Feature selective adaptation of numerosity perception

Perceptual adaptation has been widely used to infer the existence of
numerosity detectors, enabling animals to quickly estimate the number of
objects in a scene. Here we investigated, in humans, whether numerosity
adaptation is influenced by stimuli features changes as previous research
suggested that adaptation is more pronounced when adapting and test
stimuli share the same color. We tested whether such adaptation reduction
is due to a novelty effect (due to changing stimulus features) or changes
of stimuli identity. Numerosity adaptation was measured for stimuli
matched or unmatched for low-level (color, luminance, shape, motion) or
high-level (letters' identity, face emotions) features. Robust
numerosity adaptation occurred in all conditions but it was reduced when
adapting and test stimuli differed for color, luminance, and shape.
However, no reduction was observed between moving and still stimuli, a
readable change that did not affect item’s identity. Similarly, changes in
letters' spatial rotations or face features did not affect adaptation
magnitude. Image dissimilarity, quantified by Gabor filters modeling V1
predicted numerosity adaptation strength. These findings suggest that
low-level feature changes, rather than novelty per sè, affect adaptation
strength with numerosity mechanisms operating on categorized items in
addition to the total quantity of the set.