The cortical topography of visual evoked potentials elicited by chromatic and luminance motion

Abstract

When motion-onset VEPs are elicited by moving luminance patterns, the motion specific component of the response, N2, is more prominent at electrode sites that overlay the lateral occipito-parietal cortex close to area V5/MT, than over the primary visual cortex. Functional segregation suggests that colour and motion processing should take place along different ventral occipito-temporal and lateral occipito-parietal pathways, respectively. Hence, a different topographical distribution might be expected for the motion-onset VEPs elicited by chromatic and luminance motion stimuli. We recorded motion-onset VEPs to moving luminance or isoluminant chromatic sinusoidal grating stimuli from five electrodes sites located at Oz, and at four locations (T1-T4) lateral to Oz, at intervals of 5% of the head circumference. Responses were recorded from 6 subjects over a range of speeds and contrasts. The results showed that the N2 component was maximal at similar lateral electrode locations (T2) for both luminance-defined and chromatically-defined motion. The earlier P1 component was of greatest magnitude at the occipital pole (Oz) and decreased with more lateral electrode placement and again this was the same for colour and luminance responses. These similarities suggest a common origin for VEPs elicited by colour and luminance defined motion.

Keywords: Colour; motion; visual evoked potentials (VEPs)..

Fig. (1)

(a) The electrode montage used to record motion-onset VEPs from the human scalp. Starting from Oz, each of the lateral electrodes (T1-T4) were placed at intervals equal to 5% of the observers’ head circumference. The ground electrode was placed on the forehead and linked ear electrodes acted as reference. (b) Sample motion-onset VEPs recorded from a single subject which were elicited by a moving high contrast L-M chromatic stimulus (10 deg/s) recorded from the five different electrode sites (Oz - T4).

Fig. (2)

P1 (a) and N2 (b) amplitudes measured from motion-onset VEPs elicited by luminance, L-M and S/(L+M) chromatic stimuli, plotted as a function of electrode placement. This graph, like all the following, represents averages taken from 6 observers. Data are shown for high contrast stimuli of speeds ranging from 1 - 10 deg/sec. Average standard error approximated to twice the size of the data points.

Fig. (3)

P1 (a) and N2 (b) amplitudes plotted as a function of electrode location for moving chromatic stimuli of different luminance ratios. Small amounts of luminance contrast were introduced into chromatic stimuli by shifting the luminance ratio (up to +/- 0.1) away from the individually set isoluminant point. This was done for both L-M and S-(L+M) chromatic stimuli at speeds of 10, 5 and 2 deg/sec.

Fig. (4)

P1 and N2 amplitudes plotted as a function of electrode location for luminance, L-M and S/(L+M) stimuli of varying contrast. The contrast values for the luminance stimuli are in terms of Michelson contrast, those for the L-M and S-(L+M) chromatic stimuli are given in terms of a computed cone contrast value (see text for details). Data are shown for moving stimuli of relatively high (10 deg/sec), medium (5 deg/sec) and low (2 deg/sec) speed.