The human eye has a point that lacks photoreceptor cells. This is identified as:

Correct — B, Blind spot. At one place on the retina the axons of all the ganglion cells gather together and turn backwards out of the eyeball as the optic nerve, and the retinal artery and vein pass in and out at the same place. That patch is the optic disc, and because it is occupied by nerve fibres and blood vessels there is no room on it for rods or cones. Light falling there produces no signal at all, which is why it is called the blind spot. It lies about 15 degrees to the nasal side of the fovea and covers roughly 1.5 millimetres of retina, and it is the only part of the retina that is genuinely insensitive. Two things stop you noticing it. The blind spot of one eye is covered by the seeing retina of the other, and the visual cortex fills the missing patch in from the surrounding pattern, so ordinary vision has no hole in it. Edme Mariotte demonstrated the gap in 1660 with the simple trick still used today — close one eye, fix on a mark, and slide a second mark sideways until it vanishes.

• (a)Fovea — The exact opposite of what the stem describes. The fovea centralis, a small pit at the centre of the macula, is where cones are packed most densely and where vision is sharpest; the overlying layers are even pushed aside so light reaches the cones directly.
• (c)Dark spot — Not an anatomical term for any part of the eye. It is offered because it sounds as though it ought to mean the same as blind spot.
• (d)Orbit — The bony socket of the skull in which the eyeball sits, along with the muscles, fat and lacrimal gland. It is not part of the retina, so the question of photoreceptors does not arise.

The retina is the light-sensitive inner coat of the eye and carries two kinds of photoreceptor. Rods are far more numerous, work in dim light and give no colour information. Cones need brighter light, come in three types responding to different wavelengths, and give colour vision and fine detail. Their distribution is uneven: cones crowd into the fovea at the centre of the macula, rods dominate the periphery, and at the optic disc there are neither, because that is where the optic nerve leaves and the retinal vessels enter.

Every option here is a real word from the anatomy of the eye except one, so the item is testing whether the candidate can attach each name to the right place rather than recognise vocabulary. The trap is the fovea, because it is the other named point on the retina, and a candidate half-remembering that one spot is special may pick the wrong special spot. Hold the pair together as opposites — the fovea is where photoreceptors are densest and vision sharpest, the optic disc is where there are none and vision is absent. Note also that the stem says 'a point that lacks photoreceptor cells', not 'a point where vision is poor'. That wording rules out the fovea outright, since the fovea's problem in dim light is that it has no rods, not that it has no photoreceptors.

• The blind spot is the optic disc, the point where the optic nerve fibres leave the retina and the retinal vessels enter; it has neither rods nor cones.
• It lies about 15 degrees nasal to the fovea and measures roughly 1.5 millimetres across.
• The fovea centralis, at the centre of the macula lutea, holds the highest density of cones and gives the sharpest vision.
• Rods are responsible for vision in dim light and cones for colour and detail; rods far outnumber cones overall.
• Edme Mariotte demonstrated the blind spot in 1660; binocular overlap and cortical filling-in are why it goes unnoticed.

• Picking the fovea because it is the other named point on the retina; it is the densest patch of photoreceptors, not the empty one.
• Confusing the optic disc with the macula. The disc is pale and blind; the macula, just to its temporal side, is where sight is sharpest.
• Treating the orbit as a part of the eyeball. It is the bony socket that houses it.

As a straight recall item like this one, as 'the optic disc is also known as', or through the rod-and-cone distribution — which cells give colour vision, which work in dim light, and where each is concentrated.

• practice — not a real PYQ

  The region of the human retina with the highest concentration of cone cells, giving the sharpest vision, is the

  • (a)Optic disc
  • (b)Fovea centralis
  • (c)Ciliary body
  • (d)Choroid

  Answer(b) Fovea centralis — a small pit at the centre of the macula where cones are packed most densely. The optic disc is the opposite case, with no photoreceptors at all.

• practice — not a real PYQ

  The blind spot of the human eye is the point at which

  • (a)the lens is attached to the ciliary muscles
  • (b)the optic nerve leaves the retina
  • (c)the image of a distant object is formed
  • (d)the iris meets the cornea

  Answer(b) the optic nerve leaves the retina — the optic disc is occupied by nerve fibres and retinal vessels, leaving no space for rods or cones.