In a nutshell: In-phase or anti-phase electrical stimulation of the brain’s hemispheres does not affect the ability to track objects moving between the left and right fields of vision.

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When a bird flies in front of you from left to right, the image is first processed by the right hemisphere of your brain and then by the left. This is because of how the eyes send information to the brain (see figure below). The information held by each of the hemispheres must be integrated for you to know that it was the same bird, rather than two birds. Previous studies have suggested that this integration is helped by synchronized brain activity – that is, when brain cells in both hemispheres send signals at the same time.

This synchronization can be artificially induced using transcranial alternating current stimulation (tACS), in which small electrical currents are applied to the brain through the scalp. When used on both brain hemispheres at the same time (‘in-phase’), tACS increases synchronization; when applied at opposite times to each hemisphere (‘anti-phase’), tACS decreases synchronization. Brain Function CoE investigators Nicholas Bland, Jason Mattingley and Martin Sale designed an experiment to test whether tACS helped or hindered the integration of information between hemispheres.

The team instructed participants to keep track of circles moving on a screen while keeping their eyes fixed on the centre of the screen. In some cases, the circles moved freely between the left and right visual fields (which requires the brain to integrate information), while in others, they moved only within separate visual fields (which does not require integration).

Synchronization between the brain’s hemispheres is required to track movement between the left and right visual fields, but not movement within separate fields. As a result, the researchers expected in-phase tACS to only help between-field tracking and anti-phase tACS to only hinder between-field tracking.

Unexpectedly, and unlike previous work using tACS, the team found no effect of brain stimulation on the tracking task. They suggested several possible explanations for their inability to produce the expected results: tACS may have failed to reliably change brain activity in the participants; their visual tracking task may not have used the same brain networks as other tasks that have been influenced by tACS; or the task (which was very demanding) may have relied on other parts of the brain that were not directly stimulated.

Without more research, the team cannot know which explanation – if any – is correct. Nevertheless, sharing the knowledge of the circumstances in which a hypothesis doesn’t hold is equally important for advancing our understanding of how the brain works.

Next steps:
The researchers are now focusing on applying tACS while simultaneously recording brain activity, in the hope that they can directly observe the effects of tACS on the brain.

Bland, N. S., Mattingley, J. B., & Sale, M. V. (2018). No evidence for phase-specific effects of 40 Hz HD–tACS on multiple object tracking. Frontiers in Psychology, 9, 304. doi: 10.3389/fpsyg.2018.00304

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Adapted from animation, with permission from Giovanni Solarte, all rights reserved