GRADMAP Compute the gradient of an EEG spatial distribution.

Usage: >> [gradX, gradY ] = gradmap( map, filename, draw )

Inputs:
map   
level of activity (size: nbelectrodes x nbChannel)
filename   
filename (.loc file) countaining the coordinates of the electrodes, or array countaining complex positions Can also be a EEGLAB channel structure.
draw   
integer, if not nul draw the gradient (default:0)

Output:
gradX   
gradient over X
gradY   
gradient over Y (use cart2pol to get polar coordinates)

Author: Arnaud Delorme, CNL / Salk Institute, 2001

See the matlab file gradmap.m (may require other functions)

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