Comments on Paragraph 1: (other paragraphs and references to follow)
Here are several articles that indicate a loss of inhibition possibly resulting from GABAergic downregulation with some directly related to increased spontaneous activity. So far I cannot find the actual article that I read that stated the 20dB reference although I’m relatively confident that it does exist..
Morita, T.; et.al.: Enhanced activation of the auditory cortex in patients with inner-ear hearing impairment: a magnetoencephalographic study Clin Neurophysiol 2003;114: 851-859
Rajan, R.: Receptor organ damage causes loss of cortical surround inhibition without topographic map plasticity. Nature Neurosci 1998;1: 138-143
Pantev, C. & Lutkenhoner, B.: Magnetoencephalographic Studies of Functional Organization and Plasticity of the Human Auditory Cortex. J Clin Neurophysiol 2000;17: 130-142
Rauschecker, J.P.: Auditory cortical plasticity: a comparison with other sensory systems. Trends Neurosci 1999;22: 74-80
Zheng, X.Y.; et.al.: Auditory Nerve Fiber Responses Following Chronic Cochlear De-Efferentation. J Comp Neurol 1999;406: 72-86
Memory with Mild Hearing Losses:
van Boxtel, M.P.; et.al.: Mild Hearing Impairment Can Reduce Verbal Memory Performance in a Healthy Adult Population. J Clin Exp Neuropsychol 2000;22: 147-154
Pearman, A.; Friedman, L.; Brooks, J.O. & Yesavage, J.A.: Hearing Impairment and Serial Word Recall in Older Adults. Exp Aging Res 2000;26: 383-391
Gold, M.; Lightfoot, L.A. & Hnath-Chisolm, T: Hearing Loss in a Memory Disorders Clinic: A Specially Vulnerable Population. Arch Neurol 1996;53: 922-928
Palmer, C.V.; et.al.: Managing Hearing Loss in a Patient with Alzheimer disease. J Am Acad Audiol 1998;9: 275-284
Gates, G.A.; et.al.: Central Auditory Dysfunction, Cognitive Dysfunction and Dementia in Older People Arch Otolaryngol Head Neck Surg. 1996;122: 161-167
Uhlmann, R.F.; et.al.: Relationship of Hearing Impairment to Dementia and Cognitive Dysfunction in Older Adults. JAMA 1989;261: 1916-1919
Lindenberger, U.; Scherer, H. & Baltes, P.B.: The Strong Connection Between Sensory and Cognitive Performance in Old Age: Not Due to Sensory Acuity Reductions Operating During Cognitive Assessment. Psychol Aging 2001;16:196-205
This study simulated a hearing loss (resulting in a temporary acute HL – not SN) which found no decrease in memory performance. However, it is possible that the study did not take into account the physiological changes resulting from long-term sensory deprivation which may weaken synaptic density of auditory responses fibers throughout the brain. (SeeBrain Plasticity, Advances in Neurology, Vol. 73 Ed. Freund, H-J.; Sabel, B.A. & Witte, O.W. Lippincott-Raven Publishers, Philadelphia 1997 83-90 for a description of deprivation cascade.) This may refer to that known to occur after complete deafferentation, but it’s been awhile since I read through those chapters so I cannot say right now at what point or level these changes occur.
Question: Is it reasonable to assume that decreased stimulation reduces dendritic arborization which supports the Cognitive Research Hypothesis? The "Use it or Lose It" phenomenon encourages people to remain mentally and physically active. Does this not apply to maintaining the highest degree of neurophysiological integrity within the auditory system by making sure an appropriate "normal" level of auditory stimulation is maintained?