Vision

Vision uses more neurons and brain volume than any other senses in most animals.

Light has properties including

  • Intensity

    • Can vary with a factor of 1020

  • Direction

  • Colour

    • Depending on wavelength

    • Visible light between 400-700nm

  • Polarisation

    • Two components perpendicular to one another

      • Electric field

      • Magnetic field

    • Unpolarised light

      • Each light wave has its electrical field vector in a random direction

      • E.g. direct sun, moon light

    • Polarised light

      • All light waves have parallel E-vector direction

      • E.g. skylight, light reflected from a water surface

      • Humans are not sensitive to polarised light, unlike insects, crustaceans, fishes, birds, and molluscs (etc)

All eyes contain

  • A pupil

    • Light enters

  • A retina

    • Contains photoreceptors

      • Absorb the light, converting the energy to an electrical signal

  • Advanced eyes also have an optical system close to the pupul

    • Often 1 or several lenses

      • Focuses the light to create an image

        • Processed by interneurons in the retina, and then in the visual cortex in the brain (mammals)

Human eyes

  • Curved cornea provides most focussing power

  • The lens fine-tunes the focus (= accommodation)

  • The pupil controls the brightness

The retina contains two kinds of photoreceptors

  • Rods

    • Sensitive

      • Used in dim light

    • Low spatial resolution vision

    • No colour

  • Cones

    • Less sensitive

      • Used in bright light

    • High spatial resolution vision

    • Colour

2 types of connections for visual processing

  • Through-line connections

    • Transport information from one level to the next

      • From photoreceptors to bipolar cells to ganglion cells

  • Lateral connections

    • Shape and modify the through-line information via excitatory and inhibitory interactions within one level

      • Horizontal cells & amacrine cells

The ganglion cells build the last level of retinal processing, and it is their density in the retina that sets the spatial resolution of the vertebrate eye. The axons of ganglion cells constitute the optic nerve, and the information output of the eye. These axons travel along the optic tract to the lateral geniculate nucleus (LGN), and neurons here travel further to the visual cortex. The LGN and the visual cortex are retinotopically organised (signals from neighbouring photoreceptors are processed by neighbouring parts of the respective brain area).

All vertebrates have camera eyes, including cephalopods and many other molluscs.

The alternative is compound eyes, used by insects, most crustaceans, myriapods etc., and are the most widespread types of eyes among animals.

  • Composed of identical units called ommatidia

    • Each consisting of a lens element

      • Corneal lens and crystalline cone

        • Focuses light onto a bundle (rhabdom) of photoreceptors

The conversion from light to electrical signals (in the photoreceptors) is called transduction.

Vertebrate photoreceptors consists of

  • The inner segment, with the cell body

  • The outer segment, with stacks of membranes

    • The membranes are the site of transduction, and are rich in the protein rhodopsin, which absorbs the light and starts the transduction

Vertebrate photoreceptors hyperpolarise in response to light, while invertebrate photoreceptors depolarise.

Rhodopsin molecules absorb a broad range of wavelengths. with the exact interval depending on the type of rhodopsin.

The human retina has 1 type of rod an 3 types of cones

  • Blue S cones, maximally sensitive at 430 nm

  • Green M cones, maximally sensitive at 540 nm

  • Red L cones. maximally sensitive at 575 nm

If a photoreceptor absorbs a photon, the information about the wavelength is completely lose. Instead, only the type of photoreceptor that absorbed the photon is known + how many photons they absorb.

  • Our colour vision is thus based on the comparison of our visual system.

    • E.g. a wavelength of 530 nm gives it a 95% probability to be absorbed by the green cones, while only 70% and 0% for red and blue cones respectively

      • The comparison is made in the neural circuits in the retina and the visual cortex.

      • The result is an interpretation of the colour green

Animals use colour vision to find and recognize

  • Mates

  • Food

  • Shelter

  • Phototaxis

  • Other important objects for survival