Abstract: PURPOSE: See everything gray when in depressed mood. Previous studies have shown that visual perception in patients with major depressive disorder (MDD) is altered. Whether individuals with subthreshold depression (StD), who have significant depressive symptoms but do not meet the diagnostic criteria of MDD, experience abnormalities in visual perception remains largely unknown. Here we aimed to investigate whether contrast sensitivity in StD could be altered, and whether these alterations could be stable across time, using a longitudinal design.
METHODS: Thirty two subjects with StD and thirty one subjects with non-depressed were screened out to perform two tasks: contrast absolute sensitivity task (task 1) and contrast discrimination task (task 2). Using a QUEST adaptive procedure and a four- alterative-forced-choice task, we instructed the subjects to judge at which of the four alternative positions a Gabor stimulus appeared (task 1) or which of the four Gabor stimuli had higher contrast (task 2). These subjects were retested after an average time interval of about four months.
RESULTS: No difference in contrast absolute sensitivity between StD and non-depressed controls was found. However, individuals with StD demonstrated lower contrast discrimination sensitivity (i.e. higher contrast discrimination threshold) than non-depressed controls in all pedestal contrast levels across test time. Furthermore, individuals who demonstrated lower contrast discrimination sensitivity at the initial test were less likely to improve in depressive symptoms at the follow-up test.
CONCLUSIONS: Contrast discrimination sensitivity in StD was reduced, and this alteration was associated with individuals' clinical state.

Key words: Subthreshold depression, contrast absolute sensitivity, contrast discrimination sensitivity, QUEST