Covert attentional tracking with invisible phase tagging

Attention forms a fundamental component of visual processing. The locus and spread of covert attention reveals how specific locations or features in the environment are preferentially processed, however, tracking this process in a time-resolved fashion is not easy. Rapid Invisible Frequency Tagging (RIFT) is a recent advance in localizing covert attention. This technique uses stimuli that flicker at unique frequencies to invisibly "tag" areas of the visual field. The frequency-specific response elicited offers a measure of visual processing (and covert attention) over time. However, the narrow frequency band compatible with RIFT restricts the number of distinct frequency tags that can be simultaneously used. This limitation can be addressed by using oscillatory phase as an additional tagging feature. Here, we conducted a RIFT-EEG experiment (n=24) with 8 unique tags based on both frequency and phase modulation. We show that modulations in visual processing can be measured at several tagged locations with identical frequency but different phases. This also allowed for the use of combined phase- and frequency- modulated RIFT to quantify the distribution of attention across the visual field. By integrating phase-modulation into invisible frequency tagging studies of attention, we bring the field a step closer towards fine-resolution attentional tracking continuously over time and space, allowing novel insights across various domains of visual cognition.