Mirror-touch synaesthesia: the role of shared representations in social cognition

Synaesthesia is a condition in which one property of a stimulus results in conscious experiences of an additional attribute. In mirror-touch synaesthesia, the synaesthete experiences a tactile sensation on their own body simply when observing touch to another person. This thesis investigates the prevalence, neurocognitive mechanisms, and consequences of mirror-touch synaesthesia. Firstly, the prevalence and neurocognitive mechanisms of synaesthesia were assessed. This revealed that mirrortouch synaesthesia has a prevalence rate of 1.6%, a finding which places mirror-touch synaesthesia as one of the most common variants of synaesthesia. It also indicated a number of characteristics of the condition, which led to the generation of a neurocognitive model of mirror-touch synaesthesia. An investigation into the perceptual consequences of synaesthesia revealed that the presence of synaesthesia is linked with heightened sensory perception - mirror-touch synaesthetes showed heightened tactile perception and grapheme-colour synaesthetes showed heightened colour perception. Given that mirror-touch synaesthesia has been shown to be linked to heightened sensorimotor simulation mechanisms, the impact of facilitated sensorimotor activity on social cognition was then examined. This revealed that mirror-touch synaesthetes show heightened emotional sensitivity compared with control participants. To compliment this, two transcranial magnetic stimulation (TMS) studies were then conducted to assess the impact of suppressing sensorimotor activity on the expression recognition abilities of healthy adults. Consistent with the findings of superior emotion sensitivity in mirror-touch synaesthesia (where there is facilitated sensorimotor activity), suppressing sensorimotor resources resulted in impaired expression recognition across modalities. The findings of the thesis are discussed in relation to neurocognitive models of synaesthesia and of social cognition

Transcranial electric stimulation and cognitive training improves face perception

Recently, there has been much interest the effectiveness of cognitive training programmes across a variety of cognitive and perceptual domains. Some evidence suggests that combining training programmes with noninvasive brain stimulation techniques such as transcranial random noise stimulation (tRNS) can enhance training gains, but to date this has only been examined in numerosity and arithmetic tasks. In this study, we examined whether tRNS modulated the effects of a face recognition training programme. Participants completed a face discrimination training task for an hour per day over five days. Each day, training was preceded by twenty minutes of active high frequency tRNS or sham stimulation, targeted at the posterior temporal cortices or the inferior frontal gyri (IFG). Participants who received active stimulation to the posterior temporal cortices showed significant improvement on a facial identity discrimination task (the Cambridge Face Perception Test) after training, whereas those receiving sham or IFG stimulation showed no performance change. There was no evidence of an effect of stimulation on a face memory task (the Cambridge Face Memory Test). These results suggest that tRNS can enhance the effectiveness of cognitive training programmes, but further work is needed to establish whether perceptual gains can be generalised to face memory

Associations between tactile intimacy and sleep quality in healthy adults: a systematic review

Prior research suggests that different types of touch can affect sleep, but whether there is a consistent association between tactile intimacy and sleep quality is unclear. Here, we report a pre-registered systematic review (PROSPERO - CRD42020158683) of studies examining the association between tactile intimacy and sleep quality in healthy adults. The databases PsycINFO, PubMed, Web of Science, the Cochrane Central Register of Controlled Trials and EMBASE were searched on August 7th, 2020. 13 studies met the inclusion criteria and were synthesised qualitatively. Most commonly, articles researched sexual intimacy in relation to sleep quality, but some studies also investigated nonsexual affective touch and emotionally neutral touch. Some evidence for a connection between sexual function, sexual satisfaction and masturbation with sleep quality was found, however, no evidence for an association between sexual frequency or sexual positions and sleep was found. Interestingly, studies employing more subjective approaches were more likely to report an association between touch and sleep, potentially highlighting a discrepancy between self-reported and the objectively measurable association between touch and sleep

Transcranial random noise stimulation and cognitive training improves face perception

Several studies have found that cognitive training can improve face recognition. However, the effects tend to be relatively small and short-lived. Recent research has found that non-invasive brain stimulation techniques such as transcranial random noise stimulation (tRNS) can enhance and extend the effects of cognitive training in other domains, but this has not been examined for face recognition. In this study, we examined whether tRNS modulated the effects of a face recognition training program in people with typical face recognition abilities. Participants completed a face discrimination training task for one hour per day over fi ve days. Training was preceded by twenty minutes of active high frequency tRNS or sham stimulation to lateral occipitotemporal cortices. Participants completed a battery of face processing tasks assessing face memory (the Cambridge Face Memory Test, CFMT), face perception (the Cambridge Face Perception Test, CFPT), and patterns of eye-movements to faces (free-viewing of faces and social scenes); these took place before training, after training, and at a one-week follow-up session. Participants who received active stimulation showed signifi cant improvement on the CFPT following training, whereas those who received sham stimulation did not show any training gains. There was no improvement for inverted faces, and neither the active or sham stimulation group showed an improvement on the CFMT, or any change in eye-movement patterns. These results suggest that tRNS can enhance the effectiveness of face recognition training programmes, but further work is needed to establish whether perceptual gains can be generalised to memory

Pitch Memory in Nonmusicians and Musicians: Revealing Functional Differences Using Transcranial Direct Current Stimulation

For music and language processing, memory for relative pitches is highly important. Functional imaging studies have shown activation of a complex neural system for pitch memory. One region that has been shown to be causally involved in the process for nonmusicians is the supramarginal gyrus (SMG). The present study aims at replicating this finding and at further examining the role of the SMG for pitch memory in musicians. Nonmusicians and musicians received cathodal transcranial direct current stimulation (tDCS) over the left SMG, right SMG, or sham stimulation, while completing a pitch recognition, pitch recall, and visual memory task. Cathodal tDCS over the left SMG led to a significant decrease in performance on both pitch memory tasks in nonmusicians. In musicians, cathodal stimulation over the left SMG had no effect, but stimulation over the right SMG impaired performance on the recognition task only. Furthermore, the results show a more pronounced deterioration effect for longer pitch sequences indicating that the SMG is involved in maintaining higher memory load. No stimulation effect was found in both groups on the visual control task. These findings provide evidence for a causal distinction of the left and right SMG function in musicians and nonmusician

Task-dependent and distinct roles of the temporoparietal junction and inferior frontal cortex in the control of imitation

The control of neurological networks supporting social cognition is crucially important for social interaction. In particular, the control of imitation is directly linked to interaction quality, with impairments associated with disorders characterized by social difficulties. Previous work suggests inferior frontal cortex (IFC) and the temporoparietal junction (TPJ) are involved in controlling imitation, but the functional roles of these areas remain unclear. Here, transcranial direct current stimulation (tDCS) was used to enhance cortical excitability at IFC and the TPJ prior to the completion of three tasks: 1) a naturalistic social interaction during which increased imitation is known to improve rapport, 2) a choice reaction time task in which imitation needs to be inhibited for successful performance, and 3) a non-imitative control task. Relative to sham stimulation, stimulating IFC improved the context-dependent control of imitation – participants imitated more during the social interaction and less during the imitation inhibition task. In contrast, stimulating the TPJ reduced imitation in the inhibition task without affecting imitation during social interaction. Neither stimulation site affected the non-imitative control task. These data support a model in which IFC modulates imitation directly according to task demands, whereas TPJ controls task-appropriate shifts in attention towards representation of the self or the other, indirectly impacting upon imitation

Using High Frequency Transcranial Random Noise Stimulation to Modulate Face Memory Performance in Younger and Older Adults: Lessons Learnt From Mixed Findings

High-frequency transcranial random noise stimulation (tRNS) has been shown to improve a range of cognitive and perceptual abilities. Here we sought to examine the effects of a single session of tRNS targeted at the ventrolateral prefrontal cortices (VLPFC) on face memory in younger and older adults. To do so, we conducted three experiments. In Experiment 1, we found that younger adults receiving active tRNS outperformed those receiving sham stimulation (i.e., using a between-participant factor for stimulation condition; Experiment 1). This effect was not observed for object memory (car memory) in younger adults (Experiment 2), indicating that the effect is not a general memory effect. In Experiment 3, we sought to replicate the effects of Experiment 1 using a different design (within-participant factor of stimulation – active or sham tRNS to the same individual) and to extend the study by including older adult participants. In contrast to Experiment 1, we found that active tRNS relative to sham tRNS reduced face memory performance in both younger and older adults. We also found that the degree of decline in performance in the active tRNS relative to sham tRNS condition was predicted by baseline ability, with higher performing participants showing the largest decreases in performance. Overall, the results indicate that tRNS to the VLPFC modulates face memory, but that there may be performance and protocol specific moderators of this effect. We discuss these findings in the context of the broader literature showing the importance of individual variation in the outcome of non-invasive brain stimulation intervention approaches. We conclude that while tRNS may have potential as an intervention approach, generalizing from single experiment studies to wide application is risky and caution should be adopted in interpreting findings

The Oxford Face Matching Test: A non-biased test of the full range of individual differences in face perception.

Tests of face processing are typically designed to identify individuals performing outside of the typical range; either prosopagnosic individuals who exhibit poor face processing ability, or super recognisers, who have superior face processing abilities. Here we describe the development of the Oxford Face Matching Test (OFMT), designed to identify individual differences in face processing across the full range of performance, from prosopagnosia, through the range of typical performance, to super recognisers. Such a test requires items of varying difficulty, but establishing difficulty is problematic when particular populations (e.g., prosopagnosics, individuals with autism spectrum disorder) may use atypical strategies to process faces. If item difficulty is calibrated on neurotypical individuals, then the test may be poorly calibrated for atypical groups, and vice versa. To obtain items of varying difficulty, we used facial recognition algorithms to obtain face pair similarity ratings that are not biased towards specific populations. These face pairs were used as stimuli in the OFMT, and participants were required to judge whether the face images depicted the same individual or different individuals. Across five studies the OFMT was shown to be sensitive to individual differences in the typical population, and in groups of both prosopagnosic individuals and super recognisers. The test-retest reliability of the task was at least equivalent to the Cambridge Face Memory Test and the Glasgow Face Matching Test. Furthermore, results reveal, at least at the group level, that both face perception and face memory are poor in those with prosopagnosia, and are good in super recognisers

Functional lateralization of temporoparietal junction: imitation inhibition, visual perspective taking and theory of mind

Although neuroimaging studies have consistently identified the temporoparietal junction (TPJ) as a key brain region involved in social cognition, the literature is far from consistent with respect to lateralization of function. For example, bilateral TPJ activation is found during theory of mind tasks in some studies, but only right hemisphere activation in others. Visual perspective taking and imitation inhibition, which have been argued to recruit the same socio-cognitive processes as theory of mind, are associated with unilateral activation of either left TPJ (perspective taking), or right TPJ (imitation inhibition). The present study investigated the functional lateralization of TPJ involvement in the above three socio-cognitive abilities using transcranial direct current stimulation. Three groups of healthy adults received anodal stimulation over right TPJ, left TPJ or the occipital cortex prior to performing three tasks (imitation inhibition, visual perspective taking and theory of mind). In contrast to the extant neuroimaging literature, our results suggest bilateral TPJ involvement in imitation inhibition and visual perspective taking, while no effect of anodal stimulation was observed on theory of mind. The discrepancy between these findings and those obtained using neuroimaging highlight the efficacy of neurostimulation as a complementary methodological tool in cognitive neuroscience