Behavioral and neural correlates of auditory encoding and memory functions in Rhesus Macaques

Auditory recognition memory in non-human primates is not well understood. Monkeys have difficulty acquiring auditory memory tasks, and limited capability maintaining auditory information over memory delays, relative to studies of visual memory. Neural substrates of auditory discrimination and recognition memory depend on superior temporal gyrus (STG), instead of rhinal cortex necessary for visual memory (Fritz et al., 2005). The current project assessed behavioral and neural correlates of auditory processing and memory function in monkeys, particularly focusing on the dorsal temporal pole (dTP), the rostral portion of STG. Chapter 2 examined recognition memory of monkeys under influences of various sound types. In a delayed matching-to-sample (DMTS) task, rhesus monkeys were trained to determine if two sounds, separated by a 5-second delay, were same (match trials) or different (nonmatch trials). Results demonstrated monkey vocalizations served as better cues than other sound types for auditory memory performance. Memory improvements may be due to familiarity and biological significance of con-specific sounds, analogous to using facial stimuli during visual tasks. Chapter 3 examined neuronal activity of dTP, when two monkeys performed an auditory DTMS task and listened to sound stimuli. Population encoding of sample stimuli in dTP was closely associated with memory accuracy. Moreover, a suppression effect on identical sounds was present, similar to processing in the ventral visual processing stream, inferior temporal cortex (ITC) and ventral temporal pole (vTP). Delay-related activity of dTP was weak, limited and short-lived, in contrast to visual studies reporting sustained activity over memory delays in ITC, vTP and prefrontal cortex. The findings provide preliminary evidence on why monkeys show limited memory capability, compared to visual memory, for auditory information. Neurons of dTP were sound-selective, and mainly evoked by one to four discrete stimuli only. Sound types and simple acoustic properties of sound stimuli cannot completely account for response profiles of dTP neurons. The findings suggest dTP is a higher order auditory area, and receives information from various auditory areas along STG. Dorsal temporal pole fits into proposals of neural networks for auditory processing, in which a hierarchical organization of information flow exists within the primate auditory nervous system.