Colour Vision

Sensitivity & Acuity

Colour VisionTrichromatic theory of colour vision

• There is only one type of rod and this responds strongly to bluish-green light

•Cones are divided into three categories, each of which has a different sensitivity to light

• There are red light receptors, green light receptors and blue light receptors.

• These cone sensitivities support the trichromatic theory as all colours of the visible spectrum can be seen by mixing the 3 primary colours (red, blue and green)

•White objects reflect all colours to eye, black absorbs all colours so no light to the eye.

Wavelengths of light absorbed by different cones

Colour Blindness

• If you have normal vision you will see a figure seven in reddish brown dots.

• People with red-green colour blindness will not see the 7, why?

• These people lack red sensitive cones, but the green stimulated cones are stimulated by the red light, so all dots appear green

Sensitivity and Acuity• Rods and cones synapse with bipolar cells

• Bipolar cells synapse with ganglion cells

• Ganglion cells synapse with neurone fibres

• At the fovea each cone synapses individually with a ganglion cell

• This gives good Acuity (resolution). N.B Bright light needed

• Many Rods synapse with one bipolar neurone – RETINAL CONVERGENCE

•Dim light results in small amount of neurotransmitter release

•Individually, this would be insufficient to over come the threshold of the bipolar cell, but the total amount of transmitter from several rods is

• This gives less acuity but better sensitivity