Eye swapping temporally modulates potency of continuous flash suppression

Abstract

Continuous flash suppression (CFS) refers to a technique to render a monocular stimulus invisible by presenting a dynamic series of high-contrast patterns (such as Mondrian patterns) to the other eye. Despite its popularity as a tool to suppress stimulus from awareness, the suppression mechanisms underlying CFS remain not well understood. To further elucidate the suppression mechanisms, this study investigated the effects of eye swapping on CFS suppression by manipulating the eye of presentation of the suppressor and the target. Results showed that eye swapping of the suppressor and the target significantly reduced the strength of CFS suppression when swapping frequency was higher (3.5 Hz). However, strong suppression persisted at lower swapping frequency (1.2 Hz). Investigation of the time course of suppression revealed that suppression was weaker just after eye swapping but that it quickly regained strength over the monocular presentation period of the suppressor. However, this buildup seemed to not be fast enough to closely follow eye swapping at higher frequency. These findings can be better understood by the contribution of monocular processes to CFS suppression. They imply that interocular suppression caused by competition between monocular processes can mediate phenomenal suppression over multiple eye swaps when swapping frequency is low. The significance of the findings is discussed in relation to binocular rivalry and binocular switch suppression.

Figure 1.

Stimulus sequence (a) and the eye-of-presentation conditions (b) in Experiment 1. (a) The suppressor (top row) was a dynamic Mondrian pattern; it was refreshed every 94 msec, as illustrated above the suppressor image. The target (second row) was presented a few seconds after the onset of the suppressor and also temporally modulated, as illustrated below the target image. The bottom row illustrate a typical percept. (b) In the monocular condition (top row), both the suppressor (represented by blue color) and the target (hatched pattern) were presented to the same eye, whereas only the background was presented to the other eye (no interocular competition). In the dichoptic condition (middle row), the two stimuli were presented to different eyes and the eye of presentation was fixed during the trial. In the eye-swapping condition, the two stimuli were dichoptically presented and repeatedly exchanged between the eyes every 847 msec (1.2 Hz) or 282 msec (3.5 Hz). LE and RE represent the left eye and the right eye, respectively.

Figure 2.

Results of Experiment 1. Detection time in the monocular, dichoptic, and two eye-swapping conditions. For each condition, the white bar shows the detection time without the suppressor, whereas the gray bar shows the time with the suppressor. Error bars indicate +1 SEM across observers.

Figure 3.

Stimulus conditions and results of Experiment 2. (a) Schematic illustration of the target presentation in different eye-of-presentation conditions. The format of the illustration is the same as that of Figure 1(b). A brief target was presented either at an early (depicted by “E”) or a late phase (“L”) of the suppressor presentation half-cycle in the 1.2-Hz eye-swapping condition (third row). The target was presented at the corresponding timing in the other conditions. (b) Percent correct performance of the target detection task was plotted as a function of target onset phase. Different symbols represent different eye-of-presentation conditions. Error bars indicate ±1 SEM across observers. The dashed line designates the chance level of the task (50%). LE and RE represent the left eye and the right eye, respectively.