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Archive for the ‘language’ Category

broca.jpgNature is running a nice news article on the re-localization of Broca’s language area in the brain, and has as feature about it in their latest podcast.

Pierre Paul Broca originally described patient cases in which the patient suffered speech production deficits following injury to the left frontal hemisphere. However, a revisit to Broca’s original papers (see translations here and here), combined with a modern scanning of the preserved remains of Broca’s patients, has revealed that what has been called Broca’s area in modern times does not correspond to the areas implicated by Broca in his patient descriptions and neuroanatomical descriptions.

The story is interesting, but I’m amazed that the excitement is running so high. After all, lots of papers have already dethroned Broca’s (and Wernicke’s) area in the role of language processing. Take the example of the special issue of Cognition on language. Basically, what we know about language in the brain is beyond the talk (!) about Broca and Wernicke, and especially the models they suggested. Rather, both language comprehension and production require a larger neural symphony, and with substantial internal redundancy. IOW, Broca’s area can participate in comprehension, and Wernicke can play a part in production.

Nevertheless, the Nature news article is a good read, and I always recommend the Nature podcast.

-Thomas

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In an interesting paper in the latest version of Progress in Neurobiology, Yuri I. Arshavsky from UCSD writes about the epistemological dualism that exists in modern neuroscience. basically, Arshavsky claims that there is a covert dualism in the way that neuroscientists are treating mind-related topics, especially the study of “consciousness”. Indeed, as he claims:

This covert dualism seems to be rooted in the main paradigm of neuroscience that suggests that cognitive functions, such as language production and comprehension, face recognition, declarative memory, emotions, etc., are performed by neural networks consisting of simple elements.

This might initially sound a bit strange. Is not cognitive functions such as face perception due to operational simple elements? Face perception as such is a combination of many simple processes that operate in unison. So what is Arshavsky proposing? Indeed he suggests the existence of a certain kind of brain cells:

(The) performance of cognitive functions is based on complex cooperative activity of “complex” neurons that are carriers of “elementary cognition.” The uniqueness of human cognitive functions, which has a genetic basis, is determined by the specificity of genes expressed by these “complex” neurons. The main goal of the review is to show that the identification of the genes implicated in cognitive functions and the understanding of a functional role of their products is a possible way to overcome covert dualism in neuroscience.

So there should exist a subset of neurons that integrate information from a variety of input. This sounds strange, since all neurons integrate inputs from thousands of inputs, many from a large variety of inputs. So what are complex neurons? Here, we are told that:

(…) neural networks involved in performing cognitive functions are formed not by simple neurons whose function is limited to the generation of electrical potentials and transmission of signals to other neurons, but by complex neurons that can be regarded as carriers of “elementary” cognition. The performance of cognitive functions is based on the cooperative activity of this type of complex neurons.

In this way, complex neurons seem to be integrative neurons, i.e. cells that integrate information from a variety of processes. This could include the multi-modal neurons found in the functional sub-structures of the medial temporal lobe, such as the hippocampus, perirhinal, entorhinal and temporopolar cortex. But would it not mean the colour processing nodes in the visual cortex? Which IMO leads us back to a basic question: what is a functional unit in the brain. yes, the neuron is a basic building block of information processing in the brain. But what is special about language, memory and so forth in the brain?

It is possible that Arshavsky is not radical enough: what we should seek out is to avoid using generalistic and folk-psychological concepts in the first place. We should possibly not study “language”, “memory” or “consciousness”, since these concepts will always allude to fundamental assumptions of “language-ness”, “memory-ness” and “consciousness-ness”, IOW that there is something more to explain after we have found out how the brain produces what we recognize and label a cognitive function.

Maybe neuroscientists are not using a poor strategy after all? Maybe ignoring the past history of philosophy of mind is the best solution. I’m not sure (nor am I sure that I represent Arshavsky’s view properly). But how we choose to label a cognitive function depend on our past historical influence and learning, as well as our current approach.

-Thomas

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In a thorough and very good review in PLoS Genetics, James Sikela writes about the comparative genetics between the chimp and the human genome. From the article:

It has been pointed out that the primary molecular mechanisms underlying genome evolution are 1) single nucleotide polymorphisms, 2) gene/segmental duplications, and 3) genome rearrangement. In addition, a “less-is-more” hypothesis has been proposed that argues loss of genetic material may also be a source of evolutionary change. Given these factors, what are we learning about their respective roles now that we can compare multiple primate genome sequences?

As we have pointed out repeatedly in this blog, the study of genetic influence on the brain is going to change our understanding of what a human is, how and why our thoughts are formed (and restricted) the way they are. This review surely puts the finger on the pulse and notes:

One of the most important findings to emerge from the latest human and primate genome-wide studies is that a fundamental assumption underlying this model has changed: the interspecies genomic changes are numerous and diverse, and, as a result, there appear to be many additional types of genomic mechanisms and features that could also be important to the evolution of lineage-specific traits. Given this new perspective, we now know that the degree of difference between our genome and that of the chimp depends on where, and how comprehensively, we look. The multitude of genomic differences that we now know exists should provide an abundance of fertile genomic ground from which important lineage-specific phenotypes, such as enhanced cognition, could have emerged. 

Here is the abstract:

The jewels of our genome: the search for the genomic changes underlying the evolutionary unique capacities of the human brain

James M. Sikela

The recent publication of the initial sequence and analysis of the chimp genome allows us, for the first time, to compare our genome with that of our closest living evolutionary relative. With more primate genome sequences being pursued, and with other genome-wide, cross-species comparative techniques emerging, we are entering an era in which we will be able to carry out genomic comparisons of unprecedented scope and detail. These studies should yield a bounty of new insights about the genes and genomic features that are unique to our species as well as those that are unique to other primate lineages, and may begin to causally link some of these to lineage-specific phenotypic characteristics. The most intriguing potential of these new approaches will be in the area of evolutionary neurogenomics and in the possibility that the key human lineage–specific (HLS) genomic changes that underlie the evolution of the human brain will be identified. Such new knowledge should provide fresh insights into neuronal development and higher cognitive function and dysfunction, and may possibly uncover biological mechanisms for information storage, analysis, and retrieval never previously seen.

-Thomas 

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John Hawks has a very good discussion about this article. 

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In this week’s Nature a report from Kate Arnold and Klaus Zuberbühler from the Scottish Primate Research Group demonstrates that the putty-nosed guenon can combine vocalizations in order to convey different meaning. The meaning is is found between alarm calls for different situations and contexts.

From the article:

Like most forest guenons, male putty-nosed monkeys (Cercopithecus nictitans) produce two acoustically distinct loud calls (‘pyows’ and ‘hacks’) in response to a range of disturbances. Males also call spontaneously, especially during morning foraging and evening travel to sleeping sites. In addition, these calls can function as alarm calls to warn the group of an approaching predator and to discourage attack: pyows are used primarily when a leopard (Panthera pardus) is in the vicinity, and hacks are produced mainly in response to crowned eagles.

In addition to these calls, the researchers found that males often combine the same two calls into a third structure, a ‘pyow–hack’ (or P–H) sequence. The P-H sequence was emitted either to threats of leopards or eagles. In response to these calls the group of guenons respond by beginning to move. So, calls for either P or H did not significantly get the group moving. But P-H calls did.
Arnold and Zuberbühler went further on by playing different sounds to the groups (there were 17 guenon groups studied in all). Each group consisted of several adult females and one male (…hmm…).First, leopard growls were played to the group. In over half of the groups the male responded with P-H calls. In the other groups the males did call, though not with the P-H sequence.

Now wait a minute! Is this really something worth reporting in Nature? Evoking specific alarm calls in only half of the groups is not normally seen as powerful statistics. But hold on; here’s the neat part of this study. The researchers used a GPS to locate where the groups were as they moved (they are easy to locate by the calls, then move to that location and look up the GPS location). Here it was found that the groups in which males expressed the P-H combination moved for significatly longer distances. Furthermore, the groups “response to P–H sequences was not confined to the predator context, but was generally related to whether the group moved”.

In other words, this study demonstrates that not only do these putty-nosed guenons display differential vocal expressions (in males) to threats. The expressions are interpreted differently according to the context (movement). Altough it is known that syntax sets human language apart from other natural communication systems, it is also agreed that the evolutionary origins of human language are obscure. The study by Arnold and Zuberbühler sheds light on the evolutionary building blocks of language.

If you have access to Nature, you can also listen to the calls (single or combined form) through the supplementary material.

-Thomas

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Let me point you to a nice article, by Simon Fisher and Gary Marcus, in the January issue of Nature Reviews Genetics. Fisher was one of the co-discovers of the link between a verbal dyspraxia disorder in an English family and a point mutation on the FOXP2 gene. Marcus, a former student of Steven Pinker, is well-known for his critique of connectionist models of language-aquisition. Although I personally think it overstates the case for innate modules, I wholeheartedly recommend Marcus' book The Birth of the Mind which is, as far as I know, the first, and still only, popular account of what is known today about the genetic basis of brain development. Together, they have earlier written a review of the FOXP2 story in Trends in Cognitive Science. The present paper aims to show how genetical studies can aid our understading of the evolution of language.

In recent years interest in this topic has been booming. A large number of possible adaptative causes have been proposed by a range of researchers. [Language Evolution, edited by Morten Christiansen and Simon Kirby, will give you a short overview of most of these hypotheses.] There is a problem with this approach, though, in that it tends to focus on just one magical change in hominid behaviour, with the accompaying change to the brain being passed on to following generations. To take one prominent example, Robin Dunbar, for instance, sees the decisive moment in the evolution of language to be the expansion of hominid social groups, some time around the appearance of homo erectus, making it advantageous to be able to keep account of the larger number of conspecifics through communication. Dunbar speculates that a change in our ability to mentalize – sometimes also referred to as Theory of Mind – could be the new cognitive function to have facilitated this improved capacity for social communication. Another, more simpleminded hypothesis, is the still widespread idea that neuronal changes to Broca's area brought about a novel ability to string together words into syntactical phrases.

But, as more and more becomes known about the brain processes underlying language, it appears increasingly unrealistic that any such single-stroke wave of the magic wand will suffice to explain the emergence of language. The neural language system is enormously complex and encompasses, to just name a few things, auditory analysis, conceptual knowledge and memory, semantic selection processes, motor control, and a great number of other functions. It stands to reason that all these processes must have evolved on an individual basis. It is therefore much more probable that the evolution of language has gone through several different adaptive events since the last common forefather of homo and pan.

This is exactly the approach taken by Fisher and Marcus. They call it "descent with modification", and write that "[in this paper] we argue that language should be viewed not as a wholesale innovation, but as a complex reconfiguration of ancestral system that have been adapted in evolutionarily novel ways". The offshot of this approach is that the evolution of language can be studied by comparing human genetics and neurobiology to other species. Write Fisher and Marcus:

(…) although non-human primate communication shows qualitative differences from human language, studies have established that most components of language how some degree of continuity with other species. For example, the human vocal tract supports a wider epertoire of speech sounds than could be produced by other primates, but the capacity to create richly modulated formants is not unique to humans. Likewise, many animals and birds can distinguish different human speech sounds, and adult tamarin monkeys can discriminate between the distinctive rhythmic properties of different languages. Debate continues about exactly how much of the machinery of language is species – or language – specific; for example, opinion is divided over whether recursion represents the only component that is genuinely new to the human species. Nevertheless, views that consider language to be fully independent of ancestral systems are no longer tenable, and there is a growing recognition that cognitive, physiological, neuroanatomical and genetic data from non-speaking species can greatly inform our understanding of the nature and evolution of language.

The bulk of Fisher and Marcus' paper is dedicated to a review of methods for such genetical comparisons of humans to other species. But its true importance lies in the way it dismantles the adaptionist programme in evolutionary psychology. Like language, social cognition, reasoning, and other forms of cognition, are in most cases complex amalgams of a range of neurobiological processes, and therefore very unlikely the product of "wholesale innovation". A more precise picture of the evolution of human cognition is to be found through an detailed understanding of how the human brain differs from other species, and through a comparison of the genes and gene-expression systems characterising these species.
Reference

Fiser, S. & Marcus, G. (2006): The eloquent ape: genes, brains and the evolution of language. Nature Reviews Genetics 7: 9-20. [The paper can be downloaded from Marcus's homepage.]

-Martin

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The visual word form area (VWFA) is one of the most contested concepts of modern neuroscience. Its proponents claim that a dedicated slice of cortex in the occipitotemporal region of the brain – probably centered on the fusiform gyrus – underlie the ability to read. The most radical version of this hypothesis states that the VWFA is a specialized module – an idea that implicates some sort of genetical determinism. Since writing only has been around for some 6000 years many researchers doubt that there have been enough time for such a module to evolve. Instead, they expect reading to piggybackride on other, perhaps more general, visual processes.

The April 20 issue of Neuron contains a case study of man surgically treated for epilepsy that bears on these questions. As a consequence of excision of cortical tissue posterior to the putative VWFA this patient developed a case of pure alexia where, whereas before surgery he took ~600 ms to read a word, regardless of word lenght, after surgery his reading slowed to ~1000 ms for three-letter words and increased by ~100 ms per additional letter.

Crucially, postsurgical fMRI tests showed that the VWFA no longer responded more to words than to other objects, even when words were contrasted with viewing a simple fixation point. The authors suggest that this lack of activation is explained by the fact that postsurgically the VWFA is no longer receiving its normal input. Instead, a more widespread activation pattern, including frontal, parietal, and temporal sites, appear to "take over", resulting in a letter-by-letter reading strategy.

While the study clearly demonstrates that the VWFA plays some kind of (important) role in reading, it doesn't really illuminate what type of function the VWFA performs. And, at the same time, it cannot be said to settle the question of modularity, since the patient was a 46-old man who, of course, has had amble time to develop a reading skill from learning. As Alex Martin notes in a great accompanying resumé, the study do, however, leaves unresolved "the vexing problem of how to account for the intersubject consistency in the general location of the VWFA and other category-related regions in ventral occipitotemporal cortex. One possibility is that the VWFA performs a visual processing function that predisposed it to being co-opted for reading." But there may also be other reasons. In any case, it is a very nice study that will prove important for further debates on both reading and modularity.

Reference

Gaillard, R. et al. (2006): Direct intracranial, fMRI, and lesion evidence for the causal role of left inferotemporal cortex in reding. Neuron 50: 191-204.

Martin, A. (2006): Shades of Déjerine – forging a causal link between the visual word form area and reading. Neuron 50: 173-175.

-Martin

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There is now an online-only published paper in PNAS from the Max Planck Institute on the evolution of language. What is surprising is that the researchers have used functional MRI to infer the evolutionary lineage from their results. Basically, what Angela Friederici and her colleagues have done is to compare language processing that is “simple” to processing that is “complex”. While simple processing activated left frontal operculum, a phylogenetically older region of the brain, more complex language processing also activated Broca’s area, which is thought to be a more recent development specific to humans. in addition, the researchers also studied the white matter connectivity of the two brain regions by using MR tractography. Here, they found that the two regions showed different structural connectivity signatures, further supporting the functional segregation of these two areas.

This makes the researchers conclude:
“Here we report findings pointing toward an evolutionary trajectory with respect to the computation of sequences, from processing simple probabilities to computing hierarchical structures, with the latter recruiting Broca’s area, a cortical region that is phylogenetically younger than the frontal operculum, the brain region dealing with the processing of transitional probabilities”I first found this through the Max Planck Society press release page. Just reflecting briefly on this, I think that despite the study is interesting itself in terms of functional segregation of language processes, I am not convinced about the argument about the phylogeny of the two regions. As we know from research on subcortical structures such as the “limbic system“, we cannot divide between the phylogenetic “old” and limbic brain and the “newer” cortical brain. It is today considered total gibberis, because evolution of “higher” areas in the cortical surface has had a dynamic and synergetic co-evolution of cortical and subcortical areas. In similar vein, I suspect that the evolutionary trajectories of the frontal operculum and Broca’s area share a lot, and that a clear-cut division between the two areas will prove hard to make.

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