Neural correlates of visual perceptual awareness and short-term memory

The general aim of the experiments described in my Thesis is to study how the brain creates a representation of the external world. In particular I have investigated three different aspects that make possible our subjective visual experience: Visual processing, short-term memory and the resulting perceptual awareness. The first part of the Thesis deals with visual perception and visual awareness by describing experiments involving behavioral tasks in brain damaged and healthy subjects. In particular, it is focused on when and where awareness arises. We used moving stimuli, because they can reach the motion area V5 bypassing the primary visual area V1 and therefore motion represents a suitable feature to study unconscious vision. Chapter 1 deals with the differences in the event-related potential (ERP) response between static and moving gratings in healthy subjects. We found that motion selectively affects N1, i.e. a negative component mainly recorded in the posterior electrodes of the right hemisphere. This study was preparatory to the experiment described in Chapter 2 in which we tested patients with hemianopia to understand how plastic reorganization of visual areas can reinstate unconscious (“blindsight”) or conscious vision. As to the main result of this study, we found one patient who showed both implicit above chance discrimination performance with motion stimuli and an unrelated presence of degraded awareness of the stimulus presented to the damaged field. As to ERP recording, we found reliable responses correlating either with the patient’s unconscious above chance behavior (Late Posterior Negativity) or with the subjective experience of degraded vision during stimulus presentation (N1-P2a components). Taken together, these results suggest an onset of visual awareness (albeit rudimentary) during early visual processing. The second part of the Thesis deals with short-term memory (STM), that is, a crucial processing stage for the representation of the external world. The aim of this part is to investigate inter-individual differences in STM capacity and the effect of cognitive training. Instead of using a classical approach in which subjects are actively involved 7 in a task we employed a resting state paradigm. Chapter 3 describes a study using magnetoencephalography (MEG) to investigate connectivity at rest to assess if individual differences in intrinsic brain activity can predict inter-subject variability in STM. We found that intrinsic brain activity can predict the behavioral performance tested outside the scanner, and that can play a key role in performing a successful STM task. In Chapter 4 we have tested the possibility that an intensive STM training could modify the functional connectivity, that is, if there are differences in resting state networks (RSNs) before and after STM training. In particular, we asked whether three different kinds of training (Verbal, Visual and Spatial STM) could affect connectivity in the fronto-parietal RSNs and interconnected areas. We found that, at behavioral level, training had a specific effect on the different kinds of task, while at connectivity level, we did not find any changes in the RSNs. Presumably, the absence of a significant effect might be due to the use of three different types of training that increased the results variability. In addition, probably the training was too short to have effects on intrinsic brain activity, even though it was sufficient to improve behavioral performance.