Keeping our eyes focused on what we’re reaching for, whether it be an item at the supermarket or the ball your faithful pup has fetched for you, may appear seamless. But this ability is actually due to a complex neurological process that involves intricate timing and coordination.
A new study has fixed our eyes on the neurological mechanism behind “gaze anchoring” – the temporary stoppage of eye movement in order to coordinate reaches. It has found that neurons in the part of the brain used for reaching actually inhibit neural activity in the part of the brain used for eye movements.
This suppression of neuron firing inhibits eye movement, keeping our eyes locked on the target of our reach.
“Our results show that we anchor our gaze to the target of the reach movement, thereby looking at that target for longer periods,” explains Bijan Pesaran, a professor at New York University’s Centre for Neural Science in the US and one of the paper’s authors. “This is what makes our reaches much more accurate.
“The big question has been: how does the brain orchestrate this kind of natural behaviour?”
How do you study gaze anchoring?
The neuroscientists used a non-human primate model to study gaze anchoring, training two male rhesus macaques (Macaca mulatta) to perform a sequence of eye and arm movements.
The first movement was a coordinated look-and-reach to a target. Then, as little as 10 milliseconds (ms) later, a second target was presented that subjects needed to look at as quickly as possible.
These movements are similar to those made when changing the radio while driving and then focusing on a traffic light – if you quickly look away from the radio to the traffic light, you might not select the right channel (though you could also cause a crash, so keep your eyes on the road).
Reaction times to the second target were significantly longer when it appeared within 300ms of the first target, compared to when it appeared 500-800ms after, exposing the gaze anchoring effect keeping the eyes centred on the first target.
Simultaneously, they studied the firing of neurons using multiple electrodes implanted in specific areas of the brain – the parietal reach region used for reaching work and the parietal saccade region used for eye movements.
During gaze anchoring (while reaching for the object), they found that neurons in the parietal reach region suppressed the neuronal activity in the parietal saccade region and inhibited eye movement.
Importantly, the scientists note, the effects were tied to patterns of brain waves at 15-25Hz, called beta waves, that organise neural firing across the different regions of the brain.
“Beta waves have been previously linked to attention and cognition, and this study reveals how beta activity may control inhibitory brain mechanisms to coordinate our natural behaviour,” explains Pesaran.
This research offers the potential to better understand afflictions of attention and executive control that orchestrate natural behaviours – like coordinated looking and reaching. It was published in Nature.