Cross-modal interactions between auditory attention and oculomotor control.

Microsaccades are small, involuntary eye movements that occur during fixation. Their role is debated with recent hypotheses proposing a contribution to automatic scene sampling. Microsaccade inhibition (MSI) refers to the abrupt suppression of microsaccades, typically evoked within 0.1 seconds after new stimulus onset. The functional significance and neural underpinnings of MSI are subjects of ongoing research. It has been suggested that MSI is a component of the brain's attentional re-orienting network which facilitates the allocation of attention to new environmental occurrences by reducing disruptions or shifts in gaze that could interfere with processing.The extent to which MSI is reflexive or influenced by top-down mechanisms remains debated. We developed a task that examines the impact of auditory top-down attention on MSI, allowing us to disentangle ocular dynamics from visual sensory processing. Participants (N=24 and 27; both sexes) listened to two simultaneous streams of tones and were instructed to attend to one stream while detecting specific task "targets." We quantified MSI in response to occasional task-irrelevant events presented in both the attended and unattended streams (frequency steps in Experiment 1, omissions in Experiment 2).The results show that initial stages of MSI are not affected by auditory attention. However, later stages (∼0.25s post event-onset), affecting the extent and duration of the inhibition, are enhanced for sounds in the attended stream compared to the unattended stream. These findings provide converging evidence for the reflexive nature of early MSI stages and robustly demonstrate the involvement of auditory attention in modulating the later stages.Significance Statement Microsaccades are rapid eye movements occurring during fixation. Their precise role is not known but a major hypothesis is that they reflect automatic sampling of the environment. A feature of microsaccades is that they exhibit abrupt suppression (MSI) after the presentation of new stimuli. This is thought to be part of attentional re-orienting. To understand the neural circuit that controls MSI, and by extension, the brain's response to novel events in the environment, it is essential to determine which factors affect MSI. We show, for the first time, that auditory attention affects the latter (but not initial) stages of MSI. Thus, the early stages of MSI are automatic, but subsequent phases are affected by the perceptual state of the individual.