Effects of Stimulus Size and Contrast on the Initial Primary Visual Cortical Response in Humans.

Decades of intracranial electrophysiological investigation into the primary visual cortex (V1) have produced many fundamental insights into the computations carried out in low-level visual circuits of the brain. Some of the most important work has been simply concerned with the precise measurement of neural response variations as a function of elementary stimulus attributes such as contrast and size. Surprisingly, such simple but fundamental characterization of V1 responses has not been carried out in human electrophysiology. Here we report such a detailed characterization for the initial "C1" component of the scalp-recorded visual evoked potential (VEP). The C1 is known to be dominantly generated by initial afferent activation in V1, but is difficult to record reliably due to interindividual anatomical variability. We used pattern-pulse multifocal VEP mapping to identify a stimulus position that activates the left lower calcarine bank in each individual, and afterwards measured robust negative C1s over posterior midline scalp to gratings presented sequentially at that location. We found clear and systematic increases in C1 peak amplitude and decreases in peak latency with increasing size as well as with increasing contrast. With a sample of 15 subjects and ~180 trials per condition, reliable C1 amplitudes of -0.46 µV were evoked at as low a contrast as 3.13% and as large as -4.82 µV at 100% contrast, using stimuli of 3.33° diameter. A practical implication is that by placing sufficiently-sized stimuli to target favorable calcarine cortical loci, robust V1 responses can be measured at contrasts close to perceptual thresholds, which could greatly facilitate principled studies of early visual perception and attention.