Autobiographical Memory Increases Pupil Dilation

Abstract Background Pupil activity has been widely considered as a “summed index” of physiological activities during cognitive processing. Methodology We investigated pupil dilation during retrieval of autobiographical memory and compared pupil diameter with a control condition in which participants had to count aloud. We also measured pupil diameters retrieval of free (i.e., first memory that comes to mind), positive, and negative memories (memories associated, respectively, with the words “happy” and “sad”). Results Analyses demonstrated larger pupil diameters during the free, positive, and negative autobiographical memory retrieval than during the control task. Analyses also demonstrated no significant differences in pupil diameters across the three autobiographical memory conditions. Conclusion These outcomes demonstrate that, compared with counting, autobiographical retrieval results in a larger pupil size. However, the emotional valence of memories yields non-significant effect on pupil diameters. Our findings demonstrate how autobiographical memory retrieval yields pupil dilation.

dilation when participants processed old information during recognition memory tasks.
They suggested that pupil dilation mirrors mental effort related to encoding and retrieval of information from memory rather than the general level of mental effort, as proposed by the cognitive load theory of Kahneman and Beatty [13]. In a similar vein, Vo, Jacobs [22] attributed pupil dilation in recognition memory tasks to the cognitive demands of recognizing old information compared with rejecting new information. According to Vo, Jacobs [22], recognition of old information requires retrieval of qualitative contextual information about the encoding episode (e.g., when and where the information was encoded), whereas correct rejection does not require this effortful retrieval. Also, Otero, Weekes [23] suggested that pupil dilation for old information depends on the strength of memory traces upon which recognition judgment is made. These suggestions can explain why, in recognition memory, pupil dilates more for information judged as old versus information judged as new [20,[24][25][26]. Together, there is a substantial body of research on the effects of memory on pupil dilation. This research reported the increased pupil dilation in response to increased difficulty of memory processing [13][14][15][16][17][18]. This research also reported that, on recognition memory tasks, pupil dilates more when participants process old stimuli compared to new stimuli [20,[24][25][26].

The Present Study
Although the previous research is useful in understanding the effects of memory processing on pupil dilation, there is, to the very best of our knowledge, a lack of research on pupil dilation during autobiographical memory retrieval. This issue is important because the study of autobiographical memory is concerned with how people remember personal events. Autobiographical memory allows the recall of events that are relevant to one's identity and sense of self [31,32].
To this aim, we compared pupil dilation during autobiographical memory retrieval and during a control task in which participants had to count aloud. We also investigated pupil dilation following the emotional valence of memories for two reasons. First, autobiographical memory has been intimately associated with emotion [31]. Second, pupil dilation has been found to be sensitive to emotion [9][10][11]. Therefore, pupil dilation can be influenced by the emotional tone of memory.
As for hypotheses, we expected a larger pupil diameter during autobiographical memory retrieval than during the control task. We also expected the large pupil diameter during retrieval of emotional memories than during the retrieval of free memories.

Method Participants
The study included 36 graduate/undergraduate students from the University of Nantes

Procedures and Materials
Generally speaking, procedures consisted of four conditions (i.e., free autobiographical recall, positive autobiographical recall, and negative autobiographical recall, as well as counting as a control condition). During these conditions, the participants wore eye-tracking glasses and faced a white wall (see Figure 1).

Results
We compared pupil dilation across counting and the three autobiographical trials (i.e., the "free", "positive", and "negative" memories) with repeated measures ANOVA, followed up by The pupil diameter data is provided in Figure 2. Analysis showed significant differences between pupil diameters across the four trials (i.e., counting, "free", "positive", and "negative" memories), F(3, 105) = 6.14, p = .001, η 2 = .15. This effect was solely caused Figure 1. During the four conditions (i.e., free autobiographical recall, positive autobiographical recall, and negative autobiographical recall, as well as counting as a control condition), participants wore eye-tracking glasses and faced a white wall Figure 2. Means of pupil diameters during counting and during retrieval of "free" (i.e., first memory that comes to mind), and "positive" and "negative" memories (memories associated, respectively, with the words "happy" and "sad"). Error bars are 95% within-subjects confidence intervals.

Discussion
We compared pupil diameter during the retrieval of three autobiographical memories and a control task, in which participants had to count. Analyses demonstrated larger pupil diameters during free, positive, and negative autobiographical memory retrieval than during the control task. Analyses also demonstrated no significant differences in pupil diameter across the three autobiographical memory conditions (i.e., free, positive, and negative).
These outcomes demonstrate that, compared to counting, autobiographical memory retrieval results in larger pupil size. However, the emotional valence of the recalled memories yields no significant effect on pupil diameter.
The main finding is the larger pupil diameter during autobiographical memory retrieval. This study is the first one to demonstrate that the retrieval of personal information yields dilation of the pupils. More specifically, it demonstrates that pupil dilation, as observed in the literature on the memory of general information (e.g., memory for words), can also be extended to self-related information (i.e., autobiographical memory).

Effects of Memory on Pupil Dilation
To begin with the contribution of our study to the literature on the effects of memory on pupil dilation (and as mentioned in the introduction), there is a large body of research reporting increased pupil dilation in response to increased difficulty of memory processing [13][14][15][16][17][18]. This research also reported that, on recognition memory tasks, pupil dilates more when participants process old stimuli compared to new stimuli [20,[24][25][26]. This body of literature is, however, mainly concerned by recognition memory and working memory.
Research on pupil dilation and working memory has been mainly interested in how updating in working memory yields pupillary responses. In this research, updating has been defined as the ability to integrate incoming information with information that is currently held in working memory [35]. Using span tasks, research has shown increased pupil dilation with the increased number of to-be-tracked information [17,18,36]. Similar findings are reported by research using the n-back task. This working memory task requires participants to indicate whether the currently presented information is the same as information presented n trials back [35].

Pupil Dilation as Physiological Measure
Our findings demonstrate how pupil dilation can be used as a physiological evaluation of this retrieval. Physiological correlates of autobiographical memory have been mainly evaluated with regard to brain activity. In this research, autobiographical memory has been associated with a "core network" of brain areas, including the hippocampus, medial and ventrolateral prefrontal cortex, posterior cingulate, and temporoparietal junction [44,45].
Unlike the large body of research on brain activity during autobiographical memory Research has demonstrated variations of facial expressions during autobiographical memory retrieval [49][50][51]. This research has suggested that emotional facial expressions may reflect the physiological states that were experienced in the encoded events [52].
Autobiographical memory has not been only associated with the brain, cardiovascular, electrodermal, and facial expressions activities, but also with eye movements [53].

Effects of Emotion on Pupil Dilation
Although we have found that pupil diameter was larger during autobiographical memory retrieval than during the control task, our findings demonstrated no significant differences between pupil dilations across the retrieval of freely recalled, positive, and negative memories. This finding seems to contradict a body of research demonstrating how pupil dilation is dependent on emotion.
Research has demonstrated that pupils tend to dilate when participants view positive or negative images relative to neutral images [10]. This dilation has been replicated in many studies in response to emotional images [57][58][59], emotional video-clips [60], as well as facial expressions [61,62]. We would also like to emphasize that pupil dilation, as observed in our study, reflects the general physiological characteristics of this dilation. As mentioned in the introduction, pupil diameter typically varies from 1.5 to nine mm, and, in standard light conditions, pupil diameter is about 3 mm [3]. Also, the difference differences on pupil dilation between the autobiographical and control conditions mirrors that observed in research on pupil dilation and cognition; pupil typically dilates around 0.5 mm to cognitive stimulation [2].

Conclusions
Pupil activity has been considered as a "summed index" of brain activity during cognitive processing. Cognitively relevant pupil activity typically occurs following inhibition of the parasympathetic nervous system as controlled by the locus coeruleus-Translational Neuroscience norepinephrine system, which plays a key role in the regulation of cognition. By demonstrating how autobiographical memory can influence pupil activity, our study shows how pupillometry can be used as a measure of physiological responses to retrieval of selfrelated information.