Probing Sensorimotor Memory through the Human Speech-Audiomotor System.

Our knowledge of human sensorimotor learning and memory is predominantly based on the visuo-spatial workspace and limb movements. Humans also have a remarkable ability to produce and perceive speech sounds. We asked if the human speech-auditory system could serve as a model to characterize retention of sensorimotor memory in a workspace which is functionally independent of the visuo-spatial one. Using adaptation to altered auditory feedback, we investigated the durability of a newly acquired speech-acoustical memory (8- and 24-hour delay), its sensitivity to the manner of acquisition (abrupt versus gradual perturbation), and factors affecting memory retrieval. We observed extensive retention of learning (∼70%) but found no evidence for offline gains. The speech-acoustical memory was insensitive to the manner of its acquisition. To assess factors affecting memory retrieval, tests were first done in the absence of auditory feedback (with masking noise). Under these conditions, it appeared there was no memory for prior learning as if after an overnight delay, speakers had returned to their habitual speech production modes. However, when speech was reintroduced, resulting in speech error feedback, speakers returned immediately to their fully adapted state. This rapid switch shows that the two modes of speech production (adapted and habitual) can coexist in parallel in sensorimotor memory. The findings demonstrate extensive persistence of speech-acoustical memory and reveal context-specific memory retrieval processes in speech motor learning. We conclude that the human speech-auditory system can be employed to characterize sensorimotor memory in a workspace that is distinct from the visuo-spatial workspace.