Confounding of norm-based and adaptation effects in brain responses.

Separate neuroscience experiments have examined two properties of neural coding for perceptual stimuli. Adaptation studies seek a graded recovery from neural adaptation with ever greater dissimilarity between pairs of stimuli. Studies of prototype effects test for a larger absolute response to a stimulus which is distant from the center of a stimulus space. While intellectually distinct, these effects are confounded in measurement in standard neuroscience paradigms and can be mistaken for one another. Stimuli which are more distinctive are less subject to adaptation from perceptual neighbors. Therefore, a putative prototype effect may simply result from greater adaptation of prototypical stimuli by other stimuli in the experiment. Conversely, stimulus pairs which are the most perceptually distant from one another, and therefore expected to show the greatest recovery from adaptation, disproportionately draw from the extremes of the stimulus space. Thus, an apparent neural similarity effect may be created by an underlying prototype representation. We simulate BOLD fMRI results driven by each possible effect and demonstrate spurious results in support of the complementary effect. We then present an example fMRI experiment that demonstrates the confound and how it may be minimized. Finally, we discuss the implications of this intrinsic confound for studies of perceptual representation, neural coding, and category learning.

Temporally distinct neural coding of perceptual similarity and prototype bias.

Psychological models suggest that perceptual similarity can be divided into geometric effects, such as metric distance in stimulus space, and non-geometric effects, such as stimulus-specific biases. We investigated the neural and temporal separability of these effects in a carry-over, event-related potential (ERP) study of facial similarity. By testing this dual effects model against a temporal framework of visual evoked components, we demonstrate that the behavioral distinction between geometric and non-geometric similarity effects is consistent with dissociable neural responses across the time course of face perception. We find an ERP component between the "face-selective" N170 and N250 responses (the "P200") that is modulated by transitions of face appearance, consistent with neural adaptation to the geometric similarity of face transitions. In contrast, the N170 and N250 reflect non-geometric stimulus bias, with different degrees of neural adaptation dependent upon the direction of transition within the stimulus space. These results suggest that the neural coding of perceptual similarity, in terms of both geometric and non-geometric representations, occurs rapidly and from relatively early in the perceptual processing stream.