The Functional Field of View of Older Adults is Associated With Contrast Discrimination in the Magnocellular not Parvocellular Pathway

Detection and discrimination of achromatic contrast: A ganglion cell perspective

The magnocellular (MC) pathway in the primate has much higher achromatic contrast sensitivity than the parvocellular (PC) pathway, and is implicated in luminance contrast detection. But MC pathway responses tend to saturate at lower achromatic contrast than do PC pathway responses. It has been proposed that the PC pathway plays a major role in discriminating suprathreshold achromatic contrast, because the MC pathway is in saturation. This has been termed the pulsed-pedestal protocol. To test this hypothesis, responses of MC and PC pathway ganglion cells have been examined under suprathreshold conditions with stimulus configurations similar to those in psychophysical tests. For MC cells, response saturation was much less for flashed or moving edges than for sinusoidal modulation, and MC cell thresholds predicted for these stimuli were similar to psychophysical discrimination (and detection) data. Results suggest the protocol is not effective in segregating MC and PC function.

Stable abnormalities of contrast discrimination sensitivity in subthreshold depression: A longitudinal study

Subthreshold depression (StD), as a subclinical state, is highly prevalent and increases the risk for developing major depressive disorder (MDD). Although several studies have reported deficits of contrast sensitivity in MDD patients, it is unclear whether individuals with StD could demonstrate deficits of contrast sensitivity and whether the deficits could remain stable over time. Here we used a contrast discrimination task (a suprathreshold task) and a contrast detection task (a near-threshold task) to compare contrast sensitivity of the StD group with that of matched non-depressed controls. For each task, a spatial four-alternative forced-choice method and a psychophysical QUEST procedure were used to measure contrast discrimination threshold or contrast detection threshold. Participants performed an initial assessment and a follow-up assessment 4 months later. Compared to the non-depressed controls, individuals with StD demonstrated reduced contrast discrimination sensitivity, not only at the initial assessment but also at the follow-up assessment, indicating a stable abnormality. Contrast discrimination thresholds at the initial assessment did not predict changes of depression symptom severity over time. For contrast detection sensitivity, there was no significant difference between the StD group and non-depressed controls. We concluded that contrast discrimination testing might provide a trait-dependent biomarker for depression.