Origin of wavelets in the visual evoked potential

Low amplitude high frequency wavelets have been demonstrated to be ubiquitous in the visual system of animals and are observed in the ERG of man. Wavelets have also been observed superimposed upon large slow waves obtained from electrodes on occipital scalp. Presently, the rather stereotypic wavelet repetition rate permitted the use of active analog filters tuned to 100 Hz, with a 60-200 Hz bandpass which produced no measurable distortion at 100 Hz. With this method we recorded a series of wavelets in 15 of 16 subjects that usually began from 35 to 40 msec following the onset of a 10(4) troland, 25 degrees visual angle 100 msec flash. We then sought to determine the origin of these wavelets. Wavelets recorded between occiput (Oz) and vertex (Cz) had photopic spectral sensitivity that differed from that of ERG wavelets simultaneously recorded. Moreover, in a topographical study, wavelets recorded between Cz and various lateral positions reached a maximum at Oz; and when adjacent bipolar electrode pairs were used, wavelet polarities inverted as pairs were moved across the occipital region. Thus wavelets recorded between 35 and 70 msec were likely generated from occipital cortex rather than retinal, sub cortical, or diffuse cortical sites. In addition, the topographical distribution of slow wave ('P100') and wavelets differed. Wavelet latencies had a different relation to retinal illuminance than P100 latencies, suggesting that P100 and wavelets have different neurogeneses. Wavelets recorded between Oz and Cz thus reflect the earliest cortical visual processing recorded in man.