Informational Masking of Communication Signals: Behavioral Constraints and Neural Mechanisms

Vocal communication is fundamentally important in the social and sexual behaviors of humans and other animals. In many species, vocal communication occurs in large social groups, where high levels of background noise constrain a listener’s ability to hear and process vocalizations. In the context of human speech perception, these constraints are collectively known as the "cocktail party problem," which refers to the difficulty of following a conversation in a crowd.

Recognizing speech in noise is particularly challenging for people with hearing loss. At present, how the noise of a crowd constrains the brain’s ability to recognize vocalizations is poorly understood at a cellular level. Discovering the origin of these constraints and their impacts on vocal recognition is necessary to advance knowledge of the neural basis of hearing and vocal communication.

Treefrogs are one example of an animal that encounters and solves a biological analog of the human cocktail party problem. During their breeding season, male treefrogs gather around suitable bodies of water where they produce loud mating calls in dense and noisy choruses to attract females. In order to reproduce, a female treefrog must recognize the vocalizations of a calling male of her species amid the cacophony of a noisy breeding chorus.

This project integrates behavioral experiments with neurophysiological recordings from individual neurons in a subcortical area of the vertebrate brain to identify the mechanisms by which noise interferes with a female treefrog’s ability to extract information about species identity from the calls of a potential mate. The public outreach component of this project involves the creation of a new museum exhibit called “Ear of the Beholder” at the Bakken Museum.

The PI will host a formal graduate student exchange program between the labs of the PI and Co-PI and will have an extensive undergraduate research program with a special emphasis on under-represented populations in biological sciences.

Noise interferes with signal processing in two fundamentally different ways. “Energetic” masking impairs signal detection in noise and occurs when signals and noise produce competing excitation in the same peripheral auditory filter, such that they directly compete on the auditory nerve. “Informational” masking occurs when competing sounds disrupt central processes for extracting signal information, which in turn degrades signal recognition. Most behavioral studies of masking in animal communication focus on energetic masking.

Few studies have investigated the neural basis of informational masking at a cellular level in animal models. Consequently, significant gaps remain in understanding how noise causes a breakdown of neural processing to give rise to vocal constraints with evolutionary consequences. This project integrates behavioral experiments on mate choice with extracellular and in vivo, whole-cell recordings of interval-counting neurons in the anuran inferior colliculus to investigate informational masking in Cope’s gray treefrog (Hyla chrysoscelis).

Female gray treefrogs only recognize sounds as mating calls when they exceed a threshold number of pulses in length, a process mediated by interval-counting neurons. This project tests the central hypothesis that concurrent sounds impair extraction of this temporal information by disrupting sound processing by temporally tuned interval-counting neurons in the inferior colliculus.

Our working hypothesis is that informational masking increases the threshold number of pulses required for call recognition by altering the time-dependent integration of excitation and inhibition performed by interval-counting neurons. The expected outcome of this project is a better understanding of the neural mechanisms involved in hearing and vocal communication in noisy social groups.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.