April 14, 2014

On the emergence of visual cortex receptive field properties

I have always found the story of the development of our understanding of the visual cortex fascinating. Within a span of four decades we went from "The visual neurons do what?" to "That's exactly what we would do if we were to engineer a visual system." to "Look: if I show some random network some random images, it self organizes exactly in that way!" Successful scientific theories seem to end up showing us the inevitability of what we observe: "Oh, it had to be that way!" The trick is to come up with the right explanation (a la David Deutsch) for why things are the way they are...

The light sensitive neurons of the retina (120 million rods and 6 million cones) pass their electrical signals through several intermediate layers to ganglion cells, which extend about 1.5 million cell fibers (the optic nerve) into the brain for further processing. Passing through the LGN in mid-brain, the signals end up in the visual cortex at the back where their processing eventually lead to our perception of visual objects and events.

Now, the rods and cones fire when light falls on them, they are simple photoreceptors. Ganglions, on the other hand, receive inputs from a large number of photoreceptors, so we'd like to know their receptive field. The receptive field of a ganglion is the region of the retina that effects its firing when stimulated. Kuffler (1953) showed that cat ganglion cells have a receptive field with a "center surround" pattern:

Note that these cells will respond strongly to a patch of light with the right size and location, but unlike rods and cones, they will not respond strongly to a uniformly bright surface because the excitatory (+) and the inhibitory (-) areas will approximately cancel out.

Hubel and Wiesel (1959) went further and recorded from the neurons of the cat visual cortex to identify their receptive fields. Their attempts at trying to elicit response from cortical neurons with spots of light were unsuccessful in the beginning. A couple of hours into the experiment they accidentally discovered a neuron that "went off like a machine gun" when inserting a glass slide into the projector. It turned out the neuron liked the straight edge of the slide that was moving on the screen. They discovered cortical neurons (which they called simple cells) respond best not to spots, but to bars of light oriented in a particular direction.

So far we have covered the "what?" part of our story. In the following decades research on computer vision took off at the MIT AI Lab and elsewhere. The engineering approach of trying to build systems that could actually see helped us understand why nature may have "designed" the receptive fields the way it has. For example Marr and Hildreth (1980) argue that the center surround receptive fields can be thought of as convolution with the second derivative of a Gaussian and the simple cells detect the zero-crossings of this convolution which facilitate edge detection.

Engineering approaches may answer the "why" but still leave open the question of how neurons know to connect in this intricate pattern. Embryo development is still not completely understood and while it may be reasonable that the same developmental processes that can build "fingers and toes" may create some functional regions in the brain, it is probably not reasonable to expect instructions for neuron #134267542 to connect to neuron # 49726845. At this point in our story came Olshausen and Field (1996) who showed that if you train a neural network with patches of natural images and bias it to preserve information and promote sparseness, you automagically get receptive fields similar to those of Hubel and Wiesel's simple cells:

So the poor neurons had no choice in the matter. It turns out to be inevitable that under some simple conditions random networks exposed to natural images would connect up in a way that an engineer would want them to connect and perform functions similar to that of the neurons in mammalian brains!

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April 12, 2014

Our Mathematical Universe by Max Tegmark

Consider a computer simulation of a mini universe, a universe complex enough to give rise to sentient beings. As Greg Egan points out in Permutation City, (and as is obvious to anybody who has played The Sims), the subjective flow of time in the simulated universe could be many times faster or slower than ours. In fact, once we have the simulation going, we can fast forward, rewind back, play the frames out of order, and it wouldn't matter one bit to the subjective experience of the simulated folk. What if we stopped the simulation? What if we saved the whole history on a DVD and put it on a shelf? Again, the subjective experience of the simulated, which arise from the relationships between their successive moments, would be uneffected. We would just be missing the opportunity to observe them in action, and interact with them.

That brings us to the next natural question: do they need the simulation, the DVD, do they need us at all? Don't they just "exist" independently whether or not we happen to be watching, recording, interacting or anybody has ever thought of them? Max Tegmark advances the Mathematical Universe Hypothesis, which says (1) our universe at the most basic level IS (not just described by) a mathematical structure (an electron or a photon can be defined exactly by a handful of numbers and there is nothing left over), and that (2) any mathematically consistent (computable?) structure must exist in the same sense as we do!

Needless to say this has led to quite a bit of spirited discussion around the web, I would recommend Scott Aaronson's blog post and the comments therein.

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April 06, 2014

Monument Valley

For all of you Escher fans out there...

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