Contributions and Challenges of Fixational Instability to Visual Perception

Martina Poletti

Boston University

During periods of fixation, slow drifts and microscopic saccades continually perturb
the projection of the visual scene onto the retina. It has long been debated whether, during
natural viewing, fixational eye movements contribute to visual perception. We analyzed visual
discrimination in the presence and in the absence of the retinal image motion caused by
fixational eye movements. Our results show that the luminance modulations resulting from
fixational instability improve discrimination of visual detail. 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.  Results of a
modeling study, obtained by simulating the responses of retinal ganglion cells in these
experiments, reveal that fixational eye movements influence the retinal synchronous activity,
suggesting that such synchronization is a key component of the neural mechanisms that
promote perception of spatial detail. However, besides improving discrimination of details,
the incessant retinal motion introduced by ocular drift constitutes a problem for visual stability;
how does the brain know if the motion of an object on the retina is caused by fixational
instability or external motion? It has long been debated whether perceptual cancellation of this
motion relies on motor/proprioceptive information, or whether eye movements are instead
inferred directly from the retinal stimulus. In another study, we examined the mechanisms
underlying visual stability during ocular drift. In order to separate retinal and extraretinal
contributions we used a gaze-contingent display system that enabled decoupling of eye
movements from the changes in visual input that they normally cause.  We show that the
visual system relies on the spatiotemporal stimulus on the retina, rather than on extraretinal
information, to discard the motion signals resulting from ocular drift.  These results suggest
that failure to visually determine eye drift contributes to well-known motion illusions such as
autokinesis and induced movement.