• The retina – multilayered screen that lines the back surface of the eyeball and contains specialized
sensory neurons, is an extension of the brain
• Retina contains 2 types of light-sensitive receptors cells: rods and cones (120 mill rods, 6 mill cones in
• Rods – function best in dim light, black and white brightness receptors.
o 500 times more sensitive to light than cones, but do not give rise to colour sensations
o An Owl’s eye contain only rods (exceptional vision in dim light, but not colour vision)
• Cones – colour receptors, function best in bright illumination
o Pigeons and chipmunks only have cones because they’re only active by day (see the world in
colour, have poor night vision)
• Humans and animals that are active by day and night have a mix of rods and cones
• In humans rods are found everywhere in the retina except the fovea – small area in the centre of the
retina that contains only cones
• There are less cones as one moves away from the center of the retina, and more rods in the periphery
of the retina
• Rods and cones send messages to brain through 2 additional layers of cells:
• Bipolar cells: have synaptic connections with the rods and cones, they synapse with a layer of about
one million ganglion cells, whose axons are collected into a bundle to form the optic nerve
• Input from more than 126 million rods and cones funnel into 1 million traffic lanes leading out of the
retina toward higher visual centers
• Rods and cones form the rear layer of the retina, and their light sensitive ends point away from the
direction of the entering light so that they receive only a fraction of the light energy that enters the eye
• In the fovea, all the cones have their own “private line” to a single bipolar cell
• In result, our visual acuity – our ability to see in fine detail, is greatest when the visual image projects
directly on the fovea
• Predator birds such as hawks and eagles have two foveas in each eye; their visual acuity allows them to
see small prey on the ground from thousands of feet above the earth.
• Optic nerve – formed by ganglion cell axons are in the back of the eye not far from the fovea,
producing a blind spot with no photoreceptors
• We are usually unaware of our blind spot because our perceptual system “fills in” the missing part of
the visual field.
Visual Transduction: From Light to Nerve Impulses:
• Transduction – process where the characteristics of a stimuli are converted into nerve impulses
• Rods and cones translate light waves into nerve impulses through the action of protein molecules
• Light absorption by the photopigment molecules produces a chemical reaction that changes the rate of
neurotransmitter release at the receptor’s synapse with the bipolar cells
• The greater the change in transmitter release, the stronger the signal passed on to the bipolar cell and to
the ganglion cells whose axons form the optic nerve.
• If nerve responses trigger each of the 3 levels (rods or cones, bipolar cell, and ganglion cell), the
message is sent to the visual relay station in the thalamus, and then to the visual cortex of the brain
Brightness Vision and Dark Adaptation:
• Rods are more sensitive than cones are to low lighting.
• Brightness sensitivity (for both rods and cones) depends on the wavelength of the light
• Rods have a much greater brightness sensitivity than cones throughout the colour spectrum except at
the red end … rods are very insensitive
• Cones are most sensitive to low illumination in the greenish-yellow range of the spectrum
• Dark adaptation – the progressive improvement in brightness sensitivity that occurs over time under
conditions of low illumination.
• After absorbing a light, a photoreceptor’s pigment molecules are depleted for a period of time. During
dark adaptation, photopigment molecules are regenerated, and the receptor’s sensitivity increases