(Brosch, Selezneva, & Scheich, 2005). Nevertheless, it is currently thought that the neural representations of sounds and events in the primary auditory cortex are probably based on detecting relatively simple acoustic features and are not specific to speech or vocalizations, given that the primary cortex does not seem to have any obvious preference for speech over nonspeech stimuli. In the human brain, to find the first indication of areas that appear to prefer speech to other, nonspeech sounds, we must move beyond the tonotopic maps of the primary auditory cortex (Belin et al., 2000; Scott et al., 2000).
In the following sections we will continue our journey through the auditory system into cortical regions that appear to make specialized contributions to speech processing, and which are situated in the temporal, parietal, and frontal lobes. We will also discuss how these regions communicate with each other in noisy contexts and during self‐generated speech, when information from the (pre)motor cortex influences speech perception, and look at representations of speech in time. Figure 3.6 introduces the regions and major connections to be discussed. In brief, we will consider the superior temporal gyrus (STG) and the premotor cortex (PMC), and then loop back to the STG to discuss how brain regions in the auditory system work together as part of a dynamic network.
Figure 3.6 A map of cortical areas involved in the auditory representation of speech. PAC = primary auditory cortex; STG = superior temporal gyrus; aSTG = anterior STG; pSTG = posterior STG; IFG = inferior frontal gyrus; PMC = pre‐motor cortex; SMC = sensorimotor cortex; IPL = inferior parietal lobule. Dashed lines indicate medial areas.
(Source: Adapted from Rauschecker & Scott, 2009.)
What does the higher‐order cortex add?
All the systems that we have reviewed so far on our journey along the auditory pathway have been general auditory‐processing systems. So, although they are important for speech processing, their function is not speech specific. For example, the cochlea converts pressure waves into electrical impulses, whether the pressure waves are encoding a friendly ‘hello’ or the sound of falling rain; the subcortical pathways process and propagate these neural signals to the primary auditory cortex, regardless of whether they are encoding a phone conversation, barking dogs, or noisy traffic; and the primary auditory cortex exhibits a tonotopic representation of an auditory stimulus, whether that stimulus is part of a Shakespearean soliloquy or of Ravel’s Boléro. In this section, we encounter a set of cortical areas that preferentially process speech over other kinds of auditory stimuli. We will also describe deeply revealing new work into the linguistic‐phonetic representation of speech, obtained using surgical recordings in human brains.
Speech‐preferential areas
That areas of the brain exist that are necessary for the understanding of speech but not for general sound perception has been known since the nineteenth century, when the German neurologist Carl Wernicke associated the aphasia that bears his name with damage to the STG (Wernicke, 1874). Wernicke’s eponymous area was, incidentally, reinterpreted by later neurologists to refer only to the posterior third of the STG and adjacent parietal areas (Bogen & Bogen, 1976), although some disagreement about its precise boundaries continues until this day (Tremblay & Dick, 2016).
With the advent of fMRI at the end of the twentieth century, the posterior STG (pSTG) was confirmed to respond more strongly to vocal sounds than to nonvocal sounds (e.g. speech, laughter, or crying compared to the sounds of wind, galloping, or cars; Belin et al., 2000). Neuroimaging also revealed a second, anterior, area in the STG, which responds more to vocal than to nonvocal sounds (Belin et al., 2000). These voice‐preferential areas can be found in both hemispheres of the brain. Additional studies have shown that it is not just the voice but also intelligible speech that excites these regions, with speech processing being more specialized in the left hemisphere (Scott et al., 2000). Anatomically, the anterior and posterior STG receive white‐matter connections from the primary auditory cortex, and in turn feed two auditory‐processing streams, one antero‐ventral, which extends into the inferior frontal cortex, and the other postero‐dorsal, which curves into the inferior parietal lobule. The special function of these streams remains a matter of debate. For example, Rauschecker and Scott (2009) propose that the paths differ in processing what and where information in the auditory signal, where what refers to recognizing the cause of the sound (e.g. it’s a thunderclap) and where to locating the sound’s spatial location (e.g. to the west). Another, more linguistic, suggestion is that the ventral stream is broadly semantic, whereas the dorsal stream may be described as more phonetic in nature (Hickok & Poeppel, 2004). Whatever the functions, however, there appear to be two streams diverging around the anterior and posterior STG.
Over the years, these early STG results have been replicated many times using neuroimaging (Price, 2012). Each technique for observing activity of the human brain, whether it is noninvasive magnetoencephalography (MEG) or fMRI, or invasive surgical techniques such electrocorticography (ECoG; described in the next section), all have their limitations and shortcomings. It is therefore reassuring that the insights into the neuroanatomy of speech comprehension established by methods like MEG or fMRI, which can image the whole brain, are both confirmed and extended by studies using targeted surgical techniques like ECoG.
Auditory phonetic representations in the superior temporal gyrus
ECoG, which involves the placement of electrodes directly onto the surface of the brain, cannot easily record from the primary auditory cortex. This is because the PAC is tucked away inside the Sylvian fissure, along the dorsal aspect of the temporal lobe. At the same time, because ECoG measures the summed postsynaptic electrical current of neurons with millisecond resolution, it is sensitive to rapid neural responses at the timescale of individual syllables, or even individual phones. By contrast, fMRI measures hemodynamic responses; these are changes in blood flow that are related to neural activity but occur on the order of seconds. In recent years, the use of ECoG has revolutionized the study of speech in auditory neuroscience. An exemplar of this can be found in a recent paper (Mesgarani et al., 2014).
Mesgarani et al. (2014) used ECoG to learn about the linguistic‐phonetic representation of auditory speech processing in the STG of six epileptic patients. These patients listened passively to spoken sentences taken from the TIMIT corpus (Garofolo et al., 1993), while ECoG was recorded from their brains. These ECoG recordings were then analyzed to discover patterns in the neural responses to individual speech sounds (for a summary of the experimental setup, see Figure 3.7, panels A–C). The authors used a phonemic analysis of the TIMIT dataset to group the neural responses at each electrode, according to the phoneme that caused it. For examples, see panel D of Figure 3.7, which allows the comparison of responses to different speech sounds for a number of different sample electrodes labeled e1 to e5. The key observation here is that an electrode such as e1 gives similar responses for /d/ and /b/ but not for /d/ and /s/, and that the responses at each of the electrodes shown will respond strongly for some groups of speech sounds but not others. Given these data, we can ask the question: Do STG neurons group, or classify, speech segments through the similarity of their response patterns? And, if so, which classification scheme do they use?
Linguists and phoneticians often analyze individual speech sounds into feature classes, based, for example, on either the manner or the place of articulation that is characteristic for that speech act. Thus, /d/, /b/, and /t/ are all members of the plosive manner‐of‐articulation class because they are produced by an obstruction followed by a sudden release of air through the vocal tract, and /s/ and /f/ belong to the fricative