PYB102 – Week Five Revision
What is Perceptual Psychology?
Perceptual psychology is the study of how the brain receives information through the body’s
various senses. Most research in this area is conducted on hearing and vision however there
are more senses to explore!
4. Olfaction - Smell
5. Gustation - Taste
6. Orienting senses - Balance and motion
Overview of Vision
For an object to be visible it must either omit or reflect light.
The human eye can only detect light on a limited number of wavelengths.
Wavelengths are the measures of distance between the ‘waves’ of light, and
different wavelengths give rise to the sensation of different colours.
Different wavelengths give the perception of different colours.
There are a number of cells in the retina which enable the information from what we see to
pass through the optic nerve to the brain. The first points of contact to consider are rods
and cones. The signals from these rods and cones are sent to various cells before being
processed by the ganglion cells which aid in the relay of the message to the brain. Ganglion
cells do not always relay these signals to the brain.
What do Rods and Cones do?
Rods and cones are photoreceptor cells, which are neurons found in the eye. They perform
phototransduction, which is the conversion of light into chemical signals which can be
understood by the brain, by absorbing photons which then changes their membrane
potential and sends the message along.
How are they different from each other?
Rods and cones are different in shape, with rods being considerably narrower and more
heavily distributed. However, the chemical processes that both involve for
phototransduction are very similar.
Rods are extremely sensitive compared to cones, and can be triggered by as few as 6
photons. Cones on the other hand need significantly more light to see. Rods are also better able to see in the dark, but take longer to ‘kick in’ than cones do. Cones cannot see as well
in the dark, but adapt much faster.
Rods can only see in monochrome, which explains why there is no colour in the dark. Cones
can see in full colour.
Duplex Retina Theory
The differences between our two types of photoreceptors led Schultz to come up with the
Duplex Retina Theory. He believed that we actually have two separate visual systems:
1. Photopic: bright light vision via cones
2. Scotopic: dim light vision via rods
Blindness and photoreceptors
Vin Kries observed that people who were ‘night blind’ (cannot see in the dark) do not have
rods at all. The reverse applies for people who are blind during the day but can see well at
night – they do not have cones.
The ganglion cells are responsible for relaying the signals of sight to the optic nerve, which
further relays them to the brain. There are only one million ganglion cells and around 126
million photoreceptors, therefore around 126 receptors communicate with every one
Therefore there is a process of information reduction needed for this communication to
happen. This comes in the form of receptive fields. Understanding these is essential to
understanding how vision works.
Receptive fields are the regions of space in which the presence of stimulus will mean
altering the firing of that particular neuron. In the retina, the receptive field of a ganglion
cell is all of the photoreceptors which form synapses with that ganglion cell. In turn, a
group of ganglion cells will form a receptive field to another cell in the brain. This process is
Purpose of these fields in communicating with the brain
A great analogy for these receptive fields is to think of them as focus groups. Essentially, the
ganglion cell acts as the moderator of each group, and takes into account all of opinions of
the rods and cones that did and did not detect light.
The ganglion cell ultimately decides which messages received by the rods and cones will be
delivered to the brain and which won’t.
How do they determine which messages to send?
Receptive fields are circular in shape, with each circle containing millions of rods and cones.
These circles form a mosaic of receptors across the back of the retina. Often, these
receptive fields can overlap. Because of this overlap, the illumination of the center cells creates an excitatory response
(more likely to fire a signal to the brain), whereas illumination of surrounding cells are
inhibitory (less likely to do so). However, when the light illuminating the surrounding cells is
turned off, t