The Stiles Crawford effect relates to two perceptual aspects of vision. Which pair best describes these Type I and Type II categories?

• A. Type I brightness perception; Type II color perception
• B. Type I color perception; Type II brightness perception
• C. Type I brightness; Type II brightness
• D. Type I color; Type II color

Answer: (A)

The Stiles-Crawford effect shows that the way light enters the pupil changes what we perceive, and it has two distinct perceptual components.

Type I is about brightness perception. Light entering near the center of the pupil is more effective at stimulating the photoreceptors than light entering toward the edge, because photoreceptors act like directional waveguides. This makes the image appear brighter when the light hits the center, a phenomenon described as changes in perceived luminance with entry position.

Type II is about color perception. The way different cone types respond to light depends on the entry direction, so the same light can evoke slightly different hues depending on where it enters the pupil. This directional change in color appearance is the Type II component.

So the pair is brightness perception for Type I and color perception for Type II.