The Impacts Of Task Difficulty, Spontaneous Blinks And Tactile Stimuli On Microsaccades And Pupil Responses

Microsaccades and pupil responses are involuntarily physiological activities in visual system. Although humans are usually unaware of them, they are essential to vision. Microsaccades are small saccades occurring during attempted fixation. When they are counteracted, the perceived visual scenes get obscure and even disappear within an extremely short period, due to the adaptation of visual neurons. Microsaccades also precisely control the gaze location in high-acuity visual tasks. However, the visual sensitivity and neural activities along the visual path are briefly suppressed during microsaccades, in order to maintain a stable and continuous visual perception. Moreover, as an optical structure of the visual system, the pupil dilates or constricts in response to the change of luminance to allow a suitable amount of light enter into the eye. Meanwhile, the pupil size has significant effects on the image quality of retina, which further affects the visual acuity.Given the critical roles of the microsaccade and pupil response on vision, understanding which factors could modulate them, and in what manner are significant subjects in visual research. Most previous studies focused on the effect of varied parameters of visual stimuli on microsaccades, however, leaving the performance of microsaccades in non-visual cognitive activities less explored. In parallel, the impacts of non-visual external stimuli and other spontaneous physiological responses on microsaccade and pupil size also remain an open question. Moreover, related literatures revealed that microsaccades and pupil sizes could be modulated simultaneously by some external stimuli and mental activities. However, the relationship between microsaccades and pupil responses is hard to be precisely illustrated due to inconsistent experimental parameters and participants in different studies. Therefore, to address these questions, three studies were conducted in which the microsaccades and pupil responses were recorded simultaneously.Firstly, we explored the effects of task difficulty in mental calculation on microsaccades and pupil sizes. The results were listed as follows:(1) microsaccades were significantly suppressed during intense mental processing, and the extent of such suppression increased with the increased task difficulty;(2) the pupil area increased with the increased task difficulty, and the peak pupil size during a trial was significantly correlated with the response time;(3) a strong negative correlation was observed between the dynamics of microsaccade rate and pupil size.Secondly, we explored the influences of blink on microsaccades and pupil sizes, and the role of these effects in visual processing. The results are summarized as follows:(1) there was an approximate 50% reduction in the microsaccade rate starting at about 150 ms prior to blink onset. After the end of blinking, the gaze position deviated around 1° from the fixation target and then returned to the baseline within approximately 200 ms. During this process, there were large fluctuations in microsaccade rates, amplitudes, and directions, which served to correct the fixation errors;(2) we observed a marked fluctuation in pupil size, which lasted approximately 2000 ms after the end of blinking. A small pupil dilation within the initial 200 ms after the end of blinking was followed by a strong constriction. During the constricting phase, the pupil size reached the minimum at approximately 700 ms after the end of blinking;(3) we observed an improved visual acuity during the period of 500-1000 ms after the end of blinking, and the variation of visual acuity was closely related to the change in pupil size.Thirdly, we examined the effects of peripheral tactile stimuli on the microsaccade rate and pupil size, which was compared with the responses evoked by brief visual stimuli. We found that:(1) There was a sharp decrease in microsaccade rate after the onset of tactile stimuli, and the microsaccade rate reached the minimum at approximately 150 ms after the stimuli, which was quite similar to the dynamics of microsaccade rated evoked by the visual stimuli;(2) the pupil dilated approximately 300 ms after the presentation of the tactile stimuli. In contrast, the pupil constricted much later after the onset of visual stimuli.By summarizing the results listed above, it can be inferred that the non-visual factors, such as the mental calculation and tactile stimuli, could trigger similar patterns of microsaccade and pupil response, i.e., decreased microsaccade rate and dilated pupil. In contrast, the effects of visual related factors, such as blinks and briefly presented visual stimuli, on microsaccades are more complex, depending on the current requirement of visual processing.