Investigating the Architecture and Plasticity of the Auditory System through Spoken Word Recognition

Abstract

The auditory cortical system, like the visual cortical system, is thought to be organized into a dual-stream architecture consisting of an anterior ventral and a posterior dorsal stream. Under this framework, the anterior ventral stream (also known as the “what” pathway) is specialized for recognizing auditory “objects,” such as spoken words. Despite growing evidence that the anterior ventral stream is organized along a simple-to-complex hierarchy, it is still controversial whether a putative auditory lexicon for words exists and, if so, where it is located. Using an fMRI rapid adaptation experiment, inspired by prior work on visual word recognition, we found evidence that neuronal populations in the left anterior superior temporal gyrus (aSTG) are tightly tuned to individual auditory real words (RWs). In contrast, we found evidence that novel untrained auditory pseudowords (UTPWs) weakly engaged more broadly tuned neurons in this region. This is consistent with a two-stage model of category learning which predicts that auditory lexical representations develop in high-level sensory cortices through experience-dependent refinement of neuronal tuning. Also consistent with this model, we found that neuronal populations in the inferior frontal gyrus (the second stage) are also tuned to individual RWs. To directly test this hypothesis, we performed another experiment in which subjects were trained to recognize previously novel pseudowords (UTPWs). We then compared the selectivity of the aSTG to these pseudowords before and after training. This revealed that neuronal populations in this region were now tightly tuned to trained pseudowords (TPWs) but not UTPWs. In addition, we found evidence that neuronal populations in the left pre-motor cortex are tuned to individual RWs. This raises the possibility that an auditory speech production lexicon may also develop in the auditory dorsal stream.