Editha Ong

Nearwork-induced transient myopia: a critical review.

The literature on nearwork-induced transient myopia (NITM) is reviewed, with NITM being defined as the short-term myopic far point shift immediately following a sustained near visual task. A majority of these investigations demonstrated the presence of NITM for a variety of test parameters, e.g., visual acuity, contrast sensitivity and far point. Overall, these studies reported relatively small myopic shifts, with a mean of approximately 0.40 D and a range from 0.12 to 1.30 D. The subsequent decay is characterized by an exponential function with a relatively short time course. While the precise etiology and implications of NITM remain unclear, speculations regarding its origin and relevance to clinical myopia are discussed. Studies that did not demonstrate NITM are also reviewed.

Vergence adaptation and the order of clinical vergence range testing

When measuring horizontal relative vergence ranges, the conventional clinical procedure is to assess the base-in (or divergence) range before the base-out (or convergence) determination. The rationale for this order of testing is that the convergence responses stimulated during the base-out measurements may produce vergence adaptation (i.e., a fusional aftereffect), which may temporarily bias the subsequent base-in values in the base-out direction. However, the effect of reversing this order of testing (i.e., base-out before base-in) has not been directly evaluated. Accordingly, the present study compared distance base-in vergence ranges measured both before and after base-out testing at separate test sessions. Additionally, tonic vergence (TV) was assessed before and after each vergence range measurement. The results indicated that base-out vergence range testing stimulated vergence adaptation, and furthermore produced a statistically significant reduction in the subsequently measured base-in recovery value. However, the magnitude of this shift was less than the estimated degree of repeatability of the vergence range measurement. Therefore, a more appropriate order of testing would be to assess the heterophoria compensating range first in order to obtain accurate results during this more critical measurement.

Effect of luminance on visual evoked potential amplitudes in normal and disabled readers.

Background. Considerable evidence exists that some reading-disabled children have disordered visual processing, specifically in the fast processing magnocellular (M) pathway. Methods. The extent that varying luminance and temporal frequency affect amplitude and latency of visual evoked potentials (VEPs) in normally achieving and reading-disabled children grades 4 to 6 was measured. Each group consisted of approximately 30 subjects. Monocular and binocular single channel VEPs were recorded using a sinusoidal checkerboard pattern of spatial frequency 14 min arc at 3 different temporal frequencies (1, 4, and 8 Hz), and an 8 Hz flicker fusion stimulus. Stimuli were presented under high and low luminance conditions. The peak of the major positive wave component (P1Oo) of each waveform and the trough of the previous major negative wave component were identified, and the peak to trough amplitude was measured. Results. Statistical analysis of the VEP amplitudes and latencies in response to different experimental conditions was performed using a repeated measure analysis of variance (MANOVA). VEP amplitudes were significantly higher for normal readers across all conditions. Within all subjects, significant effects were found for monocular vs. binocular viewing, temporal frequencies, and high vs. low luminance. Similar analysis of latencies revealed no significant differences. Conclusions. The presence of a weaker VEP response in reading-disabled children suggests a deficit early in visual processing. The significant difference in VEP amplitudes between the two reading groups provides an objective measure of a deficit in the M pathway that has been implicated in this condition. Whether serial VEP recordings might help to assess the effects of optometric therapy by providing an independent index of therapeutic efficiency is of special interest.

Accommodative responses to naturalistic stimuli

Steady‐state accommodative responses to a wide range of commonly encountered textural and graphic stimuli were investigated in 12 young, visually normal subjects. Details of two experiments are described. In the first experiment, stimuli were varied in terms of size, contrast, spatial frequency, colour and intellectual demand. In the second experiment, only the colour of the targets was systematically altered. In both experiments the stimulus demand was maintained at 3 D, and accommodation was measured subjectively using a Hartinger coincidence optometer. No significant difference in mean accommodative response was observed between the widely differing targets adopted in either experiment. These findings suggest that an overall threshold exists for accommodative stimuli, with supra‐threshold targets of low to moderate contrast being capable of initiating and sustaining an appropriate accommodative response.

Irlen lenses do not improve accommodative accuracy at near

The purpose of the experiment was to determine the influence of Irlen lenses on steady-state accommodation in successful users, as there is speculation that near accommodative dysfunction may be a factor in these patients. Monocular steady-state accommodation in six successful Irlen patients was assessed for near blur stimuli (2, 3, and 4 D) either with nonfiltered spectacle refractive correction or their spectrally broad-band filtered spectacle refractive correction. Accommodation was measured subjectively using a Hartinger coincidence optometer. There was no significant difference in mean level of accommodation between the two conditions. Additionally, there was a small but significant increase in accommodative variability with the filtered prescription. The use of Irlen correction did not have any positive effect on monocular steady-state accommodation at near.

Effect of target proximity on transient myopia induced by equidioptric stimuli

Transient myopia may be exhibited after sustained focus on a near target. This appears to be related to the within-task accommodative response. Although proximally induced accommodation has been shown to produce considerable transient myopia under open-loop conditions, its effect under naturalistic closed-loop conditions remains unclear. Accordingly, the present study examined the effect of target distance on transient myopia for equidioptric stimuli (5 D). A Canon R-1 autorefractor was used to assess the pre- and post-task refractive state objectively. Two tasks were performed monocularly and consisted of either accommodating on a near target at 20 cm (5 D) or a far target at 6 m viewed through a −5 D lens. The former task involved both blur and proximally induced accommodation, whereas the latter consisted of a blur stimulus only. Similar amounts of transient myopia were found under both conditions. These equivalent responses may be related to the relatively small output of proximally induced accommodation believed to be present under closed-loop conditions.