Friday, August 15, 2008

Lexical phonology and the posterior STG

More interesting stuff has recently been published in JoCN by William Graves and company. Previous work by this group, highlighted here on Talking Brains, found that a regions of the pSTG showed frequency effects in naming. Now this group has used repetition priming with pseudowords to identify regions involved in lexical phonological access. Check it out:

The Left Posterior Superior Temporal Gyrus Participates Specifically in Accessing Lexical Phonology

William W. Graves1, Thomas J. Grabowski2, Sonya Mehta2 and Prahlad Gupta2
1 Medical College of Wisconsin, 2 University of Iowa

Reprint requests should be sent to William W. Graves, Medical College of Wisconsin, Neuro Lab, MEB 4550, 8701 Watertown Plank Road, Milwaukee, WI 53226, or via e-mail:

Impairments in phonological processing have been associated with damage to the region of the left posterior superior temporal gyrus (pSTG), but the extent to which this area supports phonological processing, independent of semantic processing, is less clear. We used repetition priming and neural repetition suppression during functional magnetic resonance imaging (fMRI) in an auditory pseudoword repetition task as a semantics-free model of lexical (whole-word) phonological access. Across six repetitions, we observed repetition priming in terms of decreased reaction time and repetition suppression in terms of reduced neural activity. An additional analysis aimed at sublexical phonology did not show significant effects in the areas where repetition suppression was observed. To test if these areas were relevant to real word production, we performed a conjunction analysis with data from a separate fMRI experiment which manipulated word frequency (a putative index of lexical phonological access) in picture naming. The left pSTG demonstrated significant effects independently in both experiments, suggesting that this area participates specifically in accessing lexical phonology.


Anonymous said...

Sorry, but I am not sure to understand. In both experiments, repetition priming was observed in pSTG for both pseudowords and words?

So I do not understand the term 'lexical phonology'... I need to read this paper but I think that we need a reasonable definition of what are (?) speech perception, phonological processing, lexical processing and so on (also, I am not sure that 'word frequency is (always) a putative index of lexical phonological access').

Refering of one of mine favorite paper : 'the implicit goal of speech perception studies is to understand sublexical stages in the process of speech recognition'. Not so clear, especially regarding some 'lexical/non-lexical' phonological acess'...

Marc Sato

Kenny said...

Lexical-phonological access, if I understand the term correctly, refers to phonological perception at a relatively high level of phonologic organization, eg. word-forms or holistic word perception. It can be used to avoid making claims you cannot resolve immediately about the precise level of phonological organization along linguistic divisions, phonetic, phonological, syllabic, etc., like when we find brain activity related to some aspect of speech sound processing, and it falls into the dark space of word/syllable/multiple-morphemes.

Since Graves and colleagues used repetition-priming, they are likely using single, repeating pseudowords or syllable-units of some sort to induce repetition inhibition. If they saw a reduction in activity, a conservative interpretation is not going to make specific claims about which organized phonological representation(s) was/were primed.

Brad Buchsbaum said...

just a note:

the coordinates of their repetition effect are -51, -38, 22.

see the post "where is Spt?"

In the comments of that post I noted that the average Talairach coordinates for Spt (defined mainly in working memory paradigms) were at -50, -38, 20. Nearly identical coordinates and from their image in Figure 2 it's pretty clear that the cluster in located in the posterior portion of the planum temporale.

Brad Buchsbaum

Greg Hickok said...

Geez. I'm going to have to read this paper now. This is interesting because (i) it is not where I'd expect to see activations associated with a "phonological lexicon" and (ii) this is not what we are finding in a very similar experiment. I would expect phonological word-forms to be stored/processed in the STS. This is where activations show up in speech minus non-speech comparisons and in the Okada & Hickok (2006) paper that contrasted high vs. low density words. In our own repetition priming experiment (Kenny Vaden's study -- still in the works!) we seem to be seeing STS activation. Maybe it is the non-word stimuli that are driving the Spt activation in the Graves study.

Okada, K. & Hickok, G. (2006). Identification of lexical-phonological networks in the superior temporal sulcus using fMRI. Neuroreport, 17, 1293-1296.

Greg Hickok said...

Ok, I took a quick peek at the paper. I still like it, but it isn't what I thought it was based on the abstract. I had thought it was a perceptual repetition priming effect, but it is not. It is a production effect.

Subjects listened to pseudowords and then immediately repeated each item. The manipulation was that some of the p-words were presented multiple times such that the subjects became familiar with them. This led to decreased repetition RTs and a decrease in signal in Spt.

Graves et al refer to the activation focus as "posterior superior temporal gyrus." Based on the coordinates (see Brad's comment above), and the projection of the activation onto an anatomical image, the activation is in the posterior planum temporale region: that is, in the posterior Sylvian fissure at parietal-temporal boundary (Spt).

Spt is a sensory-motor area that is not specific to speech. Although this region activates both during the perception and production of speech, in a study that Brad was deeply involved in, we showed that Spt responded just as well to perception and production of tonal/melodic stimuli as it does to speech stimuli (Hickok et al. 2003). Subsequent work by TB Wast (now former) grad student, Judy Pa, showed that this same region is modulated by output modality: Spt activates less when skilled pianists imagine playing a novel melody than when they imagine humming a novel melody (Pa & Hickok, 2008). This suggests that Spt is motor-effector specific, similar to sensory-motor areas in the parietal lobe (AIP, LIP, etc.). In other words, it walks and talks like a sensory-motor integration area...

Why does Spt show a familiarity effect in repeating non-words? We have suggested that this sensory-motor circuit is recruited for low frequency words that require sensory guidance, i.e., from phonological word forms stored (or recently activated in) the STS. So novel non-words maximally drive the system because production must be guided by the sensory representation. Once they are learned, however, "motor memory" can take some of the load and Spt activation is reduced.

All of this is consistent with the effects of lesions to this area, which are associated with condition aphasia, etc. etc. (see Hickok et al., 2003, Hickok & Poeppel, 2004, 2007 for details).

Buchsbaum, B., Hickok, G., & Humphries, C. (2001). Role of Left Posterior Superior Temporal Gyrus in Phonological Processing for Speech Perception and Production. Cognitive Science, 25, 663-678.

Hickok, G., Buchsbaum, B., Humphries, C., & Muftuler, T. (2003). Auditory-motor interaction revealed by fMRI: Speech, music, and working memory in area Spt. Journal of Cognitive Neuroscience, 15, 673-682.

Hickok, G. & Poeppel, D. (2004). Dorsal and ventral streams: A framework for understanding aspects of the functional anatomy of language. Cognition, 92, 67-99.

Hickok, G. & Poeppel, D. (2007). The cortical organization of speech processing. Nature Reviews Neuroscience, 8, 393-402.

Pa, J. & Hickok, G. (2008). A Parietal-Temporal Sensory-Motor Integration Area for the Human Vocal Tract: Evidence from an fMRI Study of Skilled Musicians. Neuropsychologia, 46, 362-368.