24/03/2009

Lexical semantics (2/4)

Let us work our way up to the next question.

Is it located at any given place in the brain?

Well, at first the answer seemed to be yes. Wernicke's area has been recognized as early as in 1874 to be associated with word meaning. Patients with lesions in this area are afflicted with a kind of condition which leaves them unable to grasp the sense of words or to produce meaningfull speech, though importantly they are essentially unimpaired in other cognitive functions. Wernicke's area is close to the auditory cortex, but the associated deficit is not limited to the oral modality and extends to written language comprehension.


But of course and as often in cognitive science, things are a great deal more complicated. Researchers now agree that semantic retrieval involves at least a network of distant regions lying in the left frontal and temporal lobes, as suggested by the picture on the right (see this 2004 brain imagery study from Noppeney et al and this Nature neuroscience review from Patterson et al -both feature authoritative collaborators).

Characterizing the neural correlates of semantics remains work in progress, and as a matter of fact entire research labs are commited to this pursuit. Let me point to one important lab at the university of Wisconsin, which should also get the credit for the artwork.

23/03/2009

Lexical semantics (1/4)

What we cognitive scientists usually mean by lexical semantics is the system responsible for storing the meaning of words. How is this system organised? How does it develop ? How does it work? And is it located anywhere in the brain in particular? How can we know? Let's start with the last question.

How can we know?
Well cognitive science uses three main classes of tools to probe the human mind: behavioral experiments, brain imagery and computational models. There is of course a continual conversation going on between the three. Behavioral experiments ask each subject in a large group to operate a simple task in controled and reproducible laboratory conditions. Responses (usually motor responses) are then treated statistically and in this way significant effects can stand out. Hypotheses are suported or disproved.

A most common behavioral experiment in lexical semantics would be semantic categorization, in which upon presentation of a word on the screen, subjects are being asked whether it belongs to one category (is this bigger than a brick?) or not. What we look at: reaction times and percent errors. You can see that the idea is to plug our measuring apparatus to the simplest and hopefully the most objective signal possible.

Brain imagery simply tries to push the logic forward: we plug our measuring apparatus directly to the brain (non-invasively of course). The brain emits all kinds of signals strong enough to be picked-up at the surface of our skulls. There are many ways to do that: EEG, or MEG, another is PET, but I guess fMRI would be the most used brain imaging technique to date. All techniques have their strengths and weaknesses, for instance they are more or less accurate as for the spatial or temporal qualities of the signal.

Once behavioral and brain imagery studies are available, computational models try to make sense of the data. Computational models can be more biologically or psychologically oriented. A good balance in this respect seems to have been found early on by connectionism, which I for one define as the minimal concession to the hardware that one should acknowledge when modeling the software: using a large number of simple units connected to one another and which can be more or less active.

The explanatory power of a computational model is, as usual, given by the number of facts it can explain relative to the number of hypotheses it makes. But explanatory power is not the whole story, one would also like a model to make predictions which could be further tested with behavioral or brain studies -we want our models to be falsifiable.

18/03/2009

14/03/2009

This just in

Ouch. Mountazer Al-Zaïdi, the Iraqi journalist famous for his wonderful shoe throwing at Bush, has been sentenced to three years in jail.. (french article).

I hope he will eventually benefit from some kind of amnesty. The guy is huge in Irak and elsewhere, and presumably the pressure of international media could help. In fact we can help right now.

It takes a lot of courage to do what he did, and three years is quite a lot of time.

13/03/2009

Links of the day

Coachsurfing seems like a nice way to travel, since one is directly plugged into the life of local people, and the network has now reached 10^6 of them.

Although..we have friends in a similar network, globalfreeloader and they have not always been exstatic about it.

Perhaps there's an equivalent community but in which one doesn't have to share one's flat..? Well sure enough there is -ahhh, how I love the 21st century!

12/03/2009

Working memory and IQ

Nice article on the times today about brain training.

The authors quickly jump to working memory, and present recent experiments with subjects whose IQ scores increase with the amount of training they get in a dual n-back task. Another group of subjects, without training, did not improve their scores.

These results are interesting, but the authors correctly point out that the control condition is not appropriate to conclude in favor of the involvment of workng memory.

09/03/2009

Sampling performance

Kutiman has done some really spectacular sampling job, exclusively based on you tube videos.
In addition to his technical mastery, he seems to be just as confortable with funk, ragga, jungle, soul etc... It is a very generous work. As for me, I especially like sample 3, at 4:36. (via Cédric)

On a totally different subject, there is also a very nice post over at "When in doubt, do", where we learn that the afore mentioned Charles Darwin was a list maker, and mind you, for not so trivial matters!

06/03/2009

Growing maps

I'm currently looking a little bit closer at growing self-organising maps for a grant proposal. These are derived from the famous Kohonen map, a kind of neural network which establishes a correspondence between a high dimensional input space and a lower dimensional one (almost always 2 dimensions, but not necessarily). Whereas soms work with a fixed number of nodes, growing soms unfold with time. This makes them useful for e.g. language acquisition modeling (especially since these are unsupervised networks).

05/03/2009

Wonderful memes

We had a splendid dinner yesterday night at the Butte-aux-cailles with some family.

The occasion was my recent PhD defense, but we also discussed tons of other subjects, especially in relation to history and art. Knowledge flows from my grand-aunt and uncle like water from a source: crystalline, distilled and enriched by years of travels. I should add that both have very impressive carreers behind them, at the Louvre for my grand-aunt and as a teacher/civil servant at the ministry of Industry for my grand-uncle.

What may have triggered it all, this time, was my not so innocent trivia that Rome, Istanbul and Paris are said to have seven hills each. After a quick count we were only able to locate 6 hills in Paris, 2 in Istanbul and 5 in Rome -well it seems Paris really has only six.

Then all bets were off, and in no time we were discussing travels, hearing about unexpected adventures, being told about grandiose peaces of architecture and to my delight simply submerged under roman history. The successive emperors Trajan, Hadrian and Marcus-Aurelius were of particular interest, and we wondered whether or not Hadrian was right to let one piece of the empire go -soon enough we agreed he was.

03/03/2009

Swimming

There is something with swimming...which I simply don't experience with any other sports.

Working out at the Gym is pretty much about what the name indicates: work. No real fun involved -except perhaps sometimes when practiced in tandem. Running is not very fun either, but at least one can enjoy the landscape and after a while a kind of meditation can take place -like a discourse between mental and physical abilities- which is really great. But only by swimming do I entirely switch off from a day of work.

This is because suddenly and for an hour or so, I don't operate in the same element anymore. Sounds have this strange distortion caused by their propagation in the liquid medium, light is diffracted, water exerts more pressure on my skin, but my weight is reduced and my balance modified -these all conspire to reset the cognitive machinery. I always come out of swimming with an open mind and clear thoughts.

Ok, off to the swimming pool! By the way, I'd like to try an endless one some day.

02/03/2009

Paris!

This is "Le petit palais", close to the "Avenue des Champs-Elysées". Let us not forget how beautiful it is, even though we spoiled parisians never wander past it anymore.


Taken by my wife, more and more inspired with her new camera!

01/03/2009

What's catastrophic forgetting?

I should describe what catastrophic forgetting means in the context of cognitive science. Well, you'd think there would be a wikipedia entry for this but there's not -should probably go and write it down myself, meanwhile here's a nice introduction paper.

Anyway, catastrophic forgetting is the phenomenon by which some neural networks completely forget past memories when exposed to a set of new ones.

Naturally, this has been of some concerns for proponents of such systems which after all aim at simulating human memory functions. Humans do not, under normal circumstances, show this behavior. However it has been suggested and pretty convincingly argued that interleaved learning could circumvent the issue, at least in feed-forward networks trained with the backpropagation rule.

There is also a more catastrophic sense in which the notion has been used, and it is to describe the complete loss of any memories, past or recent, that occurs in e.g. the Hopfield network when exposed to undue numbers of patterns (interested in reading a recent thesis on the subject?). This is a subject I find utterly fascinating, having such ramifications as the function of dream sleep and palimpsest learning. More on that later.