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.