Some remarks on the use of visually evoked potentials to measure magnocellular activity

It has been claimed that magnocellular activity can be assessed by measuring the second harmonic responses in visually evoked potentials (VEPs) to On/Off flickering stimuli. The empirical support for this claim is examined. It is noted that: (1) there is in some instances a failure to differentiate counterphase flicker from On/Off flicker. (2) The suggestion that magnocellular activity can be assessed from second harmonic VEP responses was based on the assumption that magnocellular and parvocellular responses correspond, respectively, to transient and sustained responses. This assumption has been undermined by recent quantitative research. (3) Second harmonic responses can be obtained with isoluminant color stimuli. (4) The attenuation of second harmonic responses at high temporal frequencies is not specific to chromatic stimulation. (5) Also, VEPs to contrast reversing stimuli show reduced amplitudes in the case of chromatic stimulation. It is therefore difficult to link second harmonic response to On/Off flicker specifically to magnocellular activity. It is concluded that second harmonic responses in VEPs should only be used with caution, if at all, to assess magnocellular activity.