Eccentric fixation with hemianopic field defects: A valuable strategy to improve reading ability and an indication of cortical plasticity

Abstract

Purpose: Reading disability is the main complaint in patients with hemianopic field defects (HFD). The analysis of reading strategies and of potential adaptive mechanisms is of interest for improvement of the reading process. Methods: A modified model 101 SLO was used to image the fundus (HeNe or IR laser) and to present the stimuli (HeNe laser with acousto optic modulator). Retinal fixation locus (RFL) was related to fixation targets of different size and shape and to texts. RFL and eye movements during reading were recorded simultaneously on video tape. Visual fields were examined by Tübingen Manual and/or Automated Perimetry. Results: Of 36 patients with HFD, ten showed eccentric fixation. The new RFL was shifted 1^o-2^o towards the healthy hemiretina, corresponding to a shift of the field defect towards the hemianopic side. RFL depended on stimulus size: central fixation was used for stimuli requiring high resolution, eccentric fixation was used for large targets and reading (shown by the landing points of the foveola on the text). Reading speed was increased in patients with eccentric fixation. Conclusion: The SLO method permits the simultaneous assesssment of RFL and stimulus with high spatial resolution and is a helpful method to study fixation behavior and reading in HFD. Eccentric fixation is a valuable strategy to improve reading performance in HFD by creating a sufficient reading visual field. Eccentric fixation despite intact visual acuity indicates a high degree of cortical plasticity.

Key Words:

• Eccentric fixation
• hemianopia
• reading
• cortical plasticity
• scanning laser ophthalmoscope