Wednesday, January 16, 2008

Mis-represented positions on semantic organization in the brain

I noted this in previous posts, but thought, for emphasis sake, it would be worth dedicating a single entry to the issue of what our (Hickok and Poeppel) claims actually are regarding "semantics." In fact, our claims regarding the role of the posterior temporal lobes are rather circumscribed. Here's some quotes:

"This [ventral] pathway appears to be important for interfacing sound-based representations of speech with widely distributed conceptual representations, and therefore is involved in tasks that require access to the mental lexicon." HP 2000, p. 131

"The ventral stream projects ventro-laterally... These pITL [posterior inferior temporal lobe] structures serve as an interface between sound-based representations of speech in STG and widely distributed conceptual representations..." HP 2004, p. 72

"Our claim is simply that there exists a cortical network which performs a mapping between (or binds) acoustic-phonetic representations of the one hand, and conceptual-semantic representations on the other." HP 2004, p. 81

"The more posterior regions of the ventral stream, posterior middle and inferior portions of the temporal lobes correspond to the lexical interface, which links phonological and semantic information..." HP 2007, p. 395

While our terminology changes slightly -- reflecting the fact that we don't really know what's being computed -- we have been very clear that this is not a model of semantic memory/conceptual knowledge organization, but rather a claim about the area that is particularly important for interfacing such information (located elsewhere) with acoustic/phonological representations of speech.

Despite this, we have come across examples of researchers generalizing our claims to a theory of semantics. Here's one from the semantic dementia readings:

"Although the data arising from semantic dementia clearly implicate the temporal poles, bilaterally, in semantic representation, these areas are often overlooked or even disputed in other research on semantic memory [HP 2007 cited here]." Pobric et al., 2007, p. 20137

And here is an older one from Friedemann Pulvermuller's lab:

"In contrast to other authors who suggest that semantics is represented in meaning-specific brain regions that process all words alike (Hickok and Poeppel, 2000; Lichtheim, 1885; Price et al., 2001; Scott and Johnsrude, 2003; Wernicke, 1874), we proposed that semantic representations are distributed in a systematic way throughout the entire brain." Hauk, et al., 2004, p. 305.

I won't speak for Price et al., or Scott and Johnsrude, but I will defend the dead guys, Wernicke and Lichtheim. Not only did Hauk et al. mis-attribute such a claim about semantic representation to us, they completely missed the boat even on these classical authors. Here's a quote from Wernicke, 1874/1977, p.117:

"The concept of the word 'bell', for example, is formed by the associated memory images of visual, tactual and auditory perception. These memory images represent the essential characteristic features of the object, bell."

And a quote from Wernicke's later work, 1885-1886/1977, p. 177:

"... the memory images of a bell ... are deposited in the cortex and located according to the sensory organs."

For details on Wernicke's rather sophisticated distributed theory of conceptual representation of the brain, including a new translation into English of a paper by Wernicke on the topic, see Gage & Hickok, 2005. Seriously, check it out -- you will be surprised to learn that Wernicke even postulated "Hebbian learning" mechanisms decades before Hebb.

So Wernicke clearly did not believe in a focal meaning specific brain area. Now to defend Lichtheim -- here's a quote from his 1885 paper, On Aphasia, in the journal, Brain, p. 477:

"Though in the diagram B is represented as a sort of centre for the elaboration of concepts, this has been done for simplicity's sake; with most writers, I do not consider the function to be localised in one spot of the brain, but rather to result from the combined action of the whole sensorial sphere."

The point here is not so much to criticize anyone for misquoting -- it is easy to misquote, and I'm sure we are all guilty -- rather the point is (i) to make it perfectly clear what David and I are actually claiming w.r.t. semantics because I think some people are missing the distinction between a semantic memory system and an interface between this system and speech, and more importantly, (ii) to (re-)emphasize that when you are talking about "semantics", be very clear regarding what part of semantics you're talking about.


Gage, N., & Hickok, G. (2005). Multiregional Cell Assemblies, Temporal Binding, and the Representation of Conceptual Knowledge in Cortex: A Modern Theory by a "Classical" Neurologist, Carl Wernicke. Cortex, 41, 823-832.
Hauk, O., Johnsrude, I., & Pulvermuller, F. (1994). Somatotopic representation of action words in human motor and premotor cortex. Neuron, 41, 301-307.
Hickok, G., & Poeppel, D. (2000). Towards a functional neuroanatomy of speech perception. Trends in Cognitive Sciences, 4, 131-138.
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. Nat Rev Neurosci, 8(5), 393-402.
Lichtheim, L. (1885). On aphasia. Brain, 7, 433-484.
Pobric, G., Jefferies, E., & Lambon Ralph, M.A. (2007). Anterior temporal lobes mediate semantic representation: Mimicking semantic dementia by using rTMS in normal participants. PNAS, 104:20137-41.
Wernicke, C. (1874/1977). Der aphasische symptomencomplex: Eine psychologische studie auf anatomischer basis. In G. H. Eggert (Ed.), Wernicke's works on aphasia: A sourcebook and review (pp. 91-145). The Hague: Mouton.
Wernicke, C. (1885-1886/1977). Einige neuere Arbeiten ueber Aphasie. In G. H. Eggert (Ed.), Wernicke’s works on aphasia: A sourcebook and review. The Hague: Mouton.

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