A classification-image-like method reveals observers’ strategies in two-alternative forced choice tasks R Murray, L Pritchett |
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Centre for Vision Research, York University, ON, Canada
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| There is still uncertainty about how observers perform even the simplest tasks, such as making 2AFC decisions. We demonstrate a novel method of using classification images to calculate “proxy decision variables” that estimate an observer’s decision variables on individual trials, which provides a new way of investigating observers’ decision strategies. We tested three models of the mapping from decision variables to responses. METHOD. Observers viewed two disks in Gaussian noise, to the left and right of fixation, and judged which had a contrast increment. For each trial we calculated the cross-correlation of the classification image with the two disks, providing a proxy decision variable for each alternative. After several thousand trials we mapped the observer’s decision space: we plotted the probability of choosing the right-hand disk as a function of the two decision variables. We tested the hypotheses that observers base their decisions on (a) the difference between the two decision variables, (b) independent yes-no decisions on the two decision variables, or (c) just one of the decision variables. RESULTS. Decision space maps showed that observers use the difference between the decision variables. We conclude that the difference model favoured by detection theory is a valid model of 2AFC decisions. |
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