Functional imaging of the auditory processing applied to speech sounds

Abstract

In this paper, we describe domain-general auditory processes that we believe are prerequisite to the linguistic analysis of speech. We discuss biological evidence for these processes and how they might relate to processes that are specific to human speech and language. We begin with a brief review of (i) the anatomy of the auditory system and (ii) the essential properties of speech sounds. Section 4 describes the general auditory mechanisms that we believe are applied to all communication sounds, and how functional neuroimaging is being used to map the brain networks associated with domain-general auditory processing. Section 5 discusses recent neuroimaging studies that explore where such general processes give way to those that are specific to human speech and language.

Figure 1

Four representations of the anatomical connections of the temporal lobe in the macaque brain. (a) The anatomical organization of the auditory cortex is consistent with at least four levels of processing, including core regions (darkest shading) belt regions (lighter shading), parabelt regions (stripes) and temporal and frontal regions that interconnect with belt and parabelt (lighter shading). (Adapted from Kaas et al. (1999) and Hackett & Kaas (2004)). Dotted lines indicate sulci that have been opened to show auditory regions. Regions along the length of (b) superior temporal gyrus and (c) dorsal bank of the superior temporal sulcus connect with prefrontal regions in a topographically organized anterior-to-posterior fashion. (b) Adapted from Petrides & Pandya (1988, p. 64); (c) adapted from Seltzer & Pandya (1989a). (d) Connectivity of auditory belt and parabelt; adapted from Hackett & Kaas (2004). AF, arcuate fasciculus; AS, arcuate sulcus; CS, central sulcus; Extm Cap, extreme capsule; IOS, inferior occipital sulcus; IPS, intraparietal sulcus; LF, lateral fissure; LS, lunate sulcus; PS, principal sulcus; SLF, superior longitudinal fasciculus; STG, superior temporal gyrus; STS, superior temporal sulcus; UnBd, uncinate bundle. (Note. Abbreviations are not spelt out if they are the conventional label for a microanatomically or physiologically defined area).

Figure 2

Internal structure of voiced sounds illustrating the size factors: pulse rate and resonance rate. (a,b) Glottal pulse rate and (c,d) vocal-tract length have a major effect on both the waveform and the spectrum of the sound, but human perception is extremely robust to changes in both of these factors.

Figure 3

(a) The neural activity pattern and (b) the auditory image produced by the /a/ of ‘hat’. Note that the abscissa of the auditory image (b) is ‘time interval’ rather than time itself.

Figure 4

A summary of the results of Patterson et al. (2002). Group activation for four contrasts from Patterson et al. (2002), using a fixed-effects model, rendered onto the average structural image of the group (threshold p<0.05, corrected for multiple comparisons across the whole brain). The position and orientation of the sections are shown in the bottom panels of the figure. The axial sections show the activity in a plane parallel to the surface of the temporal lobe and just below it. The highlighted regions in the structural sections show the average position of HG in the two hemispheres; they are replotted under the functional activation in the axial sections above. The functional activation shows that, as a sequence of noise bursts acquires the properties of melody (first pitch and then changing pitch), the region sensitive to the added complexity changes from a large area on HG and planum temporale (blue), to a relatively focused area in the lateral half of HG (red), and then on out into surrounding regions of the planum polare (PP) and STG (green and cyan mixed). The orderly progression is consistent with the hypothesis that the hierarchy of melody processing that begins in the brainstem continues in auditory cortex and subsequent regions of the temporal lobe. The activation is largely symmetric in auditory cortex and becomes asymmetric abruptly as it moves on to PP and STG with relatively more activity in the right hemisphere.