Involvement of target contrast in visual masking location tasks deficients in myopia

Abstract

Purpose: There have been few studies of visual temporal processing
of myopic eyes. This study investigated the visual performance
of emmetropic and myopic eyes using a backward visual masking
location task.
Methods: Data were collected for 39 subjects (15 emmetropes, 12
stable myopes, 12 progressing myopes). In backward visual
masking, a target’s visibility is reduced by a mask presented in
quick succession ‘after’ the target. The target and mask stimuli
were presented at different interstimulus intervals (from 12 to
300 ms). The task involved locating the position of a target letter
with both a higher (seven per cent) and a lower (five per cent)
contrast.
Results: Emmetropic subjects had significantly better performance
for the lower contrast location task than the myopes
(F2,36 = 22.88; p < 0.001) but there was no difference between
the progressing and stable myopic groups (p = 0.911). There were
no differences between the groups for the higher contrast location
task (F2,36 = 0.72, p = 0.495). No relationship between task
performance and either the magnitude of myopia or axial length
was found for either task.
Conclusions: A location task deficit was observed in myopes only
for lower contrast stimuli. Both emmetropic and myopic groups
had better performance for the higher contrast task compared to
the lower contrast task, with myopes showing considerable
improvement. This suggests that five per cent contrast may be the
contrast threshold required to bias the task towards the magnocellular
system (where myopes have a temporal processing
deficit). Alternatively, the task may be sensitive to the contrast
sensitivity of the observer.