Abstract:We happened to observe that text that was reflected about either the horizontal or vertical axis was more difficult to read than text that was reflected about first one and then the other, which amounts to a 180-degree rotation. In this article, we review a number of studies that examine the nature of recognizing reflected and inverted letters, and the frequency of mirror reversal errors (e.g., confusing 'b' for 'd') in children and adults. We explore recent ideas linking the acquisition of literacy with the loss of mirror-invariance, not just for text, but for objects in general. We try to connect these various literatures to examine why certain transformations of text are more difficult to read than others for adults. Keywords: mirror-reversal; left-right reversal; reading; reversal errors; mirrored textRecently, one of us held up a page of printed text ( Figure 1A) to their webcam to take a picture. The camera software by default created a mirror-image snapshot (i.e., the text was reflected or mirrored about the vertical axis, Figure 1B) thereby rendering the text virtually illegible. Correcting or undoing this sort of reflection can be trivially accomplished using any number of image processing software packages that include built-in functions such as 'reflect vertical', 'reflect horizontal', 'rotate left', and 'rotate right'. It was with some surprise that when the wrong function was accidentally applied and instead of reflecting along the vertical axis, the image was reflected along the horizontal axis, the text, although now reflected twice, was now more easily legible ( Figure 1D). It is of note that this 'double reflection' is equivalent to a 180 • rotation of the text. With only a few mouse clicks, it was clear that when normally-oriented text is mirror reflected about the horizontal axis it again becomes virtually illegible ( Figure 1C). This brief series of webcam-picture inspired observations demonstrated a basic property of text perception and reading: the legibility of text is reflection variant and rotation invariant.Inspired by these observations, we conducted a review of the corresponding work on the intersection of symmetry, reflections, and the legibility of text. We found a large body of literature on mirror-reading and writing; mirror-reversal errors of single letters in reading, copying, and writing; many studies on perception of normal and mirrored single characters; but only a small number of studies on combinations of geometric transformations including both inversion (up-down mirroring) and reflection (left-right mirroring). In this paper, we review the literature on mirror-reversals and inversions of text and synthesize some recent work and ideas on how representational mirror-invariance not only for letters but also for objects may be lost when someone learns how to read.