lecture 1- introduction and measuring perception

Question 1

what assumption was made until the mid-1800s?

Answer 1

assumptions that mind and body are separate from each other, body= physical: can be measured and studied
mind= non-physical/invisible: cannot be measured and studied

Question 2

what are the 5 senses?

Answer 2

sight, hearing, smell, taste and touch

Question 3

1850 (22nd of october) Gustav fechner

Answer 3

• body and mind are not independent
• mind can be studied by measuring the relationship between changes in physical stimulation (body) and a persons experience (mind)

Question 4

1860, what did Fechner publish?

Answer 4

“elements of psychophysics”: proposes a number of methods to study the relationship between the mental and physical including proposal of 3 techniques to measure thresholds

Question 5

what is the definition of psychophysics?

Answer 5

the scientific study of the relationship between the physical stimulus and the perceptions evoked by it.

Question 6

what are the two different types of thresholds?

Answer 6

measure the limits of the sensory system

absolute thresholds- measure the minimum of a stimulus that can be perceived by one of our senses (eg smallest amount of light energy we can see, smallest concentration of chemicals we can smell or taste ect…)

difference thresholds- measure the smallest difference between two stimuli a person can detect.

Question 7

psychophysics deconstructed:

Answer 7

• (physical) stimulus
  method
  how do we select and present the stimulus?
• method pf limits
• method of adjustment
• method of constant stimuli (classical psychophysics)

task
what does the participant have to do?
- magnitude estimation
- detection
- discrimination
- matching
- recognition

(behaviour/experience)
-measure
how do we characterise behaviour?= dependant variable
- phenomenological report
- reaction time
- physical tasks and judgments

Question 8

what are the three psychophysical methods to measure thresholds?

Answer 8

classically used to determine absolute thresholds

• methods of limits
• method of adjustment
• method of constant stimuli

Question 9

method of limits

Answer 9

• experimenter presents stimuli in ascending (intensity increases) or descending (intensity decreases) order
• determines “cross-over” points- where participants perception changes (from noticed to unnoticed or vice versa)
• several runs starting equally often above and below threshold
• threshold= average-cross over points

Question 10

method of adjustment

Answer 10

• the participant adjusts the stimulus intensity
• relatively fast method as participants can adjust the stimulus themselves in a few trials
• however is “frowned” upon (ie., quick and dirty approach)

Question 11

method of constant stimuli

Answer 11

• experimenter presents stimuli of different intensity in random order- participants indicate their perception
• stimuli must be chosen so that the weakest is never detected and the strongest always detected (typically 5-9) different intensities
• many repeated presentations- determine the percentage of correct detections
• threshold= intensity at which stimulus is detected 50% of all times

Question 12

method of constant stimuli (cont)

Answer 12

• more accurate than method of limits and adjustment
• involves more observations and stimuli are presented in random order (reduces order and expectations effects)
• very time consuming
  => choice of methods is usually determined by accuracy requirements and the amount of time available
• some of the problems related to expectations of subjects and order effects can be solved using: forced-choice procedures (suggested by Fechner)
• Adaptive staircase method: faster than method of constant stimuli (fever trials) - stimulus magnitude is selected on each trial based on the observers previous response (developed with increasing computer power)

Question 13

forced-choice procedures (2-AFC and 2-IFC)

Answer 13

example- forced-choice procedure patterns are presented side by side (or one after the other), participants task which side (or time interval) contains the grating (until the grating is faint to see)
- if participants cant see the grating at all they should obtain about 50% correct (guessing chance), Threshold determined at 75% correct

Question 14

adaptive staircase procedure

Answer 14

• to get reliable threshold measures many repetitions are required
• present intensities that actually include the threshold
• staircase deals with those issues: starts with clearly perceivable stimulus: if correctly identified decreases the intensity- until an error occurs => increase intensity again (fast and very few trials required)
• averaging over the transition or reveral points to determine threshold

issues:
- where to start? which step size? which procedure? when to stop? participant expectation…

Question 15

what are the three classical psychophysical tasks to determine threshold?

Answer 15

detection- “is the stimulus there?” 2-AFC: on which side is the stimulus presented?
discrimination- “which stimulus is stronger?” 2-AFC: which side contains the higher contrast stimulus?
matching: “adjust one out of two stimuli so that they look/sound/feel the same”

Question 16

what is the smallest difference between stimuli that humans can detect?

Answer 16

• to work this out use Webers law
• participants are asked to detect a difference between two stimuli
• size of the difference threshold depends on the size of the standard stimulus

Question 17

what is the definition of Webers law?

Answer 17

the ratio between the difference threshold and the intensity of the standard is constant
- webers law is true for most senses (as long as the stimulus intensity is not too close to the absolute threshold)
- Webers fraction depends on the sensory dimension measured

Question 18

what is the equation for Webers law?

Answer 18

K= DT/S

k= constant
DT= difference threshold
S= value of the standard stimulus

Question 19

measuring perception above threshold (Stevens, 1957)

Answer 19

magnitude estimation: participants rate the size or intensity of a stimulus on a numerical scale
- measures the relationship between physical stimulus and the perception of their magnitude
- doubling physical intensity does not necessary cause double the perceptual experience

Question 20

measuring perception above threshold cont

Answer 20

response compression:
increase in perceived magnitude is smaller than the increase in stimulus intensity

response expansion:
increase in perceived magnitude is larger than the increase in stimulus intensity

• relationship between intensity of a stimulus and perception of its magnitude can be described by a power function: stevens power law: k*Sn
• exponent (n) <1 = response compression
• exponent (n) >1 = response expansion

Question 21

issues when measuring perception

Answer 21

• different response criteria may lead to erroneous conclusions about perceptual sensitivity
• not relevant when testing different sensitives within the same person or averaging across larger groups
• important if we want to compare responses of two/ few people

Question 22

signal detection experiment:

Answer 22

• typical experiment: stimulus is either present or not (eg tumour in a brain scan) and has to be detected
• note, in signal-detection experiment only one stimulus intensity is present (either presnt or absent) (difference to threshold experiment)
• 4 possible outcomes between presences/absence of the stimulus and a persons response
• mistakes due to our perception being “noisy” (internal noise in neural responses vs. external noise eg disortion in image, glitch in the radar…)