A compression of peripheral visual space as found in previous studies could underlie the atypical horizontal–vertical asymmetry in RM. Previous studies have shown that perceptual space is distorted along both the horizontal and vertical meridians in peripheral vision (
Osaka, 1977;
Sheth & Shimojo, 2001;
Wang, Murai, & Whitney, 2020;
Yildirim et al., 2019). For example, a target that was briefly presented on the horizontal meridian or the vertical meridian was systematically mislocalized as closer to the center of gaze, indicating a compression of visual space between the target and fixation (
Sheth & Shimojo, 2001). In another peripheral localization study, observers were asked to fixate a point and to manually point at a target stimulus that appeared briefly at large eccentricities (10°–50°) along the vertical and horizontal meridians (
Osaka, 1977). The observers made systematic errors, reporting the target location closer to fixation than its actual location, indicating again that visual space between fixation and the target was compressed. The magnitude of mislocalizations depended on visual field location, with larger mislocalizations seemingly occurring on the horizontal meridian than on the vertical meridian (
Osaka 1977), a significant effect of location but no comparisons between the locations were reported. In a position matching task, participants indicated the position of a target (shown at 48 different angular positions) with a mouse cursor after the target disappeared (
Wang et al., 2020). Calculating the angular distance between two adjacent reported locations revealed whether visual space was compressed (when smaller distances were reported) or expanded (when larger distances were reported). It was found that on average visual space was compressed along the horizontal meridian and expanded along the vertical meridian. We found the same pattern of compression along the horizontal meridian in a previous study on RM (
Yildirim et al., 2019). In two RM experiments, observers were asked to report the spacing between the two outermost lines (that is, the overall extent of the array) or the spacing between adjacent lines. We found that observers reported the spacing between the outermost of three lines (presented on the horizontal meridian) as smaller than the actual spacing and the spacing between adjacent lines as larger than the actual spacing when RM occurred, but not when no RM occurred (
Yildirim et al., 2019). Importantly, the spacing estimations in RM trials were approximately the same in both experiments, indicating that the perceived spacing between the two remaining (of the three presented) lines was similar for two adjacent and the two outermost lines (
Yildirim et al., 2019). In contrast, in “correct” trials, the spacing between two adjacent lines was accurately estimated while the spacing between the two outermost lines was overestimated. There are two alternative explanations for the observed results: either an outer line was redundancy masked, corresponding to an expansion of space, or the central line was masked, corresponding to a compression of space. An experiment assessing the perceived centroid of the line arrays ruled out that an outer line was masked; whether or not RM occurred, observers reported the centroid of the line arrays similarly accurately, indicating the loss of the central line and compression of space in RM (
Yildirim et al., 2019). Taken together, we suggest that greater spatial compression on the horizontal meridian compared with the vertical meridian might underlie the reverse horizontal–vertical asymmetry we found in RM. Note that spatial compression and reduced capacities to extract regularities are not mutually exclusive. Although it is unclear how the two mechanisms are related, they may well be correlated (strong spatial compression going hand in hand with superior regularity extraction), for example, because of irregular spatial compression. Investigating to what extent regularity perception and spatial compression correlate will shed light on the relation of the two mechanisms.