A hallmark of intelligent behavior is that we can separate intention from action. To understand the mechanism that gates the flow of information between motor planning and execution, we compared the activity of frontal eye field neurons with motor unit activity from neck muscles in the presence of an intervening delay period in which spatial information regarding the target was available to plan a response. Whereas spatially-specific delay period activity was present in the activity of frontal eye field neurons, it was absent in motor unit activity. Nonetheless, motor unit activity was correlated with the time it took to initiate saccades. Interestingly, we observed a heterogeneity of responses amongst motor units, such that only units with smaller amplitudes showed a clear modulation during the delay period. These small amplitude motor units also had higher spontaneous activity compared to the units which showed modulation only during the movement epoch. Taken together, our results suggest the activity of smaller motor units convey temporal information and explains how the delay period primes muscle activity leading to faster reaction times.

A hallmark of intelligent behavior is that we can separate intention from action. To understand the mechanism that gates the flow of information between motor planning and execution, we compared the activity of frontal eye field neurons with motor unit activity from neck muscles in the presence of an intervening delay period in which spatial information regarding the target was available to plan a response. Whereas we could infer spatially-specific delayed period activity from the activity of frontal eye field neurons, neck motor unit activity during the delay period could not be used to infer the direction of an upcoming movement, Nonetheless, motor unit activity was correlated with the time it took to initiate saccades. Interestingly, we observed a heterogeneity of responses amongst motor units, such that only units with smaller amplitudes showed a clear modulation during the delay period. These small amplitude motor units also had higher spontaneous activity compared to the units which showed modulation only during the movement epoch. Taken together, our results suggest that the temporal information is visible in the periphery amongst smaller motor units during eye movement planning and explains how the delay period primes muscle activity leading to faster reaction times.

Understanding how motor plans are transformed into appropriate patterns of muscle activity is a central question in motor control. While muscle activity during the delay period has not been reported using conventional EMG approaches, we isolated motor unit activity using a high-density surface EMG signal from the anterior deltoid muscle to test whether heterogeneity in motor units could reveal early preparatory activity. Consistent with our previous work (Rungta et al., 2021), we observed early selective recruitment of small-amplitude size motor units during the delay period for hand movements like early recruitment of small-amplitude motor units in neck muscles of non-human primates performing delayed saccade tasks. This early activity was spatially specific and increased with time and resembled an accumulation to threshold model that correlated with movement onset time. Such early recruitment of ramping motor units was observed at the single trial level as well. In contrast, no such recruitment of large amplitude size motor units, called non-rampers, was observed during the delay period. Instead, non-rampers became spatially specific and predicted movement onset time after the delay period. Interestingly, spatially specific delay period activity was only observed for hand movements but was absent for isometric force-driven cursor movements. Nonetheless, muscle activity was correlated with the time it took to initiate movements in both task conditions for non-rampers. Overall, our results reveal a novel heterogeneity in the EMG activity which allows the expression of early motor preparation via small amplitude size motor units which are differentially activated during initiation of movements.

Understanding how the brain transforms motor plans into the appropriate pattern of muscle activity is a central question in motor control. Whereas some neurophysiological studies in non-human primates have shown the presence of early recruitment of motor units, other studies have reported recruitment of motor units to occur just prior to movement initiation. To address this issue in humans, and test whether early recruitment is task dependent, we studied anterior deltoid muscle activity using high density surface EMG during visually guided delayed and immediate movement tasks while subjects used either hand or isometric cursor movements. Whereas spatially specific delay period activity was present for hand movements, it was absent for isometric movements. Nonetheless, muscle activity was correlated with the time it took to initiate movements in both the task conditions. Further, we used an accumulator framework to assess initiation of movements in different task contexts. Interestingly, we found consistent but different parameters of the accumulator model to play an important role in initiating hand and cursor movements: whereas the growth rate of muscle activity was an important determinant of reaction times in delayed and immediate hand movement tasks, the onset of muscle activity recruitment played a consistent and important role in determining the reaction time for initiating cursor movements during the delayed and immediate tasks. Overall, our results reveal an unexpected heterogeneity of early recruitment as seen with high density surface EMG that may relate to the differential extent of planning engendered during actual hand movements versus isometric movements.