Human brain metabolic responses to familiarity during lexical decision

Local cerebral glucose metabolic rate was measured using F18-DG/PET in 11 normal righthanded male volunteers performing two versions of a visual lexical decision task differing only with respect to whether the stimuli were all novel and shown only once, or whether they were all repeated many times. Each subject was also scanned in a resting state (eyes open and ears unplugged). Scalp recordings of cognitive evoked potentials in the same subjects and tasks confirmed that the effects of familiarity were present during the entire 30 min FDG uptake period. The N4 (a left temporoparietal negativity 400 ms after stimulus onset) decreased with repetition, and the P3b (a 600 ms latency parietal positivity) increased. The pattern of metabolic activation was also significantly different between the three experimental conditions. Planned comparisons of the tasks with the resting condition revealed significant relative activations of visual and motor areas. Novel words evoked significantly greater activation in the left and right anterior cingulate gyri and right hippocampal formation than did repeated words in the same task. Relative glucose metabolism in the left angular gyrus was significantly greater to novel words than to resting. Thus, two tasks equated in sensory, motor, and decision processes, but differing in the familiarity of the stimuli, evoke significantly different patterns of brain activation. The patterns observed provided evidence that recent episodic memory for words engages a system that includes the bilateral anterior cingulate gyri, the medial temporal lobe, and the left angular gyrus. © 1994 Wiley-Liss, Inc.