Chapter 4: Sensation and Perception
Psychophysics: Basic Concepts and Issues
Sensation is the stimulation of sense organs
Perception is the selection, organization, and interpretation
Sensation involves the absorption of energy, such as light or sound waves, by sensory
organs such as the eyes & ears where as perception involves organizing & translating
sensory input into external stimuli.
Psychophysics is the study of how physical stimuli are translated into psychological
Thresholds: Looking for Limits
Sensation begins with a stimulus, which is any detectable input from the environment. A threshold is a dividing point between energy levels that do and do not have a
An absolute threshold for a specific type of sensory input is the minimum amount of
stimulation that an organism can detect
Absolute thresholds are not really absolute
A just noticeable difference (JND) is the smallest difference in the amount of
stimulation that a specific sense can detect
JNDs vary by sense & the smallest detectable difference is a fairly stable
proportion of the size of the original stimulus.
As stimuli increase in magnitude, the JND become larger.
Webers law states that the size of a just noticeable difference is a constant proportion of
the size of the initial stimulus
The constant proportion is called the Weber fraction and applies to all senses.
Fechners law asserts that larger and larger increases in stimulus intensity are required to
produce just noticeable differences in the magnitude of sensation.
Constant increments in stimulus intensity produce smaller and smaller increases
in the perceived magnitude of sensation.
Perceptions cant be measured on absolute scales, everything is relative.
Signal Detection Theory:
Proposes that detection of stimuli involves decision processes as well as sensory
processes, which are both influenced by a variety of factors besides stimulus
Response to a stimulus of a specific nature is pre set by a criterion which involves
higher mental processing and expectations rather then raw sensations
Performance is also dependant on the amount of noise in the system as it becomes
more difficult to pick up a weak system when there is more noise.
Perception without Awareness
Subliminal (below threshold) perception is the registration of sensory input without
Persuasive because people supposedly are defenceless against appeals operating
below their threshold of awareness.
In recent years, it has become apparent that perception can occur without awareness Sensory adaptation is a gradual decline in sensitivity due to prolonged stimulation as a
result of adapting to the stimulus.
Automatic, built-in process that keeps people focused on changes opposed to
constants in their sensory input.
Allows people to ignore the obvious & focus on other changes in their
Likely a trait developed via natural selection as a behavioural adaptation.
This is an example of how there is no one-one correspondence b/w sensory input
Our Sense of Sight: The Visual System
The Stimulus Light
Light is a form of EM radiation travelling as a wave at the speed of light
Varies in amplitude(height) & wavelength (distance b/w peaks)
Amplitude affects perception of brightness while wavelength affects perception of
Light that we see is only small portion of EM spectrum of all wavelengths.
The Eye: A Living Optical Instrument
Light varies in terms of wavelength, amplitude, and purity
Eye serves 2 purposes:
Channels light to neural tissue that receives it called the retina
House retina tissue
The lens is the transparent eye structure that focuses the light rays falling on the retina,
forms an upside down image of objects onto the retina.
Made of soft tissue, facilitates a process called accomadation that occurs when
the curvature of the lens adjusts to alter visual focus. In nearsightedness, close objects are seen clearly but distant objects appear blurry
Caused b/c focus of the light from distant objects falls short to the retina when the
cornea or lens bends the light too much or when the eyeball is too long
In farsightedness, distant objects are seen clearly but close objects appear blurry
Caused b/c focus of light from close objects falls behind the retina, typically
occurring when the eyeball is too short
The pupil is the opening in the centre of the iris that helps regulate the amount of light
passing into the rear chamber of the eye
Constricting pulpils allows less light into the eye, but sharpens the image falling
on the retina
Dialating pupils allow more light but the image is less sharp.
Tiny imperceptible eye movements are referred to as saccades
These saccades are essential to good vision and w/o vision begins to decay
Light enters the eye through the transparent cornea and pupil and is focused on the retina
by the lens