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Foveation dynamics in congenital nystagmus I: Fixation

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Abstract

Congenital nystagmus (CN) has been described as a ‘fixation’ nystagmus implying an inability to fixate a target. However, each cycle of CN contains a target-foveation period during which the eye velocity is at, or near, zero. Prolongation of foveation time, reduction of retinal image velocity and cycle-to-cycle foveation repeatability all contribute to increased visual acuity. We developed several methods to accurately measure the dynamics of foveation in CN; their use is illustrated on an individual with typical idiopathic CN and no afferent defects. During eight 5-second intervals of fixation on a stationary target, the horizontal standard deviation (SD) of the mean foveation position (FPOS) was 12.82 minarc and the SD of foveation velocity was 118.36 minarc/sec. The SD of the means of total eye position and of the non-foveating peak of the CN were 43.17 and 25.32 minarc respectively. The mean foveation-time interval (eye velocity ⩽4°/sec) was 57.27 msec. The SD FPOS for the best 1-second interval (4 successive CN cycles), in a typical 5-second record, was 0.71 minarc. Histograms revealed peaks of eye position at 0 ± 10 minarc and of eye velocity at 0 ± 240 minarc/sec. The small vertical component of the CN (16 minarc peak-to-peak) had a SD of 6.56 minarc. A nystagmus foveation function related to visual acuity was derived that was more sensitive than CN intensity. The increased visual acuity resulting from the use of convergence or base-out prisms was due to increased foveation time. Although it might appear that CN is a defect of fixation, this individual with CN had strong fixation reflexes in the sense that he was able to accurately (within 1 minarc) achieve (interbeat) and maintain (intrabeat) target foveation for appreciable periods of time. Our data support the hypothesis that individuals with idiopathic CN do not have a primary disturbance of fixation.

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Abbreviations

AVG DIST:

Average values during distance fixation

CN:

Congenital nystagmus

DIST + PRS:

Fixation at distance with prisms

GANGLE:

Gaze angle

NFF:

Nystagmus foveation function

SD:

Standard deviation (STD)

Jef:

Jerk with extended foveation

LPC:

Left pseudocycloid

Pfs:

Pendular with foveating saccades

PPfs:

Pseudopendular with foveating saccades

EPOS:

Mean eye position

FPOS:

Mean foveation period position

freq:

Mean frequency of CN

FVEL:

Mean foveation period velocity

I:

CN intensity (PPA*freq)

NFP:

Mean non-foveating peak position

PPA:

Mean peak-to-peak amplitude

PSVEL:

Mean post-saccadic velocity

REPF:

Mean range of eye position during foveation period

TFOV:

Mean time (duration) of foveation period

%F/E:

The SD of FPOS as a percentage of EPOS

%N/E:

The SD of NFP as a percentage of EPOS

%REP/PPA:

REP as a percentage of PPA

%TFOV:

TFOV as a percentage of the CN period (1/freq)

Tf/SDFPV:

The NFF = (TFOV)(freq)/[(SD FPOS)(SD FVEL)]

kTf/SDFP:

Approximation to the NFF = k(TFOV)(freq)/(SD FPOS)

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Dell'osso, L.F., Van Der Steen, J., Steinman, R.M. et al. Foveation dynamics in congenital nystagmus I: Fixation. Doc Ophthalmol 79, 1–23 (1992). https://doi.org/10.1007/BF00160130

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