Monday, October 20, 2008

Pushing needles

In my last post Now you see it, now you don't, I passed on the editor's summary of an experiment to research how the brain can combine different views of an object into a single object representation. The researchers inserted needles in the inferior temporal cortex of two monkeys and recorded neurons while the monkeys saccaded to an extra-foveal symbol while the symbol was changed during the saccade.

Reading it like that, pushing needles into the cortex sounds pretty scary, but in reality it is not. Thirty years ago a friend of mine was working for his postdoc with a troglodyte to find the pathway from the visual cortex to the arm's motoric control when a banana was presented. The troglodyte was not in discomfort and actually happily outlived Dr. Gisin, who died at a young age from a brain tumor.

Although Dr. Gisin's team developed a relation with their subject, the problem with such experiments is that we cannot really know what the monkey is thinking. At that time, it would not have been ethically possible to use humans for the experiments. However, in the 30 years since, the medical technology has progressed so much that needles are used clinically to excite neurons in patients with acute forms of diseases like Parkinson's and epilepsy.

Compared to exciting neurons, just recording from them is a minor invasion. Thus data from experiments piggy-backing on clinical procedures are starting to become available. The quantum leap with human subjects is that they can describe what they are thinking.

In Internally Generated Reactivation of Single Neurons in Human Hippocampus During Free Recall, published in Science 3 October 2008: Vol. 322. no. 5898, pp. 96 - 101, a team from UCLA and the Weizman Institute report on recordings from epileptic patients during clinical procedures. In short, they were able to determine that in the hippocampus the neurons are reactivated in the same pattern when a person sees a scene and when the person recalls the scene.

Thus, when we reason on the model of what is happening in the world outside our body, we are in the same state as when we actively scan and analyze the outside world. We cannot immediately discriminate between what we see and what we recall. This is the basis for such effects as memory color.

This is why in general, preferred color reproduction yields more pleasing images than colorimetric color reproduction. And this is also one of the things that makes color science such a challenging discipline.

Here is the Science editor's summary of the report:

The neural correlates of remembering can only be studied with complete confidence in humans, because the subjects can verbally report their internal experience. Brain surgery in which therapeutic electrodes are implanted in the brain of patients with intractable epilepsy provides an opportunity for doing such studies. Gelbard-Sagiv et al. report that neurons in and near the hippocampus of these patients showed specific patterns of activation for each episode of the television show The Simpsons. Later, when these same episodes were brought to mind by free recollection, the same pattern of neural activity was seen, demonstrating that, at least in the hippocampus, recall of a stimulus is accompanied by activation of the same neurons that were activated during the initial experience.

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