This is great video provided by Brain Highways introducing primitive reflexes and provides a great visual of how the higher brain cannot do it's work when primitive reflexes are still present in the lower survival brain.  In the video, they speak mostly about learning difficulties, however they don't touch upon their causes  or the solution to remediate primitive reflexes that are present in a child beyond age three.

This is another great video provided by Brain Highways introducing the proprioceptive system. The funny thing about the proprioceptive sense is that when it is diminished or lost, no one
- parent or child - realizes it because it is the one sense that is mostly subconscious. We are much more familiar and consciously aware of the other senses such as vision, hearing, smell, taste, and touch. Proprioception, however, is more essential to the brain because it is the only sense that is constant working very closely with the vestibular system , which also is mostly subconscious.

Children with many retained reflexes are functioning at the brain stem level, where the pons are located, which is our involuntary, immature, survival brain. Brain Highways shows in this video how it looks when a child is operating from the fight-flight survival brain effecting learning, focus and behavior.

In this video Brain Highways shows wonderfully what can happen to a child when they do not have the proper highways established connecting to the speech and language areas of the brain.  What they do not mention is that these speech and language areas are called the Broca and Wernicke's areas of the brain and can be made available for speech and comprehension of speech by doing  neurodevelopmental movements that makes these highways in early infancy in the first place.

This video gives a wonderful 'sense' of what the function is of our sensory systems, which is to process incoming auditory, visual, touch, smell and taste information from our environment as well as the two movement senses that processes where we are in space with our body. Then it goes on to show what happens when our sensory systems cannot process properly.
It however doesn't mention the cause of why there can be a Sensory Processing Disorder with a child. Again it is the primitive reflexes that assists in setting up the proper sensory pathways early on in development, from the womb to age 3. By integrating the primitive reflexes through neurodevelopmental movements we can help sensory processing difficulties.