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 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 visual stimulus while the monkey was fixating the small bright target. Changing the viewing distance resulted in significant changes in OA level for more than half of the cells (41 of 78 or 53%). The most common effect was an increase in OA level at the shorter distance. The modulations of both visual responsiveness and OA could occur simultaneously, although they often had opposite signs. Indeed, the two effects were statistically independent of each other, i.e., modulations of visual responses were not related to the level of excitability 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 responsiveness were independent of the dot density. Seventeen cells were also tested for a possible effect of vergence by the use of prisms. When there was an effect of distance, it could be closely or partially reproduced 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, probably proprioceptive. 6. The modulation of visual responsiveness by the viewing distance in the primary visual cortex indicates that integration 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 cortical circuitry is discussed that shows cortical areas where disparity selectivity and modulations of visual activity by the angle of gaze have been described so far.

Eye-centered, head-centered, and intermediate coding of remembered sound locations in area LIP.

1. The lateral intraparietal area (LIP) of the posterior parietal cortex lies within the dorsal cortical stream for spatial vision and processes visual information to plan saccadic eye movements. We investigated how LIP neurons respond when a monkey makes saccades to the remembered location of sound sources in the absence of visual stimulation. 2. Forty-three (36%) of the 118 neurons sampled showed significant auditory triggered activity during the memory period. This figure is similar to the proportion of cells showing visually triggered memory activity. 3. Of the cells showing auditory memory activity, 44% discharged in an eye-centered manner, similar to the way in which LIP cells discharge for visually initiated saccades. Another 33% responded in head-centered coordinates, and the remaining 23% had responses intermediate between the two reference frames. 4. For a substantial number of cells in all three categories, the magnitude of the response was modulated by eye position. Similar orbital "gain fields" had been shown previously for visual saccades. 5. We propose that area LIP is either at the origin of, or participates in, the transformation of auditory signals for oculomotor purposes, and that orbital gains on the discharge are part of this process. 6. Finally, we suggest that, by the level of area LIP, cells are concerned with the abstract quality of where a stimulus is in space, independent of the exact nature of the stimulus.

Coordinate transformations in the representation of spatial information.

Coordinate transformations are an essential aspect of behavior. They are required because sensory information is coded in the coordinates of the sensory epithelia (e.g. retina, skin) and must be transformed to the coordinates of muscles for movement. In this review we will concentrate on recent studies of visual-motor transformations. The studies show that representations of space are distributed, being specified in the activity of many cells rather than in the activity of individual cells. Furthermore, these distributed representations appear to be derived by a specific operation, which systematically combines visual signals with eye and head position signals.

Modulation of neural stereoscopic processing in primate area V1 by the viewing distance.

Accurate binocular depth perception requires information about both stereopsis (relative depth) and distance (absolute depth). It is unclear how these two types of information are integrated in the visual system. In alert, behaving monkeys the responsiveness of a large majority of neurons in the primary visual cortex (area V1) was modulated by the viewing distance. This phenomenon affected particularly disparity-related activity and background activity and was not dependent on the pattern of retinal stimulation. Therefore, extraretinal factors, probably related to ocular vergence or accommodation, or both, can affect processing early in the visual pathway. Such modulations could be useful for (i) judging true distance and (ii) scaling retinal disparity to give information about three-dimensional shape.