Inter- and intra-individual variability in perception: quantification approaches and investigations into modulating factors

The work presented in this thesis aims to explore human perceptual processes and their variability within and across individuals. Chapter 1 (literature review) presents the two main methodological approaches I adopted for studying perception: the oddball task with the associated P300 event-related potential, and the short-term monocular deprivation paradigm with the associated transient shift in ocular dominance. While both approaches have been traditionally discussed in connection with perceptual and cognitive processes, I discuss possible links with the physiological context at large, including autonomic and metabolic factors. Chapter 2 and Chapter 3 focus on the oddball task as a model of the “belief-updating” process, defined as the process through which we regulate our responses to external stimuli based on a combination of the stimuli themselves and our expectations generated from past experience. Results reported in Chapter 2 highlight the large inter-individual variability in the context-updating process and the consequent enhancement of responses to unexpected sensory stimuli. They suggest that three physiological indices reliably track this inter-individual variability: pupil dilation, EEG-P300 event related potential, cortical-desynchronization in the EEG beta band. Based on this evidence, I submit that pupil dilation responses provide a useful ad simple index for quantifying the perceptual and cognitive processes underlying belief updating. In Chapter 3, I apply this method in a clinical population with mild cognitive impairment or Alzheimer’s disease. The results indicate that pupil dilation responses provide a reliable read-out of cognitive impairment that is linked with degeneration in the noradrenergic-Locus Coeruleus system. Chapter 4 and Chapter 5 focus on short-term monocular deprivation, a well-established paradigm for testing plasticity in the adult human visual cortex. Chapter 4 presents a quantification of the inter-individual variability in this form of plasticity and links it with performance in other visual tasks – suggested to index intra-cortical inhibition. In Chapter 5, I report unprecedented evidence that metabolic factors affect this form of plasticity and generate considerable variability even within individuals, as plasticity is transiently reduced following overnight fasting. Chapter 6 presents a general discussion of the above results, in which I highlight the commonalities in the two paradigms used: both depending on the history of stimulation, either in the form of regularities generating expectations or in the form of sensory deprivation. I speculate on the role that the Locus Coeruleus - arousal system may play in weighting the impact of the stimulation history on current behavior, mediating the influences of contextual and metabolic effects on perception.