Is the perception of illusions abnormal in schizophrenia?

There seems to be no common factor for visual perception, i.e., performance in visual tasks correlates only weakly with each other. Similar results were found with visual illusions. One may expect common visual factors for individuals suffering from pathologies that alter brain functioning, such as schizophrenia. For example, patients who are more severely affected by the disease, e.g., stronger positive symptoms, may show increased illusion magnitudes. Here, in the first experiment, we used a battery of seven visual illusions and a mental imagery questionnaire. Illusion magnitudes for the seven illusions did not differ significantly between the patients and controls. In addition, correlations between the different illusions and mental imagery were low. In the second experiment, we tested 59 patients (mostly outpatients) with ten visual illusions. As for the first experiment, patients and controls showed similar susceptibility to all but one visual illusion. Moreover, there were no significant correlations between different illusions, symptoms, or medication type. Thus, it seems that perception of visual illusions is mostly intact in schizophrenia.

About individual differences in vision.

In cognition, audition, and somatosensation, performance strongly correlates between different paradigms, which suggests the existence of common factors. In contrast, visual performance in seemingly very similar tasks, such as visual and bisection acuity, are hardly related, i.e., pairwise correlations between performance levels are low even though test-retest reliability is high. Here we show similar results for visual illusions. Consistent with previous findings, we found significant correlations between the illusion magnitude of the Ebbinghaus and Ponzo illusions, but this relationship was the only significant correlation out of 15 further comparisons. Similarly, we found a significant link for the Ponzo illusion with both mental imagery and cognitive disorganization. However, most other correlations between illusions and personality were not significant. The findings suggest that vision is highly specific, i.e., there is no common factor. While this proposal does not exclude strong and stable associations between certain illusions and between certain illusions and personality traits, these associations seem to be the exception rather than the rule.

Linking perceptual learning with identical stimuli to imagery perceptual learning.

Perceptual learning is usually thought to be exclusively driven by the stimuli presented during training (and the underlying synaptic learning rules). In some way, we are slaves of our visual experiences. However, learning can occur even when no stimuli are presented at all. For example, Gabor contrast detection improves when only a blank screen is presented and observers are asked to imagine Gabor patches. Likewise, performance improves when observers are asked to imagine the nonexisting central line of a bisection stimulus to be offset either to the right or left. Hence, performance can improve without stimulus presentation. As shown in the auditory domain, performance can also improve when the very same stimulus is presented in all learning trials and observers were asked to discriminate differences which do not exist (observers were not told about the set up). Classic models of perceptual learning cannot handle these situations since they need proper stimulus presentation, i.e., variance in the stimuli, such as a left versus right offset in the bisection stimulus. Here, we show that perceptual learning with identical stimuli occurs in the visual domain, too. Second, we linked the two paradigms by telling observers that only the very same bisection stimulus was presented in all trials and asked them to imagine the central line to be offset either to the left or right. As in imagery learning, performance improved.