It is difficult to relate neurophysiology to the responses of human EEG recorded VEP due to the close proximity of multiple cortical areas with temporally overlapping activity, and the human cortex’s complex and variable folding. Non-human primate single-cell studies are the primary means to investigate this relationship. Inferring from primate recordings researchers develop stimuli that target visual pathways and areas. The effects of the Magnocelluar and Parvocellular pathways on the VEP have received much attention in this way. (Spekreijse et al., 1977; Tobimatsu et al., 1995, 1996; Klistorner et al., 1997; Baseler and Sutter, 1997; Rudvin et al., 2000; Ellemberg et al., 2001; Rudvin and Valberg, 2006; Foxe et al., 2008).
The Magno and Parvo pathways are particularly suited to segregation by stimulus manipulation. Magno cells respond with high gain to luminance contrast, but poorly to color only differences. Parvo cells are less sensitive to luminance contrast, but respond well to chromatic differences due to their spectral opponency. Magno cells give a high gain response to contrast, but saturate at low contrast levels (10-15%) (Livingstone and Hubel, 1987, 1988; Zemon and Gordon, 2006). Parvo cells do not saturate as contrast increases, however, they do not respond well to contrasts lower than 10-20% (Lee et al., 1993).
Less attention has been dedicated to the V1 and V2 influences on the human VEP. Hood et al. (2006) relate human mfVEP to a V1 contrast response model created by Heeger et al. (2000) that combines human fMRI and monkey single-cell recordings. They found the model predicts the mfVEP response well up to 40% contrast, with deviations afterwards. They also noticed a difference in latency of the mfVEP and the single-cell recordings. Where the mfVEP changed in latency with contrast by less than 5 ms; single-cell recording latency decreases by nearly 40 ms over the range of 5% to 95% contrast. Park et al. (2008) performed a similar contrast response-function comparison of mfVEP and fMRI from 4% to 90% contrast in human. They found a linear relationship between BOLD response in V1 to the mfVEP response. They also determined Heeger et al.’s model did not fit their results.
Park et al. reason that while their and Heeger et al.’s models are both relating electrical activity to the BOLD response, Heeger et al.’s use of single-cell recording relies on spike activity, while the BOLD and VEP are more likely driven by synaptic activity and local field potentials as they are slower potentials (Logothetis et al., 2001; Logothetis and Wandell, 2004). This difference highlights the difficulty in attempting to relate single-cell results to non-invasive human responses. Decomposing the VEP to directly determine its V1 and V2 components has only relatively recently met with good success, (Di Russo et al., 2001; Hagler et al., 2008; Goh, 2008; Ales et al., 2007, 2010).
This study used the dipole models created with the Interactive Dipole Fitting and Simultaneous Retinotopy (IDSR) method to measure stimulus effects on human V1 and V2 activity. Current source analysis was performed on PPmfVEP recordings of luminance contrast and red-green isoluminant stimuli.
Inverso, S. A. “Evoked Currents in Human Visual Cortex.” (PhD Dissertation) The Australian National University, Canberra, ACT, AU (2010).