The ability to recognise what someone is feeling from non-verbal cues, such as their facial expression, is a core social cognitive skill. Emotion recognition a crucial aspect of interpersonal function and as such, emotion recognition deficits can lead to diminished quality of life. There is substantial evidence that healthy older adults are not as accurate as younger adults at recognising facial expressions of emotions, which may make them vulnerable to impaired interpersonal functioning. However, there is debate about whether this age-related difficulty is due to biological decline or a shift in motivation, or a combination of these two factors. The overarching aims of this thesis were to clarify the pattern and magnitude of older adults’ difficulty recognising different facial expressions of emotions, and to advance our understanding of the underlying mechanisms of older adults’ difficulty in recognising emotions with a focus on the oxytocin system. This was achieved through three studies. The first was a comprehensive meta-analytic review of the facial emotion recognition and ageing literature, investigating the moderating effects of task characteristics (Chapter 3). The second was a clinical trial of the modulatory effects of oxytocin nasal spray administration on young and older adults’ facial emotion recognition accuracy (Chapter 6), and the third was a comparison of young and older adults’ peripheral oxytocin levels as measured from saliva samples (Chapter 7).
The meta-analytic review served to clarify the pattern of age-effects across emotion types, but also to identify if there were specific task features that moderated this variance. This was an important avenue of inquiry due to the numerous different task designs used to measure facial emotion recognition in the field of ageing, and the varied outcomes in terms of the magnitude and direction of age-effects reported for each emotion. The meta-analytic review included 102 data sets from 81 studies comparing older and young adults’ facial emotion recognition accuracy. The combined variance, accounted for by the number of response options included in the task, the stimulus format (e.g., videos, photographs of full-intensity expressions), and the image set from which the stimuli were sourced, was significant. Notably, videos produced relatively moderate age-effects across all emotions, which challenged the previously reported positivity effects in ageing. For disgust recognition, older adults demonstrated superior accuracy to young adults for the most common image set (Pictures of Facial Affect), but older adults were less accurate than young adults at recognising disgust on all other stimulus formats and image sets. This finding suggests that older adults do no retain disgust recognition. The overall finding that task characteristics impact on older adults’ facial emotion recognition performance transforms our understanding of their deficits, but may also have broader implications given these task are used across many different clinical populations.
In terms of biological decline, research has largely investigated neurobiological changes in brain function and structure in relation to age-related emotion recognition ability. However, the potential modulatory effects of neurotransmitters and neuropeptides linked to social cognitive function remain to be thoroughly examined. Specifically, the neuropeptide oxytocin is known to play a role in social processes, such as emotion recognition, and is also thought to decline in healthy ageing. Yet, minimal research has been conducted investigating the role that oxytocin might play in older adults’ emotion recognition accuracy. Thus, one aspect of the empirical research component of this thesis was to test the role of oxytocin, when administered intranasally, in modulating young and older adults’ facial emotion recognition accuracy. Sixty young adults (18 – 33 years) and 60 older adults (65 – 80 years), with equal proportions of men and women, completed a facial emotion recognition task of six basic emotions (anger, fear, sadness, disgust, happiness and neutral) in a placebo-controlled cross-over randomised trial of 24 IU of oxytocin nasal spray versus placebo nasal spray. Mixed-effects models revealed that oxytocin had no impact on older adults’ facial emotion recognition accuracy, contrary to the expectations. Oxytocin did improve young females’ sadness recognition, however, this effect was small. The findings from this study suggest that intranasal oxytocin had no age-related benefit on recognising facial expressions of emotion in older adults, and this thesis, therefore, does not provide evidence in support of oxytocin being a biological mechanism behind these difficulties.
The second aspect of the empirical research component of this thesis was a comparison of young and older adults’ baseline peripheral oxytocin concentration levels from saliva samples. Although peripheral oxytocin concentration levels cannot be used to infer oxytocin concentration in the central nervous system (the oxytocin pathway of primary interest to social cognitive research), it has been postulated that intranasal oxytocin exerts effects on the central oxytocin system via indirect peripheral effects. Thus, when investigating the effects of oxytocin on central processes, it is important to have an understanding of the oxytocin system as a whole in the population of interest. Based on animal research, an age-related decline of peripheral oxytocin has been speculated, but there is little research confirming this trajectory. The results of this thesis indicated that older and young adults of both sexes have similar peripheral oxytocin concentration levels, which suggests that parts of the oxytocin system may be maintained in healthy ageing.
Overall, this thesis provides new information to the understanding of the mechanisms behind older adults’ facial emotion recognition difficulties. This thesis does not provide direct support for either motivation or biological models of emotion recognition decline in healthy ageing, as the findings have implications for both perspectives. Specifically, the task characteristics that were shown to moderate older adults’ performance on measures of facial emotion recognition accuracy vary in terms of the cognitive load required but also the level of motivation engagement. As such, the findings from the meta-analytic review have implications for both motivational and biological models. Intranasal oxytocin not modulating older adults’ facial emotion recognition, and parts of the oxytocin system appearing to be maintained in healthy ageing, may initially seem to contradict biological models and in turn, provide support for motivational models. However, the findings from this thesis are not enough to eliminate the oxytocin system as a biological mechanism behind older adults’ facial emotion recognition difficulties, as more direct testing of the central oxytocin system is required. While this thesis does not provide conclusive evidence for or against oxytocin playing a role in older adults’ emotion recognition deficits, it makes important contributions to the growing foundation of knowledge, which will serve to inform future research.