Post by Elisa Guma
What's the science?
The visual system is organized retinotopically, such that mapping of visual inputs from retina to neurons within the visual cortex is spatially organized. In humans, these maps are commonly identified using functional magnetic resonance imaging. However, the relationship of these between these maps and those identified in humans and animals electrophysiologically requires further investigation. It is thought that visual attention is related to the strength of alpha band activity (which is measured electrophysiologically) in the cortex. This week in Human Brain Mapping Popov and colleagues investigated whether alpha-band activity changes are retinotopically distributed using magnetoencephalography (MEG), a technique used to measure magnetic fields produced by electric currents in the brain.
How did they do it?
Thirty healthy participants (15 males/15 females) underwent MEG scanning while performing a visual response inhibition task (similar to the Eriksen flanker task). Briefly, participants were asked to fixate their gaze on a central white square. Next, a spatial cue, either a barrel or bowtie, was presented to them at one of 16 locations. Following a short interval (2.5 seconds) in which participants had to maintain their fixation on the cue, a target shape appeared at the same location in which the spatial cue was presented. The targets were flanked by either the same shape (bowtie-bowtie), or the opposite shape (bowtie-barrel). The goal of the task was for participants to identify whether the target was a bowtie or a barrel. The authors used spatial location tuning functions to associate spatial location of alpha frequency brain activity to the spatial location of the visual cues. Finally, they mapped the activation patterns onto a probabilistic atlas of the human brain.
What did they find?
The authors found that maintenance of the cue position when the cue was in a location in the left half of the visual field was associated with a decrease in alpha band power on the opposite (right) side, and an increase in alpha power on the same side (left side). Next, they observed an increase in response time and a decrease in accuracy when the targets were flanked by the opposite shape. This was reflected in patterns of alpha activity such that when participants were not distracted by flankers of the opposite shape, but rather saw the same object for target and flank, they were better able to maintain alpha activity in the spatial location of the target. Further, the slower the reaction time of the participants, the less focal the alpha-band activation. Finally, using a brain atlas the authors show that their maps of alpha activity map onto areas of the visual cortex in the brain in a retinotopic fashion: Changes in the location of the target stimuli in the visual field were reflected by spatial differences in the activation pattern in the visual cortex..
What's the impact?
This study found that alpha-band activity in a spatial attention task had spatial specificity and was affected by participants’ distractibility and response time. These findings demonstrate that the alpha-band activity is critical in allocation of the brain’s resources in directing spatial attention. A deeper understanding of the way in which neural activity underlies visual attention may help us understand the mechanisms underlying visual perception and attention.
Popov et al. Spatial specificity of alpha oscillations in the human visual field. Human Brain Mapping (2019). Access the original scientific publication here.