Does Semantic Context Benefit Speech Understanding through “Top–Down” Processes? Evidence from Time-resolved Sparse fMRI

Abstract

Abstract■ When speech is degraded, word report is higher for semantically coherent sentences (e.g., her new skirt was made of denim) than for anomalous sentences (e.g., her good slope was done in carrot). Such increased intelligibility is often described as resulting from "top-down" processes, reflecting an assumption that higher-level (semantic) neural processes support lower-level (perceptual) mechanisms. We used time-resolved sparse fMRI to test for top-down neural mechanisms, measuring activity while participants heard coherent and anomalous sentences presented in speech envelope/spectrum noise at varying signal-to-noise ratios (SNR). The timing of BOLD responses to more intelligible speech provides evidence of hierarchical organization, with earlier responses in peri-auditory regions of the posterior superior temporal gyrus than in more distant temporal and frontal regions. Despite Sentence content × SNR interactions in the superior temporal gyrus, prefrontal regions respond after auditory/perceptual regions. Although we cannot rule out top-down effects, this pattern is more compatible with a purely feedforward or bottom-up account, in which the results of lower-level perceptual processing are passed to inferior frontal regions. Behavioral and neural evidence that sentence content influences perception of degraded speech does not necessarily imply "top-down" neural processes. ■

Semantic versus perceptual interactions in neural processing of speech-in-noise

“…Error bars indicate SEM, corrected for within-subject comparisons (Loftus and Masson, 1994). Davis et al, 2011;Giraud et al, 2004;Narain et al, 2003;Obleser et al, 2007a;Okada et al, 2010;Scott et al, 2000Scott et al, , 2006. Consistent with these studies, our results show that activation in a fronto-temporal network including the left IFG and the medially adjacent anterior insula, as well as left posterior STS/MTG is modulated by speech intelligibility.…”

“…A subset of the studies on speech intelligibility have also found the left anterior hippocampus to be sensitive to speech intelligibility, and this finding has been related to the role of this brain region in verbal memory and in processing meaning in verbal stimuli (Davis and Johnsrude, 2003;Davis et al, 2011). Together, this literature suggests that left frontal regions work in concert with left anterior to posterior temporal regions involved in higher-level binding of meaning in connected speech to assist the effortful comprehension of degraded speech (Davis and Johnsrude, 2003;Davis et al, 2011;Obleser et al, 2007a;Okada et al, 2010).…”

“…Left temporal cortex activations in the above studies range from the anterior to posterior superior temporal gyrus (STG), superior temporal sulcus (STS), and middle temporal gyrus (MTG), with the posterior activations extending to the classic temporo-parietal Wernicke's area (Davis et al, 2011;Narain et al, 2003;Okada et al, 2010;Scott et al, 2000). A subset of the studies on speech intelligibility have also found the left anterior hippocampus to be sensitive to speech intelligibility, and this finding has been related to the role of this brain region in verbal memory and in processing meaning in verbal stimuli (Davis and Johnsrude, 2003;Davis et al, 2011).…”

“…These studies have shown that the left inferior frontal gyrus (IFG), as well as regions along the left anterior to posterior temporal lobe are more activated when speech is degraded (challenging) but intelligible compared to when hearing unintelligible sounds matched for complexity (Davis and Johnsrude, 2003;Narain et al, 2003;Obleser and Kotz, 2010;Obleser et al, 2007a;Scott et al, 2000Scott et al, , 2006. Two studies explicitly also examined activations during processing of clear speech, and found the left IFG to be more strongly recruited if sentences were degraded but intelligible compared to when they were unintelligible but also compared to when they were clearly audible, suggesting that this region is recruited most strongly when successful but effortful speech processing is required (Davis and Johnsrude, 2003;Davis et al, 2011). Even in the absence of sentence-level context, it has been shown that the LIFG responds significantly more to potentially-intelligible noise-vocoded words than to clear words (Hervais-Adelman et al, 2012).…”

How to study spoken language understanding: a survey of neuroscientific methods

“…For experiments on speech comprehension, common experimental manipulations might be to compare speech stimuli that are comprehended or not comprehended due to auditory degradation, or that vary in the ease of comprehension due to the presence/ absence of lexical or semantic anomaly or ambiguity (e.g. Davis, Ford, Kherif, & Johnsrude, 2011;Rodd, Davis, & Johnsrude, 2005;Scott, Blank, Rosen, & Wise, 2000). It is also possible to contrast responses to a single set of stimuli while manipulating the behavioural response required (e.g.…”