Spatiotemporal neural correlates of confidence in perceptual decision making

In our interactions with the environment, we often make inferences based on noisy or incomplete perceptual information - for example, judging whether the person waving their hand in the distance is someone we know (as opposed to a stranger, greeting the person behind us). Such judgments are accompanied by a sense of confidence, that is, a degree of belief that we are correct, which ultimately determines how we act, adjust our subsequent decisions, or learn from errors. Neuroscience has only recently begun to characterise the representations of confidence in the animal and human brain, however the neural mechanisms and network dynamics supporting these representations are still unclear. The current thesis presents empirical findings from three studies that sought to provide a more complete characterisation of confidence during perceptual decision making, using a combination of electrophysiological and neuroimaging methods. Specifically, Study 1 (Chapter 2) investigated the temporal characteristics of confidence in relation to the perceptual decision. We recorded EEG measurements from human subjects during performance of a face vs. car categorisation task. On some trials, subjects were offered the possibility to opt out of the choice in exchange for a smaller but certain reward (relative to the reward obtained for correct choices), and the choice to use or decline this option reflected subjects’ confidence in their perceptual judgment. Neural activity discriminating between high vs. low confidence trials could be observed peaking approximately 600 ms after stimulus onset. Importantly, the temporal profile of this activity resembled a ramp-like process of evidence accumulation towards a decision, with confidence being reflected in the rate of the accumulation. Our results are in line with the notion that neural representations of confidence may arise from the same process that supports decision formation. Extending on these findings, in Study 2 (Chapter 3) we asked whether rhythmic patterns within the EEG signals may offer additional insights into the neural representations of confidence. Using an exploratory analysis of data from Study 1, we identified confidence-discriminating oscillatory activity in the alpha and beta frequency bands. This was most prominent over the sensorimotor electrodes contralateral to the motor effector that subjects used to indicate choice (i.e., right hand), consistent with a motor preparatory signal. Importantly however, the effect was transient in nature, peaking long before subjects could execute a response, and thus ruling out a direct link with overt motor behaviour. More intriguingly, the observed confidence effect appeared to overlap in time with the non-oscillatory representation of confidence identified in Study 1. In line with the view that motor systems track the evolution of the perceptual decision in preparation for impending action, results from Studies 1 and 2 open the possibility that confidence-related information may also be contained within these signals. Finally, following on from our work in the first study, we next aimed to capitalise on the single-trial neural representations of confidence obtained with EEG, in order to identify potentially correlated activity with high spatial resolution. To this end, in Study 3 (Chapter 4) we recorded simultaneous EEG and fMRI data while subjects performed a speeded motion discrimination task and rated their confidence on a trial-by-trial basis. Analysis of the EEG revealed a confidence-discriminating neural component which appeared prior to participants’ overt choice and was spatiotemporally consistent with our results from the first study. Crucially, we showed that haemodynamic responses in the ventromedial prefrontal cortex (VMPFC) were uniquely explained by trial-to-trial fluctuations in these early confidence-related neural signals. Notably, this activation was additional to what could be explained by subjects’ confidence ratings alone. We speculated that the VMPFC may support an early and/or automatic readout of perceptual confidence, potentially preceding explicit metacognitive appraisal. Together, our results reveal novel insights into the neural representations of perceptual confidence in the human brain, and point to new research directions that may help further disentangle the neural dynamics supporting confidence and metacognition