Cross-modal processing of auditory and visual symbol representations in the temporo-parietal cortex

Numeracy and literacy are fundamental cognitive skills that rely on associating visual symbols with their spoken representations. Prior research has identified the posterior temporal-parietal cortex as a key neural region for the cross-modal transformation of these audio-visual alphanumeric symbols. However, the modality-dependent and cross-modal cortical activation patterns underlying these transformations remain unclear. In this slow-event-related 3T fMRI experiment, twenty-one participants were presented with auditory or visual letters and numbers while performing a passive listening/viewing task. We found overlapping activation across auditory cortical regions for auditory letters/numbers and across ventral visual regions for visual letters/numbers. In particular, activity in superior temporal cortical regions such as A5/A4/Parabelt exhibited high reliability for auditory stimuli, whereas activity in occipital and ventral temporal cortical regions such as V3/V4/PH demonstrated high reliability for visual stimuli. The temporo-parieto-occipital junction (TPOJ) showed overlapping responses with similar amplitudes for both auditory and visual stimuli. Despite this global similarity in responses, multivariate analysis revealed that the right TPOJ successfully differentiated between visual and auditory stimuli. Our findings reinforce the TPOJ’s role in the cross-modal processing of symbolic representations and may have implications for developmental learning difficulties such as dyslexia, where cross-modal integration may form a challenge for acquiring reading fluency.