Introduction to Perception Flashcards
What is Perception
Psychological process and underlying mechanisms by which we gain knowledge of the world via sense organs
Are our perceptions of the world an accurate reflection of reality?
Philosophical issue but must be ‘yes’ most of time, otherwise it would be disastrous for our survival (e.g. crossing the road, etc.)
Do our perceptions of the world always match physical reality
no can be created by our senses (e.g. visual illusions)
what is the muller-Lyer illusion
- example of misperception
- Same physical length but the “fins” cause the lines to appear unequal
- What we perceive does not always correspond exactly to physical reality
is perception passive or active?
- it’s an active process of interpreting sensory information to guide our interactions with the environment
why does perception appear easy?
- due to specialised ‘neural circuitry’ which processes sensory information
how many photoreceptors are in each retina?
100, 000, 000
how many cells in the cortex of the brain?
100, 000, 000, 000
how many connections to other cells in each nerve cells
4000
what are the 5 sensory modalities
- taste, smell, touching, hearing and vision.
- we know the most about vision
What is psychophysics?
The study of the quantitative relationship between sensory experience (‘psycho-‘) and environmental stimulation (‘-Physics’)
How is psychophysics tested?
- subject (usually human) reports when he/she detects the presence of a sensory stimulus. Measures perceptual performance of entire organism
- central concept is the measurement of thresholds (when you can just about do it)
what is a detection threshold?
weakest stimulus (e.g. light) reliably evokes a sensation in the observer
what is a discrimination threshold?
smallest difference between 2 stimuli along a particular dimension (e.g. pitch) that can be detected (‘just noticeable difference (JND))
Methods of limits for measuring detection threshold
change stimulus strength (intensity) until subject says just detectable/undetectable
methods of adjustment for measuring detection threshold
subject adjusts stimuli until just detectable/undetectable
method of constant stimuli for measuring detection threshold
present subject with a fixed (constant) set of stimulus strengths in random order and ask them each time whether or not they detect it
problem with measuring detection thresholds
bias or subjective way of behaving
solution for problem with measuring detection thresholds
use a forced-choice task. E.g. present dim stimulus at the top and bottom of the screen and ask p where it was on the screen. If they can actually see the stimulus they will know where it was.
- can be used to calculate guess rate and a graph can be plotted
measuring discrimination thresholds
- use same method as for measuring detection threshold BUT 2 stimuli, 1 fixed and 1 variable
- ideally use a forced-choice task
- on each trial present 2 stimuli consecutively
- one stimulus (chosen at random) is presented first and the the other second
- subject reports which stimulus (‘first’ or ‘second’) has the greater strength
- calculate % correct responses and estimate the threshold
- take half way between guessing and perfect performance
- repeat many times to come up with appropriate results
Neurophysiological/electrophysiological approach (single cell responding)
- involves recording electrical activity of cells in sensory (e.g. visual) pathways
- Extracellular recording from single cells in sensory (e.g. visual) areas of brain commonly used to study perceptual apparatus
- determines a cell’s preferred stimulus by recording action potentials (electrical impulses) elicited by a range of visual stimuli
- Very fine-tipped wire (microelectrode) is surgically placed into the area of the visual system under study
- The microelectrode tip is slowly positioned next to the axon of a cell so that it picks up action potentials ideally from only that cell
- weak electrical signals are amplified to loud speakers and recorded
- experimenter can then map receptive field of a cell
what does a Peri-Stimulus-Time-Histogram (PSTH) do?
a plot that shows how the firing rate of a cell changes during the time a stimulus is presented within its receptive field
why is neurophysiology reductionist?
as it uses single cell recoding it attempts to reduce a complex problem into a set of more basic problems to be studied in a lab
pros of neurophysiology
As cells are the basic functional units of the brain, by studying individual cells we may gain an insight into the fundamental processes of say vision
cons of neurophysiology
Tells us little about perceptual processes which rely on the combined activity of many neurones (neural networks in the brain)
are individual cells considered feature detectors?
No, they do not simply signal the presence of a specific ‘feature’ in the world
Neurophysiological approach (e.g. Hubel & Wiesel, 1959)
Typical orientation selectivity (tuning) of a neuron to a bar-shaped stimulus in its receptive field
PET & fMRI
active brain areas use more
glucose & oxygen (increased blood flow)
PET
requires radioactive tagging, has
poor spatial accuracy
fMRI
link between fMRI & activity not well
understood but good spatial accuracy
EEG/MEG
Measures electrical activity
across brain (fast but poor spatial resolution)
