Thursday, May 18, 2006

Interpolation of things past   posted by Coffee Mug @ 5/18/2006 04:13:00 PM

Saw a talk yesterday by a guy named David M. Eagleman. He’s down at UT-Screwston. He is involved in that tricky cognitive neuropsych / philosophical area that you can talk and write about for a long time and end up with a more convoluted version of the same idea you had to begin with. Apparently worked with Crick some before he passed away. This stuff isn’t exactly in my area, but it’s kind of interesting, so I will give you the rundown of what I understood.

How is it that we integrate information from all these different sensory modalities into a coherent continuous conscious experience? It’s bad enough that sound is processed in a different place in the brain than vision, but also the transmission and processing speeds for these two different modalities are different. You’d think our sensory experience would be smeared, but it’s not. In fact we can tell which event of two came first even when they are in the milli- to micro-second range.

Eagleman points out that of course its very important for us to know which came first because that’s how we determine causality. His example is that when you’re walking in the woods you might have cause to be anxious if you hear a twig snap before your foot hits the ground. In fact, it appears that motor interaction with the world is one of the best ways to calibrate the system.

We’re trying to pull off this incredibly important task with shoddy equipment. We simply can’t expect visual and motor information to show up at the same time. We can’t even expect events in the same sensory modality to show up at the processing centers at the same time. For instance, cells in one of the first stopover points for visual information (the lateral geniculate nucleus of the thalamus) respond at different times to different-intensity light flashes. The first response is some 80 milliseconds slower for dim flashes than for bright ones.

So our integrated subjective experience has to live in the past a little bit. Eagleman calls this post-diction. Stuff from after a salient event is integrated into the perception of the earlier event. He demonstrates this just within the visual system using a temporal illusion called the flash-lag effect.

You can view some movies and get his explanation here. In fact, check out his whole webpage if you’re interested in this stuff. He was a literature major in undergrad, which I guess translates to some facility with communication. Anyway. The flash-lag effect is this: When a flash of light is shone in the middle of a ring that you are watching, its perceived relation to the ring depends on where the ring is directly after the flash. It seems we if we scootch our perception of the ring movement forward in time to map onto the flash better or vice versa. Stuff from ~80 ms after the flash is projected back in perceptual time, and yes he does find it interesting that the number ‘80’ shows up here and in a number of other cognitive timing paradigms.

He talked about a lot of other experiments and illusions dealing with how the system works intra-modally versus cross-modally and how intentional motor acts are good markers for temporal integration because we just did something and dammit! we expect something to happen. He even had some fMRI data from during the perception of these temporal illusions, but it was still under review, so I doubt he wants be blurting it out across the blogosphere.

The basic idea is here though. We are forced to balance between our need to have the highest temporal resolution possible to get causality right and the fact that our equipment just isn’t really up to snuff. So we do the best we can with what we got and it works pretty well blurring the world in little 80 millisecond chunks until some meddling cognitive scientist comes along and mucks things up.

Here’s a review from late last year for free in Journal of Neuroscience.