Perception is subjective. Even basic judgments, like those of visual object size, vary substantially between observers and also across the visual field within the same observer. The way in which the visual system determines the size of objects remains unclear, however. We hypothesize that object size is inferred from neuronal population activity in V1 and predict that idiosyncrasies in cortical functional architecture should therefore explain individual differences in size judgments. Here we show results from novel behavioural methods and functional magnetic resonance imaging (fMRI) demonstrating that biases in size perception are correlated with the spatial tuning of neuronal populations in healthy volunteers. To explain this relationship, we formulate a population read-out model that directly links the spatial distribution of V1 representations to our perceptual experience of visual size. Taken together, our results suggest that the individual perception of simple stimuli is warped by idiosyncrasies in visual cortical organization.

Population receptive field (pRF) analysis is a popular method to infer spatial selectivity of voxels in visual cortex. However, it remains largely untested how stable pRF estimates are over time. Here we measured the intersession reliability of pRF parameter estimates for the central visual field and near periphery, using a combined wedge and ring stimulus containing natural images. Sixteen healthy human participants completed two scanning sessions separated by 10–114 days. Individual participants showed very similar visual field maps for V1-V4 on both sessions. Intersession reliability for eccentricity and polar angle estimates was close to ceiling for most visual field maps (r>.8 for V1-3). PRF size and cortical magnification (CMF) estimates showed strong but lower overall intersession reliability (r≈.4–.6). Group level results for pRF size and CMF were highly similar between sessions. Additional control experiments confirmed that reliability does not depend on the carrier stimulus used and that reliability for pRF size and CMF is high for sessions acquired on the same day (r>.6). Our results demonstrate that pRF mapping is highly reliable across sessions.

14 Perception is subjective. Even basic judgments, like those of visual object size, vary substantiallyHow do we perceive the size of an object? A range of recent observations have lent support to the 26 hypothesis that the visual system generates the perceived size of an object from its cortical 27 representation in early visual cortex 1 . In particular, the spatial spread of neural activity in visual 28 cortex has been related to apparent size under a range of contextual modulations [2][3][4][5][6][7] . The strength of 29 contextual size illusions has further been linked to the cortical territory in V1 that represents the 30 central visual field 8,9 . These findings suggest that lateral connections in V1 may play a central role in 31 size judgments because these interactions are reduced when V1 surface area is larger. Indeed, 32 similar interactions have been argued to underlie the strength of the tilt illusion 10,11 , perceptual 33 alternations in binocular rivalry 12 , the influence of distractors in visual search tasks 13 , and visual 34 working memory capacity 14 . Even the precision of mental imagery co-varies with V1 area 15 35 suggesting V1 may be used as a 'workspace' for storing mental images whose resolution is better 36 when surface area is larger. 37. CC-BY-NC 4.0 International license not peer-reviewed) is the author/funder. It is made available under a The copyright holder for this preprint (which was . http://dx.doi.org/10.1101/026989 doi: bioRxiv preprint first posted online Sep. 16, 2015; 2 However, these previous findings do not demonstrate that V1 representations per se are relevant for 38 size judgments, and in particular for subjective judgments of object size. If V1 signals were indeed 39 the basis for these judgments then variations in the functional architecture of V1 should explain 40 idiosyncratic biases in basic size perception (i.e. size judgements that occur in the absence of any 41 contextual/illusory effects). To date this prediction remains untested. Previous neuroimaging 42 experiments have focused on modulations of apparent size that must involve additional processing, 43 either due to local interactions between adjacent stimuli in V1 or by a context that likely involves 44 processing in higher visual areas. Others have shown that the objective ability to discriminate subtle 45 differences between stimuli is related to cortical magnification and spatial tuning in early visual 46 cortex 11,16,17 . However, no experiment to date has shown a relationship between V1 and subjective 47 perceptual biases in the absence of any contextual interaction, even though there are considerable 48 individual differences in perceptual biases. 49It is well established that subjective size judgments for simple, small stimuli can vary substantially 50 between observers and even across the visual field within the same observer. Previous behavioral 51 research has shown that small visual stimuli appear smaller when they are presented in the 52 periphery [18][19][20] . A ...

Numerosity, the set size of a group of items, helps guide behaviour and decisions. Non-symbolic numerosities are represented by the approximate number system. However, distinct behavioural performance suggests that small numerosities, i.e. subitizing range, are implemented differently in the brain than larger numerosities. Prior work has shown that neural populations selectively responding (i.e. hemodynamic responses) to small numerosities are organized into a network of topographical maps. Here, we investigate how neural populations respond to large numerosities, well into the ANS. Using 7 T fMRI and biologically-inspired analyses, we found a network of neural populations tuned to both small and large numerosities organized within the same topographic maps. These results demonstrate a continuum of numerosity preferences that progressively cover both the subitizing range and beyond within the same numerosity map, suggesting a single neural mechanism. We hypothesize that differences in map properties, such as cortical magnification and tuning width, underlie known differences in behaviour.

Visual spatial attention concentrates neural resources at the attended location. Recently, we demonstrated that voluntary spatial attention attracts population receptive fields (pRFs) toward its location throughout the visual hierarchy. Theoretically, both a feed forward or feedback mechanism could underlie pRF attraction in a given cortical area. Here, we use sub-millimeter ultra-high field functional MRI to measure pRF attraction across cortical depth and assess the contribution of feed forward and feedback signals to pRF attraction. In line with previous findings, we find consistent attraction of pRFs with voluntary spatial attention in V1. When assessed as a function of cortical depth, we find pRF attraction in every cortical portion (deep, center and superficial), although the attraction is strongest in deep cortical portions (near the gray-white matter boundary). Following the organization of feed forward and feedback processing across V1, we speculate that a mixture of feed forward and feedback processing underlies pRF attraction in V1. Specifically, we propose that feedback processing contributes to the pRF attraction in deep cortical portions.