EFFECTS OF TIMING AND DURATION OF COGNITIVE ACTIVATION IN [O-15]WATERPET STUDIES

The multiple injection O-15water method offers unique opportunities for studying cognitive processing by the human brain. The influence of the duration and temporal placement of an activation task, in relatio n to the arrival of the radiotracer in the brain, is a fundamental met hodologic question for cognitive activation studies. A quantitative po sitron emission tomography (PET) study of five normal volunteers was p erformed in which the stimulation consisted of a visual activation tas k (alternating checkerboard pattern) superimposed on an auditory basel ine task (syllable monitoring). Ten injection conditions, with varying duration and timing of the visual activation, were used. Regional CBF (rCBF) in visual cortex was measured quantitatively using the autorad iographic method. A 20-s stimulation, centered on the bolus arrival in the brain, produced significant changes in rCBF. Because varying the duration and timing of the activation task technically violates the te mporal homogeneity assumption of the autoradiographic model, a mathema tical simulation was formulated to evaluate the potential influence of these variations. Results of the simulation are consistent with the P ET data and suggest that activation can be limited to a narrow tempora l window centered on the radiotracer uptake. The ability to observe si gnificant changes in rCBF with short stimulation intervals is of parti cular interest in the use of O-15water PET for studies of cognitive processes with a short course.