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The neuronal network involved in a precise type of calculation procedure, mental subtraction, was investigated by means of functional magnetic resonance imaging. Two tasks were used requiring covert production of numbers: (1) with calculation; (2) without calculation. During the first task, activation was observed in the left dorsolateral prefrontal and premotor cortices, in Broca's area and bilaterally in the inferior parietal cortex. During the second task, activation was mainly observed in Broca's area and to a less extent in the left prefrontal and premotor cortices. Statistical comparison of data in the two situations revealed that the procedure of mental subtraction is mediated by a distributed system which includes predominantly the left dorsolateral prefrontal cortex and the inferior parietal cortex bilaterally.

Effects of magnetization transfer were evaluated in vitro on circulating blood. Various velocities were tested from 0 to 72 cm/second. Decrease signal intensity caused by magnetization transfer effects was inversely proportional to inflow velocity. It reached 10% at very low velocities and disappeared at velocities higher than 30 cm/second.

)werful inhibitions of dorsal horn neurons are triggered in animals by noxious stimulation of large cutaneous areas. This tenomenon described as “Diffuse Noxious Inhibitory Controls” (DNIC) involves supraspinal structures and can be observed, beit indirectly, in man. The question arises as to whether cortical network can be modified by the involvement of DNIC. The asent study was designed to examine the central effects of noxious heat applied to large areas, namely 560 (one hand) or 1120 n2 (two hands) using WRI.

The cerebral blood volume (CBV) has been measured by transverse axial tomography. The attenuation coefficient of 100 “pixels” located in a well determined region was measured before and after injection of a contrast material (Telebrix 38). The attenuation coefficient of blood samples was measured by CT in a phantom. Blood samples were drawn before and after injection at the same time as the tomographic section (time: half rotation of the scanner). The volume was calculated by the formula: CBV ml%=difference in Acta Number of tissue (before-after injection)difference in Acta Number of blood (before-after injection) The values obtained with very early measurements (1–5 min after injection), were very close to those measured by other methods (3–5 ml/100 ml). For 5 patients, the CBV was measured by the same method at different times: 20 measures between 2 and 50 min after the injection. The CBV seemed to vary in function with time according to the equation: CBV in ml%:ate-bt + V0 (t = time in minutes; a = integration constant, a = 1.94; b = time constant, b = 0.089; V0 = real CBV). The apparent variation of CBV was the result of two factors related to the blood-brain barrier, (1) water movement from the extravascular space towards the intravascular space, related to the hyperosmolarity of the contrast medium. The variations of osmolarity and haematocrit values measured after contrast injection did confirm this hypothesis; and (2) iodine movement from the intravascular space towards the extravascular space, due to the toxicity of the material (hyperosmolarity, Na ions, etc.). The measurement of CBV by CT is only possible if the contrast materials do not modify the blood-brain barrier. The use of actual materials allows the study of the blood-brain barrier behaviour and enables to make an estimate of the cerebral extra-cellular space. But CT is not a right method of CBV measurement with the contrast medium in use nowadays.