PSYC323 - Psychophysics

Sensation

Physical feeling from something that comes into contact with the body

Perception

The organization, identification, and interpretation of sensory information in order to represent and understand the presented information or environment.

An internalist

Assumes that perceptions of objects, in the environment, and knowledge or beliefs about those object are aspects of a persons mind

An externalist

Objects/observations constitute real aspects of the world, i.e. a true representation of the world

Naive realism

Mind-independent existence. i.e. objects in the environment exist even in the absence of any mind perceiving it.

The belief that people have direct access to reality; our senses give us an accurate representation of what is ‘out there’. Idea/view that we can percive reality as it is without any bias or distortion.

Arguments against naive realism

1. Humans have cognitive biases which can distort their perception of reality. i.e. our own perceptions act on information

2. Theory of relativity = our perception of space and time are not objective realities, but creations of our minds.

John Lockes Stages of Perception

1. Causal chain that leads from the objects to the observer; a form of direct perception i.e. ideas of an object are thought to resemble qualities of the object

2. Subjective ideas of the object do NOT resemble the qualities of the object.

In sum: Perceptual experience of an object is a copy of the objects primary qualities, mixed with subjective elements caused by the observer.

Rene Descartes …

Dualism

Mind-body distinction. Attempts to overcome the question of how matter (brain) can give rise to inner mental life. Role of the soul/mind.

William James

Monism

Only the brain gives rise to perception

Has become more acceptable due to recent advancements in cognitive neuroscience and greater conceptual clarity.

Corpus Callosum

Large bundle of fibres that connects left and right cerebral hemispheres of the brain. Largest commissure (crossing tract)

involved in many functions that require both sides of the brain together.

Recognition

Process of identifying and/or categorising our perceptions

Cognitive neuroscience

The mapping of structure to function and vice versa

Neuropsychological patient work

Patients with brain abnormalities reveal functions attributed to the damaged parts.

Transduction

(Sensation)

Sensory signal (physical stimulus) is converted into an electrical/neural signal

Transmutation

(Perception —→ Recognition)

Transmission, integration and processing of the neural signal to produce an emergent psychological phenomena.

Modulated by cognition: Top-Down Processing

What are Webers Two Key Concepts?

1. The Absolute Threshold (Detection)

2. The Relative Threshold (Discrimination)

The Absolute Threshold

The minimal limit of detection i.e. the minimum stimulus that produces sensation 50% of the time.

The Relative Threshold

The minimal change in stimulus required to elicit a change in sensation 50% of the time.

= Just Noticeable Difference (JND)

Weber-Fechner Law

The JND between 2 stimuli is a function of the magnitude of the original stimulus

i.e. The larger the stimulus magnitude, the greater the amount of difference needed to produce a JND

Weber constant

= k

A constant proportion of the initial stimulus value that represents the JND (Webers Law: Change in Stimulus intensity / Original intensity = k)

Adaptation

Ekman et al (1967)

Perceived magnitude (intensity) decreases over time if stimulus is constantly present.

Psychophysics

Quantitative study of how environmental stimuli gets translated into psychological experience.

Need to measure Thresholds and a Method

Threshold Methods

1. Fechners 3 methods

2. Signal Detection

Fechners 3 methods

1. Constant Stimuli

2. Limits

3. Adjustment

Fechners Method: Constant Stimuli

absolute threshold:

• stimuli 1,2,3,4, … present a set of stimuli multiple times, random order

• ask if you detect. Answer with yes or no

JND:

• Stimulus standard versus comparison stimulus a,b,c,d

• which stimulus is weaker? is it stornger? is it the same?

Fechners Method: Limits

Absolute threshold:

• stimuli 1,2,3,4 … present in ascending/descending order

• “tell me when you detect” ——> “NOW” answer

JND:

• standard versus comparator stimuli a,b,c,d … in ascending value

• tell me when comparator goes from weaker to stronger ——> “NOW” answer.

Fechner Method: Adjustment

Absolute threshold:

• adjust the stimulus until you can just detect it

JND:

• standard stimulus provided

• adjust the comparator stimulus until it matches the standard

• Calculate the JND based on the responses.

Noise

Anything that interferes with the detection of the stimuli

Type I error in signal detection

False Alarm

Type II error in signal detection

Miss

What is the aim of signal detection

increase the likelihood of getting a hit or correct rejection while decreasing the likelihood of a mistaken judgement (miss or false alarm)

Acquisition of information assists in this process.

Signal detection theory

allows for considering criteria/bias as seperate from perceptual sensitivity.

D’

Perceptual sensitivity

Measure of ability to detect signal from noise

Large d’ =

Less noise and stronger signal. Easier to discriminate signal from the noise

Small d’ =

More noise and a weaker signal. Harder to discriminate signal from noise

Yes Bias

A participant is motivated to say yes = hit rate and false alarm rate both increase

B moves to the left, and threshold for saying yes goes down.

No bias

a participant is motivated to say no = miss and correct rejection rates both increase. B moves to right, threshold for saying yes goes up.

Magnitude estimation

Participants each assign numbers to a stimulus intensity based on their own individual frame or reference.

Dates back to Stevens

Experimenter presents a tone (intensity controlled) as a standard. Then presents another tone. Instructions are: What is your estimate of the intensity of the second tone in relation to the standard, presented as a number? Lets call the standard a 10 for simplicity. 20 means twice as loud

Basic Stevens Magnitude Estimation Experiment (1956)

Stevens’ Power Law

Left: Perceived magnitude increases non linearly at different rates in different sensory modalities.

Right: Same data plotted on logarithmic axes - perceived magnitude now increases linearly, showing the power-law relationship between stimulus intensity and perceived magnitude.

What is the importance of the sensory systems?

We need to constantly sample the environment and update our representations of it in order to respond appropriately.