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.

在稳定固视状态下，观察者会产生微小的固视性扫视（fixational saccade），其发生速率约为每秒1~2次。视觉刺激的呈现会触发固视性扫视速率的双相调制：初始阶段表现为抑制，随后进入速率升高的时段，最终恢复至基线水平。本研究表明，在被动观看过程中，该速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“存在刺激”与“不存在刺激”的试次进行分类，我们直接对比了固视眼动的对比度敏感性与个体的心理物理判断结果。分类准确率与心理物理测试表现高度契合，且相较于使用该特征的特定维度所得到的阈值估计，该方法对个体阈值的预测偏差与整体误差均更小。该分类器的性能对训练集（新被试和/或新对比度条件）的变化具备鲁棒性，仅需数量可行的试次即可获得良好的预测准确率。本研究结果显示，视觉知觉判断的敏感性与固视眼动控制机制之间存在紧密的耦合关系。这一发现提出了一种新的可能性：固视性扫视可作为一种全新的客观手段，用于评估视觉对比度敏感性，而无需观察者做出任何外显判断。