Frequency-tagging of spatial attention using periliminal flickers
Authors: Ladouce S , Dehais F 
Published: March 2024

Steady-State Visually Evoked Potentials (SSVEP) manifest as a sustained rhythmic activity that can be observed in surface electroencephalography (EEG) in response to periodic visual stimuli, commonly referred to as flickers. SSVEPs are widely used in fundamental cognitive neuroscience paradigms and Brain-Computer Interfaces (BCI) due to their robust and rapid onset. However, they have drawbacks related to the intrusive saliency of flickering visual stimuli, which may induce eye strain, cognitive fatigue, and biases in visual exploration. Previous findings highlighted the potential of altering features of flicker stimuli to improve user experience. In this study, we propose to reduce the amplitude modulation depth of flickering stimuli down to the individuals’ perceptual visibility threshold (periliminal) and below (subliminal). The stimulus amplitude modulation depth represents the contrast difference between the two alternating states of a flicker. A simple visual attention task where participants responded to the presentation of spatially-cued target stimuli (left and right) was used to assess the validity of such periliminal and subliminal frequency-tagging probes to capture spatial attention. The left and right sides of the screen, where target stimuli were presented, were covered by large flickers (13 and 15 Hz respectively). The amplitude modulation depth of these flickers was manipulated across three conditions: control, periliminal, and subliminal. The latter two levels of flickers amplitude modulation depth were defined through a perceptual visibility threshold protocol on a single-subject basis. Subjective feedback indicated that the use of periliminal and subliminal flickers substantially improved user experience. The present study demonstrates that periliminal and subliminal flickers evoked SSVEP responses that can be used to derive spatial attention in frequency-tagging paradigms. The single-trial classification of attended space (left versus right) based on SSVEP response reached an average accuracy of 81.1% for the periliminal and 58% for the subliminal conditions. These findings reveal the promises held by the application of inconspicuous flickers to both cognitive neuroscience research and BCI development.

Highlights

Frequency-tagging of spatial attention can be achieved through the presentation of flickering visual stimuli (flickers) whose contrast is reduced down to the individual’s perceptual visibility threshold revealing the potential of periliminal flickers as reliable frequency-tagging probes of spatial attention

Below this perceptual visibility threshold, the signal-to-noise ratio of SSVEP responses was not sufficient to reliably distinguish the field upon which participants directed their attention

The subliminal and periliminal flickers ameliorated the overall user experience and represent effective solutions to reduce bottom-up distraction, eye strain, and fatigue related to the presentation of flickering stimulation

The present findings have implications for the design of minimally intrusive frequency-tagging probes used within the frame of both fundamental cognitive neuroscience research and Brain Computer Interface