Role of the frontal eye field in human microsaccade responses: A TMS study

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. Microsaccadic inhibition (MSI) refers to the abrupt suppression of microsaccades, typically evoked within 0.1 s 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 (∼0.25 s postevent 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.

Visual feature tuning properties of stimulus-driven saccadic inhibition in macaque monkeys

Saccadic inhibition refers to a short-latency transient cessation of saccade generation after visual sensory transients. This oculomotor phenomenon occurs with a latency that is consistent with a rapid influence of sensory responses, such as stimulus-induced visual bursts, on oculomotor control circuitry. However, the neural mechanisms underlying saccadic inhibition are not well understood. Here, we exploited the fact that macaque monkeys experience robust saccadic inhibition to test the hypothesis that inhibition time and strength exhibit systematic visual feature tuning properties to a multitude of visual feature dimensions commonly used in vision science. We measured saccades in three monkeys actively controlling their gaze on a target, and we presented visual onset events at random times. Across seven experiments, the visual onsets tested size, spatial frequency, contrast, orientation, motion direction, and motion speed dependencies of saccadic inhibition. We also investigated how inhibition might depend on the behavioral relevance of the appearing stimuli. We found that saccadic inhibition starts earlier, and is stronger, for large stimuli of low spatial frequencies and high contrasts. Moreover, saccadic inhibition timing depends on motion direction and orientation, with earlier inhibition systematically occurring for horizontally drifting vertical gratings. On the other hand, saccadic inhibition is stronger for faster motions and when the appearing stimuli are subsequently foveated. Besides documenting a range of feature tuning dimensions of saccadic inhibition to the properties of exogenous visual stimuli, our results establish macaque monkeys as an ideal model system for unraveling the neural mechanisms underlying a ubiquitous oculomotor phenomenon in visual neuroscience.NEW & NOTEWORTHY Visual onsets dramatically reduce saccade generation likelihood with very short latencies. Such latencies suggest that stimulus-induced visual responses, normally jump-starting perceptual and scene analysis processes, can also directly impact the decision of whether to generate saccades or not, causing saccadic inhibition. Consistent with this, we found that changing the appearance of the visual onsets systematically alters the properties of saccadic inhibition. These results constrain neurally inspired models of coordination between saccade generation and exogenous sensory stimulation.

Human microsaccade cueing modulation in visual- and memory-delay saccade tasks after theta burst transcranial magnetic stimulation over the frontal eye field

Microsaccades that occur during periods of fixation are modulated by various cognitive processes and have an impact on visual processing. A network of brain areas is involved in microsaccade generation, including the superior colliculus and frontal eye field (FEF) which are involved in modulating microsaccade rate and direction after the appearance of a visual cue (referred to as microsaccade cueing modulation). Although the neural mechanisms underlying microsaccade cueing modulations have been demonstrated in monkeys, limited research has investigated a causal role of these areas in humans. By applying continuous theta-burst transcranial magnetic stimulation (cTBS) over the right FEF and vertex, we investigated the role of human FEF in modulating microsaccade responses after the appearance of a visual target in a visual- and memory-delay saccade task. After target appearance, microsaccade rate was initially suppressed but then increased in both cTBS conditions. More importantly, in the visual-delay task, microsaccades after target appearance were directed to the ipsilateral side more often with FEF, compared to vertex stimulation. Moreover, microsaccades were directed towards the target location, then to the opposite location of the target in both tasks, with larger effects in the visual-, compared to, memory-delay task. This microsaccade direction modulation was delayed after FEF stimulation in the memory-delay task. Overall, some microsaccade cueing modulations were moderately disrupted after FEF cTBS, suggesting a causal role for involvement of the human FEF in microsaccade generation after presentation of salient stimuli.

Investigating the role of human frontal eye field in the pupil light reflex modulation by saccade planning and working memory

The pupil constricts in response to an increase in global luminance level, commonly referred to as the pupil light reflex. Recent research has shown that these reflex responses are modulated by high-level cognition. There is larger pupil constriction evoked by a bright stimulus when the stimulus location spatially overlaps with the locus of attention, and these effects have been extended to saccade planning and working memory (here referred to as pupil local-luminance modulation). Although research in monkeys has further elucidated a central role of the frontal eye field (FEF) and superior colliculus in the pupil local-luminance modulation, their roles remain to be established in humans. Through applying continuous theta-burst transcranial magnetic stimulation over the right FEF (and vertex) to inhibit its activity, we investigated the role of the FEF in human pupil local-luminance responses. Pupil light reflex responses were transiently evoked by a bright patch stimulus presented during the delay period in the visual- and memory-delay tasks. In the visual-delay task, larger pupil constriction was observed when the patch location was spatially aligned with the target location in both stimulation conditions. More interestingly, after FEF stimulation, larger pupil constriction was obtained when the patch was presented in the contralateral, compared to the ipsilateral visual field of the stimulation. In contrast, FEF stimulation effects were absence in the memory-delay task. Linear mixed model results further found that stimulation condition, patch location consistency, and visual field significantly modulated observed pupil constriction responses. Together, our results constitute the first evidence of FEF modulation in human pupil local-luminance responses.

Optic Flow Speed and Retinal Stimulation Influence Microsaccades

Microsaccades are linked with extraretinal mechanisms that significantly alter spatial perception before the onset of eye movements. We sought to investigate whether microsaccadic activity is modulated by the speed of radial optic flow stimuli. Experiments were performed in the dark on 19 subjects who stood in front of a screen covering 135 × 107° of the visual field. Subjects were instructed to fixate on a central fixation point while optic flow stimuli were presented in full field, in the foveal, and in the peripheral visual field at different dot speeds (8, 11, 14, 17, and 20°/s). Fixation in the dark was used as a control stimulus. For almost all tested speeds, the stimulation of the peripheral retina evoked the highest microsaccade rate. We also found combined effects of optic flow speed and the stimulated retinal region (foveal, peripheral, and full field) for microsaccade latency. These results show that optic flow speed modulates microsaccadic activity when presented in specific retinal portions, suggesting that eye movement generation is strictly dependent on the stimulated retinal regions.