![]() ![]() Participants maintained central fixation during the trial, following the presentation of different emotional face stimuli with carefully controlled low-level visual properties (Figure 1). Emotional face stimuli were used to induce arousal fluctuation because they are often used to evoke emotional arousal. To index sympathetic and parasympathetic activity, HR and GSR were recorded concurrently with pupil size. The goal of this study is to investigate trial-by-trial fluctuations in sympathetic and parasympathetic modulation of pupil size. Notably, in previous research, factors such as visual contrast, spatial frequency, color, and eye movements are usually not adequately controlled for and may have confounded observed effects between pupil size and arousal level. Therefore, distinct patterns of eye movements in different conditions could influence pupil size differently via this pathway. Furthermore, eye movements influence not only the accuracy of pupil size measurement in the video-based eye tracking system, but also the parasympathetic and sympathetic activity via the pathway through the midbrain superior colliculus ( 27– 30). Moreover, pupil size is modulated by low-level visual properties such as luminance, color, visual contrast, and spatial frequencies ( 23– 26). Nevertheless, many studies have concurrently recorded these measures to mostly investigate the task-dependent modulation in these physiological indexes. Although an increasing number of studies have used pupil size to index the level of arousal ( 8– 15), limited research has focused on examining the relationships between pupil size, HR and GSR. Pupil size is also modulated by the balanced activity of the parasympathetic and sympathetic nervous systems ( 6, 7). Although HR is predominantly linked to the parasympathetic system and parasympathetic activation decreases HR, it is antagonistically controlled by both sympathetic and parasympathetic activity which can produce increased or decreased HR, respectively ( 3, 4). The sympathetic nervous system controls sweat gland activity, and increases in sympathetic activity produce corresponding increases in GSR ( 5). Galvanic skin response (GSR) is an independent index of sympathetic activity while heart rate (HR) is predominantly controlled by the parasympathetic nervous system ( 1– 4). Fluctuations in arousal are commonly linked to changes of the sympathetic and parasympathetic activity in the autonomic nervous system. Physiological arousal constantly changes throughout the day, and this fluctuation greatly influences behavior and performance. Overall, these results demonstrated a trial-by-trial relationship between pupil size and HR or GSR, suggesting that pupil size can be used as an index for arousal level involuntarily regulated by the autonomic nervous system. However, task-evoked pupil responses after face presentation only correlated with HR. Trial-by-trial positive correlations between pupil diameter and HR and GSR were found before face presentation, with larger pupil diameter observed on trials with higher HR or GSR. We hypothesized that a positive correlation between pupil size and HR or GSR would be observed both before and after face presentation. Human participants fixated various emotional face stimuli, of which low-level visual properties were carefully controlled, while their pupil size, HR, GSR, and eye position were recorded simultaneously. The goal of this study was to understand how pupil size is modulated by autonomic arousal. Although a growing number of studies have used pupil size to indicate the level of arousal, research that directly examines the relationship between pupil size and HR or GSR is limited. Fluctuations in arousal are regulated by the autonomic nervous system, which is mainly controlled by the balanced activity of the parasympathetic and sympathetic systems, commonly indexed by heart rate (HR) and galvanic skin response (GSR), respectively. 3Research Center of Brain and Consciousness, Taipei Medical University, Shuang Ho Hospital, New Taipei City, TaiwanĪrousal level changes constantly and it has a profound influence on performance during everyday activities.2Graduate Institute of Humanities in Medicine, Taipei Medical University, Taipei, Taiwan.1Centre for Neuroscience Studies, Queen's University, Kingston, ON, Canada.Chin-An Wang 1,2,3 * †, Talia Baird 1 †, Jeff Huang 1, Jonathan D.
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