Visual processing abnormalities in schizophrenia are commonly found, and contribute to poor social function.
There is evidence that schizophrenia affects perception of contrast and motion, control of eye movements, detection of visual contours, and recognition of faces or facial expressions. The specificity of many visual processing abnormalities in schizophrenia is still an area of active debate within the scientific community.
Perception of contrast
Contrast sensitivity
Contrast is a feature of visual stimuli that characterizes the difference in brightness between dark and light regions of an image. Perception of contrast is affected by the temporal frequency and spatial frequency properties of a stimulus, and the sensitivity to contrast in sine wave stimuli is characterized by the contrast sensitivity function. Contrast sensitivity has been shown to be impaired in schizophrenia. There is evidence that these impairments may be more severe among people with predominantly negative symptoms, or those who are not medicated. Butler and colleagues have proposed that people with schizophrenia may have a specific deficit in the magnocellular visual processing pathway, and electroencephalography (EEG) data have been presented that may support this view. Results from pharmacological studies in cats have demonstrated the role of NMDA in contrast perception of magnocellular-tuned stimuli. Application of drugs that deactivate this glutamate receptor type led to reduced neural responses in the visual system of cats, and some argue this suppression is similar to the reduced behavioral responses observed among people with schizophrenia. They claim these results are consistent with the glutamate hypothesis of schizophrenia, which proposes that dysfunction in this neurotransmitter system leads to abnormal neural activity underlying this disorder. Skottun and colleagues dispute the magnocellular deficit theory however, saying that there is not enough evidence from different research groups to support it, and that the experiments focused on this topic have shown very mixed results.
Surround suppression
The perceived contrast of a stimulus is sometimes suppressed when another stimulus is presented surrounding it, an effect known as surround suppression (see Figure 1), which is similar to the simultaneous contrast illusion. In schizophrenia, estimations of perceived contrast in surround suppression are less suppressed than for healthy adults. Further, the magnitude of this perceptual suppression effect has been shown to correlate with the concentration of GABA (γ-aminobutyric acid), an inhibitory neurotransmitter, in the visual cortex. These results may illustrate the role of GABA in mechanisms that regulate the overall level of neural activity in visual cortex, and it has been suggested that such mechanisms may be disrupted in schizophrenia. Such a disruption would be consistent with the GABA hypothesis of schizophrenia, which states that dysfunctional GABAergic inhibition may disrupt neural activity in subjects with this disorder, and this in turn may lead to visual processing abnormalities.
Motion processing
Motion perception is an important visual function and occurs from the earliest stages of cortical visual processing, with individual neurons being tuned to a preferred direction of motion. The cortical area MT (medial temporal cortex, also known as V5) plays a significant role in motion processing, and deactivation of this region using Transcranial magnetic stimulation can affect perception of motion. Subjects with schizophrenia have shown abnormalities in perceptual judgments of motion, speed and direction, with deficits in these judgments generally being reported. It has been suggested that these findings are related to the aforementioned magnocellular deficit purported to exist in this disorder. Inhibition of motion perception by the addition of a surround stimulus has also been examined in schizophrenia, with one group finding evidence both of impaired motion perception and weaker perceptual suppression effects in schizophrenia. This agrees with the findings mentioned previously related to weaker suppression of perceived contrast in this disorder. However, another recent report has disputed this finding, instead showing evidence consistent with stronger surround influence on motion perception in schizophrenia.