BIOC44 Vision Notes

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🎴 CUE CARD 1 — Photoreceptors (Rods & Cones)

What they are

  • Rods: very sensitive, work in low light; many rods converge onto a single bipolar cell.

  • Cones: less sensitive but high acuity; responsible for color and fine detail.

Function

  • Transduction: convert light into electrical signals.

  • Rods: sensitivity over specificity → detect faint light.

  • Cones: specificity over sensitivity → color and detail.

Why they matter

  • They are the first step of the visual pathway and determine how much light info enters the system.

🎴 CUE CARD 2 — Horizontal Cells

What they are

  • Lateral inhibitory interneurons connecting photoreceptors to each other and to bipolar cells.

  • Separate horizontal circuits for rods vs cones.

Function

  • Gain control: adjust signals based on overall luminance.

  • Perform lateral inhibition to help define edges.

Why they matter

  • Keep the retina from saturating in bright light.

  • Enhance contrast by comparing neighboring photoreceptors.

🎴 CUE CARD 3 — Bipolar Cells (ON & OFF Types)

What they are

  • Relay cells between photoreceptors and ganglion cells.

  • Two major types:

    • ON bipolar cells: use metabotropic (mGluR6) receptors → invert the photoreceptor signal.

    • OFF bipolar cells: use ionotropic (AMPA/kainate) receptors → preserve photoreceptor signal.

Function

  • “Flip the synapse”:

    • ON bipolars activate when light increases.

    • OFF bipolars activate when light decreases.

Why they matter

  • Create parallel pathways for detecting light increments vs light decrements, improving contrast and temporal precision.

  • They are the first layer that begins feature extraction.

🎴 CUE CARD 4 — Amacrine Cells

What they are

  • Highly diverse group of mostly inhibitory interneurons.

  • More than 30 types in many vertebrates.

Function

  • Shape timing, motion detection, direction selectivity, contrast enhancement, and temporal filtering.

  • Provide inhibitory modulation to bipolar and ganglion cells.

Why they matter

  • They prove the retina is not a camera—it computes.

  • They control Signal-to-Noise Ratio (SNR) by sharpening temporal signals and suppressing noise.

🎴 CUE CARD 5 — Retinal Ganglion Cells (RGCs)

What they are

  • The output neurons of the retina.

  • Their axons form the optic nerve.

  • Many types: ON-center, OFF-center, direction-selective, motion-sensitive, looming detectors, etc.

Function

  • Encode features, not raw pixels.

  • Convert processed retinal information into spike trains sent to the brain.

Why they matter

  • Show that the retina performs feature extraction such as edges, motion, direction, and looming threats.

  • The retina sends a compressed, meaningful representation of the visual world—not a photograph—to the brain.

🌟 ULTRA-BRIEF SUMMARY CARD

Retina = layered computational system

  • Photoreceptors (rods & cones): transduction, sensitivity vs specificity

  • Horizontal cells: luminance correction & gain control

  • Bipolar cells (ON/OFF): parallel channels, light increments vs decrements

  • Amacrine cells: inhibitory shaping, motion/temporal processing

  • Ganglion cells: feature extraction + output to brain

Overall: Retina improves SNR, performs gain control, and extracts features before sending signals to the brain.