People consider visual perception to be one of the most trustworthy means of gathering information about the world. However, striking limitations in visual processing occur daily, distorting the precision and completeness of the visual experience and demonstrating that substantial changes in a visual scene often go undetected. In particular, the role of attention in perceptual analysis is paramount for full and accurate comprehension of a visual scene. If certain aspects of visual attention are absent, or at minimum not applied specifically, then change blindness—the inability to discern large changes that occur in clear view of an observer—can occur. Remarkably, the phenomenon of change blindness can be elicited under circumstances in which the changes are made repeatedly and the observer knows that they will occur. See also: Cognition; Neurobiology; Perception; Psychology; Vision; Visual attention
Under a variety of experimental scenarios, researchers have determined that change blindness is a common shortcoming of the visual system: Only those parts of a visual scene that are firmly focused on and attended to are likely to be remembered by an observer; moreover, if something remains outside of the attentional sphere, little or no information is retained. One methodology that is used to highlight the occurrence of change blindness is known as the flicker technique. In this technique, an image of a real-world scene is presented for about a half second; this is followed by the presentation of the same image that has been altered in some way (for example, by changing the image's color or size, or by removing a part of the image). Importantly, a blank field is displayed for a fraction of a second before the appearance of each alternate image. Observers are asked to watch this flickering display and identify the change. Surprisingly, although observers predict that they will easily see any change that is sufficiently significant, experimental results indicate that the change (even a seemingly obvious change) remains unseen for long stretches of time. In fact, most observers require 10 to 20 seconds (constituting 20 to 40 alternations of the images) before the change is noticed. See also: Information processing (psychology); Memory
The key determinant in causing change blindness appears to be the effective removal of motion signals (shifts in characteristics of the retinal image) that accompany a change, either by overwhelming the local changes by global motion signals or by hiding them altogether. Typically, a local motion signal created by a change automatically draws an individual's attention, allowing it to be seen. However, if this signal is lost, attention will no longer be automatically drawn to the change. Without this visual attention, the observer will not see the change. Instead, to find the change, the viewer must direct his or her attention around the scene on a time-consuming item-by-item basis, until the changed item has been pinpointed. In the aforementioned case of the flicker technique, the insertion of the blank field produces numerous global motion signals that swamp the local signals, thereby making the change more difficult to notice. Change blindness also occurs under other conditions—for example, when a scene is interrupted by an outside factor or when a person is focused on another task. Thus, the study of change blindness and visual attention has particular applicability in determining the effects of attentional distractions on real-world tasks, such as the effects of cellphone use on driving. See also: Cellphone use and driving; Eye (vertebrate)