PSYC2390 Lecture 3: Psychophysics I

Psychophysics

The relationship between the physical, external intensity of a stimulus and our physiological reaction to it (internal)

Absolute Threshold

Intensity of the weakest stimulus you can sense

High absolute threshold

Low sensitivity; you need to have a very strong stimulus before you can sense it

Low absolute threshold

High sensitivity; you don’t need a very strong stimulus to sense it

Smoking Cigarettes/Drinking Alcohol

Increases absolute threshold for tastes; reduces sensitivity to taste

Estrogen and Olfaction (Sense of Smell)

Estrogen reduces threshold for olfaction, more or less sensitivity to smells with amount of estrogen in system

Testosterone and Olfaction (Sense of Smell)

Testosterone increases absolute threshold for olfaction; less sensitivity

How are Absolute Thresholds Measured?

1. Method of Limits

2. Method of Adjustment

3. Method of Constants

Method of Limits

Present person with range of intensities in order from high to low or vice versa; at each sound intensity ask if they hear it or not. Alternate between ascending and descending trials

Method of Adjustment

Person sits with volume dial. Set up to high volume, and person lowers volume until they can just hear it; fastest, but least precise

Method of Constants

Take 5-9 consecutive volumes and present them in a random order. Person tells you if they hear it or not, and record the response, measure % of time they say “Yes I heard it”’ most precise way but the slowest

Method of Constants Absolute Threshold

Intensity perceived 50% of the time

Method of Limits/Adjustment Absolute Threshold

Average of crossover points

Difference Threshold

The smallest difference between two stimuli that you can just perceive, also called JND. Measures discrimination

Cigarettes and Difference Threshold

Smoking increases the difference threshold for brightness, reducing contrast sensitivity

Pelli-Robson Contrast Sensitivity Chart

Individuals with high difference thresholds cannot see smaller letters, but those with lower difference thresholds can

Measuring Difference Threshold

Can use methods used to measure absolute threshold but adapt to enable comparison to the standard (constant)

Method of Limits Difference Threshold

Participants weigh the standard and comparison in their hand, decides whether the two weights are the same

Method of Adjustment Difference Threshold

Using relative brightness of 2 lights; standard light brightness is adjusted until it is perceived to be the same intensity as the standard

Method of Constants Difference Threshold

In each trial, participants first weigh standard in hand and then comparison

Negative Time Error

We tend to underestimate the intensity of stimuli perceived in the past

Weber’s Law

As the intensity of the standard goes up, so does the size of the JND

Weber’s Law Formula

K (Constant) = JND/Standard

Motivations to lie about sensitivity

To keep driver’s license, profession that needs sensory acuity, to avoid hearing aid or glasses