Human vision includes the ability to perceive the world in three dimensions (height, width, and depth), rather than as a flat, two-dimensional image. This capacity, known as depth perception, is crucial for judging distances and navigating the environment.

Human Depth Perception

Based on the principles of how color is perceived, explain why the individual sees these specific new colors (red and blue) in the afterimage.

Explaining a Visual Phenomenon

A colored-dot number plate can form a hidden number by arranging many small circles of different shades and sizes so that the number is visible only because its circles have a different color from the background circles.

Colored-Dot Number Plate Example

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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- 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

Spielman, R. M., Jenkins, W. J., & Lovett, M. D. (2020, April 22). Psychology (2nd ed.). OpenStax. https://openstax.org/books/psychology-2e/pages/1-introduction

OpenStax Psychology (2nd ed.) Textbook

- 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

- 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

Area V4: Shape and 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. 

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