Slow binocular reading during rapid serial visual presentation (RSVP) in children with amblyopia and the role of fixation instability

Children with amblyopia read slower than their peers during binocular viewing. Ocular motor dysfunction typical of amblyopia may cause slow reading. It is unclear whether this is due to fixation instability or increased forward saccades. We examined whether removing the requirement of inter-word saccades helps children with amblyopia read at a similar rate as controls using a rapid serial visual presentation (RSVP) task. We also assessed whether reading rate was related to fixation instability. Children with amblyopia (n = 32) and control (n = 30) children ages 8-12 years silently read sentences presented in RSVP (single word presentation at screen center) during binocular viewing. Exposure time per sentence changed with a 2 - down 1 - up staircase to obtain reading speed thresholds (log words/minute [WPM]). Eye movements were tracked to determine fellow eye (FE) and amblyopic eye (AE) fixation stability during RSVP reading. Children with amblyopia read slower than controls (2.75 ± 0.47 log WPM vs 3.06 ± 0.40 log WPM), and had increased AE fixation instability (0.21 ± 0.39 log deg² vs - 0.20 ± 0.18 log deg²) and increased FE fixation instability (-0.03 ± 0.34 log deg² vs - 0.20 ± 0.15 log deg²) during RSVP reading. Reading rate in amblyopic children with good FE stability (n = 11) did not differ from controls and was faster than those with poor FE stability (n = 21). Children with poor FE stability read slower than controls. Removing the need for inter-word saccades (i.e., RSVP reading) did not help children with amblyopia read at control speeds. Our data support FE fixation instability as a source of slow reading in amblyopia.