Most recent update:
June 10, 2003.|
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Timothy C. Hain, MD, Most recent update:
June 10, 2003.
Visual stimulation is often a trigger for dizziness. Patients often complain of disturbances related to
This must be considered in the context that persons with dizziness may have
Visual dependence is the general term for persons who have increased their weighting of vision, perhaps due to lack of confidence in vestibular or somatosensory input.
Visual dependence is a subtype of the general category of "sensory integration disorders". In order to talk about this we have to make a diversion into systems physiology.
Estimation of self and external motionlts the head), or noisy (such as when a sensor has been damaged)?
A mechanism is neededthat combines sensory inputs, weights them according to their relevance and reliability, and provides a reasonable estimate of orientation in space, even without any recent sensory input. In engineering terms, we are discussing an “estimator.”
Navigating the space shuttle involves similar problems. The shuttle has dozens of sensors and motors. Some sensors respond quickly, and some slowly. They may differ in accuracy, scaling, coordinate frame, timing, and noise characteristics. No single sensor can provide a complete picture of the shuttle’s state. A mechanism is needed to integrate sensor output and to develop an internal estimate of the state of the system (i.e., position, velocity, acceleration) in order to keep the shuttle on the desired course and heading.
The engineering solution to this problem developed out of work performed by Kalman and is often called a Kalman filter. It is also commonly called an “optimal estimator” or an “internal model.” There is considerable evidence that mechanisms similar to Kalman filters are used for human sensorimotor processing.
The Kalman filter is far more powerful than a simple reflex. Several key concepts must be considered before one can understand how it is superior.
- Internal models of sensors and motor output are used to develop an estimate of the current sensory and motor state. These internal models are adjusted according to experience and must track changes in bodily function. It seems likely that vestibular rehabilitation affects internal models.
- Sensory input is not used to directly compute body state, but rather, the difference between sensor input and predicted sensor input is used to correct the current estimate of body state. This design allows the Kalman filter to easily combine multiple sensor inputs—from eyes, ears, and somatosensors. The Kalman filter continues to work even in the absence of a sensory input, because it uses its estimate when the sensor is missing. Both of these highly desirable features make the Kalman filter far superior to a simple assemblage of reflexes.
- The Kalman gain weights the extent to which a sensory input affects the ongoing state estimate. This weighting provides a method of adjusting for the salience and reliability of sensory streams. It seems highly likely that vestibular rehabilitation adjusts the Kalman gain.
Overall, this sort of mechanism is clearly far superior to vestibular reflexes: Although not as fast, it can be far more accurate, it functions even in the absence of sensory input, and it is modifiable by experience and rehabilitation.
With this background, it is now possible to see why visual dependence is so common. We all use internal estimators that combine sensory information into a best "estimate" as to the self and world motion. These estimators consider several streams of input -- visual, vestibular, body sensation, and internal knowledge. When vestibular sensation is unreliable, our internal estimator adjusts the weighting of other senses.
Usually this is a good idea. Sometimes though, it gets people into trouble. When vision does not correctly reflect self or world motion, visual input can create dizziness. Thus visual dependence is a sensory integration disorder.
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Flow field similar to that used in the Microsoft Windows starfield screen saver. Divergence is zero at the center of the field and maximum towards the extremes. Figure from http://cog.brown.edu:16080/~warren/ |
This has to do with how things "stream" past. When one is walking through the aisles of a grocery store, there is a "flow field" of visual motion which can be visualized like water going past the prow of a ship. In other words, there is divergence. Objects to the left side are projected as moving leftward on the periphery of the eye, and objects on the right are projected as moving rightward on the periphery of the eye. Ordinarily, the two flow fields cancel out so that there is no net sensation of rotation. When flow processing is asymmetrical or tilted, movement through such environments may induce vertigo. Also, persons with impaired vestibular input may weight visual sensation greater than normal persons and become unsteady due to the visual impact. Many times persons with this symptom become faint and have to leave the environment. There is often a panic component. Another example of a flow field is the "starfield" screen saver distributed with Microsoft Window's software.
Patients with vestibular disorders often exhibit increased sway during optic flow. (Redfern et al. 1994).
The flow situation is actually quite a bit more complex than suggested above-- flow fields can be described in terms of four components: translation, expansion, rotation, and shear. Mathematically they can be described using the terms divergence, curl, and deformation of flow with respect to the position of the eye.
These sorts of symptoms can usually be managed by avoidance, conditioning, and dark glasses.
The brain must separate out visual motion related to self movement from environmental motion in order to remain well oriented in space and to determine whether other sensory inputs such as the ear and legs are accurate. Persons who have impaired vestibular inputs may inappropriately depend on visual input, and confuse environmental movement with self motion. Most people who drive have experienced the illusion of movement when they are stopped at an intersection, and another car starts to slowly creep forward. This is an example of environmental motion being confused with self motion.
Visual stimulation can also result in postural sway. Normal persons adapt over about 30 seconds to visual input and stop swaying, while persons with vestibular deficits do not adapt. (Loughlin et al. 1996; Loughlin et al. 2001)
While these sorts of symptoms are well understood, and are often treated by vestibular rehabilition, it is not entirely clear how effective this treatment is.
Some people with vestibular disorders become unable to work on computer screens. The precise reason for this is not entirely clear, but some have speculated that it relates to flicker (refresh rate -- see the VEDA newsletter, Vol 16, #4, 1999). According the authors of the VEDA newsletter (optometrists), one should use faster refresh rates, dim and small displays (such as laptop displays), and make sure that your vision is otherwise normal. We have found that some people do better using projection systems -- the combination of a large "screen" and a LED type projector can be useful.
Some people just can't tolerate bright light. This is called photosensitivity. It is most commonly seen in migraine sufferers, who are actually wired differently (DaSilva et al, 2007), but sometimes it also is found in people with dizziness, especially after head injuries. The treatment is avoidance, migraine medications, and dark glasses.
This is an unfinished and mainly theoretical discussion about treatment. There are many potential avenues
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