Embodying functionally relevant action sounds in patients with spinal cord injury

Abstract

Growing evidence indicates that perceptual-motor codes may be associated with and influenced by actual bodily states. Following a spinal cord injury (SCI), for example, individuals exhibit reduced visual sensitivity to biological motion. However, a dearth of direct evidence exists about whether profound alterations in sensorimotor traffic between the body and brain influence audio-motor representations. We tested 20 wheelchair-bound individuals with lower skeletal-level SCI who were unable to feel and move their lower limbs, but have retained upper limb function. In a two-choice, matching-to-sample auditory discrimination task, the participants were asked to determine which of two action sounds matched a sample action sound presented previously. We tested aural discrimination ability using sounds that arose from wheelchair, upper limb, lower limb, and animal actions. Our results indicate that an inability to move the lower limbs did not lead to impairment in the discrimination of lower limb-related action sounds in SCI patients. Importantly, patients with SCI discriminated wheelchair sounds more quickly than individuals with comparable auditory experience (i.e. physical therapists) and inexperienced, able-bodied subjects. Audio-motor associations appear to be modified and enhanced to incorporate external salient tools that now represent extensions of their body schemas.

Figure 1

Action sound discrimination task. In each trial, following the presentation of a sample sound, two subsequent probe sounds were presented. Only one of the two probe sounds was specifically related to the sample sound. In the set of lower limb actions (e.g. “male footsteps on a glass surface” [the sample sound]), one probe sound represented the same action as the sample sound but was produced using a different source (e.g. “female footsteps on a wood surface”), whereas the other probe sound represented a totally different action produced using the same body part (e.g. “running”). No image associated with an aural action was provided.

Figure 2

Latency in action-sound discrimination. The mean latency for each sound category (upper (U_RAS) and lower (L_RAS) limb-related action sounds, wheelchair-related action sounds (W_RAS), and animal action-related sounds (NH_RAS)) in the three subject groups (healthy individuals, physical therapists and individuals with spinal cord injuries). The error bars indicate the standard error of the mean (SEM). The asterisk (*) indicates significant results from the post hoc comparisons (p < 0.05).

Figure 3

Subjective ratings of action-sound familiarity and perceived motor intensity. The mean subjective Visual Analog Scale (VAS) ratings for auditory familiarity and perceived movement for each sound category (upper (U_RAS) and lower (L_RAS) limb-related action sounds, wheelchair-related action sounds (W_RAS), and animal action-related sounds (NH_RAS)) in the three subject groups (healthy individuals, physical therapists and individuals with spinal cord injuries). The error bars indicate the standard error of the mean (SEM). The asterisks (*) indicate significant results from the post hoc comparisons (p < 0.05).