Effect of Pupil Size on the Amygdala of the Beholders
Effect of Pupil Size on the Amygdala of the Beholders
In the present study, we confirmed significantly greater activity in response to large vs small pupil human faces in the amygdala, which is in accordance with a previous study by Demos et al. (2008). Against our assumption that the effect is gender dependent, no effect or interactions with subject or stimuli gender were found. These results confirmed that the differences in the amygdala activity induced by pupil size do not reflect preference or primarily sexual interests, but rather to facial signals that might indicate heightened vigilance on the part of conspecifics.
Furthermore, equivalent effect was not confirmed when cat faces were employed as stimuli, despite more explicit changes in the pupil size, suggesting that the response differences for human face stimuli according to the pupil size is not a mere response to eye stimuli, which supports the idea that the amygdala is one component of a circuit that is important for processing not only fear but rather for biological relevance (Adolphs, 2008).
Although, it was not of our primary interests, we also performed a whole-brain parametric analysis including attractiveness ratings as a modulation parameter to evaluate the impact of the attractiveness on the brain activity. The main purpose of this additional analysis was to test the validity of attractiveness ratings by examining the responses in the regions that have been shown to exhibit positive correlations with attractiveness ratings. Attractiveness ratings are subjective, and thus are fragile variables in any analysis. We, therefore, considered it possible that the rating system similarly employed by Demos et al. (2008) could not fully reflect explicit or implicit preference. Regions exhibiting linearly increasing responses to attractiveness ratings for human face stimuli included the dorsal anterior cingulate cortex, which is implicated in the process of affection. Previous studies (Winston et al., 2007; Cloutier et al., 2008), indeed, showed that increased activity in response to increased facial attractiveness was seen in the dorsal anterior cingulate cortex. Human face stimuli are, therefore, considered to have induced proper emotional responses and have been rated properly as far as we can assess. In contrast, no equivalent results were found for cat faces despite higher attractiveness ratings. A possible explanation is that it reflects comparatively less interests in biologically less relevant cat faces; although, we cannot exclude other possibilities such as methodological problem of this rating system for cat faces, or influence of the task differences or of the order of the sessions.
Despite some differences in the study design; for example, we showed both large and small pupil version of all the faces to all the subjects, a previous study by Demos et al. (2008) and the present study similarly showed that larger pupil faces induce greater responses in the amygdala, while a previous studies by Harrison et al. (2006) found no equivalent results for neutral faces. They found a trend of greater amygdala responses for larger pupil faces with happy or angry expression, and decreased responses for larger pupil faces with neutral faces, which did not reach the statistical threshold. Possible explanations for the incongruent results include differences in study design. For example, while we showed a total of 100 human faces (50 male and 50 female faces with large or small pupils), Harrison et al. used 10 male and 10 female faces with 4 expressions with 4 different sizes of pupils, and thus images of an identical person appeared 16 times. Since familiarity is known to influence the amygdala activity (Dubois et al., 1999), this might have influenced the sensitivity of the amygdala by decreasing its responses to repeatedly presented faces.
More recently, Harrison et al. (2009) evaluated the brain responses to pupils dynamically changing their size either by accurately mirroring changes in subject's pupils (positive feedback), or the opposite (negative feedback) employing faces with neutral expressions as stimuli. In contrast to their previous study (Harrison et al., 2006) showing no significant activity differences of the amygdala in response to the varied pupil size when faces with neutral expressions were employed, they found that the discordance between observed and observer's papillary changes enhanced activity in the left amygdala. With regard to the changes in subjects' pupil size, no entrainment effect following changes in size of the stimulus pupil was seen (i.e. increasing pupil size of the stimuli did not increased subject's pupil size, and vice versa). Their attempt to evaluate the implication of the amygdala activity for social interactions is interesting. However, lack of significant activity difference in the amygdala in response to observed pupil size for neutral faces in their static study (Harrison et al., 2006) make it difficult to compare our results with their findings.
In their dynamic study, the left amygdala was significantly activated only in the negative feedback, but no entrainment effect or significant difference in mean pupil size, change in pupil size or variance of pupil size across conditions between positive vs negative feedback conditions were seen (Harrison et al., 2009), while amygdala's activity was associated with entrainment effect or significantly reduced pupil size when static sad face stimuli were employed (Harrison et al., 2006). These discrepancies might suggest that the activity of the amygdala reflect the sensitivity to different factors or mechanisms of activation between their static (Harrison et al., 2006) and dynamic (Harrison et al., 2009) studies, which is not necessarily limited to face expressions. Therefore, although it is possible that the increased amygdala activity induced by larger pupils in the study by Demos et al. (2008) or in the present study was also influenced by the discordance effect to some extent, it seems difficult to explain all the effect. Although in the context that they all support the idea that the amygdala is sensitive to the changes in the pupil size of the others, which is not necessarily due to sudden change in ambient light that would affect observer and observed pupil sizes equally, they are not contradictory to each other.
While it is well-known that the pupillary size (both baseline size and the dilatation in the dark) is reduced as a function of age (Birren et al., 1950), pupil dilatation in response to attention–arousal stimulation does not differ significantly with age (Kim et al., 2000). Larger pupils in the light could, therefore, be a facial signal indicating heightened vigilance irrespective of the age, and we consider this as the primary cause of enhanced amygdala activity for larger pupils in the present study.
Another factor that might have influenced the result is the size of the corneal light reflection in the stimuli faces that seems positively correlated with the size of the pupil in the previous study by Demos et al. (2008) and in the present study, whereas it seems negatively correlated in the study by Harrison et al. (2006). Since catchlight (a photography term for artificial corneal light reflection) is often used to draw attention to the eye and to make a photograph aesthetically desirable, it is possible that larger corneal light reflection helped activate the amygdala by directing or for directing subjects to the salient stimuli, the eyes of human faces.
In parallel to previous studies (Harrison et al., 2006; Demos et al., 2008), subjects did not notice the pupil size manipulation in the present study. Responses in the brain to facial expressions can be so rapid that they could not plausibly be based on conscious awareness of the stimulus (Adolphs, 2006), and lines of evidence indicate involvement of the amygdala in such automatic neural responses to stimuli (Morris et al., 1998a, b, 2001; Whalen et al., 1998, 2004).
Finally, it might also be noteworthy that all the subjects in the present study were young adult Japanese who have grown up in a 'pupil-naïve' culture; Most Japanese have black to very dark brown irises that makes their pupil size hardly noticeable to each other. They are therefore considered to have comparatively fewer chances to acquire or maturate the sensitivity to pupil size of others. Still, the amygdala was sensitive to differences of the observed pupil size of human faces, but not of cat faces. These results support the generality of the finding across cultures.
In conclusion, in the present study, we confirmed that large pupil size of human faces is such a salient stimuli that provokes greater amygdala responses in the absence of explicit knowledge. The effect is irrespective of the gender of observed face or of observer, suggesting that this is not primarily based on sexual interests. Rather, based on the results that corresponding activity difference was not seen for cat faces, these data, together with previous studies, support the idea that the amygdala is responsive not only to explicit or implicit fear, abhorrence or preference, but also to other elements that might suggest heightened vigilance of biologically relevant stimuli.
Discussion
In the present study, we confirmed significantly greater activity in response to large vs small pupil human faces in the amygdala, which is in accordance with a previous study by Demos et al. (2008). Against our assumption that the effect is gender dependent, no effect or interactions with subject or stimuli gender were found. These results confirmed that the differences in the amygdala activity induced by pupil size do not reflect preference or primarily sexual interests, but rather to facial signals that might indicate heightened vigilance on the part of conspecifics.
Furthermore, equivalent effect was not confirmed when cat faces were employed as stimuli, despite more explicit changes in the pupil size, suggesting that the response differences for human face stimuli according to the pupil size is not a mere response to eye stimuli, which supports the idea that the amygdala is one component of a circuit that is important for processing not only fear but rather for biological relevance (Adolphs, 2008).
Although, it was not of our primary interests, we also performed a whole-brain parametric analysis including attractiveness ratings as a modulation parameter to evaluate the impact of the attractiveness on the brain activity. The main purpose of this additional analysis was to test the validity of attractiveness ratings by examining the responses in the regions that have been shown to exhibit positive correlations with attractiveness ratings. Attractiveness ratings are subjective, and thus are fragile variables in any analysis. We, therefore, considered it possible that the rating system similarly employed by Demos et al. (2008) could not fully reflect explicit or implicit preference. Regions exhibiting linearly increasing responses to attractiveness ratings for human face stimuli included the dorsal anterior cingulate cortex, which is implicated in the process of affection. Previous studies (Winston et al., 2007; Cloutier et al., 2008), indeed, showed that increased activity in response to increased facial attractiveness was seen in the dorsal anterior cingulate cortex. Human face stimuli are, therefore, considered to have induced proper emotional responses and have been rated properly as far as we can assess. In contrast, no equivalent results were found for cat faces despite higher attractiveness ratings. A possible explanation is that it reflects comparatively less interests in biologically less relevant cat faces; although, we cannot exclude other possibilities such as methodological problem of this rating system for cat faces, or influence of the task differences or of the order of the sessions.
Despite some differences in the study design; for example, we showed both large and small pupil version of all the faces to all the subjects, a previous study by Demos et al. (2008) and the present study similarly showed that larger pupil faces induce greater responses in the amygdala, while a previous studies by Harrison et al. (2006) found no equivalent results for neutral faces. They found a trend of greater amygdala responses for larger pupil faces with happy or angry expression, and decreased responses for larger pupil faces with neutral faces, which did not reach the statistical threshold. Possible explanations for the incongruent results include differences in study design. For example, while we showed a total of 100 human faces (50 male and 50 female faces with large or small pupils), Harrison et al. used 10 male and 10 female faces with 4 expressions with 4 different sizes of pupils, and thus images of an identical person appeared 16 times. Since familiarity is known to influence the amygdala activity (Dubois et al., 1999), this might have influenced the sensitivity of the amygdala by decreasing its responses to repeatedly presented faces.
More recently, Harrison et al. (2009) evaluated the brain responses to pupils dynamically changing their size either by accurately mirroring changes in subject's pupils (positive feedback), or the opposite (negative feedback) employing faces with neutral expressions as stimuli. In contrast to their previous study (Harrison et al., 2006) showing no significant activity differences of the amygdala in response to the varied pupil size when faces with neutral expressions were employed, they found that the discordance between observed and observer's papillary changes enhanced activity in the left amygdala. With regard to the changes in subjects' pupil size, no entrainment effect following changes in size of the stimulus pupil was seen (i.e. increasing pupil size of the stimuli did not increased subject's pupil size, and vice versa). Their attempt to evaluate the implication of the amygdala activity for social interactions is interesting. However, lack of significant activity difference in the amygdala in response to observed pupil size for neutral faces in their static study (Harrison et al., 2006) make it difficult to compare our results with their findings.
In their dynamic study, the left amygdala was significantly activated only in the negative feedback, but no entrainment effect or significant difference in mean pupil size, change in pupil size or variance of pupil size across conditions between positive vs negative feedback conditions were seen (Harrison et al., 2009), while amygdala's activity was associated with entrainment effect or significantly reduced pupil size when static sad face stimuli were employed (Harrison et al., 2006). These discrepancies might suggest that the activity of the amygdala reflect the sensitivity to different factors or mechanisms of activation between their static (Harrison et al., 2006) and dynamic (Harrison et al., 2009) studies, which is not necessarily limited to face expressions. Therefore, although it is possible that the increased amygdala activity induced by larger pupils in the study by Demos et al. (2008) or in the present study was also influenced by the discordance effect to some extent, it seems difficult to explain all the effect. Although in the context that they all support the idea that the amygdala is sensitive to the changes in the pupil size of the others, which is not necessarily due to sudden change in ambient light that would affect observer and observed pupil sizes equally, they are not contradictory to each other.
While it is well-known that the pupillary size (both baseline size and the dilatation in the dark) is reduced as a function of age (Birren et al., 1950), pupil dilatation in response to attention–arousal stimulation does not differ significantly with age (Kim et al., 2000). Larger pupils in the light could, therefore, be a facial signal indicating heightened vigilance irrespective of the age, and we consider this as the primary cause of enhanced amygdala activity for larger pupils in the present study.
Another factor that might have influenced the result is the size of the corneal light reflection in the stimuli faces that seems positively correlated with the size of the pupil in the previous study by Demos et al. (2008) and in the present study, whereas it seems negatively correlated in the study by Harrison et al. (2006). Since catchlight (a photography term for artificial corneal light reflection) is often used to draw attention to the eye and to make a photograph aesthetically desirable, it is possible that larger corneal light reflection helped activate the amygdala by directing or for directing subjects to the salient stimuli, the eyes of human faces.
In parallel to previous studies (Harrison et al., 2006; Demos et al., 2008), subjects did not notice the pupil size manipulation in the present study. Responses in the brain to facial expressions can be so rapid that they could not plausibly be based on conscious awareness of the stimulus (Adolphs, 2006), and lines of evidence indicate involvement of the amygdala in such automatic neural responses to stimuli (Morris et al., 1998a, b, 2001; Whalen et al., 1998, 2004).
Finally, it might also be noteworthy that all the subjects in the present study were young adult Japanese who have grown up in a 'pupil-naïve' culture; Most Japanese have black to very dark brown irises that makes their pupil size hardly noticeable to each other. They are therefore considered to have comparatively fewer chances to acquire or maturate the sensitivity to pupil size of others. Still, the amygdala was sensitive to differences of the observed pupil size of human faces, but not of cat faces. These results support the generality of the finding across cultures.
In conclusion, in the present study, we confirmed that large pupil size of human faces is such a salient stimuli that provokes greater amygdala responses in the absence of explicit knowledge. The effect is irrespective of the gender of observed face or of observer, suggesting that this is not primarily based on sexual interests. Rather, based on the results that corresponding activity difference was not seen for cat faces, these data, together with previous studies, support the idea that the amygdala is responsive not only to explicit or implicit fear, abhorrence or preference, but also to other elements that might suggest heightened vigilance of biologically relevant stimuli.