Ocular Speech Tracking Persists in Blindness, but Its Dynamics and Oculo-Cerebral Connectivity Depend on Visual...

While eye movements have been shown to track the speech envelope, it is unknown whether this reflects a hard-wired mechanism or one shaped by (lifetime) audiovisual experience. Further, questions remain about whether ocular tracking is modulated by speech intelligibility and which brain regions drive these synchronized eye movements. Here, we investigate ocular speech tracking in 47 (20 male), blindfolded early blind, late blind, and sighted individuals using magnetoencephalography and source-reconstructed oculomotor signals while participants listened to narrative speech of varying intelligibility. We find that oculomotor activity tracks acoustic speech features; however, while neural speech tracking is modulated by intelligibility, ocular tracking patterns remain ambiguous. Interestingly, we find effects reflected in two frequency-specific components: a low-frequency (∼1 Hz) effect present across all groups, indicating that visual experience is not required, and a high-frequency (∼6 Hz) effect reduced in early and late blind individuals. Moreover, this finding is not driven by cerebro-ocular connectivity, as late blind individuals exhibit stronger connectivity between the eyes and the left temporal cortices without a corresponding increase in ocular tracking. In conclusion, ocular speech tracking seems to respond selectively to acoustic features of speech, and does not require visual experience to develop. It may thus represent a hard-wired oculomotor mechanism within the oculo-cerebral network involved in speech processing.