Brain Stimulation

The brain can be stimulated in various ways. The first involves sensory stimulation. For example, by presenting flickering visual stimuli that produce a so-called steady-state visually evoked potential (SSVEP) response. The second involves electrical stimulation, which includes both non-invasive brain stimulation such as transcranial direct or alternating current stimulation (tDCS/tACS) and invasive micro-stimulation. We have mainly worked on sensory stimulation so far, but have started doing non-invasive brain stimulation and plan to start micro-stimulation studies in the future.

Research Highlights

We studied whether two visual stimuli flickering at different frequencies interact with each other using invasive recordings in monkeys. We found that low-frequency SSVEPs caused much greater suppression than high-frequency ones, which further depended on the relative orientations of the stimuli (Salelkar and Ray, 2020, Scientific Reports).

Next, we replicated these interactions in human EEG recordings. Then, we mapped the response suppression of one stimulus (target) by a competing stimulus (mask) over a much wider range than the previous study. Surprisingly, we found that the suppression was not stronger at low frequencies as shown earlier, but systematically varied depending on the target TF, indicating local interactions between the two competing stimuli. These results were confirmed in both human EEG and monkey LFP and electrocorticogram (ECoG) data (Liza and Ray, 2022, Journal of Neuroscience). Our results show that sensory interactions between multiple SSVEPs are more complex than shown previously and are influenced by both local and global factors, underscoring the need to cautiously interpret the results of studies involving the SSVEP paradigm.