StartReact effects in first dorsal interosseous muscle are absent in a pinch task, but present when combined with elbow flexion

Abstract

ObjectiveTo provide a neurophysiological tool for assessing sensorimotor pathways, which may differ for those involving distal muscles in simple tasks from those involving distal muscles in a kinetic chain task, or proximal muscles in both.MethodsWe compared latencies and magnitudes of motor responses in a reaction time paradigm in a proximal (biceps brachii, BB) and a distal (first dorsal interosseous, FDI) muscle following electrical stimuli used as imperative signal (IS) delivered to the index finger. These stimuli were applied during different motor tasks: simple tasks involving either one muscle, e.g. flexing the elbow for BB (FLEX), or pinching a pen for FDI (PINCH); combined tasks engaging both muscles by pinching and flexing simultaneously (PINCH-FLEX). Stimuli were of varying intensity and occasionally elicited a startle response, and a StartReact effect.ResultsIn BB, response latencies decreased gradually and response amplitudes increased progressively with increasing IS intensities for non-startling trials, while for trials containing startle responses, latencies were uniformly shortened and response amplitudes similarly augmented across all IS intensities in both FLEX and PINCH-FLEX. In FDI, response latencies decreased gradually and response amplitudes increased progressively with increasing IS intensities in both PINCH and PINCH-FLEX for non-startling trials, but, unlike in BB for the simple task, in PINCH for trials containing startle responses as well. In PINCH-FLEX, FDI latencies were uniformly shortened and amplitudes similarly increased across all stimulus intensities whenever startle signs were present.ConclusionsOur results suggest the presence of different sensorimotor pathways supporting a dissociation between simple tasks that involve distal upper limb muscles (FDI in PINCH) from simple tasks involving proximal muscles (BB in FLEX), and combined tasks that engage both muscles (FDI and BB in PINCH-FLEX), all in accordance with differential importance in the control of movements by cortical and subcortical structures.SignificanceSimple assessment tools may provide useful information regarding the differential involvement of sensorimotor pathways in the control of both simple and combined tasks that engage proximal and distal muscles.

Cumulative distribution functions: An alternative approach to examine the triggering of prepared motor actions in the StartReact effect

“…Note that the Tresch et al (2014) dataset includes data collected from participants with stroke which were reported separately in Honeycutt et al (2014). For the sake of brevity, we have limited the report within the main body to the analysis of a single dataset provided by Castellote and Kofler (2018). This task recorded EMGs from the biceps brachii (BB) in a flex-only task, first dorsal interosseous (FDI) in a pinch-only task, and both BB and FDI in a combined pinch-flex task.…”

“…With our categorised data, we conducted a linear mixed-effects model with Kenward-Roger approximation for degrees of freedom using the lmer function (lmerTest package; v2.0-36; Kuznetsova et al, 2017) on the Castellote and Kofler (2018) data as a representative dataset. Percentile categorisation (fast onset/slower onset) and task type were set as fixedfactors in the model and participants were set as a random factor.…”

“…Given some movement types showed a large proportion (max = 56.3%; see Table 2) of trials in the non-startle categorisation of RT which occurred with SCM activity, we conducted a Bayesian test of association (Albert, 1997) to examine whether the presence of SCM activity differs across our startle and non-startle RT categories. The analysis of Castellote and Kofler's (2018) data resulted in BF = 2.5 x 10 12 for BB in the flex-only task, BF = 6 for FDI in the pinch-only task, BF = 2 x 10 25 for BB in the combined pinch-flex task, and BF = 8.2 x 10 16 for FDI latency in the combined pinch-flex task. BFs > 100 indicate decisive evidence against the null hypothesis (Jeffreys, 1961) and as such, the flex-only, BB pinch-flex, and FDI pinch-flex tasks show decisive 15 evidence for the dependence of percentile categorisation on SCM activity.…”

“…The percentage of responses within each category after splitting the data into startle and nonstartle categories (see Figure 1) are also presented in Table 2. The CDFs calculated for the Castellote and Kofler (2018) data are plotted along with the mean latency of SCM+ and SCMresponses in Figure 2, and the distribution of SCM+ responses within the startle and non-startle categories can further be seen in Figure 3. Extended analyses are presented in Appendix C. latency in flex-only task.…”

“…CDFs allow an examination of how trials with the fastest RTs differ from those with slower RTs which would be considered unrepresentative of the StartReact effect, whilst avoiding the pitfalls of relying on SCM activity as an indicator of StartReact mechanisms which have been outlined previously (Marinovic & Tresilian, 2016). We re-analysed data from seven studies (Castellote & Kofler, 2018;Honeycutt, Kharouta, & Perreault, 2013;Marinovic, de Rugy, Riek, & Tresilian, 2014;Marinovic, Milford, Carroll, & Riek, 2015;Ossanna, Zong, Ravichandran, & Honeycutt, 2019; which have investigated differences in response times across trials in the presence and absence of SCM activity. We used our method to evaluate the utility of separating trials on the basis of SCM activity by examining the distribution of SCM activity across the spectrum of RTs and evaluating the relationship between RT and the presence of SCM activity.…”