The many faces of attention and emotion: Behavioural and functional imaging studies in neglect patients
Patient with hemispatial neglect may lose awareness of stimuli contralateral to their brain lesion (most often involving the right parietal lobe) when a competing stimulus is simultaneously presented in the ipsilesional field - a phenomenon called visual extinction. However, in keeping with the fact that many early visual areas are still intact in these patients, several neuropsychological studies have shown that extinguished stimuli may still be processed at a preattentive or even unconscious level, sometimes up to a stage where shape, colour, or semantic features can be extracted, yet without awareness. I will present a series of behavioural experiments showing that the nature of the task and/or the nature of the stimuli can determine whether contralesional events are perceived or "extinguished" by the patients. For example, some aspects of face processing seem to occur automatically in the contralesional hemifield despite contralesional inattention and extinction, including extraction of emotional significance. This suggests that some (preattentive or unconscious) processing may still occur in ventral occipito-temporal pathways and amygdala. More recent studies using event-related fMRI and evoked potentials studies in patients with neglect and extinction have investigated the neural fate of contralesional visual stimuli when these either escape or reach awareness, and confirmed that unseen stimuli can still produce activity in the ventral visual system, amygdala, and even orbitofrontal cortex without awareness. On the other hand, awareness appears to be specifically associated with greater activity in distributed brain areas at both early and later stages of visual processing.