NEURAL PROCESSING OF STEREOPSIS AS A FUNCTION OF VIEWING DISTANCE IN PRIMATE VISUAL CORTICAL AREA V1

1. The influence of viewing distance on disparity selectivity was inve stigated in area V1 of behaving monkeys. While the animals performed a fixation task, cortical cells were recorded extracellularly in the fo veal representation of the visual yield. Disparity selectivity was ass essed by using static random dot stereograms (RDSs) through red/green filters flashed over the central fixation target. 70 determine the inf luence of the viewing distance, a color video monitor was positioned a t lived distances of 20, 40, or 80 cm. The same RDSs with the same ang ular size of dots were used at the three distances. 2. Disparity sensi tivity was tested on 139 cells, of which 78 were analyzed at two or mo re distances and the rest (61) at a single distance. When disparity se lectivity 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 greater than or equal to 1.5 times more common than far cel ls ar all three distances, The latency distribution of the responses o f disparity-selective (DS) cells was similar at all three distances, w ith 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 ch anged, Disparity selectivity could be present at a given distance and absent at other(s), with often a loss of visual response. This emergen ce of disparity coding was the strongest effect (25 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 al all recorded distances (13 of 78 or 17%), changing the viewing distance also affected the sh arpness (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 ch ange, Finally, the activity of four DS cells was not affected at all b y the viewing distance. 4. Another effect concerned the level of ongoi ng activity (OA), defined as being the neural activity in darkness pre ceding the flash of the visual stimulus while the monkey was fixating the small bright target. Changing the viewing distance resulted in sig nificant changes in OA level fur more than half of the cells (41 of 78 or 53%). The most common effect was an increase in OA. level at the s horter distance. The modulations of both visual responsiveness and OA could occur simultaneously, although they often had opposite signs. In deed, the two effects were statistically independent of each other, i. e., modulations of visual responses were not related to the level of e xcitability of the neurons. 5. Control experiments were performed that showed that the effects of changing the viewing distance were not due to the retinal patterns in that the modulations of visual responsiven ess were independent of the dot density. Seventeen cells were also tes ted for a possible effect of vergence by the use of prisms. When there was an effect of distance, it could be closely or partially reproduce d by using prisms. These controls. together with the effects observed on OA. strongly suggest that the modulations of neural activity of the V1 neurons by the viewing distance are extraretinal in origin, probab ly proprioceptive. 6. The modulation of visual responsiveness by the v iewing distance in the primary visual cortex indicates that integratio n of information from both retinal and extraretinal sources can occur early in the visual processing pathway for cortical representation of three-dimensional space. A functional scheme of three-dimensional cort ical circuitry is discussed that shows cortical areas where disparity selectivity and modulations of visual activity by the angle of gaze ha ve been described so far.