Neuromagnetic evidence for early access to cognitive representations.

How do physical and cognitive properties of stimulus words influence the neuromagnetic response of the human brain? Are the physiological correlates of these properties dissociable and at which latencies can they be observed? Short and long words, as well as rare and common words, were repeatedly presented in a memory task while neuromagnetic brain responses were recorded using MEG. Word length and frequency were reflected by brain responses at overlapping but distinct intervals. The influence of the physical factor, length, started at approximately 100 ms after onset of written words, immediately followed by a physiological manifestation of the non-physical cognitive stimulus property, word frequency, which was first apparent at 120-160 ms. There was a differential frequency effect: neurophysiological correlates of short words showed the frequency influence much earlier than did longer words. These data indicate that non-physical cognitive aspects of word stimuli can be reflected in early neuromagnetic responses, and that the latency of these physiological correlates of cognitive stimulus properties may depend on the physical stimulus make-up.