Visual Acuity and Contrast Sensitivity Quiz

Special Visual Function

• Visual acuity is the most commonly used measure of visual resolution
• It's used in clinical diagnosis, evaluation, legal screening, and professional communication
• Visual acuity assessment is helpful in correcting refractive errors
• Under certain circumstances, standard measurements of visual acuity do not predict performance on tasks like target detection and identification

Objectives

• Contrast sensitivity function
• Dark adaptation
• Dynamic vision
• Glare

Contrast Sensitivity

• Contrast sensitivity is a crucial parameter for evaluating vision
• Clinically, visual acuity often involves high contrast (e.g., black letters on a white background)
• In reality, objects have varying contrast
• Understanding the relationship between visual acuity and contrast provides a detailed understanding of vision

Contrast Sensitivity (C)

• Contrast sensitivity (C) can range from 0.0 to 1.0
• Sometimes also referred to as modulation or Michelson contrast
• Luminance of contrast gratings varies sinusoidally
• This allows for contrast to change without altering the average luminance across the screen

Sine Wave Gratings and Spatial Frequency (SF)

• Sine wave gratings are used to determine spatial frequency
• The size of grating bars is expressed as the number of cycles per degree
• Spatial frequency refers to the fineness or coarseness of the grating; units are cycles per degree

Spatial Frequency and Contrast

• The degree to which frequencies are amplified or reduced by the optical systems of the eye is described in modulation transfer function (MTF)
• Behavioral analogy of eye to MTF is the contrast sensitivity function (CSF)
• CSF describes how sensitive a person is to different spatial frequencies of sine wave gratings
• CSF is measured using contrast detection experiments

Contrast Sensitivity Function (CSF)

• Under photopic conditions (normal daylight vision), contrast sensitivity measurements display a band-pass function when using sinusoids
• The CSF peak is in the mid-spatial frequency range
• Maximum resolution is observed under high contrast conditions

Spatial CSF

• Spatial CSF is band-pass
• Mid-range spatial frequencies (3-10 cycles/degree) are perceived best

Factors for the shape of CSF

• Mean luminance of the grating (whether sinusoidal or square waveforms)
• Level of defocus
• Clarity of eye optics
• At low light levels, maximum contrast sensitivity is approximately 8%, and maximum resolution is approximately 6 cycles per degree
• As light levels increase, the peak of contrast sensitivity function is closer to 0.5% contrast
• At higher levels, the cutoff frequency for high spatial frequencies is around 50 to 60 cycles per degree

Examples of How Refractive Error or Disease Alters CSF

• Refractive errors and eye diseases can affect the contrast sensitivity function (CSF)
• Patients with multiple sclerosis, and those with cataracts typically have reduced contrast sensitivity
• Mild refractive errors and mild amblyopia result in a mild effect on CSF
• More severe conditions result in further reduction

Multiple Spatial Frequency Channels

• CSF is not primarily a single neuron, but rather an envelope of sensitivity across several underlying mechanisms
• Neurons have varying preferred spatial frequencies
• Differing receptive field sizes correspond to these varying spatial frequencies

Dark Adaptation

• The eye adjusts to varying light levels
• The minimum light intensity needed for vision is determined
• Dark adaptation measures the recovery of sensitivity of the eye in a dark room

Factors Affecting Dark Adaptation

• Intensity and duration of pre-adapting light
• Size and location of the retinal region used to register test spot
• Wavelength of the threshold light
• Rhodopsin regeneration

Test Spot Location on Retina

• When small test spots are measured in the fovea, only one branch reflecting cone responses is evident
• When larger test spots are measured in the periphery, both rod and cone responses are present, and a notable break is seen

Test Spot Size on the Retina

• Small test spots primarily activate cones, and large test spots activate both cones and rods

Wavelength of Threshold Light

• Rods and cones have varying sensitivity to different wavelengths of light
• Rods are more sensitive to shorter wavelengths
• Rods are more sensitive to shorter wavelengths than cones at higher levels of adaption

Rhodopsin Regeneration

• Dark adaptation depends on rhodopsin regeneration,
• Bleaching rhodopsin reduces sensitivity
• 1% rhodopsin bleaching reduces sensitivity by 10 log units
• Differences in rod and cone sensitivities due to bleaching are linked to receptor-level processes

Summary

• The dark adaptation curve illustrates the transition between cone-mediated vision (starting point) and rod-mediated vision (later part of curve)

Red Light at Night

• Red light use at night does not significantly affect rod function

Dynamic Visual Acuity

• DVA assesses visual details in dynamic (moving) conditions

Kinetic VA

• KVA is ability to recognize objects moving horizontally or vertically

Vestibulo-Ocular Reflex

• The vestibulo-ocular reflex (VOR) helps maintain visual stability during head movements
• The reflex activates extraocular muscles to compensate for head movements

Potential Application

• DVA measurements are relevant for vision training, sports vision, and cases of low vision or traumatic brain injury