Modulação cognitiva do processamento auditivo no colículo inferior

Abstract

In auditory system, top-down effects could be defined as observers intrinsic variables which enable neural coding of sounds to be modifiable based on the context of sensory interaction and on previous experiences. Since inferior colliculus (IC) receive projections from both ascending auditory system and areas related to processing of behavioral contexts it is reasonable to figure that its patterns of activity to sounds could be modified once these sounds acquire biological relevance. To evaluate this hypothesis, Wistar rats with electrodes placed on CI to record local field potentials (LFP) were submitted to an auditory fear conditioning protocol. The experimental group (n = 5) was presented to five acoustic conditioning stimuli (CS, 10 kHz pure tone amplitude modulated at 53.71 Hz, duration of 30s and intensity of 82 dB) paired with foot shocks (400 A, 2s), while the control group (n=5) was presented to the same number of stimuli, although they did not coincided temporally. IC responses and percentage of freezing behavior were evaluated in sessions 24 h before (pre-conditioning) and 24 h after (test) conditioning, which consisted of five trials of CS presentation. Behavioral differentiation between groups was not significant on pre-conditioning, but, on experimental group, associative learning led to conditioning of freezing behavior to CS on test. IC response to CS was a neural oscillation following the modulation frequency, and was evident by time-frequency analysis of LFP as increases of both amplitude and phase stability at 53.71 Hz (auditory steady-state responses, ASSR). Both amplitude and phase components of ASSR were modified as an effect of associative learning. When animals were reexposed to CS on test, phase component consistently shifted from its preconditioning values, on first and second CS presentations, but with repeated presentations it returned progressively to baseline values and at the fifth trial the initial shift was completely reversed. At the same time that phase returned to baseline, progressively increased the ability of the neural circuit to sustain the phase component extremely invariant, around baseline values, during the whole period of CS presentation. This enhanced phase stability was concurrent in the CS presentation sequence with the augmentation of ASSR amplitude. The effects observed on experimental group were not evident on control group, suggesting that the modifications on IC processing of CS were due to acquisition of biological relevance by the acoustic stimulus as a consequence of associative learning. The aforementioned results suggest that associative learning determines the modulation of both the temporal structure and the amplitude of neural responses of IC to sounds that acquired biological relevance. Even though these two effects are evident at different moments of test, the issue of their causal relationship and the possibility of being outcomes of distinct global patterns of sensory processing are matter for future investigations.