Abrupt Transitions in the Perception of Illusory Contours Triggered by Specific Visual Stimuli

Abstract

Two forms of learning have been identified in previous studies. Gradual improvement is observed for many perceptual tasks; it is often stimulus-specific and is consequently thought to involve synaptic modifications in early visual cortical areas. In contrast, sudden transitions, either cognitive (‘insight’) or perceptual (eg in the Dalmatian dog picture) are generally thought to occur more centrally.

Here we report that a sudden transition in the perception of illusory-contour stimuli can be triggered by specific visual stimuli, rather than by verbal instructions, leading to an abrupt improvement in psychophysical performance.

Subjects discriminated between two classes of Kanizsa-like shapes where the (illusory) edges were slightly curved into ‘thin’ or ‘fat’ figures (D L Ringach, R Shapley, 1993 Perception 22 Supplement, 51). In the first (‘test’) block, only small amounts of curvature (1° – 3°) were given; subjects performed poorly in that block. In the second (‘training’) block, stimuli with larger curvature (4° – 6°) were added to the set. Performance for the 1° – 3° stimuli improved abruptly—often within seconds—compared to the first block: the pooled responses of eight naive subjects showed a marked jump, followed by maintained elevation, of the fraction-correct responses as a function of time. Good performance was retained in a third (re-test) block, with a stimulus set identical to that of the first block. Control experiments verified that the subjects understood the task from the beginning. Furthermore, the learned performance did not transfer to a different (smaller) retinal size. Thus the jump in performance is a perceptual, stimulus-specific phenomenon (rather than a cognitive one), although it exhibits characteristics of ‘insightful’ understanding.