Functional gradients through the cortex, multisensory integration and scaling laws in brain dynamics

In the context of the increasing number of works on multisensory and cross-modal effects in cerebral processing, a review is made on the functional model of human brain proposed by Justo Gonzalo (1910–1986), in relation to what he called central syndrome (caused by unilateral lesion in the parieto-occipital cortex, equidistant from the visual, tactile and auditory projection areas). The syndrome is featured by a bilateral, symmetric and multisensory involvement, and by a functional depression with dynamic effects dependent on the neural mass lost and related to physiological laws of nervous excitability. Inverted or tilted vision as well as tactile and auditive inversion, under minimum stimulus, appears as a stage of incomplete integration, being almost corrected under higher stimulus or facilitation by multisensory integration. The syndrome reveals aspects of the brain dynamics that suggest a functional continuity and unity of the cortex. A functional gradients scheme was proposed in which the specificity of the cortex is distributed with a continuous variation. This syndrome is interpreted as a scale reduction in the nervous excitability of the system, the different sensory qualities being affected allometrically according to scaling laws. A continuity from lower to higher sensory functions was proposed. The sensory growth by an increase of the stimulus or by multisensory facilitation is found to follow approximately power laws, that would reflect basic laws of biological neural networks. We restrict the analysis to the visual system.