CNS186-Intro     1.15.08

Introduction to Psychophysics (Shimojo)

0. Definition of Psychophysics
Behavioral and objective study of sensation, perception and behavior. An indispensable component in cognitive and neurosciences.

1. Historical Overview
The term "psychophysics" may sound peculiar in the modern world, but it stemmed from German philosophical tradition.
In short, "psycho" is the dependent variable (sensation) and "physics" is the independent, and it is a study of the functional relationship, or the parallelism between them.
    -Fechner, G. T. (1860) Elements of Psychophysics.
        -Sensation is yielded by interaction between the physical and the psychological
         world (the body & the mind, the material & the mental, etc.). Dualism.
        -The concept of threshold
        -To search for equations which relates the two world, by quantifying, or
         measuring sensations.
    -Psychophysical Laws
         -Weberís law: constant ratio between the stimulus (baseline energy) and the
          relative threshold.
         -Guilfordís law: a generalized version of Weberís law.
         -Fechnerís law: systematic relationship between the magnitude of stimulus and
          that of sensation.
    -Stevens' new psychophysics
         -Magnitude estimation
         -Cross-modal matching (eg. grip meter)
         -(a)reliable(repeatable), (b)consistent across subjects
    - Stevens' Law (a modified version of Fechnerís).
         - Approximated by a line in log-log plot.
In short, the classical psychophysics is characterized by dualism, quantitative measurement, and mathematical laws.

2. Modern psychophysics in the context of cognitive and neurosciences
In a more modern sense, psychophysics typically refers to behavioral and cognitive study of brain/mind functions in both humans and animals.
    -Modern psychophysics as a part of behavioral sciences
          (cf. behaviorism, animal psychophysics)
    -Psychophysics today: Main body of visual sciences
         Helmholtz "unconscious inference"
         Marr "ambiguity solving by constraints"
         Pylyshyn "cognitive impenetrability"
         Fodor "modularity"
     -Study of competence as opposed to that of performance (Chomsky)
     -Constraining (and being constrained by) theory/algorighm/implementation (Marr,
         1982)

3. Concept of Threshold
The original meaning of threshold is a border between undetectable (unnoticeable) and detectable (noticeable) in a stimulus energy domain. However as below, there are many different ways to use it now. Note (important!) that it is a physical quantitiy, not a mental, although it is related to mental experience by definition.
(1) Absolute threshold (Stimulus threshold)
     -Threshold of detection
     -Supraliminal/subliminal
     -Sensitivity = 1 / stimulus threshold
(2) Relative threshold (Difference threshold, Differential
     threshold, DL, Just noticeable difference, JND)
     -Threshold of discrimination (increment/decrement)
(3) Terminal threshold (Terminal stimulus)
     -Minimum intensity of stimulus for maximum sensation
(4) Recognition threshold
     -Just recognizable
(5) Reaction threshold
     -Minimum intensity of stimulus to yield any bodily response

4. Methods
Psychophysical method basically aims to accomplish experimental designs which satisfy objectiveness, statistical significance, and control of artifacts.
    -X-Alternative, forced-choice (XAFC)
    -Constant Stimuli
    -Staircase, Double staircase
    -Adjustment
    -Magnitude estimation

5. Psychometric Functions and Values
Psychophysical experiments can also be categorized as to whether it intends to measure a (detection) threshold, or to measure above-threshold performance of some kind.
    -Threshold vs. super-threshold
         eg. reaction time, percentage correct
    -Probit analysis
    -Mean and threshold
        eg. Mueler-Lyer illusion
    -Cancellation and PSE(Point of Subjective Equality)
        eg. Simultaneous color contrast

6. Signal Detection
Without detecting a sensory signal, perception cannot begin.
Thus, this is the simplest possible framework to approach it. Signal detection theory provides a solid mathematical as well as experimental bases for it. Performance of human observers and machines can be compared on the identical bases.
   -Signal detection theory
        describing signal receiver's behavior mathematically, and providing a mean of
        analyzing the essential structure of the observer's decisions in psychophysical
        tasks and specifies the optimal or ideal detection process in a
        variety of task situations.
   -Accuracy of detection and ROC (Receiver Operating Characteristic)
   -Criterion
   -dí
   -Probability summation
       = the inherent advantage of detection under conditions of multiple, independent
        opportunities, relative to a single opportunity. Important for integration process
        from channels.
       ex. binocular summation
         Pb = 1 - (1 - Pl)(1 - Pr)
         P(D) = 1 - [1 - Pi(ri - 1)]
       Threshold models of visual target detection

7. Examples
    -Contrast sensitivity, acuity, hyperacuity in human infants
    -Crossmodal interactions/integrations
    -Combinations with neuroscience methods, such as TMS, fMRI and EEG

References
Palmer, S. E. (1999) Visual Science - Photon to Phenomenology. Sensation & Perception.
   Cambridge, MA, MIT Press. (Chapter 1 & 2)
Green, D. M. & Swets, J. A. (1966) Signal Detection Theory and Psychophysics. New
   York: John Wiley.
Jones, F. N. (1974) History of psychophysics and judgment. In E. C. Carterette & M. P.
   Morton (Eds.) Handbook of  Perception, Vol.2, Psychophysical judgment and
   measurement. New York: Academic Press.
Lindsay, P. H. & Norman, D. A. (1977) An Introduction to Psychology. London, Acad.
   Press. (Appendix A.)
Shimojo, S., Paradiso, M. and Fujita, I. (2001) What Visual Perception Tells Us About
   Mind and Brain. Proceedings of National Academy of Science, 98,12340-2341.