Pupil Diameter Tracks Statistical Structure in the Environment to Increase Visual Sensitivity

Pupil diameter as an indicator of sound pair familiarity after statistically structured auditory sequence

Inspired by recent findings in the visual domain, we investigated whether the stimulus-evoked pupil dilation reflects temporal statistical regularities in sequences of auditory stimuli. We conducted two preregistered pupillometry experiments (experiment 1, n = 30, 21 females; experiment 2, n = 31, 22 females). In both experiments, human participants listened to sequences of spoken vowels in two conditions. In the first condition, the stimuli were presented in a random order and, in the second condition, the same stimuli were presented in a sequence structured in pairs. The second experiment replicated the first experiment with a modified timing and number of stimuli presented and without participants being informed about any sequence structure. The sound-evoked pupil dilation during a subsequent familiarity task indicated that participants learned the auditory vowel pairs of the structured condition. However, pupil diameter during the structured sequence did not differ according to the statistical regularity of the pair structure. This contrasts with similar visual studies, emphasizing the susceptibility of pupil effects during statistically structured sequences to experimental design settings in the auditory domain. In sum, our findings suggest that pupil diameter may serve as an indicator of sound pair familiarity but does not invariably respond to task-irrelevant transition probabilities of auditory sequences.

Interpersonal eye-tracking reveals the dynamics of interacting minds

The human eye is a rich source of information about where, when, and how we attend. Our gaze paths indicate where and what captures our attention, while changes in pupil size can signal surprise, revealing our expectations. Similarly, the pattern of our blinks suggests levels of alertness and when our attention shifts between external engagement and internal thought. During interactions with others, these cues reveal how we coordinate and share our mental states. To leverage these insights effectively, we need accurate, timely methods to observe these cues as they naturally unfold. Advances in eye-tracking technology now enable real-time observation of these cues, shedding light on mutual cognitive processes that foster shared understanding, collaborative thought, and social connection. This brief review highlights these advances and the new opportunities they present for future research.

Location-specific deviant responses to object sequences in macaque inferior temporal cortex

Many species learn temporal regularities in their visual environment, demonstrating visual statistical learning. In this study, we explored the sensitivity of macaque inferior temporal (IT) cortical neurons to transition probabilities of sequentially presented visual images, presented at different locations in the visual field. We exposed monkeys to sequences of two images, where the first image was presented either foveally or peripherally, and the second image was consistently presented foveally. Following several weeks of exposure, we recorded IT responses to assess differences between the exposed (Fixed) and new, Deviant sequences, where the identity of the first image in a sequence differed from the exposure phase. While enhanced responses to Deviant sequences were observed when both images of a pair were foveally presented during exposure, no such deviant responses were present when the first image was presented peripherally. This finding challenges the notion that mere exposure to image sequences always leads to deviant responses in macaque IT. The results highlight the complexity of the mechanisms underlying statistical learning in primates, particularly in the context of peripheral image presentations, emphasizing the need for further investigation into the origins of these responses in the IT cortex.

Inferior temporal cortex leads prefrontal cortex in response to a violation of a learned sequence

Primates learn statistical regularities that are embedded in visual sequences, a form of statistical learning. Single-unit recordings in macaques showed that inferior temporal (IT) neurons are sensitive to statistical regularities in visual sequences. Here, we asked whether ventrolateral prefrontal cortex (VLPFC), which is connected to IT, is also sensitive to the transition probabilities in visual sequences and whether the statistical learning signal in IT originates in VLPFC. We recorded simultaneously multiunit activity (MUA) and local field potentials (LFPs) in IT and VLPFC after monkeys were exposed to triplets of images with a fixed presentation order. In both areas, the MUA was stronger to images that violated the learned sequence (deviants) compared to the same images presented in the learned triplets. The high-gamma and beta LFP power showed an enhanced and suppressed response, respectively, to the deviants in both areas. The enhanced response was present also for the image following the deviant, suggesting a sensitivity for temporal adjacent dependencies in IT and VLPFC. The increased response to the deviant occurred later in VLPFC than in IT, suggesting that the deviant response in IT was not inherited from VLPFC. These data support predictive coding theories that propose a feedforward flow of prediction errors.

Synchrony to a beat predicts synchrony with other minds

Synchrony has been used to describe simple beat entrainment as well as correlated mental processes between people, leading some to question whether the term conflates distinct phenomena. Here we ask whether simple synchrony (beat entrainment) predicts more complex attentional synchrony, consistent with a common mechanism. While eye-tracked, participants listened to regularly spaced tones and indicated changes in volume. Across multiple sessions, we found a reliable individual difference: some people entrained their attention more than others, as reflected in beat-matched pupil dilations that predicted performance. In a second study, eye-tracked participants completed the beat task and then listened to a storyteller, who had been previously recorded while eye-tracked. An individual's tendency to entrain to a beat predicted how strongly their pupils synchronized with those of the storyteller, a corollary of shared attention. The tendency to synchronize is a stable individual difference that predicts attentional synchrony across contexts and complexity.

Eye Movement and Pupil Measures: A Review

Our subjective visual experiences involve complex interaction between our eyes, our brain, and the surrounding world. It gives us the sense of sight, color, stereopsis, distance, pattern recognition, motor coordination, and more. The increasing ubiquity of gaze-aware technology brings with it the ability to track gaze and pupil measures with varying degrees of fidelity. With this in mind, a review that considers the various gaze measures becomes increasingly relevant, especially considering our ability to make sense of these signals given different spatio-temporal sampling capacities. In this paper, we selectively review prior work on eye movements and pupil measures. We first describe the main oculomotor events studied in the literature, and their characteristics exploited by different measures. Next, we review various eye movement and pupil measures from prior literature. Finally, we discuss our observations based on applications of these measures, the benefits and practical challenges involving these measures, and our recommendations on future eye-tracking research directions.

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.

Memory guidance of value-based decision making at an abstract level of representation

Value-based decisions about alternatives we have never experienced can be guided by associations between current choice options and memories of prior reward. A critical question is how similar memories need to be to the current situation to effectively guide decisions. We address this question in the context of associative learning of faces using a sensory preconditioning paradigm. We find that memories of reward spread along established associations between faces to guide decision making. While memory guidance is specific for associated facial identities, it does not only occur for the specific images that were originally encountered. Instead, memory guidance generalizes across different images of the associated identities. This suggests that memory guidance does not rely on a pictorial format of representation but on a higher, view-invariant level of abstraction. Thus, memory guidance operates on a level of representation that neither over- nor underspecifies associative relationships in the context of obtaining reward.

Pupil Diameter Tracks Statistical Structure in the Environment to Increase Visual Sensitivity

Pupil diameter determines how much light hits the retina and, thus, how much information is available for visual processing. This is regulated by a brainstem reflex pathway. Here, we investigate whether this pathway is under the control of internal models about the environment. This would allow adjusting pupil dynamics to environmental statistics to augment information transmission. We present image sequences containing internal temporal structure to humans of either sex and male macaque monkeys. We then measure whether the pupil tracks this temporal structure not only at the rate of luminance variations, but also at the rate of statistics not available from luminance information alone. We find entrainment to environmental statistics in both species. This entrainment directly affects visual processing by increasing sensitivity at the environmentally relevant temporal frequency. Thus, pupil dynamics are matched to the temporal structure of the environment to optimize perception, in line with an active sensing account.SIGNIFICANCE STATEMENT When light hits the retina, the pupil reflexively constricts. This determines how much light and thus how much information is available for visual processing. We show that the rate at which the pupil constricts and dilates is matched to the temporal structure of our visual environment, although this information is not directly contained in the light variations that usually trigger reflexive pupil constrictions. Adjusting pupil diameter in accordance with environmental regularities optimizes information transmission at ecologically relevant temporal frequencies. We show that this is the case in humans and macaque monkeys, suggesting that the reflex pathways that regulate pupil diameter are under some degree of cognitive control across primate species.