Cognitive Psychology 2: perception

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Who developed detection thresholds?
Fechner
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What are the limits?
They change stimulus strength (intensity) until subject says just detectable/undetectable
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What is the adjustment?
As above but subject adjusts stimulus until just detectable/undetectable
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What is the constant stimuli?
Present subject with a fixed constant set of stimulus strengths in random order and ask them each time whether or not they detect it
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What is the problem with this?
The subject may be biased or not telling the truth
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What is the solution to this problem?
Use a forced choice task
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What is this?
Ask the subject to report something about the stimulus
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On each trial what is presented?
The stimulus in one of two temporal intervals
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When does the subject respond?
first or second interval
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What is calculated?
% of correct responses and threshold
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What is the threshold?
Stimulus strength producing 75% correct performances (eg.5 units)
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What is the detection threshold of: vision?
Candle flame seen at 30 miles on dark, clear night
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Hearing:
Tick of a watch from a distance of 20 foot
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Taste:
1 Tsp sugar in 2 gallons of water
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Smell:
1 drop perfume diffused in volume of 3 room flat
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Touch:
Insect wing falling on your back from height of 1cm
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What did Galanter suggest?
Impressive detection sensitivity of human perceptual mechanisms
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How does an experimenter measure discrimination thresholds psychophysically?
Use same methods as previous slide (2 stimuli, 1 fixed and 1 variable), ideally use a forced choice task
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On each trial present the two stimuli
Consequently
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What does subject reports?
which stimuli has the greater strength
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What is calculated?
correct responses and estimate the threshold
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What do opticians use?
A crude variant of this technique to assess which spectacle correction is best for you
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What is the threshold?
Stimulus strength producing 75% correct performance
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What was properties of perceptual mechanisms inferred?
From changes in thresholds (colour vision, dark adaptation, motion perception)
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For the same stimulus strength, how did the subject respond?
Correctly and sometimes incorrectly, so transition from chance performance to perfect performance is gradual not abrupt
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What is this explained by?
Signal detection theory
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What are sensory systems?
Imperfect and are inherently noisyv(cells in sensory pathways exhibit some degree of random firing
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What does the internal noise infer?
Our perceptual decisions about the world when the stimulus is weak
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What does this lead to?
incorrect decisions some of the time, thresholds re probablistic not absolute measures of performance (75% correct)
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What is the neurophysiological approach?
It is called the single cell recording
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What is the extracellular recording?
from single cells in sensorys areas of brain commonly used to study perceptual apparatus
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What is the technique used?
To determine a cells preferred stimulus by recording action potentials elicited by a range of visual stimuli
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Where is the very fine tipped wire plce?
surgically placed into the area of the visual system under study
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Where was the microelectrode tip positioned?
Next to the axon of a cell so that it picks up action potentials ideally from only that cell
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What is the peri - stimulus histogram?
A plot showing how the firing rate of a cell changes during the time a stimulus is presented within its receptive fields
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Using neural responses to calculate thresholds mean?
Statistical techniques can be used to construct neurometric function similar to psychometric function plotted by psychophysicists
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What do shows cells potential detect/
A stimulus can be used to derive neural thresholds
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Why is neurophysiology reductionist?
It attempts to reduce a complex problem (i.e. how we perceive) into a set of more basic tractable problems that can be studied in the lab
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As cells are basic functional units of the brain, what does this show?
individual cells we may gain an insight into the fundamental processes of say vision
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What are limitations?
: Tells us little about perceptual processes which rely on the combined activity of many neurones (neural networks in the brain)
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What can indiviudal cells not be considered?
Feature detectors because they do not simply signal the presence of a specific feature in the world
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What do cells typically respond better to?
Some stimuli than others their firing rate depends on many other factors
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A visual cell that responds the best to say vertical edges in its receptive field which might give only what?
a weak response when the contrast of a vertical edge is low
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However, what could also respond?
A horizontal edge if the contrast is very high
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What must consequently the brain do?
What is in the world by the patterns of activity within populations of neurons
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What did Hubel and wiesal find?
Typical orientation selectivity (tuning) of a neuron to a bar-shaped stimulus in its receptive field,
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What do the cells respond to?
The vertical stimulus
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What is the limit of brain injury?
many perceptual functions may not be anatomically localised, other brain areas may compensate for damaged area
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How does a PET & fMRI scan do?
Active brain areas use more glucose and oxygen
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Wht does a PET scan require?
Radioactive tagging has poorspatial accuracy
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What is fMRI link?
Activity not well understood but good spatial accuracy
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What does EEG measure?
Measures electrical activity across brain (fast but poor resolution
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Other cards in this set

Card 2

Front

What are the limits?

Back

They change stimulus strength (intensity) until subject says just detectable/undetectable

Card 3

Front

What is the adjustment?

Back

Preview of the front of card 3

Card 4

Front

What is the constant stimuli?

Back

Preview of the front of card 4

Card 5

Front

What is the problem with this?

Back

Preview of the front of card 5
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