Subjective Size Perception Depends on Central Visual Cortical Magnification in Human V1

Abstract

In the Ebbinghaus illusion, the context surrounding an object modulates its subjectively perceived size. Previous work implicates human primary visual cortex (V1) as the neural substrate mediating this contextual effect. Here we studied in healthy adult humans how two different types of context (large or small inducers) in this illusion affected size perception by comparing each to a reference stimulus without any context. We found that individual differences in the magnitudes of the illusion produced by either type of context were correlated with V1 area defined through retinotopic mapping using functional MRI. However, participants' objective ability to discriminate the size of objects presented in isolation was unrelated to illusion strength and did not correlate with V1 area. Control analyses showed no correlations between behavioral measures and the overall V1 area estimated probabilistically on the basis of neuroanatomy alone. Therefore, subjective size perception correlated with variability in central cortical magnification rather than the anatomical extent of primary visual cortex. We propose that such changes in subjective perception of size are mediated by mechanisms that scale with the extent to which an individual's V1 selectively represents the central visual field.

Perceptual Organization and Consciousness

“…In subsequent experiments we further show that this correlation is present for both components of the Ebbinghaus stimulus, that is, both for contexts with both small and large inducers. Our results further support the interpretation that the cortical distance over which the contextual interaction occurs is a major factor determining illusion strength (Schwarzkopf and Rees 2013). While correlational studies like this cannot resolve the question of causality and the specific circuits mediating the illusion remain to be identified, our findings suggest that the surface area of V1 at least in part reflects the subjective awareness of object size.…”

Unique semantic space in the brain of each beholder predicts perceived similarity

“…For example, the size of primary visual cortex varies across individuals by a factor of about 2.5 (37,38). Interindividual variation in V1 cortical magnification predicts variation of the magnitude of visual size illusions across subjects (3,39 Fig. 4.…”

Optimal experimental design for MAPS

“…Visual abilities and experiences of the environment vary considerably between individuals (Halpern, Andrews, & Purves, 1999), and also covary with inter-individual differences in brain structure and function (Genç et al, 2011(Genç et al, , 2011Genç, Bergmann, Singer, & Kohler, 2014;Schwarzkopf & Rees, 2013;Schwarzkopf, Song, & Rees, 2011;Verghese, Kolbe, Anderson, Egan, & Vidyasagar, 2014) (Genç et al, 2011(Genç et al, , 2011Genç, Bergmann, Singer, & Kohler, 2014;Schwarzkopf & Rees, 2013;Schwarzkopf, Song, & Rees, 2011;Verghese, Kolbe, Anderson, Egan, & Vidyasagar, 2014). As well as varying between individuals, perceptual biases vary substantially within the visual field for a single person.…”

“…There is also more recent evidence for idiosyncratic patterns of perceptual biases across the visual field in different observers, including the perception of high-level attributes like facial age and gender as well as lower level features like aspect ratio, spatial frequency, orientation, color (Afraz, Pashkam, & Cavanagh, 2010), position (Kosovicheva & Whitney, 2017), and size (Moutsiana et al, 2016;Schwarzkopf & Rees, 2013). Most of this spatial heterogeneity in perceptual bias could be due to "under-sampling" of the visual field by neurons tuned to these stimulus features (Afraz et al, 2010).…”