Dysfunction of early-stage visual processing in schizophrenia: harmonic analysis.

Schizophrenia is associated with severe neurocognitive deficits that constitute a core feature of the disorder. Deficits have been most extensively studied in relationship to higher-order processes. This study evaluated the integrity of early visual processing in order to evaluate the overall pattern of visual dysfunction in schizophrenia. Steady-state visual-evoked potentials (ssVEPs) were recorded over the occipital cortex (Oz) in patients with schizophrenia and schizoaffective disorder (N=26) and in age-matched comparison volunteers (N=22). Two stimuli were used: windmill-dartboard and partial-windmill, which are contrast-reversing ( approximately 4 Hz), radial patterns with dominant low spatial-frequency content. Each stimulus was presented for 1 min. Fourier analysis was performed on the ssVEP data to extract the relevant temporal frequency (i.e., harmonic) components. Magnitude-squared coherence (MSC) was computed to estimate the relative signal level for each frequency component. The patients showed reduced amplitude and coherence of second harmonic responses in both conditions, but intact first harmonic responses in the windmill-dartboard condition. This finding of a differential deficit may indicate a significant loss in the magnocellular pathway, which contributes to the generation of the second harmonic component under these conditions. Early sensory deficits may lead to impairments in subsequent stages of processing.