Inefficient visual search for second-order motion

Visual search rate was used to assess attentional resources required for de tection of opposing motions defined either by luminance or by modulations o f texture contrast, flicker, or size. Though luminance-based targets were d etected quickly, search through second-order motion was slow. Control exper iments ruled out stimuli visibility, complexity, eccentricity sensitivity, and attributes of the carrier as possible accounts. Results suggest separat e processing of the two types of stimuli: Luminance-based motion is detecte d by spatiotemporal filters, whereas second-order motion is likely processe d by a capacity-limited, later stage. Rate-reducing effects of increased co ntrast and speed mirrored previous research suggesting that effortful featu re tracking may be the mechanism. (C) 2001 Optical Society of America.