Source derivation of the visually evoked potential.

There has been much recent conjecture over the apparent source location of the visually evoked potential, particularly in considering the lateralization found in halffield pattern reversal stimulation. This paper presents an alternative by directly identifying the sinks or sources present in the scalp current flow as these correspond to the maximal current flow into or out of the cortex (Nunez, 1981). Hjorth (1975) introduced a practical procedure for source derivation which has proved useful in the interpretation of the electroencephalogram (Wallin and Stalberg, 1980). We have applied source derivaton to cortical evoked potentials and present results obtained from right and left half-field stimulation, (Clement, Flanagan and Harding, 1985) and upper and lower field stimulation in normal subjects and patients with visual deficits.