The Nature of Cognitive Impairment
Obviously, intelligence does not work that way. We don't have a 'dial' that goes from profound cognitive impairment to profound giftedness. Intelligence is also not like height, in that we can't clearly identify a single dimension that people of different intelligence levels vary along in a linear fashion. Cognitive tests involve a wide variety of specific cognitive functions which, in NTs, emerge at many different ages, and individuals are scored based on how many of these cognitive functions are operational and how consolidated they are (as measured by whether they perform certain tasks correctly).
But what is different in the cognitive process of an intellectually disabled person as opposed to an NT? There are hundreds of different possible etiologies of cognitive disability. Even if you only look at genetic causes, genes that are implicated in cognitive development may perform many different specific tasks. LIS1, on chromosome 17p13.3, influences a process called 'neuronal migration', which takes place early on in brain development - deleting this gene causes lissencephaly, where the brain is smooth instead of wrinkly, and results in profound cognitive impairment, severe intractable seizures and severe cerebral palsy. In contrast, the DLG3 gene on Xq13 influences how neurotransmitter recepters are clustered, and mutations cause moderate cognitive disability in males. And MECP2, on Xq28, doesn't do anything directly, instead regulating the activating of a wide variety of other genes, and mutations in MECP2 can cause a wide variety of different cognitive disabilities depending on the specific kind of mutation - in males, MECP2 mutations have been linked to a severe neurodegenerative disorder with death in infancy at the one extreme, and mild cognitive disability at the other extreme. Each of these mutations have very different effects on the structure and function of the brain, but they all cause cognitive disability of varying degrees.
So my question is, what specific cognitive processes, if impaired, could cause wide-ranging, pervasive effects on development?
Koike et al (1992) studied 22 individuals, ranging from 8 to 57 years old, who were all identified as profoundly intellectually disabled, with a wide range of underlying causes. None had independent mobility, only one had any useful handskills, and all were profoundly impaired in both receptive and expressive language. Important to the testing procedure is the fact that none of them showed any visual impairment and all of them blinked when an object moved rapidly towards their eyes.
Each participant was seated in front of a blind and a teacher held the participant's head in position to watch it. At each trial, the blind was removed and they could see an object with one of two patterns (a circle or a cross) on it move slowly back and forth. Their visual tracking of the object was recorded by two experimenters as well as by electrodes attached to the participants' faces. Each individual saw the first pattern on six trials in a row, and the second pattern for the last two trials.
In most people, if you present them with the exact same thing over and over, they get bored. This is measured as a reduction in response to the stimulus - in this study, reduction in visual tracking - and is known as habituation. If you then change the stimulus in some way that the person can perceive, they will 'perk up' and responding will increase - this is dishabituation.
Eleven participants showed that exact pattern, of habituation followed by dishabituation. Since this pattern is shown even by newborns, that's not all that surprising. What is surprising is that the other 11 didn't. Two showed habituation, but then didn't dishabituate when the stimulus was changed, and the other 9 didn't habituate - in other words, they showed just as much interest the sixth time they saw the stimulus as they'd shown the first time. These eleven, incidentally, had significantly poorer communication skills than the participants who habituated.
Now, there are many possible explanations for the observed failure to habituate. Maybe they had more subtle visual problems that interfered with perception of the two patterns. Maybe visual tracking was automatically elicited by movement whether or not they were interested in it. Or maybe they have severe difficulties remembering visual shapes.
But let's go on a thought experiment. What if a child was born, who was almost completely normal in cognitive potential, except that the child couldn't habituate to repeated stimuli? In other words, to this child, newness doesn't 'pop out' and catch his/her attention because everything feels 'new' even if he/she has seen it a hundred times. And not because the child can't remember seeing it before, but because seeing it before doesn't make it any less inherently interesting to this child.
If you attend to a stimulus, you can learn stuff about it. But within a very short time, you run out of things you can learn from a single unchanging stimulus. At this point, most children will disengage from the stimulus and start ignoring it, or else do something to change the stimulus in some way. Furthermore, when they've got something tuned out, they no longer expend any significant mental effort in processing it.
So the net effect of habituating is that two things happen: you stop expending effort attending to something when there's no benefit to attending to it, and you shift focus to some new stimulus that can teach you something else.
If a child does not habituate, therefore, they'll spend an awful lot of time paying attention to the exact same things. (Like a norn pushing the elevator button over and over.) And they will spend less time attending to new things - things they can learn from. As a result, their overall learning opportunities will be reduced.
In addition, the desire for novelty provides a strong motivation to learn certain skills, because many skills help a child expose him/herself to something new. Reaching allows the child to get tactile stimuli from objects within reach, and grasping enables them to move the object: maybe chewing on it, maybe throwing or dropping it, maybe banging it against something. Mobility allows the child to bring things into reach that weren't previously in reach, and to change what appears in their visual field and possibly the texture of the surface they're on - and not only that, they get kinesthetic stimuli from performing the movement! Communication skills allow them to request that others provide them with stimuli (eg giving them an object they can't reach), and also allows them to get novel information from the messages that others convey.
A typical young child will quickly realize that learning new skills will bring them opportunities to explore new things more easily, and will try very hard to learn those new skills. A child who isn't interested in new things, in contrast, won't try as hard to learn new skills. He/she may try to do so for comfort reasons, such as shifting position so he/she doesn't have one arm twisted underneath his/her body, or learning how to say 'no' so people will take things away that the child finds upsetting, but he/she will have one less motivation to learn skills, and will therefore be less willing to exert a lot of effort to learn those skills.
This might set off a cascade of missed opportunities throughout the child's life, such that the child falls further and further behind in development. Could this result in some cases of profound intellectual disability?