Abstract:
Although the contrast sensitivity function (CSF) is a particularly useful way of characterising functional vision, its psychophysical measurement relies on observers being able to make reliable perceptual reports. This can be challenging e.g. when testing children. Here we describe a system for measuring the CSF without observer-report using an automated analysis of optokinetic nystagmus (OKN), an oscillatory eye movement made in response to moving stimuli (here, spatial-frequency - SF - band-pass noise). We show that predicting perceived direction using the proportion of eye movements that are consistent with OKN in the stimulus direction allows us to make an unbiased estimate of contrast sensitivity across SF. We next compare CSFs of 25 observers derived using either OKN or perceptual report. Both approaches yield near-identical CSFs that capture subtle inter-observer variations in acuity (R=0.80, p< 0.0001) and contrast sensitivity (R=0.80, p< 0.0001) amongst observers with ostensibly normal vision. A trial-by-trial analysis reveals that, even when observers' perceptual report is at chance, there is a very high correlation between our OKN-derived measure and observers' perceptual report. This indicates that OKN and self-report are likely tapping into a common neural mechanism providing further support for the proposal that OKN is a valid alternative to the current gold standard measures of CSF based on perceptual report. We discuss how our approach can be paired with an efficient psychophysical method to derive rapid automated measures of the CSF and other measures of functional vision.
