Participants
Twenty-four participants took part in the study. Two groups of blind individuals (one congenitally blind and one late blind) and 2 groups of sighted adults took part in the experiment. The sample size was derived from previous studies that tested blind individuals (see e.g., Büchel, Price, Frackowiak & Friston, 1998; Guerreiro, Putzar & Röder, 2016), which had on average 5-6 blind individuals, as this population is not as large as the sighted one. Furthermore, we calculated a sensitivity power analysis based on (minimum) required effect size, given the sample size, and obtained that for Mann-Whitney tests with 2 groups, α = 0.05 and Power = 0.80, and 6 participants per group, an effect size of 2 is required. Note that all the significant comparisons between the blind and their respective controls exceeded this effect size.
The congenitally blind group consisted of 5 adults (3 females, mean age: 35.4, range: 26-40 years), and its sighted aged-matched control group comprised 5 adults (3 females, mean age: 36.4, range: 26-47). All congenitally blind participants were totally blind from both eyes since birth and had no pattern vision. Blindness was due to peripheral reasons in all participants: retinopathy of prematurity (N = 3), optic atrophy (N = 1) and unknown peripheral defect (N = 1).
The late blind group comprised 7 adults (2 females, mean age: 38.7, range: 19-60), and its sighted aged-matched control group comprised 7 adults (2 females, mean age: 38.8, range: 20-54). All late blind participants were totally blind from both eyes and had no pattern vision at the time of testing. Blindness was due to peripheral reasons in all participants, including retinitis pigmentosa (N = 3), macular degeneration (N = 2), glaucoma (N = 1), and unknown peripheral defect (N = 1). The blindness onset in the late blind group varied between 5 and 16 years of age, and the mean duration of blindness before taking part in the study varied between 14 and 50 years. All blind participants were Braille-readers at the time of testing, but they started at different stages in development depending on blindness onset. All participants were right-handed as assessed by self-report and with absence of tactile perception disorders. The study was approved by the Ethic Committee of the University of Milano-Bicocca, and all participants signed an informed consent before starting the experiment.
Stimuli and Procedure
The task consisted in drawing the outline of two bodies on tracing paper. Participants were told that one body had to be their own, as they imagine it to be; the other body had to be an ideal body, i.e., a body that participants imagined to have perfect proportions.
Before starting to draw, sighted participants were blindfolded, and all participants were allowed to explore the tracing paper to be able to fit the drawing within the paper’s space. Furthermore, all participants performed two training-drawings before the target drawings, to familiarize with the procedure.
Participants were told that the bodies had to be a one-line drawing, i.e., to be performed without letting the marker detach from the paper, and that they had to draw only the external shape of the body, without details of inner parts of the body (e.g., mouth, eyes, hands, feet, etc). Participants were asked to start from the head and to keep the sheet right in front of them throughout the drawing. Half of the participants started by drawing their own body, while the other half started by drawing the ideal body. A representative picture of the original draw for each group is depicted in Figure 1.
Data analysis
Each body draw was sampled using 62 landmarks representative of the different body parts as shown in Figure 1 (last panel). A picture of each draw was taken keeping the camera flattened aligned on the top of the sheet at a constant distance of 25 centimetres (cm). Subsequently, the experimenter, by using an image modification software (i.e., Microsoft Paint) extracted the values of each point in pixel coordinates. Pixel coordinates of landmarks were coded and averaged, thus resulting in a map for each draw. Then, for visualization purposes, we used Generalized Procrustes Analysis (GPA) to average the different configurations for each group across participants. This procedure aligns sets of homologous landmarks, removing differences in location and rotation thus highlighting differences in shape (Bookstein, 1997). Note that we kept the information about scale. To perform the GPA we used Shape, a MATLAB toolbox from Dr. Preston (https://www.maths.nottingham.ac.uk/personal/spp/shape.php), based on an algorithm derived from Gower (1975) and Ten Berge (1977). Based on the approach adopted in previous studies (Medina, Tamè & Longo, 2018; Tamè, Bumpus, Linkenauger & Longo, 2017), using a similar logic we focused our analyses on the size of the body comparing it between the different groups. First, we investigated whether the overall configuration of the 62 landmarks representing the whole body was changing in size as a function of the different group tested (i.e., congenitally blind vs control and late blind versus control) for the two type of requested drawings. Similarly to Medina et al. (2018), we quantified the size of each configuration by calculating the centroid size, as well as the root mean square of the distance from each of the 62 points from their centre of mass (or centroid, see Bookstein 1997; Zelditch, Swiderski & Sheets, 2012). For each participant, we calculated the centroid size (in pixel units) for the different configurations of points for the whole body as well as for three set of body parts, namely hands, feet and head. We focused on these body parts, because we hypothesised that hands, selectively, and not other body parts, would be distorted in blind individuals. Thus, feet and head were taken as control body parts to observe whether the hypothesised change in hand representation is highly specific or generalises to other body parts. The rational of this comparison was dictated by the fact that human hands and feet are serially homologous structures that have co-evolved (Rolian, Lieberman & Hallgrìmsson, 2010), showing several similarities (Manser-Smith, Tamè, Longo, 2018; Manser-Smith, Tamè, Longo, 2019) and some differences (Manser-Smith, Romano, Tamè, Longo, 2021) in their representations.
To further explore the hypothesis that hands, but not other body parts, could be differently represented in blind individuals, we conducted a second analysis to calculate the estimated size of the fingers of the hands. On such configurations, we calculated the differences in the estimation of the fingers’ length between groups (i.e., congenitally and late blind individuals vs sighted controls) and drawing conditions (i.e., own vs ideal body). As shown in the graphical depiction of Figure 2, for each finger, we calculated the middle points (B) between the base of the most proximal phalange of each finger (distance between A and C). Then, to estimate the finger’s length, we calculated the Euclidean distance between this middle point (B) and the tip of the finger (D) using the pdist Matlab function (MathWorks, Natick, MA). The raw data have been made publicly available via the Open Science Framework and can be accessed at https://osf.io/ahuyp/.
Body sizes were then compared between groups by means of Mann–Whitney U test, considering the size of the drawn whole bodies and the single body parts (hands, feet and head); single body part sizes were computed as the proportion between single body part and whole body (i.e., % single body part = single body part/whole body*100).