Dependence of the stimulus-driven microsaccade rate signature in rhesus macaque monkeys on visual stimulus size and polarity

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.

Sensory tuning in neuronal movement commands

Movement control is critical for successful interaction with our environment. However, movement does not occur in complete isolation of sensation, and this is particularly true of eye movements. Here, we show that the neuronal eye movement commands emitted by the superior colliculus (SC), a structure classically associated with oculomotor control, encompass a robust visual sensory representation of eye movement targets. Thus, similar saccades toward different images are associated with different saccade-related "motor" bursts. Such sensory tuning in SC saccade motor commands appeared for all image manipulations that we tested, from simple visual features to real-life object images, and it was also strongest in the most motor neurons in the deeper collicular layers. Visual-feature discrimination performance in the motor commands was also stronger than in visual responses. Comparing SC motor command feature discrimination performance to that in the primary visual cortex during steady-state gaze fixation revealed that collicular motor bursts possess a reliable perisaccadic sensory representation of the peripheral saccade target's visual appearance, exactly when retinal input is expected to be most uncertain. Our results demonstrate that SC neuronal movement commands likely serve a fundamentally sensory function.

Stimulus salience conflicts and colludes with endogenous goals during urgent choices

Selecting where to look next depends on both the salience of objects and current goals (what we are looking for), but discerning their relative contributions over the time frame of typical visuomotor decisions (200-250 ms) has been difficult. Here we investigate this problem using an urgent choice task with which the two contributions can be dissociated and tracked moment by moment. Behavioral data from three monkeys corresponded with model-based predictions: when salience favored the target, perceptual performance evolved rapidly and steadily toward an asymptotic level; when salience favored the distracter, many rapid errors were produced and the rise in performance took more time-effects analogous to oculomotor and attentional capture. The results show that salience has a brief (∼50 ms) but inexorable impact that leads to exogenous, involuntary capture, and this can either help or hinder performance, depending on the alignment between salience and ongoing internal goals.

Saccadic inhibition during free viewing in macaque monkeys

Through the process of saccadic inhibition, visual events briefly suppress eye movements including microsaccades. In humans, saccadic inhibition has been shown to occur in response to the presentation of parafoveal or peripheral visual distractors during fixation and target-directed saccades and to physical changes of behaviorally relevant visual objects. In monkeys performing tasks that controlled eye movements, saccadic inhibition of microsaccades and target-directed saccades has been shown. Using eye data from three previously published studies, we investigated how saccade rate changed while monkeys were presented with visual stimuli under conditions with loose or no viewing demands. In two conditions, animals passively sat while an LED lamp flashed or screen-wide images appeared in front of them. In the third condition, images were repeated semiperiodically while animals had to maintain their gaze within a wide rectangular area and detect oddballs. Despite animals not being required to maintain fixation or make saccades to particular targets, the onset of visual events led to a temporary reduction of saccade rate across all conditions. Interestingly, saccadic inhibition was found at image offsets as well. These results show that saccadic inhibition occurs in monkeys during free viewing.NEW & NOTEWORTHY We investigated the time courses of saccade rate following visual stimuli during three conditions of free viewing in macaque monkeys. Under all conditions, saccade rate decreased transiently after the onset of visual stimuli. These results suggest that saccadic inhibition occurs during free viewing.

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

Microsaccade is a type of fixational eye movements that is modulated by various sensory and cognitive processes, and impact our visual perception. Although studies in monkeys have demonstrated a functional role for the superior colliculus and frontal eye field (FEF) in controlling microsaccades, our understanding of the neural mechanisms underlying the generation of microsaccades is still limited. By applying continuous theta-burst stimulation (cTBS) over the right FEF and the vertex, we investigated the role of the FEF in generating human microsaccade responses evoked by salient stimuli or by changes in background luminance. We observed higher microsaccade rates prior to target appearance, and larger rebound in microsaccade occurrence following salient stimuli, when disruptive cTBS was applied over FEF compared to vertex stimulation. Moreover, the microsaccade direction modulation after changes in background luminance was disrupted with FEF stimulation. Together, our results constitute the first evidence of FEF modulation in human microsaccade responses.

Time-dependent inhibition of covert shifts of attention

Visual transients can interrupt overt orienting by abolishing the execution of a planned eye movement due about 90 ms later, a phenomenon known as saccadic inhibition (SI). It is not known if the same inhibitory process might influence covert orienting in the absence of saccades, and consequently alter visual perception. In Experiment 1 (n = 14), we measured orientation discrimination during a covert orienting task in which an uninformative exogenous visual cue preceded the onset of an oriented probe by 140-290 ms. In half of the trials, the onset of the probe was accompanied by a brief irrelevant flash, a visual transient that would normally induce SI. We report a time-dependent inhibition of covert orienting in which the irrelevant flash impaired orientation discrimination accuracy when the probe followed the cue by 190 and 240 ms. The interference was more pronounced when the cue was incongruent with the probe location, suggesting an impact on the reorienting component of the attentional shift. In Experiment 2 (n = 12), we tested whether the inhibitory effect of the flash could occur within an earlier time range, or only within the later, reorienting range. We presented probes at congruent cue locations in a time window between 50 and 200 ms. Similar to Experiment 1, discrimination performance was altered at 200 ms after the cue. We suggest that covert attention may be susceptible to similar inhibitory mechanisms that generate SI, especially in later stages of attentional shifting (> 200 ms after a cue), typically associated with reorienting.

Instantaneous movement-unrelated midbrain activity modifies ongoing eye movements

At any moment in time, new information is sampled from the environment and interacts with ongoing brain state. Often, such interaction takes place within individual circuits that are capable of both mediating the internally ongoing plan as well as representing exogenous sensory events. Here, we investigated how sensory-driven neural activity can be integrated, very often in the same neuron types, into ongoing saccade motor commands. Despite the ballistic nature of saccades, visually induced action potentials in the rhesus macaque superior colliculus (SC), a structure known to drive eye movements, not only occurred intra-saccadically, but they were also associated with highly predictable modifications of ongoing eye movements. Such predictable modifications reflected a simultaneity of movement-related discharge at one SC site and visually induced activity at another. Our results suggest instantaneous readout of the SC during movement generation, irrespective of activity source, and they explain a significant component of kinematic variability of motor outputs.

Dissociable Cortical and Subcortical Mechanisms for Mediating the Influences of Visual Cues on Microsaccadic Eye Movements

Visual selection in primates is intricately linked to eye movements, which are generated by a network of cortical and subcortical neural circuits. When visual selection is performed covertly, without foveating eye movements toward the selected targets, a class of fixational eye movements, called microsaccades, is still involved. Microsaccades are small saccades that occur when maintaining precise gaze fixation on a stationary point, and they exhibit robust modulations in peripheral cueing paradigms used to investigate covert visual selection mechanisms. These modulations consist of changes in both microsaccade directions and frequencies after cue onsets. Over the past two decades, the properties and functional implications of these modulations have been heavily studied, revealing a potentially important role for microsaccades in mediating covert visual selection effects. However, the neural mechanisms underlying cueing effects on microsaccades are only beginning to be investigated. Here we review the available causal manipulation evidence for these effects' cortical and subcortical substrates. In the superior colliculus (SC), activity representing peripheral visual cues strongly influences microsaccade direction, but not frequency, modulations. In the cortical frontal eye fields (FEF), activity only compensates for early reflexive effects of cues on microsaccades. Using evidence from behavior, theoretical modeling, and preliminary lesion data from the primary visual cortex and microstimulation data from the lower brainstem, we argue that the early reflexive microsaccade effects arise subcortically, downstream of the SC. Overall, studying cueing effects on microsaccades in primates represents an important opportunity to link perception, cognition, and action through unaddressed cortical-subcortical neural interactions. These interactions are also likely relevant in other sensory and motor modalities during other active behaviors.