Visual Acuity




(1)
University of Sydney, Sydney, Australia

 




Overview






  • Visual acuity (VA) is a measure of keenness of sight.


  • It is a relationship between the size of a stimulus and visual detection of that stimulus [1].


  • A high acuity implies a low threshold to detecting the stimulus.


  • VA is affected by:

    (a)

    Optical factors that influence the quality of light reaching the retina

     

    (b)

    Physiological factors that determine photoreceptor sensitivity and neural processing [2]

     


Visual Angle






  • Stimulus size is measured by the angle subtended at the nodal point of eye (Fig. 19.1) [3].

    A347009_1_En_19_Fig1_HTML.gif


    Fig. 19.1
    Nodal point of the eye


  • The minimum angle of resolution (MAR) is the smallest visual angle resolvable by that eye.


  • It is mostly determined by the foveal photoreceptor density.


Types of Visual Acuity (Table 19.1; Fig. 19.2)





Table 19.1
Types of visual acuity [1, 2, 4–9]






































 
Minimum visible
Minimum resolvable (ordinary visual acuity)
Minimum discriminable (hyperacuity)

Task
Determine presence or absence of a target
Distinguish features of target (e.g., form, shape, pattern)
Determine relative location of >1 visible features

Example
Is there a dot?
Is that an E or an F?
Is the upper line to the left or right of the lower line?

Influential stimulus factors
Contrast

Size
Contrast

Size

Spacing
Relative location

Vertical separation

Influential physiological factors
Sensitivity of photoreceptors to light
Density of foveal photoreceptors
Neural processing:

 Retinal ganglion cell center surrounds antagonist receptive fields

 Cortical linear receptive fields

Best threshold
~1 s of arc
~1 min of arc
~3 s of arc


A347009_1_En_19_Fig2_HTML.gif


Fig. 19.2
(a) Minimal visible; (b) minimal resolvable; (c) minimal discriminable visual acuity





  • There are various types of visual acuity that differ according to visual task and threshold.

    (i)

    Minimum visible

     


  • This refers to detecting the presence of a visual stimulus [4].

    (ii)

    Minimum resolvable

     


  • This refers to distinguishing details (form, shape, pattern) of a visual stimulus [1, 5].

    (iii)

    Minimal discriminable (hyperacuity, vernier)

     


  • This refers to detecting a discontinuity of alignment (relative location of more than one object) [6].


  • It has a lower threshold than minimum resolvable VA and is not limited by foveal photoreceptor density.


  • It involves enhanced border contrast sensitivity by ganglion cell and cortical processing [7–9].


  • Hyperacuity is less affected by optical blur than minimal resolvable VA [10].


Factors Influencing Visual Acuity




1.

The point spread function

The point spread function defines how the optical components (clear media) of the eye process light.



  • Consider light from a distant point source image (e.g., a star).


  • Due to imperfections of the optics of the eye, the light does not reach the retina as a point; rather the light falls on the retina in a distribution similar to that shown in Fig. 19.3a, b [11, 12].

    A347009_1_En_19_Fig3_HTML.gif


    Fig. 19.3
    The point spread function. (a) Good optical transmission; (b) poorer optical transmission than in a; (c) two points at a fixed retinal distance x can be discriminated by system (a); (d) at the same distance x the two points cannot be discriminated by the poorer optical system (b) (Based on Levi [2])


  • This distribution is the point spread function (Fig. 19.3) [2].

 

2.

Optical factors

Optical factors affect the point spread function of the retinal image.

(i)

Refractive error



  • Roughly 0.5 diopters of spherical refractive error blurs the VA by 1 Snellen line [13].


  • Cylindrical refractive error can also reduce VA [14].

 

(ii)

Media opacities [15]

 

(iii)

Pupil size



  • VA is maximal through pupil size 2–6 mm.


  • A large pupil (>6 mm) reduces VA by increasing spherical and higher-order aberrations [16].


  • A small pupil (<2 mm) reduces VA by increasing diffraction of light [17, 18].

 

(iv)

Wavelength



  • VA is marginally better for monochromatic light; however, this is most noticeable at low contrast.


  • This is because chromatic aberration leads to image degradation [19, 20].

 

 

3.

Physiologic factors

(i)

Foveal cone density



  • The density of foveal cone packing is a critical determinant of fine visual resolution.


  • This is because the more closely packed the photoreceptors, the better the visual system’s ability to discriminate differences in light distribution [5, 21].


  • At least two cones for each cycle of a test sinusoid are required for good resolution (Fig. 19.4) [2].

    A347009_1_En_19_Fig4_HTML.gif


    Fig. 19.4
    Foveal photoreceptor density determines the limits of visual acuity resolution; testing sine waves below (a) and at (b) the limits of resolution are faithfully resolved; (c) frequencies with <2 cones per cycle cannot be resolved (Based on Levi [2]) [2]


  • Human foveal cones are separated by approximately 30 s of arc; hence, MAR is 1 min [5].

 

(ii)

Cone to ganglion cell convergence



  • The fovea is characterized by 1:1:1 cone-bipolar-ganglion cell convergence [22, 23].


  • This reduces receptive field size to maximize image resolution.

 

(iii)

Retinal illumination



  • At low (scotopic) luminance levels mediated by rods, VA is reduced; however, it increases with increasing retinal illumination.


  • Under moderate photopic luminance conditions (ranging from full moonlight to a sunny sky), VA remains fairly constant [2, 18].

 

(iv)

Contrast (see Chap. 20. Contrast Sensitivity)



  • Contrast is the difference between stimulus and background illumination.


  • Reduced contrast between stimulus and background can lead to reduced VA [24, 25].


  • VA is generally tested under conditions of high contrast, e.g., black letters on a white screen.


  • Contrast sensitivity is our ability to detect a change in luminance over a uniform background.


  • It is best described using a spatial contrast sensitivity function reflecting contrast detection threshold at different spatial frequencies [26].


  • Individuals with subnormal contrast sensitivity (e.g., from cataract) can have normal or reduced VA.

 

(v)

Retinal eccentricity



  • In photopic conditions, maximal VA occurs when using the central fovea; VA falls rapidly with eccentric fixation [24, 27].


  • This is due to reduced cone density, increased convergence, and summation of neural pathways with increased eccentricity (see Chap. 8. The Retina) [21, 23].

 

(vi)

Target and eye movement (see Chap. 22. Temporal Properties of Vision)



  • Visual sensitivity decreases during saccades and during significant object movement [28].


  • However, there is increased visual sensitivity immediately prior to the saccade [29].


  • Microsaccades do not detract from acuity; they are necessary to prevent Troxler’s phenomenon [30–32] and precisely locate gaze for high-acuity tasks [33].

 

(vii)

Duration of exposure (see Chap. 22. Temporal Properties of Vision)

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Oct 28, 2016 | Posted by in OPHTHALMOLOGY | Comments Off on Visual Acuity

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