What causes errors in a near-threshold forced-choice shape orientation task? S Heinrich1, S Giesemann2, M Bach3 |
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1Dept. of Ophthalmology, University of Freiburg, Germany |
| Recently, we found that false responses in a near-threshold 8-alternative forced-choice Landolt C shape orientation task are not equally distributed, as would be assumed by psychometric theory. Rather, response orientations adjacent to the displayed orientation occur 3 times as often as other false orientations. To better understand this effect, we assessed how precisely subjects are able to assess the orientation of threshold-sized Landolt Cs in the first place. We presented threshold-sized stimuli at 360 possible orientations to 15 subjects, who provided an orientation response at 1-degree resolution via a rotary knob that controlled an orbiting orientation marker on the screen. The width of the response distribution relative to the display orientation was determined by fitting a raised cosine. The standard case of 8 orientations and an above-threshold 360-orientation task served as references to disentangle response imprecision from perceptual resolution. In all subjects, the response distribution in the near-threshold 360-orientation task extended substantially into what would be the catchment intervals of the adjacent orientation responses of an equivalent standard 8-orientation task. The data quantitatively explain the unequal distribution of false responses in the 8-orientation task and suggest that perceptual resolution, rather than response imprecision, is the dominant factor. |
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