The long-term objective of this proposal is to identify the fundamental neural principles that allow hearing sounds in noisy background. Although we are normally adept at extracting and hearing sounds that are in noisy backgrounds, listeners with hearing impairments have great difficulty understanding speech and other communications signals in such backgrounds. Despite a large and important literature on the psychophysics of hearing in noisy backgrounds and the responses of neurons to sounds in noisy backgrounds, the most fundamental question remains: What is the relationship between single-neuron activity in the cortex and the perception of sounds that are in noisy backgrounds? Unlike previous work, in this proposal, the PI directly tests the relationship between (1) the acoustic stimulus, (2) single-neuron activity, and (3) subjects'behavioral reports. This proposal tests the role that three brain regions in the ventral auditory 'object'pathway have in hearing sounds in noisy backgrounds: (1) the primary auditory cortex, (2) the secondary auditory cortex-specifically the anterolateral belt region of the auditory cortex, and (3) the ventrolateral prefrontal cortex. The hypotheses of this proposal are tested by recording extracellularly from neurons in these three brain regions while the monkeys report whether or not they hear a target stimulus that is in a noisy background. The target sound is a tone burst (Aim 1) or a vocalization (Aim 2). In both Aims, the noisy background is either comodulated or unmodulated noise.
Both Aims test psychophysical and neurophysiological hypotheses. For both Aims better we expect better detection thresholds in a background of comodulated noise than in a background of unmodulated noise.
Aim 1 hypothesizes that neural activity in the primary auditory cortex is reliably modulated by the acoustic properties of a sound, independent of behavior.
This Aim also hypothesizes that neural activity in the secondary auditory cortex-specifically the anterolateral belt region of the auditory cortex-reflects the sensory evidence used to form a perceptual decision but does not reflect the decision process.
Aim 2 hypothesizes that neural activity in the ventrolateral prefrontal cortex represents, on a trial-by-trial basis, the decision process that converts incoming auditory evidence into a categorical choice. Finally, for both Aims, we hypothesize that neural representations are dependent on the nature of the noisy background.
The results of this grant proposal will reveal strategies used by the brain to hear in noisy acoustic backgrounds. These findings may provide a fuller understanding of why hearing-impaired listeners have difficulty understanding speech in noisy backgrounds. The insights from these studies will also improve the design of hearing prosthetics, diagnostic assays, and the development of rehabilitation programs.