Onset Of Visual Stimulation Research Articles

An increase in spontaneous activity mediates visual habituation.

SUMMARYThe cerebral cortex is spontaneously active, but the function of this ongoing activity remains unclear. To test whether spontaneous activity encodes learned experiences, we measured the response of neuronal populations in mouse primary visual cortex with chronic two-photon calcium imaging during visual habituation to a specific oriented stimulus. We find that, during habituation, spontaneous activity increases in neurons across the full range of orientation selectivity, eventually matching that of evoked levels. This increase in spontaneous activity robustly correlates with the degree of habituation. Moreover, boosting spontaneous activity with two-photon optogenetic stimulation to the levels of visually evoked activity accelerates habituation. Our study shows that cortical spontaneous activity is linked to habituation, and we propose that habituation unfolds by minimizing the difference between spontaneous and stimulus-evoked activity levels. We conclude that baseline spontaneous activity could gate incoming sensory information to the cortex based on the learned experience of the animal.

Spiking Suppression Precedes Cued Attentional Enhancement of Neural Responses in Primary Visual Cortex.

Attending to a visual stimulus increases its detectability, even if gaze is directed elsewhere. This covert attentional selection is known to enhance spiking across many brain areas, including the primary visual cortex (V1). Here we investigate the temporal dynamics of attention-related spiking changes in V1 of macaques performing a task that separates attentional selection from the onset of visual stimulation. We found that preceding attentional enhancement there was a sharp, transient decline in spiking following presentation of an attention-guiding cue. This disruption of V1 spiking was not observed in a task-naïve subject that passively observed the same stimulus sequence, suggesting that sensory activation is insufficient to cause suppression. Following this suppression, attended stimuli evoked more spiking than unattended stimuli, matching previous reports of attention-related activity in V1. Laminar analyses revealed a distinct pattern of activation in feedback-associated layers during both the cue-induced suppression and subsequent attentional enhancement. These findings suggest that top-down modulation of V1 spiking can be bidirectional and result in either suppression or enhancement of spiking responses.

Neuronal oscillations form parietal/frontal networks during contour integration.

The ability to integrate visual features into a global coherent percept that can be further categorized and manipulated are fundamental abilities of the neural system. While the processing of visual information involves activation of early visual cortices, the recruitment of parietal and frontal cortices has been shown to be crucial for perceptual processes. Yet is it not clear how both cortical and long-range oscillatory activity leads to the integration of visual features into a coherent percept. Here, we will investigate perceptual grouping through the analysis of a contour categorization task, where the local elements that form contour must be linked into a coherent structure, which is then further processed and manipulated to perform the categorization task. The contour formation in our visual stimulus is a dynamic process where, for the first time, visual perception of contours is disentangled from the onset of visual stimulation or from motor preparation, cognitive processes that until now have been behaviorally attached to perceptual processes. Our main finding is that, while local and long-range synchronization at several frequencies seem to be an ongoing phenomena, categorization of a contour could only be predicted through local oscillatory activity within parietal/frontal sources, which in turn, would synchronize at gamma (>30 Hz) frequency. Simultaneously, fronto-parietal beta (13–30 Hz) phase locking forms a network spanning across neural sources that are not category specific. Both long range networks, i.e., the gamma network that is category specific, and the beta network that is not category specific, are functionally distinct but spatially overlapping. Altogether, we show that a critical mechanism underlying contour categorization involves oscillatory activity within parietal/frontal cortices, as well as its synchronization across distal cortical sites.

Two distinct neural mechanisms in early visual cortex determine subsequent visual processing

Neuroscience research has conventionally focused on how the brain processes sensory information, after the information has been received. Recently, increased interest focuses on how the state of the brain upon receiving inputs determines and biases their subsequent processing and interpretation. Here, we investigated such 'pre-stimulus' brain mechanisms and their relevance for objective and subjective visual processing. Using non-invasive focal brain stimulation [transcranial magnetic stimulation (TMS)] we disrupted spontaneous brain state activity within early visual cortex (EVC) before onset of visual stimulation, at two different pre-stimulus-onset-asynchronies (pSOAs). We found that TMS pulses applied to EVC at either 20 msec or 50 msec before onset of a simple orientation stimulus both prevented this stimulus from reaching visual awareness. Interestingly, only the TMS-induced visual suppression following TMS at a pSOA of −20 msec was retinotopically specific, while TMS at a pSOA of −50 msec was not. In a second experiment, we used more complex symbolic arrow stimuli, and found TMS-induced suppression only when disrupting EVC at a pSOA of ∼ −60 msec, which, in line with Experiment 1, was not retinotopically specific. Despite this topographic unspecificity of the −50 msec effect, the additional control measurements as well as tracking and removal of eye blinks, suggested that also this effect was not the result of an unspecific artifact, and thus neural in origin. We therefore obtained evidence of two distinct neural mechanisms taking place in EVC, both determining whether or not subsequent visual inputs are successfully processed by the human visual system.

Neural Representation During Visually Guided Reaching in Macaque Posterior Parietal Cortex

Visually guided hand movements in primates require an interconnected network of various cortical areas. Single unit firing rate from area 7a and dorsal prelunate (DP) neurons of macaque posterior parietal cortex (PPC) was recorded during reaching movements to targets at variable locations and under different eye position conditions. In the eye position-varied task, the reach target was always foveated; thus eye position varied with reach target location. In the retinal-varied task, the monkey reached to targets at variable retinotopic locations while eye position was kept constant in the center. Spatial tuning was examined with respect to temporal (task epoch) and contextual (task condition) aspects, and response fields were compared. The analysis showed distinct tuning types. The majority of neurons changed their gain field tuning and retinotopic tuning between different phases of the task. Between the onset of visual stimulation and the preparatory phase (before the go signal), about one half the neurons altered their firing rate significantly. Spatial response fields during preparation and initiation epochs were strongly influenced by the task condition (eye position varied vs. retinal varied), supporting a strong role of eye position during visually guided reaching. DP neurons, classically considered visual, showed reach related modulation similar to 7a neurons. This study shows that both area 7a and DP are modulated during reaching behavior in primates. The various tuning types in both areas suggest distinct populations recruiting different circuits during visually guided reaching.

Light-induced Down-regulation of the Rat Class 1 Dynein-associated Protein robl/LC7-like Gene in Visual Cortex

Light-induced Down-regulation of the Rat Class 1 Dynein-associated Protein robl/LC7-like Gene in Visual Cortex

Light-induced down-regulation of the rat class 1 dynein-associated protein robl/LC7-like gene in visual cortex.

Dynein and kinesin are the main microtubule-dependent motors that mediate intracellular movement in eukaryotic organisms. We have cloned a full-length cDNA encoding rat dynein light chain protein, robl/LC7-like (class 1), from visual cortex. We found that rat robl/LC7-like gene is highly expressed in neocortex and displays the unusual feature of being rapidly down-regulated by sensory stimulation. This effect was seen at both mRNA and protein levels in visual cortex, being detectable in as little as 45 min after the onset of visual stimulation. Down-regulation by sensory stimulation was also found within ocular dominance columns of area V1 in monocularly deprived monkeys. Our results suggest a high turnover rate of the robl/LC7-like protein and the presence of a repressor mechanism in neurons that is tightly coupled to synaptic stimulation.

Plasticity- and neurodegeneration-linked cyclic-AMP responsive element modulator/inducible cyclic-AMP early repressor messenger RNA expression in the rat brain

In order to explore the role of CREM (cyclic-AMP responsive element modulator) gene expression in the function of the central nervous system, the gene transcripts were investigated in the rat brain in several conditions linked to increased neuronal activity. Up-regulation of CREM messenger RNA levels in the hippocampus was found to follow intraperitoneal administration of kainate (10 mg/kg). This increase was observed in both the dentate gyrus and hippocampus proper (CA subfields) and reached its maximum at 6 h after the treatment. Intrahippocampal injection of N-methyl- d-aspartate (200 nmol) resulted in elevated CREM messenger RNA expression as well. A similar increase of the messenger RNA abundance was also observed in the retrosplenial cortex after treating the female rats with a high dose (5 mg/kg) of dizocilpine maleate, an N-methyl- d-aspartate receptor antagonist. All these conditions are linked to neuronal excitation and neurodegeneration. However, an increase in CREM messenger RNA accumulation was also observed in the visual cortex after exposure of dark-adapted animals to the light, a procedure linked to neuronal plasticity. In the latter condition, it was found that CREM messenger RNA reached its highest levels at 6 h, i.e. later than the maximal increase of expression of immediate early genes such as c-fos, jun B and zif 268, observed 45 min following the onset of visual stimulation. The ICER (inducible cyclic-AMP early repressor) form of CREM messenger RNA was identified to be induced by the light exposure. Finally, it was also found that cycloheximide, an inhibitor of protein synthesis, overinduces CREM/ICER gene expression. Together, these data suggest that CREM/ICER may be responsive to neuronal activation. Furthermore, given that CREM products have been shown previously to down-regulate expression of immediate early genes in vitro, they suggest that ICER may function as a molecular switch involved in down-regulation of immediate early gene expression in the rat brain.

The consequences of inactivating areas V1 and V5 on visual motion perception.

We studied the capacity of normal humans to discriminate the direction of motion of visual stimuli when areas V1 or V5 were reversibly inactivated with transcranial magnetic stimulation. We found that (i) magnetic stimulation of V5 at intervals of -20 to +10 ms before or after the onset of visual stimulation was effective in abolishing motion perception--other delays were not; (ii) magnetic stimulation of V1 abolished motion perception only marginally and at delays which were significantly different from those obtained with V5, the stimulation now being effective only at delays of 60-70 ms after the onset of visual stimulation. We conclude (i) that stimulation of V5 is a much more potent way of inducing akinetopsia (motion imperception) than stimulation of V1; (ii) that perceptually effective visual motion signals reach V5 at or before 30 ms and reach V1 at or before 60 ms--consequently, perceptually effective motion signals reach V5 before they reach V1; (iii) that, given the time course of arrival of signals in V1 and V5, it takes about 30-50 ms for signals from V1 to reach V5. We conclude further that there are probably two components reaching V5 from the retina, a fast one which bypasses V1 and a slow one which reaches it through V1.

Temporal dynamics of visual-evoked neuromagnetic sources: Effects of stimulus parameters and selective attention

Results are reviewed from several neuromagnetic studies which characterize the temporal dynamics of neural sources contributing to the visual evoked response and effects of attention on these sources. Different types of pattern-onset stimuli (< or = 2 degrees) were presented sequentially to a number of field locations in the right visual field. Multiple dipole models were applied to a sequence of instantaneous field distributions constructed at 10 ms intervals. Best-fitting source parameters were superimposed on Magnetic Resonance images (MRI) of each subject to identify the anatomical structure(s) giving rise to the surface patterns. At least three sources, presumably corresponding to different visual areas, were routinely identified from 80-150 ms following the onset of visual stimulation. This observation was consistent across subjects and studies. The temporal sequence and strength of activation of these sources, however, were dependent upon the specific stimulus parameters used to evoke the response (e.g., eccentricity) and on the relevance of the stimulus to the subject. In addition, our results provide evidence for the recurrence of activity in striate and extrastriate regions, following the initial cycle of responses.

Development of retinal synaptic arrays in the inner plexiform layer of dark-reared mice

In the central area of the retina of mouse the rate of synaptogenesis in the inner plexiform layer (IPL) drops precipitously at about the time the eyes open. To determine if the visual input at eye opening provides a signal for the neurons to stop adding synapses, mice were raised in darkness during the period of maximal synaptogenesis and through eye opening. Retinal synaptic arrays of dark-reared and normally reared animals were compared quantitatively. The rate of synaptogenesis after eye opening in dark-reared mice indicated that the onset of visual stimulation was not the cue to stop synaptogenesis. However, the synaptic arrays of the IPL of dark-reared mice consistently had more conventional synapses than those of normally reared mice. It is concluded that the number of conventional synapses in the central retina was increased by dark-rearing.