Overview

We have all seen/or can imagine a frog catching a fly, a cat pouncing on a mouse or a lion chasing a deer. The question that I find interesting is - while the predator waits for their prey to get closer, do they simply wait or prepare a plan to attack? The hallmark of behaviour is that we can separate these mental processes like making decisions and plans from being executed depending on different conditions and circumstances. Previous studies have suggested that the gating of this information between intention and action can happen at different levels in our central nervous system involving the motor cortex, subcortical structures such as the basal ganglia and even in the spinal cord. Based on the outline provided here, I have examined the extent of this gating and its modulation by task context in case of eye and hand movements independently.

I begin my thesis with Chapter 1, by reviewing the relevant literature on possible ways for how this gating might happen in case of both gaze (eye and head) and hand movements. In Chapter 2, I show that information does leak through into periphery while planning to make an upcoming movement. Here, I have also described the methods that were used during my research. I analysed data from neck muscles and neural recordings from frontal eye fields while the two monkeys were being trained to perform on a memory guided task under head restrained conditions. Significant number of units showed modulation in both central (FEF) and peripheral (neck muscles) nervous system. I also collected surface electromyographic (sEMG) signals using high density electrode arrays from shoulder muscles while human subjects were involved in making reach movements. For both, the activity during this interval was found to be correlated with the time it took to initiate an impending saccade. I also observed that the motor units that showed modulation during the delay period had smaller amplitudes relative to the units that showed modulations only after the delay period. Accumulator models (neural activity increases towards a threshold) have been associated with motor preparation in frontal eye fields. I tested, whether this same framework could be extended during the delay time to explain the activity pattern of neurons in central and recruitment of motor units in peripheral systems. In Chapter 3, I looked at the sources of variability for initiating saccades under different task contexts. I showed that using delayed conditions one could account for the variability due to two different kinds of computational processes carried out centrally, i.e., ‘where’ and ‘when’ to initiate a saccade. Secondly, I also show that task context does modulate the information that reaches to the periphery to initiate saccades and reach movements. I then summarised and concluded in Chapter 4, discussing how we can assess some aspects of central planning by looking at the recruitment pattern of motor units and further how its modulation is dependent on task context. I have also discussed the limitations of my study and the possible avenues that could be explored in future.