Vision & Phototransduction Tournament
Two onboarding diagrams orient you in the sensory transduction landscape and the visual cycle. Then eight MCAT-DoK quiz rounds: transducin vs. the gustducin trap, why the light receptor hyperpolarizes in light, the full cascade from photon to synapse, retinal anatomy and the two cortical streams, how many cone opsins you carry, the rod/cone division of labor, the refractive path through the eye, and vitamin A deficiency.
Where the Visual phototransduction fits in Sensory perception
Reactome's Sensory Perception map puts the four sensory transduction systems side by side - vision, taste, smell, and hearing - and three of them are GPCR cascades built on the same parts list. The highlighted visual phototransduction panel is the best-characterized of them, and the only one where the second messenger is destroyed rather than made. Click the highlighted Visual phototransduction panel to enter the tournament.
Click the highlighted Visual phototransduction box to continue.
What this tournament tests
Each task maps to a distinct MCAT cognitive demand. The first two orient you in the broader topology; the next eight test the high-yield mechanism, regulation, sequence and quantitative reasoning that consistently appear on test day.
The Bigger Picture
Anchor visual phototransduction inside Reactome's sensory perception map.
Whole-Pathway Overview
Pan and zoom the curated WikiPathways visual cycle figure before you start answering.
Fill in the Blank
Recall transducin as rhodopsin's G protein - and why gustducin is the trap.
Disruptor
Explain the dark current and why light HYPERpolarizes the photoreceptor.
Sequence Ordering
Trace photon -> metarhodopsin II -> transducin -> PDE6 -> cGMP falls -> channels close -> hyperpolarization -> less glutamate.
Match the Pairs
Pair each structure or theory (fovea, chiasm, dorsal/ventral streams, trichromatic, opponent-process, myopia, hyperopia) with its description.
Numeric Input
Recall how many cone opsins support normal trichromatic color vision.
Select All That Apply
Identify TRUE statements about retinal layering, rod convergence, the blind spot, and scotopic vs. photopic vision.
Odd One Out
Separate the refractive path (cornea, aqueous humor, lens) from the opaque sclera.
Vitamin A Disruptor
Link fat malabsorption to a starved visual cycle, night blindness, and xerophthalmia.
Public leaderboard
Your score posts to a global, persistent leaderboard scored by points first, time as tiebreaker.
Vision in 60 seconds
Light travels cornea -> aqueous humor -> pupil -> lens -> vitreous humor -> retina, and the cornea supplies roughly two thirds of the total refraction while the lens supplies the adjustable part. During accommodation for near objects the ciliary muscle CONTRACTS, slackening the suspensory ligaments so the lens rounds up. Myopia focuses in front of the retina and needs a minus (diverging) lens; hyperopia focuses behind it and needs a plus lens.
The retina is famously inverted: light passes the ganglion and bipolar layers before reaching the receptors, which sit against the pigment epithelium. Rods are the high-sensitivity, high-convergence, single-pigment scotopic receptors (no color). Cones are the low-convergence, photopic receptors packed in the fovea for acuity, with three opsins (S, M, L). The optic disc has no receptors at all - the physiological blind spot.
Phototransduction is a GPCR cascade that works by removing its second messenger. A photon isomerizes 11-cis retinal, rhodopsin becomes metarhodopsin II, which activates transducin -> PDE6 -> cGMP falls -> cGMP-gated Na+/Ca2+ channels close -> the cell hyperpolarizes and releases LESS glutamate. Photoreceptors never fire action potentials; they modulate tonic release, and the bipolar cell's receptor type (inhibitory mGluR6 for ON cells, ionotropic for OFF cells) sets the sign.
Downstream, nasal fibers cross at the optic chiasm so each hemisphere sees the contralateral field, and cortex splits into the dorsal "where/how" stream (parietal) and the ventral "what" stream (temporal). Color needs both classic theories: trichromatic at the receptor level and opponent-process for the antagonistic channels that explain negative afterimages. Clinically, vitamin A deficiency starves the visual cycle and hits rods first as night blindness.
FAQ
Why does the photoreceptor hyperpolarize in response to light?
Because it is depolarized in the dark. High cGMP holds cation channels open, producing a steady inward dark current that keeps the cell near -40 mV and tonically releasing glutamate. Light activates PDE6, cGMP is hydrolyzed, those channels close, and unopposed K+ efflux drives the membrane negative. Light is therefore encoded as a decrease in tonic transmitter release, not as an increase in firing.
Do you need trichromatic theory or opponent-process theory for color vision?
Both, at different levels. Trichromatic (Young-Helmholtz) theory is about the retina's three cone opsins with different peak wavelengths, which explains color mixing. Opponent-process theory is about the paired antagonistic red-green and blue-yellow channels built downstream in ganglion cells and the LGN, which explains why staring at red produces a green afterimage. The MCAT typically rewards recognizing that they are complementary rather than competing.
Why is red-green color blindness so much more common in men?
The M (green) and L (red) opsin genes sit adjacent on the X chromosome and are highly similar, so they recombine and hybridize easily. The resulting defects are X-linked recessive, and because males have only one X, a single affected allele produces the phenotype. Blue-yellow deficits are far rarer because the S opsin gene is autosomal.
Why does rod convergence increase sensitivity but reduce acuity?
Many rods feed a single bipolar and ganglion cell, so their small photon-driven signals summate and even a few photons can produce a detectable output. The cost is spatial resolution: the brain cannot tell which rod in that pool was stimulated. Foveal cones do the opposite, wiring nearly one-to-one to ganglion cells, which is why the fovea has the highest acuity and why you fixate directly on what you want to read.
Do I need an account to play?
No. The tournament is fully public. You get a randomized handle and your score posts to the public leaderboard at the bottom of this page.
Keep going
The general receptor logic behind phototransduction - G protein subunits, second messengers, and amplification.
The glutamate synapse that carries the visual signal from photoreceptor to bipolar to ganglion cell.
Overview diagram: Reactome Pathway R-HSA-9709957, licensed CC BY 4.0.