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Laura Renninger, Anna Ma-Wyatt, Donald C. Fletcher; Misdirected Saccades With The PRL Lead To Reach Delays In Age-related Macular Degeneration. Invest. Ophthalmol. Vis. Sci. 2012;53(14):4393.
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© ARVO (1962-2015); The Authors (2016-present)
Last year we presented a case study of oculomotor and reaching behavior in a patient with age-related macular degeneration (AMD; Renninger, Ma-Wyatt & Fletcher, ARVO 2011). As observed in that study and others (Timberlake, et.al. 2011), reach latencies are elevated in AMD although reach durations appear normal. This study examines to what extent the elevation of reach latencies is attributed to changes in target visibility and localization and whether eye-hand coordination is disrupted with central scotoma.
Our AMD observer had long-standing bilateral wet AMD and an established preferred retinal locus (PRL). Central field loss was greater than 10° in diameter as measured with microperimetry. During experimental sessions, the observer first fixated a central target with the PRL and pressed a button to begin the trial. A high contrast dot appeared at distances ranging from 2°-12° in a block design, in one of eight directions with equal probability. The observer was instructed to make a rapid point to this target. The reach endpoint was recorded with a touch sensitive LCD and eye movements were tracked. All trials in which the target fell into the central scotoma were discarded from the analysis. The same protocol was carried out with an age-matched control observer with healthy central vision.
Saccadic reaction times were similar between observers. Reach latencies were found to be a weak linear function of target eccentricity, however AMD observer latencies were greater than would be predicted by increases in eccentricity alone. While bringing the PRL to the target, the AMD observer made more saccades and the eye was in flight longer on average as compared to the control observer. The control observer typically launched his hand during the first saccade. The AMD observer often launched his hand with later saccades. We evaluated the tortuosity of the overall eye movement path and found that for all target distances, the AMD observers’ saccades were significantly more misdirected.
The AMD observer has adapted eye-hand coordination to utilize the PRL instead of the fovea. Normal saccade latencies suggest that visual detection is intact for targets appearing outside the central scotoma, however saccadic localization of the target with the PRL is significantly misdirected. The reduction of oculomotor control with the PRL appears to be an important cause of reach delays.
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