Light Adaptation and Vision

Light Adaptation

• The visual system changes to different illumination levels through this process.
• This is studied with an increment threshold procedure.
• A threshold is determined, then the background intensity is increased, and the measurement is repeated.
• This happens for light levels ranging from darkness to extreme brightness.
• Light adaptation curve can be split into sections: Dark light, Square Root Law, Weber's Law and Saturation.

Scotopic Portion

• This section has a slope (m) of zero.
• Light levels in this range are limited by the neural noise within the visual system.
• The background is practically black.
• Neural noise produces as much "dark light" as the background itself.

Square Root Law

• The curve has a slope of ½.
• It reflects quantal fluctuations in the background.
• The background is dim enough that fluctuations inherent in the light source producing the background determine the threshold.
• This is expressed as the De-Vries-Rose law: ΔI∝√I.
• For brief, small test spots, increment thresholds rise as the square root of the background over the entire photopic range.

Weber's Law

• It covers a 4 log-unit range and has a slope of approximately 1.
• A slope of 1 means this law is followed.
• As the background brightness is increased, the increment intensity must be proportionately increased.
• The ratio of increment intensity to background intensity remains constant: ΔI/I=k
• This constant ratio is referred to as Weber's fraction or Weber's constant.
• The Weber's fraction for scotopic vision is about 0.14.
• The Just Noticeable Difference (JND) increases as illumination increases.
• Following Weber's law means the threshold contrast remains constant as illumination changes.
• Weber's law indicates increasing background illumination corresponds to increasing increment threshold (JND).
• Increasing illumination reduces the absolute sensitivity of the visual system.
• Sensitivity regulation occurs at the level of photoreceptors with calcium playing an important role.

Saturation

• Saturation has a slope of infinity, which indicates that rods are saturated.
• At high background illuminations, rods cannot signal the stimulus.
• The brightness of the background overwhelms the rods.
• Bleaching of a rhodopsin molecule closes sodium channels in the rod outer segment.
• This reduces sodium flow into the outer segment, leading to rod hyperpolarization that is dependent on the stimulus's intensity, up to a point.
• The limited number of sodium channels in a rod outer segment limits the magnitude of rod hyperpolarization.
• When about 10% of rhodopsin molecules are bleached, a critical number of sodium channels are closed.
• Consequently, bleaching of rhodopsin produces no further hyperpolarization and the rods are saturated.

Photopic Portion

• The indicated slope of 1 indicates Weber's law for the curve.
• The photopic Weber's fraction is about 0.015.
• The photopic system is more sensitive to contrast than the scotopic system.
• Cones do not show a saturation effect.

Convergence in the Retina

• 126 million rods and cones converge to 1 million ganglion cells.
• Rods have a higher convergence than cones.
• 120 rods to one ganglion cell, compared to six cones to one ganglion cell
• Cones in the fovea have a one-to-one relation with ganglion cells.

Spatial Resolution vs Summation

• Visual resolution is superior under daylight conditions (photopic VA=20/20, scotopic VA is 20/200).
• Sensitivity occurs in scotopic conditions with that spatial resolution tradeoff.
• A trade off between resolution and sensitivity is due to how rods and cones connect to the postreceptoral elements of the retina.
• Rods are connected in such a way as to sum up information over space.
• The scotopic system has spatial summation and manifests greater summation.
• Cones maximize visual resolution at the expense of sensitivity.
• The scotopic system produces excellent sensitivity, but poor spatial resolution.
• The photopic system shows less spatial summation, but is capable of excellent spatial resolution (20/20).

Spatial Resolution - Wiring

• Rods left in picture connect wiring, cones to the right
• Numbers generated by each indicate “spot” of intensity
• The number present the number of units units in response

Absolute Sensitivity of Vision

• Activation of one rod is due to one quantal absorption.
• The stimulus is detected when ten such activated rods activate a ganglion cell.
• A stimulus emitting only 10 quanta is not visible due to quanta reflected/absorbed.
• Less than 20% of the quanta incident on the retina are absorbed by rhodopsin.

Temporal Resolution vs Summation

• The scotopic system sums up information over time to a greater extent than the photopic system, indicating temporal summation.
• The photopic system is better able to distinguish two flashes of light separated by a brief interval.
• This shows that the photopic system has superior temporal resolution.
• The high degree of temporal summation of the scotopic system is consistent with its greater sensitivity, and sums information over both space (spatial summation) and time (temporal summation) to obtain excellent sensitivity.
• In contrast, the photopic system shows limited spatial and temporal summation and offers the system excellent spatial and temporal resolution.
• It is evident that there is a trade-off between summation and resolution

Stiles-Crawford Effect of the First Kind

• Rods and cones respond in a different manner to light rays that do not strike them perpendicular (orthogonal) to their surfaces.
• Under scotopic conditions, changes to brightness levels are low since the angle at which light rays are incident on rods is relatively insignificant.
• Under photopic conditions, light rays that strike cones perpendicularly are perceived as brighter, the Stiles Crawford effect.
• This effect is due to waveguide properties of the cones.
• The physical dimension of a quantum of light makes the angle of the funnel cone critical for rays.
• Deviation from orthogonal entry reduces effectiveness of light rays and brightness of the rays.
• Cones always point to the center of the pupil to maximize effectiveness.
• A congenital or secondary from trauma from stiles effects occur in the cones.
• Stiles crawford effect points to maximize the effectiveness of rays capturing the light.

Rod vs Cone

• Rods: Rod shape morphology and free disks with 0.14 ratio. Sensitive to dim light Cones: Conical attached disks with 0.015 ration. Not sensitive to bright light.