In contrast to the traditional view that all aspects of speech processing are strongly left dominant, we have argued in several papers that the recognition of speech sounds is supported by auditory regions in both hemispheres (Hickok & Poeppel, 2000, 2004, 2007). The evidence for this view comes from neuropsychological studies:
1. Chronic damage to the left superior temporal gyrus alone is not associated with auditory comprehension deficits or speech perception deficits, but instead is associated with speech production deficits (conduction aphasia).
2. More extensive chronic damage to the left temporal lobe IS associated with auditory comprehension deficits (e.g., in Wernicke's aphasia), but these deficits are not predominantly caused by difficulties in perceiving speech sounds. Instead, post phonemic deficits appear to account for a majority of the auditory comprehension deficit in aphasia. Evidence for this conclusion comes from the fact that such patients tend to make more semantic than phonemic based errors on auditory word-to-picture matching tests with semantic and phonemic foils.
3. In contrast to the relatively minimal effects of unilateral damage on speech sound recognition, damage to superior temporal regions in both hemisphere's IS associated with a profound deficit in perceiving speech sounds (e.g., word deafness).
One criticism of this body of neuropsychological data is that it involves patients with chronic lesions, and therefore the possibility of compensatory reorganization of speech recognition processes. For example, it could be that speech recognition is strongly left dominant in the intact brain, but following chronic left hemisphere injury, the right hemisphere gradually assumes speech recognition function.
Two new studies argue against this view. Both examine the effects of acute left hemisphere disruption on auditory word comprehension; one uses Wada methods, the other acute stroke. Both find that (i) auditory word-level comprehension deficits tend to be relatively mild, and (ii) reflect primarily post phonemic deficits.
Evidence from Wada procedures
This study (Hickok, et al. 2008) looked at the ability of patients undergoing clinically indicated Wada procedures to comprehend auditorily presented words with either their left or right hemispheres anesthetized. Patients listened to a stimulus word and were asked to point to the matching picture from a four-picture array that included the target, a semantic foil, a phonemic foil, and an unrelated foil. The basic results are provided in the figure below. Overall, errors were more common following left hemisphere anesthesia, but when errors occurred, they tended to be semantic (>2:1 ratio). Notice that the overall phonemic error rate with left disruption is less than 10%. This indicates that even acute disruption of left hemisphere function does not profoundly affect speech sound recognition during auditory comprehension.
Evidence from acute stroke
One could argue that evidence from Wada procedures may not be generalizable to the population as a whole given that Wada patients have a pre-existing neurological condition. Studies of patients in the acute phase of stroke avoid this potential complication. In a collaborative study with Argye Hillis at Johns Hopkins, we examined the auditory comprehension abilities of 289 patients who were within 24 hours of hospital admission for stroke (Rogalsky, et al. 2008). For this study we used a picture verification paradigm: subjects heard a word and were shown a picture that either matched the word, was a phonological foil, or was a phonemic foil. Subjects were asked to decide if the word and picture matched. We used a signal detection-based analysis to determine how well subjects were discriminating matches from non-matches. The top panel of the figure below shows the distribution of patients across the different performance levels. Notice that a very small fraction of the entire group scored worse than 80% correct overall (~7%). The bottom panel shows how well subjects in each of these performance bins could discriminate targets from semantic versus phonemic foils (y-axis = A-prime scores which approximates % correct). At every performance level, semantic confusions dominated (i.e., scores are lower for semantic foils). Within the bottom 7% of subjects -- those who scored worse than 80% correct -- performance was better on phonemic foils than semantic foils by 10 percentage points (72% vs. 62% correct, respectively), and well above chance (50%).
We conclude that the processing of speech sounds during auditory word comprehension is not profoundly impaired by left hemisphere damage either in chronic or acute stages of insult. This in turn indicates that both hemispheres of the intact brain have the capacity for processing speech sounds during comprehension. In other words, speech sound processing is bilaterally organized to some extent. This stands in sharp contrast to the impact of unilateral lesions on speech production, which can lead to profound deficits.
Hickok, G., Okada, K., Barr, W., Pa, J., Rogalsky, C., Donnelly, K., Barde, L. & Grant, A. (in press). Bilateral capacity for speech sound processing in auditory comprehension: Evidence from Wada procedures. Brain and Language,
G Hickok, D Poeppel (2000). Towards a functional neuroanatomy of speech perception Trends in Cognitive Sciences, 4 (4), 131-138 DOI: 10.1016/S1364-6613(00)01463-7
G Hickok, D Poeppel (2004). Dorsal and ventral streams: a framework for understanding aspects of the functional anatomy of language Cognition, 92 (1-2), 67-99 DOI: 10.1016/j.cognition.2003.10.011
Gregory Hickok, David Poeppel (2007). The cortical organization of speech processing Nature Reviews Neuroscience, 8 (5), 393-402 DOI: 10.1038/nrn2113
C ROGALSKY, E PITZ, A HILLIS, G HICKOK (2008). Auditory word comprehension impairment in acute stroke: Relative contribution of phonemic versus semantic factors Brain and Language DOI: 10.1016/j.bandl.2008.08.003