Perception Lecture 2- Psychophysics and Thresholding

•The distinction between sensation and perception • The concept of thresholding • Types of threshold approaches (pros and cons) • Why thresholding is important • Weber and Fechner’s laws

Perception

The act of giving meaning to a detected sensation. The process of organizing and interpreting sensory information in the brain to make sense of the external world (knowledge driven)

What is physically happening is not the same as what we perceive (unconscious inferences based on our biology, info our senses received, educated guesses)

Sensation

The ability to detect a stimulus. Sensory receptors and the nervous system receive and represent stimulus information from the environment (information-driven)

Prior knowledge (memory)

Enables us to recognize meaningful objects, faces, scenes, and events

Psychophysics

The science defining quantitative relationships between physical stimuli and psychological (subjective) events, created by Gustav Fechner (1801-1887) thought about the philosophical relationship between mind and matter.

How do we measure perception?

We cannot directly compare physical and psychological intensities or directly measure psychological sensations

sk a participant to report if they perceive a sensation or if they can discriminate between two or more sensations (so indirect measuring)

Name different experimental approaches

Same/different method: Are these 2 stimuli the same or different? – discrimination.

Matching to sample (XAB task). Is stimulus A or stimulus B the same as stimulus X? – discrimination and recognition

Alternative forced choice (AFC): 2 AFC means a participant must choose between one of two possibilities e.g. Is the correct stimulus the one to the left or the right? • Detection, discrimination, recognition etc

What is thresholding?

• measuring the sensitivity of the senses to find the limits of what can be perceived.

• individual differences: In the population, there can be marked variability in sensory experiences and sensitivities.

Detection threshold

The lowest intensity level at which a physical stimulus can be detected.

Difference threshold

The smallest difference in intensity between two stimuli that a person can detect; also known as the just noticeable difference (JND).

Recognition threshold

The lowest intensity level at which a detected stimulus can be identified or recognized.

Absolute threshold

The minimum amount of stimulation required for an observer to detect a stimulus 50% of the time.

Just Noticeable Difference (JND)

the smallest detectable difference between two stimuli; also known as difference threshold – are these stimuli different?

Ernst Weber

Weber's Law

The principle stating that the just noticeable difference is a constant fraction of the reference stimulus intensity, represented as $\Delta I / I = K$.

Point of Subjective Equivalence (PSE)

The point at which two physically different stimuli are perceived as subjectively equal.

Method of adjustment

A psychophysical threshold estimation procedure in which the observer directly alters the stimulus intensity until it is barely detectable or matches a criterion.

Fechner's Law

The principle stating that perceived sensation magnitude grows logarithmically as physical stimulus intensity increases, represented as $S = k \log(I)$.

Method of constant stimuli

A psychophysical method where a set of predetermined stimulus intensities, ranging from subthreshold to super-threshold, are presented in random order.

Method of limits

A psychophysical method where stimulus intensity is incrementally increased or decreased in fixed ascending or descending series until the participant's response changes.

Adaptive staircase method

A psychophysical method where stimulus intensity changes based on previous participant responses, reversing direction when threshold boundaries are crossed and narrowing step sizes for efficiency.

Magnitude estimation

A scaling method in which participants assign numerical values directly proportional to their subjective perception of stimulus intensity.

Cross-modality matching

A psychophysical task where an observer adjusts the intensity of a stimulus in one sensory modality to match the perceived intensity of a stimulus in a different modality.

Matching to sample (XAB task)

An experimental task where participants determine whether stimulus A or stimulus B matches a standard reference stimulus X.