Blind individuals, particularly congenitally blind children, lack peripheral retinal processing which is responsible for subconscious navigation. Peripheral vision normally works in conjunction with the cerebellum and vestibular system (via specialized neural pathways) to align the body and position it in space. The cerebellum relies in part on the peripheral retina to tell it where the body is in space, where it is going, and how fast and in what direction. Visual input is coordinated with muscle signals and vestibular inflow. When peripheral vision is lost, the cerebellum must make these calculations based solely on muscular and vestibular data. In a way, blindness causes a functional loss of ability in the cerebellum. It is as if the cerebellum had sustained damage so that it cannot do its full job (this is speculation on my part, I am not a cerebellar expert).
Cerebellar damage causes a characteristic gait; a wide stance and a shuffling walk. This sounds like (and looks like) what we have come to call the blind gait. We reason that blind children shift their center of gravity back because they are blind; they move their face out of harms way. This leads to a widened stance and a shuffling gait. There may, however, be another (deeper?) explanation for the blind gait. It may come from the loss of the peripheral visual processing system, and its link with the cerebellar pathways. It is also interesting to note that cerebellar damage causes a characteristic veer either to the right or left. It could be that the veering tendencies of some blind individuals are also due to the disruption of the cerebellar pathways. The low tone seen in many congenitally blind children may also be related to this loss of peripheral, subconscious processing, since low tone is a consequence of cerebellar damage.
Nystagmus also results from cerebellar damage. It is interesting to speculate that nystagmus may also be related to the disruption of the subconscious pathways for navigation. The very fine micro-oscillations of the eyeballs that are necessary for perception are controlled by the cerebellum. Disruption of the cerebellar pathways would lessen smooth control, resulting in a coarser, widened ocular oscillation.
The pathway from the retina to the vestibular system to the cerebellum is called the vestibulo-ocular pathway. One of its functions is to rapidly refixate, so that the eye is able to track a moving object while the head is also turning. Damage to the vestibulo-ocular pathway causes an inability to smoothly track moving objects.
The older cerebellar brain includes the basal ganglia which abut against the cerebral ventricles. Subconscious motor and navigational pathways flow to and from the basal ganglia. When a child has hydrocephalus there is pressure on the ventricles which then expand, killing cells that line the walls. This may explain why children with a history of hydrocephalus have not only poor motor control, but also have characteristic problems with navigation (spatial awareness, left right discrimination, etc.).
When the cerebellum is damaged (or its pathways disrupted) then subconscious, automatic movements are reduced or lost. This means that children with reduced cerebellar function must will actions. They must use higher cognitive areas and voluntarily command their muscles to perform functions. This is obviously an inefficient and exhausting task. Watching children with cerebral palsies or other neuromotor damage, you quickly see how debilitating loss of cerebellar functioning can be.
I noticed something else about the use of vision in physically impaired children. I used to say that many of these kids did not know where their eyes were in their heads; that they did not have ocular proprioception. The eyes of these kids seemed to drift all over the place. I had to repeatedly ask them to look at me or an object, which they seemed fleetingly able to do. I now realize that when asked to voluntarily direct their eyes they can do so. It is hard and exhausting, but they can do it. Most of the time however, the subconscious lower pathways of the cerebellum are inoperable; so most of the time oculomotor control is on "drift".
notes:
The eyes have a strong connection to the amygdala (part of the basal ganglia) that is related to strong emotional response.
Navigational tracts interconnect the eyes, the vestibular inner ear systems, the cerebellum, the pons, and the basal ganglia. From the basal ganglia the tracts go through the thalamus to the frontal cortex (frontal eye fields, premotor and motor areas). The basal ganglia are a collection of paired cell clusters buried deep below the cortex in white matter. The basal ganglia are spaced at a distance from each other but are interconnected by neural tracts. The basal ganglia include the corpus striatum, the globus pallidus, the substantia nigra, and the subthalamic nucleus. Together, these ganglia control complex patterns of movements such as walking. They control smooth voluntary movements.
Return to the index page
Return to the top of this page.
baldwin@cris.com
Back: "Brain" home page