Studies in binocular accommodation.
A study of the binocular accommodation response is presented for
normal and amblyopic observers to selected stimulus conditions
using a binocular infra-red optometer and a commercially-available
autorefractor. The work reviews the neural control of the near
triad and discusses the historical development of models of mutual
interaction between accommodation and convergence, presenting
experimental evidence to support or refute each proposition. The
basic characteristics of the accommodation response are reviewed
along with the influencing factors.
A central feature of this work is the evaluation of the
correlation present between the eyes for both step-wise changes in
target vergence and steady-state viewing.
Reaction times for visually normal subjects were found to be
similar to those found by previous workers and were independent of
both size and direction of the step change. Response times for a
mean step size of approximately 2.5D exhibited a marked degree of
intersubject variability, particularly for the decreasing response
and were step-size dependent. Eye dominancy was not found to be
a significant factor in the overall response time.
The binocular accommodation responses were found to have a high
level of correlation to step-wise changes in target vergence.
This in, itself, is perhaps not surprising in view of the
anatomical similarities between the eyes and the relatively large
dioptric changes induced. To obtain a clearer picture of the
control of accommodation assessment of the microfluctuations was
A high degree of correlation between amblyopic eyes and their
fellow normal eyes is reported for both reaction and response
times, although response times are longer than those for normal
eyes. Reaction times for four subjects were not significantly
different to those of the dominant eye. The subject presenting
with the deepest amblyopia did have a significantly increased
reaction time and a relationship with minimum angle of resolution
Steady-state viewing shows the microfluctuations to have a high
level of coherence, suggesting the control of accommodation to be
at or above the point at which the IIIrd nerves are conjoint.
Increasing target vergence causes an increase in the rms amplitude
of the microfluctuations, binocular viewing not influencing the
response characteristics. As target luminance decreased, rms
values and low frequency drifts increased.
Amblyopic eyes show an increase in the magnitude of the low
frequency components of the microfluctuations for moderate to high
stimulus vergences. The presence of different behaviour to that
observed in normals supports a role for the microfluctuations.
The response of amblyopic eyes to coloured stimuli results in an
increase of the low frequency component to targets at the
extremities of the visual spectrum, furthering the argument for a
positive role for the fluctuations. The steady-state response to
coloured stimuli differed from that found in normal eyes in that
the appropriate response to overcome the chromatic interval was
not observed for moderate to high stimulus vergences. The
overall anomalous response could not use the additional
information provided by coloured targets.
Finally the detectibility of defocus was tested with sine waves
and using signals derived from the microfluctuations. The
threshold of detection for the microfluctuations is similar to
that for sine waves, but is thought to be due to the presence of
discontinuities and abrupt shifts in the response level. This
adds support to Crane's(1966) hypothesis of 'accommodative