Neurological
-They are "split-brained" (the two hemispheres don't communicate well).
-Mostly subcortical (neurons of cortex lack myelin).
-Mostly reflexive (react to stimuli but can't control what they do).
Respiratory
-Infant lungs are proportionally larger.
-Infants use more pressure and have less efficient control over expiration.
-Paradoxing (Abdomen and chest may move in opposite directions when babies/children breath. Gone by 3 years old).
Respiratory differences are thought to account for slower rate and less coarticulation in children.
Oral/Pharyngeal/Laryngeal
-Oral cavity and pharynx not well differentiated (gradual bend).
-Mandible and tongue both very high
-Oral cavity flat and almost filled by the tongue.
-Pharynx is very short. Larynx is very high.
-Arytenoid cartilages are proportionally large (so less vibratory tissue).
-Poor differentiation of the vocal fold layers.
These 2 factors result in little pitch control. Adult proportions by age 1 year (vocal tract proportions of adult by 6-8 months).
The largest changes in the physical/neurological speech system occur from 0-12 months and 12-24 months.