Perception - Lecture 2

Entoptic phenomena are illusory perceptions produced by normal and abnormal structures within the eye or visual system. Issue: what perceptual phenomena are patients likely to report as the result of normal vs. abnormal ocular structures?

1. Phenomena associated with pre-retinal structures.* 2. Phenomena associated with retinal structures.* 3. Phenomena associated with cortical events. *Special illumination or viewing conditions often required.

Illumination condition: a small light source or pinhole (PH) at the anterior focal point of the eye - uniform field of approximately collimated light.

-Note the round disk of light from the PH. -Occulde/unocclude the opposite eye and note changes in the size of the bright disk. -Shine a penlight into the opposite eye and note changes in the size of the bright disk. The disk corresponds to this iris shadow; if the inner margin of the iris is not smooth, the edge of the disk will appear irregular.

-Note the round disk of light from the PH. -Slowly close the eyelid of the eye viewing thru the PH and note the motion of the shadow across the disk. -Shadow appears to move up from the bottom.

Because of the inversion of the retinal projections into space. Note that although produced by structures in or on the eye, entopic phenomena are perceived in external space.

-Opacities in the ocular media cast shadows on the retina. -If the light source is very small, then the shadow is sharp (umbra); if the source is larger, then the sharp umbra is surrounded by graded penumbra.

In normal viewing, the pupil limits the light reaching the retina. With a normal pupil, the umbra is short so the shadows have very low visibility.

1. When the opacity is large 2. When the opacity is near the retina 3. When the pupil size is small

PH motion causes the shadow of an ocular opacity to move relative to the bright PH field. 1. "With" motion for anterior segment 2. "Against" motion for posterior-segment opacities 3. No motion for opacities in the pupillary (nodal) plane.

See pic Motion of the pinhole causes the shadow of an anterior segment opacity to undergo percieved "with" motion, relative to the bright pinhole field.

See pic Motion of the pinhole causes the shadow of a posterior segment opacity to undergo perceived against motion, relative to the bright pinhole field.

Motion of the pinhole causes the shadow of an opacity in the nodal plane to undergo no percieved motion, relative to the bright pinhole field.

-Floaters are the most commonly seen entopotic shadows. -Noticeable with bright empty field (constricted natural pupils) or in a microscope (small exit pupil) -Visibility of floaters during ordinary viewing indicates that they are attributable to structures close to the retina

-Normal floaters are attributed to small pieces of debris. ex. decomposed RBCs in the posterior vitreous. -the bright center is from diffraction around the translucent RBC.