Abstract
Abstract: :
Purpose: Colour-defined coherent motion breaks down when the moving pattern (made up of randomly distributed checks) is buried in dynamic luminance contrast (LC) noise. In such displays the perceived spatial position of any coloured check is poorly defined. Our aim was to find out whether the accurate position information of coloured checks is preserved and available in other visual tasks, in spite of the perceived uncertainty of spatial location. Methods: A dynamic texture of LC defined checks (size 0.25 x 0.25) was generated by allowing the luminance of each check to vary randomly with equal probability within a range specified as a % of background luminance. Three experiments were carried out to establish how the amplitude of dynamic LC noise affects thresholds : A) for perception of colour-defined motion. B) for detecting a change in the position of discrete coloured checks and, C) for vernier misalignment in vertically oriented coloured checks. Results: Experiment A confirms previous findings in that thresholds for detection of colour-defined motion increase systematically with LC noise provided the coloured checks are distributed randomly. The results obtained from experiment B show that when the subject's task is to detect a sudden change in the position of one of four discrete, colour-defined checks, the corresponding thresholds also increase linearly with dynamic LC noise. Perceptually, any of the the coloured targets appears to occupy a number of positions around its mean location, joining in the ongoing random motion of LC checks. In marked contrast to the findings from experiments A & B, the results from experiment C show that the subjects have no problem with vernier alignment. The colour detection thresholds in this task and the corresponding standard deviations remain completely unaffected by the presence of LC noise. These spatial alignment thresholds remain invariant with LC noise amplitude for both short (∼80ms) and long (∼300ms) presentation durations. Conclusion: The results of this study suggest that colour signals can be used in parallel pathways and that accurate colour-defined position information is available and can be used in certain visual tasks in spite of the perceived uncertainty of spatial location. Since "Blindsight" labels the ability to make use of visual information in the absence of conscious perception, "Double-Blindsight" may be the appropriate label to describe the ability to override perception (i.e., to discard misleading, consciously perceived visual signals) and to make good use of accurate information, in spite of misleading percepts.