Thursday, January 8, 2009

Functional organization of the planum temporale

This is the title of a talk I'm giving at the Auditory Cognitive Neuroscience Society meeting tomorrow in Tucson. What I'm going to argue is that there is no such thing. Let me explain...

The planum temporale is a gross anatomical feature. Although it is often referred to and studied as a functional region -- e.g., The Planum Temporale as a Computational Hub (Griffiths and Warren, 2002) among many other papers -- there is no evidence to support this view. Cytoarchitectonic data indicate at least four distinct fields within the PT that themselves do not respect the anatomical boundaries of the region; i.e., the cytoarchitectonic areas extend beyond the PT to include the lateral STG, parietal operculum, and supramarginal gyrus. Further, although the PT is often referred to as "auditory cortex" only the anterior portion of it appears to be auditory cortex proper.

In other words, the PT is not a functionally unitary region, and we should stop trying to characterize it as such.

Griffiths and Warren (2002) noticed correctly that a range of different types of stimuli can activate the PT. To account for this observation they proposed that the PT functions as a computational hub, which they envision as a kind of pattern matcher/router: various kinds of input come in and get sorted and routed to the appropriate processing systems. I've looked closely at two functions that have consistently implicated the PT region, spatial hearing and sensory-motor integration. It turns out that if you look at the activation maps associated with these two functions on a within subject basis, they lite up distinct areas of the PT region. Spatial hearing-related activations (listening to sounds coming from a variety of spatial locations > from a single location) activate a more anterior location that is likely within the auditory cortex portion of the PT and the sensory motor activations (regions that activate both during the perception and covert production of speech) are more posterior, likely within the non-auditory regions of the PT. This kind of result provides further evidence for a functionally heterogenous PT, and argues against hypotheses like the Griffith and Warren's computational hub.

T Griffiths, J Warren (2002). The planum temporale as a computational hub Trends in Neurosciences, 25 (7), 348-353 DOI: 10.1016/S0166-2236(02)02191-4


Jonas said...

Greg, I am glad that 2009 finally sees the planum speak up at full volume here at talking brains. A very interesting point you raise, and I am very sorry to miss your talk tomorrow. Could you kindly post the cytoarchitecture reference you refer to? I also like your idea of finally getting beyond the PT. I would love to see my own various PT activations being mapped onto your suggested anterior–posterior delineation. Best wishes, J.

Greg Hickok said...

The classic Galaburda & Sanides (1980, THE JOURNAL OF COMPARATIVE NEUROLOGY 190:597-610) paper is clear on this point. From page 609:

"Area Tpt represents a transitional type of
cortex between the specialized isocortices of
the auditory region and the more generalized
isocortex (integration cortex) of the inferior
parietal lobule. By this statement it is meant
that it lacks specialty features of sensory cor-
tex, i.e., emphatic granulation and light in-
fragranular layers. It corresponds in location
and appearance to Economo's and Koskinas's
TA, ('25) and Brodmann's area 22 at its poste-
rior end ('09). Area Tpt often extends beyond
the caudal end of the temporal lobe to occupy
variable amounts of suprasylvian cortex."

Tpt is a region occupying the posterior portion of the PT but extending beyond it. More recent data from monkey (see work by Hackett) corroborates this view.

yisroel said...

The point raised here is yet another illustration of the need for a more fine grained breakdown of cortical regions based on up to date cytoarchitectonic analyses that could be adopted as a standard for the field.

Jonas said...

Thanks indeed, Greg.

Anonymous said...

I think the Griffiths & Warren (2002) statement about variation in function within PT could be true in that there is variation in thickness therein.