Building a Visual Scene: Pathways, Maps and Receptive Fields
Beyond the retina the visual system is a set of maps in which neighbouring cells look at neighbouring parts of the world, and each cell responds best to a specific feature in its own small window. What a cell prefers gets more elaborate at each stage, from edges to objects.
What a learner can do afterwards
- Define a receptive field and describe how one is measured
- Trace the pathway from retina to primary visual cortex, including what the crossing achieves
- Explain how selectivity for an oriented edge could be built from simpler inputs
1 · Read
A neuron's receptive field is simply the patch of world it watches: the exact visual pattern that excites it best. This is why you can spot a friend's face in a crowd or catch a ball moving toward you: cells tuned to just the right pattern fire the instant it appears. Hubel and Wiesel mapped these fields with an electrode in cortex, flashing spots and bars and listening for bursts of spikes.
From the retina, ganglion axons bundle into the optic nerve and meet at the chiasm, where the nasal halves cross and the outer halves stay. The thalamus relay, the LGN, keeps the eyes in separate layers while preserving the retinal map. In people it stacks six layers, keeping the two eyes apart. Each brain half thus views the opposite visual field through both eyes, and a lesion on one side blinds the opposite side.
Cortical fields stop being round. A simple cell owns stretched side-by-side on and off zones, so only a bar tilted to match, parked exactly on its edge, drives it hard. It is built by lining up several LGN center-surround fields in a row. Complex cells keep the tilt but accept it anywhere in the field.
Complex cells keep the tilt rule but drop the parking rule: an edge at the right angle fires them anywhere inside the field, and many prefer motion one way. Higher areas elaborate further toward objects and faces, splitting into a ventral what-stream and a dorsal where-stream.
Fields are mapped by listening for the best stimulus, and cortex builds oriented edges from lined-up retinal maps.
2 · Watch
Take it off screen
Where it sits
Where this leads
Jobs that lean on this skill. Follow one to see everything it is built on.
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.