Áreas culturales: la dimensión espacial (Herskovits)

2.3.2.1 Visual Illusions Task.

The visual illusions task is a computer-based program, presented in Flash (as used in Chouinard & Noulty, 2013). Participants were presented with 13 different visual illusions tapping into different neural mechanisms (see Table 3 for a list of all illusions included in the task). Each illusion was presented four times for a total of 52 trials on a computer screen. The illusions were presented in a random order generated by the program and participants were instructed to perform a size-matching task, adjusting the length of a comparison stimulus ‘A’ to match the size of a second standard stimulus ‘B’ (see Figure 1). For each visual illusion the comparison stimulus was counterbalanced, appearing twice in the position of stimulus ‘A’ and twice as stimulus ‘B’. The comparison stimulus was presented either 20% larger or 20% smaller than the target stimulus.

Participants adjusted the comparison stimulus by pressing the left hand button on a mouse to make the stimulus bigger and the right hand button to make it smaller. Each press of the mouse decreased or increased the size of the comparison stimulus by two pixels. Participants were also instructed to judge the size of comparison stimulus without using any other approaches (e.g. measuring the length with their finger). At the beginning of each trial, instructions were presented at the top of the screen. Three buttons were presented to

participants at the bottom of the screen: “Increase”, “Decrease”, and “Done”. Once

participants had adjusted the comparison stimulus so that they perceived it to be the same size as the standard stimulus they were asked to press “Done” to indicate they had finished.

Participants had as much time as they required to complete the task. On average the task took approximately 25 minutes to complete. A susceptibility index was calculated on completion of the programme for each visual illusion for each participant:

[(Perceived Size in Configuration A – Perceived Size in Configuration B)/ (Perceived Size in Configuration A + Perceived Size in Configuration B)].

An ‘overall susceptibility’ score was generated by summing the susceptibility scores for individual visual illusions and calculating the mean for each participant. This calculation is based on previous studies using the same method (e.g. Chouinard, Noulty, Sperandio, & Landry, 2013). The susceptibility index provides a score regarding the magnitude of the difference between the size of the comparison stimulus and the standard stimulus once the participant has finished their adjustments. The susceptibility score therefore relates to how closely the participant has size-matched the comparison stimulus with the target stimulus, measured in pixels. The susceptibility score ranges from 0 – 1, with lower scores being indicative of lower susceptibility to visual illusions (e.g. a score of 0 would mean that the

comparison stimulus was accurately matched to the same size as the target stimulus). Individuals who are less susceptible to illusions will be more accurate in size matching the two stimuli. Visual illusion tasks using the same method of adjustment have been widely used in visual psychophysics (e.g. Chouinard et al., 2013). Both clinical and non-clinical groups completed the visual illusions task.

Table 3

The 13 Illusions included in the Visual Illusion Task

Visual Illusion Diagram Description

Ebbinghaus In the Ebbinghaus illusion, the

two inner circles in both images are physically the same size. Contextual cues from the outer circles lead to the inner circle on the right being perceived as larger than the inner circle on the left.

Muller-Lyer In the Muller-Lyer illusion,

two identically sized horizontal lines are presented with an arrowhead at either end. On the top line both arrowheads point inwardly and on the bottom line both arrowheads point outwardly. Due to the orientation of the arrowheads, the image appears larger when they point inwards and smaller when they point outwards.

Ponzo In the Ponzo illusion, the top

and bottom line are identical in size. The converging lines give the appearance of depth and the top line appears longer in length than the bottom line.

Delbeouf In the Delbeouf illusion, the two inner circles are the same size. Due to contextual cues from the surrounding circle the inner circle on the right appears bigger than the inner circle on the left.

Ehrenstein In the Ehrenstein illusion lines

A and B, on the left and right hand side of the yellow square are indentifical in size. Due to contextual cues from the converging lines, line A appears longer in length than line B.

Helmholtz-Square In the Helmholtz-Square

illusion the horizontal and vertical length of the square (comprised of vertical lines) is identical in size. Due to the orientation of the lines, the length of line B appears greater than line A.

Horizontal-Vertical In the Horizontal-Vertical

illusion both lines are identical in length. Based on prior knowledge of horizontal and vertical objects, the vertical line is perceived as longer in length than the horizontal line.

Jastrow In the Jastrow illusion the two

figures are identical. Due to their orientation, the lower figure is perceived as bigger than the upper figure

Oppel-Kundt In the Oppel-Kundt illusion the distance represented by line A is equal to the distance represented by line B. When the distance is filled with vertical lines (image A) the length appears longer than the empty space (image B).

Poggendorf In the Poggendorf illusion, a

straight line set at a 45 degree angle is concealed by an intervening rectangle. The line is misinterpreted as being offset whilst the true image is linear

Sanders In the Sanders illusion, the

two diagonal lines are equal in length. Based on depth cues from the surrounding from the purple lines, line A is perceived as longer than line B.

Shepard In the Shepard illusion, the

mid sections (indicated by the white lines) are identical for both parallelograms. Due to the orientation of the shapes line B appears longer in length than line A.

Square-Diamond In the Square-Diamond

illusion, both squares are identical in size. When the square is in a diamond position (image B) it appears larger than its standard position (image A).