The Role of Alpha Synchrony in Spatial Attention During Neurofeedback Training

Post by Shireen Parimoo

What's the science?

Neural oscillations in the alpha frequency range (8-12Hz) are associated with cortical inhibition and visuospatial attention. Higher alpha power (i.e. synchronization) is related to decreased neuronal firing and thought to suppress sensory processing, whereas reduced alpha power (i.e. desynchronization) is thought to facilitate sensory processing. Although brain stimulation studies provide support for a causal role of alpha oscillations in visuospatial attention, the widespread and non-specific spatiotemporal effects of stimulation make it difficult to infer causality. Neurofeedback training is a technique that is used to alter brain activity endogenously by monitoring neural activity and using real-time feedback to allow the participant to achieve the desired brain state. This week in Neuron, Bagherzadeh and colleagues used magnetoencephalography (MEG) and neurofeedback training to modulate parietal alpha activity and investigate its causal role in visuospatial attention.

How did they do it?

Twenty participants completed neurofeedback training with a match-to-sample task during MEG scanning. Half of them were trained to increase alpha power in the left hemisphere (LNT group) and the rest were trained to do the same in the right hemisphere (RNT group). During the task, participants were shown grated stimuli (see picture) and trained to modulate their alpha power. Stimulus visibility changed with alpha power, providing real-time feedback to participants. The MEG recordings were used to compute an alpha asymmetry index, which was the difference in alpha activity between the hemispheres ipsilateral and contralateral to the training direction. The authors used dynamic statistical parametric mapping – a statistical method used to map neural activity to brain regions – to determine whether participants successfully modulated alpha power in the parietal cortex. To examine whether alpha modulation changed over the course of training, they compared alpha asymmetry in the first block to the last block of the training task.

A subset of the participants also completed a Posner cueing task (n=14) and a free-viewing task (n=6) before and after neurofeedback training, which allowed the authors to compare the effects of training on alpha modulation and behavioral performance. The Posner cueing task is a target detection task in which valid, invalid, or neutral spatial cues indicate which side of the screen to attend. To evaluate preparatory spatial attention in response to the cues, the authors computed an attentional modulation index (AMI) for cortical activity in each hemisphere as the difference in alpha power between left-cued trials and right-cued trials. They also calculated the difference in AMI before and after neurofeedback training to assess the effect of training on alpha modulation. Finally, participants completed a free-viewing task in which they freely explored images of scenes and fractals while their eye movements were recorded. Eye movements during visual exploration were examined to determine whether neurofeedback training led to a spatial bias in the absence of explicit spatial instructions.

What did they find?

Participants in both groups successfully modulated parietal alpha synchrony in the relevant hemisphere during neurofeedback training by increasing ipsilateral alpha, reducing contralateral alpha power, or a mix of the two strategies. Alpha modulation improved with training, as alpha asymmetry was higher at the end of the training session than at the beginning. The LNT group, who trained to increase alpha power in their left hemisphere, had higher alpha power in the left parietal cortex but not the right parietal cortex. Interestingly, the RNT group decreased left parietal alpha over the course of training rather than increasing alpha power in the right parietal cortex. Thus, the effects of training were specific to the left parietal cortex in both groups. In the Posner cueing task, the AMI reflected visual attention in response to each cue. Specifically, the AMI was positive in the left parietal cortex, indicating greater alpha activity in response to the left cue than the right cue, and negative in the right parietal cortex, indicating greater alpha activity in response to the right cue. After training, the LNT group exhibited larger left parietal AMI, whereas the RNT group had lower AMI in the right parietal cortex. This means that the effects of neurofeedback training on alpha power in the trained hemisphere persisted into a subsequent spatial attention task.

Neurofeedback training was not related to performance on the spatial trials of the Posner cueing task. However, specifically on neutral trials, participants were faster at detecting targets ipsilateral to the trained hemisphere than to contralateral targets. For example, participants in the LNT group responded faster to targets presented in the left visual hemifield than in the right visual hemifield. Additionally, there was a positive relationship between alpha modulation and reaction time, since those who showed the largest change in alpha modulation over the course of neurofeedback training also had the largest difference in reaction times between ipsilateral and contralateral targets. Finally, neurofeedback training also modulated eye movement behavior on a non-spatial task. After training, participants in the LNT group showed a bias toward exploring the left visual hemifield whereas the RNT group was biased toward the right visual hemifield. Thus, neurofeedback training of parietal alpha activity modulated alpha oscillations across multiple tasks and was related to visuospatial behavior.  

What's the impact?

This study is the first to use neurofeedback techniques to demonstrate the causal role of endogenous parietal alpha oscillations in visuospatial attention, even in non-spatial tasks. The results provide further insight into the effect of alpha synchrony on top-down and bottom-up attention and pave the way for future research on the applications of neurofeedback training for psychiatric disorders.

Bagherzadeh et al. Alpha synchrony and the neurofeedback control of spatial attention. Neuron (2019). Access the original scientific publication here.