Summary and conclusion

For this thesis, I had set out to study whether information leaks into the peripheral nervous system. To study this, I looked at responses from the neck and shoulder muscles most likely to be involved in orienting eye, head and neck and arm movements to a visual target in macaques and human subjects, respectively. In chapter 2, a delay time was used in the memory guided saccade task to separate the effects due to visual related (where) and motor related (where) of the responses, while recording simultaneously and independently from frontal eye fields (fef) centrally and from neck muscles peripherally. This allowed me to show that temporal (when) but not spatial (where) information leaks into the periphery during the delay period. Furthermore, I also showed that temporal information leaked continuously and this information was reflected largely in smaller motor units, which presumably code for less force and reflect the order of recruitment in the motor periphery. Similar results were also obtained in the human part of the experiments involved in making reach movements. Interestingly, in human studies, I also did see the leakage of spatial information during the delay time, even though motor unit activity was assessed with surface electromyography. Also, consistent with the idea that the smaller motor units code for less force, I also observed that slower movements were recruited slowly and early during the delay period. I also tested whether LATER framework could be used to understand movement planning in fef and movement preparation to overcome inertia of the head during the delay period. I tested this using two different perspectives (i) reaction time and (ii) delay time. Interestingly, both the approaches gave similar results, centrally and peripherally. The activity at the time of the GO cue was higher for fast reaction times and longer delay periods. These difference in activity could be accounted for primarily by changes in slope and onsets in the neurons and motor units during the delay period. In Chapter 3, I tested the validity of the LATER model during an immediate reaction time task. as well. Irrespective of the task context, the LATER frame work could be used to explain the reaction time differences due to systematic changes in different components of the LATER model. Furthermore, I tested and observed task specific changes indifferent parameters like baseline, onsets, slopes and thresholds. These changes were found both centrally and peripherally and could explain shortening in reaction time due for immediate saccades relative to delayed saccades. Moreover, I showed that different type of units that were observed during the delay time, i.e., the rampers and non-rampers, could parcel out the variability due to separate decisions of where and when to make a movement. In humans, task context played an important role only for onsets. The non rampers were recruited much later after the go cue when compared to the delayed condition.

Open questions and possible future directions

One of the main shortcomings was that these studies were done on head restrained monkeys trained to make smaller saccades of 12 degrees eccentricity. Natural behaviour entails saccades of much higher amplitude and this could have been a possible reason for not seeing spatial information leaking through in our study, as discussed in detail in Chapter 1. Thus, studies regarding testing for leakage of specific signals could incorporate these changes to address this issue in future work. Secondly, even though our goal was to show that LATER approach could be used as a linking hypothesis to study movement preparation between centre and periphery, more causal approaches such as micro-stimulation might provide a more rigorous method tolnik activity in FEF and neck muscles. In addition, the use of granger causality could be used to further study how information from different type of neurons showing modulations with and without visual activity during the delay period are connected to motor units in periphery. Also, many other groups have also used recorded from neck and different set of axial muscles like deltoid, pectoralis, biceps and triceps to address related questions. Further, the results from this study opens an avenue of questions that could be addresses in future experiments. For example, one could test whether leakage can be modified in disease states characterized by the attenuation of inhibitory control. The use of TMS to disrupt inhibitory control model can also be used perhaps to study more nuanced aspects of central processing such as decision making and changes of mind by recording from the periphery.