Biomedical subjects

Gordon Heron

Publications and source records attributed to Gordon Heron.

4 recordsLinked to original sources

PubMed2005

The effect of monocular and binocular viewing on the accommodation response to real targets in emmetropia and myopia.

PURPOSE: Decreased blur-sensitivity found in myopia has been linked with reduced accommodation responses and myopigenesis. Although the mechanism for myopia progression remains unclear, it is commonly known that myopic patients rarely report near visual symptoms and are generally very sensitive to small changes in their distance prescription. This experiment investigated the effect of monocular and binocular viewing on static and dynamic accommodation in emmetropes and myopes for real targets to monitor whether inaccuracies in the myopic accommodation response are maintained when a full set of visual cues, including size and disparity, is available. METHODS: Monocular and binocular steady-state accommodation responses were measured with a Canon R1 autorefractor for target vergences ranging from 0-5 D in emmetropes (EMM), late-onset myopes (LOM), and early-onset myopes (EOM). Dynamic closed-loop accommodation responses for a stationary target at 0.25 m and step stimuli of two different magnitudes were recorded for both monocular and binocular viewing. RESULTS: All refractive groups showed similar accommodation stimulus response curves consistent with previously published data. Viewing a stationary near target monocularly, LOMs demonstrated slightly larger accommodation microfluctuations compared with EMMs and EOMs; however, this difference was absent under binocular viewing conditions. Dynamic accommodation step responses revealed significantly (p < 0.05) longer response times for the myopic subject groups for a number of step stimuli. No significant difference in either reaction time or the number of correct responses for a given number of step-vergence changes was found between the myopic groups and EMMs. CONCLUSION: When viewing real targets with size and disparity cues available, no significant differences in the accuracy of static and dynamic accommodation responses were found among EMM, EOM, and LOM. The results suggest that corrected myopes do not experience dioptric blur levels that are substantially different from emmetropes when they view free space targets.

Accommodation, Ocular↗

PubMed2003

Retinotopic accommodation responses in myopia.

PURPOSE: A reduced sensitivity to retinal image blur has been reported in myopes. Diminished blur detection reduces the error signal to the retinotopic (blur-induced) accommodation system and results in impaired accommodation responses under retinotopic conditions. This study was conducted to investigate retinotopic accommodation responses in emmetropia and myopia under dynamic conditions. METHODS: Static accommodation responses to a blur-only target with vergences of 0 to 4.5 D were measured with an optometer. Microfluctuations of accommodation were recorded with the subject viewing the target at a vergence of 4 D, and dynamic step responses were measured for step stimuli from 2.5 to 3.5 D and 2.0 to 4.0 D, with the optometer in dynamic recording mode. Measurements were obtained from a group of 32 visually normal emmetropes (EMMs) and subjects with progressing myopia. RESULTS: Stimulus-response curves were not significantly different between the refractive groups. Subjects with late-onset myopia (LOMs) demonstrated significantly larger accommodation microfluctuations compared with emmetropes and subjects with early-onset myopia (EOMs). Fourier analysis revealed that the increase in the magnitude of the fluctuations was mediated by the low-frequency components. Accommodation step responses revealed longer reaction times in LOMs. Further analysis showed that LOMs responded to accommodation step stimuli only between 43% and 64% of the time. In contrast, the other groups showed a response rate of almost 100%. CONCLUSIONS: The experiments demonstrate a reduction in retinotopic processing in LOMs, which results in an increased variability in their dynamic accommodation response to stationary near targets and reduced performance for dynamic step tasks. The results demonstrate a reduced blur appreciation under dynamic conditions in these refractive groups that may lead to periods of retinal image blur of varying magnitude during near work.

Accommodation, Ocular↗

PubMed2002

Visual latency in the spontaneous Pulfrich effect.

BACKGROUND: Interocular visual latency differences in patients with traumatic optic neuropathy due to midface injury were compared using both measurements of the delay from the spontaneous Pulfrich effect and pattern visual evoked potentials (VEPs). METHODS: Six patients with a spontaneous Pulfrich effect following midfacial injury observed a target which oscillated sinusoidally with an elliptical path in the frontal plane. The spontaneous Pulfrich delay was calculated from the size of the minor axis of the target ellipse on an XY plotter adjusted until the patient judged that no depth (i.e. no ellipse) was seen. Six separate pattern-reversal VEPs were recorded monocularly, with an artificial pupil, for a 4 cycle/degree grating. The mean P100 peak latency for each eye was used to calculate interocular latency differences. For both the Pulfrich effect and VEP measures normal subjects were included for comparison. RESULTS: Patients had P100 delays ranging from 2.8 to 17.8 ms, whereas Pulfrich delays were much shorter, ranging from 0.14 to 1.3 ms. A significant positive correlation was found between the two measures of delay, but the magnitude of the interocular difference in VEP was much greater than the delay calculated for the spontaneous Pulfrich effect. Tints used to correct the spontaneous Pulfrich effect in the patients were generally of high transmission (75-85%) and not dense enough to provoke an effect in normal observers. CONCLUSION: We have described a technique to quantify interocular delay associated with a spontaneous Pulfrich effect in patients with midfacial trauma. Interocular delays measured from pattern-reversal VEPs are similar in direction and relative severity but are larger in absolute terms.

Adult↗

PubMed2002

Correction of the Pulfrich phenomenon by surgery and laser.

We describe 2 patients who presented with asymmetrical media changes and a spontaneous Pulfrich phenomenon. In the first case, the phenomenon was due to cataract formation and an interocular latency difference of 1.49 ms +/-0.07 (SD) was found. After cataract surgery, the symptoms resolved and no interocular latency difference could be measured. In the second case, the phenomenon was caused by posterior capsule opacification and an interocular latency difference of 1.16 +/- 0.08 ms was found. The symptoms were alleviated and no interocular latency difference could be measured after a neodymium:YAG laser capsulotomy was performed. The symptomatology of the Pulfrich phenomenon is often difficult for patients to explain. Clinicians should be aware of the phenomenon with cataract patients.

Cataract↗