BRAIN-BEHAVIOR RELATIONSHIPS - EVIDENCE FROM PRACTICE EFFECTS IN SPATIAL STIMULUS-RESPONSE COMPATIBILITY

1. We measured relative cerebral blood flow (rCBF) changes with positr on emission tomography and (H2O)-O-15 in six normal subjects repeatedl y performing a spatial stimulus-response compatibility task. Subjects had two motor response conditions. They were instructed to respond wit h the left hand to a left visual held light stimulus and with the righ t hand to a right visual field light stimulus (compatible condition), and with the right hand to a left visual held light stimulus and with the left hand to a right visual held light stimulus (incompatible cond ition). Six rCBF measurements per condition were performed in each sub ject. 2. Reaction times were faster (P < 0.0005) in the compatible (28 7 ms) than the incompatible (339 ms) condition (spatial stimulus-respo nse compatibility effect). A bilateral increase (P < 0.05) in rCBF in the superior parietal lobule of the two hemispheres was observed durin g the incompatible condition when compared with the compatible one. No rCBF decreases were observed. Reaction times correlated (P < 0.0001) with the rCBF in the two activated superior parietal lobule areas. 3. Reaction times decreased with practice according to a Linear trend (P < 0.05). Practice-related linear rCBF increases (P < 0.05) were observ ed in the dorsolateral prefrontal, premotor, and primary motor cortex of the left hemisphere. No significant rCBF decreases were observed. 4 . Practice did not affect the spatial stimulus-response compatibility effect. A parallel shortening of reaction times was observed in both c ompatible and incompatible conditions, in both left and right hand res ponses, and in both left and right visual fields. Accordingly, when rC BF was analyzed, the spatial stimulus-response compatibility by practi ce interaction did not show any significant activated area. 5. These f indings suggest that the two activated areas in the left and right sup erior parietal lobules subserve the mapping of the visual stimulus spa tial attributes onto the motor response spatial attributes and that th e rCBF increases in the incompatible response condition represent the more complex computational remapping required when stimuli and respons e do not match spatially. 6. The dorsolateral prefrontal, premotor, an d motor rCBF linear increases in the left hemisphere seem to reflect t he effect of practice on cortical processes common to both compatible and incompatible response conditions. These cortical processes presuma bly strengthen the links between stimuli and responses under different stimulus-response compatibility conditions. The lateralization of the rCBF increases suggests a left hemisphere superiority in these proces ses.