Damera, S. R. | Search bioRxiv

bioRxiv2022

Evidence for a Spoken Word Lexicon in the Auditory Ventral Stream

The existence of a neural representation for whole words (i.e., a lexicon) is a common feature of many models of speech processing. Prior studies have provided evidence for a visual lexicon containing representations of whole written words in an area of the ventral visual stream known as the "Visual Word Form Area" (VWFA). Similar experimental support for an auditory lexicon containing representations of spoken words has yet to be shown. Using fMRI rapid adaptation techniques, we provide evidence for an auditory lexicon in the "Auditory Word Form Area" (AWFA) in the human left anterior superior temporal gyrus that contains representations highly selective for individual spoken words. Furthermore, we show that familiarization with novel auditory words sharpens the selectivity of their representations in the AWFA. These findings reveal strong parallels in how the brain represents written and spoken words, showing convergent processing strategies across modalities in the visual and auditory ventral streams. HighlightsO_LIIndividual auditory word form areas (AWFA) were defined via an auditory localizer C_LIO_LIThe AWFA shows tuning for individual real words but not untrained pseudowords C_LIO_LIThe AWFA develops tuning for individual pseudowords after training C_LI

neuroscience↗

bioRxiv2021

Metamodal Coupling of Vibrotactile and Auditory Speech Processing Systems Through Matched Stimulus Representations

It has been postulated that the brain is organized by "metamodal", sensory-independent cortical modules implementing particular computations, leading to the intriguing hypothesis that brain areas can perform tasks (such as word recognition) not just in "standard" sensory modalities but also in novel sensory modalities. Yet, evidence for this theory, especially in neurotypical subjects, has been variable. We hypothesized that effective metamodal engagement of a brain area requires congruence between the novel and standard sensory modalities not only at the task level (e.g., "word recognition") but critically also a match at the algorithmic level (in Marrs terminology), i.e., at the level of neural representation of the information of interest. To test this hypothesis, we trained participants to recognize vibrotactile versions of auditory words using two encoding schemes. The vocoded approach preserved the dynamics and representational similarities of auditory speech while the token-based approach used an abstract phoneme-based code. Although both groups learned the vibrotactile word recognition task, only in the vocoded group did trained vibrotactile stimuli recruit the auditory speech network and lead to increased coupling between somatosensory and auditory speech areas. In contrast, the token-based encoding appeared to rely on paired-associate learning. Thus, matching neural input representations is a critical factor for assessing and leveraging the metamodal potential of cortical modules.

neuroscience↗