Visual stability across saccades while viewing complex pictures.

Abstract

As people visually examine their world, the proximal stimulus pattern changes position on their retinae with every saccade, but they still perceive the world as being stable. This phenomenon of space constancy or visual stability was explored by making changes in natural, full-color pictures during selected saccades as observers examined them for 20 s in preparation for a recognition test. In Experiment 1, the pictures were displaced up, down, left, or right by 03, 0.6, or 1.2 deg. In Experiment 2, the pictures were expanded or contracted in size by 10% or 20%. As a secondary task, subjects pressed a button whenever they detected a change in the picture. Three results from previous studies with simpler stimuli did not generalize to this situation. Evidence suggests that subjects' detection of image displacements as they examine complex pictures primarily involves the use of local information in the region of the saccade's landing position. A saccade target theory of visual stability is proposed.Visual Stability Across Saccades -2 VISUAL STABILITY ACROSS SACCADES WHILE VIEWING COMPLEX PICTURESMaking a saccadic eye movement causes a displacement of the light pattern across the retinae. If a similar retinal displacement occurs during an eye fixation, there is perception of movement: that is, the world appears to jump. However, the same pattern of motion on the retinae, occurring as a consequence of making a saccade, is not perceived and the world appears stable. 1 This phenomenon, referred to traditionally as space constancy, and which we will call visual stability, permits people to visually explore the world with a moving sensory matrix without mis-attributing self-induced stimulus motion on the matrix to the world itself. How the visual system achieves this stability has been a matter of speculation and research since Helmholtz (1866/1963) discussed the problem.Two classes of theories have been proposed to explain this visual stability. One class of theories assumes that the characteristics of the proximal stimulus alone are sufficient to distinguish between retinal change resulting from saccadic eye movements and change resulting from movement in the world. The other class of theories assumes that some additional, non-retinal information is required, though individual theories differ in the nature of the information proposed. Proximal Stimulus Alone: GibsonGibson (1966) argued that the transformation of the visual array over time that results from making a saccade is different from that which typically results from motion in the world. A saccadic eye movement produces a rigid displacement of the entire light pattern on the retinae. Thus, this type of transformation specifies a self-induced stimulus displacement rather than a change in the world. Furthermore, though not stated by Gibson, saccadic suppression (Volkman, Schick, & Riggs, 1968), together with visual masking from pre-and post-saccadic visual fields, prevents the perception of the stimulus motion on the retinae that results from a saccade-prod...

Transsaccadic memory of position and form

“…Our results necessitate some modification of the reference object theory that we described earlier McConkie and Currie, 1996). The visual system need not be committed to a single identified reference object before the saccade begins, for a task-irrelevant non-target object can become the reference object, and the system does not know in advance which object will be appropriate as the reference object.…”

“…In more recent work, we (Deubel and Schneider, 1994;Deubel et al, , 1998) developed a 'reference object theory' that assumes that pre-and postsaccadic visual 'snapshots' are linked by means of the saccade target which is assumed by the visual system as being stable. In a very similar theoretical approach, the 'saccade target theory ' (McConkie and Currie, 1996;Currie et al, 2000) also assigns a privileged status to the object that constitutes the target for the saccade. Both theories assume that with each new fixation the visual system runs through a sequence of processing steps which starts with the selection of one object as the target for the next saccade.…”

“…In scenes consisting of only two objects like in the experiments of and Verfaillie and De Graef (2000) it is likely that both objects are attended before the saccade, so both stimuli are encoded with sufficient accuracy in transsaccadic memory. The more complex stimuli of Irwin (1992), McConkie and Currie (1996) and Currie et al (2000), however, exceed the capacity of the visual memory; hence only a selected subset of the stimuli, preferentially around the saccade target area, will be still in memory for further processing after the saccade.…”

“…According to our theoretical interpretation, the visual system searches for the presaccadic object of attention within a spatiotemporal 'constancy window', comparing the stored features with the new image around the saccade goal immediately after a saccade (Deubel et al, 1984;Irwin et al, 1994). If this target is found within a certain spatial and temporal window, the visual system assumes it to have remained stable during the saccade, and the target becomes a 'reference object' to determine the positions of other objects and textures McConkie and Currie, 1996). According to this theory, then, extraretinal signals are not used for transsaccadic integration under normal circumstances, because the reference object usually is found.…”

“…This suggests that information near the saccade target is more likely to be encoded in transsaccadic memory than information from more distant locations. McConkie and Currie (1996) used full-colour pictures of natural scenes which changed during the saccade. In their Experiment 2, the scene expanded or contracted during the eye movement.…”