Abstract: Whether attention can change our subjective perception of stimulus appearance is an everlasting research topic in psychology. In the last two decades, a series of psychophysical studies from the Carrasco lab have shown that involuntary attention triggered by an exogenous visual cue can enhance the perceived brightness of the subsequent visual stimulus. More importantly, several recent studies have found that the exogenous visual cue could also expand the perceived range of the stimulus's boundary, leading to a "size illusion". Accordingly, these studies have consistently suggested that the exogenous cue induces the size illusion also by triggering involuntary attentional orienting. However, it is noteworthy that when the target at the cued location was perceived as being larger than the same-size target at the uncued location in these studies, the cue was unexceptionally smaller than the subsequent target that appeared in the same location. Therefore, the perceptual bias for the size of the cued target may not be entirely due to the involuntary attentional orienting induced by the cue; instead, it may partially result from sensory contrast induced by the temporal and spatial proximity between the small cue and the large target that was cued.
Given that salient unilateral auditory stimuli have been shown to be effective at triggering involuntary attentional orienting, Experiment 1 replaced the previous visual cue with an auditory cue (Fig. 1A) to determine whether involuntary attentional orienting contributes to the exogenous-cue-induced size illusion at all under the premise of excluding sensory contrast in size between cue and target. Besides, Experiment 1 also designed a visual-cue task (Fig. 1B) to further clarify the degree to which involuntary attentional orienting (and hence sensory contrast) contributes to the previously reported visual-cue-induced size illusion, which was done by calculating the correlation between the magnitudes of size illusions in the visual-cue and auditory-cue tasks. On each trial of Experiment 1, a spatially nonpredictive cue (i.e., a pure tone in the auditory-cue task, a small dot in the visual-cue task) was presented on the left or right side shortly before the appearance of a pair of red and blue visual disks (targets), with one at the cued location and the other at the uncued location. Meanwhile, the diameter of one disk was fixed at 2.30° (standard stimulus) whereas that of the other disk varied among 1.38°, 1.84°, 2.30°, 2.76° and 3.22° (test stimulus). The task for participants was to indicate the color (red v.s. blue) of the disk that appeared to be larger in size. Experiment 2 was similar to the auditory-cue task in Experiment 1, except that participants were required to indicate the color of the larger disk in half of the blocks and indicate the color of the smaller disk in the other half of the blocks. Experiment 2 aimed to examine whether the auditory-cue-induced size illusion holds true after the cue-related response bias, if any, was fully counterbalanced.
Experiment 1 showed that the point of subjective equality (PSE) was significantly smaller when the test target was cued than when it was uncued not only in the visual-cue task but also in the auditory-cue task (Fig. 2B). Furthermore, the size illusion induced by the auditory cue was replicated in Experiment 2 (Fig. 3B). However, the correlation analysis in Experiment 1 showed that there was no significant correlation between the magnitudes of the size illusion in the visual-cue and auditory-cue tasks (Fig. 2C). These novel findings indicate that on the one hand, involuntary attentional orienting can indeed contribute to the exogenous-cue-induced size illusion; on the other hand, the contribution of sensory contrast in size between cue and target is much larger than that of involuntary attentional orienting in the previously reported visual-cue-induced size illusion. Therefore, the current study explicitly reveals for the first time that there are dual psychological mechanisms for the exogenous-cue-induced size illusion.

Key words: exogenous cue, auditory cue, visual cue, size illusion, involuntary attentional orienting, sensory contrast

摘要： 以往研究认为外源性线索通过引起非随意注意定向来诱发大小错觉。然而这些研究中线索通常小于随后呈现于同侧的目标,因此线索诱发的对目标大小的知觉偏差可能仅源于小线索与大目标之间的继时感觉对比。研究引入听觉线索以排除感觉对比,结果显示听觉线索亦能诱发大小错觉,但听觉线索与视觉线索诱发的大小错觉之间无显著相关。研究结果一方面证明外源性线索能通过非随意注意定向来诱发大小错觉,另一方面表明先前研究中视觉线索诱发的大小错觉主要源于线索与目标之间的感觉对比,故明确揭示了外源性线索诱发的大小错觉存在双重机制。

关键词: 外源性线索, 听觉线索, 视觉线索, 大小错觉, 非随意注意定向, 感觉对比