Multistable visual phenomena. The role of executive functions in the perceptual reversal tasks.

My thesis concerns multistable visual phenomena. Multistability refers to the phenomenon of perceptual switching perception between possible interpretations of an unchanging stimulus.This topic has received attention especially during the last half of the XX century but still there is no common agreement about the underlying processes. Indeed the multistable perception processes have been categorized as bottom-up and top-down theories (automatic vs. cognitive processes). In order to investigate the neural substrates involved in the perceptual reversal tasks, two experiment were performed by means transcranial magnetic stimulation (TMS). In the first experiment has been investigated the involvement of the right dorsolateral prefrontal cortex and occipitotemporal cortex in perceptual reversal, by means repetitive transcranial magnetic stimulation (rTMS). Thirty right- participants (fourteen males) took part in the study. 1 Hz rTMS (off-line, 10% submotor threshold) was administered for 600s before the task Subjects were divided in three groups depending on the stimulation condition: Group 1: rTMS applied over the right dorsolateral prefrontal cortex; Group 2: occipito-temporal cortex; Group 3: Sham. After rTMS, a total of 110 black-white images (6X9, 9X6, 9x9) were randomly presented: 34 figure-ground reversible images, the same 34 disambiguate images, and 21 non-reversible images and the same 21 neutral modified images. The subject had to decide whether the image was reversible or neutral. A repeated measures Anova with group (frontal, occipito-temporal, sham) as between subjects variable and image (reversible, disambiguated, neutral) as within subjects variable was run on RTs of correct trials and on accuracy. Both the main effect of group and image were significant (p= .011; p< .0001) the interaction group x image was also significant (p= .001) on RTs. Post-hoc tests highlighted slower RTs for disambiguated and neutral images in the frontal group as compared to the occipito-temporal group (p= .014 and p= .016) and sham group (p= .019 and p= .006). In the case of accuracy, the main effect of image was significant (p< .001). The aim of the second experiment was to clarify the results of the previous one. Specifically, the goal was to understand if the presence of disambiguated and neutral image could influence the recognition of reversible pictures. Then in this experiment only reversible image were showed. 1 Hz rTMS (off-line, 10% submotor threshold) was administered for 900s before the task. Subjects were divided as in the previous experiment. After rTMS, a total of 84 figure-ground reversible pictures (6X9, 9X6, 9x9) were randomly presented. The stimuli were all black and white. Each trial consisted of a fixation cross at the center of the screen for 500 ms, followed by the appearance of the stimulus. The subject had to press (with his/her right hand) the button on the keyboard when he/she was able to reverse the figure and ground. Then, the subject had to describe both percepts in the picture. Several practice trials were presented, in order to familiarize with the procedure and the type of stimuli. A one-way Anova with group (frontal, occipito-temporal, sham) as independent variable showed significant effects of group on RTs of correct trials and on accuracy. The frontal group showed slower RTs than occipito-temporal and sham group (both p < 0.05) and a mean of correct responses significantly lower than other two groups (p< 0.05). Results showed that the dorsolateral prefrontal cortex is involved in perceptual reversal processes. rTMS applied on the dorsolateral prefrontal cortex affects the ability to switch between percepts, and to select between the neutral and reversible image. Results supported the "top-down" hypothesis, which suggests that reversal takes place during perceptual interpretation, therefore, at the highest level of visual hierarchy.