Area Spt is one of my favorite brain locations. We've been working on characterizing its response properties ever since we reported its existence in two papers back in 2001 and 2003 (Buchsbaum, Hickok, and Humphries, 2001; Hickok, Buchsbaum, Humphries, & Muftuler, 2003). Spt is located in the left posterior Sylvian region at the parietal-temporal boundary. The defining feature of Spt is that it activates both during the perception of speech and during (covert) speech production. Subsequent work has found that Spt is not speech-specific as it responds also during tonal melodic perception and production (humming), and that Spt is relatively selective for vocal tract gestures in that it responds more during perception and covert humming of tonal melodies than during perception and imagined piano playing of tonal melodies (Pa & Hickok, 2008) . On the basis of evidence like this, I have argued that Spt is a sensory-motor integration area for the vocal tract motor effector, just like monkey area LIP is a sensory-motor integration area for the eyes, and the parietal reach region (or area AIP) is a sensory-motor area for the manual effectors.
One nagging objection that has been raised more than once is this: "Isn't your 'motor' activity just auditory imagery?" That is, maybe during covert rehearsal there is some kind of motor-to-sensory discharge that serves to keep active sensory representations of speech in auditory cortex (i.e., Spt). Another possible objection that is less often raised is that the "sensory" activity we see in Spt isn't really sensory but is really motor preparation.
Just yesterday we got a paper accepted in the Journal of Neurophysiology that I think rules out these kinds of objections (Hickok, Okada, & Serences, in press). Here's the logic. If Spt really is just like other sensory-motor integration areas (e.g., LIP, AIP), it will be composed of a population of sensory-weighted cells, motor-weighted cells, and truly sensory-motor cells. Two things follow, (i) the BOLD response to combined sensory-motor stimulation should be greater than the BOLD response to either sensory or motor activation alone (because sensory-motor stimulation activates a larger cohort of cells than either sensory or motor alone), and (ii) the pattern of activity within Spt may be different during sensory activation than during motor activation (on the assumption that sensory and motor weighted cells are not perfectly distributed across the sampled voxels within Spt. If we can show that the response to sensory stimulation and motor stimulation are different, then Spt activity can't be all sensory or all motor; it must be sensory-motor.
Here's how we tested these predictions using fMRI. Subjects either listened to a 15s block of continuous speech (continuous listen), listened to 3s of speech and then rested for 12s (listen+rest), or listened to 3s of speech and then covertly rehearsed that speech for 12s (listen+rehearse):
First the BOLD results. Spt was identified separately in each subject by the subtraction, listen+rehearse minus listen+rest. This picks out areas that are more active during rehearsal than rest. Here's the BOLD activation for each condition in each subject's Spt ROI:
In the listen+rehearse condition, we predict that the BOLD response will be dominated by sensory stimulation during the first phase of the trial, will be a mix of sensory and rehearsal response during the middle phase of the trial (because the sensory response hasn't yet decayed while the rehearse response starts kicking in), and then will be dominated by the rehearsal response during the final phase of the trial. If you look at the listen+rehearse response curve compared to the continuous listen curve you can see how this prediction is born out: responses are equal in the first phase (because both conditions involve identical sensory stimulation), then activity in the continuous listen condition saturates and maintains roughly the same activity level until the end of the trial whereas activity in listen+rehearse continues to elevate (presumably because the sensory and motor-rehearsal responses are summing) then falls back down toward the end of the trial (presumably because the sensory signal is decayed). So, the BOLD predictions pan out.
Next we used pattern classification analysis to see if the pattern of the response in Spt was different during sensory stimulation versus motor activation. Amplitude information was removed from the data for this analysis. We trained a Support Vector Machine to classify the two conditions (continuous listen vs. listen+rehearse) on data from all but one run then tested its classification accuracy in the remaining run. This hold-one-out procedure was repeated until all runs had been classified. In addition, we performed this classification in three different time bins within the trial: early, middle, and late. The prediction is that classification accuracy should be maximal when the two conditions are maximally dominated by different signal sources, i.e., in the final time bin, and should be no better than chance in the first time bin when both signals are predominantly sensory. Here's what we found (blue lines represent upper and lower 5% boundaries for classification accuracies determined via a permutation test):
Classification accuracy for the continuous listen vs. listen+rehearse conditions was significantly above chance only in the last time bin, that is when the BOLD signal was maximally dominated by distinct input sources, one sensory the other motor. Notice too that at this time point the BOLD amplitude in these two signal are the same, which provides additional evidence that classification accuracy has nothing to do with amplitude.
If the pattern of activity in Spt is different during sensory stimulation compared to during motor stimulation (and independent of amplitude), then Spt activity can't be all sensory or all motor. This, together with the range of supporting evidence indicates that Spt is indeed a sensory-motor area.
Buchsbaum B, Hickok G, and Humphries C. Role of Left Posterior Superior Temporal Gyrus in Phonological Processing for Speech Perception and Production. Cognitive Science 25: 663-678, 2001.
Hickok G, Buchsbaum B, Humphries C, and Muftuler T. Auditory-motor interaction revealed by fMRI: Speech, music, and working memory in area Spt. Journal of Cognitive Neuroscience 15: 673-682, 2003.
Hickok, G., Okada, K., & Serences, J. (in press). Area Spt in the human planum temporale supports sensory-motor integration for the speech processing. Journal of Neurophysiology
Pa J, and Hickok G. A parietal-temporal sensory-motor integration area for the human vocal tract: Evidence from an fMRI study of skilled musicians. Neuropsychologia 46: 362-368, 2008.