Psychologia, 1998, 41, 199-202

A STATIC IMAGE OF A RAPIDLY MOVING PATTERN CAN BE PERCEIVED BY MAKING A SACCADE Yuji TAKEDA, Masayoshi NAGAI, Koji KAZAI and Akihiro YAGI Kwansei Gakuin University, Japan

Ten undergraduate and two graduate students were required to identify the moving patterns. The two patterns were made of triangles; one with apices pointing up and the other with apices pointing down. A given pattern was moved rapidly on a CRT and the subject was asked to watch and say whether the triangles pointed up or down. When they fixated a single point, the correct responses were near chance. When they made a saccade across the moving pattern, their detection performance was significantly better. The subjects reported that they could perceive the static image of the pattern when the saccade was made. This result might be explained by the hypothesis that saccadic suppression reduces excessive visual input.

Key words:

saccade, moving pattern, pattern identification, static image, saccadic suppression

If you ride on a bicycle, you may observe the following phenomenon: When you gaze at the tread of tire on the rolling wheel of your own bicycle, you may not recognize the tread pattern on the tire. However, you can recognize the tread pattern at the moment if you make a saccade from the tread to another location. Thus, the saccade enhanced the ability to resolve the static pattern on the rapidly moving surface in apparent contrasts with the well known phenomenon of saccadic suppression (e.g., the reduction of visual sensitivity during saccadic eye movements. Latour, 1962; Volkmann, 1962). Saccades also decrease the sensitivity to motion perception (Bridgeman, Hendry, & Stark, 1975; Shioiri & Cavanagh, 1989; Stark, Kong, Schwartz, Hendry, & Bridgeman, 1976). This decrease in sensitivity begins before the onset of a saccade and lasts after the offset of the saccade (Shioiri & Cavanagh, 1989; Volkmann, Schick, & Riggs, 1968; Wallach & Lewis, 1966). The mechanism underlining saccadic suppression is not clear. However, there are two major possible explanations. The first hypothesis is that a "corollary discharge» related to the motor act suppresses the visual sensitivity (Riggs, Merton, & Morton, 1974; Sperry, 1950; Volkmann, 1962; Volkmann, Riggs, White, & Moore, 1978). The second is that the suppression is a form of masking secondary to overlapping retinal images during saccades. During saccades, not only is retinal sensitivity reduced, but also visual information processing seems to be suppressed at higher levels. For example, it was demonstrated that the identification of degraded patterns or the processing of mental We thank Dr. Enoch Callaway for a review of a preliminary version of the manuscript. Correspondence: Yuji Takeda, Department of Psychology, Kwansei Gakuin University, Uegahara 1155, Nishinomiya, Hyogo 662-8501, Japan (E-mail: [email protected]). 199

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rotation was suppressed during saccades (Sanders & Houtmans, 1985; Sanders & Rath, 1991). These properties of saccadic suppression seem to contradict with the bicycle tire phenomenon. Saccadic suppression generally ca,uses the reduction of sensitivity to visual stimuli during the saccade. On the contrary, saccadic eye movements improve the pattern identification in the bicycle tire phenomenon. In fact, however, they may be related each other. The present experiment offers a quantitative laboratory analog of the bicycle tire phenomenon. However, in the bicycle tire phenomenon, the visual image of the rapidly moving pattern is present only at a pre-saccadic time. In this experiment, the retinal image of the moving pattern is present before during and after the saccade.

METHOD

Twelve undergraduate and graduate student volunteers (8 males and 4 females) from Kwansei Gakuin University participated in this experiment as subjects. All had normal or corrected-to-normal vision. Stimuli were generated and displayed on a IWATSU ISEL AV-Tachistoscope system (IS-701A) equipped with P31 phosphor. The stimuli were presented in a 31.25 x 6.25° display field (500 x 100 pixels) with green dots (34.7 cd/m 2). Subjects observed all stimuli at a 50 cm viewing distance in darkness. Each trial began with a tone pip and a visual warning stimulus; the latter was a green square filling the whole display field upon which a black fixation point was present. Each fixation point was located 7.5 ° horizontally from the center of the display field, either to the left or to the right. After the warning stimulus, a scrolling stimulus composed of a 25 X 5 array of triangles (upward or downward) was presented with the green fixation points for Is (see Fig. 1). During presentation of the scrolling stimulus, elements of the stimulus scrolled upward 31.25°/s (1 pixell 2 ms). Following the scrolling stimulus, the green square filling the display field was displayed until subject responded. Subjects were required to judge whether the triangle elements of the stimulus were pointing up or down, and to press one of two keys using two alternative forced choice. After approximately 1s from a response, the warning stimulus of the next trials was presented. During a presentation of the scrolling stimulus, subjects were required to gaze on one of the fixation points under fixation conditions (right-fixation conditions and left-fixation conditions), and to make a

the array of ~s

the array of \l s

15° Fig. 1.

The scrolling stimuli. The scrolling stimulus which was a 25 x 5 array of triangles was scrolled upward 31.25°/s. A pair of "+"s were fixation points.

PERCEPTION BY A SACCADE

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saccade from one of the fixation points to another one under saccade conditions (right-to-Ieft saccade conditions and left-to-right saccade conditions). The saccade was began after the onset of the scroll stimulus and ended before the offset of it. Subjects were instructed to restrain their eye blinks while the scrolling stimuli were being presented. Each session consisted of 10 practice and 20 real trials. Subjects were tested in two sessions for each conditions in random order. Eye movements and eye blinks were monitored using EOG electrodes. Subjects received feedback information when incorrect responses were made .

. RESULTS

Mean percentages of correct responses for the fixation conditions and the saccade conditions are shown in Table 1 (collapsed across two fixation points for the fixation conditions and two directions of the saccadic eye movements for the saccade conditions). No vertical eye movements were observed in any of the conditions. Correct responses under the saccade condition averaged 70.8%. Correct responses under the fixation condition averaged approximately 50%. Thus, the recognition of scrolled elements in the saccade conditions was less difficult than that in the fixation conditions. A t-test indicated a significant difference between the fixation conditions and the saccade conditions (t( 11) == 3.66, P< 0.01).

DISCUSSION

This experiment demonstrates that elements of a moving pattern that cannot be resolved by fixed gaze become recognizable when a saccade is made. The recording of the vertical EOG showed that no vertical movement occurred, when subjects made saccades horizontally. Thus, there were no visual pursuit movements that followed the pattern movement. Previous studies reported that the visual perception during saccades was inhibited (Latour, 1962; Volkmann, 1962). However, the present study demonstrated that saccadic eye movements could facilitate the identification for the rapidly moving patterns. This phenomenon could not be explained by any of previously proposed theories (e.g., Volkmann, 1986). We suggest that subjects could not recognize the rapidly moving elements in the fixation conditions because the rate of input exceeded information processing capacity. We will use the term "traffic" to indicate the rate of information being delivered, as that term is used in a similar way in discussing computer networks. Table 1.

Mean Percentages and SDs of Correct Responses SACCADE

FIXATION

mean

0.708

0.531

SD

0.137

0.071

Note: The data of the "SACCADE" was collapsed across two directions of saccade, and that of the "FIXATION" was collapsed across two fixation points. The statistical analysis shows significant difference.

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When traffic exceeds information processing capacity, processing probably does not go on beyond a low-level. Although we can make a spatial selection of information by attention, we cannot make a temporal selection of information. Thus temporal overload might simply prevent the system from operating properly. Improved recognition during a saccade may reflect a reduction of traffic and correction of overload by means of saccadic suppression. In the bicycle tire phenomenon, saccadic suppression might provide a 'snap shot' of the moving pattern by suppressing the weaker visual information from the period just preceding the saccade, such as a strobe light provides a kind of 'snap shot' of a moving object. In the bicycle tire phenomenon, there is no overloading information following the saccade, nor is there information on the nature of moving pattern. In the present experiment, the overloading input is present before, during and after the saccade. Under these conditions, it remains to be determined whether the snap shot involves data from before, during or after the saccade.

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