What the exam asks
In Section I, expect scenarios comparing rods and cones in dim versus bright settings, questions linking afterimages or color vision deficiency to the two theories of color vision, and vignettes describing patients with prosopagnosia or blindsight. In Section II, you may need to operationalize visual acuity or color detection in an Article Analysis Question (AAQ) or use visual transduction concepts to explain research findings in an EBQ.
Core ideas
Focusing light and transduction
Light enters the eye and is focused by the lens onto the retina, the light-sensitive inner surface at the back of the eye.
- Accommodation is the process by which the lens changes its curvature and thickness to focus near or distant objects on the retina.
- Nearsightedness (myopia) occurs when the lens focuses light rays from distant objects in front of the retina; nearby objects remain clear, while distant objects look blurry.
- Farsightedness (hyperopia) occurs when light rays from nearby objects reach the retina before coming into focus (forming a focal point behind the retina); distant objects remain clear, while nearby objects look blurry.
At the retina, transduction converts light energy into neural impulses. Light activates photoreceptors (rods and cones), which trigger bipolar cells, which in turn activate ganglion cells. The axons of ganglion cells bundle together to form the optic nerve, carrying visual information to the brain (via the thalamus to the primary visual cortex in the occipital lobe).
Where the optic nerve exits the eye, there are no receptor cells, creating a blind spot. You do not notice this gap in everyday vision because your two eyes have overlapping visual fields and your brain automatically fills in the missing information using surrounding visual context.
Rods versus cones
| Feature | Rods | Cones |
|---|---|---|
| Location | Concentrated in the periphery of the retina | Concentrated in the fovea (center of the retina) |
| Function | Detect black, white, gray, shapes, and movement | Detect fine detail and color |
| Lighting | High sensitivity; essential for low-light (dim) vision | Low sensitivity; require bright light to function well |
| Adaptation | Adapt slowly in the dark (up to 20–30 minutes) | Adapt quickly in the dark (within a few minutes) |