Is there an optimal stimulation rate for frequency-tagged visual word-selective responses?

The existence of inter-individual variability in hemispheric dominance for word recognition processes still needs to be understood. Unfortunately, there are methodological and practical limitations in using neuroimaging techniques and linguistic tasks to determine language lateralisation. Fast Periodic Visual Stimulation (FPVS) coupled with EEG provides objective and sensitive measures of visual recognition processes without explicit tasks required. Indeed, by using a visual word oddball paradigm in which every 5th of the 10 images presented in one second (10Hz base stimulation), a word is inserted (2Hz) in rapid streams of non-words or pseudo-words, a robust index of lexical representation in the left occipito-temporal cortex has been revealed (Lochy et al., 2015).
Here we use this promising technique, to determine if there is an optimal stimulation rate for frequency-tagged visual word-selective responses. For this, words are embedded in streams of non-words (i.e., pre-lexical discrimination) or pseudo-words (i.e., lexical discrimination) at a periodic rate, giving rise to frequency-tagged word-selective responses. We used four different frequencies (4 to 20Hz base frequency), to assess if word responses vary in amplitude and topography according to stimulation rate. For all frequencies, words are inserted at 1Hz, at the last frame of the base frequency except for one frequency at 10Hz (i.e., base stimulation) with 2Hz for the oddball frequency. 41 participants were tested both in EEG-FPVS and with reading tests. Reading performance was measured to see if visual word-selective response is modulated by reading speed and/or performance.
We found differences in amplitude according to the level of discrimination of the word with a significantly higher amplitude at the visual word-selective response for pre-lexical than lexical discrimination. For both conditions, amplitude and topography differ according to stimulation rate with a strong left activation found at 4Hz and at 10Hz stimulation rate (2Hz at the oddball frequency). Even if responses are more bilateral at 4Hz in non-words condition, no significant difference was found between the left amplitude of these two frequencies. We then found a significant correlation only at 4Hz for lexical word discrimination with reading speed. The faster a person reads a word, the higher the amplitude of the word-selective response in a lexical discrimination over the occipitotemporal cortex.
These results suggest that optimal frequencies of stimulation as well as lateralisation vary with the type of word-selective response. However, 4Hz seems to be the most adapted frequency to lead to a stronger left response for word-selective discrimination depending on the reading performance.