No Causal Role for Premotor Cortex in the Perception or Misperception of Degraded Speech: Evidence from Transcranial Magnetic Stimulation

Although listeners can enhance perception by using prior knowledge to predict the content of degraded speech signals, this process can also elicit "misperceptions." The neurobiological mechanisms responsible for these phenomena remain a topic of debate. There is relatively consistent evidence for involvement of the bilateral posterior superior temporal gyri (pSTG) in speech perception in noise; however, a role for the left premotor cortex (PMC) is debated. In this study, we employed transcranial magnetic stimulation (TMS) and a prime-probe paradigm for the first time to investigate causal roles for the left PMC and pSTG in speech perception and misperception. To produce misperceptions, we created partially mismatched phonologically similar pseudosentence probes (e.g., She moved into her apartment soon after signing the lease-Che moffed inso har apachment sool amter siphing tha leals). All probe sentences were then spectrotemporally degraded and preceded by a clear prime sentence. Compared with a control site (vertex), inhibitory stimulation of the left pSTG selectively disrupted priming of real but not pseudosentences. However, inhibitory stimulation of the left PMC did not significantly influence perception of either real sentences or misperceptions of pseudosentences. These results confirm a role for the left pSTG in the perception of degraded speech. However, they do not support a role for the left PMC in either lexical or sublexical processing during perception of degraded speech using ecologically valid sentence stimuli. We discuss the implications of these findings for neurobiological models of speech perception.