Despite the fact that g [general intelligence] is commonly assessed with tests of vocabulary, memory for associations, reasoning ability on the Raven’s Progressive Matrices (where subjects must discover a visual pattern within a matrix of stimuli, and select what the next pattern in the sequence would look like), and a wide variety of other very abstract and untimed tests, it appears that the variance they share can be reliably and accurately indexed by reaction time on a task where subjects must merely press a lighted button. The correlations between such simple tasks and g is around .62, which is higher than the correlation between many subscales of IQ tests and the g factor to which they contribute.
The essence of the argument is that the faster you process information coming into the brain, the more information you have at your disposal, enabling better decision making. Imagine filling a bottle with water from a tap. How fast you can fill the bottle depends on how fast the water comes out of the tap and how wide the neck of the bottle is. If the tap is dripping slowly then the size of the bottleneck doesn’t matter. As the tap is opened wider a critical point is reached when the flow from the tap exceeds the capacity of the bottleneck. However if the bottleneck is widen then the bottle can continue to catch all the water coming out of the tap.
This analogy holds true for the cerebellum hypothesis of dyslexia and ADHD. A lot of sensory information comes into the cerebellum so if it is underdeveloped it acts as a bottleneck. Any sensory input that cannot be coped with will be discard rather than processed. So when an ADHD child doesn’t hear your instructions, it maybe because they ‘heard’ the sounds but they were discarded before the signals reached the conscious, processing parts of the brain.
At the risk of stretching the idea to breaking point, the same logic applies to a dyslexic’s problems with phonetics. In a simple word such as CAT, there are three sounds (C – A – T) that need to be processed in a very short space of time. If the bottleneck of the cerebellum is too small and the tap of the phonetic sounds is flowing fast, then some of these sounds will be lost. When learning to read this means the dyslexic has incomplete information available to them when trying to match the sounds they hear with the letters on the page.
There is research that indicates that people with ADHD or dyslexia have slower reaction times than the general population (see below). The cerebellum is one of the areas of the brain involved with making quick response so a poor reaction time is consistent with the idea that a weak or underdeveloped cerebellum is partly to blame.
Research: Mean response times, variability, and skew in the responding of ADHD children: a response time distributional approach.
Reaction time indices of attention deficits in boys with disruptive behavior disorders.