Perception lecture 1

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What is perception?
The relationship between brain function and behaviour
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What is there?
No direct contact with the physical world, only information conveyed by our sense organs to our brain. All knowledge of outside world depends on senses
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What is perception?
Psychological processes and underlying physiological mechanisms by which we gain knowledge of the world via sense organs
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Are our perception of the world an accurate reflection of reality?
Philosophical issue but must be ‘yes’ most of the time, otherwise it would be disastrous for our survival (crossing the road)
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However our perceptions of the world are what?
Do not always match the physical reality
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What is a common example of misperception?
Muller-Lyer illusion
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What is perception?
not simply passive registering of information about the world. It is an active process of interpreting sensory information to guide our interactions with the environment
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Why does perception appear easy?
we have so much specialised ‘neural circuitry’ devoted to processing sensory information
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What is the visual brain area?
- 1 000 000 000 photoreceptors in each retina to receive light - 1000 000 000 00 cells in cortex of the brain - Each nerve cell makes 4000 connections to other cells
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What is the theoretical motivations for studying perception?
As much of the cortex is involved in processing sensory information by studying perception we will gain an understanding of how the brain functions in general
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What is the physiological approach to studying perception?
- Psychophysics: The study of the quantitative relationships between sensory experience and environmental stimulation
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Who invented psychophysics?
- Oldest branch of experimental psychology based on the ideas of Weber (1834) and subsequent techniques developed by Fechner (1860) - Subject reports when he or she detects the presence of a sensory stimulus. Measures perceptual performance
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What is the central concept?
Measurement of thresholds
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What is a detection threshold?
Weakest stimulus reliably evokes a sensation in the observer
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What is a discrimination threshold?
smallest difference between 2 stimuli along a particular dimensions that can be detection (just noticeable difference
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What is the method of limits?
change stimulus strength (intensity) until subject says just detectable/undetectable
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What is the method of adjustment?
As above but subject adjusts stimulus until just detectable/undetectable
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What is the method of 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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When to use force choice task?
If subjects are bias and not telling the truth
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What is a force choice task?
Ask subject to report something about the stimulus, One each trial present the stimulus in one of two temporal intervals, Subject responds ‘first’ or ‘second’ interval, Calculate % correct responses and threshold, Thresholds: Stimulus strength=75%
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Vision detection
Candle flame seen at 30 miles dark; clear night
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Hearing
Tick of a watch from a distance of 20 foot
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Taste:
1 teaspoon 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 1 cm
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How to measure discrimination thresholds?
2 stimuli, 1 fixed, 1 interval, force choice, one stimulus is presented first and the other second, subject reports which stimulus has the greater strength, calculate % correct responses and estimate threshold
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What do opticians do?
- Opticians use a crude variant of this technique to assess hich spectacle correction is best for you
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What are the discrimination thresholds between 2 stimuli
Taste: 8.3%, brightness: 7.9%, loud; 4.8%, vibration: 3.6%, heaviness: 2% and electric shock:1.3%
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For same stimulus strength what?
The subject sometimes responds correctly and sometimes incorrectly. Transition from chance to perfect is gradual not abrupt
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What is this explained by.
Signal detection theory
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What is the Signal detection theory
- Sensory systems are imperfect and are inherently ‘noisy’ (E.G cells in sensory pathways exhibit some degree of random firing) - This internal noise interferes with 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. Therefore, thresholds are probabilistic not absolute measures of performance (eg. 75% correct)
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What is the neurophysiological approach?
Single cell recording
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What does this involve?
- Involves recording electrical activity of cells in sensory pathways
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What is the technique used to determine?
cell’s preferred stimulus by recording action potentials (electrical impulses) elicited by a range of visual stimuli - Very fine tipped wire is surgically place into the area of the visual system under study
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When is 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 happens next?
- The weak electrical signals are then amplified usually send to a loud speaker and then recorded - The experimental animal faces a screen on which a range of visual patterns are displayed
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What does this enable?
- Enables the experimenter to map the receptive field of a cell (Pattern of visual stimulation on the retina that changes the cell’s firing rate)
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What is a peri stimulus time histogram
a plot showing how the firing rate of a cell (normally action potentials per second) changes during the time a stimulus is presented within its receptive field
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What is statistics techniques and what can they be used to do?
‘neurometric function’ similar to the psychometric function plotted by psychophysicists - Based on signal-detection-theory (SDT) of Green & Swets (1966)
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What does this show?
Shows cell’s potential to (say) detect stimulus. Can be used to derive neural thresholds (Parker & Newsome, 1998)
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What is wrong with neurophysiology?
It is reductionist
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Why?
- It attempts to reduce a complex problem into a set of more basic tractable problem that can be studied in the lab - As cell are the basic functional units of the brain, by studying individual cells we may gain an insight into vision
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What is a limitation?
Tell 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 individual cells be considered as?
Feature detectors (They do not simply signal presence of a specific feature in the world
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Why?
• This is because the activity of ANY one cell is inherently ambiguous with regard to the actual stimulus within its receptive field that made it respond
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Cells typically what?
• Although cells typically respond better to some stimuli than others (exhibit tuning) their firing rate depends on many other factors (e.g. contrast)
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What is a visual cell?
responds best to say vertical edges in its receptive field might give only a weak response when the contrast of a vertical edge is low
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What must the brain work out?
what is in the world by the patterns of activity within populations of neurones (coding by ensemble activity)
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What is the neurophysiological approach?
- Typical oritentation selectivity of a neuron to a bar shaped stimulus in its receptive field - Activity of neurons in inferotemporal cortex to profiles
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Why should human studies be carefully explained?
Due to use of only animal studies
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What patients are normally studied?
• Lesion studies/neuropsychological patients
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However?
typically diffuse (limits usefulness), many perceptual functions may not be anatomically localised, other brain areas may compensate for damaged area
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PET
requires radioactive tagging, has poor spatial accuracy
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fMRI
link between fMRI & activity not well understood but good spatial accuracy
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EEG and MEG
Measures electrical activity across brain (fast but poor spatial resolution) • Measurements can be made while subject performs a perceptual task
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Card 2

Front

What is there?

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No direct contact with the physical world, only information conveyed by our sense organs to our brain. All knowledge of outside world depends on senses

Card 3

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What is perception?

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Card 4

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Are our perception of the world an accurate reflection of reality?

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Card 5

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However our perceptions of the world are what?

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