More about the senses
Many people on the spectrum have problems with processing sensory stimuli. The estimates vary between about 70 to 97%. The more severe the autism, the greater the sensory deviations.
With people on the spectrum things go wrong with stimulus processing. So, let’s first get a short impression of how this works.
The processing of external stimuli
However different our seven senses may be, the processing of sensory stimuli in the brain works in the same way. It always starts with sensory input which is received by the nervous system for processing. Stimuli run from receptors (little sensing organs) to the thalamus, a brain center where perceptual stimuli are selected (smell being the only exception) and are subsequently transferred to several other centers in the cortex which can then ‘order’ them to be suppressed and become conscious to us or not.
(Here is one of the sources.)
The aim of this information processing is to alert the nervous system, especially in case of danger, for there may be reasons for immediate action. Secondly ‘maps’ (representations of internal and external situations) are being put together which enable the nervous system to organize and plan.
The brain employs several mechanisms to ensure that the received and processed information is prioritized for optimal functioning. One form of optimization is the suppression of less important stimuli in order to pay sufficient attention to the most important ones. Another series of mechanisms involves maintaining a balance between activity and passivity. Some parts of the brain initiate movement and others suppress it. Normally, there is a fine balance which enables the nervous system to fine-tune responses to meet all environmental demands.
Divergence – another mechanism – ensures the information received in the nervous system from one source is simultaneously sent to many other parts of the system; for instance, to generate ‘fight or flight’ responses.
Convergence provides the assurance that the system will not react on the basis of insufficient information. For example, a specific neuron (brain cell) may activate only if it receives three or more types of input.
Finally, it’s informative to know that integration occurs from stimuli of different senses which are combined in order to arrive at the most complete brain-maps.
We have summed up just a few of the vastly varied mechanisms involved in the processing of sensory stimuli to convey an impression of the complexities. With this in mind, it’s easier to appreciate how much is required to make these processes run perfectly and how common it may be to upset them. ‘Connectivity’, or the way information is processed in the brain, is a rapidly growing research field which seems very promising for unraveling a number of mysteries around autism.
Here we’re just concerned with one type of malfunctioning in the processes which explain part of the sensory difficulties many people on the spectrum have.
Over- and under-responsivity, modulation problems
People on the spectrum suffer remarkably often from over- and/or under-responsivity.
One of the many disturbances of the senses is modulation (think of volume and tone switches on your stereo). Mostly two types of modulation disorders are distinguished:
- overresponsivity, e.g. exaggerated, rapid onset and/or prolonged reactions to sensory stimulation (i.e. distress from loud noises),
- under-responsivity, e.g. unawareness or slow response to sensory input (i.e. tends to walk into things). (Derived from Ben-Sasson et al.)
In the first case, the neurological thresholds are too low, in the second, they are too high. These thresholds are higher when more stimuli are needed for a response. If the central nervous system responds very rapidly, then the threshold is low. Everyone needs a balance between high and low: sufficient to notice enough in order to remain aware and alert, but not so much that one is engulfed by too much information.
Everyone can be characterized according to one’s position on two dimensions: the height of one’s sensory thresholds and the degree to which one actively compensates for its results.
Engaging in high stimulus activity as part of a strategy to compensate for a lack of stimulus response is commonly called ‘sensation seeking’.
People engaging in an active strategy to lower stimuli in the case of a low threshold can be called ‘sensation avoiders'; they often exhibit ritualistic, rule-bound behavior in which they allow as many familiar stimuli and as little unfamiliar ones as possible.
One who responds passively to high thresholds and so does not try to find more stimuli, seems uninterested and emotionally flat and misses a lot of what happens.
Children who respond passively to low thresholds, and, in doing so, don’t fight the excess stimuli, are easily distracted and hyper-active.
As mentioned, most people on the spectrum suffer from over- and/or under-sensitivity. Modulation disorders are more severe in kanner-autism than in Asperger’s or PDD-NOS and so tend to vary with the severity of the autism. This mainly concerns overresponsivity however. In case of under-responsivity the severity of the autism doesn’t matter that much.
Although it may be counter-intuitive, research shows under-responsivity to be slightly more common in autism than overresponsivity. Autistic children between 6 and 9 years old are considerably more often under-responsive than the reverse.
If this finding is surprising, it’s understandable because people who are extremely bothered by sound, light, smell and so on, attract considerably more attention than their opposites. Moreover, when taking measures to ameliorate these disorders it’s, for instance, much easier to create noise than to keep it at bay. Nevertheless, problems of under-responsiveness can be considerable and should not be underestimated. Take, for instance, undersensitivity to temperature which holds dangers for over- and under-heating. More generally: responding passively to high thresholds leads to an impression of being uninterested and emotionally flat, as just mentioned. Many autistic children thus miss a lot of what’s going on around them.
In case one has heightened perception, and can turn low thresholds into an advantage, one has sensory hypersensitivity. For this see here.