Abstract: Neural activity appears to be essential for the normal development of the orientation-selective responses of cortical cells. It has been proposed that the correlated activity of LGN cells is a crucial component for shaping the receptive fields of cortical simple cells into adjacent, oriented subregions alternately receiving ON- and OFF-center excitatory geniculate inputs. After eye opening, the spatiotemporal structure of neural activity in the early stages of the visual pathway depends not only on the characteristics of the environment, but also on the way the environment is scanned. In this study, we use computational modeling to investigate how eye movements might affect the refinement of orientation tuning in the presence of a Hebbian scheme of synaptic plasticity. Visual input consisting of natural scenes scanned by varying types of eye movements was used to activate a spatiotemporal model of LGN cells. In the presence of different types of movement, significantly different patterns of activity were found in the LGN. Specific patterns of correlation required for the development of segregated cortical receptive field subregions were observed in the case of micromovements, but were not seen in the case of saccades or static presentation of natural visual input. These results suggest an important role for the eye movements occurring during fixation in the refinement of orientation selectivity.
Abstract: The barn owl is a nocturnal predator that is able to capture mice in complete darkness using only sound to localize prey. Two binaural cues are used by the barn owl to determine the spatial position of a sound source: differences in the time of arrival of sounds at the two ears for the azimuth (interaural time differences (ITDs)) and differences in their amplitude for the elevation (interaural level differences (ILDs)). Neurophysiological investigations have revealed that two different neural pathways starting from the cochlea seem to be specialized for processing ITDs and ILDs. Much evidence suggests that in the barn owl the localization of the azimuth is based on a cross-correlation-like treatment of the auditory inputs at the two ears. In particular, in the external nucleus of the inferior colliculus (ICx), where cells are activated by specific values of ITD, neural activation has been recently observed to be dependent on some measure of the level of cross-correlation between the input auditory signals. However, it has also been observed that these neurons are less sensitive to noise than predicted by direct binaural cross-correlation. The mechanisms underlying such signal-to-noise improvement are not known. In this paper, by focusing on a model of the barn owl's neural pathway to the optic tectum dedicated to the localization of the azimuth, we study the mechanisms by which the ITD tuning of ICx units is achieved. By means of analytical examinations and computer simulations, we show that strong analogies exist between the process by which the barn owl evaluates the azimuth of a sound source and the generalized cross-correlation algorithm, one of the most robust methods for the estimate of time delays.
Abstract: It is known that the perceived color of an object depends on the context in which it is viewed, its reflectance properties and the spectral distribution of the illuminating light. What is not known, however, is how the visual system functions so that color percepts depend upon the integration of local and contextual cues. While phenomenological theories of color vision exist, robust neurally based theories consistent with psychophysical observations are sparse. In the present study we develop such a theory and establish its self-consistency by computer simulations of cerebral cortical areas involved in color perception. The simulations test the hypothesis that long-range reciprocal connections within and between cortical areas mediate a dynamic process of reentry that integrates contextual cues into the color percept. When stimuli similar to those used in psychophysical testing of contextual influence were used, firing patterns consistent with psychophysical data on color constancy and color induction in humans were observed. Selective disruption of reciprocal inter- or intra-areal connections reduced the correspondence between the model's responses and the psychophysical data. The findings are consistent with the proposal that reentrant interactions within and between cortical areas provide a major basis for the context-sensitive aspects of color vision.
