We are interested in how the brain can rapidly translate visual input into motor output. And here, we describe the emerging target paradigm, which elicits quantifiable and reliable fast visuomotor responses. The fast visuomotor responses listed by the paradigm include stimulus-locked responses, which are short bursts of muscle activity tied to visual stimulus onset.
Having a paradigm that reliably elicits fast visuomotor responses potentiates the study of the underlying neural mechanisms across the lifespan and in neurological disorders like Parkinson's disease. Many aspects of the emerging target paradigm could be modified to better understand the sensory, cognitive, and motor factors which influence the fast visuomotor system. Begin by applying electromyography or EMG sensors to the targeted upper limb muscle involved in the reaching movement being studied.
Visualize the target muscle by requesting an action known to recruit the muscle of interest. For the clavicular head of the pectoralis major muscle, ask the participant to relax their elbows at their sides and push their palms together. If it is difficult to visualize the target muscle, palpate the area of interest while having the participant repeatedly perform the requested action.
Target areas with notable changes in the muscle for electrode placement. Use alcohol swabs to clean the skin areas where electrodes will be located and apply adhesives and electrode gel to the surface sensors. Ask the participant to perform the action associated with the muscle recruitment again and adhere sensors over the muscle belly, positioning them to lie in parallel with the direction of the fibers of the targeted muscle.
Place the ground electrode on the clavicle contralateral to the reaching arm, then secure sensors and ground electrodes to the surrounding skin with medical tape. Turn on the EMG system to allow for EMG collection throughout the experiment. Check the quality of the EMG signal with a desktop monitor or oscilloscope connected to the system.
Have the participant perform a reaching movement into or opposite from the preferred direction of the muscle of interest and ensure that the EMG activity increases or decreases respectively. Next, set up the participant in a robotic reaching apparatus that allows reaching movements in a horizontal plane and the application of force to the manipulandum. Seat the participant in the experimental chair, maximizing their comfort to minimize changes of posture throughout the experiment.
Click start on the associated software which initiates the first trial and force generated by the robotic reaching apparatus applied to the participant's upper limb. Verbally instruct the participant to start the first trial by bringing the real-time cursor or RTC into the start position for a duration of one to 1.5 seconds. The occluder changes color to instruct the subject that the upcoming trial requires a pro or anti-reach.
Determine if the participant is able to generate a visually-guided reach depending on the color of the occluder. When the occluder is green, the participant intercepts the moving target with the RTC. When the occluder is red, the participant moves the RTC away from the target.
Ensure that the moving target, which was stationary and visible to the participant at the top of the inverted Y, begins moving towards the participant along the path of the inverted Y.When the moving target moves behind the occluder, it should move at a constant speed of 30 centimeters per second along the y-axis and should be invisible to the participant. During this interval, the participant should maintain the hand position at the imagined start position. Once the moving target reaches half the length of the occluder, it bifurcates along one of the inverted Y outputs with an additional X velocity component.
The target vanishes for a constant delay of approximately half a second, with the delay depending on the size of the occluder and the speed of the target. When the moving target reaches the edge of the occluder, make sure that the software keeps it invisible until the full target has emerged and only then presents it to the participant. The target should be presented to a randomized side at one of the inverted Y paths while the participant's hand remains at the starting position.
The exact time of target emergence must be known to ensure accurate measurement of fast visuomotor responses. Here, we present another visual stimulus, which is not seen by the participant, to a photodiode located in the corner of the screen at the exact time of target emergence. This photodiode provides an analog signal for alignment of muscle activity and reaction times.
Depending on the participant's reaching behavior, this software provides feedback as text written on the occluder during the inter-trial interval. The feedback says hit for correct interception, wrong way for incorrect direction of reach, or miss for neither correct nor incorrect responses detected during the inter-trial interval. The moving target and the starting position reappear at their respective original locations 200 milliseconds after the participant's reach behavior is completed.
Start the next trial when the participant brings the RTC to the starting position. Minimize the participant's movement between each block to ensure consistency of recordings. After verbal confirmation that the participant is ready to begin the next block, initiate the next block and continue to monitor participant performance and EMG output.
Stimulus-locked responses or SLRs depended on target location, with SLRs on the right pectoralis major consisting of an increase or decrease in muscle recruitment following leftward or rightward target presentation respectively. To detect SLRs, separate time series receiver operating characteristic analyses were performed on trials with earlier or later than average reaction times. This analysis indicated whether electromyography onset was invariant to stimulus or movement onset, which was determined by the slope of the line connecting early and late discrimination times plotted as a function of reaction time.
Data from two subjects reaching toward a stationary object or moving target is shown here. Participant one does not exhibit an SLR in the static paradigm, but a clear SLR can be seen in the emerging target paradigm. The SLR was also apparent in the mean electromyography traces for participant one in the emerging target paradigm.
While participant two exhibited an SLR in both the static and emerging target paradigms, the magnitude of the SLR was much greater in the emerging target paradigm. On average, SLR magnitude was considerably larger in the emerging target versus the static paradigm. In contrast, the latency of detected SLRs did not differ between the two paradigms.
SLRs were detected in all five participants in the emerging target paradigm, but only in three participants in a paradigm with static targets. SLR magnitudes in the anti-reach condition were muted compared to that in the pro-reach condition. The paradigm could be varied to better understand the sensory, cognitive, or motor factors that influence fast visuomotor responses.
Future experiments could vary target speed, alter the time behind the occluder, or require participants to choose which hand to move. We've started to use reliable responses elicited by the emerging target paradigm to study the fast visuomotor system in elderly participants and in patients with Parkinson's disease.
