Neural Centers and Perceptual Characteristics of Auditory Short-term Memory Anna A. Dreyer Studies of auditory perception rarely incorporate findings regarding storage of short-term auditory memory. Investigators rarely incorporate auditory working memory into the neural circuits and anatomical regions they hypothesize underlies perception. This review focuses on the brain regions, neural circuits and perceptual characteristics of human auditory working memory, the storage of auditory percepts necessary to perform short-term, trial-by-trial tasks, in contrast to global, permanent memory formation. The neural areas participating in auditory working memory greatly overlap with those implicated in auditory perception, suggesting the importance of incorporating working memory into our understanding of auditory perception. Investigators interested in elucidating the mechanisms of auditory memory have focused on characterizing the perceptual limitations and neurophysiological responses. Psychophysical studies seek to understand human properties of the representation, single-cell recordings look for responses characteristic of short-term retention and imaging studies implicate the brain regions necessary to perform various short-term memory tasks. In addition, lesion studies have been important in examining the perceptual and neurophysiological defects when a key area implicated in short-term auditory memory has been removed. This review will describe how each methodology has shaped the current understanding of the mechanisms underlying auditory memory. Retention of non-verbal, perceptual information needed in pitch and lateralization discrimination is discussed to allow for comparisons between neurophysiological studies in non-human primates and human perceptual and imaging studies. Psychophysical studies find parametric segregation of short-term memory stores Psychophysical studies have explored system storage capacities in delayed performance of tasks with respect to variation in sensory parameters, such as duration, pitch, spatial location and memory load of auditory stimuli. In particular, investigators have discovered that auditory memory has separate stores for important auditory parameters. Anourova et al. (1999) conducted experiments testing whether pitch and location short-term memory can be disrupted with pitch and location-based distracters. Subjects were asked to identify whether two stimuli, separated by a delay, matched in frequency, in cases where either a stimulus of a different pitch or in a different spatial location served as a distracter during the delay. Anourova et al. found that only stimuli of the same modality (pitch of spatial location) degraded performance, concluding that pitch and location are contained in different auditory stores. Findings of Clarke et al. (1999) corroborate those findings. Semal and Demany (1991) suggest that pitch and timbre also have distinctly independent representations in auditory working memory. Other studies characterize the pitch or location-based stimuli that serve as the most distracting. For instance Deutch (1972) also asked subjects to compare two test tones and played distracter tones between the two test tones. Subjects were told to ignore the distracter tones and indicate whether the two tones were similar or different in pitch. However, as also found by Anourova et al (1999) and Clarke et al. (1999), performance degraded with distracter tones having similar pitches to the test tones. Performance worsened dramatically when the distracter tones were 2/3 tone away from the test tone, regardless of the tone frequency, and improved for distracter tones higher and lower in frequency. These findings indicate that distracter tones are Harvard-MIT Division of Health Sciences and TechnologyHST.722J: Brain Mechanisms for Hearing and SpeechCourse Instructors: Bertrand Delgutte, M. Christian Brown, Joe C. Adams, Kenneth E. Hancock, Frank H. Guenther, Jennifer R. Melcher, Joseph S. Perkell, David N. CaplanAnna A. Dreyer October 26, 2005 most effective when they are close in frequency to the test tones. This study and another conducted by the author a year later (Deutsch, 1973) also adds that the auditory memory storage is logarithmically organized, since the errors scaled with tonal distance between the distracter and test tone. As evidence of further parameter segregation in auditory working memory, Clement et a. (1999) suggested that pitch and loudness memory processing have separate representations and possibly different processing centers. This study measured performance during a frequency and an intensity discrimination task and found that the patterns of performance degradation as a function of delay between the two test stimuli differs substantially, as shown in Figure 1. Figure 1: Discrimination performance (d’) as a function of separation time between the two stimuli for frequency and intensity discrimination. Results averaged for four subjects in each condition. Results show that working memory degrades at different rates for the two conditions suggesting different underlying storage mechanisms. Adapted from Clement et al. (1999). This finding is analogous to different working memory domains processing contrast and spatial frequency in the visual system, and provides further evidence of different memory stores for important auditory working memory parameters. Neurophysiological studies find neural correlates of short-term retention in medial geniculate body and auditory cortex Relatively few studies have directly recorded neural responses from auditory cortical areas participating in short-term memory tasks. Studies have implicated the auditory cortex in short-term memory processing in cats (Neff et al., 1975), dogs (Chorazyna and Stepien, 1961) and monkeys (Stepien et al., 1960). This relatively primitive stage or cortical processing was thoroughly investigated by Gottlieb et al., (1989) who concluded that the cortical neurons retain information about the first tone frequency during interstimulus intervals (ISI) using a rate code. Gottlieb et al. trained monkeys to discriminate two sounds with a delay in a performance and a passive listening condition, and found that the firing rate during the ISI correlates with the first tone in both conditions. In addition, they found