1.3 Change Blindness and Inhibition of Return

There are two phenomena related to visual attention that people experience every day. One is that a person turns a blind eye to some change in their environment. The phenomenon is called change blindness. The other is that attention focus never fixes a location for a long time, and there exists a mechanism to inhibit the fixation returning back to the original location. In the following sections, we explain the two phenomena.

1.3.1 Change Blindness

Change blindness (CB) is defined as the induced failure of observers to detect a change in a visual display [39]. This invisible change often happens in the alternating images with a blank fields (about 80 ms or more). Two nearly identical scenes with a certain background change appear one by one. The changes can be noticed easily since the change location can be popped out at alternate times. However, when a transient blank frame (more than 80 ms) is inserted between the two nearly identical scenes, the change is often not noticed by observers. In another setting, two nearly identical pictures with some changes are displayed side by side as shown in Figure 1.5. Pointing the difference between them at a glance is difficult. Once the large change is detected in Figure 1.5, most people are amazed at failing to notice it. The cause of change blindness is human selective visual attention. Some changes in no-attention regions are ignored, as shown in Figure 1.3, in which many regions of the family picture are not reached by the eye saccade. Figure 1.5 is a similar case since the change occurred in the inattention background. The other cause is that only one feature or object can be focused on by observers for a transient time. If the alternating images that accompany the change are unable to provide location information in a short time, as mentioned above – for example, a full blank field is inserted between two successive frames with little difference – then the change cannot be picked up by the HVS.

Figure 1.5 An example of change blindness, by glancing at the two nearly identical pictures. Reproduced with permission from Christopher G. Healey, ‘Perception in Visualization,’ North Carolina State University, http://www.csc.ncsu.edu/faculty/healey/PP/index.html (accessed October 1, 2012)

How can change blindness be so easily induced? The main reason is that the focused attention only operates on one item at a time. In the real world, there are a lot of items which can attract observers. If the change between two images is not within the focus of attention, the change information is often swamped. In general, for a complex scene, eye scanning usually needs to take considerable time by serial processing to find the change.

1.3.2 Inhibition of Return

Attention shift (both overt and covert) is a phenomenon of the visual system as mentioned in Section 1.2.4. After people view a scene, the saliency can be inhibited for the currently selected location. So the fixated location will move to its peripheral location. Why can the attention focus leave the most salient location in the visual field and not come back to the location immediately? Weakening of the information novelty over a long period of staring is a reason for the situation. Is there another physiological cause for the phenomenon? The first physiological experiment was described in 1984 by Posner and Cohen [40]. They had measured an inhibitory aftereffect at the original location in delayed response to subsequent stimuli [41]. The phenomenon was later called inhibition of return (IoR). Some studies reported that the inhibitory signal may come from the superior colliculus in the brain [41, 42]. Whatever the principle of IoR is, the effect of discouraging attention from returning back to the original attended location is very useful for object search in visual fields. Most computational models proposed later have used the IoR mechanism.