Colour Vision Overview 1

The Mystery of Color Perception

The subjective experience of color, particularly the "redness of red," presents a challenge in defining the qualitative nature of conscious experience. While individuals with normal color vision are assumed to share similar color sensations, it remains uncertain whether these experiences are indeed qualitatively identical across individuals. One person's perception of red might be another's perception of blue, raising questions about the true nature of shared sensory experiences.

Light and Electromagnetic Radiation

To understand the neuroscience of color vision, it's crucial to understand the properties of light as electromagnetic radiation. Light is a portion of the electromagnetic spectrum, emitted from sources like the sun in packets of energy called photons. These photons travel at the speed of light and vibrate at a frequency related to their energy. Electromagnetic radiation can be described by:

  • Wavelength

  • Frequency

The relationship between these is given by:

c=νλc = \nu \lambda

Where:

  • cc is the speed of light,

  • ν\nu is the frequency,

  • λ\lambda is the wavelength.

Rearranging, we find that wavelength and frequency are inversely proportional.

λ=cν\lambda = \frac{c}{\nu}

The Electromagnetic Spectrum and Visible Light

The electromagnetic spectrum ranges from high-energy, short-wavelength gamma rays to low-energy, long-wavelength radio waves. Visible light occupies a narrow band within this spectrum, ranging from approximately 400 nanometers (violet) to 700 nanometers (red).

While the visible spectrum is roughly the same across different animals, some species can perceive slightly into the ultraviolet range, while others may not see as far into the red.

Dimensions of Light as a Stimulus

Light as a stimulus varies in two key dimensions:

  1. Intensity: Measured as energy (e.g., watts per unit area), intensity relates to subjective brightness within the visible spectrum.

  2. Wavelength/Chromaticity: This relates to color. Shorter wavelengths appear violet or blue, mid-range wavelengths appear green to yellow, and longer wavelengths appear orange to red.

It's important not to confuse light intensity with the energy inherent in photons:

  • Light Intensity: The energy of light measured in watts per unit area representing brightness.

  • Photon Energy: The energy of individual photons, where higher frequency photons carry more energy.

For a given light source, the energy measured in watts per unit area would depend on the number and energy of the photons. High-energy photons (short wavelengths) mean fewer photons are needed to achieve the same energy level compared to low-energy photons (long wavelengths).

Light Intensity Levels

Light intensity varies over an enormous range, approximately 12 orders of magnitude. Luminance, measured in candelas per square meter, ranges from 10−610^{-6} to 10610^{6}, corresponding to light levels from a dark, moonless night to bright sunlight.

Visual function operates across much of this range:

  • Scotopic Vision: Operates in low-light conditions (starlight), primarily using rods.

  • Photopic Vision: Operates in bright-light conditions (indoor lighting, sunlight), primarily using cones.

  • Mesopic Vision: An intermediate range where both rods and cones contribute.

Measuring Light Intensity

Light intensity can be measured in three primary ways:

  1. Radiant Energy: Measured in watts per square meter, this applies to all electromagnetic radiation, including visible light, UV, and X-rays.

  2. Luminance: Measured in candelas per square meter, this is radiant energy scaled according to the spectral sensitivity of the typical human eye. Humans are most sensitive to light in the green-yellow region of the spectrum.

  3. Photon Flux: Measures the flow rate of individual photons, quantified as photons per unit area per unit time. This is crucial for visual neuroscientists quantifying photoreceptor responses, as the number of photons absorbed determines the response.

The energy per photon varies across the spectrum. At longer wavelengths (red end), photons have lower energy, resulting in higher photon flux for a given level of radiant energy. Conversely, at shorter wavelengths (blue end), photons have higher energy, resulting in lower photon flux for the same radiant energy.

Photoreceptors: Rods and Cones

Vertebrates have a duplex retina, containing:

  • Rods: For scotopic vision (low light).

  • Cones: For photopic vision (bright light).

Humans have a single spectral class of rod, with peak sensitivity around 500 nm. There are three spectral classes of cones:

  • Short Wave (Blue) Cones: Sensitive to shorter wavelengths.

  • Middle Wave (Green) Cones: Sensitive to middle wavelengths.

  • Long Wave (Red) Cones: Sensitive to longer wavelengths.

Spectral sensitivity refers to the wavelengths of light at which a photoreceptor is most sensitive. Photon absorption is a stochastic process, with the highest probability of absorption at the peak sensitivity wavelength.

The spectral peaks of cones are broadly tuned:

  • The blue cone absorbs photons most effectively around 420 nm but can still absorb photons at other wavelengths.

  • The shapes of sensitivity curves dictate color perception deriving from the ratios of activation across the three cone types.

Luminance and Chromaticity Signalling

Chromaticity (color) is signaled by the comparison of cone signals. Greater signals from long-wave cones relative to middle-wave cones indicate orange to reddish chromaticities. The converse indicates greenish to bluish regions.

Luminance (brightness) is determined by the sum of photoreceptor signals across all classes. Brightness increases as the total signal from photoreceptors increases.

What is Color?

Color is not simply wavelength. Hue, or color appearance, changes with wavelength. As wavelength increases, hue changes from violet to blue to green to yellow to red.

  • Spectral Colors: Colors associated with specific wavelengths (e.g., 450-470 nm for blue, 540 nm for green).

  • Non-Spectral Colors: Colors that cannot be equated with a specific wavelength (e.g., purple, pink, brown, black, white). Black and white are achromatic but are still used as color descriptors.

Color depends on wavelength but cannot be directly equated with it. The complexity of color perception will be further explored in the next segment.