Full analysis for viewing behaviour in this study is included in the electronic supplementary material. Dilation of the pupils is an indicator of an observer's sexual interest in other people, but it remains unresolved whether this response is strengthened or diminished by sexually explicit material. To address this question, this study compared pupillary responses of heterosexual men and women to naked and dressed portraits of male and female adult film actors. Pupillary responses corresponded with observers' self-reported sexual orientation, such that dilation occurred during the viewing of opposite-sex people, but were comparable for naked and dressed targets.
These findings indicate that pupillary responses provide a sex-specific measure, but are not sensitive to sexually explicit content. The presentation of male and female adults elicits dilation in observers' pupils to the people category that matches their sexual interest.
This effect is observed with a variety of stimuli, ranging from static images of partially dressed adults [ 1 — 3 ] to photographs of nudes [ 3 — 5 ], and sexually explicit video [ 6 , 7 ]. High sexual explicitness is necessary for eliciting a response pattern that reveals observers' sexual preferences with other physiological approaches, such as phallometric measures [ 8 , 9 ]. This raises ethical concerns and limits the use of such measures in applied and research settings [ 10 ].
It remains unclear, however, whether the level of sexual exposure also modulates pupillary responses, which level of exposure provides the strongest index of sexual interest with this method, and whether exposure interacts with observer sex. An early investigation provides evidence that nude images selectively enhance pupillary responses to people of sexual interest [ 3 ]. In this study, heterosexual female observers viewed images of two male and two female models presented in various stages of undress.
Observers exhibited greatest pupillary dilation to images of naked men in comparison to partially and fully dressed men. However, this effect was only present for some of the male images, and no difference was found for images of women. Moreover, male observers were not included in this study, and pupillary responses were measured crudely with a manual technique.
In a subsequent study, naked images elicited a generalized pupil dilation response in heterosexual men and women that did not differentiate target sex [ 11 ]. In contrast to Hamel's [ 3 ] findings, this indicates that nudity might also interfere with the measurement of sex preference effects. However, more recent investigations with more precise eye-tracking equipment also indicate that the pupillary responses of heterosexual male observers to nude [ 6 , 7 ] and partially nude people [ 1 , 2 ] reflect their sexual interests.
By contrast, the pupils of heterosexual female observers dilated indiscriminately for both sexes [ 1 , 6 ], or more to same-sex stimuli [ 7 ]. However, these studies did not directly compare responses to nude and partially nude stimuli with images of dressed persons and therefore cannot address whether these image types provide different indexes of sexual interest.
To provide a more direct comparison, a recent investigation contrasted observers' pupillary responses to video footage of nude persons performing sexual acts with footage of dressed persons discussing the weather [ 12 ]. In this study, pupil dilation patterns were stronger for sexually explicit stimuli, and these materials also yielded clearer sex differences. However, the sexually explicit and non-explicit stimuli were not systematically matched for person identity and scene content in this study, which raises the possibility that these factors contributed to the difference in pupillary response patterns.
Consequently, these findings cannot reveal fully whether more explicit sexual information yields stronger pupil dilation patterns linked to sexual orientation. To systematically investigate how level of nudity affects pupillary responses, the current study employed highly controlled portraits of dressed and naked adults, which were matched for identity, pose and image content.
An intermediate stage of nudity was also presented, by blurring genital and chest areas of the naked stimuli. Observers' fixations on these images were analysed briefly to confirm that they were looking at the person content. We then analysed pupillary responses and correlated these with observers' sexual appeal ratings of the depicted adults.
Fifty-two 28 females, 24 males students from the University of Kent, with a mean age of To confirm sexual orientation on taking part in the experiment, participants again completed the Kinsey scale.
Responses for sexual attraction and fantasies were combined and revealed means of 1. Participants with a score that was three standard deviations above these means were excluded from further analysis. A further female participant that produced pupillary responses that were three standard deviations above the mean pupillary responses was excluded. This resulted in the exclusion of experimental data for four male and three female observers. To create an intermediate nudity condition, the pelvic region of the naked male targets and the breast and pelvic region of the naked female targets were blurred using a graphics software Adobe Photoshop CS3, Gaussian Blur with pixel radius.
This resulted in a total of 36 photographs, comprising 12 images for each of three exposure conditions dressed, blurred and naked. The content of the resulting images is no longer recognizable but colour and mean image luminance are retained for similar approaches, see [ 1 , 17 ]. Example stimuli of dressed, blurred and naked women and men, and the corresponding control images. All rights reserved. The Eyelink measures corneal reflection and dark pupil with a video-based infrared camera, and computes the number of pixels that are occluded by participants' pupils.
Viewing was binocular but only participants' left eye was tracked. Participants were invited to take part in an experiment on sexual interest that involved viewing images of dressed and naked men and women, but were kept naive to the full purpose until the end. Subjects were seated in a quiet windowless room with consistent artificial lighting. The participants' left eye was tracked and calibrated using the standard Eyelink procedure.
Thus, participants fixated a series of nine target points on the display monitor. Fixation accuracy was then validated against a second series of nine targets. Calibration was repeated if poor measurement accuracy less than 0. Participants were instructed to rate the personal sexual appeal of all 72 images. Participants were instructed to keep their fingers on these keys at all times. The intact scenes and control images were randomly intermixed for each participant by the Eyelink software and interspersed by a short break every 24 trials.
On completion of the eye-tracking task, participants completed the Kinsey scale and MKSOG to confirm sexual orientation see Participants section.
Sexual appeal ratings for male a and female b observers. Error bars represent the standard error of the means. Mean response times ms for all stimulus categories for male a and female b observers. Error bars represent standard errors of the means. The fixation locations were analysed to confirm that observers looked at the person content of the stimuli, and the sexually relevant regions of the nude images.
Blinks and fixations outside the display monitor were excluded. These data demonstrate that observers fixated the targets' faces in all conditions but increased attention to the chest and pelvis in the naked conditions a detailed analysis of the percentage fixations to the head, chest and pelvis is reported in the electronic supplementary materials available online.
Distribution of fixations for a female top row in panel and a male bottom row in panel target in the dressed, blurred and naked exposure conditions for male and female observers. Pupillary responses were then computed by calculating the mean pupil size for all fixations across the duration of the stimulus displays.
These values were used to compute an overall mean, across all stimuli, for each participant. These pupillary responses were evaluated for outliers, which resulted in the exclusion of one female participant with a score three standard deviations above the group mean.
The percentage difference i. For the resulting scores, a value of zero indicates no change in pupil size and positive or negative scores reflect relatively larger dilation or smaller constriction pupil sizes for a stimulus category for similar approaches, see [ 1 , 21 , 22 ]. Percentage pupillary change for all stimulus categories for male a and female b observers. Error bars represent standard error of the means. To determine whether this pattern could be accounted for by differences in response times to the experimental conditions, one-way ANCOVAs were conducted on the pupillary data for images of men and women with response time as the covariate.
These analyses therefore suggest that any differences in viewing time across experimental conditions cannot explain the pupil dilation patterns. Sexual appeal ratings were also correlated with mean pupillary change.
For this analysis, the control conditions were excluded and the data for male and female targets was combined. The distribution of sexual appeal ratings was skewed. Therefore, non-parametric Spearman's correlations are reported. This study examined whether pupillary responses to the visual presentation of men and women are influenced by different levels of sexual exposure.
More specifically, we sought to determine whether one of these conditions dressed, partially naked or naked provides a clearer index of sexual interest, and whether this interacts with observer sex. This experiment showed pupillary responses that were consistent with observer's self-reported sexual preferences. Thus, pictures of women elicited a clear pupillary dilation in heterosexual male observers that was not present when viewing men or control images.
In contrast, pupil size was largest in heterosexual female observers during the viewing of men compared to women and control images. When pupillary responses were broken down by exposure condition, strong dilation patterns for both dressed and naked persons emerged.
Only a small set of studies have directly compared pupillary responses to such images, with inconsistent results. One study assessed pupillary responses of heterosexual female observers and found enhanced dilation for naked male images [ 3 ]. However, a later study revealed a generalized dilation response for naked stimuli of both sexes in heterosexual males and females [ 11 ].
Several factors could account for these discrepancies. For example, such a discrepancy in findings might reflect the use of different eye-tracking methods for measuring pupil size, which range from elementary pupillometry systems that record pupil diameter only every minute [ 3 ] or every 0.
Furthermore, it is unclear whether these studies controlled for stimulus factors such as identity, colour and pose. Strong differences in colour tone arising from such a mixture of identities and race could have interfered with pupillary responses to the sexual content of these images [ 23 , 24 ].
Similarly, Watts et al. The current study improves on these previous attempts by using sophisticated contemporary eye-tracking technology in combination with highly controlled stimuli. Under these conditions, pupillary responses to images of men and women appear to be sex-specific but not sensitive to the sexual explicitness of the materials. Naked images of people have been shown to elicit a stronger recording of arousal than dressed images when this is measured with other physiological measures, such as genital response and skin conductance [ 8 , 25 — 27 ].
It is unclear why a similar pattern is not found with pupillary responses here. Pupil dilation is an instantaneous response [ 28 ], so it is possible that a change in pupil size is elicited with lower levels of sexual arousal than is necessary for other physiological measures. As such, images of dressed people may provide sufficient arousal for eliciting a similarly strong dilation response to naked images under the current conditions.
The responses of male observers to stimuli depicting women converge with previous research, which has also shown increases in pupil size to such content [ 1 , 2 , 5 — 7 ]. In the current study, female observers also showed stronger dilation for photographs depicting the opposite sex. In the sex literature, there is mixed evidence with regard to the response patterns of heterosexual women.
Some studies have revealed pupil dilation in female observers that is indistinguishable to sexual content of men and women [ 1 , 6 , 7 ] or stronger for the opposite sex [ 3 , 4 , 12 , 22 ]. In light of these differences, the current study also investigated whether nudity influences the pupillary responses of heterosexual females by enhancing [ 3 ] or diminishing any sexual preference effects [ 11 ].
In this experiment, these observers recorded clear dilation patterns for images depicting persons of the opposite sex, consistent with their sexual orientation. More importantly, this pattern was present for naked and dressed images.