Our eyes are constantly moving while receiving visual input from our surroundings. This information is used by the brain to create a mental representation of the world. Even when our gaze is fixated on an object, miniature eye movements remain. Although these movements go unnoticed, without them, the visual neurons in our eyes would become used to the stationary image, and the world would fade from our view. Like other sensory systems, when exposed to a constant stimulus, neurons adapt and cease to respond (for instance, you feel your clothes when you first put them on, but for the majority of the day you don’t notice them).
Microsaccades are the largest and fastest of these involuntary eye movements, and occur once or twice per second during gaze fixation. Although scientists still debate their exact function, studies have found evidence for their role in preventing visual fading of stationary images. One of the areas of the brain associated with microsaccades is the middle temporal (MT) area, which is located in the visual cortex (at the back of the brain) and is important for perceiving motion. Neurons in the MT, like other neurons in the visual system, are prone to adaptation. A prime example is the “waterfall illusion,” where staring unblinkingly at a moving waterfall and then shifting gaze to an unmoving object will make that object appear to be moving upward. The MT is one brain area that Eric Cook, a researcher in McGill’s Physiology department, and his lab are studying in order to understand conscious visual perception.
In a study published this month in the McGill Science Undergraduate Research Journal (MSURJ), two students in the Cook lab used small electric currents to stimulate neurons through electrodes implanted in area MT of monkeys – a technique known as microstimulation – to see if it affected the production of microsaccades. Haider Riaz, a U2 student in physics and Ashkan Golzar, a PhD student in physiology, conducted this experiment, which is a part of a larger study in the lab looking at whether microstimulation could influence perception.
The researchers had two monkeys participate in a random dot motion task, which involves identifying the direction of motion of a small group of dots in a larger cloud of randomly moving dots. The task is relatively easy if a large portion of the dots is moving together and becomes more difficult as the number of coherently moving dots become smaller. Experimenters recorded the monkeys’ eye movements and microstimulated neurons in area MT while they were carrying out the task. They found that microstimulation resulted in an increased frequency of microsaccades.
If the MT stimulation was the cause of the eye movements, there would be a clear temporal link between the two, which would look like a small “pulse” associated with the microstimulation shortly before the microsaccade. The experimenters looked for this effect but could not find a significant association. “The answer I got was ambiguous. There is more analysis that can be done, [but] microstimulation could cause an increase in microsaccades – we haven’t ruled it out,” explained Riaz.
One of the biggest problems faced by brain researchers is how to determine a causal link between brain activity and behaviour. Functional magnetic resonance imaging, a popular method for studying the brain, is only able to correlate the two, without establishing whether the activity was necessary to cause the behaviour. On the other hand, with electrical microstimulation, if stimulating a neuron results in a specific action, there is a strong case for a causal relationship between the two.
People have long believed eyes to be the “windows” to the soul. Since scientists first identified eye tracking as a way to determine where a person is looking, they have slowly been able to peek inside. Microstimulation may allow scientists to not only observe what’s happening behind these windows, but also influence it from within.