Retina and Visual Neuroscience
Retina and visual neuroscience
This domain tests retinal circuitry, phototransduction, receptive fields, and cortical processing. Questions often ask you to predict a response from a stimulus or to separate rod from cone function.
Core rules
- Phototransduction: light isomerizes 11-cis retinal to all-trans, activating rhodopsin, transducin, and phosphodiesterase; cGMP falls and cation channels close, producing hyperpolarization.
- ON bipolar cells respond to light increments via metabotropic glutamate receptors; OFF bipolar cells respond to decrements via ionotropic receptors.
- Ganglion cells have centre-surround receptive fields; a small centred spot is optimal, while uniform illumination produces little response.
- Rods saturate in daylight; cones provide colour and high acuity. Colour opponency begins in the retina and is elaborated in cortex.
- V1 neurons encode orientation and spatial frequency; dorsal and ventral streams support motion and object recognition.
Worked example
A retinal ganglion cell has an excitatory centre and inhibitory surround. Which stimulus gives the strongest response: a small spot covering only the centre, a large spot covering centre and surround, or diffuse light? The small centred spot wins because it excites the centre without recruiting the inhibitory surround. A large spot adds surround inhibition, reducing the response; diffuse light drives centre and surround nearly equally, so the net response is weak. This is the logic behind Mach bands and contrast enhancement.
Traps
- Saying photoreceptors depolarize to light; they hyperpolarize.
- Mixing ON/OFF pathways with rods/cones.
- Assuming the fovea has high convergence; it has low convergence and high acuity.
- Confusing cortical magnification with retinal magnification.