To improve the signal to noise and to make it easier for the reader to see the differences among subjects, the individual mfVEP responses in
Figure 2 were grouped as shown in
Figure 1B and summed. In particular, the 36 sectors of the three most peripheral rings, falling between 5.8° and 22.2°, were divided into six groups of six sectors and their responses summed. These are the responses falling outside of the blue circles in
Figures 2A and
2B. The central 4 sectors, falling within 1.2° of the foveal center and within the black circles in
Figures 2A and
2B, were divided into two groups as shown in
Figure 1B. (Notice the grouping is different for channel 1 as opposed to channels 2 and 3.) Finally, the remaining 20 sectors of the middle 2 rings, falling between 5.8° and 1.2° and between the black and blue circles in
Figures 2A and
2B, were divided into two groups of 4 sectors and four groups of 3 sectors and their responses summed. For all groups, the responses were summed within regions producing responses of similar waveforms in most individuals (
Klistorner & Graham, 1999;
Hood, Odel, et al., 2000,
Hood, Zhang, et al., 2000). Further, in the case of channel 1, the responses from the upper and lower fields are, in general, reversed in polarity as expected from the topography of V1 in the calcarine fissure (
Baseler et al., 1994; and see Figure 8 in
Hood & Zhang, 2000). Although the precise relationship between the mfVEP and the traditional pattern-reversal VEP (e.g.,
Harding, Odom, Spileers, & Spekreijse, 1996) has yet to be determined, the first prominent peak of the mfVEP appears to correspond to N75 and the second to P100. In channel 1, the polarity of N75 tends to be positive in the lower field and negative in the upper field, although variations exist across the hemi-fields due to variations in the folding of local regions of the cortex (see
Figures 3,
5, and
8 in
Hood & Zhang, 2000). Similarly, the responses for channels 2 and 3 typically are reversed in polarity as the vertical midline is crossed. Thus, for the central responses, the upper two and lower two were grouped together in the case of channel 1 and the left and right two in the case of channels 2 and 3.