Physical Energy + Psychological Experience Psychophysics

Absolute threshold Minimum amount of energy needed that can be detected 50% of the time. Is this really “absolute”? Stimulus @ intensity level 3 is detected 20% of the time. A stimulus at level 5 is above the threshold but is only detected 80% of the time. Why does it vary?

Example Think of an electric burner on a stove. Imagine turning that burner on and then placing your hand directly on it. At first you won't feel much heat because is takes time for the burner to heat up. But at some point it will get hot enough for you to detect…meaning, there is some temperature that is just hot enough for you to notice it. This isn't the point at which you get burned, but the point at which it is just hot enough for you to detect the presence of the heat.

Absolute threshold

Subliminal stimuli & Supraliminal Stimuli Weak stimuli or stimuli too brief to be perceived consciously. Below A.T. Marketing Supraliminal Stimuli Strong & constantly perceived Above A.T.

Subliminal messages? Return

Signal detection theory Explains why the A.T. varies Intensity of signal Capacity of sensory systems Amount of background stimulation (noise) Sensitivity Willingness to respond to stimuli Motivation (wants/needs) Expectancy Response Criterion “Capacity” of sensory system refers to how “good” vision or hearing is in the individual.

Signal detection theory Provides a way to predict and understand responses to stimuli Four elements in S.D. trials to determine response False Alarm Miss Hit Correct Rejection False Alarm: An error that occurs when a stimulus is high enough to make you report a signal even when no signal is present. Miss: Signal occurs but is so faint it does not produce enough stimulation for you to detect it. Hit: Correctly detecting a stimulus Correct Rejection: No signal occurs and you don’t report the signal.

Tornado warning S.D. example Figure B (Yellow) Represents outcomes of a HIGH response criterion. Figure C (Green) represents outcomes of a LOW response criterion. Radar systems are very sensitive (they detect even the slightest weather patterns in addition to swirling dust and insect clouds). A tornado’s “signature” may or may not be picked up. Depends the forecaster’s sensitivity & response criterion. In establishing the criterion for sounding the warning signal the forecaster must consider the consequences. Low Criterion: False alarms will unnecessarily disrupt people’s lives & cause low response to the signal later. High Criterion: The warning signal occurs only when the forecaster is certain of a tornado, could allow a dangerous storm to go unreported. Expectations can change the forecaster’s response: If the forecaster knows it’s the time of year when tornado’s only occur 50% of the time, a high response criterion would be necessary. Bottom Line: There will be a trade off! If tornados are present 50% of the time the false alarm and correct rejection rate is higher. However, when tornados are present 90% of the time the false alarm rate goes up even higher and the correct rejection rate goes down. The Hit rate is higher however, and the miss rate is lower. High Response Criterion Low Response Criterion

Uses of s.d. theory Generally used to help understand why people make mistakes Spotting tornados Inspecting luggage at airport security check points Diagnosing medical conditions Searching for oil Looking for clues at a crime scene Psychologists recommend that manufacturers place flawed items among a batch of objects to be inspected. Why?

Differences in Stimuli Illustrated by Weber Just-Noticeable Difference (JND) Also known as the Difference Threshold The weaker the stimuli the easier it is to detect differences Quarter Demo Real Life Application: Change in prices. If you experience a $10 change in the price of gas you would notice the difference more intensely than if the price of your rent went up $10.

Weber’s Law Law States JND = KI K is the Weber’s constant for a particular sense. I is the amount, or intensity, of the stimulus.

Weber’s law example The value of K for loudness is .10. If a stimuli is 100 decibels, you would have to add or remove 10 decibels in order for there to be a DETECTABLE change in its loudness.

Magnitude estimation Illustrated by Fechner Explains Weber’s law How our perception of stimulus intensity is related to the actual strength of the stimulus. “Fechner’s Law”

Magnitude estimation It takes only a small amount of sound to make a soft sound seem twice as loud. It takes a substantial amount of sound to make a loud sound seem twice as loud. Magnitude Estimation Demo

Weber & Fechner’s Law Weber’s Law Detecting the smallest difference in stimulus energy. Weber’s Law Difference b/w ACTUAL physical magnitude of a stimulus & PERCIEVED magnitude. Fechner’s Law

Big picture Humans do well at estimating differences between stimuli.

Big picture However…perception is relative & can be disrupted.