The influence of kinesthetic motor imagery and effector specificity on the long-latency stretch response

Abstract

The long-latency \"reflexive\" response (LLR) following an upper-limb mechanical disturbance is generated by neural circuitry shared with voluntary control. This feedback response supports many task-dependent behaviours and permits the expression of goal-directed corrections at latencies shorter than voluntary reaction time. An extensive body of literature has demonstrated that the LLR shows flexibility akin to voluntary control, but it has never been tested whether instruction-dependent LLR changes can also occur in the absence of an overt voluntary response. The present study used kinesthetic motor imagery (Experiment 1) and instructed participants to execute a voluntary response in a non-stretched contralateral muscle (Experiment 2) to explore the relationship between the overt production of a voluntary response and LLR facilitation. Activity in stretched right wrist flexors were compared to standard \"not-intervene\" and \"compensate\" conditions. Our findings revealed that on ~40% of imagery and ~50% of contralateral trials, a partial voluntary response \"leaked-out\" into the stretched right wrist flexor muscle. On these \"leaked\" trials, the early portion of the LLR (R2) was facilitated and displayed a similar increase to compensate trials. The latter half of the LLR (R3) showed further modulation, mirroring the patterns of voluntary response activity. By contrast, the LLR on \"non-leaked\" imagery and contralateral trials did not modulate. We suggest that even though a hastened voluntary response cannot account for all instruction-dependent LLR modulation, the overt execution of a voluntary response in the same muscle(s) as the LLR is a pre-requisite for facilitation of this rapid feedback response.\n\nKey Points SummaryO_LIThe long-latency \"reflexive\" response (LLR) in the upper-limb modulates based on volitional intent.\nC_LIO_LIWe used two novel approaches, kinesthetic motor imagery and execution of a contralateral movement, to examine the relationship between the overt production of a voluntary response and LLR facilitation.\nC_LIO_LIOur findings reveal an indissociable link between LLR facilitation and the appearance of a voluntary response in stretched muscle; specifically, the LLR was only facilitated on imagery or contralateral trials when a voluntary response \"leaked-out\" into the same muscle.\nC_LIO_LIThe first half of the LLR (R2) displayed a general increase on all trials with a voluntary response, while the latter portion (R3) mirrored the patterns of voluntary response activity.\nC_LIO_LIWe suggest that a voluntary response specific to the same muscle (as the LLR) is a pre-requisite for LLR facilitation\nC_LI