Brain Imaging of Auditory Learning Functions in Rats: Studies with Fluorodeoxyglucose Autoradiography and Cytochrome Oxidase Histochemistry

For brain imaging of learning functions, the primary goal of the study is to detect learning-related changes in quantitative functional measures of neural activity. Both of the imaging techniques used in the studies reported here operate on the basis that activity of brain cells is coupled to energy metabolism. Functional changes are considered as evidence for the participation of a brain region in a learning paradigm relative to its activity in a control condition that differs only on the learned relation between the stimuli. Isolating the underlying metabolic processes contributing to the activity change related to learning is not, therefore, central to the goal of these studies. The findings presented here illustrate how an acoustic stimulus modifies the activity of the auditory system when the stimulus acquires different behavioral roles through learning. The studies also map, in detail, the rat brain to identify the learning effects of the acoustic stimulus outside the auditory system. Two learning paradigms were used: long-term habituation of the acoustic startle reflex, and differential Pavlovian conditioning of acoustic stimuli. These represent nondiscriminative and discriminative forms of auditory learning depending on the loudness or frequency of the acoustic stimuli, respectively. Brain imaging was done with two techniques recently combined and adapted to map learning effects on the same brain. They incorporate fluorodeoxyglucose autoradiography and cytochrome oxidase histochemistry as described in detail here. These mapping techniques provided functional images of how entire neural systems change, and were used to quantify relative changes in the operations of each neural system involved in the learning paradigms. Together, the experiments support the conclusion that learning-related changes in stimulus significance occur in the auditory system and influence anatomically linked structures distributed in neural systems with specific functional contributions to mediate the behavioral change produced by each specific form of learning. Functional brain imaging techniques provided a more complete picture of how the rat brain works as a unit to modify behavior during learning in the intact organism.