Formation and decay of auditory short-term memory in the macaque monkey.

Echoic memory (EM) is a short-lived, precategorical, and passive form of auditory short-term memory (STM). A key hallmark of EM is its rapid exponential decay with a time constant between 1 and 2 s. It is not clear whether auditory STM in the rhesus, an important model system, shares this rapid exponential decay. To resolve this shortcoming, two rhesus macaques were trained to perform a delayed frequency discrimination task. Discriminability of delayed tones was measured as a function of retention duration and the number of times the standard had been repeated before the target. Like in the human, our results show a rapid decline of discriminability with retention duration. In addition, the results suggest a gradual strengthening of discriminability with repetition number. Model-based analyses suggest the presence of two components of auditory STM: a short-lived component with a time constant on the order of 550 ms that most likely corresponds to EM and a more stable memory trace with time constants on the order of 10 s that strengthens with repetition and most likely corresponds to auditory recognition memory. NEW & NOTEWORTHY This is the first detailed quantification of the rapid temporal dynamics of auditory short-term memory in the rhesus. Much of the auditory information in short-term memory is lost within the first couple of seconds. Repeated presentations of a tone strengthen its encoding into short-term memory. Model-based analyses suggest two distinct components: an echoic memory homolog that mediates the rapid decay and a more stable but less detail-rich component that mediates strengthening of the trace with repetition.