Academic journal article Attention, Perception and Psychophysics

Detection and Identification of Spatial Offset: Double-Judgment Psychophysics Revisited

Academic journal article Attention, Perception and Psychophysics

Detection and Identification of Spatial Offset: Double-Judgment Psychophysics Revisited

Article excerpt

Published online: 10 My 2014

(Q> The Psychonomic Society, Inc. 2014

Abstract In a bull's-eye acuity task, we asked observers to identify in which direction, to the left or the right, a spot had been displaced from the center of a circle and-after that, in the same trial-to detect which of the two presented circles contained the displaced spot. Replicating our previous findings (Allik, Dzhafarov, & Rauk, 1982), the spatial offset direction identification probability was higher than the probability with which the correct observation interval could be detected. All data were explained by a Thurstonian model, according to which the spatial positions of both spots are projected onto an internal axis of representation as two random numbers, .r and y, drawn from a random distribution with a fixed standard deviation ζ (final sigma). The observed identification and detection probabilities were accurately reproduced, provided that the observer tested two different inequalities: x + y > 0 for the identification, and x^sup 2^ - y^sup 2^ > 0 for the detection. In order to eliminate small discrepancies between the predicted and the observed data, we proposed that the positional error increases with increasing distance from the center of the annulus. It was concluded that, to explain the superiority of the identification over the detection effect, there is no need to propose separate axes of representation for mono- and bipolar information, as is usually postulated in double-judgment psychophysics.

Keywords Double-judgment psychophysics * Detection and recognition * Spatial intervals * Thurstonian psychophysics * Bull's-eye acuity * Simulation modeling

Psychophysicists usually prefer the two-altemative forced choice (2AFC) over the classical yes-no or adjustment pro- cedure for determining sensory thresholds, because it allows for ehmination of, or at least control over, response biases (Ulrich & Miller, 2004). In a 2AFC detection task, two stim- uli-a reference and a comparison-are presented in two successive or simultaneous observation intervals, with the order or position of presentation varying randomly from trial to trial. Participants are asked to indicate which of the two observation intervals contained the comparison stimulus. However, this method says very little about the attributes by which these two observation intervals were actually distin- guished. In order to leam about those attributes that have been used effectively, it is necessary, in addition to the detection task, to ask participants to identify the comparison stimulus. For this purpose, at least two categories of the comparison stimulus, A and B, are presented, and participants are asked to identify to which category the presented comparison stimulus belonged and, of course, in which observation interval the comparison stimulus was located. Since two different tasks are solved in a single trial, this procediue has been called a double-judgment experiment, sometimes abbreviated 2 x 2AFC (Klein, 1985).

According to a widely held belief, detection is easier than identification. For example, it takes less energy, contrast, or time to detect the presence of an object than to identify its form (Handel & Christ, 1969; Straube & Fahle, 2011). Researchers also believe that there is no simple relationship between detection and identification. The only obvious rela- tionship that could determine the ratio between detection and identification is any similarity between the categories A and B. If these two categories become increasingly similar to each other, it becomes potentially almost impossible to tell them apart (Thomas, 1985). Double-judgment psychophysics en- tered into the limelight when researchers tried to find evidence for the existence of detectors sensitive to a narrow range of visual features, such as spatial frequency, orientation, and polarity of contrast (e.g., Blakemore & Campbell, 1969). Perhaps the most impressive application of double-judgment psychophysics so far has been finding out which visual fea- tures an observer uses to decide the presence or absence of visual stimuli. …

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