Chapter 4- Sensation and Perception
Sensation is the stimulation of sense organs.
Perception is the selection, organization, and interpretation of sensory input.
Sensation involves the absorption of energy, such as light or sound waves, by sensory
organs, such as the ears and eyes.
Perception involves organizing and translating sensory input into something meaningful,
such as your best friends face or other environmental stimuli.
Psychophysicsthe study of how physical stimuli are translated into psychological
Thresholds: Looking for Limits
Sensation begins with a stimulus.
A threshold is a dividing point between energy levels that do and do not have
An absolute threshold for a specific type of sensory input is the minimum amount of
stimulation that an organism can detect.
Absolute thresholds define the boundaries of an organisms sensory capabilities.
As stimulus intensity increases, subjects probability of responding to stimuli gradually
Weighing the Differences: The JND
A just noticeable difference (JND) is the smallest difference in the amount of
stimulation that a specific sense can detect.
Webers law states that the size of a just noticeable difference is a constant
proportion of the size of the initial stimulus.
This constant proportion is called the Weber fraction. It applies to not only weight
perception but to all the senses.
Fechners law, which states that the magnitude of a sensory experience is
proportional to the number of JNDs that the stimulus causing the experience is
above the absolute threshold.
Constant increments in stimulus intensity produce smaller and smaller increases in the
perceived magnitude of sensation.
Signal-detection theory proposes that the detection of stimuli involves decision
processes as well as sensory processes, which are both influenced by a variety of
factors besides stimulus intensity.
Signal-detection theory attempts to account for the influence of decision-making
processes on stimulus detection.
Your responses will depend in part on the criterion you set for how sure you must feel
before you act. Setting this criterion involves higher mental processes rather than raw
sensation and depends on your expectations and on the consequences.
Your performance will also depend on the level of noise in the system. Noise comes
from all of the irrelevant stimuli in the environment and the neural activity they elicit.
Perception without Awareness
Subliminal perceptionthe registration of sensory input without conscious
awareness (limen is another term for threshold, so subliminal means below threshold).
There is evidence that perception without awareness can take place. The effects of subliminal stimuli turn out to be nearly as subliminal as the stimuli
themselves. Subliminal stimulation generally produces weak effects.
These effects can be detected only by very precise measurement under carefully
controlled laboratory conditions in which subjects are asked to focus their undivided
attention on visual or auditory materials that contain subliminal stimuli.
Sensory adaptation is a gradual decline in sensitivity due to prolonged stimulation.
Sensory adaptation is an automatic, built-in process that keeps people tuned into changes
rather than the constants in their sensory input.
The Stimulus: Light
Light is a form of electromagnetic radiation that travels as a wave, moving, naturally
enough, at the speed of light.
Light waves vary in amplitude (height) and in wavelength (the distance between peaks).
Amplitude affects mainly the perception of brightness, while wavelength affects mainly
the perception of colour.
Humans normally see mixtures of several wavelengths. Light can also vary in its purity
(how varied the mix is).
Purity influences perception of the saturation, or richness, of colours. Saturation is hard to
The Eye: A Living Optical Instrument
The eyes serve two main purposes: They channel light to the neural tissue that receives it
called the retina and they house that tissue.
Light enters through the cornea. The cornea and the crystalline lens, located behind it,
form an upside down image of objects on the retina.
The lens is the transparent eye structure that focuses the light rays falling on the
The lens is capable of adjustments that facilitate a process called accommodation.
Accommodation occurs when the curvature of the lens adjusts to alter visual focus.
In nearsightedness, close objects are seen clearly but distant objects appear blurry
because the focus of light from distant objects falls a little short of the retina. This
focusing problem occurs when the cornea of 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
because the focus of light from close objects falls behind the retina. Problem typically
occurs when the eyeball is too short.
The iris is the coloured ring of muscle surrounding the pupil, or black centre of the eye.
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. When the pupil constricts, it lets less
light into the eye but it sharpens the image falling on the retina. When the pupil dilates
(opens), it lets more light in but the image is less sharp.
Eye movements are referred to as saccades. These tiny movements are essential to
It has been suggested that one form of saccade may give away your covert gaze even
when you are looking somewhere else.
The Retina: The Brains Envoy in the Eye The retina is the neural tissue lining the inside back surface of the eye; it almost
absorbs light, process images, and sends visual information to the brain.
The optic disk is a hole in the retina where the optic nerve fibres exit the eye. Because
the optic disk is a hole in the retina, you cannot see the part of an image that falls on it.
Hence, it is known as the blind spot.
Visual Receptors: Rods and Cones
The retina contains two types of receptors: rods and cones.
Cones are specialized visual receptors that play a key role in daylight vision and
colour vision. Cones handle most of our daytime vision, because bright lights dazzle the
rods. Play a major role in the perception of colour. However, cones do not respond well
to dim light, which is why you dont see colour very well in low illumination. Cones
provide better visual acuitythat is, sharpness and precise detailthan rods.
The fovea is a tiny spot in the centre of the retina that contains only cones; visual
acuity is greatest at this spot.
Rods are specialized visual receptors that play a key role in night vision and
peripheral vision. Rods handle night vision because they are more sensitive than cones
to dim light.
Dark and Light Adaptation
Dark adaptationthe process in which the eyes become more sensitive to light in low
Dark adaptation is virtually complete in about 30 minutes, with considerable progress
occurring in the first 10 minutes. Cones adapt more rapidly than cones.