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Susana T. Chung, Girish Kumar; Dynamics of Fixational Eye Movements in the Presence of Central Vision Loss. Invest. Ophthalmol. Vis. Sci. 2012;53(14):3148.
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Fixation stability is known to be poor for people with central vision loss (CVL) and has been suggested as a contributing factor for the poor visual performance for these individuals. However, little is known about how the dynamics of the saccades during fixation (referred to simply as saccades here) and slow drift components of fixational eye movements are affected in the presence of CVL, and which component of the fixational eye movements plays a major role in limiting fixation stability.
Twelve observers with long-standing central vision loss (age: 58-87, logMAR acuity: 0.48-1.32) and 14 age-matched older adults (controls) with normal vision (age: 62-84, logMAR acuity: ≤0.0) participated in this study. Observers were asked to maintain fixation on the center of a 1° cross (2° for some observers with CVL) projected on the retina of the better-seeing eye of observers with CVL, or a randomly chosen eye of the control observers, using a Rodenstock scanning laser ophthalmoscope (SLO), for trials of 30 s. The retinal image and the position of the cross were recorded digitally at a frame rate of 30 Hz. Eye movements were extracted from the recorded videos at a sampling rate of 540 Hz using a cross-correlation technique for epochs of 10 s. A velocity criterion of 10°/s was used to differentiate between saccades and slow drifts.
Compared with control observers, observers with CVL on average exhibited greater fixation variability (BCEA: 1.40±1.45 deg2 vs. 0.06±0.04 deg2, p<0.005) and made more saccades during fixation (2.42±1.25/s vs. 1.49±1.05/s, p=0.001). The magnitude of these saccades were greater (2.35±2.41° vs. 0.20±0.23°, p<0.005), and were associated with higher velocity (horizontal: 32.4±12.6°/s vs. 18.5±5.4°/s, p<0.005; vertical: 9.51±3.43°/s vs. 5.51±2.38°/s, p<0.005) for observers with CVL, compared with control observers. However, assuming that these saccades were targeted toward the preferred fixation location, the errors (magnitude: 0.14±0.52° vs. -0.03±0.13°, p=0.06; direction: -0.02±0.13° vs. 0.002±0.04°, p=0.26) made were not different between the two groups of observers. The slow drift velocities (horizontal: 6.02±2.86°/s vs. 5.65±1.87°/s, p=0.51; vertical: 4.06±1.52°/s vs. 3.88±1.04°/s, p=0.56) were also not different between the two groups, but the inter-saccadic interval was shorter for observers with CVL (0.53±0.62 s vs. 1.48±1.35 s, p<0.005).
The greater fixation instability found in observers with CVL could not be explained by the slow drift components of fixational eye movements. Instead, it is likely that the increased rate, larger magnitude and higher velocity of fixation saccades contribute to the increased fixation instability for people with CVL.
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