Magnetic and electrical brain responses to chromatic contrast in human

Differences between magnetic responses to red-green chromatic gratings and yellow-black luminance gratings were: (1) response waveforms differed considerably; (2) at some recording sites the chromatic grating response was considerably greater than the sum of responses to the red and green components of the chromatic grating; (3) the latencies of the successive peaks in the response to the onset of chromatic contrast were greater than the latencies of the corresponding peaks in the response to luminance contrast onset; (4) chromatic grating responses were lowpass with respect to spatial frequency while luminance grating responses were bandpass; (5) chromatic grating responses attenuated more steeply with increasing frequency above 2 c/deg than did luminance grating responses. Items (3)-(5) above are consistent with well-known psychophysical findings that contrast sensitivity is lowpass for chromatic gratings and chromatic responses are more sluggish than luminance responses. In subsidiary experiments we found that magnetic responses to red-green and blue-yellow equiluminant gratings had similar waveforms in all six subjects tested, but the topographical distributions were different in three subjects. The results of comparing magnetic and electrical responses to the onset and offset of contrast can be understood in terms of the considerable intersubject variability in the relation between neuroanatomy and cortical function that has been demonstrated by other techniques.