Wernicke's area revisited: parallel streams and word processing

Abstract

Auditory word-form recognition was originally proposed by Wernicke to occur within left superior temporal gyrus (STG), later further specified to be in posterior STG. To account for clinical observations (specifically paraphasia), Wernicke proposed his sensory speech center was also essential for correcting output from frontal speech-motor regions. Recent work, in contrast, has established a role for anterior STG, part of the auditory ventral stream, in the recognition of species-specific vocalizations in nonhuman primates and word-form recognition in humans. Recent work also suggests monitoring self-produced speech and motor control are associated with posterior STG, part of the auditory dorsal stream. Working without quantitative methods or evidence of sensory cortex' hierarchical organization, Wernicke co-localized functions that today appear dissociable. "Wernicke's area" thus may be better construed as two cortical modules, an auditory word-form area (AWFA) in the auditory ventral stream and an "inner speech area" in the auditory dorsal stream.

Fig. 1. A composite illustration of human auditory cortex and macaque auditory fields

Relative to the macaque, human auditory cortex is rotated ∼45° off the anterior-posterior axis of the superior temporal plane (Galaburda & Sanides, 1980; Rademacher et al., 2001; Fullerton & Pandya, 2007; Hackett, 2011) with primary auditory cortex [core, Brodmann’s area (BA) 41] located along Heschl’s gyrus (HG) and secondary auditory cortex (lateral and medial belt, BA 42 and 52, respectively) located in planum polare (PP) and planum temporale (PT). To facilitate comparisons with the macaque literature, names of functionally-defined macaque subfields are shown on a flatmap of human anatomy (core: A1, R, RT, RTp; lateral belt: CL, ML, AL, RTL; medial belt: CM, MM, RM, RTM) (A). Subfield delineation is estimated from relative field sizes in the macaque, scaled with respect to the volume of human core (Penhune et al., 1996; Rademacher et al., 2001) and functionally localized according to tuning characteristics (Rauschecker et al., 1995; Chevillet et al., 2011). The composite figure implies a course for the human ventral and dorsal streams along the superior temporal plane. Fields exhibiting heightened selectivity for monkey calls are shown in yellow: lateral belt field AL (Tian et al., 2001; Tsunada, Lee & Cohen, 2011) and area RTp (Kikuchi, Horwitz & Mishkin, 2010). For orientation, the cortical patch shown in flatmap (A) is outlined on the cortical surface (dashed line with scissor markers) (B). Additional points of reference include the circular sulcus (CS), insular cortex (Ins),

Fig 2. Anatomical predictions for the site of auditory word-form recognition

(A) In the macaque, communication call processing is strongly associated with anterior-lateral portions of the superior temporal plane, specifically areas AL (circled) (adapted from Rauschecker & Tian, 2000). (B) The putatively homologous human site resides at the anterior-lateral aspect of Heschl’s gyrus (circled) (adapted from Galaburda & Sanides, 1980). (C) This site is within the territory originally proposed by Wernicke (shaded region marked x) (adapted from Wernicke, 1881) but (D) is inconsistent with the location given for Wernicke’s area by Geschwind (shaded region marked 4) (adapted from Geschwind, 1969).

Fig. 3. Meta-analyses of auditory-word processing

Analyses of studies comparing brain response to speech stimuli versus matched control sounds (A–C), indicative of selectivity for speech sounds, found a leftward bias and an anterior progression in peak effects with phoneme-length studies’ peak focus density in left mid-STG (A), word-length studies’ peak density in left anterior STG (B), and phrase-length studies’ peak density in left anterior STS (C). Peak density for studies investigating phonetically specific adaptation (D), indicative of invariant representation, was found in left mid- to anterior STG. Peak density for areal specialization studies (E), which compared brain response to speech stimuli versus other natural non-speech sounds, also indicative of selectivity for speech sounds, was greatest in left STG. Intensity represents ALE value. Adapted from DeWitt & Rauschecker (2012).