Retinotopic and spatiotopic accommodation responses in emmetropia and myopia
The incidence of myopia has been rising steadily over the last century. Today the
prevalence of the condition ranges from under 1% in remote parts of the world to over
90% in some highly industrialised urban areas. The associated pathological risk factors
in connection with the continuous increase in the number of myopic individuals have put
increasing pressure on the health and social systems worldwide. The aetiology and
course of the condition has therefore always been of interest to patients, ophthalmic
practitioners and researchers alike.
Reduced accommodation responses as a precursor to or even a cause of myopia have
been proposed for a long time. Increased lags of accommodation found in myopes,
causing a reduction in the quality of the retinal image, seem to support animal models,
which have suggested that retinal defocus if present over longer periods of time can lead
to the elongation of the globe.
In this work different aspects of the accommodation response have been studied in
emmetropes (EMM), early-onset (EOM) and late-onset myopes (LOM) under a variety
of viewing conditions.
The steady state accommodation response, the microfluctuations of accommodation and
accommodation step responses of different magnitudes (1-dioptre and 2-dioptre steps)
were examined for retinotopic (eye referenced) and spatiotopic (body-referenced)
viewing conditions. Retinotopic conditions involved viewing targets in Badal systems.
Spatiotopic viewing involved real targets presented in free space thus providing size and
proximity cues to the accommodation controller. The free space targets were viewed
monocularly and binocularly, which examined the importance of vergence
accommodation for the different refractive groups.
Steady state accommodation responses were found to be similar across the refractive
groups for all viewing conditions. LOM subjects however, showed higher variability of
their steady-state response in Badal systems but not in free space viewing. LOMs also
demonstrated significantly greater microfluctuations of accommodation in the retinotopic
condition, while the fluctuations were of similar magnitude for all groups for free space
viewing. While EMMs and EOMs responded promptly and quickly to step changes in
target vergence in both retinotopic and spatiotopic conditions, LOMs showed slower
reaction and response times in the blur-only environment. Analysis of the response rate
(percentage of step changes made for a given number of stimulus changes) revealed that
some LOMs hesitated to adjust their accommodation to small step changes in target
distance when only retinotopic cues were present. Real targets alleviated these
differences between the groups and increased the LOMs response rate considerably. The
minimum blur threshold of the accommodation controller was examined presenting
sinusoidal stimuli of very small dioptric magnitude in a Badal system. Fourier analysis
revealed that EMMs and EOMs responded to stimuli < 0.15 D while LOMs required
stimuli > 0.3 D. Cross-correlating the findings of the different experiments showed that
LOMs with inaccurate and more variable accommodation responses in retinotopic
conditions improve substantially with the introduction of real world targets, whereas
EMMs and EOMs with sufficient blur accommodation show no significant improvement.
The findings indicate a deficient blur processing in LOMs but demonstrate that there are
no significant differences in accommodation performance between emmetropes and
myopes in real world conditions. The implications of the findings on the development
and progression of myopia are discussed at the end of the thesis.