Long–range signal interactions in V2 neurons of macaque monkeys.

Abstract

Abstract: : Purpose: Neurons’ responses to stimuli in the classic receptive–field (CRF) of V1 neurons are modulated by stimuli beyond the CRF and this ability to integrate signals over distance is thought to be important for perceptual integration and binding (e.g., contour integration). However, nothing is known about the long–range interactions beyond V1, and therefore, we investigated the center–surround interactions of V2 neurons. Methods: Microelectrode recording experiments were conducted under anesthesia and paralysis in three macaque monkeys. By presenting drifting circular sine–wave gratings (TF=3.0 HZ) that were optimized for orientation and spatial frequency on the center of the CRF and varying stimulus diameters, we investigated the center–surround interactions of V1 and V2 neurons. Stimulus contrast was either 20% (low contrast) or 80% (high contrast). Results: For high contrast gratings, the average center size of V2 neurons was larger than that of V1 but the surround size was similar for V1 and V2. The surround influence over the CRF responses was purely inhibitory. As previously reported in V1, neurons in both areas had larger excitatory centers when low contrast gratings were used, because stimulation of the surround near the CRF exerted excitatory influence over the CRF responses. This zone was larger in V2 than in V1. Conclusions: The neuronal network underlying the long–range signal integration of V2 units appears to be similar to that of V1 units (Cavanaugh et al 2002; Angelucci et al, 2002). In both areas, the feed–forward connections are primarily responsible for the CRF responses. The contrast–depended excitatory or inhibitory influences from the near surround appear to be mediated by the intrinsic long–range horizontal connections in respective areas. The predominantly suppressive surround in both V1 and V2 appears to depend on the feedback signals from the common source(s), i.e., higher order visual areas (e.g., MT).

Keywords: visual cortex • receptive fields • electrophysiology: non–clinical