Spatial characteristics of center-surround antagonism in motion discrimination

Abstract

In young observers, the stimulus duration required to correctly discriminate the direction of low contrast moving targets decreases (i.e., performance improves) as stimulus area increases (spatial summation). However, at high contrast stimuli, duration thresholds increase with increasing stimulus size (spatial suppression), a result thought to be linked to center-surround antagonism in non-classical receptive fields (Tadin et al., 2003). Previous research from our lab suggests that such center-surround antagonism changes across the life span, as inhibitory mechanisms in visual cortex decline (Betts et al., 2005). Here we examine the extent to which spatial summation and suppression, and age-related changes in both, depend on stimulus spatial frequency. Stimulus duration thresholds for 0.5, 1, 2, and 4 c/deg Gabors drifting at 2 deg/s were measured over a range of contrasts (2.8% – 46%) and sizes (0.7–5 visible cycles) for younger (19–30 years) and older (63–75 years) observers. In younger observers, spatial summation and suppression occurred in all spatial frequency conditions, with summation occurring over a larger range of cycles as frequency increased for low contrast stimuli, and similar levels of suppression across all frequencies for high contrast stimuli. Older observers showed a similar pattern of results at low contrast, but as contrast increased, suppression was reduced compared to younger observers across all spatial frequencies. Our results suggest that patterns of spatial summation and suppression are preserved across low and medium spatial frequencies, and that the age-related reduction in spatial suppression is independent of stimulus spatial frequency.