Sensation & Perception

A.P. Psychology

Chapter 4

Drill 1-12-2010

• Read the case of Dr. P…..

• What accounts for his inability to recognize faces and familiar objects by sight?

• How would your world be different if you were unable to sense the objects & people around you?

• Give examples of how your life would change if you were to lose your sense of sight?

Visual Agnosia

• The inability to recognize objects through sight

• The person’s eyes function properly, but there seems to be a problem with the processing of what is being seen

• Seems to be the result of damage somewhere in the visual pathway

Sensation & Perception

• Sensation• The stimulation of the

sense organs• Involves the

absorption of energy, such as light or sound waves by sensory organs

• Perception• The selection,

organization, and interpretation of sensory input

• Involves translating sensation into something that is meaningful

What do you see?

Psychophysics

• The study of how physical stimuli are translated into psychological experience– Gustav Fechner (1860’s) – German

– Sensation begins with a Stimulus» Any detectable input from the

environment– Wanted to know - For any given sense, what

is the weakest detectable stimulus?

Thresholds

• Threshold ~ a dividing point between energy levels that do and do not have a detectable effect

• Absolute Threshold ~ for a specific type of sensory input is the minimum amout of stimulation that an organism can detect

• Define the boundaries of an organism’s sensory capabilities

Examples of Absolute Thresholds

Vision A candle flame seen at 30 miles on a dark clear night

Hearing A tick of a watch under quiet conditions at 20 feet

Taste One teaspoon of sugar in two gallons of water

Smell One drop of perfume diffused into entire the entire volume of a six-room apartment

Touch The wing of a fly falling on your cheek from a distance of 1 centimeter

The Just-Noticeable Difference

• The smallest difference in the amount of stimulation that a specific sense can detect

• Similar to absolute thresholds – absolute thresholds are the JND from no stimulus input

• Smallest detectable difference is a fairly stable proportion

• Ernst Weber • Fechner’s brother-in-law• First to demonstrate the JND• Weber’s Law ~ the size of a JND is a constant proportion

of the size of initial stimulus (Weber’s fraction)» Ex. The Weber fraction for lifting weights is

approximately 1/30 which means that you should be able to detect the difference between 30 ounce weight & 31 ounce weight, but not a 90 & 91 ounce weight (difference is 3 ounces for 90)

Fechner’s Law• States that the magnitude of a sensory

experience is proportional to the number of JND’s that the stimulus causing the experience is above the absolute threshold

• Constant increments in stimulus intensity produce smaller & smaller increases in the perceived magnitude of sensation

» Ex. You are in a dark room with a single lamp that has 3 bulbs of the same wattage. You turn a switch and one bulb lights. After a dark room, the difference is striking. Turn again, and a second bulb comes on. The amount of light is doubled, but the room does not seem twice as bright. And so on….

Homework

• The Secret Life of James Thurber ~ Article & Questions

• Sensation & Perception Passage