Neural processing of stereopsis as a function of viewing distance in primate visual cortical area V1

Y. Trotter, S. Celebrini, B. Stricanne, S. Thorpe and M. Imbert Centre de Recherche Cerveau et Cognition, Faculte de Medecine de Rangueil, Universite Paul Sabatier, Toulouse, France. 1. The influence of viewing distance on disparity selectivity was investigated in area V1 of behaving monkeys. While the animals performed a fixation task, cortical cells were recorded extracellularly in the foveal representation of the visual field. Disparity selectivity was assessed by using static random dot stereograms (RDSs) through red/green filters flashed over the central fixation target. To determine the influence of the viewing distance, a color video monitor was positioned at fixed distances of 20, 40, or 80 cm. The same RDSs with the same angular size of dots were used at the three distances. 2. Disparity sensitivity was tested on 139 cells, of which 78 were analyzed at two or more distances and the rest (61) at a single distance. When disparity selectivity was analyzed at a given distance, about half the cells were found to be selective at 40 or 80 cm, but only a third at 20 cm. Near cells were > or = 1.5 times more common than far cells at all three distances. The latency distribution of the responses of disparity-selective (DS) cells was similar at all three distances, with a mean distribution centered around 60 ms. 3. Changing the viewing distance drastically affected the neural activity of the V1 neurons. The visual responsiveness of 60 of 78 cells (77%) was significantly changed. Disparity selectivity could be present at a given distance and absent at other(s), with often a loss of visual response. This emergence of disparity coding was the strongest effect (28 of 78 or 36%) and occurred more frequently from short to long distances. Among the cells that remained disparity insensitive at all recorded distances (31 of 78 or 40%), about half also showed modulations of the amplitude of the visual response. For cells that remained DS at all recorded distances (13 of 78 or 17%), changing the viewing distance also affected the sharpness (or magnitude) of disparity coding in terms of level of visual responsiveness and those changes were often combined with variations in tuning width. In only two cells did the peak of selectivity type change. Finally, the activity of four DS cells was not affected at all by the viewing distance. 4. Another effect concerned the level of ongoing activity (OA), defined as being the neural activity in darkness preceding the flash of the v