Abstract
The eyes are never still during maintained gaze fixation. When microsaccades are not occurring, ocular position exhibits continuous slow changes, often referred to as drifts. Unlike microsaccades, drifts remain to be viewed as largely random eye movements. Here we found that ocular position drifts can, instead, be very systematically stimulus-driven, and with very short latencies. We used highly precise eye tracking in three well trained macaque monkeys and found that even fleeting (~8 ms duration) stimulus presentations can robustly trigger transient and stimulus-specific modulations of ocular position drifts, and with only approximately 60 ms latency. Such drift responses are binocular, and they are most effectively elicited with large stimuli of low spatial frequency. Intriguingly, the drift responses exhibit some image pattern selectivity, and they are not explained by convergence responses, pupil constrictions, head movements, or starting eye positions. Ocular position drifts have very rapid access to exogenous visual information.
Highlights
Eye position changes subtly even when perfect gaze fixation is attempted (Barlow, 1952; Nachmias, 1959, 1961; Ratliff & Riggs, 1950; Steinman, Haddad, Skavenski, &Wyman, 1973)
Such “fixational” eye movement comes in two primary flavors: microsaccades, which resemble large saccades (Hafed, 2011; Rolfs, 2009; Zuber, Stark, & Cook, 1965) and rapidly shift gaze position by a minute amount; and ocular position drifts, which are even smaller and slower eye movements (Barlow, 1952; Ditchburn & Ginsborg, 1953; Martins, Kowler, & Palmer, 1985; Nachmias, 1959, 1961; Poletti & Rucci, 2015; Ratliff & Riggs, 1950; Skinner, Buonocore, & Hafed, 2019; St Cyr & Fender, 1969)
The drift response is neither an eye tracking artifact nor a convergence reflex We considered the possibility that the stimulus-driven drift response was part of a near reflex triad (Myers & Stark, 1990; Nachmias, 1961)
Summary
Such “fixational” eye movement comes in two primary flavors: microsaccades, which resemble large saccades (Hafed, 2011; Rolfs, 2009; Zuber, Stark, & Cook, 1965) and rapidly shift gaze position by a minute amount; and ocular position drifts, which are even smaller and slower eye movements (Barlow, 1952; Ditchburn & Ginsborg, 1953; Martins, Kowler, & Palmer, 1985; Nachmias, 1959, 1961; Poletti & Rucci, 2015; Ratliff & Riggs, 1950; Skinner, Buonocore, & Hafed, 2019; St Cyr & Fender, 1969). The movements could still be random walks but have statistics sufficient to modulate retinal image information for the purposes of aiding spatial vision (Kuang et al, 2012; Rucci & Victor, 2015) This leaves the question of how precisely these eye movements may potentially be controlled still unanswered. Their amplitudes are sufficient to move images across individual cone photoreceptors in the fovea or multiple rod photoreceptors in some peripheral zones
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