Bihemifield visual stimulation reveals reduced lateral bias in dyslexia.

Orton (1936) observed that dyslexic readers display not only obvious linguistic processing errors, but also diminished lateralized specialization of other cerebral hemispheric functions. To explore his "intergrading" hypothesis, six developmental dyslexics (DDs) and a group of good readers (GRs) were tested on measures of interhemispheric coordination. All subjects (ages 16 to 47) demonstrated normal oculomotor control and visual acuity prior to testing. Subjects were instructed to track three different point-light source patterns, (single stimulus in one hemifield, dual stimuli in one hemifield and a pair of simultaneous, symmetric, bihemifield stimuli [SSBS]), presented in random sequence and arrayed horizontally at ±5, ±10, and ±15 degrees eccentricity. Tested with unihemifield stimuli, all subjects showed normal saccadic latencies and trajectories. In response to SSBS, all GRs showed pronounced directional preference, choosing largely to track one side over the other. DDs showed reduced laterality bias (p<.025). DDs showed significantly longer response latencies to SSBS than to unihemifield stimulation (p<.01) and differed significantly from GRs (p<.05).