Dictionary tag: P-Q-R-S-T

Trivariance

Trivariance

The term trivariance is used to refer to this first stage of the trichromatic process. It refers to both the phototransductive response of the cone cells themselves and to the three separate channels used to convey their colour information forward to subsequent levels of neural processing.

Each channel conveys information about the response of one cone-type to both the wavelength of the incoming light it is tuned to and to its intensity. In both physiological and neurological terms this process is exclusively concerned with trivariance – three discernible differences in the overall composition of light entering the eye.

It is the separation of the signals produced on each channel that accounts for the ability of our eyes to respond to stimuli produced by additive mixtures of wavelengths corresponding with red, green and blue primary colours. But more of that later!

By way of summary, the rod and trivariant cone systems are composed of photoreceptors with connections to other cell types within the retina. Both specialize in different aspects of vision. The rod system is extremely sensitive to light but has a low spatial resolution. Conversely, the cone system is designed to function in stronger light. As a result, cones are relatively insensitive compared with rods but have a very high spatial resolution. It is this specialisation that results in the extraordinary detail, resolution and clarity of human vision.

Rod System Cone System
High sensitivity, specialized for night vision Lower sensitivity specialized for day vision
Saturate in daylight Saturate only in intense light
Achromatic Chromatic, mediate colour vision
Low acuity High acuity
Not present in the central fovea Concentrated in the central fovea
Present in larger number than cones Present in smaller number than rods

Caption

Trough

A trough is the point on a wave with the maximum value of downward displacement within a wave-cycle. A crest is the opposite of a trough, so the maximum or highest point in a wave-cycle.

  • On a wave at sea, the trough is the lowest point in the wave cycle, where the water displacement is furthest down from its rest position. A crest, on the other hand, is the highest point where the displacement is furthest up.
  • For electromagnetic waves, which have electric and magnetic fields, a trough on either axis represents the point where the field reaches its minimum value in the downward direction. A crest represents the point of maximum value in the upward direction.
  • Wavelength refers to a complete wave-cycle from one crest to the next, or one trough to the next.
  • Frequency refers to the number of wave cycles that pass a given point in a given amount of time.
  • The amplitude of a wave is a measurement of the distance from the centre line (or the still position) to the top of a crest or to the bottom of a corresponding trough.
  • Amplitude is related to the energy a wave carries. The energy a wave carries is related to frequency and amplitude. The higher the frequency, the more energy, and the higher the amplitude, the more energy.
Related diagrams

Each diagram below can be viewed on its own page with a full explanation.

References

Trough

A trough is a point on a wave with the maximum value of downward displacement within a wave-cycle. A crest is the opposite of a trough, so the maximum or highest point in a wave-cycle.

  • On a wave at sea, the trough is the lowest point in the wave cycle, where the water displacement is furthest down from its rest position. A crest, on the other hand, is the highest point where the displacement is furthest up.
  • For electromagnetic waves, which have electric and magnetic fields, a trough on either axis represents the point where the field reaches its minimum value in the downward direction. A crest represents the point of maximum value in the upward direction.