Accepted paper: Spontaneous and deterministic fluctuations of pre-stimulus alpha power govern biases in visual line bisection

Chris Benwell’s most recent study shows that an observer’s brain state, just before seeing a transected line, influences their judgment of its centre. Also, these critical brain states do not seem to fluctuate randomly from instant to instant, but trend systematically over time.

Brain states can be defined by rhythmic activity in characteristic frequency bands. We know that the alpha rhythm (roughly ten cycles per second) plays its role in how we take in the visual world around us. For example, strong alpha can shield us from visual impressions. Weak alpha allows for more sensory intake.

Until recently this role of alpha had mostly been shown for briefly presented light flashes just bright enough to be on the verge of being visible. But Chris’ study suggests that alpha may be a gatekeeper for other aspects of visual processing in the brain, too. Here, observers judged whether a line was transected left or right of its centre. Crucially, alpha power before line presentation influenced this judgment.

Line stimuli – Observers had to judge whether lines were interrupted left or right of their actual centre.

More surprising however was that fluctuations in alpha power didn’t come as spontaneous or random as previously thought. Instead, the time observers spent on the task seemed to play a role – at least to some extent. Alpha simply increased over the course of the experiment. And here’s the novel aspect: This deterministic trend predicted a gradual shift in line centre judgments from trial to trial. On average, observers judged the centre to be more to the right than it actually was.

In brief, Chris’ experiment showed that alpha not only influences whether we see very faint stimuli but also how we make judgments about the centre of a visual object. Also, alpha does not just fluctuate randomly but has a deterministic component: When performing a task for an extended period of time alpha inclines gradually. This leads to a sustained and predictable change in our visual perception.


Schematic of spontaneous and deterministic influences of alpha (a neurometric) on reported line center (a psychometric) – the colour gradient towards purple indicates that observers showed an increased rightward bias over time meaning that they increasingly judged the line centre more right than it actually was.

These findings beg an interesting question for future research: Can we find other determinants of alpha (i.e. brain state) fluctuations? Moreover, can all ‘spontaneous’ fluctuations ultimately be described by deterministic processes?

The paper has just been accepted for publication in the European Journal of Neuroscience, and can be found here.

Benwell CSY, Keitel C, Harvey M, Gross J, Thut G (accepted) Trial-by-trial co-variation of pre-stimulus EEG alpha power and visuospatial bias reflects a mixture of stochastic and deterministic effects. European Journal of Neuroscience

This paper is part of the EJN special issue “Neural Oscillations”.