Abstract
Does ventral visual cortex contain separate regions with specialized representations for specific categories of stimuli, or is there a general representational space for all categories shared across regions? Here, we examined this long-standing question by considering representations of visually-presented letters, which are known to drive selective responses in a circumscribed region of inferotemporal cortex. Using fMRI, we measured cortical responses to each lowercase letter individually, across twenty fonts, in 14 participants. We conducted a searchlight analysis comparing the two halves of the dataset, revealing moderately reliable RDMs for letters extensively throughout ventral visual cortex. We even found RDMs with moderate split-half reliability in regions selective for other categories (fusiform face area: left: ρ=0.38, right: ρ=0.53 and parahippocampal place area: left: ρ=0.43, right: ρ=0.43). Do any of these reliable brain regions have representational structure corresponding to behavioral signatures of letter similarity? To measure the perceptual similarity of letters, we asked participants (n=517) to categorize single letters rapidly with a yes/no response (e.g. is this an “a”?), where the time it took to reject a letter was considered a measure of its similarity to the target. We found that regions throughout ventral visual cortex, including bilateral FFA and PPA, all correlated with the behaviorally-measured similarity of letters. Additionally, extensive brain-behavior correlations were found when letter similarity was measured with a second behavioral task, visual search (n=222). These results demonstrate that the behaviorally-measured representational geometry of letters is found throughout ventral visual cortex, rather than being isolated to a specific region. Broadly, these findings join previous research (Cohen et al, 2016; Prince & Konkle, 2020) suggesting that inferotemporal cortex contains an integrated feature space, with tuning relevant to all kinds of visual stimuli evident throughout this cortex.