T Technical Aspects of Auditory Processing:The auditory system of vertebrates evolved after the appearance of the vestibular system and out of what was part of that system.


As man's brain evolved, the cortical auditory area did not increase in its capacity for auditory perception ?..but instead it developed a speech center. That center evolved topographically next to the sensory area for hearing and the motor and somatosensory area for the lower face (V and VII). Both phylogenetically and ontogenetically language development shows a dependence upon auditory processes. (Ayres SI and LD p 237)


The most primitive structures for auditory functions lie in the brain stem. Auditory coding begins soon after stimuli enter the brain and many connections with other ongoing processes offer an opportunity for further processing before the stimuli reach the cortex. At the brain stem level the processes are not lateralized as to dominance in the same sense that speech is lateralized.


In other words below the cortex, both sides of the brain are involved in similar auditory tasks. Although speech production in the adult requires cortical action, the vocalizations at and soon after birth reflect brain stem function, indicating presence of motor as well as sensory mechanisms at that level, and serving as a reminder that the brain stem carries mechanisms for well integrated sensorimotor responses of a primitive type.


The reticular system, offers a major opportunity for integration of auditory information with that from other sensory sources. A considerable number of auditory fibers terminate in the reticular formation and presumably are involved in the total integrative activity there.The mesencephalic reticular formation is the seat of considerable sensory convergence and it is to this area that many auditory stimuli are sent as well as vestibular and possibly other proprioceptive information.


A uditory stimuli are among the most commonly reported modalities showing convergence, suggesting both the importance and the means of establishing associations between auditory stimuli and other sensory modalities. Vestibular and acoustic stimuli are commonly found to activate the same neural structure.


It is conjectured that intersensory integration is an important contributant to the total auditory-language developmental sequence and can be utilized effectively in the intervention program.Another critical role of the brain stem and thalamic area in auditory-language function is that which has been hypothesized by many to be a major system for integrating cortical with sub-cortical information for final analysis. (Centrencephalic system - Penfield)


Myers (1967) Work on lower Primates - Conjectured that the method of organizing the different portions of the cortex for speech production is through systems projecting down to brain stem centers as opposed to organizing them through association fibers in the pre-central somatic motor cortex.General evidence from the study of man favors an interpretation that man's speech relies on both sub-cortical as well as callosal communication.


Ayres hypothesizes that it was these sub-cortical mechanisms that accounted for the enhanced reading scores of children with auditory-language problems who received sensory integration therapy.One of the earliest insights as to the involvement of lower brain centers in language was made by Penfield and Roberts (1959) who asserted that comprehension of speech occurred only after impulses were received in the higher brain stem and both cortical areas and while there was interaction between the higher brain stem and left hemisphere. Speech itself occurred after interaction between the higher brain stem and left hemisphere; the impulses traveled to both cortical motor areas directing muscles used in speech.