Search

The role of intrinsic cortical dynamics is a debatable issue. A recent optical imaging study (Kenet et al., 2003) found that activity patterns similar to orientation maps (OMs), emerge in the primary visual cortex (V1) even in the absence of sensory input, suggesting an intrinsic mechanism of OM activation. To better understand these results and shed light on the intrinsic V1 processing, we suggest a neural network model in which OMs are encoded by the intrinsic lateral connections. The proposed connectivity pattern depends on the preferred orientation and, unlike previous models, on the degree of orientation selectivity of the interconnected neurons. We prove that the network has a ring attractor composed of an approximated version of the OMs. Consequently, OMs emerge spontaneously when the network is presented with an unstructured noisy input. Simulations show that the model can be applied to experimental data and generate realistic OMs. We study a variation of the model with spatially restricted connections, and show that it gives rise to states composed of several OMs. We hypothesize that these states can represent local properties of the visual scene.

Binocular disparities applied to large-field patterns elicit vergence eye movements at ultra-short latencies. We used the electromagnetic search coil technique to record the horizontal and vertical positions of both eyes while subjects briefly viewed (150 ms) large patterns that were identical at the two eyes except for a difference in position (binocular disparity) that was varied in direction from trial to trial. For accurate alignment with the stimuli, the horizontal and vertical disparity vergence responses (HDVRs, VDVRs) should vary as the sine and cosine, respectively, of the direction of the disparity stimulus vector. In a first experiment, using random-dots patterns (RDs) with a binocular disparity of 0.2 degrees , this was indeed the case. In a second experiment, using 1-D sine-wave gratings with a binocular phase difference (disparity) of 1/4-wavelength, it was not the case: HDVRs were maximal when the grating was vertical and showed little decrement until the grating was oriented more than approximately 65 degrees away from vertical, whereas VDVRs were maximal when the grating was horizontal and began to decrement roughly linearly when the grating was oriented away from the horizontal. We attribute these complex directional dependencies with gratings to the aperture problem, and the HDVR data strongly resemble the stereothresholds for 1-D gratings, which are minimal when the gratings are vertical and remain constant for orientations up to approximately 80 degrees away from the vertical when expressed as spatial phase disparities [Morgan, M. J.,Castet, E. (1997). The aperture problem in stereopsis. Vision Research, 37, 2737-2744.]. To explain this constancy of stereothresholds, Morgan and Castet (1997) postulated detectors sensitive to the phase disparity of the gratings seen by the two eyes (rather than their linear separation along some fixed axis, such as the horizontal). However, because (1) our VDVR data with gratings did not show this constancy and (2) the available evidence strongly suggests that there are no major differences in the disparity detectors mediating the initial HDVR and VDVR, we sought an alternative explanation for our data. We show that the dependence of the initial HDVR and VDVR on grating orientation can be successfully modeled by a bias in the number and/or efficacy of the detectors that favors horizontal disparities.

Previous studies of the ferret visual cortex indicate that the development of direction selectivity requires visual experience. Here, we used two-photon calcium imaging to study the development of direction selectivity in layer 2/3 neurons of the mouse visual cortex in vivo. Surprisingly, just after eye opening nearly all orientation-selective neurons were also direction selective. During later development, the number of neurons responding to drifting gratings increased in parallel with the fraction of neurons that were orientation, but not direction, selective. Our experiments demonstrate that direction selectivity develops normally in dark-reared mice, indicating that the early development of direction selectivity is independent of visual experience. Furthermore, remarkable functional similarities exist between the development of direction selectivity in cortical neurons and the previously reported development of direction selectivity in the mouse retina. Together, these findings provide strong evidence that the development of orientation and direction selectivity in the mouse brain is distinctly different from that in ferrets.

In the central part of the visual field, horizontal retinal disparity is directly involved in stereoscopic vision processing. Its role in the peripheral visual field is unclear as that of vertical disparity. By performing extra-cellular recordings in behaving monkeys, we show that horizontal disparity is specifically encoded in the foveal representation of the visual field in the primary visual cortex (area V1). In the peripheral field however, we find that vertical disparity is truly encoded with tuning profiles similar to those of horizontal disparity. At such retinal eccentricies both types of disparities interact strongly, dependently on the preferred orientation axis of the receptive fields as predicted by the binocular energy model. This functional organization should participate to the neural processing involved in constructing stereoscopic percept at large retinal eccentricities. In addition to the modulation of visual activity by the gaze direction that we also show in the peripheral area V1, our results demonstrate that the primary visual cortex is organized functionally for an early neural processing of visual 3-D space reconstruction.; Dans la représentation centrale du champ visuel, la disparité rétinienne horizontale est impliquée directement dans la perception stéréoscopique. Son implication en vision périphérique est moins évidente tout comme celui de la disparité verticale dont le rôle fonctionnel reste controversé.Par des enregistrements extra-cellulaires réalisés chez le singe vigile, nous montrons que la disparité horizontale est codée de façon préférentielle dans la représentation fovéale du champ visuel du cortex visuel primaire (aire V1). En périphérie, les interactions fortes entre disparités horizontales et verticales et leur lien étroit avec la sélectivité à l'orientation (en accord avec le modèle d'énergie binoculaire) suggèrent leur implication dans la construction du percept stéréoscopique dans les zones excentrées du champ visuel. De plus les modulations de l'activité visuelle des neurones par la direction du regard que nous observons dans l'aire V1 également, nous permettent de conclure que le cortex visuel primaire participe aux mécanismes neuronaux de reconstruction de l'espace tridimensionnel.