Color Vision




(1)
University of Sydney, Sydney, Australia

 




Overview






  • Color is a subjective sensory phenomenon, not a physical attribute of an object.


  • Color perception arises from stimulation of cones by light.


  • Color perception varies with:

    (a)

    The spectral composition of light reflected from object

     

    (b)

    The ambient light surrounding the object

     

    (c)

    The subject’s level of visual adaptation

     


  • Humans can distinguish possibly 7–10 million colors [1].


Color and Light






  • Monochromatic light is colored light of a single wavelength (Table 24.1).


    Table 24.1
    Wavelengths corresponding to spectral colors




























    Spectral color
    Wavelength (nm)

    Violet
    430

    Blue
    460

    Green
    520

    Yellow
    575

    Orange
    600

    Red
    650


  • White light can be decomposed into a spectrum of colors using a prism [2].


  • A wide range of colors can be reproduced by an appropriate combination of the additive primary colors: blue, green, and red.


  • Complementary colors are two appropriately selected colors which mix to produce white light.


  • Metamers are physically distinct combinations of light that appear identical; e.g., monochromatic yellow light is a metamer of yellow produced by red and green light combined.


Perception of Colors


Colors can be subjectively appraised and graded by three qualities: hue, saturation, and brightness.

(i)

Hue



  • Hue is the aspect of color allowing it to be assigned a position on a color spectrum.


  • It is related to the wavelength of monochromatic light.


  • In paint theory, hue is often referred to as a “pure color.”

 

(ii)

Saturation



  • Color saturation is determined by dilution of hue by white.


  • Pure hue is complete saturation; it can be progressively desaturated until white is reached.

 

(iii)

Brightness



  • Brightness is the apparent intensity of color: varying from very dim to dazzling.


  • It is ‑related to the object’s radiant energy.

 


Phenomena in Color Perception




1.

Colour inconstancy



  • An object’s apparent color changes by altering background spectral composition (Fig. 24.1).

    A347009_1_En_24_Fig1_HTML.gif


    Fig. 24.1
    Color inconstancy. The inner square is identical on either side of the image. It appears pale blue against a deep orange background; it appears darker against a pale blue background


  • Similarly, the color can appear to remain the same despite changes in ambient light effecting the spectral composition of light from the object and its background [3, 4].


  • This is because color perception is not due to the absolute spectral composition of light from an object, but the spectral composition relative to the background.

 

2.

The Abney effect:



  • Desaturating a specific wavelength by adding white can change the apparent hue [5].


  • The desaturated stimulus is perceived as a new hue possibly because of post-receptoral mechanisms that are necessary for maintaining color constancy [6].

 

3.

Bezold-Brucke effect:



  • Hues appear to change with changes in light intensity: [7, 8]


  • As intensity increases, spectral colors shift toward:

    (a)

    Blue (for wavelengths below 500 nm)

     

    (b)

    Yellow (wavelengths above 500 nm)

     


  • At lower intensities, the red/green axis dominates.

 


Trichromacy: Cone Transmission of Color






  • Normal color vision is trichomatic, mediated by three types of cone receptor distinguishable by their spectral sensitivity:

    (a)

    Short-wavelength-sensitive (SWS or S) cones

     

    (b)

    Middle-wavelength-sensitive (MWS or M) cones

     

    (c)

    Long-wavelength-sensitive (LWS or L) cones [9]

     


  • Each type has a distinctive photoreceptor pigment that determines spectral sensitivity.


  • There is considerable overlap in spectral sensitivity between the three cone populations; however, each has a specific peak spectral sensitivity (Table 24.2, Fig. 24.2).


    Table 24.2
    Spectral sensitivity of three types of cone receptors
























    Cone population
    Spectral sensitivity peak (nm)
    Major color sensitivity

    Short wavelength sensitive (S)
    440–450
    Blue

    Middle wavelength sensitive (M)
    535–550
    Green

    Long wavelength sensitive (L)
    570–590
    Red


    A347009_1_En_24_Fig2_HTML.gif


    Fig. 24.2
    Overlapping spectral sensitivity curves for SWS, MWS, LWS cones and rods


  • The wavelength of light determines the likelihood of stimulating each cone type.


  • Most cones are either M or L; S cones make up 5–10 % and are not found within the central fovea [10].


  • Trichromacy allows a full range of colors to be distinguished [11].


Opponent Processes: Color Processing in the Inner Retina and Lateral Geniculate Nucleus






  • The three cone types give rise to perception of hues arranged in two opponent pairs:

    (a)

    Red/green (R/G)

     

    (b)

    Blue/yellow (B/Y)

     


  • Opponent processing is found in inner retinal circuitry and the lateral geniculate nucleus.


1.

Inner retinal color processing



  • Inner retinal color processing occurs through distinct R/G and B/Y opponent channels.


(i)

Red/green opponency



  • R/G perception is conveyed by color opponent midget ganglion cells (MGCs) with center–surround antagonistic receptive fields (CSARFs) [12, 13].


  • These cells compare M and L cone inputs [14–16].


  • Color opponent midget cell CSARFs are organized such that the center and surrounds are dominated by opposing M and L cone types; i.e., M–center/L–surrounds or L–center/M–surrounds.

 

(ii)

Blue/yellow opponency



  • B/Y opponency is conveyed through small bistratified ganglion cells that receive:

    (a)

    ON signal from S cone inputs (the blue signal)

     

    (b)

    OFF signal from summated M and L cone inputs (the yellow signal) [17]

     


  • In addition, melanopsin–containing ganglion cells convey B/Y information [18].


  • Other combinations of S cone input with M and/or L cone inputs are reported [19] but not yet well understood.

 

(iii)

Achromatic information



  • Achromatic information is conveyed through parasol ganglion cells [20, 21].

 

 

2.

Lateral geniculate nucleus (LGN) color processing



  • Color opponent LGN cells are parvocellular cells in laminae 3–6 that receive MGC projections [22].


  • They have similar receptive field properties to the MGCs that provide their input.


  • Most R/G parvocellular cells transmit color opponency; these have CSARFs which have color opponency to large spot sizes and spatial luminance sensitivity (acuity) to small spots [23].


  • Some koniocellular LGN cells receive small bistratified ganglion cell B/Y opponent information [24].

 


Color Processing in the Visual Cortex




1.

The primary visual cortex (V1) (see Chap. 14, The Primary Visual Cortex)



  • Chromatic projections arrive in V1 along separate LGN R/G and B/Y channels [16].


  • Information from parvocellular channels projects to V1 layers 2 and 3; parvocellular projections are used for both achromatic luminance sensitivity and R/G color processing [25].


  • B/Y signal is conveyed via koniocellular channels that project to superficial layers of V1 [16, 24].


  • There is considerable overlap between color and spatial processing in V1: most V1 neurons convey color information, and most of these are also selective for spatial properties and orientation [26–30].

 

2.

Color-sensitive neurons in V1



  • Sensitivity to color in V1 occurs predominantly through the combined activity of two kinds of neurons: single–opponent and double–opponent cells.


  • These have distinct functions: the single-opponent cells respond to large areas of color, while double-opponent cells respond to color boundaries, patterns, and textures.

    (i)

    Double-opponent cells



    • They make up the majority of color-sensitive neurons in V1 layers 2 and 3 [23, 25].


    • Their receptive fields are both chromatically and spatially opponent [23, 31].


    • They respond strongly to color bars but weakly to full-field color stimuli [30].


    • Most have red-cyan color opponency (L versus M + S input); a minority are blue-yellow opponent (S versus M + L) [19, 23, 30].


    • Because of their specialized receptive field structure, they are candidates for the neural basis for color contrast and color constancy [32–34].

     

    (ii)

    Single-opponent cells



    • These have center-surround properties without orientation selectivity [30].


    • Different from double opponent cells, they are stimulated by large homogenous fields of color [35].

     

    (iii)

    Complex opponent cells



    • These cells respond to color contrast without having double-opponent receptive fields [23].


    • Color stimuli from a wide range of visual field loci can elicit a response.


    • They are analogous to complex cells with specific orientation selectivity over a large area of visual field (see Chap. 14).

     

 

3.

The extrastriate visual cortex (see Chap. 15, The Extrastriate Cortex)

(i)

V2

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Oct 28, 2016 | Posted by in OPHTHALMOLOGY | Comments Off on Color Vision

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