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The fame of Paul Broca as a precursor of cognitive neuroscience is mainly due to his groundbreaking work in describing brain lesions, most notably patient Leborgne’s speech disorder, and the correlation he drew with the left inferior frontal lesion that caused it, in a region known ever since as Broca’s area.1 Based on further cases of aphasia, he was also able to elucidate the left hemisphere’s systematic specialisation for language. Less recognised is his pioneering interest for in vivo brain imaging. He used measures of scalp temperature for the early diagnosis of brain lesions, as well as for functional imaging in normal subjects, which are today the two main applications of techniques such as magnetic resonance imaging.2 His imaging ideas were inspired from studies at the time that measured skin temperature to infer the localisation and mechanism of arterial lesions of the limbs, and to guide amputation whenever necessary.
In order to apply this approach to the field of neurology, Broca devised a “thermometric crown, allowing the simultaneous application of six thermometers around the head. It is made of a series of small identical cotton pockets, tied together with a circular band of elastic material, with thermometers placed inside the pockets”.2 He later improved this apparatus by means of two additional thermometers critically located over the inferior frontal gyrus “which is assigned to language”.
Broca was well aware that “the dura mater, the skull, the scalp constitute an obstacle to heat radiations”, and acknowledged that it was “an open question to what extent thermometric exploration could be used for the general diagnosis of brain diseases”.2 He nevertheless gained some optimism from the obstinate attempts he made in patients and normal subjects.
Thus, “when the sylvian artery is occluded [by an embolism], the temporal thermometer should show a lower temperature than on the healthy side”.2 Despite its obvious limitations, Broca tried to make the most of the coarse spatial resolution of the technique, observing that following sylvian infarcts “blood is provided by collateral pathways, ie...branches of the anterior and posterior cerebral arteries...which then become the site of an exaggerated blood flow. Hence one observes a higher temperature on the occipital and even more on the frontal thermometers, relative to the healthy side”.2
Broca also tried to infer underlying pathophysiological mechanisms, and surmised that progressive thrombosis, contrary to embolism, leads to increased rather than to reduced temperatures. “One of the first patients in which I made this exploration was a doctor from the provinces who was introduced to me by his son, himself a doctor. Speech was not abolished, but it was already severely impaired; there was no disorder of sensation or motility. I easily recognized with my hand that the temperature of the left temporal region was notably increased, and I observed, using two thermometers applied on the temporal regions, that this increase was of 3 degrees, an excessive figure which I never observed again.”2 He predicted that the condition should worsen as a consequence of an extension of a “congestive softening” to the whole hemisphere. Indeed, the patient’s son soon informed Broca that “speech was entirely abolished, that intelligence was deeply impaired, that the patient was bed-ridden”, and three months later that his father eventually died.
Broca also hypothesised that brain temperature should increase with the execution of cognitive tasks by normal subjects. Importantly, he thought that those variations should be local, mostly affecting frontal regions dedicated to higher brain functions. He observed that “following intellectual work, the frontal temperature increases more than the temporal temperature, and the latter more than the occipital temperature. One may confirm this by placing the thermometric crown on a resting subject. After about 20 minutes, when all thermometers have reached equilibrium, one asks the subject to perform a mental task. If he is only half-literate, if he does not read very fluently, one asks him to read a text aloud, and after a few minutes, one sees the thermometers rise, mainly the frontal ones”.2 Stressing the influence of cultural acquisitions on brain function, Broca noted that “the result is nil or extremely minuscule if the subject can read without difficulty. To make the cerebral temperature rise [in medical students], I had to give them a more arduous work, generally consisting in adding thirty five digit numbers. Then the thermometers, mainly the frontal ones, rose substantially”.
Because of a regrettable lack of confidence in this venerable thermometric technique, we did not endeavour to replicate Broca’s findings. Nevertheless, we deemed it justified to acknowledge his open mindedness and curiosity. There is no doubt Broca would have felt both perfectly comfortable and immensely excited with the present plethora of brain imaging techniques.