Decreases in frontal and parietal lobe regional cerebral blood flow related to habituation.

We previously reported decreased mean CBF between consecutive resting conditions, ascribed to habituation. Here we address the regional specificity of habituation over three consecutive flow studies. Regional CBF (rCBF) was measured in 55 adults (12 right-handed men, 12 right-handed women, 14 left-handed men, 17 left-handed women), with the 133Xe inhalation technique, during three conditions: resting, verbal tasks (analogies), and spatial tasks (line orientation). Changes in rCBF attributable to the cognitive tasks were eliminated by correcting these values to a resting equivalent. There was a progressive decrease in mean rCBF over time, reflecting habituation. This effect differed by region, with specificity at frontal (prefrontal, inferior frontal, midfrontal, superior frontal) and inferior parietal regions. In the inferior parietal region, habituation was more marked in the left than the right hemisphere. Right-handers showed greater habituation than did left-handers. There was no sex difference in global habituation, but males showed greater left whereas females showed greater right hemispheric habituation. The results suggest that habituation to the experimental setting has measurable effects on rCBF, which are differently lateralized for men and women. These effects are superimposed on task activation and are most pronounced in regions that have been implicated in attentional processes. Thus, regional decrement in brain activity related to habituation seems to complement attentional effects, suggesting a neural network for habituation reciprocating that for attention.