Data from: Fixational eye movements predict visual sensitivity

During steady fixation, observers make small fixational saccades at a rate of around 1-2 per second. Presentation of a visual stimulus triggers a biphasic modulation in fixational saccade rate – an initial inhibition followed by a period of elevated rate and a subsequent return to baseline. Here we show that, during passive viewing, this rate signature is highly sensitive to small changes in stimulus contrast. By training a linear support vector machine to classify trials in which a stimulus is either present or absent, we directly compared the contrast sensitivity of fixational eye movements with individuals’ psychophysical judgements. Classification accuracy closely matched psychophysical performance and predicted individuals’ threshold estimates with less bias and overall error than those obtained using specific features of the signature. Performance of the classifier was robust to changes in the training set (novel subjects and/or contrasts) and good prediction accuracy was obtained with a practicable number of trials. Our results indicate a tight coupling between the sensitivity of visual perceptual judgements and fixational eye control mechanisms. This raises the possibility that fixational saccades could provide a novel and objective means of estimating visual contrast sensitivity without the need for observers to make any explicit judgement.

在稳定注视期间，受试者会产生小幅注视性眼跳（fixational saccades），频率约为每秒1~2次。视觉刺激的呈现会引发注视性眼跳频率的双相调节：初始阶段出现抑制，随后进入频率升高的阶段，最终恢复至基线水平。本研究发现，在被动观看过程中，该频率特征对刺激对比度的微小变化具有极高敏感性。通过训练线性支持向量机（linear support vector machine）对"刺激存在/缺失"的试次进行分类，我们直接比较了注视性眼球运动的对比度敏感性与受试者的心理物理判断表现。分类准确率与心理物理任务表现高度吻合，且相较于使用该特征的特定维度所得到的阈值估计结果，该模型对个体阈值的预测偏差与总体误差均更小。该分类器的表现在训练集变更（如使用全新受试者或全新对比度条件）时仍保持稳健，且仅需少量可行数量的试次即可获得良好的预测准确率。本研究结果表明，视觉知觉判断的敏感性与注视性眼球运动控制机制之间存在紧密的耦合关系。这一发现提示，注视性眼跳可作为一种全新的客观手段，用于评估视觉对比度敏感性，而无需受试者做出任何外显判断。