Babies have a curious set of reflexes: lightly brush their palms, or the soles of their feet, and they will immediately grasp whatever caused the contact. In the case of feet, it’s more of an attempt than a successful grasping; human feet, while far more flexible and manipulative than most creatures’, are no longer the virtual hands possessed by our tree-dwelling ancestors and relatives.
These and a few other basic responses are commonly called the “primitive, or infantile, reflexes“, and are unusual for a variety of reasons. For one thing, they’re not permanent. As babies age, the reflexes disappear.
But they’re not gone. Unlike many other reflexes, they don’t originate in the peripheral nerves, but the central nervous system. The reflex patterns don’t cease to exist, and they don’t cease to act. They’re eventually inhibited by more sophisticated parts of the brain associated with the frontal cortex. We know that the reflexes don’t cease to exist because there are conditions that cause them to reappear in adults; most of them involve major brain damage, particularly to the frontal areas, and are used to diagnose the severity of injury in cases of head trauma.. People with cerebral palsy frequently possess the responses as well, although they can often learn to control and prevent the reflexes consciously.
These points illustrate a very important basic principle: the mind is made out of ‘layers’ of modules and functions, starting with the most rudimentary, basic, and primitive, and moving to the most complex and subtle. At no point do the lower levels cease to exist or to produce output; we can act in complex ways only because the more basic reactions are held back and prevented from exerting control.
As various factors reduce the efficiency and health of our nervous system, it’s the most complex subsystems that fail first. The more basic, the more hardwired, and the less emulated the system, the less vulnerable it is to widespread damage or malfunctioning. This has long been observed with intoxicants and conditions that impair central nervous system functioning, and is one of the ways neuroscientists understand how the brain creates such complex behaviors as a sense of humor. (Curiously, that's not an aspect of the more modern and recent neurological modules, but is associated with very primitive responses. That may be discussed later.)
But all inhibition can fail. The more powerful the activity of the lower processes, the less likely it will be that the frontal lobes will be able to control them. Faced with more than it can handle, the 'angel brain' can be overwhelmed, letting the more basic modules to influence behavior and thinking.
This is the primary reason why IQ isn’t adequate to access someone’s intellectual capacity, a topic I will address further in another post.