problem 3 - signal detection & color perception

Question 1

the absolute threshold

Answer 1

the smallest amount of stimulus energy necessary to detect a stimulus

Question 2

method of limits

for determining the absolute threshold

Answer 2

the experimenter presents stimuli in either ascending order (intensity is increased) or descending order (intensity is decreased) - e.g. a tone

1. tone is presented
2. observer inficates if they can hear it
3. another tone is presented at a lower intensity & observer responds
4. repeated until observer can no longer hear

next series of trials begins below the observer’s threshold, so that he says “no” on the first trial, and continues until he says “yes”

crossover point = change from yes to no

Question 3

method of adjustment

for determining the absolute threshold

Answer 3

• Stimulus intensity is continuously adjusted until the observer can just barely detect the stimulus
• Observer does not respond as each tone is presented (like in the method of limits) - instead he simply adjusts the intensity until he can just barely hear the tone
• This just barely audible intensity is taken as the absolute threshold

Question 4

method of constant stimuli

for determining the absolute threshold

Answer 4

• Experimenter presents 5-9 stimuli with diff intensities in random order
• E.g. presenting six light intensities 10 times each and determining the percentage of times that the observer perceived each intensity
• Threshold usually defined as the intensity that results in detection on 50% of trials

Question 5

difference threshold

Answer 5

the smallest difference between two stimuli that a person can detect (aka just noticeable difference)
* Determined by asking participants to indicate whether they detect a difference between 2 stimuli - e.g. sensing a difference in weight
* Findings: when difference between the standard and comparison weights was small observers found it difficult to detect the difference in the weights (easily detected larger differences)

Question 6

signal detection theory

Answer 6

Threshold determined by methods like constant stimuli can depend on whether the subject is a conservative or liberal responder
* Difference between 2 ways of responding = response criterion

Question 7

liberal vs conservative responders

Answer 7

liberal = say ‘yes’ when they may have possibly seen the light

conservative = say ‘yes’ only when they are certain they have seen the light

threshold appears lower for conservative responders although difference is actually caused by difference in response criteria

Question 8

payoffs (signal detection experiments)

Answer 8

can change the percentage of hits and false alarms by manipulating each person’s motivation

Add financial reward for correct responses and penalize for incorrect responses

Question 9

trichromatic theory of color vision

main idea

Answer 9

States that color vision depends on the activity of 3 different receptor mechanisms

Young & Helmholtz

Question 10

monochromatism

Answer 10

A rare form of color blindness that is usually hereditary and occurs in about 10/1 million people - missing 2 or 3 cones

Usually have no functioning cones = vision has the characteristics of rod vision in both dim and bright lights - see everything in shades of lightness (white, black & gray)

Have poor visual acuity and are very sensitive to bright lights - the rod system is not designed to function in bright light = becomes overloaded in strong illumination, creating a perception of glare

Question 11

dichromatism

Answer 11

Experience some colors, though a lesser range than trichromats - missing one cone

Three major forms:
1. protanopia: missing L cones/red receptors
2. deuteranopia: missing M cones/green receptors
3. tritanopia: missing S cones/blue receptors (most rare)

Question 12

opponent-process theory of color vision

main idea

Answer 12

States that color vision is caused by opposing responses generated by blue and yellow and by red and green

Question 13

simultaneous color contast

(opponent-process theory of color vision)

Answer 13

an effect that occurs when surrounding an area with a color changes the appearance of the surrounded area

Question 14

3 mechanisms of opponent-process theory

Answer 14

Hering proposed three mechanisms, each of which responds in opposite ways to different intensities or wavelengths of light:
1. The Black (-) White (+) mechanism responds positively to white light and negatively to the absence of light
1. Green (-) Red (+) responds positively to red and negatively to green
1. Blue (-) Yellow (+) responds negatively to blue and positively to yellow

(-) = inhibition
(+) = activation

Question 15

opponent neurons

(opponent-process theory of color vision)

Answer 15

found in the retina & LGN - respond with an excitatory response to light from one part of the spectrum and with an inhibitory response to light from another part

Question 16

trichromatic theory vs opponent-process theory

Answer 16

Both trichromatic and opponent-process theories are correct and each one describes physiological mechanisms at different places in the visual system

• Trichromatic theory describes what is happening at the beginning of the visual system, in the receptors of the retina
• Opponent-process theory describes events later in the visual system