Processing time of contour integration: the role of colour, contrast, and curvature

We investigated the temporal properties of the red-green, blue-yellow, and luminance mechanisms in a contour-integration task which required the linki ng of orientation across space to detect a 'path'. Reaction times were obta ined for simple detection of the stimulus regardless of the presence of a p ath, and for path detection measured by a yes/no procedure with path and no -path stimuli randomly presented. Additional processing times for contour i ntegration were calculated as the difference between reaction times for sim ple stimulus detection and path detection, and were measured as a function of stimulus contrast for straight and curved paths. We found that processin g time shows effects not apparent in choice reaction-time measurements. (i) Processing time for curved paths is longer than for straight paths. (ii) F or straight paths, the achromatic mechanism is faster than the two chromati c ones, with no difference between the red-green and blue-yellow mechanisms . For curved paths there is no difference in processing time between mechan isms. (iii) The extra processing time required to detect curved compared to straight paths is longest for the achromatic mechanism, and similar for th e red-green and blue-yellow mechanisms, (iv) Detection of the absence of a path requires at least 50 ms of additional time independently of chromatici ty, contrast, and path curvature. The significance of these differences and similarities between postreceptoral mechanisms is discussed.