Waterfalls, adaptation and light

Firstly, you’ll have to excuse the somewhat ‘in house’ nature of this post, as it’s me writing about Christian writing about Tom. It’s an account of Tom giving an address to the Association for the Teaching of Psychology where he conducted a fantastic demonstration of how you can test out whether your brain adapts to certain visual conditions ‘locally’ on an eye-by-eye basis, or ‘centrally’ in eye independent perceptual brain areas.

Moments into the keynote talk, the teachers and I found ourselves blinded by darkness. As our eyes adjusted, we were told to cover one eye with our hands before the lights were raised again. A little wait for our open eyes to become light-adjusted and then the lights re-dimmed. What would happen to our vision this time? The answer depends on whether adaptation to light levels occurs centrally, in the brain, or locally in each eye. The audience tested this, looking through each eye one at a time and discovering the strange experience of having one eye adapted to the light and one to the dark, thus showing that light adaptation occurs locally. Both eyes open led to a strange, grey, grainy, effect. ‚ÄúWhoever said psychology isn’t useful is wrong,‚Äù Stafford said. ‚ÄúYou now have the perfect strategy for visiting the toilet in the night and finding your way back to your bed in the dark.‚Äù

Light adaptation may well occur locally, but what about adaptation to motion? A huge video of a waterfall filled the screen. After a minute staring at the cascading water, the video was stopped and the audience experienced the well-known illusion of the water appearing to flow upwards. But what if the flowing water was watched with just one eye (with the other covered), with the paused video then observed through the previously covered eye? The illusion was still experienced, thus showing that in this case, adaptation to motion had occurred centrally, in the brain.

If you don’t have a waterfall handy, you may be interested to know it’s a form of ‘motion after effect‘ illusion and there’s a similar demonstration online that you can try. If you go to that link, click ‘detach’ and resize the window to get a bigger version.

You’ll need to supply the room and light yourself though. The hall full of teachers is optional.

Link to BPSRD on visual adaptation.
Link to motion after effect example.

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