4. These results suggest that 5-HT plays a role in suppressing the induction of LTP in the rat visual cortex.

Animals↗
PubMed1996

Deficient plasticity in the primary visual cortex of alpha-calcium/calmodulin-dependent protein kinase II mutant mice.

The recent characterization of plasticity in the mouse visual cortex permits the use of mutant mice to investigate the cellular mechanisms underlying activity-dependent development. As calcium-dependent signaling pathways have been implicated in neuronal plasticity, we examined visual cortical plasticity in mice lacking the alpha-isoform of calcium/calmodulin-dependent protein kinase II (alpha CaMKII). In wild-type mice, brief occlusion of vision in one eye during a critical period reduces responses in the visual cortex. In half of the alpha CaMKII-deficient mice, visual cortical responses developed normally, but visual cortical plasticity was greatly diminished. After intensive training, spatial learning in the Morris water maze was severely impaired in a similar fraction of mutant animals. These data indicate that loss of alpha CaMKII results in a severe but variable defect in neuronal plasticity.

Animals↗
PubMed1999

Visual attention: spotlight on the primary visual cortex.

Visual search tasks appear to involve spatially selective attention to the target, but evidence for attentional modulation in the visual area with the most precise retinotopic organization V1 has been elusive. Recent imaging studies show that spatial attention can indeed enhance visual responses in human V1.

Animals↗
PubMedDate not supplied

Heterogeneity in local distributions of orientation-selective neurons in the cat primary visual cortex.

We have employed the tetrode technique, which allows accurate discrimination of individual neuronal spike trains from multiunit recordings, in order to examine the variation of orientation selectivity among local groups of neurons. We recorded a total of 321 cells from 62 sites in area 17 of halothane-anesthetized cats; each site contained between three to ten neurons that were estimated to be less than 65 microns away from the tetrode tip. For each cell, we determined the orientation tuning in response to moving bars. Of the cells tested, 8.4% were unresponsive, 22.7% had no preferential response to any particular orientation, while 68.8% were tuned. The average difference in preferred orientation between cell pairs recorded at the same site was 10.7 deg, but the variance in preferred orientation differences differed significantly among sites. Some clusters of cells exhibited the same or nearly the same orientation preference, while others had orientation preferences that differed by as much as 90 deg. Our data demonstrate that the tuning for orientation is more heterogeneously distributed at a local level than previous studies have suggested.

Action Potentials↗
PubMedDate not supplied

Activity correlates of cytochrome oxidase-defined compartments in granular and supragranular layers of primary visual cortex of the macaque monkey.

To determine if changes in metabolic capacity revealed by cytochrome oxidase (CO) histochemistry are related to sustained changes in energy-utilizing neuronal activity, we assayed CO levels and recorded multiunit firing rates along nearly tangential penetrations of V1 in seven adult macaque monkeys before and after single, monocular injections of TTX. Within as little as 14 h, TTX blockade began to reduce CO staining in zones of layer 4C that received dominant input from the injected eye. Since simple monocular occlusion has only minor effects on cortical CO levels (Trusk et al., 1990), the changes in activity that were specifically associated with CO depletion were isolated by comparing spike rates during monocular TTX blockade and during monocular occlusion. Five second samples of multiunit spike rate were obtained after 2-min adaptation to each of four adapting fields: black, gray, white, and textured. Results were similar for these four conditions. In layer 4C, ocular dominance zones with input from the TTX eye had ongoing spike rates that were 48% of the rates in zones with input from a normal but occluded eye. In six animals, it was possible to record activity at a single site before, during, and after the onset of TTX blockade. Background activity at these interpuff sites decreased as much as 3-fold in less than 1 h but stabilized within 3-4 h to an average of 53% of pre-TTX rates. These data support the interpretation that energy utilization linked to sustained spike rates partially regulates CO levels under normal conditions, at least in layer 4. Furthermore, changes in neuronal activity induced by retinal TTX preceded the detectable reduction in CO activity in V1 suggesting that the adjustment of CO levels was in response to the altered activity.

Action Potentials↗
PubMed2000

Inhibitory threshold for critical-period activation in primary visual cortex.

Neuronal circuits across several systems display remarkable plasticity to sensory input during postnatal development. Experience-dependent refinements are often restricted to well-defined critical periods in early life, but how these are established remains mostly unknown. A representative example is the loss of responsiveness in neocortex to an eye deprived of vision. Here we show that the potential for plasticity is retained throughout life until an inhibitory threshold is attained. In mice of all ages lacking an isoform of GABA (gamma-aminobutyric acid) synthetic enzyme (GAD65), as well as in immature wild-type animals before the onset of their natural critical period, benzodiazepines selectively reduced a prolonged discharge phenotype to unmask plasticity. Enhancing GABA-mediated transmission early in life rendered mutant animals insensitive to monocular deprivation as adults, similar to normal wild-type mice. Short-term presynaptic dynamics reflected a synaptic reorganization in GAD65 knockout mice after chronic diazepam treatment. A threshold level of inhibition within the visual cortex may thus trigger, once in life, an experience-dependent critical period for circuit consolidation, which may otherwise lie dormant.

Aging↗
PubMed1992

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.

Afferent Pathways↗
PubMed1993

Preferential representation of the fovea in the primary visual cortex.

The retinal fovea, which corresponds to the central degree or so of vision, is spatially over-represented in the visual cortex. It is about 0.01% of retina area, but at least 8% of the striate cortex. Does this simply reflect an equivalently uneven distribution of ganglion cells in the retina, or is the cortical representation of the fovea preferentially expanded? The answer hinges on the resolution of long-standing discrepancies between the retinal and cortical magnification factors. We approached the problem in a different way, using a retrograde transneuronal tracer from cortex to retina to relate directly the number of ganglion cells projecting to marked areas of striate cortex. We report here that ganglion cells near the fovea were allocated 3.3 to 5.9 times more cortical tissue than more peripheral ones, and conclude that the cortical representation of the most central retina is much greater than expected from the density of its ganglion cells.

Animals↗
PubMed2000

Activity in primary visual cortex predicts performance in a visual detection task.

Visual attention can affect both neural activity and behavior in humans. To quantify possible links between the two, we measured activity in early visual cortex (V1, V2 and V3) during a challenging pattern-detection task. Activity was dominated by a large response that was independent of the presence or absence of the stimulus pattern. The measured activity quantitatively predicted the subject's pattern-detection performance: when activity was greater, the subject was more likely to correctly discern the presence or absence of the pattern. This stimulus-independent activity had several characteristics of visual attention, suggesting that attentional mechanisms modulate activity in early visual cortex, and that this attention-related activity strongly influences performance.

Action Potentials↗
PubMed1993

Activation of human primary visual cortex during visual recall: a magnetic resonance imaging study.

The degree to which the process involved in visual perception and visual imagery share a common neuroanatomical substrate is unclear. Physiological evidence for localization of visual imagery early in the visual pathways would have important bearing on current theories of visual processing. A magnetic resonance imaging technique sensitive to regional changes in blood oxygenation was used to obtain functional activation maps in the human visual cortex. During recall of a visual stimulus, focal increases in signal related to changes in blood flow were detected in V1 and V2 cortex in five of seven subjects. These experiments show that the same areas of the early visual cortex that are excited by visual stimulation are also activated during mental representation of the same stimulus. Some of the processes used in topographically mapped cortical areas during visual perception may also be utilized during visual recall.

Adult↗
PubMed1998

Task-dependent influences of attention on the activation of human primary visual cortex.

There has been a good deal of controversy over whether attention influences area V1-the first cortical area onto which information from the retina is projected. Attention to motion has been found to modulate monkey area MT and the human homolog of MT/MST. Here we show that activation of V1 by attention to motion is task dependent. Our stimulus consisted of a group of translating random dots superimposed over another group of random dots executing expansion motion. Subjects were instructed to pay attention selectively to the translation, expansion, or neither in particular (passive condition). The activity in the human MT/MST homolog measured by functional magnetic resonance imaging (fMRI) was significantly higher in both the translation and the expansion conditions than in the passive condition, while the activity in area V1 was significantly higher only in the translation condition. These results show that attention to motion modulates area V1, and more interestingly that high-level cognitive processing such as attention may directly or indirectly determine the retroactive extent of feedback within the motion pathway in a manner dependent on the type of motion attended.

Cognition↗
PubMed1999

Functional MRI reveals spatially specific attentional modulation in human primary visual cortex.

Selective visual attention can strongly influence perceptual processing, even for apparently low-level visual stimuli. Although it is largely accepted that attention modulates neural activity in extrastriate visual cortex, the extent to which attention operates in the first cortical stage, striate visual cortex (area V1), remains controversial. Here, functional MRI was used at high field strength (3 T) to study humans during attentionally demanding visual discriminations. Similar, robust attentional modulations were observed in both striate and extrastriate cortical areas. Functional mapping of cortical retinotopy demonstrates that attentional modulations were spatially specific, enhancing responses to attended stimuli and suppressing responses when attention was directed elsewhere. The spatial pattern of modulation reveals a complex attentional window that is consistent with object-based attention but is inconsistent with a simple attentional spotlight. These data suggest that neural processing in V1 is not governed simply by sensory stimulation, but, like extrastriate regions, V1 can be strongly and specifically influenced by attention.

Attention↗
PubMedDate not supplied

Layer IVA of rhesus monkey primary visual cortex.

Layer IVA of rhesus monkey striate cortex contains pyramidal cells arranged in distinct groups. Their cell bodies are in a configuration of flat cones, each with an average diameter of 60 microns, and their apical dendrites aggregate into bundles that ascend toward the pial surface. Nissl-stained sections suggest that these pyramidal cell cones have their bases in layer IVB, with their tops extending into layer IVA. The neurons in the cones are readily apparent in MAP2 antibody-stained material, and in cytochrome oxidase-reacted tissue it is evident that the pyramidal cell cones occupy the pale spaces that are surrounded by the darkly reactive honeycomb lattice. This lattice of neuropil around the cones contains some axons and boutons that are immunoreactive for parvalbumin, and it is within the lattice that other investigators have shown afferents from the parvocellular (P)-layers of the dLGN to terminate. Because of this input, it is likely that the pyramidal cell cones of layer IVA are involved with color and form perception. The relationship between the layer IVA cones of neurons and the underlying system of previously described pyramidal cell modules (Peters and Sethares, 1991) is discussed, as well as the possibility that the pyramidal cell cones might represent aggregations of neurons, which receive input from basic sets of P-like afferents originating from color-responsive ganglion cells of the retina, as described by Schein and de Monasterio (1987).

Animals↗
PubMed2000

Moving illusory contours activate primary visual cortex: an fMRI study.

Identifying the cortical areas activated by illusory contours provides valuable information on the mechanisms of object perception. We applied functional magnetic resonance imaging to identify the visual areas of the human brain involved in the perception of a moving Kanizsa-type illusory contour. Our results indicate that, in addition to other cortical regions, areas V5 and V1 are activated. Activity in area V1 was particularly prominent.

Adult↗