Under scotopic conditions, a faint star visible in peripheral vision disappears when the gaze is directed straight at it. The explanation is:
- A Direct fixation stimulates rods, whose sensitivity falls further in darkness
- B Fixation increases intraocular pressure and blurs the image
- C The fovea contains only cones, which are too insensitive for dim light ✓
- D Accommodation for near viewing removes the star from focus
Correct answer: C. The fovea contains only cones, which are too insensitive for dim light
Explanation
The foveola is populated exclusively by cones, which require higher light levels than rods, so a star at threshold intensity is invisible under direct fixation. Rods peak in density in a ring around the fovea roughly 15 to 20 degrees eccentric, which is why off-centre viewing works for dim targets. Accommodation is irrelevant for distant objects and fixation does not raise intraocular pressure, so the distractors fail on basic ocular physiology.
Reference: Guyton and Hall Textbook of Medical Physiology, 14th ed.
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