What’s Going On?



What role might the brain stem nuclei play in causing or contributing to autism?

If we go by the assumption that autism is caused by sensory impulses not completing the full processing circuit, so that they are not properly integrated into coherent, meaningful perceptions, it stands to reason that the earlier these impulses are interrupted in the processing circuit, the more basic the information that is going to be sacrificed. The most basic information the brain receives is from our skin. Sensations of touch, temperature, texture, pain and pressure are conveyed from skin receptors through the peripheral nerves to the spinal cord and up to the brain stem.

At the brain stem, these sensations feed into the reticular core or formation; a tangled web of nuclei that acts as an arousal center for the nervous system. When stimulated these nuclei wake, calm, alert and excite us. They send messages on to the hypothalamus to regulate heartbeat, breathing and digestion. They also send messages up to the cortex, signaling it to shift our attention from one thing to another. If a person’s reticular processes are not operating smoothly, focusing attention and maintaining control would be very difficult.14

Sensations of touch also feed into another even more complex and critical network of nuclei at the brain stem level. These vestibular nuclei have interconnections with almost every part of our brain and body. They process and coordinate sensations from the gravity and movement receptors in our inner ears with information from all our other sense receptors. This processing determines our basic relationship to gravity and the physical world, and this gravitational security serves as a reference point for our interpretation of all sensory experience.15  A child who feels uncertain about his connection to the earth is likely to feel uncertain and insecure about just about everything else.16

If the vestibular nuclei are not functioning properly, the registration and integration of all sensory messages would be “off.” The soothing sensations from normal mother-child bonding, from holding, cuddling and snuggling, would be sabotaged before they could wend their way to the limbic system, so that the basic connections that inspire kindness and empathy would never have a chance to develop. 

Sensations from the eyes and ears would not be processed into clear-cut perceptual information if that input was not properly integrated with basic body information at the brain stem-vestibular level. A child who is uncertain about what he sees and hears because he is missing some relevant visual, spatial or auditory clue would find it very difficult to attend to either words or deeds in a typical manner.

How might the poor modulation of sensory input impact vestibular function in autism?

If certain inhibitory neurotransmitters are not doing their job of inhibiting and certain excitatory neurotransmitters are not doing their job of facilitating, the vestibular nuclei cannot do their job of integrating and passing along organized, modulated sensory information to all the body and brain systems and structures that need it. Because of too little facilitation, information from the vestibular receptors does not flow smoothly to the spinal cord and muscles, undermining balance and muscle tone and making a child gravitationally insecure, thereby insuring that the operation of all sensory processes will be out of sync with the normal state.17  Because of too little inhibition, rather than being eliminated, the disorganized sensory impulses that pass through the vestibular nuclei are fed into the reticular arousal system and the brain becomes overstimulated.

How might the thalamus contribute to autism?

After leaving the brain stem, sensory impulses move on to another cluster of nuclei within the thalamus. Located in the core of the brain, these nuclei control what and how impulses are transmitted and integrated beyond the brain stem level. They are differentiated and specialized to correspond with and relay information reciprocally to and from the sensory lobes and association areas of the cortex.

This two-way connection network is not a genetic given. In fact, we are born into a state where the sensory areas of the brain are “cross-wired.” As we develop, our growing experience and consciousness separates and straightens out this crucial wiring.18  But if that experience is lagging, the specialized sensory wiring of either the cortex or thalamus might remain skewed, skewing their crucial two-way interconnections and causing problems with input registration. 

Or, the specialized nuclei in the thalamus might act on their own to suppress or selectively pass along sensory input and cortical feedback. Thalamic nuclei working in a particularly selective mode would account for the failure of many autistic children to attend to the things that normal children readily notice. If the thalamus is acting in its gatekeeping role to suppress some sensory impulses while exaggerating the importance of others, then not only would autistic attention be limited and selective, but it would likely be odd as well. The brain would have a hard time knowing what information was important and what was not.

How are sensory impulses transformed into motor responses?

Tactile impulses from the hands, fingers and mouth and proprioception from the muscles and joints wend their way from the brain stem, through the thalamus, to the somatosensory cortex in the parietal lobe, where details about the type of touch we feel or movement our muscles make enter our conscious awareness.19

Once we become aware of sensations from our body, we must decide what to do about them. This is the job of the association cortex. The association nuclei send signals to the basal ganglia and to the cerebellum, where precise motor commands are formulated and passed on to the thalamus. The thalamus receives these signals, coordinates them with incoming sensory information and forwards messages up to the motor cortex, where the commands are executed.20 

The motor cortex actually consists of two neural strips located atop and down the sides of the brain’s frontal lobes, with different sized patches or sections devoted to specific body parts according to their degree of sensitivity. The left motor cortex controls the muscles on the body’s right side and the right one those on the body’s left side. The hands, feet and mouth have the largest cortical patches, affording us greater precision in the control of these muscles.21

What might go awry in the carrying out of motor responses?

Once a movement is begun it is continuously monitored by the cerebellum, which receives instructions from the motor cortex and proprioceptive feedback from the muscles and joints. The cerebellum compares and assesses this information and sends signals to the motor cortex to modify or fine tune the movement.22  If the nuclei in the cerebellum are not operating to full capacity, information relayed to the motor cortex might be vague or inaccurate. 

The movement process might be further complicated if proprioceptive feedback is weak because muscles and joints are not receiving a healthy flow of input from the vestibular nuclei to keep them toned. Also, if tactile sensations from the mouth and hands fail to register in the somatosensory cortex as intact, clearly defined perceptions, the result would be a diminished sense of oral awareness and tactile discrimination, which would make the planning and carrying out of the movements involved in speech and fine motor tasks difficult.

The bottom line, if the sensations in a child’s brain aren’t feeding him back good, precise information about his body, he will have a poorly developed sense of self overall.

How does stimulating the body help to develop and organize the brain?

Every time the cerebellum actively plans and sends out a motor command, the tactile, vestibular and proprioceptive input that feeds back into the brain as a result of this intentional movement adds information to the ‘strip’ map of the motor cortex. The more information fed into each patch, the more detailed and accurate this motor map or body percept will become. And, the more accurate and detailed a person’s body percept, the better he will be at learning and carrying out other intentional movements.

Stimulating a body physically, either through exercise or sensory integrative therapy, forms and stimulates synapses. When synapses are stimulated, used repeatedly for sensori-motor functions, neural memories of those functions are created.23  Neural memories are the basis for all learning, both mental and physical. On the physical side of the equation, neural memories translate into motor skills; actions that no longer require planning or conscious attention.24  A child with many motor skills is able to do many things without thinking about them, while one with fewer motor skills has to think a lot about everything he does and consequently doesn’t find doing many things enjoyable.

Why do autistic children often have so little motivation or interest in doing new things?

One explanation is that many autistic children have a paucity of motor skills causing them to have a difficult time figuring out how to do things, and because of this they have little desire to try anything new or different. But the problem probably originates in the association cortex; the primary response planning region of the brain.

In the autistic brain, sensory impulses flow unimpeded until they reach the nuclear networks in the brain stem where, if either network is malfunctioning but particularly the vestibular one, they begin to unravel, to scatter or fragment. Those impulses that do make it on to the thalamus are filtered further through selective registration or through skewed interconnections to the cerebral cortex. The result is that fewer intact sensations actually make it all the way to the final association phase where a decision can be made to act upon them. 

Because their developing brains are only fully registering and integrating a selected portion of the sensations that pass through them, autistic children are not fully capable of making a well considered decision to do or not to do something. The shortcircuiting of cerebral neural impulses not only results in a lack of inner drive, it also causes an inability to think things through; to consider all factors of a decision before taking action. Hence their predisposition to impulsivity.

Even if the association cortex in an autistic brain does make a decision to do something, plans an action or response, there is a good chance that the message it sends on to the cerebellum and the cerebellum sends on to the motor cortex will be obfuscated by being routed through skewed thalamic synapses. And, if a motor command is initiated at the cortical level, the message still has to pass through the brain stem en route to the spinal cord and muscles, so there is further likelihood that it could get fouled up. In short, autistic children lack an inner drive because they lack an intact, fully functioning sensory processing system.

How does their restrictive thought pattern affect the play style of autistic children?

Because they cannot register the meaning or potential use of toys, young children with autism have little incentive to explore or play with them. They generally find activities that challenge them to experience new sensations and develop new skills confusing and off-putting. They would rather be left alone to do nothing in particular or to stick to the familiar, repetitive actions and routines that they know can’t hurt them and won’t overwhelm them. 

The trouble is that the limited “play” strategies they come up with -- strategies that include holding, gazing at, twirling, mouthing or lining up toys -- do hurt them because they do not challenge them to learn anything. By not trying to figure out more sophisticated or purposeful ways to manipulate their toys autistic children set themselves further behind developmentally by failing to process the necessary sensations and develop the neural connections that would help to better organize their brains.25

What is the critical Catch-22 situation in which young people with autism find themselves?

Because the synapses in the brains of autistic children are not working efficiently or effectively, fewer intact perceptions reach the association or planning processing areas, causing them to have less motivation or inner drive. Consequently, they plan and carry out less intentional actions, resulting in the formation of fewer neural memories and motor skills. This further impedes the growth of the crucial neural interconnections that would help to develop and organize their brains so that they could properly process sensations into the perceptions or adaptive responses that would make learning -- planning and carrying out actions or thinking meaningful thoughts -- easier for them.

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