Pulvermuller = Wernicke - Lichtheim ?
A Comparison Of Brain Language Models
- Response to Hickok and Poeppel -
Hickok and Poeppel (*, **) recently attacked the neurobiological model of language my colleagues and I developed over the last 20 years (Braitenberg & Pulvermüller, 1992; Pulvermüller, 1996, 1999, 2001, 2005; Pulvermüller, Kherif, Hauk, Mohr, & Nimmo-Smith, 2009; Pulvermüller & Preissl, 1991). Their key statement is that what they call the "Pulvermuller model" is identical to the language model put forward by Wernicke and modified by Lichtheim. Their key statement is expressed in the following equation:
Pulvermuller = Lichtheim – Wernicke
Although I am honoured by the comparison of my humble work with that of such great neurologists and cognitive neuroscientists – and dare say that I am proudly wearing Lichtheim's beard on appropriate occasions since – I still feel an urge to make it clear that the equation and comparison are entirely inappropriate. Because Hickok and Poeppel's misunderstanding might have arisen from an imprecise statement on my part, I would like to point out similarities and differences between the two models below. I will also refer to Hickok and Poeppel's own model to illustrate that some of the weaknesses of classic models are still present in current theorising.
Similarities between the Pulvermuller and Lichtheim-Wernicke models
True, Lichtheim, Wernicke, myself and probably most brain-language theorists subscibe to the important status of the left perisylvian language cortex – which is best defined, as Wernicke did, as the first convolution surrounding the Sylvian fissure (Bogen & Bogen, 1976; Wernicke, 1874). True also that we believe that semantic processes are in some way related to the reactivation of memories that draw on widespread cortical areas/circuits – although the classics seem to mention only sensory areas, whereas I believe that motor, premotor and prefrontal neurons (in and outside the perisylvian areas) are relevant, too. However, here the analogy between the Wernicke-Lichtheim and Pulvermuller models ends.
Three main differences between the Pulvermuller and Lichtheim-Wernicke models
A range of differences between the "Pulvermuller model" of distributed neuronal circuits and the Wernicke-Lichtheim theory are quite obvious, especially the fact that one is a neuronal model (specifying neurons, their connections, and neuronal assemblies) also implemented in computer models (Garagnani, Shtyrov, & Pulvermüller, 2009; Garagnani, Wennekers, & Pulvermüller, 2009; Pulvermüller & Preissl, 1991; Wermter et al., 2004), whereas the other is a model of brain centres and connections. I will not discuss such obvious differences. Instead, I will highlight three critical features of the distributed neuronal circuit model setting it apart from Lichtheim's and Wernicke's models. These features are critical for explaining a range of recent – and exciting – research results the Lichtheim-Wernicke model (and many others following it) do not explain. The "Pulvermuller model" had actually predicted these results in the first place (e.g., Pulvermüller, 1999).
The Pulvermuller model posits
- Functional interdependence between inferiorfrontal and superiortemporal circuits in speech production and perception
- Category-specific semantic circuits with specific cortical distributions, and
- Linguistic circuits distributed over both hemispheres.
1. Inferiorfrontal and superiortemporal circuits are functionally interdependent in speech production and perception
My colleagues and I once claimed functional inter-dependence of neuronal assemblies in inferiorfrontal and superiortemporal language areas (Braitenberg & Pulvermüller, 1992; Braitenberg & Schüz, 1992), BECAUSE this interdependence follows from a neurobiological principle, that of Hebbian correlation learning, and the presence of strong reciprocal connections between inferiorfrontal and superiortemporal areas. Frontotemporal interdependence is critical for explaining the multimodal character of aphasias along with double dissociations between aphasia syndromes (Pulvermüller & Preissl, 1991). Meanwhile, additional experimental evidence has confirmed that the two sets of areas are indeed functionally interdependent in speech processing (Fadiga, Craighero, Buccino, & Rizzolatti, 2002; Meister, Wilson, Deblieck, Wu, & Iacoboni, 2007; Paus, Perry, Zatorre, Worsley, & Evans, 1996; Pulvermüller et al., 2006; Pulvermüller, Shtyrov, & Ilmoniemi, 2003; Wilson, Saygin, Sereno, & Iacoboni, 2004; Zatorre, Evans, Meyer, & Gjedde, 1992). Especially, magnetic stimulation of the motor cortex – specifically tongue and lip motor areas – led to better processing of the concordant phonemes, but impaired processing of discordant ones. So, surprisingly, when the lip area was stimulated, subjects tended to misperceive "t" as "p", and, vice versa, lip area stimulation led to bias towards "t" perception (Fazio et al., 2009). One reason why these results were so surprising is the fact that they contradicted old believes grounded in the Lichtheim-Wernicke type. Note that Lichtheim (following Broca) had written that "understanding of spoken words" is preserved in motor or Broca's aphasia (related to inferiorfrontal lesions), a view also Dres. Hickok and Poeppel seem to share. However, this position is difficult to reconcile with much clinical evidence (e.g., Basso, Casati, & Vignolo, 1977; Moineau, Dronkers, & Bates, 2005; Yee, Blumstein, & Sedivy, 2008) and, importantly, the recent TMS results demonstrate the influence of inferiorfrontal/premotor neuronal activity on speech perception. Functional changes in the motor system do have a profound – and specific and actually linguistically interesting – effect on speech perception and understanding. The "Pulvermuller model" neurocomputational model by Pulvermüller and Preissl had predicted this outcome (see, for example, Pulvermüller & Preissl, 1991).
We have discussed the relationship between the "Pulvermuller model" and other theories explaining a motor influence on speech perception elsewhere (D'Ausilio et al., 2009; Pulvermüller et al., 2006). Interestingly, in a recent paper Dr. Hickok and his colleagues chose to reiterate a number of points we had made about limitations of the motor theory of speech perception (see Lotto, Hickok, & Holt, 2009; see Pulvermüller et al., 2006). However, they did not discuss the neurobiological circuit model, the "Pulvermuller model", which is consistent with the data.
2. Semantic circuits in the brain are category-specific and have specific cortical topographies
The distributed circuit model further claims functional interdependence of perisylvian language cortex on the one hand and sensorimotor and multimodal areas on the other in the processing of signs and their semantically related concepts. This explicit neuronal version of a sensorimotor account of category-specific semantic processes explains why a lesion in inferiortemporal higher-visual cortices or frontocentral motor systems can, respectively, degrade specifically the processing of action or visually-related words (Damasio & Tranel, 1993; Daniele, Giustolisi, Silveri, Colosimo, & Gainotti, 1994; Gainotti, 2006; Neininger & Pulvermüller, 2003; Shallice, 1988; Warrington & McCarthy, 1983, 1987; Warrington & Shallice, 1984), why motor neuron disease impairs action verb and action concept processing (Bak, O'Donovan, Xuereb, Boniface, & Hodges, 2001), why magnetic stimulation of the motor system differentially affects the processing of arm and leg related words (Pulvermüller, Hauk, Nikulin, & Ilmoniemi, 2005) and why arm and leg motor areas "light up" instantaneously when we understand a word like "pick" or "kick" (Aziz-Zadeh, Wilson, Rizzolatti, & Iacoboni, 2006; Hauk, Johnsrude, & Pulvermüller, 2004; Hauk & Pulvermüller, 2004; Pulvermüller, Härle, & Hummel, 2000; Shtyrov, Hauk, & Pulvermüller, 2004) or a concrete or abstract sentence including it (Boulenger, Hauk, & Pulvermüller, 2009; Buccino et al., 2005; Tettamanti et al., 2005). Note that a range of unexpected and therefore exciting new results had been predicted a priori on the basis of the distributed circuit model (Pulvermüller, 1999, 2005), although we recently found that the semantic circuits are even more far-scattered than previously thought (Pulvermüller, Kherif, Hauk, Mohr, & Nimmo-Smith, 2009).
The Wernicke model and equally Lichtheim's proposal do indeed claim some kind of functional relationship between their language centres and sensory processes in other areas. However, these models lack one important feature in this respect, namely specificity. Why would arm words spark the lateral motor cortex, leg words dorsal motor sites, colour and form words different parts of inferiortemporal cortex and olfactory words olfactory brain regions (Gonzalez et al., 2006)? Because there are lexicosemantic cell assemblies WITH SPECIFIC CORTICAL TOPOGRPHIES driven by action-perception information (Pulvermüller, 2005). Once again, the Pulvermuller model had predicted this. Lichtheim's model – and equally Hickok and Poeppel's – would require revision to provide accounts for these data.
A final point on the relationship between sign and meaning: We assume that the semantic parts of distributed lexicosemantic circuits are critical for their activation. This leads to a (very specific) version of a sensorimotor account of word-category deficits after lesions outside the perisylvian language areas (for recent evidence, see Pulvermüller et al., 2009). Lesions to Lichtheim's hypothetical concept centre "B" (Begriffszentrum) or in sensory brain areas were not assumed to lead to language deficits (for related points, see Caplan, 1987; for related points, see Freud, 1891).
3. Linguistic circuits are distributed over both hemispheres
Language areas were exclusively left-hemispheric in Lichtheim's model. It was later found that also the right hemisphere may include a lexicon of its own (Zaidel, 1990) and that the two lexica actually cooperate, help each other, in lexical processing (Mohr, Pulvermüller, Rayman, & Zaidel, 1994; Mohr, Pulvermüller, & Zaidel, 1994). This finding led to the proposal that word-related (lexical) cell assemblies are strongly lateralised circuits involving neurons in both left and right perisylvian cortex; in contrast, semantic circuits were assumed to be symmetric or lateralised to a lesser degree (Pulvermüller, 1999; Pulvermüller & Mohr, 1996). The proposal that language-related circuits are distributed over both hemispheres and that there are phonological, lexical, morphosyntactic and semantic representations for words with different degrees of laterality (for recent fMRI evidence, see Pulvermüller, Kherif, Hauk, Mohr, & Nimmo-Smith, 2009) was, as to the best of my knowledge, not part of the Lichtheim-Wernicke proposal, although it was later adopted by others – notably by Hickok and Poeppel (Hickok & Poeppel, 2000, 2004, 2007).
This is not an exhaustive list of differences between the Lichtheim and Pulvermuller models. But the three differences mentioned seem fundamental enough to make evident that Hickok and Poeppel's equation "Pulvermuller = Lichtheim-Wernicke" is false.
At best unclear ...
A general comment on Poeppel's exegesis of Lichtheim's work (**): I do not agree with a number of his points, which I skip for brevity. As Caplan pointed out in his excellent textbook, a major deficit of Lichtheim's paper in 1885 is that major features of the model are "at best unclear and at worst contradictory". One of these features is the direction in which activity is allowed to travel along pathways, and this directly impacts on the model's inability to explain speech perception and comprehension deficits in motor or Broca's aphasia: To explain speech production deficits in "sensorial" or Wernicke's aphasia, activation flow from motor to acoustic speech centres is allowed (an new assumption added to the model, which seems somewhat ad hoc, or "forced", as the author himself indeed notes, p. 439), but discussion of the corresponding reciprocal (motor-to-acoustic) activation is not mentioned when it comes to the explanation of Broca's aphasia. A more detailed treatment of achievements and shortcomings of Wernicke's and Lichtheim's models can be found in Caplan's seminal work (Chapters 4 and 6, Caplan, 1987).
A problem with Lichtheim's paper I saw, and still see, emerges from the use of the term "centre" in the context of semantic processing (Lichtheim, 1885a, 1885b). In my view, this term suggests that there is indeed one local module for meaning. (Note that the paper was written at a time when a dominant view in neurology still held that higher functions cannot be localised; the evidence for functionally specific local *centres* such as speech was still relatively new.) Lichtheim indeed bows towards Wernicke later-on in his article, saying that concepts are distributed across "the whole sensorial sphere" (p. 477). In my view, this statement is difficult to reconcile with the use of the term "centre". Are we looking at one more example where Lichtheim's paper is "at best unclear and at worst contradictory"?
Be this as it may. I still believe that some of my colleagues may have taken the Begriffszentrum, or concept area, quite literally, adopting it in their own thinking. Actually, Hickok and Poeppel are a nice example here. They postulate an area which is similar in function to a Begriffszentrum, which they label as "lexical interface" or "sound meaning interface". If there is such a privileged place in our brains for word-to-meaning binding – why should it be in one particular place (in the middle of the temporal lobe)? Connections between the perisylvian cortex (also the posterior temporal cortex) and the sensory cortices run through a range of areas and fibre bundles and it is, in my view, *not* plausible to assume a *single* binding site. Neuroanatomical data indicate that there are multiple binding sites – or convergence zones as Antonio Damasio once called them (Damasio, 1989; Damasio, Grabowski, Tranel, Hichwa, & Damasio, 1996). Because the entire cortex appears to be a huge associative memory (Braitenberg & Schüz, 1998), all areas, even the auditory, motor and olfactory cortices – can function as binding sites for specific combinations of lexical and semantic information.
In sum, I highlighted three features that distinguish the "Pulvermuller model" from that by Wernicke-Lichtheim:
- Frontotemporal interdependence: inferiorfrontal language production circuits and superiortemporal speech perception circuits depend on each other functionally,
- Semantic topography: the meaning of words is carried by neuronal assemblies whose cortical distribution indexes aspects of the words' meaning, and
- Differential laterality: phonological, lexical, morphological, syntactic and semantic circuits are distributed over both hemispheres with different degrees of laterality.
The other differences (coverage of syntax, implementation in explicit neural simulation etc.) are so obvious that discussion is probably redundant (Garagnani, Shtyrov, & Pulvermüller, 2009; Pulvermüller, 2003).
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