In Ales et al (2010a), scalp topographies resulting from activation of discrete visual areas V1, V2 and V3 were simulated based on retinotopic mapping data of 27 subjects. Consistent with the known projections of the lower visual field to the dorsal, mostly upward-facing division of V2/V3, and of the upper field to the ventral, mostly downward-facing division (Wandell et al., 2009), they found that simulated V2/V3 scalp topographies inverted in polarity for upper and lower locations. Upper-lower field polarity inversion is one of the more popularly known properties of the earliest component of the human visual evoked potential (VEP), “C1” (e.g. Martinez et al., 1999; Di Russo et al., 2002) and one of several properties comprising the classic “cruciform model” which proposes a primary visual cortical (V1) source for the C1 (Jeffreys and Axford, 1972a,b; Clark et al., 1995). Combined with the results of constrained source modeling from another study indicating simultaneous onset of V1 and V2 (Ales et al., 2010b), Ales et al. (2010a) took this extrastriate polarity inversion as evidence that the C1 may be generated in V2 and/or V3 rather than in V1. In our original comment (Kelly et al., 2013) we contested this claim, specifically criticizing 1) their critically incomplete definition of the cruciform model, 2) their inappropriate use of large stimuli that blur V1-consistent topographical shifts, and 3) their neglect of intracranial findings in non-human primates. In their reply, Ales et al (2013) contest the first issue on the basis of the neuroanatomical literature, and remedy the second and third issues by showing the results of new simulations of smaller appropriately-placed stimuli, and by reviewing the relevant human and non-human literature. In the following, we first examine the new simulation results with reference to empirical data for the same visual field locations (Di Russo et al., 2002); we then discuss the systematic within-quadrant topographical shifts that form a critical but neglected part of the unabridged cruciform model, and illustrate them using population-averaged VEP data and anatomical MRI; finally, we briefly address some of the conclusions made by Ales et al. (2013) on the non-human primate literature.
['Simon P. Kelly', 'M. Isabel Vanegas', 'Charles E. Schroeder', 'Edmund C. Lalor']
The cruciform model of striate generation of the early VEP, re-illustrated, not revoked: A reply to Ales et al. (2013)☆