Introduction to Perception

Question 1

What is Perception

Answer 1

Psychological process and underlying mechanisms by which we gain knowledge of the world via sense organs

Question 2

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

Answer 2

Philosophical issue but must be ‘yes’ most of time, otherwise it would be disastrous for our survival (e.g. crossing the road, etc.)

Question 3

Do our perceptions of the world always match physical reality

Answer 3

no can be created by our senses (e.g. visual illusions)

Question 4

what is the muller-Lyer illusion

Answer 4

• 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

Question 5

is perception passive or active?

Answer 5

• it’s an active process of interpreting sensory information to guide our interactions with the environment

Question 6

why does perception appear easy?

Answer 6

• due to specialised ‘neural circuitry’ which processes sensory information

Question 7

how many photoreceptors are in each retina?

Answer 7

100, 000, 000

Question 8

how many cells in the cortex of the brain?

Answer 8

100, 000, 000, 000

Question 9

how many connections to other cells in each nerve cells

Answer 9

4000

Question 10

what are the 5 sensory modalities

Answer 10

• taste, smell, touching, hearing and vision.
• we know the most about vision

Question 11

What is psychophysics?

Answer 11

The study of the quantitative relationship between sensory experience (‘psycho-‘) and environmental stimulation (‘-Physics’)

Question 12

How is psychophysics tested?

Answer 12

• 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)

Question 13

what is a detection threshold?

Answer 13

weakest stimulus (e.g. light) reliably evokes a sensation in the observer

Question 14

what is a discrimination threshold?

Answer 14

smallest difference between 2 stimuli along a particular dimension (e.g. pitch) that can be detected (‘just noticeable difference (JND))

Question 15

Methods of limits for measuring detection threshold

Answer 15

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

Question 16

methods of adjustment for measuring detection threshold

Answer 16

subject adjusts stimuli until just detectable/undetectable

Question 17

method of constant stimuli for measuring detection threshold

Answer 17

present subject with a fixed (constant) set of stimulus strengths in random order and ask them each time whether or not they detect it

Question 18

problem with measuring detection thresholds

Answer 18

bias or subjective way of behaving

Question 19

solution for problem with measuring detection thresholds

Answer 19

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

Question 20

measuring discrimination thresholds

Answer 20

• 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

Question 21

Neurophysiological/electrophysiological approach (single cell responding)

Answer 21

• 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

Question 22

what does a Peri-Stimulus-Time-Histogram (PSTH) do?

Answer 22

a plot that shows how the firing rate of a cell changes during the time a stimulus is presented within its receptive field

Question 23

why is neurophysiology reductionist?

Answer 23

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

Question 24

pros of neurophysiology

Answer 24

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

Question 25

cons of neurophysiology

Answer 25

Tells us little about perceptual processes which rely on the combined activity of many neurones (neural networks in the brain)

Question 26

are individual cells considered feature detectors?

Answer 26

No, they do not simply signal the presence of a specific ‘feature’ in the world

Question 27

Neurophysiological approach (e.g. Hubel & Wiesel, 1959)

Answer 27

Typical orientation selectivity (tuning) of a neuron to a bar-shaped stimulus in its receptive field

Question 28

PET & fMRI

Answer 28

active brain areas use more
glucose & oxygen (increased blood flow)

Question 29

PET

Answer 29

requires radioactive tagging, has
poor spatial accuracy

Question 30

fMRI

Answer 30

link between fMRI & activity not well
understood but good spatial accuracy

Question 31

EEG/MEG

Answer 31

Measures electrical activity
across brain (fast but poor spatial resolution)