Top-down knowledge supports the retrieval of lexical information from degraded speech

How is it that the human brain is capable of making sense from speech under many acoustically compromised conditions? The support through top-down knowledge is inevitable but can we identify brain measures of this matching process between degraded auditory input and possible meaning? To answer these questions, the present study investigated the modulation of the induced gamma-band activity (GBA) in the auditory domain in response to degraded speech. During an EEG experiment subjects first listened to digitally degraded unintelligible speech signals (derived from German nouns). In an exposure sequence, half of the nouns were presented in a non-degraded intelligible format and memorized, while in the crucial test sequence subjects listened to all degraded speech signals again and were asked to identify the words. The induced GBA (40-Hz range) showed an increase at left temporal electrode sites around 350 ms only for words correctly identified in the test sequence. No differences in induced GBA were evident in the baseline sequence; neither did the evoked brain potentials yield any comparable effect. We conclude that the observed enhancement in induced gamma-band activity reflects a matching process of top-down lexical memory traces with degraded sensory input to form a comprehendible speech percept. The findings are highly corroborant to analogous studies in the visual system. They lend further plausibility to a left-lateralized fronto-temporal network enabling lexically guided speech perception, and they demonstrate the complementary role of time-sensitive brain analyses in discerning the functional neuroanatomy of speech.