Brownian thought space

Cognitive science, mostly, but more a sometimes structured random walk about things.

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Location: Rochester, United States

Chronically curious モ..

Friday, July 11, 2008


Here's a nice illusion:

Stare at the central dot in the image. 
Wait for the image to switch over to the color-inverted version. Keep staring at the dot.
Now, when the image switches back, you should see a 'normally' colored picture (the colors might be a bit washed out).
But! Look around the pic now - it is still a grayscale image!

How I think it works:
The reversed-color image causes specific (spatially congruent) bits of the retina to adapt to the color at that point. So, when the inverted-color image goes off, you get a recovery, which is the true color, and that 'paints' the grayscale :)

(Photo (c) me)

Thursday, July 10, 2008

Astrocyte Computing

The term is supposed to parallel 'Neural Computing', and is intended to work out how astrocytes in the brain are capable of doing computations. 

In the past, I've wondered about astrocytes in relation to hemodynamics in the brain. The standard (T2*) BOLD response in fMRI is primarily caused by local changes in the OxyHb:DeoxyHb ratio. The assumption has been that this ratio reflects local neural activity, through a coupling between neural activity and blood flow. Other studies have shown that the coupling is largely mediated through astrocytes. 

But do astrocytes actually perform any computations beyond just recruiting blood flow? Not entirely clear; but a new paper from the Mriganka Sur group shows that astrocytes in the ferret visual cortex have properties similar to neurons.

Here are stimulus-dependent fluorescence changes in neurons and astrocytes: Notice that the time scale is comparable- astrocytes are off by just a couple of seconds. Turns out that the astrocytes are also more sharply tuned (e.g., to orientation, in these studies).

And another really cool result: the authors add a glutamate transporter antagonist (TBOA). This has the effect of not clearing glutamate from the synaptic cleft. This causes (a) a reduction in the activity of the astrocytes and (b) an increase and prolongation in the activity of neurons due to extra glutamate in the cleft. But, look at what happens to the blood volume change (measured as an intrinsic optical signal at 546-nm): it decreases! 

I.e., although neuronal firing increases, blood volume change decreases. Of course, this is in the presence of TBOA, but it makes you wonder if there are physiological conditions under which the glu-transporters are downregulated (perhaps at a different timescale), leading to a decrease in blood-flow changes.

Of course, blood-flow and BOLD are themselves not super-directly related. But what these studies show is that hemodynamic measures might be actually capturing astrocyte activity, rather than neural activity.

It remains to be seen how much of the computation in the brain is attributable to the astrocytes. And also what 'neural network' models might really mean ;)