Many things happen at once - there are always abundant stimuli to be perceived, items to be remembered, and courses of action to be planned. Considerable research on attention has explored how our limited-capacity brains screen out the onslaught. The flip side of this problem is that sometimes information about multiple stimuli must be preserved despite limitations in processing capacity. This proposal explores potential brain mechanisms capable of preserving multiple auditory items of information simultaneously. We seek to understand how different types of neural codes accomplish this feat. We focus on sound location, which is critical for communicating and avoiding hazards in complex scenes. Localizing multiple sounds involves two types of codes in auditory brain areas: a firing-rate code for horizontal sound location which is superimposed on a map for sound frequency. A critical limitation of the firing- rate code for location, however, is that the representation of multiple iems would appear to be impossible because neurons cannot discharge at more than one firing rate at a time. We will therefore investigate two possibilities: that the code preserves multiple items via segregation into different hills of activity as afforded by the associated frequency map (aim 1), and that the code preserves multiple items by some form of switching behavior, in which individual neurons encode only one stimulus at a time but multiple sounds are represented at the population level and/or across time (aim 2). These experiments will be carried out in the inferior colliculus (IC), an essential processing stage of the auditory pathway: nearly all information reaching higher auditory areas must pass through this structure. Thus, what the IC can encode places boundary conditions on the information that is available to subsequent stages of processing. We will conduct single and multiple electrode recordings of spiking and local field potential activity during performance of a task involving eye movements to multiple sounds. These experiments will yield important insights into many aspects of normal perception and cognition as well as related disorders. The ability to keep multiple items in mind is central t communication, working memory, attention, and sensory-motor skills, and it may be adversely affected in disorders such as attention-deficit disorder, autism, central auditory processing disorder, and age- related hearing loss.
This project investigates how the brain preserves information about multiple sound sources. Deficits in this ability contribute to poor communication performance in noisy scenes in central auditory processing disorder and age-related hearing loss. Insights from these experiments could ultimately lead to improvements in the treatment of these and related conditions.