Fixation Locus in Patients with Bilateral Central Scotomas for Targets that Perceptually Fill In

Abstract

Purpose In this experiment we investigated whether target type affects the retinal fixation location and stability in patients with bilateral central scotomas, and specifically whether targets expected to perceptually fill in are imaged at or near the vestigial fovea. Methods The retinal location and stability of fixation were measured using the Nidek MP-1 microperimeter in twelve patients with bilateral central scotomas for six types of fixation target, three expected to fill in and three that included letters. The approximate position of the vestigial fovea was delineated in 10 of the patients either by using residual retinal landmarks or by locating the residual foveal pit in a dense macular scan, obtained with a Spectralis Optical Coherence Tomographer. Fixation location and stability were compared for the different target types and referenced to the position of the vestigial fovea. Results All of the subjects except one fixated consistently on targets that included a letter using peripheral retinal locations outside of the central scotoma. Eleven of the twelve subjects used a retinal location closer to the vestigial fovea to fixate targets expected to fill in, compared to letters. Although four of the subjects imaged the fill-in targets at or within a half degree of the vestigial fovea, six other subjects imaged the fill-in targets at a retinal locus removed from the vestigial fovea. Target type produced no overall significant difference in fixation stability, specified in terms of bivariate contour ellipse area (BCEA). However, in some individual subjects, fixation tended to be more stable on letter targets than on fill-in targets. Conclusions In patients with central field loss, letter targets generate more consistent fixation behavior than fill-in targets and should be used for eccentric viewing training and perimetry.

Repeatability of Nidek MP-1 Fixation Measurements in Patients With Bilateral Central Field Loss

“…And, finally, in some of the patients the foveal scotoma may have been smaller than the size of the fixation target, which would have allowed these patients to fixate using the fovea on a target that was perceptually filledin. 23 We suggest that estimates of fixation parameters, such as log BCEA, PRL eccentricity, and PRL meridian, in patients with bilateral central field loss might best be determined from two or more successive brief examinations and examined for concordance. When repeated estimates do not agree, the examiners should be alert to the possibility that the patients either do not have a firmly established PRL or switch between multiple PRLs.…”

“…Off-line calculation of the BCEAs using the fixation data exported from the MP-1, after excluding points more than 63 SD from the mean fixation position, confirmed the calculated BCEA values. We did not attempt to correct the calculated BCEA value 23,25 for the one patient who exhibited multiple PRLs (see below). The log BCEAs were transformed logarith-mically to reduce positive skew and produce an approximately normal distribution of the data.…”

Scotoma Visibility and Reading Rate with Bilateral Central Scotomas

“…The PRL is frequently located at or very near the border of the scotoma, which may be problematic for efficient reading. 2,33 In normal reading, visibility of the letters to the right and, to a lesser extent, the letters to the left of fixation are important for achieving optimal reading speed. 34 For this reason, it is likely that the fraction of a word that a patient with central vision loss can see during a single fixation will exert an influence on reading speed.…”

Neuroplasticidad en personas con baja visión: locus preferente retinal y su potencial para la rehabilitación visual

“…El entrenamiento en el uso de los LPRs como estrategia para mejorar el desempeño en tareas visuales específicas ha sido probado para tareas que requieren una alta agudeza visual tanto en controles sanos (37,47,(49)(50)(51)(52)(53) , como en pacientes (28,48,(54)(55)(56)(57)(58)(59)(60)(61)(62)(63)(64)(65)(66)(67)(68)(69)(70)(71)(72) .…”

“…Por otra parte, en pacientes el entrenamiento puede realizarse en el LPR espontáneamente formado por el paciente (61,64,65,68) o en uno escogido según criterios clínicos, el cual se denomina TRL (del inglés, trained retinal locus) (54,63,67,71,73,74) .…”