Critical Period - Auditory Processing

Auditory Processing

Many studies have supported a correlation between the type of auditory stimuli present in the early postnatal environment and the development on the topographical and structural development of the auditory system.

First reports on critical periods came from deaf children and animals that received a cochlear implant to restore hearing. Approximately at the same time, both an electroencephalographic study by Sharma, Dorman and Spahr and an in-vivo investigation of the cortical plasticity in deaf cats by Kral and colleagues demonstrated that the adaptation to the cochlear implant is subject to an early, developmentally-sensitive period. These data demonstrate, both for human children and for animals, that understanding the critical period has consequences for medical therapy of hearing loss.

Recent studies have examined the possibility of a critical period for thalamocortical connectivity in the auditory system. For example, Zhou and Merzenich (2008) studied the effects of noise on development in the primary auditory cortex in rats. In their study, rats were exposed to pulsed noise during the critical period and the effect on cortical processing was measured. Rats that were exposed to pulsed noise during the critical period had cortical neurons that were less able to respond to repeated stimuli; the early auditory environment interrupted normal structural organization during development.

In a related study, Barkat, Polley and Hensch (2011) looked at how exposure to different sound frequencies influences the development of the tonotopic map in the primary auditory cortex and the ventral medical geniculate body. In this experiment, mice were reared either in normal environments or in the presence of 7 kHz tones during early postnatal days. They found that mice that were exposed to an abnormal auditory environment during a critical period P11- P15 had an atypical tonotopic map in the primary auditory cortex. These studies support the notion that exposure to certain sounds within the critical period can influence the development of tonotopic maps and the response properties of neurons. In general, the early auditory environment influences the structural development and response specificity of the primary auditory cortex.