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Summary 2.4. Problem 3 & 4
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This is a detailed summary of problems 3 & 4 for course 2.4 Perception.
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Summary Sensation & Perception chapter 8 - 15 (excl. 11), ISBN: 9781605358758
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Problem 3: Perception and decisionLearning goals: case A makingquestions:
Coordinator & Problem 4: Colour
1. What influences decision making when detecting a noise? 1. How does the pay-off structure affect the response
Learning goal: case B criterion that a participant uses in making decisions
1. What does the graph show (curves, axis, grey area)? about the presence or absence of a stimulus? How
Learning goal: case C does it affect the hit rate, false alarm rate, and rate
1. How do we detect differences in weight? Are there of misses and correct rejections?
common characteristics between stimuli in detection? 2. Can you see colours if you have only one type of cone
Problem 4: receptor (instead of three)? Why/why not?
Learning goals:
1. How do we perceive colour? (theories on colour
perception)
2. What is colour blindness (and related disorders/deficits)?
Source: Coren, 5th edition, chapter 2, 15-31p (Problem 3)
Detection
- Our sensory systems are responsive to energy changes in the environment. Energy
changes may take the form of electromagnetic (light), mechanical (sound, touch,
movement, muscle tension), chemical (tastes, smells), or thermal (heat, cold)
stimulation. The problem of detection is the problem of how much an energy
change, starting from zero, is necessary for a sensory system to register its presence.
(e.g. for an individual to sense it).
- Absolute threshold: the minimal amount of energy change from zero. “Lifted the
sensation over the threshold of consciousness” below some critical intensity of the
stimulus, a person would not be able to detect it. As soon as this threshold intensity
is exceeded, we would expect the observer to always detect its presence.
- Psychometric function: This hypothetical relation can be described using a graph
called psychometric function.
Method of constant stimuli: experiment that measures the absolute threshold of hearing.
The listener sits in a quiet room wearing headphones. The experimenter selects a set of
, tones to present. The tones differ in their intensity, some clearly easily heard, some not
heard at all. She presents these tones, one at a time, to the listener. Each is presented many
times in an irregular order. The listener is required to respond yes when he detects the
stimulus and no when he does not. A fixed or constant set of stimuli is chosen in advance.
as the stimulus energy increases, the proportion of yes responses, indicating that the person
heard this stimulus, gradually increases.
although the method of constant stimuli can produce useful estimates of absolute
threshold, it does have some drawbacks. time consuming pretesting needed, many
trials etc.
One way to avoid this:
Method of limits: to focus on stimuli near the absolute threshold rather than trace out the
entire psychometric function. experiment begins with a stimulus that is easily heard and
decreases intensity gradually until the observer is unable to hear it. (descending series) or
the other way around (ascending series)
On average the descending series yield lower thresholds than the ascending series. It
seems that the threshold varies from measurement to measurement, or from moment to
moment.
- The method of limits is still somewhat inefficient because it is only the stimuli that
bracket the threshold (the last two in each testing series) that give any information;
the rest tell us nothing.
Adaptive testing
- Keeps the test stimuli “hovering around” the threshold by adapting the sequence of
stimulus presentations to the observer’s responses.
- Staircase method: start with descending series of stimuli. Each time observer says
yes, we decrease stimuli intensity by one step. When the stimulus is too weak for the
observer to hear we don’t end the series, but reverse its direction, increasing the
stimulus intensity by one step. Continue as long as required. This is the simplest
example of the use of adaptive testing to find thresholds. Using the observer’s
previous responses to determine the stimulus series allows the experimenter to zero
in on the threshold quickly and efficiently, with few wasted trials and with a high
degree of reliability. most efficient
Signal detection theory:
Catch trials: no stimulus presented, into the series of trials. Accurate observers would
always respond to “no” on catch trials because no stimulus was present; a “yes” response
on such a trial would be just a guess. Thus, if observers responded “yes” too often on catch
trials they were cautioned against guessing, the calculated threshold was adjusted to
account for the guesses, or the data were discarded altogether.
Signal detection theory attempts to deal with this problem. This is a mathematical
theory that was derived from the same statistical decision theory that is used to analyse
scientific experiments. This theory assumes that any stimulus must be detected against the
background of endogenous noise in our sensory systems. Thus, on each trial the observer
has to decide whether the signal was present in all that noise or whether there was only the
usual fluctuating neural noise.
- In signal detection theory there is no absolute threshold; there is only a series of
observations, each of which must be categorized as either signal present or signal
absent. A series of such decisions can be used to deduce how sensitive a person is to
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