DEC 14, 2011
This study published in October in Investigative Ophthalmology & Visual Science used eye movement recordings to examine the adaptive strategies used by individuals with infantile nystagmus syndrome (INS) during reading. The authors found that these individuals learn to manipulate their nystagmus using a range of strategies to acquire visual information from the text.
These strategies include taking advantage of the stereotypical and periodic nature of involuntary eye movements to allow them to achieve the desired goal. The versatility of these adaptations yields reading speeds that are often much better than expected, given the degree of foveal and ocular motor deficits.
The authors recorded eye movements at 500 Hz in 25 volunteers with INS and seven healthy controls when reading paragraphs of text centered at horizontal gaze angles of 20°, 10°, 0°, 10° and 20°. For each location they measured reading speeds along with logMAR visual acuity and nystagmus during gazeholding.
Median reading speeds were 204.3 words per minute in individuals with INS and 273.6 words per minute in controls, with several individuals with INS reading more rapidly than controls. Adaptive strategies used by INS subjects include: suppression of corrective quick phases allowing involuntary slow phases to achieve the desired goal, voluntarily changing the character of the involuntary slow phases using quick phases, and correction of involuntary slow phases using quick phases.
The authors conclude that humans take advantage of involuntary movement in the ocular motor system to achieve desired goals presumably through skills acquired during visual development. Individuals with nystagmus appear to be able to control the absence or presence, timing, amplitude and direction of nystagmus quick phases leading to modulation of involuntary slow oscillations.
The variability in reading speeds is likely related to the wide range of strategies adopted to counteract the underlying drift of the eyes. The results also indicate that reading speed is slower during left-beating compared to right-beating jerk nystagmus, presumably, the authors say, because the former relies on the slow phase to move the eyes across the text. Factors leading to differences in visual acuity, such as foveal hypoplasia, also likely account for the range of observed reading speeds.