Implicit perceived vocal trustworthiness negatively correlates with amygdala activation

It has long been established that people make rapid judgements about another’s personality and that these judgements have lasting influence on subsequent decisions and interactions. Particularly, in voice research, it has been shown that one word of less than 500 ms is sufficient for forming first impressions of a speaker, and that listeners highly agree on who sounds trustworthy, or dominant. Furthermore, the rapid first impressions formed within a listener, still hold true after a prolonged exposure of approximately 3 seconds. It has also been suggested that numerous personality traits can be summarised in a two-dimensional space of trustworthiness, and dominance. Given our intrinsic need of survival, and self-preservation, first impressions are aiding decisions as to whether to approach or avoid a person. Neurological evidence however has linked activation in the amygdala (more precisely in the superficial (SF) subdivision) to perceived trustworthiness rather than dominance, implying that the amygdala assists in approach/ avoidance decisions but not in identifying whether a person is physically capable of carrying out threatening behaviour. Despite this clear relationship having been extensively researched with face stimuli, the connection between amygdala activation and perceived vocal trustworthiness is poorly understood. Thus, the current study investigated whether amygdala activation correlated with varying levels of vocal trustworthiness. Furthermore, as there has been an ongoing debate as to whether response patterns were linear or quadratic polynomials in face research, a secondary aim of the current study was to explore response patterns. To achieve that, the study was divided into three experiments. In Experiment 1, vocal word stimuli (‘hello’) were pre-validated online for perceived trustworthiness, and 15 voices per voice sex were selected for the fMRI experiments. Experiment 2 focussed on recording amygdala activity (in the whole and the SF part of the amygdala) across two implicit task designs – a 1-back task (Experiment 2a) requiring a high level of attention and cognitive load, and a PureTone detection task in which attention and cognitive load were lower. It was hypothesised that amygdala activation would be negatively correlated to perceived vocal trustworthiness in male and female voices, irrespective of task. Overall, the hypotheses in these experiments were only partially confirmed as significant correlation values were found for male voices but not female voices. Furthermore, results were task dependent with significant results being observed in the high attention/ cognitive load paradigm (Experiment 2a) but not in the PureTone detection task (Experiment 2b). This suggests that the amygdala is sensitive to modulations in socially relevant vocal characteristics related to approach/avoidance decisions, however, a more varied approach of stimuli selection might be required. Given this study was exploratory in nature, these results should be replicated in a confirmatory analysis on an independent data set with more participants. Furthermore, since this study employed univariate methods, multivariate whole brain analysis would aid in establishing additional neural areas involved in processing vocal trustworthiness.