Introduction to perception Flashcards
What is perception?
The psychological processes and underlying physiological mechanisms by which we gain knowledge of the world.
How accurate are our perceptions of the world?
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)
Perception is not simply the passive registering of information about the world, but…
an active process of interpreting sensory information to guide our interactions with the environment.
Because perception appears easy (largely automatic and effortless), what trap is it easy to fall into?
Believing that there’s nothing to explain. (Gregory, 1977 - there are problems to be solved, vision is nothing short of a miracle)
Why does perception appear easy?
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.
How many photoreceptors are there in each retina?
1 thousand million, 10^9
How many cells are there in the cortex of the brain?
100 thousand million, 10^11
How many connections to other cells does each nerve cell make?
4000
What is the main theoretical motivation for studying perception?
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.
What are some practical motivations for studying perception?
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.
Which is the most well studied/understood sensory modality?
Vision.
What is psychophysics?
The study of the quantitative relationship between sensory experience and environmental stimulation.
What is the oldest branch of experimental psychology, and whose ideas is it based on?
Psychophysics, based on Weber (1834)’s ideas and subsequent techniques developed by Fechner (1860).
What is the main experimental technique in psychophysics?
The observer reports when they detect the presence of a sensory stimulus - measures perceptual performance of entire organism.
What is the central concept of psychophysics?
The measurement of thresholds.
What different thresholds are measured in psychophysics?
Detection and discrimination thresholds.
Define detection threshold.
The weakest stimulus value (e.g. Intensity of light, tactile pressure) that can just reliably evoke a sensation in the observer.
Define discrimination threshold.
The smallest difference between two stimuli along a particular dimension (e.g. pitch) that can be detected.
What does JND stand for?
Just noticeable difference in a discrimination threshold task.
What detection threshold measurement methods were developed by Fechner?
- 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.
What is a problem with Fechner’s methods?
The subject could be biased or lying. This can be solved by using a forced-choice task.
Describe how a forced-choice task is used to measure detection thresholds.
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.
According to Galanter, 1962, what are our detection thresholds equivalent to for the main 5 senses?
- 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.
Describe how a forced-choice task is used to measure discrimination thresholds.
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.
According to Teghtsoonian, 1971, what are our discrimination threshold values (as a percentage difference between stimuli) under ideal conditions?
- taste - 8.3%
- brightness - 7.9%
- loudness - 4.8%
- vibration - 3.6%
- heaviness - 2%
- electric shock - 1.3%
What theory explains why transition from chance performance to perfect performance on threshold perception tasks is gradual rather than abrupt?
Signal Detection Theory (Green and Swets, 1966).
What is the main premise of signal detection theory?
- 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.
What does the neurophysiological approach to studying perception involve?
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.
What does single cell recording enable the experimenter to do?
Map the receptive field of a cell (e.g. Pattern of visual stimulation on the retina that changes the cell’s firing rate).
With single cell recording, how can the stimuli be changed?
- absence and presence differences - peri-stimulus-time-histogram (PSTH)
- find optimal stimulus that causes greatest change in firing rate.
What does PSTH stand for and what is a PSTH?
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.
What threshold calculation technique which uses neural responses is based on SDT?
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)
Why is neurophysiology reductionist?
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.
Why can individual cells not be considered as feature detectors?
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.
What is coding by ensemble activity?
How the brain interprets sensory information by the patterns of activity within populations of neurons.
What is a problem with neurophysical research, aside from its reductionist nature?
The experiments are carried out on animals, so caution should be exercised when using these studies to explain human perception due to species differences.
Lesion studies and neuropsychological patients have shown that perception can be disrupted by brain damage. What are the problems with this method of study?
- 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.
What is particularly useful about using functional imaging techniques to investigate perception?
Measurements can be made while a subject performs a perceptual task - good for ‘when’ and ‘where’.
What is a problem with using PET to study human perception?
It requires radioactive tagging and has poor spatial accuracy.
What are the advantages and disadvantages of using fMRI to study human perception (Wandell, 1999)?
+ has good spatial accuracy
- link between fMRI and activity not well understood.
What are the advantages and disadvantages of using EEG/MEG to study perception?
- based on correlates of electrical activity across the brain - poor spatial resolution
+ very fast
