Similar contrast sensitivity functions measured using psychophysics and optokinetic nystagmus.

Steven C Dakin,Philip R K Turnbull

doi:10.1038/srep34514

Abstract

Although the contrast sensitivity function (CSF) is a particularly useful way of characterising functional vision, its measurement relies on observers making reliable perceptual reports. Such procedures can be challenging when testing children. Here we describe a system for measuring the CSF using an automated analysis of optokinetic nystagmus (OKN); an involuntary oscillatory eye movement made in response to drifting stimuli, here spatial-frequency (SF) band-pass noise. Quantifying the strength of OKN in the stimulus direction allows us to estimate contrast sensitivity across a range of SFs. We compared the CSFs of 30 observers with normal vision measured using both OKN and perceptual report. The approaches yield near-identical CSFs (mean R = 0.95) that capture subtle intra-observer variations in visual acuity and contrast sensitivity (both R = 0.84, p < 0.0001). Trial-by-trial analysis reveals high correlation between OKN and perceptual report, a signature of a common neural mechanism for determining stimulus direction. We also observe conditions where OKN and report are significantly decorrelated as a result of a minority of observers experiencing direction-reversals that are not reflected by OKN. We conclude that there are a wide range of stimulus conditions for which OKN can provide a valid alternative means of measuring of the CSF.

Highlights

The contrast sensitivity function (CSF) is a useful way of characterising functional vision, its measurement relies on observers making reliable perceptual reports
Note that the peaks of these functions are shifted in relation to one another: at high spatial frequencies (SFs) most optokinetic nystagmus (OKN) is elicited at low velocities, while for low SFs more OKN is elicited at high velocities
We have described an automated method for measuring a contrast sensitivity function (CSF) using an automated analysis of optokinetic nystagmus (OKN) and compared the results to a CSF obtained using conventional perceptual report (ΨCSF)

Summary

Introduction

The contrast sensitivity function (CSF) is a useful way of characterising functional vision, its measurement relies on observers making reliable perceptual reports. In the lab one measures the minimum contrast (threshold) supporting detection of a pattern at different spatial frequencies (SFs), to derive a psychophysical contrast sensitivity function ΨCSF2 Such a procedure is too slow to use in the clinic, and adaptive alternatives[3,4] use Bayesian inference to determine the optimal combination of contrast and SF to present on a given trial. Alternative, objective estimates of the CSF have been obtained with scalp-recordings of visual-evoked potentials (VEPs) that correlate well with psychophysical measurements at lower SFs (transient VEPs, R2 = 0.81 with dynamic stimuli[9]) Both pattern-reversal VEPs10,11 and “sweep” VEPs (that cycles through a set of SFs and contrasts) produce CSFs of similar shape to the ΨCSF, but with substantially lower sensitivity (e.g. 0.62–0.79 log units[12]). Like OFR, MT/MST is involved in generation of OKN e.g. lesions in these areas in macaques leads to inaccurate tracking during OKN21

Methods

Results

Conclusion