Evidence for Criterion Shifts in Visual Perceptual Learning: Data and Implications

Article excerpt

Work on visual perceptual learning for contrast detection has shown that reliable decreases in detection thresholds are accompanied by reliable increases in false alarm rates (Wenger & Rasche, 2006). The present study assesses the robustness and replicability of these changes, demonstrating that they are independent of a variety of task demands (i.e., the specific method used for perceptual practice and threshold estimation) and the presence or absence of trial-by-trial feedback and that the source of the increases can be found in shifts in changes in sensitivity and in bias for detection, identification, or both. Although the increase in false alarm rates suggests a strategic shift in response criteria for detection, we demonstrate that there are multiple potential explanations, including explanations that do not require strategic shifts on the part of the observer. The empirical evidence and analysis of alternative explanations reinforce the inference that visual perceptual learning may involve more than changes in perceptual sensitivity and that cortical circuits beyond the primary visual areas may be involved.

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Human perceptual abilities are remarkably responsive to the effects of practice. A broad and accumulating literature on perceptual learning (for reviews, see Ahissar & Hochstein, 2004; Fahle & Poggio, 2002; Gibson, 1953, 1969; Gibson, Walk, Pick, & Tighe, 1958; Herzog & Fahle, 1998; Jacobs & Michaels, 2006; Sinha & Poggio, 2002) documents the range of tasks in which practice on elementary visual judgments can produce dramatic improvements in performance. This behavioral evidence has been used to motivate the development of a range of models for the mechanisms of perceptual learning (e.g., Adini & Sagi, 2001; Adini, Sagi, & Tsodyks, 2002; Gold, Bennett, & Sekuler, 1999; Gold, Sekuler, & Bennett, 2004; Petrov, Dosher, & Lu, 2005; Yu, Klein, & Levi, 2004), with the majority of these models focused on how perceptual sensitivity to either the presence of or a variation in a defining characteristic changes with experience. The work presented here explores the possibility that the changes observed in visual perceptual learning may also reflect changes in response (decisional) criteria for both detection and identification, in addition to or instead of changes in perceptual sensitivity.

In order to ask these questions, it is critical to consider the extent to which the behavioral evidence available in any study of perceptual learning can speak in a reasonably restrictive manner to changes in perceptual sensitivity (Fahle & Poggio, 2002; Sagi & Tanne, 1994; Zenger & Sagi, 2002). If that evidence can reflect changes due to factors other than changes in sensitivity-such as changes in biases for detection, identification, response selection, or all three-inferences regarding the underlying mechanisms may be in error. Consider, in particular, the possibility that experience may produce changes in both sensitivity and bias. The latter could suggest important roles for selective attention, memory, or other influences, such as might be available by way of back-projections within the visual system (e.g., Di Lollo, Enns, & Rensink, 2000; Lamme & Roelfsema, 2000; Li, Piëch, & Gilbert, 2004; Ling & Carrasco, 2006; Sigman & Gilbert, 2000; Sigman et al., 2005). However, if the experimental approach does not provide the data necessary to measure both types of potential changes, or if it does not adequately control for the possible effects of different types of response bias, any inferences regarding any single basis for change is potentially suspect.

In a large number of the studies on perceptual learning, an experimental approach-an adaptive version of a two-alternative forced choice (2AFC) method-has been used that is widely assumed to minimize or obviate potential influences of response bias (see also Klein, 2001, for a much more complete methodological discussion of these issues). …


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