How Fixational Eye Movements are Related to Covert Attention
Post by Lina Teichmann
The takeaway
Microsaccades are small involuntary eye movements that occur when we are attending to peripheral things in our environment. These eye movements correlate with alpha-band activity (a particular frequency of brain activity) in the brain - a neural signature of spatial attention - but are not necessary for neural modulation to occur.
What's the science?
When driving a car, we usually attend to what is right in front of us, but at the same time, we need to allocate some of our attentional resources to other things in our periphery like pedestrians or cyclists. This process is known as covert spatial attention and allows us to prioritize relevant information that is not currently in fixation. Covert attention is associated with directional biases in fixational eye movements or microsaccades, which are small jerk-like, involuntary eye movements. In the human brain, alpha activity has commonly been associated with spatial attention. This week in Nature Communication, Liu and colleagues examined if and how the commonly observed biases in microsaccades and the modulation of alpha activity in the brain are related when deploying covert attention.
How did they do it?
Participants completed a spatial memory task while their brain activity data was recorded with electroencephalography (EEG) and their eye movement data was recorded with an eye tracker. To compare the spatial modulation of the alpha activity, the researchers looked at lateralization (i.e., comparing the strength of alpha over the right and left posterior EEG sensors which is typically modulated by whether attention is deployed to the left or right visual field).
In the memory task, participants were asked to recall the orientation of one of two lines which were shown on right and the left of the screen. The researchers then measured whether (1) the presence and direction of eye movements during recall were indicative of performance and (2) whether lateralized alpha activity – a marker for spatial attention – is correlated with attention-directed microsaccades.
What did they find?
Overall performance was not dependent on microsaccades, as participants recalled the orientation of the line equally well regardless of the presence of an eye movement. However, when an eye movement was present, the direction was linked to performance, with eye movements towards the location of the to-be-recalled item resulting in better performance.
Looking at the neural data, the researchers found that there was clear lateralized alpha (a marker for spatial attention) activity when eye movements occurred towards the location of the to-be-recalled item. However, a highly similar pattern was observed when no eye movement was present, highlighting that eye microsaccades are not necessary for spatial attention. Considering only trials where a microsaccade occurred, the data showed that eye movements away from the location of the to-be-recalled item reduce the lateralized alpha modulation. This highlights that both markers for covert attention, microsaccades, and lateralized alpha-band activity are correlated, but that microsaccades are not necessary for spatial attention.
What's the impact?
Measuring and understanding covert attention is critical for gaining insights into how we navigate through our visual world. Liu and colleagues have found an elegant way to show that there is a correlational but not necessarily an obligatory link between microsaccades and lateralized alpha activity in the brain, which is frequently used as a marker of spatial attention.