Coordination of Pupil and Saccade Responses by the Superior Colliculus

Pupil size correlates with heart rate, skin conductance, pulse wave amplitude, and respiration responses during emotional conflict and valence processing

Pupil size is a non-invasive index for autonomic arousal mediated by the locus coeruleus-norepinephrine (LC-NE) system. While pupil size and its derivative (velocity) are increasingly used as indicators of arousal, limited research has investigated the relationships between pupil size and other well-known autonomic responses. Here, we simultaneously recorded pupillometry, heart rate, skin conductance, pulse wave amplitude, and respiration signals during an emotional face-word Stroop task, in which task-evoked (phasic) pupil dilation correlates with LC-NE responsivity. We hypothesized that emotional conflict and valence would affect pupil and other autonomic responses, and trial-by-trial correlations between pupil and other autonomic responses would be observed during both tonic and phasic epochs. Larger pupil dilations, higher pupil size derivative, and lower heart rates were observed in the incongruent condition compared to the congruent condition. Additionally, following incongruent trials, the congruency effect was reduced, and arousal levels indexed by previous-trial pupil dilation were correlated with subsequent reaction times. Furthermore, linear mixed models revealed that larger pupil dilations correlated with higher heart rates, higher skin conductance responses, higher respiration amplitudes, and lower pulse wave amplitudes on a trial-by-trial basis. Similar effects were seen between positive and negative valence conditions. Moreover, tonic pupil size before stimulus presentation significantly correlated with all other tonic autonomic responses, whereas tonic pupil size derivative correlated with heart rates and skin conductance responses. These results demonstrate a trial-by-trial relationship between pupil dynamics and other autonomic responses, highlighting pupil size as an effective real-time index for autonomic arousal during emotional conflict and valence processing.

Age-related changes in pupil dynamics and task modulation across the healthy lifespan

The pupil is modulated by luminance, arousal, bottom-up sensory, and top-down cognitive signals, and has increasingly been used to assess these aspects of brain functioning in health and disease. However, changes in pupil dynamics across the lifespan have not been extensively examined, hindering our ability to fully utilize the pupil in probing these underlying neural processes in development and aging in healthy and clinical cohorts. Here, we examined pupil responses during the interleaved pro-/anti-saccade task (IPAST) in healthy participants across the lifespan (n = 567, 5-93 years of age). Based on the extracted measurements of pupil dynamics, we demonstrated age-related changes in pupil measures and task modulation. Moreover, we characterized the underlying factors and age-related effects in components of pupil responses that may be attributed to developmental and aging changes in the associated brain regions. Finally, correlations between factors of pupil dynamics and saccade behaviors revealed evidence of shared neural processes in the pupil and saccade control circuitries. Together, these results demonstrate changes in pupil dynamics as a result of development and aging, providing a baseline with which altered pupil responses due to neurological deficits at different ages can be studied.

Pupil Size and Eye Movements Differently Index Effort in Both Younger and Older Adults

The assessment of mental effort is increasingly relevant in neurocognitive and life span domains. Pupillometry, the measure of the pupil size, is often used to assess effort but has disadvantages. Analysis of eye movements may provide an alternative, but research has been limited to easy and difficult task demands in younger adults. An effort measure must be sensitive to the whole effort profile, including "giving up" effort investment, and capture effort in different age groups. The current study comprised three experiments in which younger (n = 66) and older (n = 44) adults listened to speech masked by background babble at different signal-to-noise ratios associated with easy, difficult, and impossible speech comprehension. We expected individuals to invest little effort for easy and impossible speech (giving up) but to exert effort for difficult speech. Indeed, pupil size was largest for difficult but lower for easy and impossible speech. In contrast, gaze dispersion decreased with increasing speech masking in both age groups. Critically, gaze dispersion during difficult speech returned to levels similar to easy speech after sentence offset, when acoustic stimulation was similar across conditions, whereas gaze dispersion during impossible speech continued to be reduced. These findings show that a reduction in eye movements is not a byproduct of acoustic factors, but instead suggest that neurocognitive processes, different from arousal-related systems regulating the pupil size, drive reduced eye movements during high task demands. The current data thus show that effort in one sensory domain (audition) differentially impacts distinct functional properties in another sensory domain (vision).

Post-Saccadic Oscillations of the Pupil and Lens Reduce Fixation Stability in Retinitis Pigmentosa and Age-Related Macular Degeneration

Purpose

Post-saccadic oscillations (PSOs) reflect movements of gaze that result from motion of the pupil and lens relative to the eyeball rather than eyeball rotations. Here, we analyzed the characteristics of PSOs in subjects with age-related macular degeneration (AMD), retinitis pigmentosa (RP), and normal vision (NV). Our aim was to assess the differences in PSOs between people with vision loss and healthy controls because PSOs affect retinal image stability after each saccade.

Methods

Participants completed a horizontal saccade task and their gaze was measured using a pupil-based eye tracker. Oscillations occurring in the 80 to 200 ms post-saccadic period were described with a damped oscillation model. We compared the amplitude, decay time constant, and frequency of the PSOs for the three different groups. We also examined the correlation between these PSO parameters and the amplitude, peak velocity, and final deceleration of the preceding saccades.

Results

Subjects with vision loss (AMD, n = 6, and RP, n = 5) had larger oscillation amplitudes, longer decay constants, and lower frequencies than subjects with NV (n = 7). The oscillation amplitudes increased with increases in saccade deceleration in all three groups. The other PSO parameters, however, did not show consistent correlations with either saccade amplitude or peak velocity.

Conclusions

Post-saccadic fixation stability in AMD and RP is reduced due to abnormal PSOs. The differences with respect to NV are not due to differences in saccade kinematics, suggesting that anatomic and neuronal variations affect the suspension of the iris and the lens in the patients' eyes.

Brain-wide arousal signals are segregated from movement planning in the superior colliculus

The superior colliculus (SC) is traditionally considered a brain region that functions as an interface between processing visual inputs and generating eye movement outputs. Although its role as a primary reflex center is thought to be conserved across vertebrate species, evidence suggests that the SC has evolved to support higher-order cognitive functions including spatial attention. When it comes to oculomotor areas such as the SC, it is critical that high precision fixation and eye movements are maintained even in the presence of signals related to ongoing changes in cognition and brain state, both of which have the potential to interfere with eye position encoding and movement generation. In this study, we recorded spiking responses of neuronal populations in the SC while monkeys performed a memory-guided saccade task and found that the activity of some of the neurons fluctuated over tens of minutes. By leveraging the statistical power afforded by high-dimensional neuronal recordings, we were able to identify a low-dimensional pattern of activity that was correlated with the subjects' arousal levels. Importantly, we found that the spiking responses of deep-layer SC neurons were less correlated with this brain-wide arousal signal, and that neural activity associated with changes in pupil size and saccade tuning did not overlap in population activity space with movement initiation signals. Taken together, these findings provide a framework for understanding how signals related to cognition and arousal can be embedded in the population activity of oculomotor structures without compromising the fidelity of the motor output.

Time-on-task effects on human pupillary and saccadic metrics after theta burst transcranial magnetic stimulation over the frontal eye field

Pupil size undergoes constant changes primarily influenced by ambient luminance. These changes are referred to as the pupillary light reflex (PLR), where the pupil transiently constricts in response to light. PLR kinematics provides valuable insights into autonomic nervous system function and have significant clinical applications. Recent research indicates that attention plays a role in modulating the PLR, and the circuit involving the frontal eye field (FEF) and superior colliculus is causally involved in controlling this pupillary modulation. However, there is limited research exploring the role of the human FEF in these pupillary responses, and its impact on PLR metrics remains unexplored. Additionally, although the protocol of continuous theta-burst stimulation (cTBS) is well-established, the period of disruption after cTBS is yet to be examined in pupillary responses. Our study aimed to investigate the effects of FEF cTBS on pupillary and saccadic metrics in relation to time spent performing a task (referred to as time-on-task). We presented a bright stimulus to induce the PLR in visual- and memory-delay saccade tasks following cTBS over the right FEF or vertex. FEF cTBS, compared to vertex cTBS, resulted in decreased baseline pupil size, peak constriction velocities, and amplitude. Furthermore, the time-on-task effects on baseline pupil size, peak amplitude, and peak time differed between the two stimulation conditions. In contrast, the time-on-task effects on saccadic metrics were less pronounced between the two conditions. In summary, our study provides the first evidence that FEF cTBS affects human PLR metrics and that these effects are modulated by time-on-task.

Eye Movements Decrease during Effortful Speech Listening

Hearing impairment affects many older adults but is often diagnosed decades after speech comprehension in noisy situations has become effortful. Accurate assessment of listening effort may thus help diagnose hearing impairment earlier. However, pupillometry-the most used approach to assess listening effort-has limitations that hinder its use in practice. The current study explores a novel way to assess listening effort through eye movements. Building on cognitive and neurophysiological work, we examine the hypothesis that eye movements decrease when speech listening becomes challenging. In three experiments with human participants from both sexes, we demonstrate, consistent with this hypothesis, that fixation duration increases and spatial gaze dispersion decreases with increasing speech masking. Eye movements decreased during effortful speech listening for different visual scenes (free viewing, object tracking) and speech materials (simple sentences, naturalistic stories). In contrast, pupillometry was less sensitive to speech masking during story listening, suggesting pupillometric measures may not be as effective for the assessments of listening effort in naturalistic speech-listening paradigms. Our results reveal a critical link between eye movements and cognitive load, suggesting that neural activity in the brain regions that support the regulation of eye movements, such as frontal eye field and superior colliculus, are modulated when listening is effortful.SIGNIFICANCE STATEMENT Assessment of listening effort is critical for early diagnosis of age-related hearing loss. Pupillometry is most used but has several disadvantages. The current study explores a novel way to assess listening effort through eye movements. We examine the hypothesis that eye movements decrease when speech listening becomes effortful. We demonstrate, consistent with this hypothesis, that fixation duration increases and gaze dispersion decreases with increasing speech masking. Eye movements decreased during effortful speech listening for different visual scenes (free viewing, object tracking) and speech materials (sentences, naturalistic stories). Our results reveal a critical link between eye movements and cognitive load, suggesting that neural activity in brain regions that support the regulation of eye movements are modulated when listening is effortful.

The Costs of Paying Overt and Covert Attention Assessed With Pupillometry

Attention can be shifted with or without an accompanying saccade (i.e., overtly or covertly, respectively). Thus far, it is unknown how cognitively costly these shifts are, yet such quantification is necessary to understand how and when attention is deployed overtly or covertly. In our first experiment (N = 24 adults), we used pupillometry to show that shifting attention overtly is more costly than shifting attention covertly, likely because planning saccades is more complex. We pose that these differential costs will, in part, determine whether attention is shifted overtly or covertly in a given context. A subsequent experiment (N = 24 adults) showed that relatively complex oblique saccades are more costly than relatively simple saccades in horizontal or vertical directions. This provides a possible explanation for the cardinal-direction bias of saccades. The utility of a cost perspective as presented here is vital to furthering our understanding of the multitude of decisions involved in processing and interacting with the external world efficiently.

Fixation offset decreases pupillary inhibition of return

Inhibition of return (IOR) is reflected as a slower manual or saccadic response to a cued rather than an uncued target (manual IOR and saccadic IOR, respectively), and as a pupillary dilation when a bright, relative to a dark side of a display is cued (pupillary IOR). The aim of this study was to investigate the relation between an IOR and oculomotor system. According to the predominant view, only the saccadic IOR is strictly related to the visuomotor process, and the manual and pupillary IORs depend on non-motor factors (e.g., short-term visual depression). Alternatively, the after-effect of the covert-orienting hypothesis postulates that IOR is strictly related to the oculomotor system. As fixation offset affects oculomotor processes, this study investigated whether fixation offset also affects pupillary and manual IORs. The results show that fixation offset decreased IOR in pupillary but not manual responses, and provides support for the hypothesis that at least the pupillary IOR is tightly linked to eye movement preparation.

Neural α Oscillations and Pupil Size Differentially Index Cognitive Demand under Competing Audiovisual Task Conditions

Cognitive demand is thought to modulate two often used, but rarely combined, measures: pupil size and neural α (8-12 Hz) oscillatory power. However, it is unclear whether these two measures capture cognitive demand in a similar way under complex audiovisual-task conditions. Here we recorded pupil size and neural α power (using electroencephalography), while human participants of both sexes concurrently performed a visual multiple object-tracking task and an auditory gap detection task. Difficulties of the two tasks were manipulated independent of each other. Participants' performance decreased in accuracy and speed with increasing cognitive demand. Pupil size increased with increasing difficulty for both the auditory and the visual task. In contrast, α power showed diverging neural dynamics: parietal α power decreased with increasing difficulty in the visual task, but not with increasing difficulty in the auditory task. Furthermore, independent of task difficulty, within-participant trial-by-trial fluctuations in pupil size were negatively correlated with α power. Difficulty-induced changes in pupil size and α power, however, did not correlate, which is consistent with their different cognitive-demand sensitivities. Overall, the current study demonstrates that the dynamics of the neurophysiological indices of cognitive demand and associated effort are multifaceted and potentially modality-dependent under complex audiovisual-task conditions.SIGNIFICANCE STATEMENT Pupil size and oscillatory α power are associated with cognitive demand and effort, but their relative sensitivity under complex audiovisual-task conditions is unclear, as is the extent to which they share underlying mechanisms. Using an audiovisual dual-task paradigm, we show that pupil size increases with increasing cognitive demands for both audition and vision. In contrast, changes in oscillatory α power depend on the respective task demands: parietal α power decreases with visual demand but not with auditory task demand. Hence, pupil size and α power show different sensitivity to cognitive demands, perhaps suggesting partly different underlying neural mechanisms.

Differential aspects of attention predict the depth of visual working memory encoding: Evidence from pupillometry

What determines how much one encodes into visual working memory? Traditionally, encoding depth is considered to be indexed by spatiotemporal properties of gaze, such as gaze position and dwell time. Although these properties inform about where and how long one looks, they do not necessarily inform about the current arousal state or how strongly attention is deployed to facilitate encoding. Here, we found that two types of pupillary dynamics predict how much information is encoded during a copy task. The task involved encoding a spatial pattern of multiple items for later reproduction. Results showed that smaller baseline pupil sizes preceding and stronger pupil orienting responses during encoding predicted that more information was encoded into visual working memory. Additionally, we show that pupil size reflects not only how much but also how precisely material is encoded. We argue that a smaller pupil size preceding encoding is related to increased exploitation, whereas larger pupil constrictions signal stronger attentional (re)orienting to the to-be-encoded pattern. Our findings support the notion that the depth of visual working memory encoding is the integrative outcome of differential aspects of attention: how alert one is, how much attention one deploys, and how long it is deployed. Together, these factors determine how much information is encoded into visual working memory.

Salient omissions-pupil dilation in response to unexpected omissions of sound and touch

Introduction

Recent theories describe perception as an inferential process based on internal predictive models adjusted by means of prediction violations (prediction error). To study and demonstrate predictive processing in the brain the use of unexpected stimulus omissions has been suggested as a promising approach as the evoked brain responses are uncontaminated by responses to stimuli. Here, we aimed to investigate the pupil's response to unexpected stimulus omissions in order to better understand surprise and orienting of attention resulting from prediction violation. So far only few studies have used omission in pupillometry research and results have been inconsistent.

Methods

This study adapted an EEG paradigm that has been shown to elicit omission responses in auditory and somatosensory modalities. Healthy adults pressed a button at their own pace, which resulted in the presentation of sounds or tactile stimuli in either 88%, 50% or 0% (motor-control) of cases. Pupil size was recorded continuously and averaged to analyze the pupil dilation response associated with each condition.

Results

Results revealed that omission responses were observed in both modalities in the 88%-condition compared to motor-control. Similar pupil omission responses were observed between modalities, suggesting modality-unspecific activation of the underlying brain circuits.

Discussion

In combination with previous omission studies using EEG, the findings demonstrate predictive models in brain processing and point to the involvement of subcortical structures in the omission response. Our pupillometry approach is especially suitable to study sensory prediction in vulnerable populations within the psychiatric field.

Pupillary dynamics reflect the impact of temporal expectation on detection strategy

Everyday life's perceptual decision-making is informed by experience. In particular, temporal expectation can ease the detection of relevant events in noisy sensory streams. Here, we investigated if humans can extract hidden temporal cues from the occurrences of probabilistic targets and utilize them to inform target detection in a complex acoustic stream. To understand what neural mechanisms implement temporal expectation influence on decision-making, we used pupillometry as a proxy for underlying neuromodulatory activity. We found that participants' detection strategy was influenced by the hidden temporal context and correlated with sound-evoked pupil dilation. A model of urgency fitted on false alarms predicted detection reaction time. Altogether, these findings suggest that temporal expectation informs decision-making and could be implemented through neuromodulatory-mediated urgency signals.

Building a decoder of perceptual decisions from microsaccades and pupil size

Many studies have reported neural correlates of visual awareness across several brain regions, including the sensory, parietal, and frontal areas. In most of these studies, participants were instructed to explicitly report their perceptual experience through a button press or verbal report. It is conceivable, however, that explicit reporting itself may trigger specific neural responses that can confound the direct examination of the neural correlates of visual awareness. This suggests the need to assess visual awareness without explicit reporting. One way to achieve this is to develop a technique to predict the visual awareness of participants based on their peripheral responses. Here, we used eye movements and pupil sizes to decode trial-by-trial changes in the awareness of a stimulus whose visibility was deteriorated due to adaptation-induced blindness (AIB). In the experiment, participants judged whether they perceived a target stimulus and rated the confidence they had in their perceptual judgment, while their eye movements and pupil sizes were recorded. We found that not only perceptual decision but also perceptual confidence can be separately decoded from the eye movement and pupil size. We discuss the potential of this technique with regard to assessing visual awareness in future neuroimaging experiments.

Fatigue and Arousal Modulations Revealed by Saccade and Pupil Dynamics

Saccadic eye movements are directed to the objects of interests and enable high-resolution visual images in the exploration of the visual world. There is a trial-to-trial variation in saccade dynamics even in a simple task, possibly attributed to arousal fluctuations. Previous studies have showed that an increase of fatigue level over time, also known as time-on-task, can be revealed by saccade peak velocity. In addition, pupil size, controlled by the autonomic nervous system, has long been used as an arousal index. However, limited research has been done with regards to the relation between pupil size and saccade behavior in the context of trial-to-trial variation. To investigate fatigue and arousal effects on saccadic and pupillary responses, we used bright and emotional stimuli to evoke pupillary responses in tasks requiring reactive and voluntary saccade generation. Decreased voluntary saccade peak velocities, reduced tonic pupil size and phasic pupillary responses were observed as time-on-task increased. Moreover, tonic pupil size affected saccade latency and dynamics, with steeper saccade main sequence slope as tonic pupil size increased. In summary, saccade dynamics and tonic pupil size were sensitive to fatigue and arousal level, together providing valuable information for the understanding of human behavior.

Pupillometry as an integrated readout of distinct attentional networks

The course of pupillary constriction and dilation provides an easy-to-access, inexpensive, and noninvasive readout of brain activity. We propose a new taxonomy of factors affecting the pupil and link these to associated neural underpinnings in an ascending hierarchy. In addition to two well-established low-level factors (light level and focal distance), we suggest two further intermediate-level factors, alerting and orienting, and a higher-level factor, executive functioning. Alerting, orienting, and executive functioning - including their respective underlying neural circuitries - overlap with the three principal attentional networks, making pupil size an integrated readout of distinct states of attention. As a now widespread technique, pupillometry is ready to provide meaningful applications and constitutes a viable part of the psychophysiological toolbox.

Pupil-linked Arousal Signals in the Midbrain Superior Colliculus

The orienting response evoked by the appearance of a salient stimulus is modulated by arousal; however, neural underpinnings for the interplay between orienting and arousal are not well understood. The superior colliculus (SC), causally involved in multiple components of the orienting response including gaze and attention shifts, receives not only multisensory and cognitive inputs but also arousal-regulated inputs from various cortical and subcortical structures. To investigate the impact of moment-by-moment fluctuations in arousal on orienting saccade responses, we used microstimulation of the monkey SC to trigger saccade responses, and we used pupil size and velocity to index the level of arousal at stimulation onset because these measures correlate with changes in brain states and locus coeruleus activity. Saccades induced by SC microstimulation correlated with prestimulation pupil velocity, with higher pupil velocities on trials without evoked saccades than with evoked saccades. In contrast, prestimulation absolute pupil size did not correlate with saccade behavior. However, pupil velocity correlated with evoked saccade latency and metrics. Together, our results demonstrated that small fluctuations in arousal, indexed by pupil velocity, can modulate the saccade response evoked by SC microstimulation in awake behaving monkeys.

A unified model of the task-evoked pupil response

The pupil dilates and reconstricts following task events. It is popular to model this task-evoked pupil response as a linear transformation of event-locked impulses, whose amplitudes are used as estimates of arousal. We show that this model is incorrect and propose an alternative model based on the physiological finding that a common neural input drives saccades and pupil size. The estimates of arousal from our model agreed with key predictions: Arousal scaled with task difficulty and behavioral performance but was invariant to small differences in trial duration. Moreover, the model offers a unified explanation for a wide range of phenomena: entrainment of pupil size and saccades to task timing, modulation of pupil response amplitude and noise with task difficulty, reaction time-dependent modulation of pupil response timing and amplitude, a constrictory pupil response time-locked to saccades, and task-dependent distortion of this saccade-locked pupil response.

Influence of the Location of a Decision Cue on the Dynamics of Pupillary Light Response

The pupils of the eyes reflexively constrict in light and dilate in dark to optimize retinal illumination. Non-visual cognitive factors, like attention, arousal, decision-making, etc., also influence pupillary light response (PLR). During passive viewing, the eccentricity of a stimulus modulates the pupillary aperture size driven by spatially weighted corneal flux density (CFD), which is the product of luminance and the area of the stimulus. Whether the scope of attention also influences PLR remains unclear. In this study, we contrasted the pupil dynamics between diffused and focused attentional conditions during decision-making, while the global CFD remained the same in the two conditions. A population of 20 healthy humans participated in a pair of forced choice tasks. They distributed attention to the peripheral decision cue in one task, and concentrated at the center in the other to select the target from four alternatives for gaze orientation. The location of this cue did not influence participants' reaction time (RT). However, the magnitude of constriction was significantly less in the task that warranted attention to be deployed at the center than on the periphery. We observed similar pupil dynamics when participants either elicited or canceled a saccadic eye movement, which ruled out pre-saccadic obligatory attentional orientation contributing to PLR. We further addressed how the location of attentional deployment might have influenced PLR. We simulated a biomechanical model of PLR with visual stimulation of different strengths as inputs corresponding to the two attentional conditions. In this homeomorphic model, the computational characteristic of each element was derived from the physiological and/or mechanical properties of the corresponding biological element. The simulation of this model successfully mimicked the observed data. In contrast to common belief that the global ambient luminosity drives pupillary response, the results of our study suggest that the effective CFD (eCFD) determined via the luminance multiplied by the size of the stimulus at the location of deployed attention in the visual space is critical for the magnitude of pupillary constriction.

A Novel Tectal/Pretectal Population of Premotor Lens Accommodation Neurons

Purpose

Under real-world conditions, saccades are often accompanied by changes in vergence angle and lens accommodation that compensate for changes in the distance between the current fixation point and the next target. As the superior colliculus directs saccades, we examined whether it contains premotor neurons that might control lens compensation for target distance.

Methods

Rabies virus or recombinant rabies virus was injected into the ciliary bodies of Macaca fascicularis monkeys to label circuits controlling lens accommodation via retrograde transsynaptic transport. In addition, conventional anterograde tracers were used to confirm the rabies findings with respect to projections to preganglionic Edinger–Westphal motoneurons.

Results

At time courses that rabies virus labeled lens-related premotor neurons in the supraoculomotor area and central mesencephalic reticular formation, labeled neurons were not found within the superior colliculus. They were, however, found bilaterally in the medial pretectal nucleus continuing caudally into the tectal longitudinal column, which lies on the midline, between the colliculi. A bilateral projection by this area to the preganglionic Edinger–Westphal nucleus was confirmed by anterograde tracing. Only at longer time courses were cells labeled in the superior colliculus.

Conclusions

The superior colliculus does not provide premotor input to preganglionic Edinger–Westphal nucleus motoneurons, but may provide input to lens-related premotor populations in the supraoculomotor area and central mesencephalic reticular formation. There is, however, a novel third population of lens-related premotor neurons in the tectal longitudinal column and rostrally adjacent medial pretectal nucleus. The specific function of this premotor population remains to be determined.

Increasing pupil size is associated with improved detection performance in the periphery

Visible light enters our body via the pupil. By changing its size, the pupil shapes visual input. Small apertures increase the resolution of high spatial frequencies, thus allowing discrimination of fine details. Large apertures, in contrast, provide a better signal-to-noise ratio, because more light can enter the eye. This should lead to better detection performance of peripheral stimuli. Experiment 1 shows that the effect can reliably be demonstrated even in a less controlled online setting. In Experiment 2, pupil size was measured in a laboratory using an eye tracker. The findings replicate findings showing that large pupils provide an advantage for peripheral detection of faint stimuli. Moreover, not only pupil size during information intake in the current trial n, but also its interaction with pupil size preceding information intake, i.e., in trial n-1, predicted performance. This suggests that in addition to absolute pupil size, the extent of pupillary change provides a mechanism to modulate perceptual functions. The results are discussed in terms of low-level sensory as well as higher-level arousal-driven changes in stimulus processing.

Enlargement of female pupils when perceiving something cute

It is reported that women's pupils dilate when they see a baby; it is unclear if this pupillary response is caused by the perception of cuteness itself. Since many objects besides babies can be perceived as cute, we investigated whether the perception of cuteness, or the type of object observed, is related to pupil dilation. In the first experiment, female participants were requested to rate the subjective cuteness of greyscale pictures of objects such as animals and foods; their pupil sizes were measured. The results showed a significant positive correlation between perceived cuteness and participants' pupil dilation. In the second experiment, participants rated the cuteness of images of female faces. Results revealed a significant negative correlation between perceived cuteness and pupil dilation. In our study, perceiving cuteness enlarged female observers' pupils except when observing female faces. Positive reactions associated with cuteness may be premised on the existence of unconscious perceptual alterations and physical responses.

Linking Pupil Size Modulated by Global Luminance and Motor Preparation to Saccade Behavior

Saccades are rapid eye movements that are used to move the high acuity fovea in a serial manner in the exploration of the visual scene. Stimulus contrast is known to modulate saccade latency and metrics possibly via changing visual activity in the superior colliculus (SC), a midbrain structure causally involved in saccade generation. However, the quality of visual signals should also be modulated by the amount of lights projected onto the retina, which is gated by the size of the pupil. Although absolute pupil size should modulate visual signals and in turn affect saccade responses, research examining this relationship is very limited. Besides, pupil size is associated with motor preparation. However, the role of pupil dilation in saccade metrics remains unexplored. Through varying peripheral background luminance level and target visual contrast in the saccade task, we investigated the role of absolute pupil size and baseline-corrected pupil dilation in saccade latency and metrics. Higher target detection accuracy was obtained with lower background luminance level, and larger absolute pupil diameter correlated with smaller saccade amplitude and higher saccade peak velocities. More interestingly, the comparable modulation between pupil dilation and stimulus contrast was obtained, showing larger pupil dilation (or higher contrast stimuli) correlating with faster saccade latencies, larger amplitude, higher peak velocities, and smaller endpoint deviation. Together, our results demonstrated the influence of absolute pupil size induced by global luminance level and baseline-corrected pupil dilation associated with motor preparation on saccade latency and metrics, implicating the role of the SC in this behavior.

Differentiating global luminance, arousal and cognitive signals on pupil size and microsaccades

Pupil size reflects a proxy for neural activity associated with global luminance, arousal and cognitive processing. Microsaccades are also modulated by arousal and cognitive processing. Are these effects of arousal and cognitive signals on pupil size and microsaccades coordinated? If so, via what neural mechanisms? We hypothesized that if pupil size and microsaccades are coordinately modulated by these processes, pupil size immediately before microsaccade onset, as an index for ongoing cognitive and arousal processing, should correlate with microsaccade responses during tasks alternating these signals. Here, we examined the relationship between pupil size and microsaccade responses in tasks that included variations in global luminance, arousal and inhibitory control. Higher microsaccade peak velocities correlated with larger pre-microsaccade pupil response related to arousal and inhibitory control signals. In contrast, pupil responses evoked by global luminance signals did not correlate with microsaccade responses. Given the central role of the superior colliculus in microsaccade generation, these results suggest the critical involvement of the superior colliculus to coordinate pupil and microsaccade responses for arousal and inhibitory control modulations, but not for the pupil luminance modulation.

Visual exploration dynamics are low-dimensional and driven by intrinsic factors

When looking at visual images, the eyes move to the most salient and behaviourally relevant objects. Saliency and semantic information significantly explain where people look. Less is known about the spatiotemporal properties of eye movements (i.e., how people look). We show that three latent variables explain 60% of eye movement dynamics of more than a hundred observers looking at hundreds of different natural images. The first component explaining 30% of variability loads on fixation duration, and it does not relate to image saliency or semantics; it approximates a power-law distribution of gaze steps, an intrinsic dynamic measure, and identifies observers with two viewing styles: static and dynamic. Notably, these viewing styles were also identified when observers look at a blank screen. These results support the importance of endogenous processes such as intrinsic dynamics to explain eye movement spatiotemporal properties.

Pupil dynamics: A potential proxy of neural preparation for goal-directed eye movement

The pupils reflexively constrict or dilate to regulate the influx of light on the retinae. Pupillary light reflex (PLR) is susceptible to many non-visual cognitive processes including covert orientation of attention and planning rapid saccadic eye movement. The frontal eye field (FEF) and superior colliculus (SC), which also send projections to the PLR pathway, are two important areas in primate's brain for planning saccade and orientation of attention. The saccadic reaction time (SRT) and the rate of increase in activity of movement neurons in these areas are inversely correlated. This study addressed how pupil dynamics, activity in the FEF and SC and SRT are related in a saccadic decision-making task. The rate of visually evoked pupil constriction was found inversely related to SRT. This was further verified by simulating a homeomorphic biomechanical model of pupillary muscle plants, wherein we projected signals similar to build-up activity in the FEF and SC to the parasympathetic (constriction) and sympathetic (dilation) division of the PLR pathway, respectively. A striking similarity between simulated and observed dynamics of pupil constriction suggests that PLR is a potential proxy of saccade planning by movement neurons in the FEF and SC. Indistinguishable pupil dynamics when planned saccades were elicited versus when they were cancelled eliminated the possibility that the obligatory pre-saccadic shift of attention alone influenced the rate of pupil constriction. Our study envisages a mechanism of how the oculomotor system influences the autonomic activity in an attempt to timely minimize saccadic visual transients by regulating the influx of light.

Role of the frontal eye field in human pupil and saccade orienting responses

The appearance of a salient stimulus evokes a series of orienting responses including saccades and pupil size to prepare the body for appropriate action. The midbrain superior colliculus (SC) that receives critical control signals from the frontal eye field (FEF) is hypothesized to coordinate all components of orienting. It has shown recently that the FEF, together with the SC, is also importantly involved in the control of pupil size, in addition to its well-documented role in eye movements. Although the role of the FEF in pupil size is demonstrated in monkeys, its role in human pupil responses and the coordination between pupil size and saccades remains to be established. Through applying continuous theta-burst stimulation over the right FEF and vertex, we investigated the role of the FEF in human pupil and saccade responses evoked by a salient stimulus, and the coordination between pupil size and saccades. Our results showed that neither saccade reaction times (SRT) nor pupil responses evoked by salient stimuli were modulated by FEF stimulation. In contrast, the correlation between pupil size and SRTs in the contralateral stimulus condition was diminished with FEF stimulation, but intact with vertex stimulation. Moreover, FEF stimulation effects between saccade and pupil responses associated with salient stimuli correlated across participants. This is the first transcranial magnetic stimulation (TMS) study on the pupil orienting response, and our findings suggest that human FEF was involved in coordinating pupil size and saccades, but not involved in the control of pupil orienting responses.