Introduction to perception

Question 1

What is perception?

Answer 1

The psychological processes and underlying physiological mechanisms by which we gain knowledge of the world.

Question 2

How accurate are our perceptions of the world?

Answer 2

They must be a reasonably accurate reflection of reality, otherwise it would be disastrous for out survival, but they don’t always match up (e.g. Visual illusions such as the Muller-Lyer illusion)

Question 3

Perception is not simply the passive registering of information about the world, but…

Answer 3

an active process of interpreting sensory information to guide our interactions with the environment.

Question 4

Because perception appears easy (largely automatic and effortless), what trap is it easy to fall into?

Answer 4

Believing that there’s nothing to explain. (Gregory, 1977 - there are problems to be solved, vision is nothing short of a miracle)

Question 5

Why does perception appear easy?

Answer 5

Because we have so much specialised neural circuitry devoted to processing sensory information, e.g. 30 separate ‘visual cortical areas’ identified so far, taking up about 50% of the cortex.

Question 6

How many photoreceptors are there in each retina?

Answer 6

1 thousand million, 10^9

Question 7

How many cells are there in the cortex of the brain?

Answer 7

100 thousand million, 10^11

Question 8

How many connections to other cells does each nerve cell make?

Answer 8

4000

Question 9

What is the main theoretical motivation for studying perception?

Answer 9

As so 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 - perception as a tool.

Question 10

What are some practical motivations for studying perception?

Answer 10

Discoveries have been put to good use, e.g. Primary colours and television, phones use knowledge that intelligible speech is possible through transmitting only a small portion of the total sound pattern.

Question 11

Which is the most well studied/understood sensory modality?

Answer 11

Vision.

Question 12

What is psychophysics?

Answer 12

The study of the quantitative relationship between sensory experience and environmental stimulation.

Question 13

What is the oldest branch of experimental psychology, and whose ideas is it based on?

Answer 13

Psychophysics, based on Weber (1834)’s ideas and subsequent techniques developed by Fechner (1860).

Question 14

What is the main experimental technique in psychophysics?

Answer 14

The observer reports when they detect the presence of a sensory stimulus - measures perceptual performance of entire organism.

Question 15

What is the central concept of psychophysics?

Answer 15

The measurement of thresholds.

Question 16

What different thresholds are measured in psychophysics?

Answer 16

Detection and discrimination thresholds.

Question 17

Define detection threshold.

Answer 17

The weakest stimulus value (e.g. Intensity of light, tactile pressure) that can just reliably evoke a sensation in the observer.

Question 18

Define discrimination threshold.

Answer 18

The smallest difference between two stimuli along a particular dimension (e.g. pitch) that can be detected.

Question 19

What does JND stand for?

Answer 19

Just noticeable difference in a discrimination threshold task.

Question 20

What detection threshold measurement methods were developed by Fechner?

Answer 20

• Limits - change intensity until subject says it’s just detectable/undetectable.
• Adjustment - subject adjusts stimulus intensity until just detectable/undetectable.
• Constant stimuli - subject presented with a fixed set of stimulus intensities in a random order and asked whether they can detect it.

Question 21

What is a problem with Fechner’s methods?

Answer 21

The subject could be biased or lying. This can be solved by using a forced-choice task.

Question 22

Describe how a forced-choice task is used to measure detection thresholds.

Answer 22

The stimulus is presented randomly in one of two temporal intervals, and the subject responds with either the first or second interval. The percentage of correct responses is used to calculate the threshold.

Question 23

According to Galanter, 1962, what are our detection thresholds equivalent to for the main 5 senses?

Answer 23

• vision - a candle flame 30 miles away on a dark, clear night.
• hearing - watch ticking from 20 feet away.
• taste - 1 tsp of sugar in 2 gallons of water.
• smell - one drop of perfume diffused in a 3 room flat
• touch - an insect wing falling on back from distance of 1cm.

Question 24

Describe how a forced-choice task is used to measure discrimination thresholds.

Answer 24

Two stimuli are presented consecutively on each trial, observer reports which is stronger. The threshold is estimated through the percent of correct responses plotted on a psychometric function.

Question 25

According to Teghtsoonian, 1971, what are our discrimination threshold values (as a percentage difference between stimuli) under ideal conditions?

Answer 25

• taste - 8.3%
• brightness - 7.9%
• loudness - 4.8%
• vibration - 3.6%
• heaviness - 2%
• electric shock - 1.3%

Question 26

What theory explains why transition from chance performance to perfect performance on threshold perception tasks is gradual rather than abrupt?

Answer 26

Signal Detection Theory (Green and Swets, 1966).

Question 27

What is the main premise of signal detection theory?

Answer 27

• sensory systems are imperfect and inherently ‘noisy’ (cells in sensory pathways exhibit some degree of random firing)
• this interferes with our perceptual decisions when a stimulus is weak or the difference is small.
• leads to incorrect decisions some of the time, therefore thresholds are probabilistic not absolute measures of performance.

Question 28

What does the neurophysiological approach to studying perception involve?

Answer 28

Single Cell recording: Recording electrical activity of cells in visual pathways - extracellular recording is commonly used to study perceptual apparatus, microelectrodes record a single cell’s APs to determine the cell’s preferred stimulus.

Question 29

What does single cell recording enable the experimenter to do?

Answer 29

Map the receptive field of a cell (e.g. Pattern of visual stimulation on the retina that changes the cell’s firing rate).

Question 30

With single cell recording, how can the stimuli be changed?

Answer 30

• absence and presence differences - peri-stimulus-time-histogram (PSTH)
• find optimal stimulus that causes greatest change in firing rate.

Question 31

What does PSTH stand for and what is a PSTH?

Answer 31

peri-stimulus-time-histogram, a plot showing how the firing rate of a cell (APs/sec) changes during the time a stimulus is presented within its receptive field.

Question 32

What threshold calculation technique which uses neural responses is based on SDT?

Answer 32

Statistical methods used to calculate neurometric function, which shows a cell’s potential to detect a stimulus. Can be used to derive neural thresholds (Parker and Newsome, 1998)

Question 33

Why is neurophysiology reductionist?

Answer 33

It attempts to reduce a complex problem - how we perceive - into a set of more basic tractable problems that can be studied in the lab. By studying individual cells we may gain an insight into the fundamental processes of perception, but it tells us little about perceptual processes which rely on the combined activity of many neurons.

Question 34

Why can individual cells not be considered as feature detectors?

Answer 34

They don’t simply signal the presence of a specific feature in the world, the activity of any one cell is ambiguous with regard to the actual stimulus within its receptive field which made it respond. Although cells typically exhibit tuning (respond better to some stimuli than others), their firing rate depends on many other factors.

Question 35

What is coding by ensemble activity?

Answer 35

How the brain interprets sensory information by the patterns of activity within populations of neurons.

Question 36

What is a problem with neurophysical research, aside from its reductionist nature?

Answer 36

The experiments are carried out on animals, so caution should be exercised when using these studies to explain human perception due to species differences.

Question 37

Lesion studies and neuropsychological patients have shown that perception can be disrupted by brain damage. What are the problems with this method of study?

Answer 37

• brain injury is typically diffuse, limiting its usefulness.
• many perceptual functions may not be anatomically localised.
• other brain areas may compensate for the damaged area.

Question 38

What is particularly useful about using functional imaging techniques to investigate perception?

Answer 38

Measurements can be made while a subject performs a perceptual task - good for ‘when’ and ‘where’.

Question 39

What is a problem with using PET to study human perception?

Answer 39

It requires radioactive tagging and has poor spatial accuracy.

Question 40

What are the advantages and disadvantages of using fMRI to study human perception (Wandell, 1999)?

Answer 40

+ has good spatial accuracy

- link between fMRI and activity not well understood.

Question 41

What are the advantages and disadvantages of using EEG/MEG to study perception?

Answer 41

• based on correlates of electrical activity across the brain - poor spatial resolution
  + very fast