Saccadic Suppression of Displacement: Effects of Illumination and Background Manipulation

Abstract

In contrast to other functions which are suppressed during saccades, saccadic suppression of displacement (SSD--a decrease in sensitivity to visual displacements during saccades) has often been considered to be due to efferent processes rather than to visual masking. The aim of this study was to explicitly assess the importance of visual conditions in SSD. In two experiments, a small computer-generated target made random horizontal jumps. An infrared eye tracker was used to detect the saccade toward the new position, triggering a smaller centripetal displacement of the target. Subjects reported awareness of these intrasaccadic displacements by pressing a key. In the first experiment, the task was performed in both a well-lit environment and in darkness. In the second experiment these conditions were replicated and additional factors such as the contrast of the background and the effect of moving the target spot alone or the target plus the entire background were investigated. Unlike other forms of saccadic suppression, SSD was stronger in the dark, although subjects also had a greater bias to report detections in that condition. Other background manipulations had no effect. The effect of ambient lighting on SSD is small and subtle. Effects of other background manipulations may be overridden by the focusing of attention on a small moving target.

Apparent motion during saccadic suppression periods

“…The detection rate was higher than we expected based on previous reports from which detection probability could be inferred (Irwin 1992;MacAskill et al 2003;Triesch et al 2003). In this experiment, subjects were encouraged to view the scenes freely and to make many saccades.…”

“…We conWrmed that observers are not sensitive to such displacement when it occurs during a saccadic eye movement. This conWrms earlier Wndings of saccadic suppression of displacement with smaller, simpler stimuli 3 (Bridgeman et al 1975;Li and Matin 1997;MacAskill et al 2000MacAskill et al , 2003Anand and Bridgeman 2002;Deubel et al 2004). Bridgeman and Fisher (1990) reported that sensitivity to trans-saccadic target displacement depended on target eccentricity with more suppression for central than peripheral targets.…”

“…This motion blur could obscure the more subtle apparent motion associated with the image displacement stimulus. If the apparent motion signal or even visibility of the image were essentially below threshold during the saccade due to blur, masking or active suppression then estimation of trans-saccadic displacement would necessarily depend on comparison of image position before and after the saccade (MacAskill et al 2003). The loss of this motion signal and the requirement to carry out these comparisons in other frames of reference than retinocentric coordinates (see below) likely makes the image translation easier for the visual system to discount during saccades.…”

“…Ilg and HoVmann (1993) had similar Wndings and noted that their subjects were responding to the relative motion of the saccade target compared to the background. MacAskill et al (2003) argued that degree of saccadic suppression of displacement is larger when subjects judge the whole motion of a display compared to the relative motion of a saccade target to a background. Our experiments conWrm the reduced salience and phenomenological slowing of transsaccadic translations.…”

“…We varied the size of the translations as there is some evidence from experiments with small target displays that displacements that are small with respect to the size of the saccade are less detectable (Li and Matin 1990;McConkie and Currie 1996;MacAskill et al 2003).…”