Lexical word processing is unaffected by rapid invisible frequency tagging in reading: Evidence from eye movements

The aim of this study was to test whether rapid invisible frequency tagging (RIFT)-a promising tool to directly measure covert attention allocated to parafoveal words-affects lexical processing during natural reading. RIFT involves flickering the background of the target word(s) at an imperceptible frequency (≥60 Hz), but imperceptibility does not necessarily equate to the absence of a cognitive effect. The current study examined the potential impact of the RIFT paradigm on eye movements. Forty-eight participants read 474 high-frequency and low-frequency target words that were embedded in one-line sentences and displayed randomly in three condition blocks: no tagging, invisible tagging at 60 Hz (i.e., the RIFT) and visible tagging at 30 Hz. Linear mixed-effect modelling and divergence point analysis revealed a robust frequency effect, with high-frequency words processed faster than low-frequency words, but, crucially, no significant differences across the three conditions in terms of its magnitude or onset latency, except for a miniscule effect on saccade amplitude (0.06˚). Overall fixation durations (irrespective of lexical frequency) did not differ between the no-tagging and RIFT conditions, but were significantly longer in the visible tagging condition, suggesting that deliberately drawing attention to the tagging area actually slowed down reading. Furthermore, a postexperiment questionnaire indicated relatively low RIFT awareness. Altogether, preserved lexical frequency effects of comparable magnitude, similar divergence points and overall fixation and saccade patterns, suggest that RIFT is a valid tool for measuring attention during reading that is unlikely to interfere with word processing and highlight its potential for application in ecologically valid settings involving eye movements.