Mirror-invariance is not exclusively visual but extends to touch

Abstract

Mirror-invariance enables recognition of mirrored objects as identical. During reading acquisition, sighted readers must overcome this innate bias to distinguish between mirror-inverted letters ('d' vs. 'b'). Congenitally blind individuals seem to overcome mirror-invariance for Braille letters, too. Here, we investigated mirror-invariance across modalities and its modulation based on the objects' familiarity. Congenitally blind and sighted subjects participated in same-different judgment tasks using tactile (blind and blindfolded sighted subjects) and visual (sighted subjects) modalities. Stimuli included pairs of letters (Braille and Latin) and familiar non-linguistic stimuli: geometric figures and everyday objects, presented in identical ('p'/'p'), mirror ('p'/'q'), and different ('p'/'z') conditions. In the tactile modality, no group differences were found in shape judgment for non-linguistic stimuli. In the orientation-based task, higher expertise for haptic rather than visual geometric figures was observed in the sighted group. Sighted participants exhibit difficulties when judging the shape of Latin letters as identical to those presented in mirror orientation (signature of breaking mirror invariance), in comparison to the blind, who had no difficulties in mirror shape judgment for Braille and non-linguistic stimuli. Results suggest that mirror-invariance is modality-independent.

Keywords: Braille; Congenitally blind individuals; Mirror-invariance; Reading; Shape recognition.

Fig. 1

Behavioral Results – Mirror Drop in Orientation-based Tasks and Mirror Cost in Shape-based Tasks [BC and SC (visual)]. (A) Mean mirror drop (orientation-based task) was observed in the congenitally blind group (tactile) and the sighted group (visual). A more positive result suggests greater difficulty in noticing the mirror orientation of presented stimuli. Congenitally blind individuals responded ‘different’ with less difficulty to mirror pairs of Braille letters than to everyday objects, and the groups did not differ. (B) Mean mirror cost (shape-based task) was observed in the congenitally blind group (tactile) and the sighted group (visual). A more positive result indicates more significant interference of mirror orientation with shape judgment. For sighted individuals, responding ‘same’ for mirror pairs of letters was more difficult than for other stimuli. In a congenitally blind group, we did not find the same pattern of results. Threshold levels: *p < 0.05, **p < 0.01, ***p < 0.001. Error bars represent S.E.M.

Fig. 2

Behavioral Results – Mirror Drop in Orientation-based Tasks and Mirror Cost in Shape-based Tasks [BC, SC (visual), SC (blindfolded)]. (A) Mean mirror drop (orientation-based task) was observed in the experiment in groups: congenitally blind individuals, sighted individuals (visual), and blindfolded sighted individuals. A more positive result indicates greater difficulty in noticing the mirror orientation of presented stimuli. Blindfolded sighted individuals responded ‘different’ with less difficulty than the congenitally blind group. (B) Mean mirror cost (shape-based task) was observed in the experiment in groups: congenitally blind individuals, sighted individuals (visual), and blindfolded sighted individuals. A more positive result indicates greater interference of mirror orientation with shape judgment. Sighted individuals in the visual task had lower mirror cost than the congenitally blind and blindfolded sighted individuals. The congenitally blind and the sighted groups in the tactile task did not differ. Threshold levels: *p < 0.05, **p < 0.01, ***p < 0.001. Error bars represent S.E.M.

Fig. 3

Experimental design and conditions: (A) Experimental design: Congenitally blind individuals participated in the tactile experiment and were presented with three categories of stimuli: Braille letters, basic geometric figures, and everyday objects. Sighted individuals (blindfolded) also participated in the tactile experiment, with two stimulus categories: basic geometric figures and everyday objects. Additionally, sighted individuals participated in the visual experiment and were presented with four stimulus categories: Latin letters, Braille letters, basic geometric figures, and everyday objects. (B) Experimental conditions: In both experimental tasks (shape-based and orientation-based tasks), we presented all stimulus categories in identical, mirrored, and different conditions. In the figure, an example of Braille letters and their corresponding Latin alphabet equivalents is presented. (C) Tactile stimuli: Examples of tactile stimuli used in the tactile experiment.