Sensation and Perception
- Sensation: the process by which our sensory receptors and nervous system receive and
represent stimulus energies from our environment. The brain receives input from our
- Perception: the process of organising and interpreting sensory information, enabling us
to recognise meaningful objects and events. The brain understands the input from
Bottom-up processing: Taking sensory information and assembling and integrating it
Top-down processing: using models, ideas and expectations to interpret sensory information,
the process of seeing includes the process of perception. Our brain organizes its experience
from these concepts from “top-down”
How do psychologists go about studying sensation and/or perceptual processes?
Psychophysics refers to the study of psychological effects of the forms of energy (heat, light,
sound) that we can detect.
Has allowed to identify processes such as thresholds for detecting physical stimuli
o Absolute thresholds refer to the minimum level of stimulus intensity needed to
detect a stimulus on 50% of trials
o Detection above this intensity would indicate better performance than at chance
levels (i.e. half the time) subliminal is below our threshold for being able to
consciously detect a stimulus
o We aren’t consciously aware of subliminal stimuli
o We can be primed by these stimuli à the effect of subliminal can appear in our
choices and behaviour typically short-lived, not enduring à experiments
discount attempts at subliminal advertising
Research shows that:
1. We can sense something without being aware of it
2. We can be briefly primed, but not enduringly influenced, by subliminal stimuli
“just noticeable difference”
. Weber’s Law: the minimum difference (in colour, weight, temperature, etc) for a person to
be able to detect the difference between 2 stimuli 50% of the time (trials of stimulus
. principle that for two stimuli to be perceived as different, they must differ by a constant and
not a constant amount à e.g. how easily can you tell the difference between two light weights?
… and two heavy weights?
To detect novelty in our surroundings, our senses tune out a constant stimulus
Allows us to focus on relevant changes in the environment
e.g. a smoker may be insensitive to cigarette odour
Perceptual set: what we expect to perceive influences what we do perceive: context, emotions.
Motivation may influence what we perceive à reflects top-down processing
- Effect of context on perception effect: if we don’t have enough context we could
perceive it in a different way (depends on gestures, and others)
- Effect of motivation, emotional and physical state on perception: experiments show:
o Destinations seem farther when you’re tired
o A target looks farther when a crossbow is heavier
o A hill looks steeper with a backpack, after sad music is played, or when walking
o An object you desire appears to be closer to you
How do our organs respond to incoming sensory information?
From sensory organs to the brain – there are 3 stages to process the information:
1. Reception: the stimulation of sensory receptor cells by energy (sound, light, heat)
2. Transduction: transforming this cell stimulation into neural impulses
3. Transmission: delivering this neural information to the brain to be processed
Vision: energy, sensation, and perception
- Light is waves of electromagnetic radiation
- Our brains turn our eyes’ responses to these waves into visual information
- The colour of the light is related to its wavelength or frequency
- The brightness or intensity of light is related to the height or amplitude of light waves
Light is reflected from an object and
passes through the cornea and the
Direction of light is bent and focused
by the lens towards the retina at the
back of the eye.
- Light entering from top is bent down. From bottom is bent up.
- Inverted image is formed on the retina
- Stimulation of receptors on the retina begins the process of transduction into neural
impulses to be sent through the optic nerve.
1. Light enters causing chemical changes in photoreceptors which are receptor cells on
the retina. There are two types. Rods – sensitive to faint light. Cones – high detail colour
vision, there are not a lot of them.
2. Chemical changes are transduced in action potential activating bipolar cells
3. Bipolar cells activate ganglion cells (axons from optic nerve)
4. Optic nerve transmits the information to the visual cortex, through the thalamus
Information processing: The images we see are not made of light, but made of neural signals
by stimulating photoreceptors. Signals are carried from the retina through the optic nerve to
the thalamus and then to the visual cortex.
Colour vision: how do we see colour?
- We perceive colour based on the wavelengths of light reflected by objects
- Colour processing occurs in two stages
o Retina’s red, green, and blue cones respond varying degrees to a different colour
stimuli, as the young-Helmholtz trichromatic theory suggests
o Cones’ responses are then processed by opponent-process cells as opponent-
process theory proposes
young-Helmholtz trichromatic theory
3 types of colour receptors (cones) – red, green, and blue
each cone is sensitive to particular wavelengths of light: all the colours we perceive are created
by light waves stimulating combinations.
Colour blindness: people lacking cones sensitive to red or green may have difficulty
distinguishing red from green.
Opponent process theory
Neural process of perceiving white as the opposite of perceiving black, similarly yellow vs
blue and red vs green are opponent processes
Seeing one means simultaneously inhibiting
Some neurons in visual cortex will be activated by one colour, but turned off (inhibited) by its
Neurons in primary visual cortex respond to certain basic features
Orientations of lines, edges, direction of movement
In association areas of cortex, neurons respond to more complex combinations of features
Permit perception of more complex forms, such as faces
Prosopagnosia: inability to recognise faces, associated with damage to fusiform guys.