Identification of a pathway for intelligible speech in the left temporal lobe

Abstract

It has been proposed that the identification of sounds, including species-specific vocalizations, by primates depends on anterior projections from the primary auditory cortex, an auditory pathway analogous to the ventral route proposed for the visual identification of objects. We have identified a similar route in the human for understanding intelligible speech. Using PET imaging to identify separable neural subsystems within the human auditory cortex, we used a variety of speech and speech-like stimuli with equivalent acoustic complexity but varying intelligibility. We have demonstrated that the left superior temporal sulcus responds to the presence of phonetic information, but its anterior part only responds if the stimulus is also intelligible. This novel observation demonstrates a left anterior temporal pathway for speech comprehension.

Fig. 1

Spectrograms of ‘They’re buying some bread’. Time is represented on the abscissa (0.0–1.43 s) and frequency on the ordinate (0.0–4.4 kHz). The darkness of the trace in each time/frequency region is controlled by the amount of energy in the signal at that particular frequency and time. (A) Normal speech (Sp) is intelligible with clear intonation. (B) Spectrally rotated speech (RSp) is not intelligible without extensive training, though some phonetic features and some of the original intonation are preserved. (C) Noise-vocoded speech (VCo) is intelligible, has very weak intonation and a rough sound quality. (D) Spectrally rotated noise-vocoded speech (RVCo) is completely unintelligible and does not sound like a voice.

Fig. 2

Significant voxels from the three contrasts used in the analysis, co-registered on to the left (A) and right (B) lateral MRI templates that are available in the image analysis software (SPM99b). The threshold was set at P < 0.00001 uncorrected, excluding clusters with <50 adjacent voxels. At this threshold, the activations were confined to the temporal lobes. Each contrast was centred around zero, and the ordinate of each plot is the mean size of the effect for each condition ± standard error of the mean, within the peak voxel. The coordinates of the peak voxel (x, y and z) in the stereotaxic space of SPM99b, and the Z-score, are shown at the head of each plot. (A) Left temporal lobe. The contrast {(Sp + VCo + RSp) – RVCo}, colour-coded in red, showed (1) the left superior temporal gyrus, both lateral and anterior to the primary auditory cortex, and (2) a separate region in the posterior superior temporal sulcus. The contrast {(Sp + VCo) – (RSp + RVCo)}, colour-coded in yellow, showed clear separation of intelligible from non-intelligible stimuli in the anterior superior temporal sulcus (3a). In the mid-superior temporal sulcus (3b), the response profile to the stimuli appeared to be transitional between the profiles in the posterior and anterior superior temporal sulcus. This ‘transition’ pattern may reflect a change in response to the rotated speech stimuli, but could also arise as a result of the smoothing applied to the data. (B) Right temporal lobe. The only significant voxels were revealed by the contrast {(Sp + RSp) – (VCo + RVCo}, colour-coded in white. They were located in the lateral superior temporal gyrus, anterior to the primary auditory cortex.

Fig. 3

The five peak activations illustrated in Fig. 2A and B mapped on to sagittal (left column) and coronal (right column) slices of the T₁-weighted MRI template. The upper arrow points at the sylvian sulcus, the lower arrow to the superior temporal sulcus. The contrasts which reveal these activations are shown for each pair of images. The x, y and z coordinates are in millimetres relative to the plane of the anterior commissure. STS = superior temporal sulcus; STG = superior temporal gyrus.