Early visual evoked potentials: an indicator of bioelectrical activity of the lateral geniculate nucleus?

In non-human primate experiments Schroeder et al. (1992) demonstrated that visual evoked activity of the lateral geniculate nucleus (LGN) can be recorded on the frontolateral epidural surface of macaque monkeys. So far it is not known whether analogous visual evoked potentials can be found on the scalp surface of humans.

The search for early visual evoked responses was performed in healthy volunteers with the following technique: white Ganzfeld-flash stimulation (< 100 µsec, 0.81 cd/m2s, frequency 1.1 Hz), low frequency filter 1 Hz, high frequency filter 100 Hz (12 db/octave), averaging of 100 stimulations. A topographic map of responses was recorded at standard electroencephalography scalp positions. In addition to these methods the following parameters were modified: stimulation (flash versus checkerboard reversal, monocular versus binocular), recording of right and left hemisphere responses, frequency filtering and frequency analysis.

Scalp mapping of VEP responses indicated an early reproducible response with a wide frontolateral distribution consisting of a small positive and a larger negative potential (= GER - presumed geniculatum evoked response). A closer analysis of responses was performed at 6 different standard electroencephalography scalp positions electrode positions (F2, F4, FC2, FC4, FC6, C4) in 7 healthy volunteers. At FC4 the amplitude of the GER was maximal (4.8 ± 2.9 µV). The peak latencies were: positive wave 31.8 ± 2.8 msec, negative wave 43.6 ± 2.3 msec. Monocular stimulation resulted in nearly identical responses on both hemispheres. Amplitudes increase at binocular stimulation by + 44 % compared to monocular flashes. Frequency filtering and spectrum analysis clearly demonstrate that the GER mainly consists of < 30 Hz components. A comparable response with checkerboard reversal could not be detected after averaging of 100 stimulations and a rudimentary GER was found after 5000 sweeps.

An early wide spread visually evoked potential could be identified in humans that seems to be identical to the surface recorded response of the LGN that was previously described in non-human primate experiments.