Abstract: Our eyes are constantly in motion. Even during visual fixation, small eye movements continually jitter the location of gaze. It is known that visual percepts tend to fade when retinal image motion is eliminated in the laboratory. However, it has long been debated whether, during natural viewing, fixational eye movements have other functions in addition to preventing the visual scene from fading. In this study, we analyzed the influence of fixational eye movements on the discrimination of gratings masked by noise with a power spectrum similar to that of natural images. Using a new method of retinal image stabilization, we selectively eliminated the motion of the retinal image that normally occurs during the intersaccadic intervals of visual fixation. We show that fixational eye movements improve discrimination of high spatial frequency, but not of low spatial frequency, stimuli. This improvement originates from the temporal modulations introduced by fixational eye movements in the visual input to the retina, which emphasize the high spatial frequency harmonics of the stimulus. In a natural visual world dominated by low spatial frequencies, fixational eye movements appear to constitute an effective sampling strategy by which the visual system enhances the processing of spatial detail.

[TOP]

Abstract: Although it is known that images tend to disappear when they are stabilized on the retina for tens of seconds or minutes, the possible functions of fixational movements during the brief periods of visual fixation that occur during natural viewing remain controversial. Studies that investigated the retinal stabilization of stimuli presented for less than a few seconds have observed neither decrement in contrast sensitivity nor image fading. In this study, we analyzed the effect of retinal stabilization on discriminating the orientation of a low-contrast and noisy small bar that was displayed for either 500 ms or 2 s. The bar was randomly tilted by 45° either clockwise or counterclockwise. For both exposure durations, percentages of correct discrimination were significantly lower under conditions of visual stabilization than in the presence of the normally moving retinal image. These results are consistent with the predictions of recent computational models that simulated neuronal responses in the early visual system during oculomotor activity and support the
hypothesis that visual processes deteriorate rapidly
in the absence of retinal image motion.

[TOP]

Abstract: The identification accuracy of briefly flashed stimuli followed by an interstimulus interval (ISI) of variable length was compared to that obtained with longer flashes that prolonged the exposure of the stimulus throughout the ISI. The interval between the onset of the stimulus and the onset of the mask (stimulus onset asynchrony (SOA)) was the same in the two conditions. Consistent with a dependence of visual identification on SOA, the percentages of correct identification in the two conditions were approximately similar at all SOAs irrespective of the level of noise, stimulus familiarity, and stimulus complexity. However, departures from the onset-onset rule were also present. While the two conditions yielded virtually identical identification accuracy with an SOA of 80 ms, small but significant differences were found for shorter and longer intervals. Possible theoretical explanations of the results are presented.


[TOP]