Distance modulates neural activity in V1, V2, and V4: 'where' on the 'what' pathway

Purpose. Two processing streams emanate from primary visual cortex. A dorsal stream to the parietal lobe, and a ventral stream to the inferotemporal lobe. Much evidence demonstrates the presence of spatial information on the parietal pathway, however, lesion experiments show that the integrity of inferotemporal cortex is essential for object size judgements, which depend on distance estimates. Two hypotheses are suggested: (1) spatial position is estimated on the parietal pathway, and transferred to inferotemporal cortex; or (2) spatial information is common to both processing streams. We have tested these by looking for evidence of spatial modulation of visual signals at sites on the ventral pathway. Methods. Two macaque monkeys were trained to fixate a spot on a movable monitor. Following unit isolation and RF mapping, size tuning was measured with the monitor at distances of 45, 90, and 180 cm. Measurements were made under binocuiar full field conditions, and, under monocular restricted field conditions. Results. Of 125 neurons in V1, V2 and V4, approximately half of the cells in all areas show significant distance modulation (p < 0.05). Most common were cells in which response increased with Nearness or Farness. Eliminating disparity and spatial context had no effect on distance modulation in some cells, thereby ruling out both as possible explanations. Conclusions. Spatial modulation is found along the two cortical processing streams, and may represent a common code for spatial scaling and object binding.