EEG Correlates of Backward Masking in Schizotypy

Visual deficits are well documented in schizophrenia, especially in backward masking. In our task a Vernier is followed by a blank inter-stimulus interval (ISI) and then a grating mask. Observers indicate the direction of the Vernier offset (left vs right). Schizophrenia patients need ISIs that are 120ms longer than in healthy controls. Interestingly, these masking deficits are reflected in strongly reduced EEG amplitudes in patients when compared to controls. Schizophrenia is considered to be on a continuum ranging from strongly affected patients to healthy people with schizotypic personality traits. Schizotypic traits are measured with self report questionnaires such as the short O-LIFE. In analogy to patients, schizotypic traits are divided in positive traits (e.g., hallucinations), negative traits (e.g., anhedonia) and cognitive disorganization (CogDis). It has been shown that healthy students scoring high in CogDis show higher masking deficits than students scoring low in CogDis. On average, high CogDis students need ISIs that are 20ms longer than in low CogDis students. Here, we show that preselected high as compared to low CogDis students show reduced EEG amplitudes, but to a lower degree than do patients. We computed the Global Field Power (GFP), which is the standard deviation across all 192 EEG electrodes. The GFP is a measure of the overall strength of brain activity. We found deficits at 200ms after stimulus onset associated with the N1 component. This component is related to ventral stream processing and fine shape discrimination. We suggest that mechanisms necessary to boost faint stimuli are deficient in schizophrenia patients and attenuated in high CogDis students. Elevated CogDis seems to be a risk factor for schizophrenia.