A recent paper in Nature
and a recent paper in Science provide
ECog evidence for dorsal stream function and STG function, respectively.
The first paper, “Sensory–motor transformations for
speech occur bilaterally,” is from my NYU colleague Bijan Pesaran’s lab;
the first author is Greg Cogan, a post-doc with Bijan. The paper tackles the
important question of how dorsal stream structures implement sensory–motor transformations, an issue that
Greg Hickok and I have speculated about (and Greg H. has worked on
extensively). This rich paper reports a bunch of cool findings worth reading
and studying. One of the strong claims – the part of the data providing the
title – concerns the bilateral nature of (those parts of) the dorsal stream
underpinning sensory-motor transformations for speech. Previous work has argued
that output-related dorsal-stream processing is lateralized, certainly much
more strongly than ventral stream areas/functions. I still find that position
on the right track (cf. Hickok & Poeppel 2007), and I derive some special
frisson from the fact that Greg Cogan, the co-architect of this counter-argument,
was my graduate student and is an important collaborator. The data are the data
– so it’s now important to figure out the why/how/what/when of these two dorsal
streams. I am no apologist for lateralization in speech, but these data
certainly present a new interpretive challenge. Speculations, ideas, data
welcome.
The second paper, “Phonetic Feature Encoding in Human
Superior Temporal Gyrus,” is from Eddie Chang’s lab at UCSF and is
spearheaded by Nima Mesgarani (now faculty at Columbia University in the EE
department). Over the years, the evidence has steadily accumulated that STG is
the ‘home’ of acoustic-phonetic perceptual analysis. Previous ECog data,
including stimulation data, for example by Dana Boatman, Nathan Crone, and
colleagues, has been strong evidence for STG (e.g. for review, Boatman 2004, http://www.ncbi.nlm.nih.gov/pubmed/15037126).
This new work builds on those findings and demonstrates the sensitivity and
selectivity of this region. From data acquired while the patients listened to
spoken sentences (of numerous speakers), Nima et al. extracted activity
profiles of the electrodes to all English phonemes. Phonetic features turn out
to be an effective grouping principle (manner is especially prominent). Nima
had done a similar project in Shihab Shamma’s lab in his dissertation work (I
harassed him about it at his defense …) - but ferrets neither speak nor listen
to all that much human speech … In this new work, the acoustic-phonetic
encoding is elegantly described, providing some ways to think about the
intermediate representations that could link input-related spectro-temporal
processing to linguistic structures.
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