Context-dependent changes in visual sensitivity induced by Müller–Lyer stimuli

We measured the detectability of a single line (target) flanked by high-contrast inward- or outward-pointing arrowheads (context). We show that as a function of target contrast, context angle, and context position there is a continuum of contextual modulations of target detectability that vary from strong inhibition (target detection is impaired) to strong excitation (target detection is facilitated), but target detection is not affected when the context is presented at low contrasts. The results show striking correlations with the perceived length distortions in the Müller-Lyer illusion, i.e. an inward-pointing arrowhead results in improved target detectability and increased perceived length of the bar, whereas an outward-pointing arrowhead results in diminished target detectability and decreased perceived length of the bar. Both suppressive and facilitatory effects diminish as target contrast, arrowhead angle, and line-arrowhead spatial disparity are increased. At larger distances between line and arrowhead the suppressive effects become facilitatory (the Müller-Lyer illusion reverses). When concurrent Müller-Lyer extent experiments are run, we found that the perceived length of the target stimulus is overestimated or underestimated as it is flanked by high-contrast inward or outward-pointing arrowheads, the magnitude of the length distortion effects diminishing as target contrast increases. To explain the nature of both context-induced suppression and facilitation in contrast detection we present a population model of orientation detectors in visual cortex that relies on short and long-range horizontal cortical connections, and suggest that that the same type of mechanism that accounts for contrast detection may account for perceived extent.