Eye Movements Modulate the Spatiotemporal Dynamics of Word Processing

Abstract

Active reading requires coordination between frequent eye-movements (saccades) and short fixations in text. Yet, the impact of saccades on word processing remains unknown, as neuroimaging studies typically employ constant eye fixation. Here we investigate eye-movement effects on word recognition processes in healthy human subjects using anatomically-constrained magnetoencephalography, psychophysical measurements, and saccade detection in real-time. Word recognition was slower and brain responses were reduced to words presented early vs. late after saccades, suggesting an overall transient impairment of word processing after eye-movements. Response reductions occurred early in visual cortices and later in language regions, where they co-localized with repetition priming effects. Qualitatively similar effects occurred when words appeared early vs. late after background-movement that mimicked saccades, suggesting that retinal motion contributes to postsaccadic inhibition. Further, differences in postsaccadic and background-movement effects suggest that central mechanisms also contribute to postsaccadic modulation. Together, these results suggest a complex interplay between visual and central saccadic mechanisms during reading.

Left-Lateralized Contributions of Saccades to Cortical Activity during a One-Back Word Recognition Task

“…In contrast to salient external visual motion, self-induced Left-Lateralized Saccadic Contributions during Visual Word Recognition 4 saccadic visual motion is not consciously perceived, although it continues to be processed in the visual system and thus can impact information processing at subsequent fixation (Ibbotson and Cloherty, 2009). Consistent with interactions between visual signals during and after saccades, our previous results suggest that saccadic image motion modulates responses to words presented at fixation (Temereanca et al, 2012). In addition to such visual effects, central saccadic signals mediated by brain regions that control eye movements and attention are known to impact information processing in visual areas, producing effects that include transsaccadic suppression followed by postsaccadic enhancement.…”

“…However, active vision studies in humans and monkey indicate that central saccadic signals gated by brain regions that control eye movements and attention modify visual perception and cognition around the time of saccades, acting via distinct mechanisms before, during and after an eye movement (Wurtz, 2008;Berman and Colby, 2009;Ibbotson and Krekelberg, 2011). Recent evidence using magnetoencephalography (MEG) combined with saccade detection in real time supports the view that saccades also impact brain responses to words presented at fixation, revealing various degrees of saccadic modulation in early visual and higher cortical areas (Temereanca et al, 2012). An unexplored question is whether central saccadic signals impact occipitotemporal and temporal cortical areas that are implicated in visual word processing above and beyond preceding saccadic influences in occipital cortex.…”

“…Saccadic effects have been reported throughout the visual system, including early cortical and thalamic visual areas (Reppas et al, 2002;Sylvester and Rees, 2006;MacEvoy et al, 2008) as well as in areas of the temporal cortex implicated in visual recognition and memory (Sobotka et al, 1997(Sobotka et al, , 2002Purpura et al, 2003;Barlett et al, 2011;Monosov et al, 2011;Jutras et al, 2013). Using anatomically-constrained MEG source estimates to measure patterns of cortical activity with high temporal and spatial resolution, previous evidence suggests that visual and central effects of saccades also alter the dynamics of word-evoked cortical activation (amplitude, time course) at multiple stages of visual word processing (Temereanca et al, 2012). It is not known, however, if central saccadic signals impact higher stages of visual word representation in occipitotemporal and/or temporal cortex above and beyond preceding effects in occipital cortex.…”

“…Employing this approach, we compared cortical activity following the onset of saccades and spanning fixation before word appearance vs. similar external visual stimulation. We then examined whether differential responses colocalized with word repetition priming effects, as well as with known saccadic effects on responses to words (Temereanca et al, 2012).…”

Abnormal Visual Motion Processing Is Not a Cause of Dyslexia

“…However, brain imaging studies on reading primarily focus on decoding of single words rather than more ecologically valid sentences or passages, thereby avoiding the very mechanisms that are important to the understanding of the role of visual magnocellular systems in reading. Other technologies have been employed to study the role of eye movements in word processing, and could be expanded to dyslexia (Temereanca et al, 2012). To examine the possibility that there might be a direct link between neural systems underlying the linguistic aspects of reading and area V5/MT at the cortical level, we examined whether resting-state connectivity between right V5/MT and left hemisphere reading areas (i.e.…”