engleski

Visual localization of angles: The angle shifting effect

The aim of this study was to determine the existence, and measure the size, of a shift in perceived angle apex when compared to the actual location in two-dimensional space. To our knowledge, this angle shifting effect has not been described in psychological literature. Our previous experiments provided inconsistent results. Two alternative forced choice experiments determined that there was a significant shift of angle apex location towards the inner surface of the angle. The same shift was not found in visuo-motor localization experiments. For the current study we conducted two experiments using the psychophysical method of constant stimuli. In the first experiment participants (N = 35) were shown two fins (100 pixels or 6.02 degrees of visual angle) which formed a right angle for 500 ms. This was followed by a masking stimulus of the same duration. Finally, participants were shown a single dot and their task was to judge whether the presented dot was to the left or to the right of the apex of the previously presented angle. The target stimuli set consisted of seven dots spread over 30 pixels in equal increments with the middle dot appearing at the exact location of the angle apex. The leftmost and rightmost dots appeared 15 pixels to the left and right of the actual angle apex. Results showed that the point of subjective equality (PSE) was shifted by 13.77 pixels (0.41 degrees of visual angle) towards the inner surface of the angle. The offset was significant when compared to a fixed value corresponding to the objective point of equality (t(16) = 5.58, p < .01). However, a portion of the participants appeared to have a PSE which exceed the spread of stimuli values. A second experiment was conducted in order to determine whether this was due to methodological issues. For the second experiment (N = 34) we increased the spread to 60 pixels in equal increments between the seven target stimuli and the mask stimulus duration was increased to 750 ms. Results showed that the PSE was shifted towards the inner surface of the angle. The effect was indeed slightly larger than the most extreme stimulus from the first experiment (16.08 pixels equating to 0.48 degrees of visual angle). The effect was again significant (t(25) = 7.25, p < .01). There seems to be a perceived shift of angle apex location towards the inner surface of the angle. These findings have clear implications which count a partial explanation of the Muller-Lyer illusion as one of the most interesting.

angle shifting effect, angle perception, vertex localization, spatial perception

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