A quantitative analysis of eye movements during the cover test – a preliminary report

Abstract

The aim of this study was to carry out quantitative analyses of eye movements during the cover test on a group (n = 57) of asymptomatic emmetropes. Eye movements were recorded during an automated cover test while subjects fixated a distance (3.4 m) and near (0.4 m) target. There was a significant difference between the amplitude of phoria measured after 2 s of occlusion compared to 10 s (P < 0.01). The mean phoria after 10 s of occlusion was 0.1 degree (eso) for distance fixation and 1.6 degrees (exo) for near fixation. The distribution was approximately normal for both distance and near fixation but the standard deviation was greater for near (2.0 degrees) than distance fixation (1.4 degrees). The pattern of eye movements during the recovery phase was more complex than is often assumed, in many cases consisting of a variety of saccadic and vergence movements involving both the covered and 'fixing' eye. The latency of the first recovery movement was significantly shorter for exophoric than esophoric deviations (P < 0.01) and the latency of some recovery movements were very short (< 150 ms).

Effect of prolonged dissociation on the subjective measurement of near heterophoria

“…These two studies noted that the time taken for the occluded eye to reach a stable heterophoric position varied both bet\\een subjects, and between the two eyes of each individual. Barnard and Thomson (1995) reported a significant difference between the objectively measured heterciphorias obtained 2 s and 10 s following dissociation. However, dissociation periods longer than !0s were not examined.…”

“…This observation is also consistent with previous reports using objective recording techniques to examine the pattern of eye movements during the cover test (Peli and McCormack. 1983;Barnard and Thomson. 1995).…”

Near Heterophoria in Early Childhood

“…The Purkinje image tracking approach provides an objective recording that is analyzed offline to determine heterophoria. 36,39,40 The only disruption to the child during testing is the presence of a near infra-red (IR) filter in front of one eye while they view a cartoon movie (the filter provides occlusion as used in a clinical cover test, while permitting data collection from both eyes). These eye position recordings have a resolution of less than 2 pd, which is the smallest deviation detected under ideal conditions with the clinical cover test.…”

Quantification of heterophoria and phoria adaptation using an automated objective system compared to clinical methods

“…(1996) state that ‘there are no available data on validity of phoria tests’. Only a few studies have used an objective eye movement system to record the response decay to phoria or its dynamics (Peli and McCormack, 1983; Barnard and Thomson, 1995). These two studies used mechanical shutters to occlude the eye, where the experimenters could preset the amount of time the shutter was closed.…”

“…However, they did not compare the eye movement responses with other clinical methods, nor did they report the repeatability or precision of the steady‐state phoria level. Barnard and Thomson (1995) do report statistically significant differences in the phoria when comparing 10 s of occlusion vs 2 s, and hence recommend that the eye should be occluded for at least 10 s. Furthermore when evaluating precision, results should be reported on individual subjects to quantify the variability within the data.…”