In our paper, we discuss a recent fMRI study from Professor Maria Concetta Morrone’s lab in Pisa, Italy (Pooresmaeili et al., 2013) which examined the role of the primary visual cortex (V1) in size perception. Consistent with previous fMRI studies that examined V1 activation during size perception in the context of optical illusion displays (Murray et al., 2006; Fang et al., 2008) and afterimages (Sperandio et al., 2012), the results from Pooresmaeili et al.’s experiments further confirmed that patterns of activation in V1 more closely reflect the perceived size rather than retinal image size of a stimulus. More specifically, in such experiments the size of the image projected onto the retina by a stimulus remained constant while the perceived size of the stimulus varied widely, with such variations precisely corresponding to the degree of eccentricity in V1 activation. In our commentary, we questioned whether this pattern of results implies that V1 is a type of “Cartesian Theater” (Dennett, 1991) for the visual phenomenology of object size. That is, since the eccentricity of retinotopic activity in V1 closely reflects the visual experience of an object’s size and not its retinal image size, one might be tempted to think that this is the case. However, we doubt that these results should be construed in that way, and we present some considerations to support our view. Moreover, although the authors of the fMRI study did not advocate this notion explicitly, they do argue that their experimental design and choice of visual stimuli diminished the influences of top-down mechanisms on size perception and that their results therefore demonstrate that perceptual size effects are mediated primarily by feed-forward mechanisms. However, we suggest a number of reasons to be skeptical of this claim, and offered alternative explanations for the author’s results that suggest the potential influence of top-down modulation from other brain areas.
Chouinard, P. & Ivanowich, M. (2014). “Is the Primary Visual Cortex a Center Stage for the Visual Phenomenology of Object Size?” The Journal of Neuroscience 34(6): 2013-2014.
Dennett, D. (1991). Consciousness Explained. Brown and Co, Boston.
Fang, F., Boyaci, H., Kersten, D., and Murray, S.O. (2008). “Attention-dependent representation of a size illusion in human V1”. Curr Biol 18:1707-1712.
Murray, S.O., Boyaci, H., and Kersten, D. (2006). “The representation of perceived angular size in human primary visual cortex”. Nat Neurosci 9:429-434.
Pooresmaeili, A., Arrighi, R., Biagi, L., and Morrone, M.C. (2013). “Blood Oxygen Level-Dependent Activation of the Primary Visual Cortex Predicts Size Adaptation Illusion”. J Neurosci 33:15999-16008.
Sperandio, I., Chouinard, P.A., and Goodale, M.A. (2012). Retinotopic activity in V1 reflects the perceived and not the retinal size of an afterimage”. Nat Neurosci 15:540-542.