Transient motion blindness : the role of selective inhibition in visual perception
Transient motion blindness is a visual phenomenon where temporary blindness for first-order visual motion is induced in observers by placing the target motion episode within a temporal stream of irrelevant distractors (Sahraie, Milders & Niedeggen, 2001). Observers are asked to ignore all irrelevant motion occurring prior to a clear signal (the appearance of a red fixation), whereupon they are asked to pay attention to any subsequent motion and to report its direction. Under these circumstances, observers are found to be completely unaware of any motion episodes that occur within 300ms of the red cue, even though first-order motion is traditionally thought to ‘pop-out’ of visual displays without the need for attention (Dick, Ullman & Sagi, 1987). Sahraie et al (2001) proposed that motion blindness (MB) may result from the carry-over of motion distractor inhibition. The present thesis evaluated the accuracy of this hypothesis over a series of 10 experiments, finding that: 1) MB is a robust and easily replicable phenomenon; 2) MB cannot be explained purely in terms of the task switching and spatial shifting costs incurred during the task; 3) MB is contingent upon the presence of irrelevant distractor motion; 4) MB only arises when this distractor motion is selectively inhibited; 5) the severity of MB can be predicted from knowledge of an observer’s inhibitory strength; 6) perceptual motion sensitivity is measurably reduced during periods of MB; and 7) the magnitude of the MB effect varies predictably with the amount of inhibition thought to have been generated by the task. Overall, it is argued that motion blindness does arise as the result of distractor inhibition in line with the inhibitory hypothesis of Sahraie et al (2001), and that the impairment reflects general operating principles of the selective attention which controls sensory information processing.