Dual-echo fMRI can detect activations in inferior temporal lobe during intelligible speech comprehension

Abstract

The neural basis of speech comprehension has been investigated intensively during the past few decades. Incoming auditory signals are analysed for speech-like patterns and meaningful information can be extracted by mapping these sounds onto stored semantic representations. Investigation into the neural basis of speech comprehension has largely focused on the temporal lobe, in particular the superior and posterior regions. The ventral anterior temporal lobe (vATL), which includes the inferior temporal gyrus (ITG) and temporal fusiform gyrus (TFG) is consistently omitted in fMRI studies. In contrast, PET studies have shown the involvement of these ventral temporal regions. One crucial factor is the signal loss experienced using conventional echo planar imaging (EPI) for fMRI, at tissue interfaces such as the vATL. One method to overcome this signal loss is to employ a dual-echo EPI technique. The aim of this study was to use intelligible and unintelligible (spectrally rotated) sentences to determine if the vATL could be detected during a passive speech comprehension task using a dual-echo acquisition. A whole brain analysis for an intelligibility contrast showed bilateral superior temporal lobe activations and a cluster of activation within the left vATL. Converging evidence implicates the same ventral temporal regions during semantic processing tasks, which include language processing. The specific role of the ventral temporal region during intelligible speech processing cannot be determined from this data alone, but the converging evidence from PET, MEG, TMS and neuropsychology strongly suggest that it contains the stored semantic representations, which are activated by the speech decoding process.

Methods for cleaning the BOLD fMRI signal

“…As a result, the combined fMRI signal has a better contrast-to-noise ratio mainly due to the reduction in thermal noise, which can be exploited for increasing the sensitivity and specificity for task-based and resting state experiments at different field strengths. Multi-echo fMRI offers clear advantages for imaging brain regions such as the orbitofrontal cortex and inferior temporal lobes, which are prone to susceptibility distortions and signal dropouts (Halai et al, 2014; 2015). …”

Barking up the right tree: Univariate and multivariate fMRI analyses of homonym comprehension

“…Image acquisition and processing: Images were acquired on a 3T Siemens Prisma scanner using a 32-channel head coil. A dual-echo protocol was employed in which gradient-echo EPI images were simultaneously acquired at two TEs (13ms and 35ms) and a mean of the two echo series was computed during preprocessing (Halai, Parkes, & Welbourne, 2015). This approach improves signal quality in the ventral ATLs, which typically suffer from susceptibility artefacts (Ojemann et al, 1997).…”

Comprehending auditory speech: previous and potential contributions of functional MRI

“…This method has recently been shown to out-perform single spin-echo imaging, and has also been shown to ameliorate "signal drop out", in temporal lobe regions (Halai, Welbourne, Embleton, & Parkes, 2014). Indeed, using this approach a recent study revealed selective responses to intelligible speech in inferior regions of the anterior temporal lobe, as well as more superior regions typically identified in studies of speech intelligibility (Halai, Parkes, & Welbourne, 2015).…”

“…Since the establishment of fMRI over PET as the neuroimaging method of choice for studies of healthy brain function, this trend is strongly apparent in studies of speech comprehension. In general, the inferior temporal lobe has received relatively little attention in studies of speech, yet there are clues to its involvement in studies contrasting the perception of auditory speech at varying levels of intelligibility -for example, Adank's (2012) meta-analysis, which implicates a node in left fusiform gyrus, and Halai et al's (2015) finding of left inferior temporal and fusiform gyrus activation when employing dual-echo acquisition to reduce signal drop-out artefact. Beyond the issue of whether methodological limitations of fMRI have obscured the true apex of the temporal speech processing hierarchy in the ATL, the question arises as to whether this region reflects a language-specific response, or a domaingeneral semantic "hub" (Patterson et al, 2007).…”

“…This method has recently been shown to out-perform single spin-echo imaging, and has also been shown to ameliorate "signal drop out", in temporal lobe regions (Halai, Welbourne, Embleton, & Parkes, 2014). Indeed, using this approach a recent study revealed selective responses to intelligible speech in inferior regions of the anterior temporal lobe, as well as more superior regions typically identified in studies of speech intelligibility (Halai, Parkes, & Welbourne, 2015).An important distinction between auditory and visual processing in the human brain is the larger number of key relay nuclei in the ascending auditory pathway, including the cochlear nucleus, the superior olive, inferior colliculus and medial geniculate nucleus. Imaging these small structures presents challenges in functional imaging through the increased movement artefact associated with the proximity of these sites to pulsatile movement in blood vessels.…”