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Documenta Ophthalmologica

, Volume 79, Issue 1, pp 1–23 | Cite as

Foveation dynamics in congenital nystagmus I: Fixation

  • L. F. Dell'osso
  • J. Van Der Steen
  • R. M. Steinman
  • H. Coleewijn
Article

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.

Key words

Congenital nystagmus foveation fixation 

Glossary

General Terms

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)

CN Waveforms

Jef

Jerk with extended foveation

LPC

Left pseudocycloid

Pfs

Pendular with foveating saccades

PPfs

Pseudopendular with foveating saccades

Calculated (Statistical) Terms

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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Copyright information

© Kluwer Academic Publishers 1992

Authors and Affiliations

  • L. F. Dell'osso
    • 1
    • 2
    • 3
  • J. Van Der Steen
    • 5
  • R. M. Steinman
    • 4
  • H. Coleewijn
    • 5
  1. 1.Ocular Motor Neurophysiology Laboratory, Veterans Affairs Medical CenterUSA
  2. 2.Department of NeurologyCase Western Reserve University and University Hospitals of ClevelandCleveland
  3. 3.Department of Biomedical EngineeringCase Western Reserve University and University Hospitals of ClevelandCleveland
  4. 4.Department of PsychologyUniversity of MarylandCollege Park
  5. 5.Department of PhysiologyErasmus UniversityRotterdamThe Netherlands

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