Pre-Pulse Inhibition and Sensory Gating
The following was written up for several discussion groups dealing directly with these issues.

Children with attention deficiencies are easily distracted by the slightest noise.  People with obsessive-compulsive disorder are irritated by small irregularities and imperfections in their surroundings.  Autistic individuals can be set on edge by seemingly bearable stimuli they somehow perceive as beyond intense.  These appear on the surface to be a separate set of symptoms distributed across very different diagnoses.  Surprisingly, they are the manifestation of a generalized problem common in patients conditions such as ADD/ADHD, OCD, schizophrenia, and, particularly, autism (including Asperger's syndrome).

Sensory gating is what allows the majority of the population to move relatively unencumbered across an environment with varying levels of sensory input.  Though it might interrupt the conversation momentarily, most of us have no problem passing a noisy construction site.  We can step out of the office building into bright sunlight, take a moment to adjust, and be on our way.  We re-set our threshold depending on the context of the situation.

However, individuals afflicted by this symptom lack the ability to filter out extraneous stimuli without becoming agitated or simply distracted.  Some individuals may experience loud noises or bright lights as painful even at levels that seem mild or moderate, and this is a source of great anxiety to them.

The mental component that allows the average person to adjust for varying levels like this is known as pre-pulse inhibition (PPI henceforth.  The name comes from the fact that, in most individuals, a "warning shot" (i.e., pre-pulse) will prevent a second, larger stimulus from making them jump when it arrives moments later.  For example, if I say "Boo!" you might jump.  But if I say "boo" (normal speaking voice) and then give you the big scare a second later, odds are it simply won't spook you... at least as much.  This is not the case in individuals lacking this module, hence kids with ADHD attend to the least distraction, autistic individuals are bothered by physical contact and deviations from routine that present the unexpected, and so on.   Odds are you can recall experiences in which sensory stimuli were intense enough that you could not filter them out, perhaps a fire alarm going off.  In those cases, things became painful or at least intrusive enough to prevent you from concentrating. 

Indeed, in some exceptional (though not uncommon) cases, otherwise normal individuals can have their threshold-setting faculties affected.  For example, a hangover the morning after a night of heavy drinking means innocuous sounds such as the ticking of a clock or chirping of a bird can become unbearable.  Another common example are the accessories symptoms migraines sufferers experience in which sensitivity to light, sounds, or even smell are increased.  In cases such as these, individuals are forced to retreat to darker, quieter, more predictable environs, usually the bedroom to "sleep it off."

For most of the population, situations like these represent exceptions from our everyday experience.  For those afflicted by the absence of PPI, there is no gate to block intense sensations, so this is an on-going problem throughout what might otherwise be normal life.  As for what you can do about it... that's a tougher problem.  If these symptoms are part of a larger condition that can be diagnosed (e.g., ADD/ADHD), doctors can then treat the big picture and (hopefully) that component can be dealt with in connection to the rest.  In other cases, perhaps other modifications would work'

Pulling out ear plugs at a concert may not look cool, but if you are going to be miserable otherwise, which would you prefer?  Explaining what PPI is to your friends is a good idea as well.  It may be awkward, but educating your peers so you can participate can change the nature of the experience from torture to enjoyment.  Informed friends are more understanding.  In the absence of this information, it is easy to misconstrue avoidance of certain situations as rudeness if you have to distance yourself from unbearable stimuli or make a pattern of repeatedly declining invitations to certain social outings.  Indeed, the inability to filter out input can have the effect of making those with this condition appear chronically as withdrawn and introverted as the aforementioned hangover and migraine sufferers are on isolated occasions. 

While not everyone with sensory gating difficulties will be able to participate in every aspect of a normal life, having a framework for understanding the natures of some of their symptoms is a means of dealing with them.  Spreading that understanding to their peers creates a support structure that creates a hospitable environment that may render them effectively symptomless, at least with respect to their problems with sensory input.

Addendum: Some examples of input that can particularly affect individuals with poor sensory gating:
Modality Examples
Sound: telephones ringing
alarm clocks (but who doesn't hate these?)
loud music
clanking glass/silverware
machinery or appliances (e.g., vacuum cleaners)
Light: sunlight outdoors, especially in the evening when the sun is close to the horizon
headlights of oncoming traffic
exposed lightbulbs such as on desk lamps if they're tilted upwards
pulsing lights in dance clubs
"surprise" moments in movies (e.g., explosions) or displays in video games
Tactile: seemingly clothing may be irritating and feel abrasive even
gripping a writing utensil may place enough pressure against the skin that it is intense
things that have other "feels" such as "slimey" may be more than they can handle
Other senses: cold weather is miserable for me far worse than any of my peers
spicy food is much more intense than others seem to experience it

Copyright Alexplorer.
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