THRESHOLDS

A FEW DEFINITIONS Psychophysics: The study of relationships between the physical characteristics of stimuli such as their intensity, and our psychological experience of them. Absolute Threshold: The minimum stimulation needed to detect a particular stimulus about 50% of the time.

A GREAT EXAMPLE… Hearing tests measure absolute thresholds. The moment you hear that beep, that sound has crossed your absolute threshold.

SIGNAL DETECTION THEORY A theory predicting how and when we detect the presence of a faint stimulus (a signal) amid background stimulation (noise.) Assumes there is no single absolute threshold and that detection depends partly on a person’s motivation and alertness.

MORE… Signal Detection Theorists seek to understand why people respond differently to the same stimuli and why the same person’s reactions vary as circumstances change. Exhausted parents will notice the faintest whimper from a newborn’s cradle while failing to notice louder, unimportant sounds.

Myers says… “In a horror-filled wartime situation, failure to detect an intruder could be fatal. Mindful of many comrade’s deaths, soldiers and police in Iraq probably became more likely to notice—and fire at—an almost imperceptible noise. With such heightened responsiveness come more false alarms, as when the US military fired on an approaching car that was rushing an Italian journalist to freedom…

Killing the Italian intelligence officer who had rescued her Killing the Italian intelligence officer who had rescued her. In peacetime, when survival is not threatened, the same soldiers would require a signal before sensing danger.”

Signal Detection can also have life-or-death consequences when people are responsible for watching an airport scanner for weapons, monitoring patients from an intensive-care nursing station, or detecting radar blips. Studies have shown that people’s ability to catch a faint signal diminishes after about 30 minutes.

TSA TSA periodically adds images of guns, knives, and other threatening objects into bag X-Rays. When the signal is detected the system congratulates the screener and the image disappears.

More From Myers Experience matters. In one experiment, 10 hours of action video game playing—scanning for and instantly responding to any intrusion—increased novice players’ signal detection skills.

Subliminal Stimulation Hoping to penetrate our unconscious, entrepreneurs offer recordings that supposedly speak directly to our brains to help us lose weight, stop smoking, or improve our memories. Masked by soothing ocean sounds, unheard messages will influence our behavior. Such claims make 2 assumptions: 1) we can unconsciously sense subliminal (below threshold) and 2) without our awareness, these stimuli have extraordinary suggestive powers.

THE BIG QUESTION Can we sense stimuli below our absolute thresholds? In one sense, the answer is clearly “yes.” Remember, absolute threshold means that we recognize the stimulus HALF THE TIME. BELOW THIS THRESHOLD we can still detect the stimulus SOME of the time.

DIFFERENCE THRESHOLDS AKA THE JUST NOTICEABLE DIFFERENCE: The minimum difference between two stimuli required for detection 50% of the time. Sensory Adaptation: Diminished sensitivity as a consequence of constant stimulation.

Good Example of Sensory Adaptation You go to your neighbor’s house and it smells musty. You wonder how they can bare living there with that smell; but after 15 minutes you stop noticing it.

“We need above all to know about changes; no one wants or needs to be reminded 16 hours a day that his shoes are on.”

Weber’s Law The principle that, to be perceived as different, two stimuli must differ by a constant percentage (rather than a constant amount.)

Examples of Weber’s Law 1) Put a quarter in one envelope and 2 quarters in another. Telling the difference would be easy. Now put 1 quarter in a shoe and 2 in the other. That would be much more difficult. 2) If you sell someone an expensive suit, they are more likely to buy a shirt and tie AFTER they have spent more money on the suit.

SMALL GROUPS Vision Group, be sure to include: -Transduction -accommodation -Wavelength -rods -hue -cones -intensity -optic nerve -pupil -blind spot -iris -fovea -lens -Young-Helmoltz Theory -retina -Opponent-Process Theory

SMALL GROUPS Hearing Group be sure to include: -audition -conduction hearing loss -frequency -sensorineural hearing loss -pitch -cochlear implant -middle ear -cochlea -inner ear -place theory -frequency theory

FRQ FOR HOMEWORK A musician is walking home alone late one night and is startled when a dog in a yard to his left barks unexpectedly. Respond to each of the following regarding the musician’s ability to hear the bark: Trace the path that the sound waves travel as they enter the ear and proceed to the receptor cells for hearing Trace the path that neural impulses created by the bark travel from the receptor cells to the brain. Explain how the musician would know that the bark originated to his left even without seeing the dog.