Neurobiology Ph.D. candidate Shawn Willett (Groh Lab) will present his dissertation seminar Encoding of Concurrent Sounds in the Monkey Inferior Colliculus.
Please email DGSA LaDonna Huseman for connection details.
Abstract: Natural environments contain numerous sounds that overlap in time, space, and content. To survive in these environments animals must extract and encode behaviorally relevant sounds even if those sounds co-occur. How the brain encodes simultaneous sounds is ambiguous, given that sound features are thought to be rate coded and can sometimes recruit largely overlapping populations. In this work, I record the activity of neurons in the monkey inferior colliculus (IC), a coding bottleneck, while monkeys report the location of a single, or two concurrent, sound source(s). I first test the hypothesis that in the presence of multiple sounds IC neurons become more selective to sound frequency, reducing population overlap and allowing for the representation of each sound in a segregated population. I found, in contrast to this hypothesis, frequency selectivity was reduced on dual sound trials compared to single sound trials. Frequency receptive fields broadened, and sound frequency accounted for less variance in the dual sound response. These changes to the frequency response functions decreased the performance of a maximum-likelihood decoder in correctly labeling the condition of a held out dual sound trial. Nevertheless, the monkeys were able to behaviorally distinguish these sounds with high accuracy. Which suggested alternative theories, such as recent evidence of alternations in firing rate between the rates corresponding to each of the two sounds, offer a more promising approach. I then tested whether these response alterations were a general behavior observed in the IC across a large frequency range. These response alternations were observed in roughly 63% of IC neurons and their contribution to the population response remained stable across the full, 2 octave, range of frequency separations tested. Thus, response alternations are a general mechanism used by the IC to facilitate the encoding of multiple sounds and support a growing body of evidence showing response alternations in many brain regions. These results challenge the widespread notion that neurons normalize their responses in the presence of multiple stimuli, suggesting that some of that data may be better explained by alternating neural responses.