- Partial or complete loss of color vision despite presence of normal functional cones in retina
- Similar to more common cases of color blindness, except that it is due to lesions in the brain, rather than lack of specific cones
- Area of the brain associated with this disorder is area 4 (V4), suggesting that the ventral stream of the visual system is associated with color processing

Achromatopsia

- This area receives input from blob and interblob regions of V1
- Neurons in this area have larger receptive fields and many are color and orientation specific, suggesting their prominent role in the perception of color and shapes
- Damage in this area could result in achromatopsia

University of Colorado at Boulder

- Some information from the primary cortex extends ventrally towards the temporal lobe
- This is called the ventral stream
- Consists of several brain regions, thought to be involved in the perception of the visual world and the recognition of objects

Ventral Stream

- Depends on special channels from retina to area V4 of visual cortex
- 4 stages of color perception:
    - Cones in retina respond to different light wavelengths
    - Information is processed by retinal cells: retinal ganglion cells are excited by some light and inhibited by others
    - Ganglion cells send information to the LGN
    - Information goes to area V1, and then to additional cortical areas

Color perception

Bear, M. F., Connors, B. W., & Paradiso, M. A. (2016). Neuroscience: Exploring the brain. Wolters Kluwer. 

Neuroscience: Exploring the Brain

Area V4: Shape and Color Perception

- This area receives information from P cells, which are closely associated with color and shape recognition, as well as the cones of the retina.
- Cells in this area are strongly stimulated by color and abstract shapes
- Output from area IT goes to temporal lobe structures associated with learning and memory, suggesting that area IT may be important for visual perception and visual memory

Area IT - Inferior Temporal Gyrus

- Face-selective part of the brain
- This area could be responsible for facial recognition, since stroke damage here can lead to difficulty in recognizing faces (prosopagnosia)

Fusiform Face Area (FFA)

- OFA has been suggested to be sensitive to facial identity (when differing between a classmate and a professor's face, for example)

Occipital Face Area (OFA)

- Receptor cells that have differing sensitivity to different wavelengths of light
- Every cone has one out of the three pigment types
- The vast majority of visual objects stimulate at least 2 pigment types
    - Allows for good visual acuity and perception of form
- Neurons detect differences between cones, allowing for color perception

Retinal cones

Color 'blindness' is more accurately described as a color vision deficiency. The most prevalent type is red-green color blindness, an X-linked inherited condition caused by the absence of cones that process medium-wavelength light. Because the responsible gene is on the X chromosome, and men have only one X chromosome, this deficiency is far more common in men than in women. Women possess two X chromosomes, which typically provides a functional gene if the other is mutated. This condition impairs the ability to distinguish between certain colors, especially reds and greens.

Color "blindness"

A spectrum of color vision exists across mammals. There are four main categories that mammals’ color vision falls into.
- Excellent trichromatic vision-- present in diurnal primates (eg: humans and rhesus monkeys).
- Robust dichromatic vision-- present in species with two cone types and many cones (eg: dogs and pigs)
- Feeble dichromatic color vision-- present in species with two cone types and few cones (eg: house cats)
- Minimal color vision-- present in species with one cone type (eg: owls and raccoons)


Color perception in different mammalian species

Retinal ganglion cells and parvocellular LGN cells exhibit spectral opponency. This means they are activated by certain wavelengths and inhibited by others.
- Each of these ganglion cells processes information from two or three types of cones.
- Some of these cones are excitatory and some are inhibitory.
- Ganglion cells respond based on the differences between excitatory and inhibitory input.
- Proportion of spectrally opponent cells in the LGN correlates to degree of color vision.


Spectral opponency in retinal and LGN cells

- Spectrally opponent cortical cells are involved in color perception
- Wavelength information is used to separate objects from their background (color and/or intensity).
- Helps to distinguish objects from one another


Visual cortex cells involved in color perception

Stimulus such as colors might be readily encoded among individuals with Autism Spectrum Disorder because improvement in performances such as the cueing task is found when colors are involved. 

Color perception in Children with Autism Spectrum Disorder 

ASD and Perception Reference

A fundamental characteristic of human sight is color vision, the ability to perceive the world in a spectrum of colors rather than in monochrome. This allows for the differentiation of objects based on the wavelengths of light they reflect.

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