The micfogenesis of geometrical illusions: contextdependent changes in visiual sensitivity and the muller-lyer effect

Purpose, We analyze the influence of visual contexts which resemble the Muller-Lyer configuration on the detectability of a target stimulus. Methods. Six adult subjects were instructed to detect the occurrence of a target while the target and the context were briefly flashed periodically. During each trial the target was presented alone, in conjunction with the context, or neither target nor context were presented. The target occurred randomly at one of four contrasts; the context (outward or inward arrowhead) was always presented at high contrast. We measured the detection ratio (denned as proportion correct detections) for all four target contrasts in the following conditions: (a) target alone; (b) target and outward arrowhead; (c) target and inward arrowhead. We varied the angle between the arrowhead fins and the distance between the arrowhead and the target. Results. Target detection is facilitated by the inward arrowhead and suppressed by the outward arrowhead. The amount of facilitation and suppression increases monotonically with the decrease in arrowhead angie, but it diminishes as target contrast increases. When the distance between target and context increases, both inhibitory and excitatory effects diminish drastically in strength. If distance is further increased the influence of the outward arrowhead changes from suppression to facilitation. The inhibitory and excitatory effects are amplified if a symmetric arrowhead is added on the other end of the target. All observed variations in detection ratio match qualitatively the lesults obtained with the Mulier-Lyer illusion, in which a bar appears long or short when it is flanked by inward or outward arrowheads. Conclusions, (a) Previous results (e.g., Kapadia et al., 1995; Polat and Sagi, 1994) reported facilitory effects of the surround. We found that as a function of surround orientation and spatial offset there is a continuum of surround modulations ranging from strong inhibition to strong excitation; (b) Our findings are consistent with short and long-range receptive field interactions in primary visual cortex and suggest a physiological basis for surround modulation; (c) The Muller-Lyer extent illlusion is correlated with target detectability. We suggest that detectability and perceived extent are caused by the same mechanism.

Duke Scholars

Author Gregory R. Lockhead Psychology & Neuroscience