The laplacian analysis of the pattern onset response in man.

The pattern onset VEP described by means of principal component analysis was shown to be composed of at least two overlapping time components, of which one has a striate and the other an extrastriate origin. This analysis was based on a multi (24)-electrode registration. We looked for a technique that can distinguish, with a limited number of electrodes, between the striate and extrastriate components and can be applied in the clinic. The calculation of a 5-point 'Laplacian' operator over the striate area enhances the contribution of the striate source relative to the contribution of the extrastriate source that lies outside the area of the operator, and vice versa. We applied two 5-point operators, one centered on Oz and the other 6 cm left of Oz. This montage allowed for the selective recording of the striate and extrastriate activities separately. This was proved by means of an adaptation paradigm that selectively reduced the striate contribution. Application of the laplacian operator for clinical practice is exemplified by means of the VEPs of a patient suspected of psychogenic hemianopsia.