Lecture 3 the light sense pt2 dark and light

Q: What are the three lighting conditions and which photoreceptors operate in each?

Photopic (>10 cd/m²) → cones

Mesopic (10⁻³ to 10 cd/m²) → rods + cones

Scotopic (<10⁻³ cd/m²) → rods

Q: How can the retina function over such a huge range of light levels?

Adaptation occurs through:

Rod and cone systems

Pupil size changes (~1 log unit)

Photopigment bleaching/regeneration

Neural adaptation (1-2 log units)

Q: What is dark adaptation?

The recovery of visual sensitivity after exposure to bright light when entering darkness.

Q: How is dark adaptation measured?

Adapt eye to bright light (~5000 cd/m²) for ~3 min

Bleach photopigments

Turn off bleaching light

Measure threshold detection of a dim peripheral test stimulus over time

Q: Describe the dark adaptation curve.

Two phases:

Cone phase

Cones recover first

Dominate first 5-8 min

Rod phase

Rods become more sensitive

Dominate afterwards

Rod-cone break occurs at ~5-8 min.

Q: When is maximum rod sensitivity reached?

Approximately 30-35 minutes after entering darkness.

Q: Why does the test stimulus appear violet at first and colourless later during dark adaptation?

Early: cones detect the violet stimulus

Later: rods dominate vision

Rods do not provide colour perception

Q: What factors affect the shape of the dark adaptation curve?

Intensity of pre-adapting light

Duration of bleaching

Retinal location tested

Stimulus wavelength

Rhodopsin regeneration

Q: How quickly do photopigments regenerate?

Rhodopsin: ~50% regenerated in 5 min

Cone pigments: ~50% regenerated in 1.5 min

Q: What is the absolute threshold of scotopic vision?

The minimum amount of light detectable by a fully dark-adapted visual system.

Measured after ~30-40 min of dark adaptation.

Q: What are the key conclusions from absolute-threshold experiments?

5-14 quanta absorbed at threshold

A single quantum can activate a rod

A single rod cannot produce vision alone

Rod signals must be pooled

Q: What is rod pooling and why is it important?

Many rods converge onto a ganglion cell.

Advantage

Excellent light sensitivity

Disadvantage

Reduced spatial resolution

Q: What is an increment threshold (JND)?

The smallest increase in light intensity (ΔL) required to detect a difference from a background luminance (Lb).

Also called the just noticeable difference (JND).

Q: State Weber's Law.

Q: What are the important regions of the TVI (threshold versus intensity) curve?

Part 1

Threshold independent of background (dark noise dominates)

Part 2

Threshold ∝ √background luminance

de Vries-Rose Law

Part 3

Threshold ∝ background luminance

Weber's Law

High luminance

Cones determine threshold

Low luminance

Rods determine threshold

Q: How is Weber's Law used clinically?

Through static perimetry (visual field testing).

Fixed background luminance

Small light increments presented

Threshold sensitivity measured across visual field

Used to detect visual-field defects and ocular disease.

Q: Explain dark adaptation, light adaptation and Weber's Law.

Scotopic vision uses rods (<10⁻³ cd/m²).

Photopic vision uses cones (>10 cd/m²).

Mesopic vision uses both systems.

Adaptation allows vision across ~13 log units of luminance.

Adaptation occurs through rod/cone systems, pupil changes, photopigment regeneration and neural adaptation.

Dark adaptation is recovery of sensitivity following bleaching.

Cones recover first.

Rod-cone break occurs at ~5-8 min.

Rod sensitivity peaks at ~30-35 min.

Rhodopsin regenerates more slowly than cone pigments.

Rod density peaks around 15-20° eccentricity and rods are absent at the fovea.

Absolute threshold is measured after full dark adaptation.

At threshold, only a few photons are needed.

A single photon can activate a rod.

Vision requires pooling of signals from multiple rods.

Pooling increases sensitivity but reduces spatial resolution.

Increment threshold (JND) is the smallest detectable increase in luminance.

Weber's Law:

TVI curves describe light adaptation.

de Vries-Rose region: threshold ∝ √background luminance.

Weber region: threshold ∝ background luminance.

Static perimetry uses increment thresholds to assess visual field sensitivity.