Abstract
Previous studies have shown reductions of the functional magnetic resonance imaging (fMRI) signal in response to repetition of specific visual stimuli. We examined how adaptation affects the neural responses associated with categorization behavior, using face adaptation aftereffects. Adaptation to a given facial category biases categorization towards non-adapted facial categories in response to presentation of ambiguous morphs. We explored a hypothesis, posed by recent psychophysical studies, that these adaptation-induced categorizations are mediated by activity in relatively advanced stages within the occipitotemporal visual processing stream. Replicating these studies, we find that adaptation to a facial expression heightens perception of non-adapted expressions. Using comparable behavioral methods, we also show that adaptation to a specific identity heightens perception of a second identity in morph faces. We show both expression and identity effects to be associated with heightened anterior medial temporal lobe activity, specifically when perceiving the non-adapted category. These regions, incorporating bilateral anterior ventral rhinal cortices, perirhinal cortex and left anterior hippocampus are regions previously implicated in high-level visual perception. These categorization effects were not evident in fusiform or occipital gyri, although activity in these regions was reduced to repeated faces. The findings suggest that adaptation-induced perception is mediated by activity in regions downstream to those showing reductions due to stimulus repetition.
Highlights
Faces are highly similar visual objects, which may be encountered under a variety of circumstances, humans possess an extraordinary ability to discriminate among their component categories such as identity and expression
Adaptation has been shown to affect perception of viewpoint (Fang and He, 2005) and normality of distorted faces (Yamashita et al, 2005). These psychophysics studies pose a hypothesis that adaptation-induced categorization effects are associated with high level neural representations that are tuned to global stimulus categories, rather than simple visual features
We used event-related functional magnetic resonance imaging (fMRI) to examine how brain activity underlying perception of expression and identity categories in ambiguous morph faces is modulated by prior adaptation
Summary
Faces are highly similar visual objects, which may be encountered under a variety of circumstances (e.g., illumination, pose, etc.), humans possess an extraordinary ability to discriminate among their component categories such as identity and expression. A number of functional magnetic resonance imaging (fMRI) studies have studied category selectivity in regions in ventral temporal cortex by contrasting conditions in which stimulus These studies focused on simple manipulations such as position (Köhler and Wallach, 1944), orientation (Gibson and Radner, 1937), curvature (Gibson, 1933), spatial frequency (Blakemore and Sutton, 1969), motion direction (Huk et al, 2001) and color/orientation contingencies (McCollough, 1965). Adaptation has been shown to affect perception of viewpoint (Fang and He, 2005) and normality of distorted faces (Yamashita et al, 2005) These psychophysics studies pose a hypothesis that adaptation-induced categorization effects are associated with high level neural representations that are tuned to global stimulus categories, rather than simple visual features. Face aftereffects are to some extent invariant to changes in simple visual features of the adapting and test stimuli including contrast, color, size (Yamashita et al, 2005), orientation (Watson and Clifford, 2003) and translation (Fang and He, 2005; Leopold et al, 2001)
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