Representational Plasticity In The Auditory Cortex

This is the authors' abstract. We don't add key points for this paper.

Sensory cortices for touch, vision, and audition all represent their sensory receptor surfaces using topographic maps. The capacity of these topographic maps in sensory cortices to be reorganized as a function of previous experiences has been demonstrated in adult animals.1-5 In the auditory system, several studies have demonstrated that the representation of sound frequency in adult animals can be substantially remodeled following long-term conditioning with tonal stimuli paired with electrical stimulation of one of the brain's learning centers.6,7 As we learn more about the brain's remarkable ability to reorganize, it is becoming increasingly clear that aspects of the sensory input can guide dramatically different forms of plasticity.8-11 Most studies of auditory cortical plasticity have characterized changes evoked by relatively simple sounds (e.g., tones), even though most sounds encountered in the real world are broadband and have time-varying features (e.g., speech and vocalizations). To extend previous studies with simple stimuli, we have been investigating how long-term experience with complex sounds alters the tonotopic map of frequency. In one of these experiments, activation of the nucleus basalis of Meynert, located in the basal forebrain, was repeatedly paired with the presentation of the word “sash” to adult rats that were awake and unrestrained. After 4 weeks of such pairing, we reconstructed a detailed map of the auditory cortex from up to 100 microelectrode penetrations. We observed that the group conditioned with “sash” alone exhibited a large high-frequency map expansion relative to naïve controls. This robust effect may be the consequence of simple Pavlovian conditioning principles whereby the brain dynamically adapts its response to the acoustic cue that predicts the reward. Since the high-frequency “s” neurons were always activated first and were rewarded every time “sash” was presented, the auditory cortex simply expanded the high-frequency sector of the tonotopic map. To probe how non-reinforced inputs influence the expression of cortical plasticity in a subgroup of the animals conditioned with the sound “sash,” we also presented unpaired sounds. The map expansion strategy became unavailable when these unpaired background sounds (“s,” “a,” “sh,” and “sash” played backwards) were presented during the daily pairing sessions. In this group, all of the same cortical territories are engaged, but not rewarded every single trial. Thus, the two experimental groups had the same reinforced input, same number of trials, and the same activation of the brain's learning center. Interestingly, the expression of this map plasticity effect is blocked by a relatively simple manipulation in the statistics of the sensory environment: having unpaired background sounds that activate the same cortical regions of the map. This tells us that we have only scratched the surface of our understanding of basic principles underlying brain plasticity. In addition to reorganization of tonotopic maps, the central nervous system must use other strategies to learn and adapt to the environment. Much progress was made during the “Decade of the Brain,” but one of the exciting challenges for auditory neuroscientists will be to unravel the mystery of how the brain learns complex sounds and deals with the natural statistics of our sensory environment. Continued basic and clinical research will be needed to provide a precise and detailed understanding on how sensory experience guides neural plasticity. This knowledge will likely aid in the development of more effective training strategies to ameliorate neurologic deficits associated with auditory system impairments. As audiologists become more involved in auditory rehabilitation, we believe that the insights derived from neuroscience research on brain plasticity will yield promising new therapies for effective “neurorehabilitation.”Figure. PandyaPritesh K. Pandya is a PhD candidate engaged in neural plasticity research at the UT-Dallas School of Behavior and Brain Sciences. He earned a master's degree in audiology from the University of Tennessee and was employed at the National Center for Rehabilitative Auditory Research located in the Portland VA Medical Center. He was awarded the AAA Student Investigator Grant in 2001 and is a recipient of a National Research Service Award (NRSA) for individual pre-doctoral fellows from the National Institute on Deafness and other Communication Disorders. His research adviser and sponsor for the pre-doctoral fellowship is Michael Kilgard, PhD, assistant professor of cognition and neuroscience at UTD. E-mail [email protected].