The differential effects of Type 1 and Type 2 illusory contours on spatial pattern detection and discrimination

Purpose. Illusory contours (IC's), like luminance-defined contours, possess neurophysiological correlates and interact with physical stimuli. IC's produced by either offset gratings (Type 1 IC's), or by Kanizsa-type inducers (Type 2 IC's), elicit neural responses in primate visual cortical areas V1 and V2, respectively. The effects of Type 1 and Type 2 IC's on contrast discrimination thresholds for Gabor targets were assessed in order to infer the properties of the neural mechanisms which produce these contours in human observers. Methods. Type 1 and 2 IC strength was equated for all observers using a matching procedure. Increment thresholds for Gabor targets presented on Type 1 and 2 IC's were measured in four observers as a function of pedestal contrast (0-16%), spatial frequency (1.25-10 c/d, constant octave bandwidth) and orientation (0-90° relative to IC orientation). Results. Relative to control conditions which possessed inducers but lacked IC's, Type 1 IC's masked, whereas Type 2 IC's facilitated target detection and discrimination at low (<5%) pedestal contrasts. At higher pedestal contrasts this relationship was reversed. Both Type 1 and Type 2 IC's interacted maximally with targets of approximately 5 c/d which were oriented parallel to the IC's. Conclusions. Both Type 1 and Type 2 IC's interacted maximally with Gabor targets whose spatial parameters are those which optimally stimulate primary visual cortical neurons. This finding suggests that Type 1 and Type 2 IC's share a common mechanism of early contrast transduction. Type 1 and 2 IC's equated in strength have opposite effects, however, on the detection and discrimination of Gabor targets. This latter finding suggests a dissimilarity in the pooling mechanisms which underlie the formation of Type 1 and 2 illusory contours.