Nervous Excitability Dynamics in a Multisensory Syndrome and its Similitude with Normals: Scaling Laws (pp. 161-190)
Authors: (Isabel Gonzalo-Fonrodona, Miguel A. Porras, Departamento de Optica, Facultad de Ciencias Fýsicas, Universidad Complutense de Madrid, Ciudad Universitaria, Madrid, Spain, and others)
Abstract: In the context of increased number of works published on multisensory and crossmodal
effects related to human cerebral processing and brain plasticity, we review
the phenomenology and interpretation of a cortical syndrome that is associated with a
unilateral parieto-occipital lesion in a rather unspecific (or multisensory) zone of the
cortex. This syndrome is closely related to recent findings. The patients with this
syndrome suffered from bilateral and symmetric multisensory disorders with dynamic
effects dependent on the extent of nervous mass lost and the intensity of the stimulus.
They also presented cross-modal effects. The physiological interpretation of this
syndrome has led to a rather unitary conception of brain dynamics. It can help us
to understand perception laws on a physiological basis and can open new avenues of
research. A key point is the similitude of this syndrome with normals, in the sense
that this syndrome would be the result of a scale reduction in the cerebral excitability
(assumption based on neurophysiological data). This will permit us to understand
the desynchronization between different sensory qualities due to their successive loss
according to a well-defined physiological order that is determined by their different excitability
demands. The first qualities lost when the nervous excitation diminishes are
themost complex ones. This process seems to followthe allometric laws of dynamical
systems that have been subjected to a scale reduction.
The striking disorder of inverted perception (visual, tactile, or auditive) in this
syndrome is compared to other cases of visual inversion reported in the bibliography.
Special attention is also paid to the noticeable capability of improving perception by
intensification of the stimulus or by means of another type of stimulus (cross-modal),
muscular effort being one of the most efficient and less known means. This capability
was found to be greater as the nervous excitability deficit (lesion) was greater and as
the primary stimulus was weaker. Thus, in a normal subject, this capability is much
weaker (or negligible), though still perceptible for functions with high excitability demand,
as shown in recent research. We also review the proposed scheme of functional
cortical gradients where the specificity of the cortex is distributed with a continuous
variation, in relation to other proposals and recent findings. The model would account
for multisensory or cross-modal interactions.
Experimental data of perception (including cross-modal effects) in central syndrome
cases are fitted using Stevens’ power law. This law is in relation to allometric
scaling power laws dependent on the active neural mass, that seem to govern many
biological neural networks.
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