Orchestrating eye-hand movements: What mechanisms does the brain use?
The eyes and the hands work together as a team for most of the daily tasks we perform. When we pick up the morning coffee, or when we drive to work, our brains are constantly commandeering our eye and hand systems to bring about smooth, coordinated movements. We often do not consciously compute the steps required to bring them about. However, when the coordination is disrupted, even the simplest of tasks like picking up a book prove to be extremely challenging. How does the brain achieve efficient eye-hand coordination? A recent study from Prof. Aditya Murthy’s laboratory at the Indian Institute of Science, Bangalore, explores this critical question and suggests a framework to understand the control mechanisms ofcoordinated eye-hand movements.
As a part of this study, 14 participants performed an eye-hand task in the laboratory. During the task, their eye and hand movements were recorded along with task parameters, and were later analyzed computationally. The task itself required subjects to make coordinated or isolated eye and hand movements (finger-pointing) to targets appearing peripherally on a screen. The targets could appear either on the left or right side of a central fixation spot.
“The subjects heard a ‘reward beep’ if they made an eye and/orhand movement correctly to the target, starting from the central spot. In addition, to perturb the eye and hand systems, we sometimes displayed a second target in a position diametrically opposite to that of the initial target (redirect tasks). In these cases, the subjects had to successfully cancel the movement being planned to the initial target position and redirect it to the new second target, necessitating the use of higher-order control mechanisms for both the eye and hand systems.We investigated the difference between the control mechanisms in isolated and coordinated conditions”, elaborates Dr. Atul Gopal, one of the members of the study.
Computational models that mimic the brain processes were then applied on recorded data. In the instance of a redirect task, two computational processes - ‘GO’ and ‘STOP’ are instantiated. ‘GO’ is the decision for the eye or hand systems to make a movement, and ‘STOP’ is the decision to cancel a movement being planned. The decision in the redirect task is much like a race between the GO and STOP processes; whichever wins will determine the action produced.
Previous work at the lab has shown that isolated eye and hand movements have separate GO processes, which bring about independent eye and hand movements. However, during coordinated movements, the eye and hand systems share a common GO process, thus bearing the hallmark of coordination. In the computational framework, redirection of planned eye-hand movements requires an additional control signal, namely the STOP process. “We specifically wanted to check if the ‘STOP’ process is common for coordinated eye-hand movements, similar to the ‘GO’ process we found in the previous study”, says Dr. Gopal.
Behavioral analyses and computational simulations using the data obtained from the redirect taskrevealed that coordinated stopping or redirecting of eye-hand movements doesshare a common STOP process.The behavioral performance curves,which quantify the ability of a subject to stop the planned movement,were distinct for the eye and hand when these movements were executed in isolation. “This was expected since the hand movement, which takes a longer time to be initiated, is easier to stop compared to an eye movement, which is faster, hence more difficult to control”, explains Dr. Gopal. However, they were comparable for coordinated eye-hand movements. This similarity was also observed for the time taken to switch from plan 1 to plan 2 in redirect trials, only for coordinated eye and hand movements.
“The results point towards the recruitment of a common STOP signal in the case where we alter coordinated eye-hand movement plans, and yet there seems to be distinct control processes for the same movements when executed in isolation”, says Prof. Murthy. “We have shown that the mechanism of control of eye-hand movements is flexible –both common and separate strategies are recruited according to the demands of the task. Flexible control is perhaps indispensable, given the huge variety of coordinated and isolated movements we perform effortlessly on a daily basis”, he remarks.
“It is remarkable that one can get deep and wonderful insights into how the brain works just by carefully analyzing behavior. We are only beginning to understand the architecture of simple movements performed under laboratory-controlled settings. One can only try to fathom how the brain initiates, controls and coordinates the much more complicated movements that we continually execute. It requires but a lifetime of scientific inquiry!” signs off Prof. Murthy with a smile.
Dr. Aditya Murthy is a Professor at the Centre for Neuroscience at IISc, Bangalore. He can be contacted firstname.lastname@example.org
Dr. Atul Gopal was a graduate student in Prof. Murthy’s laboratory. He is currently a post-doctoral associate at the National Institutes of Health, USA.
About the paper:
The report has been published in the Journal of Neurophysiology and can be accessed online at http://jn.physiology.org/content/jn/early/2016/02/12/jn.00910.2015.full.pdf