Electrophysiological estimate of human cortical magnification.

OBJECTIVE

The cortical magnification factor characterizes the area of human primary visual cortex activated by a stimulus as a function of angular distance from an observer's line of sight. This study estimates human cortical magnification using an electrophysiological method with excellent temporal resolution: visual evoked potential (VEP) dipole source localization.

METHODS

For each of 60 independently modulated checkerboard patches within the central 18 deg of the visual field, location, orientation, magnitude, and time-course of the dipole current source that best described the VEP distribution across a multi-electrode array was obtained. At numerous eccentricities, cortical magnification was determined using two different techniques: (1) the distance between each pair of adjacent stimulus patches was matched to the corresponding distance between adjacent cortical sources; and (2) the area of each stimulus patch was matched to the magnitude of the corresponding cortical source (which was assumed to be proportional to cortical area).

RESULTS

The estimates of human cortical magnification using our electrophysiological method were similar to previous estimates from psychophysics, cortical stimulation, and functional magnetic resonance imaging.

CONCLUSIONS

The concordance of results provided by these disparate technologies, with differing spatial and temporal limitations, supports their combination in studying the spatio-temporal dynamics of human brain function.