Abstract
Recent studies using video-based eye tracking have presented accumulating evidence that postsaccadic oscillation defined in reference to the pupil center (PSOp) is larger than that to the iris center (PSOi). This indicates that the relative motion of the pupil reflects the viscoelasticity of the tissue of the iris. It is known that the pupil size controlled by the sphincter/dilator pupillae muscles reflects many aspects of cognition. A hypothesis derived from this fact is that cognitive tasks affect the properties of PSOp due to the change in the state of these muscles. To test this hypothesis, we conducted pro- and antisaccade tasks for human participants and adopted the recent physical model of PSO to evaluate the dynamic properties of PSOp/PSOi. The results showed the dependence of the elasticity coefficient of the PSOp on the antisaccade task, but this effect was not significant for the PSOi. This suggests that cognitive tasks such as antisaccade tasks affect elasticity of the muscle of the iris. We found that the trial-by-trial fluctuation in the presaccade absolute pupil size correlated with the elasticity coefficient of PSOp. We also found the task dependence of the viscosity coefficient and overshoot amount of PSOi, which probably reflects the dynamics of the entire eyeball movement. The difference in task dependence between PSOp and PSOi indicates that the separate measures of these two can be means to distinguish factors related to the oculomotor neural system from those related to the physiological states of the iris tissue.NEW & NOTEWORTHY The state of the eyeball varies dynamically moment by moment depending on underlying neural/cognitive processing. Combining simultaneous measurements of pupil-centric and iris-centric movements and a recent physical model of postsaccadic oscillation (PSO), we show that the pupil-centric PSO is sensitive to the type of saccade task, suggesting that the physical state of the iris muscles reflects the underlying cognitive processes.
Highlights
The state of an eyeball varies dynamically moment by moment, reflecting the activity of various muscles, namely, extraocular muscles, sphincter/dilator pupillae muscles, and ciliary muscles
To visualize the mutual relationships among the parameters examined so far, we present the results of a path analysis
Based on the results described earlier and the rationale behind the present study, we hypothesized the following paths: task should affect the viscosity coefficient of PSOp/PSOi and the elasticity coefficient of PSOp
Summary
The state of an eyeball varies dynamically moment by moment, reflecting the activity of various muscles, namely, extraocular muscles (controlling gaze direction), sphincter/dilator pupillae muscles (controlling pupil diameter), and ciliary muscles (controlling lens thickness). It is possible that iris-centric or search coil signals reflect predominantly the rigid movements of the entire eyeball, whereas the pupil-centric signals reflect the viscoelastic movements of the tissue inside the iris (if the oscillation becomes overt, it is called “iridodonesis” in the clinical field; Desai and Tajik 2017). This possibility is supported by a recent study, which demonstrated that a physical model incorporating the dynamic movement of the viscoelastic iris can account for the experimental observation (Bouzat et al 2018)
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