There was an intriguing citation in ASCAP from an article in "Science."
"In premammalian tetrapods, the middle ear evolved as a chain of bones attached to the mandible and cranium, but in adult mammals the chain is detached from the mandible and lies behind it. The neocortex evolved concurrently with detachment of the chain. In mammalian development the auditory chain arises connected to the mandible but later detaches, recapitulating the phylogenetic transformation, in modern didelphid (marsupial) development, the auditory chain reaches mature size by the third week after birth and is then separated from the jaw and displaced caudally as the neocortex grows for another 9 weeks." ASCAP, Sept 1996, pp 21-22. (1)
I sent for the original article, kindly supplied by Timothy Rowe along with a second paper on "Brain heterochrony and origin of the mammalian middle ear" (2) of the University of Texas at Austin. I learned that mammals were defined in Linnean tradition by the separation of the ear bones from the jaw. As time worked on all of us, our incus, malleus, & stapes became more specialized for higher frequency hearing and separated from our jaw. (3) The separation appears to be a product of cerebral enlargement. Rowe describes the growth of two bubbles above the midbrain, bubbles that push forward and backward over the original brain. The developing brain exerts hydrostatic pressure (due to closing of ventricular exits) as well as pressure secondary to the growth of neurons and forces the skull to grow larger. The growing brain thus tears the auditory structure from the jaw and moves them backward on the skull.
Mammals are also characterized by the "inside-out" development of the cortex as neurons migrate in columns upward from the ventral aspect, forming columns as they move. The sensory and motor refinements seen in mammals are a consequence of this unique movement and include "an extended range of auditory sensitivity, enhanced olfaction, and the sensory functions of hair. Also distinctively mammalian are the development of corticospinal pathways and well-developed specific motor nuclei which receive afferents from the cerebellum or basal ganglia, project to specific restricted regions of the cortex, and are situated rostrally in the ventral half of the thalamus." (4)
Much, if not most or all, of this growth occurs postnatally.
(There are soft castinets, Speculation lightly dances for me again.)
ADHD is often associated with a smaller right orbital pole (ordinarily larger than the left in humans) and changes in the size of the underlying basal ganglia. ADHD is also theorized to reflect an defect in motor inhibition, a defect that underlies subsequent ontogenetic failures in language incorporation and retrieval, planning, emotional regulation, event analysis, and problem solving. (5) Also, some 40% of ADHD kids have chronic ear infections, even requiring ear tubes to relieve hydrostatic pressure.
Hearing can be an inhibitory factor for ongoing response sequences, even ones that are visually elicited. (When running, I use my hearing to spot cars, trucks, & buses from the rear or coming around corners to the front. Tires produce a high frequency hiss that is usually more noticeable than engine noise. ADHD kids complain especially about friends talking in class as a distractor from the teacher's lecture. After stimulants are given, some of them comment that they can track both the teacher and their friend concurrently.) Still, I had earlier missed the hint in the pervasive hints thrown by client complaints about sound in their daily lives ... disruptive, disorganizing effects.
Harry can't concentrate on the television if his wife is talking in next room on the phone. Pete cranks up the television volume so that he can't hear his parents or sister in the background. Unfortunately, his parents can't concentrate themselves because of his TV noise. Walt (and I) are both annoyed by rattling change in the car box or by noisy wiper blades. Neither of us appreciate other people's talking during movies. Kids' popping their knuckles really bugs some parents. My sniffles annoyed my mother to anger. Certainly, random noise significantly interferes with "thinking," conceived as covert rehearsal of motor sequences.
I am not suggesting that ear infections "cause" ADHD, considered to be the most heritable of psychiatric disorders. But, has anyone tracked familial incidence of ear infections or auditory distractibilty in regard to familial incidence for ADHD children? Maybe it would be fruitful to separate auditory distractibility from ADHD in our analyses given that many people, including manics, also find random noise, particularly speech and high frequency sounds, to be disruptive.
It could be that those ear infections, occuring at about the same ontogenetic stage as cortical elaboration, are far more damaging than many now think.
1) Rowe T (1996) Coevolution of the mammalian middle ear and neocortex. Science, 273, 651-654.
2) Heterochrony, defined by Gould (in Keller E & Lloyd E (1992), Eds., Keywords in Evolutionary Biology, Cambridge MA: Harvard Univ Press) reflects changes in the timing of the development of individual organs in the same organism, during ontogeny. It has a second use, referring to changes in the timing of the development of corresponding organs when compared to an ancestor.
3) Hearing and feeding were mediated by the same bony structure in mammalian ancestors. It's an interesting coincidence that low frequency sounds were likely during eating. Could associated vocalizations have been paired with jaw motion to become a foundation for oral communication? Fun stuff!
4) Quoted from Rowe T, (1996) Brain Heterochrony and Origin of the Mammalian Middle Ear. In "Memoirs of the California Academy of Sciences, #20, New Perspectives on the History of Life, p. 90. The original is interlaced with extensive citations. The original is available from him at the Department of Geological Sciences and Vertebrate Paleontology, University of Texas at Austin, Austin, Texas 78712.
5) Barkley R (1996) Attention Deficit/Hyperactivity Disorder. In Mash E & Barkley R (eds) "Childhood Psychopathology," NY: Guilford. See Barkley R (1997) ADHD and the Nature of Self Control. NY: Guilford, for a fuller exposition of impairment of executive functions in ADHD.