Historical Note on Multisensory and Motor Facilitation and its Dependence on Brain Excitability Deficit

In the context of the great boom in research on multisensory processes initiated with the publication of The Merging of the Senses by Stein and Meredith (1993), and the great achievements since then, we note here the recent posthumous publication of Justo Gonzalo, which is the first English translation of his original publications. He described multisensory phenomena at a functional and macroscopic physiological level in patients with unilateral parieto-occipital cortical lesions in an associative area equidistant from the visual, tactile and auditory areas. The disorder is a multisensory and bilateral alteration called 'central syndrome'. Here we focus on some aspects related to the facilitation effect, i.e., the improvement in the perception of a test stimulus with the help of another stimulus. The greater the lesion and the lower the intensity of the test stimulus, the greater the facilitation effect. One of the most effective facilitating stimuli in these patients was found to come from the motor system, such as muscular effort. The gradation observed between different cortical syndromes led Gonzalo to introduce the concept of functional cortical gradients, whose superposition would result in multisensory zones. The fact that functional behaviour in the central syndrome is considered similar to that of a normal individual, but on a reduced scale of excitability, allows scaling concepts to be applied and some generalisations to be made.