Size and Shape Constancy are Cognitive Rather than Perceptual: Linton Size Constancy Illusion and Linton Shape Constancy Illusion

We present two new illusions that challenges our understanding of size and shape constancy, with implications for how we should think about feedback to the primary visual cortex (V1), the perception/cognition distinction, and the neural correlates of visual experience. Size constancy is traditionally thought to affect the perceived angular (2D) size of objects (Murray et al., 2006, Nat. Neurosci. 9 429-434). Indeed, observers are often surprised to find that size constancy stimuli are the same size in image pixels. This affect is now typically attributed to feedback to the primary visual cortex (V1) (Ni et al., 2015, Curr. Biol., 24(14) 1653-1658). A similar claim has been made about shape constancy (Thouless, 1931, Brit. J. Psych., 21 339-359). However, if size and shape constancy really do affect perceived angular (2D) size, then any rectangular frame placed in the same 2D region of the image should also be equally distorted. However, this is not what we find. Instead, pictorial size constancy, stereo size constancy, and stereo shape constancy all persist without distorting the bounding boxes placed around them. This leads us to four conclusions: First, size and shape constancy do not affect our visual experience (the perceived angular size of objects), but merely our judgment of their perceived angular size. Second, size and shape constancy become important paradigms for understanding just how low-level, and automatic, involuntary, and resistant to revision, ‘cognition’ can be. It also provides a clear demonstration of how ‘cognition’ can be mistaken for ‘perception’. Third, since size constancy is thought to rely on feedback processing to the Primary Visual Cortex (V1), we conclude that top-down processing to V1 is ‘cognitive’ rather than ‘perceptual’ (Linton, 2021, Neurosci. Conscious., 2 niab017). Fourth, this suggests that feedforward (rather than feedback) V1 must be the site for the neural correlates of visual experience.