Stimuli to differentiate the neural response at successive stages of visual processing using the VEP from human visual cortex.

Background: Clarifying the enigmatic relationship between stimulus property, neural response and the evoked potential is essential if non-invasive functional imaging is to make a meaningful contribution to the understanding of how maturational or degenerative processes influence brain activity. Visual cortex has proven a favourite target to elucidate this relationship. However, to date most studies involving the visual system have yielded inconsistent results or have been strongly criticised.

New Method: We developed a set of three visual stimuli, two of which either had the same low- or high spatial frequency characteristic. Adult volunteers viewed these as pattern reversing stimuli while the scalp electric potential was recorded using a 10-10 array of electrodes.

Results: Established processing mechanisms of the primate visual system enabled us to link the amplitude of the N75 and P100 to the size of the neural population processing the temporal luminance contrast, and the amplitude of the N135 and P240 to the size of the neural processing the spatial luminance contrast in our stimuli. Calculating the distribution of current source density enabled us to identify the neural source of each VEP component.

Conclusions: Demonstrating a direct relationship between the temporal- and spatial luminance contrast properties of our stimuli and the size of the neural population involved provides a better understanding of the nature of the relationship between stimulus property, neural response and the VEP. It also shows that EEG can contribute in a significant manner to the study of the influence of maturational or degenerative processes on brain activity.