This commentary considers how far one can go in making inferences about functional modularity or segregation, based on the sorts of analyses used by Caplan & Waters in relation to the underlying neuronal infrastructure. Specifically an attempt is made to relate the "functionalist" approach adopted in the target article to "neuroreductionist" perspectives on the same issue

Friston, KJ

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UCL Discovery

According to C&W, sentence (1) is made up of just one proposi-tion, but sentence (2) is made up of two. The idea seems to be thatin sentence (1) the boy is simultaneously hugging the girl and thebaby (the three are squeezed into one bear hug), but in sentence(2) the boy hugs a girl and also kisses a baby (but notice he coulddo both at the same time; he could be hugging both, but kissingonly the baby). The claim that there is only one proposition in sen-tence (1) assumes that comprehenders create a particular mentalmodel for the sentence, one in which a boy hugs both a girl and ababy at the same time. The meaning of the sentence is also com-patible, though, with a situation in which the boy hugs the girl andthen he hugs the baby, in which case clearly the sentence containstwo propositions. It could even be argued that the sentence con-tains two propositions even in the simultaneous case, because therepresentation indicating the meaning of the sentence has to in-dicate somewhere what agent acted on the baby. The point hereis not that the analysis assumed by C&W is wrong but rather thattheir position requires a set of assumptions that are vulnerable tochallenge and so must be explained in more detail.I will end by repeating that I generally like the approach the au-thors take in this work. Recently we have begun examining howunimpaired undergraduates interpret unambiguous sentencessuch as The lawyer was sued by the doctor and The dentist waspulled by the tooth. We are finding striking evidence that sen-tences that are implausible by virtue of world knowledge – in theauthors’ terminology, by post-interpretive processing – areprocessed quite differently from sentences whose sensibility canbe ruled out based on feature mismatch (a tooth cannot be anagent because agency requires animacy). Even more intriguing isour finding that, if the two participants in an implausible event(e.g., The lawyers were sued by the doctor) differ on some gram-matical feature (plurality in this case), confusion occurs much lessoften than when the noun phrases cannot be distinguished with agrammatical feature. We believe the distinguishing features allowthe two noun phrases to be unambiguously bound to their appro-priate thematic roles, and as a result misinterpretations are muchless likely to occur (the two noun phrases cannot swap positions).Data of this sort provide independent and compelling evidencefor the original and creative theoretical perspective C&W presentin their target article, because they require an explanation that as-sumes a separation between processes that compute interpreta-tions and so are internal to the language system, and processes that evaluate and modify those interpretations and are thereforeexternal to the system.Modularity, segregation, and interactionsKarl J. FristonThe Wellcome Department of Cognitive Neurology, Institute of Neurology,London WC1N 3BG, England. k.friston@fil.ion.ucl.ac.ukAbstract: This commentary considers how far one can go in making in-ferences about functional modularity or segregation, based on the sorts ofanalyses used by Caplan & Waters in relation to the underlying neuronalinfrastructure. Specifically, an attempt is made to relate the “functionalist”approach adopted in the target article to “neuroreductionist” perspectiveson the same issue.Caplan & Waters (C&W) provide compelling arguments in sup-port of the notion that there is a separate working memory systemfor assigning the syntactic structure of a sentence by noting thatthere is no interaction between structural complexity and otherprocesses that require working memory. This is a nice example of a general approach to inferring something about the cognitivearchitecture of a system given only its outputs (responses) to in-puts or stimuli that can be manipulated over a number of dimen-sions. The approach is related to Sternberg’s revision of Donder’ssubtractive method (Sternberg 1969) in which the interactionsamong various processes are used to infer something about theirfunctional organization. Both depend critically on interactions,and it is this theme that will be pursued in the context of neuro-biologically motivated analyses of functional anatomy. The keyquestion, addressed by C&W, is whether the mnemonic aspects of interpretive sentence processing are modular or functionallydissociable from other (e.g., post-interpretive) aspects. This ques-tion is purely functionalist but implies the existence of distinctneuronal systems that mediate this processing. An explicit versionof the same question is “Is there functional segregation, in termsof neuronal systems, for the structural complexity of sentences?”Much is invested in the term “functional segregation” here, and it is worthwhile considering what it means.The brain appears to adhere to two principles of organization,functional segregation (Phillips et al. 1984) and the functional in-tegration of segregated systems, such as cortical areas, subareas,neuronal populations or individual cells. “Functional segregation”refers to the selective neuronal responses to specific sensorimotorattributes. Consider the cortical area V5, a motion-sensitive area(Zeki et al. 1991) that can be characterized in terms of populationresponses (as measured with fMRI or local field potentials withelectrode recordings) to visual stimuli. To demonstrate functionalspecialization for motion, one would have to show a high degreeof mutual information between V5 responses and visual motion.For this area to be functionally segregated, there is a further re-quirement that there are no responses to other attributes, such aschanges in colour. This constraint has important consequences forthe context sensitivity of speed-dependent responses: imaginethat we characterised the receptive field f(s,l) of a V5 cell as afunction of stimulus speed (s) and wavelength (l). Under segre-gation, the responses (x) over different speeds should be the samefor any two wavelengths:x 5 f(s,l1) 5 f(s,l2) 5 f(s).In short, the speed-dependent responses would be insensitive to thecolour of the stimulus. In other words, functional segregation for aspecific attribute implies that the same responses will be elicited bychanges in that attribute irrespective of the other stimulus attrib-utes (or more generally the context). Imagine that we measured theresponse of V5 to a changes in speed using long-wavelength (red)stimuli and short-wavelength (blue) stimuli. If we found a responsedifference (i.e., a context-sensitive, speed-dependent response),then functional segregation for motion per se has to be rejected andwe would infer that this area preferred red (or blue) motion (seeFig. 1 for a schematic illustration of the receptive fields implied bythese situations). This context sensitivity is simply revealed by theinteraction between speed and wavelength in predicting the re-sponse. Precisely the same construct is used by C&W. In their for-mulation the response is in terms of speed and accuracy of sentenceprocessing and the stimulus attribute of interest is structural com-plexity. They have examined the complexity-dependent responses(low vs. high) in two contexts to see if there was an interaction. Fail-ing to find an interaction allows them to assert that functional seg-regation is a sufficient model for the observed findings. The twocontexts were low and high concurrent working memory load orsubjects with low and high capacity.The above argument suggests that functional segregationshould involve a series of simple subtractions in different contextsin order to demonstrate the context-invariant nature of selectiveresponses. In functional neuroimaging this is the tenet of “con-junction analyses” (Price & Friston 1997). A conjunction is de-fined as the presence of a main effect (e.g., of structural complex-ity) in the absence of interactions (e.g., with memory load). Itwould be interesting to revisit this issue with functional neu-roimaging using a factorial design wherein structural complexity(high and low) was crossed with memory load (sub- and supra-span). It is possible that some brain regions would show functionalsegregation for complexity (as identified with a conjunction analy-sis). However, it is likely that some regions would show regionallyspecific interactions, speaking to the integration between inter-Commentary/Caplan & Waters: Working memory and sentence comprehensionBEHAVIORAL AND BRAIN SCIENCES (1999) 22:1 99pretive sentence processing and the mnemonic processes engagedby digit recall. Finding such effects would not detract from infer-ences about segregation in other brain areas but would speak tothe integration, or coordination, of segregated processing systems(Friston et al. 1996). Although, to my knowledge this experimenthas never been carried out, it seems extremely well justified by thepsychological background provided in Caplan & Waters.Interpretative and post-interpretativeprocesses in sentence comprehensionEdward Gibsona and Rose RobertsbaDepartment of Brain and Cognitive Sciences and Department of Linguisticsand Philosophy, Massachusetts Institute of Technology, Cambridge, MA02139 and; bDepartment of Brain and Cognitive Sciences, MassachusettsInstitute of Technology, Cambridge, MA 02139. gibson@psyche.mit.edurose@psyche.mit.eduAbstract: We discuss several issues raised by Caplan & Waters’s distinc-tion between interpretative and post-interpretative processes in sentencecomprehension, including the nature and properties of the two systems,problems with measuring their respective capacities, and the relationshipbetween the hypothesized separate-language-interpretation-resource(SLIR) and the general verbal working memory system that supports post-interpretive processing.Commentary/Caplan & Waters: Working memory and sentence100 BEHAVIORAL AND BRAIN SCIENCES (1999) 22:1Figure 1 (Friston). Schematic to illustrate the relationship be-tween interactions and context-sensitive responses. The upperpanel shows the response of a neuronal system to changes in someattribute to which it is sensitive. In this instance the responses areinsensitive to the context (i.e., there are no interactions betweenattribute and context. Conversely the responses in the lower panelevidence strong interactions and context-sensitivity. These re-sponse profiles can be thought of as “receptive fields.”Text removed due to third party copyright

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