lPsy102: Introduction to Psychology
(4) Sensation and Perception
Sensation and Perception
Sensation – the process of turning physical input (such as
wavelengths of light) into the electro-chemical language of your
Perception – the process of assigning meaning to that
First step in sensation is when external energy or substance gets converted into
excitation or inhibition, called transduction.
Is it a duck (looking to the left), or a rabbit (looking
to the right)?
* The various lines, angles and shades of grey in this image
provide you with many visual sensations.
* These sensations do not differ when you see the rabbit from
when you see the duck. Instead, your perception changes.
Our brain is interpreting the sensations differently, creating
Gustav Fechner, published a landmark piece on perception.
The field psychophysics evolved out of his work, how we
perceive sensory stimuli based on their physical
Colour of light is called hue by psychologists
We’re sensitive to 3 colours, red green and blue. Mixing of these colours called
additive colour mixing can create any colour.
Mixing equal amounts of red, green and blue light produces white light.
Mixing of coloured pigments in paint of ink is called subtracting colour mixing. Vision: Creating a world of meaningful objects.
(1) Describe how light gets translated into the electrochemical
language of the brain.
(2) Explain how the essential features of the visual input, such as
colour, are extracted by the brain.
(3) Outline how a stable, meaningful interpretation of visual
information is created and why the interpretation process
sometimes leads to visual illusions.
Translating the input: Visual transduction.
* ‘Visible’ light forms just one part of the spectrum of electro-
Wavelength – the distance from one energy peak to another =
Intensity – how much energy is transmitted = brightness.
Gamma rays, x-rays, ultraviolet, infrared, radio waves
Red – high wavelength,
Violet- small wavelength
Entering the eye.
Sclera, is the white of the eye
an lipochrome for yellowish-brown.ye, pigments cause eye colour, melanin for brown
Your retina, thin membrane at the back of the eye, contains
light-sensitive receptor cells. Fovea is the central part of
retina responsible for acuity, sharpness of vision.
Light enters through pupil, which can control the amount of light streaming in to
the eye by blinking or the ‘pupilary reflex’.
Dilation is called the expansion of the pupil
Cornea, layer that covers the iris and pupil, bends incoming glight to focus it on
back of the eye.
Lens bends light, cells of light are transparent allowing light to pass through. * The first job of your eye is to focus the incoming light onto
Fovea, Optic nerve
Front part- controls the amount of light that comes in the eye.
Back part- reflects or turns the image your seeing
Red eye- reason, because your seeing the persons retina, retina
is red. Because its dark, pupil is wide open.
Accommodation: lense changes shape to focus light on back of
Light has to be focussed on your retina by the lens, which
changes shape depending on whether you are looking at
an object that is close to you or far away.
Muscles are adjusting the shape of the lens.
For near objects called myopia, the cornea being too steep
or our eyes too long.
For far objects called hyperopia, the cornea is too flat ou
our eyes too short.
The light-sensitive cells in the retina contain photopigments
that react to light, causing a chemical change in the cell
that results in neural activity. Chemical breakdown in the
Eyeball loses flexibility when we get old—poorer vision.
Receptor cells: Rods and cones.
Receptor cells located in outermost layer
* Rods: at night light falls on rod cells, do not perform colour ,
don’t pick up colour.
* 120 million
* Edge rather than centre of retina
* More sensitive than cones
* Night vision
Rods: long and narrow, enable us to see basic shapes and forms., low level light
Photopigment in rods is rhodopsin, vitamin A is needed to make rhodopsin. After staring at an image for a while and then looking at a white wall, you see a
reverse image because sense receptors become fatigued after too much
photopigment is bleached. Photopigment is bleached when light enters the rods.
Cones: shaped like small cones, colour vision, sensitive to detail,
require more light than rodes do.
Less sensitive than rods
Colour vision advantage over rod cells
Fine detail “
* The process through which you are gradually able to see
more and more effectively in the dark.
Eye starts to get full of photo pigment. Gradually chemical gets
Eyes full with photo pigment have maximum sensitivity.
Adaptation to the dark, eyes are full of photo pigment.
Cover one eye in the dark, then eyes adapt to the dark easier.
Just like pirates, sea is bright and inside of the ship is dark.
Early processing in the retina.
* By combining the activity of several receptor cells in its
‘receptive field’, a ganglion cell can detect simple features
of the array.
Ganglion cells contain axons, bundle axons together and depart the eye to reach
Ganglion cell sends on signal (positive), bipolar cells (positive and
negative) send on signal to the rods.
3 positive rods for a positive bipolar cell and 3 negative rods for a negative
bipolar cell. At the point where the optic nerve, which carries visual
information to the brain, leaves the eyeball, there is a
blind spot where no receptor cells are located.
Blind spots result, because axons of ganglion cells push everything
else aside.Point in our vision where were blind, no receptor cells
(rods and cones), nothing there to detect it.
Optic nerve contains axons of ganglion cells, takes all the messages
from the retina into the brain. Fork on the way is called optic chiasm,
half axons cross optic chiasm and other half stay on the same side.
Optic nerves turn into optic tracts, which send axons to visual
thalamus and then to primary visual cortex. Remaining axons go to
superior colliculus in the midbrain, the temporal lobe for visual form
and colour and the parietal lobe for visual form, position and motion.
Strange- you don’t experience it.
Brain takes a guess on what is there and finds image that’s a
demonstration of the blind spot.
The visual information pathway.
* Information passes through the optic nerve, over the optic
chiasm (where signals cross between the hemispheres)
and to the lateral geniculate nucleus (LGN). Left side of
space hits the right side of the retina. And the other way
Processing the visual scene.
* Simple cells can detect lines.
* By combining the input of simple cells, more elaborate
patterns can be detected. Brain damage and the visual system.
* Prosopagnosia – the inability to process faces. Visual
impairment, people can’t recognize faces.
Prosopagnosic people often use other cues to recognise
people, such as:
* Voices & gait
* General body size and shape
* Visual agnosia – the inability to recognise certain classes of
objects, often despite being able to see their form.
Able to tell us the shape and colour of an object but cant recognize i
* Akinetopsia – the inability to perceive movement.
Jumps no smooth movement, people with this condition tend to find it
difficult to interact with people, can’t perceive their movements.
* PET studies demonstrate which areas of the brain are most
active when we process visual stimuli. They show that
different parts of our brains process different information
from the visual field.
Colour vision: Trichromatic theory. We bas