Hack 26. Get Adjusted
2.15.2. Why It Works
Adaptation operates for a perceptual purpose, rather than being a reflection of neural fatigue or being a side effect of some kind of long-term memory phenomenon. It seems to be that sensory systems contain an intrinsic and ongoing mechanism for correcting drift in the performance of components of the system. Constant levels of input are an indication that either some part of the neural machinery has gone wrong and is over-responding, or at the least the input isn’t as relevant as other stuff and should be canceled out of your sensory processing to allow you to perceive variations around the new baseline.
This relates to the idea of channel decorrelation1that sensory channels, as far as possible, should be providing independent evidence, not correlated evidence about the world. If the input is correlated, then it isn’t adding any extra information, and large, constant, moving stimuli create a load of correlation, across visual space and across time, among the neurons responsible for responding to motion.
Not all cells adapt to all stimuli. Most subcortical sensory neurons don’t adapt.2 Some kinds of stimuli aren’t worth learning to ignoresuch as potentially dangerous looming stimuli [Hack #32] and so aren’t adapted to.
Adaptation lets us ignore the stuff that’s constant, so we can concentrate on things that are either new or changing. This isn’t just useful, it is essential for the constant ongoing calibration we do of our senses. Adaptation isn’t so much a reduction in response as a recalibration of our responses to account for the recent history of our sensory neurons. Neurons can vary the size of their response over only a limited range. Momentarily changing the level that the baseline of this range represents allows the neurons to better represent the current inputs.
2.15.3. In Real Life
You can see the changing baseline easily in the adaptation of our eyes to different levels of brightness. Perhaps more surprising is the adaptation to constant motion, such as you get on a boat. Continuous rocking from side to side might cause seasickness on the first day aboard, but soon adaptation removes it. Upon returning to land, many suffer a syndrome called “mal de debarquement” in which everything seems to be rocking (no doubt in the opposite direction, not that you could tell!).
The “deafening silence” which results from the disappearance of a constant sound is due to auditory adaptation. Our hearing has adapted to a loud baseline so that when the sound disappears we hear a silence more profound (neurally) than we can normally hear in continuously quiet conditions.
Adaptation allows us to ignore things that are constant or predictable. I’m guessing that this is why mobile phone conversations in public places are so distracting. Normal conversations have a near-constant volume and a timing and rhythm that allow us to not be surprised when the conversation switches between the two speakers. With a mobile phone conversation, we don’t hear any of the clues that would allow our brains to subconsciously predict when the other person is going to speak. Consequence: large and unpredictable variations in volume. Just the sort of stimuli that it’s hard to adapt to and hence hard to filter out.
2.15.4. End Notes
Barlow, H. B. (1990). A theory about the functional role and synaptic mechanism of visual after-effects. In C. Blakemore (ed.), Vision: Coding and Efficiency, 363-375. Cambridge, U.K.: Cambridge University Press.
There is a good introduction to adaptation in this paper (which is interesting in its own right too). Kohn, A., & Movshon, J. A. (2003). Neuronal adaptation to visual motion in area MT of the macaque. Neuron, 39, 681-691.
2.15.5. See Also
There’s a discussion of olfactory (smell) adaptation at the Neuroscience for Kids web site (http://faculty.washington.edu/chudler/chems.html).
Taken From : Mind Hacks
