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The effect of the basal cerebral blood flow (CBF) on both the magnitude and dynamics of the functional hemodynamic response in humans has not been fully investigated. Thus, the hemodynamic response to visual stimulation was measured using blood oxygenation level-dependent (BOLD) functional magnetic resonance imaging (fMRI) in human subjects in a 7-T magnetic field under different basal conditions: hypocapnia, normocapnia, and hypercapnia. Hypercapnia was induced by inhalation of a 5% carbon dioxide gas mixture and hypocapnia was produced by hyperventilation. As the fMRI baseline signal increased linearly with expired CO2 from hypocapnic to hypercapnic levels, the magnitude of the BOLD response to visual stimulation decreased linearly. Measures of the dynamics of the visually evoked BOLD response (onset time, full-width-at-half-maximum, and time-to-peak) increased linearly with the basal fMRI signal and the end-tidal CO2 level. The basal CBF level, modulated by the arterial partial pressure of CO2, significantly affects both the magnitude and dynamics of the BOLD response induced by neural activity. These results suggest that caution should be exercised when comparing stimulus-induced fMRI responses under different physiologic or pharmacologic states.

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Studies in primate physiology and human functional neuroimaging have convincingly shown that the area of the brain termed MT/V5(+)-which includes the middle temporal visual area MT/V5 along with adjacent motion-sensitive areas such as MST--is involved in the processing of motion information [1,2]. Tootell et al. [3] showed that the blood oxygenation level dependent (BOLD) signal measured by functional magnetic resonance imaging (fMRI) in the human MT/V5+ seemingly correlates with the strength of perceived motion aftereffect (MAE), the illusory motion of a stationary pattern that one sees after adapting to a moving pattern [4]. The signal in MT/V5+ decayed slowly during the period when the MAE was seen. It is possible that this slow decrease in MT/V5+ activity was unrelated to the perceptual experience of motion. After replicating Tootell et al.'s experiment, a modified version of the experiment was conducted in which a blank period was inserted between the adapting motion stimulus and the stationary testing pattern. The results demonstrated that MT/V5+ activity indeed decayed more slowly after an effective unidirectional motion adaptation than after bidirectional adaptation, without corresponding perception of MAE. Nevertheless, in a more conclusive experiment, we adapted observers to a unidirectional motion for a very long period and showed that the activity in MT/V5+ changed in synchrony with the presence and absence of perceived MAE, simply as a result of presenting a stationary visual stimulus in and out of the adapted retinal region.

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Effects of the size of corpus callosum measured from in vivo magnetic resonance imaging (MRI) recordings on cortical activations evaluated using functional MRI (fMRI) were analyzed during motor tasks. Twelve right-handed men performed unilateral finger movements and bilateral movements either with or without a temporal delay between left and right fingers. The size of the rostral part of corpus callosum and the anterior and posterior callosal truncus explained 11.9 and 15.2% of activation in the mesial frontal cortex in unimanual left and right finger movements, respectively. In bimanual simultaneous movements, 34.2% of the activated voxels in the mesial frontal cortex were related to the size of corpus callosum. In bimanual movements in which left finger movement preceded the onset of the right finger movement, the callosal size accounted for 88.7% of activation in the mesial frontal cortex. In contrast, when the right finger movement preceded the left, callosal size accounted for only 31.3% of the mesial frontal cortex activation. The correlations between callosal parameters and activation over the lateral cortex were sparse and occurred only in bimanual movements. The results suggest that corpus callosum modulates the activity of the supplementary motor and cingulate cortical areas depending on temporal complexity of bimanual movements.

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