Transient and steady-state VEPs—reappraisal

We report here the recent findings in transient and steady-state VEPs that were recorded in our laboratory. Firstly, we studied the neural generators of the pattern reversal transients VEPs (T-VEPs). The optic tract responses recorded during stereotactic pallidotomy showed a P50–N80 complex. Visual evoked magnetic fields (VEFs) showed that the equivalent current dipoles (ECDs) of the N75, P100 and N145 were located in the primary visual cortex (V1). Secondly, we have developed a technique using multimodality VEPs, which can evaluate the parvocellular (color and form) and magnocellular systems (motion). T-VEPs to isoluminant chromatic stimuli showed N120, which was absent or delayed in patients with acquired color deficits. T-VEPs to apparent motion (AM) showed P120. Visual evoked magnetic fields showed that the ECDs of N120 and P120 were located in V1. Abnormality rates to color and apparent motion stimuli were different in optic neuritis, suggesting that these potentials distinguish between the two parallel visual pathways. Thirdly, we studied the physiological properties of the second (2F) and fourth harmonic (4F) components of the steady-state VEPs (S-VEPs). It was found that the 2F and 4F were differently tuned to the spatial and temporal frequencies of achromatic and chromatic stimuli. The oblique effect was also clearly demonstrated in the S-VEPs. From the above findings, a combined use of T-VEPs and S-VEPs could allow us to explore the human visual function.