Visual Perception

We have begun a series of experiments that explore some properties of visual attention. The focus of this research is a phenomenon called "inhibition of return" (IOR). Previous studies have shown that when a new object appears, there is typically a short period of perceptual facilitation attached to the location that the object appeared in, followed by a longer period of relatively impaired perceptual processing there (IOR). The experiments we have conducted have explored whether this pattern is tied to the sudden appearance of a new perceptual object, or whether the same effects occur when an object suddenly disappears. Both situations entail a need to revise the observer's representation of the scene, but virtually all previous work has focussed on the effects of onsets (appearances) rather than offsets (disappearances). Our experiments also use somewhat more complex visual displays than have been used in most previous work in this area, a factor that may well be important in the allocation of visual attention.

This page provides links to small demonstrations and data summaries for four experiments that we have done in this area. Each trial in each experiment was built by presenting a series of four frames. In all experiments, the first frame was simply a small fixation cross, presented in the center of the screen, for 250 msec. Similarly, the second frame in all experiments included 8 gray circles, arranged in a circular formation, presented for 750 msec. The experiments differed in the arrangement and number of small red or blue shapes (boxes and disks) that were present in some of these gray circles (see below). The experiments also differed in the changes that took place in the third and fourth frames, changes that involved the appearance or disappearance of parts of the display. In all four experiments, the duration of the third frame was varied (from 80 msec to 610 msec), to investigate the timecourse of any attentional effects. The fourth frame in each experiment contained the target event (the appearance or the disappearance of an element of the display) that the observer was to respond to.

Each of the four experiments can be thought of as involving two Events (one that occurs in the third frame, and one that occurs in the fourth frame), with the observers always responding to the second Event. Each Event could be either the appearance of a new object in the display, or the disappearance of an object from the display. We analyzed the results in terms of the difference in time between the two Events (ISI = 80, 120, 180, 270, 410, or 610 msec), and in terms of the spatial separation of the two Events: The target Event could either occur within the same gray circle as the first Event(see above), within a gray circle 90 degrees from the first Event, or 180 degrees away from it. For a one-trial demonstration of each experiment, click on the Demo link for that experiment.

• Onset -Onset Experiment: [Graph of data]. Following the first frame's fixation cross, the second frame included four gray circles (evenly spaced) that each contained 2 "dummy" red or blue figures. The four remaining circles were the "active" fields, and were empty during this frame. The third frame was identical, except for the addition of one red or blue figure to one of the active circles. This third frame was on for one of six variable times, ranging from 80 - 610 msec. The final frame matched the third, but one more red or blue figure was added to an active gray circle. Observers were told to push a button as soon as they detected the occurrence of the second onset. [ Demo of this onset-onset experiment].
• Offset -Offset Experiment: [Graph of data]. In this experiment, the second frame also had four "active" gray circles, alternating with four inactive ones. In this case, the four inactive ones were empty, and remained so throughout the trial. Each active gray circle in the second frame had two small red or blue figures. In the third frame, one of the red or blue figures was deleted from one of the active circles. In the fourth frame, one more small figure was deleted; observers were told to respond to this second offset. Demo
• Large Offset - Onset Experiment: [Graph of data]. In this experiment, we increased the perceptual salience of the first Event, by making this Event be the disappearance of a large gray circle (rather than one of the small colored figures). As in the first experiment, four of the gray circles were "dummy" locations that each contained two red or blue figures. The post-fixation cross frame included these four inactive circles, and four active ones. Three of the active circles were empty, and the fourth contained one red or blue figure. In the third frame, one of the four active circles disappeared. In the fourth frame, one red or blue figure was added to one of the four active locations. The observers responded to this onset. Demo
• Large Offset -Offset Experiment: [Graph of data]. The four inactive locations in this experiment were empty, and stayed empty. In the frame following the fixation cross, three of the active circles had two colored figures, and one of them had one such figure. The Event in the third frame was the disappearance of one of the large gray circles. In the final frame, one of the small colored figures disappeared from one of the four active locations. Demo

  
  

  Demos made possible by Dr Clue (Ian A. Storms) and The JavaScript Source (Mike McGrath)!