Dissociating perceptual experience and sensation: the case of visual imagery and visual suppression

Behind the apparently simple process of “seeing”, there are complex neural mechanisms involved that are not completely understood. Visual perception consists of two key components: information analysis and subjective awareness. This thesis reports some behavioural experiments to cast further light on two conditions in which perceptual experience and sensation are dissociated: visual imagery and visual suppression. THE CASE OF VISUAL IMAGERY: A long standing issue is whether perception and mental imagery share similar cognitive and neural mechanisms. To cast further light on this problem we compared the effects of real and mentally generated visual stimuli on simple reaction time (RT). In five experiments we tested the effects of difference in luminance, contrast, spatial frequency, motion and orientation. With the intriguing exception of spatial frequency in all other tasks perception and imagery showed qualitatively similar effects. We found a correspondence between perception and imagery effects for luminance, contrast, speed of motion, and line orientation. In contrast, we found an interaction between perception and imagery for spatial frequency: gratings of low spatial frequency were responded to more quickly than those of higher spatial frequency only for visually presented stimuli. Thus, the present study shows that basic dependent variables exert similar effects on visual RT either when retinally presented or imagined. The present results undoubtedly provide support for some overlap between the structural representation of perception and imagery. THE CASE OF VISUAL SUPPRESSION: Binocular rivalry occurs when the inputs from the two eyes are incompatible and cannot be fused into a single, coherent percept. The visual inputs reach alternatively consciousness for a few seconds and while one percept is seen (dominant) the other is invisible (suppression). The neural mechanisms underlying binocular rivalry have been much debated. Recent evidence shows a correlation between the activity in the striate cortex and the subjectively reported state of rivalry. However, little is known about the role of subcortical processing stages. Here we provide evidence for an involvement of subcortical pathways during binocular rivalry. To this purpose, we employed the redundant target effect (RTE) with stimuli undergoing binocular rivalry. RTE simply means that response to two (or more) targets is faster than to one. It is mediated by a spatial summation process where subcortical structures, likely involving the superior colliculus, play an important role. In this study, Gabors were presented in a way to generate binocular rivalry on one side and fusion on the other side of the fixation cross. Target/s was/were a contrast increment of the Gabor that could be visible or perceptually suppressed. The effects of the same redundant target were compared when it was dominant and it was suppressed. The RTE found when the redundant target was dominant disappeared when the same target was perceptually suppressed. Since the RTE was ascribable to a neural coactivation, the present finding suggests that visual suppression involves the RTE pathway. These results support the idea that binocular suppression involves visual processes at early stages, prior to the primary visual cortex.