Craig Blackwell, MD

Santa Cruz, CA
Diplomate: American Board of Ophthalmology
Fellow: American Academy of Ophthalmology

Welcome to the Website of Craig Blackwell, MD

An Ophthalmology Practice in Santa Cruz, CA

Eye Motility

Eye Movement and Coordination

Eye movement is surprisingly complex. There are six muscles attached to the outside of each eye. Two move the eye horizontally. The other four are oriented vertically at angles and have different actions depending on which direction the eye is pointing. Consider that to move one eye in a particular direction one muscle has to contract while the paired muscle on the other side of the eye must relax. Then consider that for both eyes to turn together multiple muscles must act in concert keeping both eyes in precise alignment.


To maintain alignment the brain actively steers the eyes to keep the image from each eye exactly overlapping. Restating that, if the images from each eye are in close enough proximity the brain fuses them into a single image. Maintaining “fusion” is the driving force that keep the eyes aligned.

Depth perception is the amazing ability of the brain that takes the slightly different image from each eye and creates an accurate estimate of distance from an object.

When both eyes are open they are kept in alignment by the fusion mechanism, as mentioned above. When one eye is covered, breaking fusion, it may wander slightly inward or outward. This represents your set point. In some people the eyes wander to a greater degree and the muscles do a lot of extra work to bring them together. In that case the eye muscles may seem tired as the day progresses or with extended reading or near work and double vision may occur. In adults these problems are dealt with using glasses and sometimes prisms and exercises.

Double Vision

Some people will describe double vision when an image is out of focus. Upon closing one eye the unfocused image will appear the same. With true “double vision” there are two sets of images, and closing one eye will leave a single sharp image. The images may be side-by-side or one above the other. If that occurs it is important to distinguish between problems localized to the ocular alignment system from those caused by conditions in the central nervous system, like trauma, tumor or stroke.

If double vision is stable and the cause has been determined we can measure the amount the eyes are out of alignment. If the deviation is small and outward then eye exercises may help. If it is moderate then “prism” can be built into the glasses to realign the images. If it is a large deviation then muscle surgery may be needed.

Children represent a set of specialized concerns because the brain is still developing vision and alignment mechanisms. Screening for developmental problems begins at birth and should continue through the school years.