Sir, Sandrone et al. (2012, 2013) rediscovered, translated, and commented on the manuscripts of Angelo Mosso (1882, 1884), in which Mosso described his ‘human circulation balance’; James (1890) described this as a
‘delicately balanced table which could tip downwards either at the head or the foot if the weight of either end were increased’.
Mosso claimed that the balance allowed him to observe changes in cerebral blood volume associated with mental effort and emotional responses, and consequently the balance is regarded as the direct forerunner of modern non-invasive functional neuroimaging techniques. However, Sandrone et al. (2012, 2013) stated that
‘we have no direct evidence that the balance was really able, as stated, to measure changes in cerebral blood flow during acts of cognition … despite its proven ability to measure blood volume changes in various organs (e.g. lungs, feet, hands)’.
In our laboratory, we recently constructed a balance similar to Mosso’s, and using modern data collection and analysis methods that were unavailable to Mosso, we investigated whether the balance was sensitive to changes in cerebral blood volume produced by modulating the level of mental activity. The construction and mechanism of our balance is depicted and explained in Fig. 1, and may be compared with Figs 3 and 8 in Sandrone et al. (2013), which show Mosso’s apparatus. The balance is a class 1 lever, in which the moment of a force measured at the fulcrum is proportional to the magnitude of the force and its distance from the fulcrum. With a participant lying on the balance across the fulcrum, if mental activity produces a net shift of blood towards or away from the head then this will produce a slight change in the centre of mass of the participant relative to the fulcrum of the lever, …