Hack 26. Get Adjusted

We get used to things because our brain finds consistency boring and adjusts to filter it out.

My limbs feel weightless. I can't feel my clothes on my body. The humming of my laptop has disappeared. The flicker of the overhead light has faded out of my consciousness. I know it all must still be happeningI just don't notice it anymore.

In other words, it's just another normal day in the world with my brain.

Our brains let us ignore any constant input. A good thing too; otherwise, we'd spend all our time thinking about how heavy our hands are, how exactly our T-shirts feel on our backs, or at precisely what pitch our computers are humming, instead of concentrating on the task at hand.

The general term for this process of adjusting for constant input is called adaptation. Combined with relative representation of input, adaptation gives us aftereffects. The motion aftereffect is a good example of a complex adaptation process, so we'll walk through a detailed story about that here in a moment.

Both relative representation and the motion aftereffect are described in [Hack #25] . Simply put, how much "movement up" we perceive depends on the activation of up-sensitive neurons compared against the activation of down-sensitive neurons, not just the absolute level of activity.

Adaptation is a feature of all the sensory systems. You'll notice it (or, on the contrary, most likely not notice it) for sound, touch, and smells particularly. It affects vision [Hack #25], too. If you stop to consider it for a moment, you'll appreciate just how little of the world you actually notice most of the time.

Adaptation is a general term for number of processes. Some of these processes are very basic, are of short term, and occur at the level of the individual sense receptor cells. An example is neuronal fatigue, which means just what it sounds as if it means. Without a break, individual neurons stop responding as vigorously to the same input. They get tired. Strictly speaking, ion channels in the membrane that regulate electrical changes in the cell become inactivated, but "tired" is a close enough approximation.

The most basic form of memory is a kind of adaptation, called habituation. This is just the diminishing of a response as the stimulus that provokes it happens again. The shock of a cold shower might make you gasp at first, but with practice you can get in without flinching. It was neuroscientists using a similar kind of situationpoking sea slugs until they got used to itthat first demonstrated that learning happens due to changes in the strength and structure of connections between individual neurons.

In Action

Aftereffects are the easiest way to see adaptation occurring. You can have aftereffects with most thingssounds, touch pressure, brightness, tilt, and motion are just some. Some, like the motion aftereffect [Hack #25], are due to adaptation processes that happen in the cortex. But others happen at the point of sensation. The adaptation of our visual system to different light levels happens directly in the eyes, not in the cortex.

To see this, try adapting to a darkened room with both eyes and then walking into a bright room with only one eye open. If you then return to the darkened room, you will be able to see nothing with one eye (it has quickly adapted to a high level of light), yet plenty with the eye you kept closed in the light room (this eye is still operating at the dark-adapted baseline). The effect is very strong as you switch between having alternate eyes open and the whole lighting and tone of the room you're looking at changes instantly.

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.

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.

End Notes

1. 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.

2. 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.

See Also

· There's a discussion of olfactory (smell) adaptation at the Neuroscience for Kids web site (http://faculty.washington.edu/chudler/chems.html).

Date: 2015-12-11; view: 801