| Abstract |
In this study, simple visual reaction time measurements have been performed to study the
monocular and binocular effects of contrast and defocus on visual performance.
The binocular- and monocular (dominant eye) reaction times (RT) from 6 subjects (age: 25-40 years)
have been measured for different levels of positive defocus (up to 4.25 dpt) using a 4c/deg, 10%
contrast sine grating stimulus. Additionally the same 4c/deg sine grating stimulus was used to
measure the binocular- and monocular (dominant eye) RTs of each subject at 64 different contrast
levels (from 100% to two times threshold). The results have been used to calculate a linear RT vs.
1/contrast function.
Binocular RTs have been lower than monocular for all levels of defocus and all subjects. They reached
the same RT values as monocular at about 0.75 dpt higher defocus, indicating a binocular advantage
in defocus tolerance of 0.75 dpt.
Combining the defocus-RT data with the contrast-RT data allowed a conversion of reaction time to
perceived contrast. Perceived contrast was significantly higher for binocular compared to monocular
viewing. The binocular increase of perceived contrast was calculated and an average binocular
summation factor of 2.43 was found. There was no correlation between binocular summation and
amount of defocus.
Monocular and Binocular depth of focus has been calculated, using the defocus-RT data and
individually defined RTs as blur criteria. Depth of focus was found to be constantly higher for
binocular viewing. The magnitude of increase showed a high inter-subject variance and was in the
range from 0.18dpt to 0.92dpt. Possible reasons for these results have been discussed.
Additionally it was possible to compare the results of above described experiment for horizontally
and vertically oriented sine gratings. It turned out, that binocular reaction times for vertical gratings
are slightly lower until 1.5dpt defocus, but higher for defocus levels above this. It was found that the
insufficient spatial overlap of the binocular images caused an effective contrast loss for vertical,
compared to horizontal gratings.
At last, negative defocus and the accommodative range has been briefly investigated.
Accommodative range found to be much higher for monocular viewing than for binocular. Reason for
this discrepancy is the vergence movement of the eyes during accommodation which causes
effective diplopia for the 3m distant stimulus.
The measurement of simple visual reaction times found to be a reliable and precise psychophysical
/behavioral method to assess several aspects of visual performance.
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Investigating effect of defocus using simple visual reaction times
