The steady state visually evoked potential topography dynamics in schizophrenia

We report on the topography of the steady state visually evoked potential (ssvep) recorded from twenty normal male subjects and twenty five patients with a history of schizophrenic symptomatology; recruited at the mental health research institute. All subjects were required to visually fixate on a central point on a computer screen for two minutes while brain electrical activity was recorded from 64 scalp sites. Then the visual steady state stimulus was presented for one minute. The ssvep was elicited by superimposing a spatially uniform 13 hz flicker on the visual field and fourier techniques were used to calculate the ssvep magnitude at each scalp site. The ssvep analysis, was centred on the 60 seconds prior to and after the visual stimulus was turned on. Spherical spline interpolation techniques were employed to determine the distribution of the ssvep magnitude topography. The major difference in ssvep topography between the groups was the ratio of prefrontal to occipital ssvep magnitude; normal subjects demonstrated prefrontal ssvep magnitude approximately 60% of the maximum occipital value. By contrast, schizophrenic prefrontal ssvep values were significantly lower than the occipital values. This reduction in prefrontal ssvep will be discussed in the context of ''hypofrontality'' and possible medication effects