RESUMO
Our environment is made of a myriad of stimuli present in combinations often patterned in predictable ways. For example, there is a strong association between where we are and the sounds we hear. Like many environmental patterns, sound-context associations are learned implicitly, in an unsupervised manner, and are highly informative and predictive of normality. Yet, we know little about where and how unsupervised sound-context associations are coded in the brain. Here we measured plasticity in the auditory midbrain of mice living over days in an enriched task-less environment in which entering a context triggered sound with different degrees of predictability. Plasticity in the auditory midbrain, a hub of auditory input and multimodal feedback, developed over days and reflected learning of contextual information in a manner that depended on the predictability of the sound-context association and not on reinforcement. Plasticity manifested as an increase in response gain and tuning shift that correlated with a general increase in neuronal frequency discrimination. Thus, the auditory midbrain is sensitive to unsupervised predictable sound-context associations, revealing a subcortical engagement in the detection of contextual sounds. By increasing frequency resolution, this detection might facilitate the processing of behaviorally relevant foreground information described to occur in cortical auditory structures.
RESUMO
Echolocating big brown bats fly, orient, forage, and roost in cluttered acoustic environments in which aggregate sound pressure levels can be as intense as 100 to 140 dB SPL, levels that would impair auditory perception in other terrestrial mammals. We showed previously that bats exposed to intense wide-band sound (116 dB SPL) can navigate successfully through dense acoustic clutter. Here, we extend these results by quantifying performance of bats navigating through a cluttered scene after exposure to intense band-limited sounds (bandwidths 5-25 kHz, 123 dB SPL). Behavioral performance was not significantly affected by prior sound exposure, with the exception of one bat after exposure to one sound. Even in this outlying case, performance recovered rapidly, by 10 min post-exposure. Temporal patterning of biosonar emissions during successful flights showed that bats maintained their individual strategies for navigating through the cluttered scene before and after exposures. In unsuccessful flights, interpulse intervals were skewed towards shorter values, suggesting a shift in strategy for solving the task rather than a hearing impairment. Results confirm previous findings that big brown bats are not as susceptible to noise-induced perceptual impairments as are other terrestrial mammals exposed to sounds of similar intensity and bandwidth.