Categorical Perception The creatures outside looked from pig
Categorical Perception The creatures outside looked from pig to man, and from man to pig, and from pig to man again; but already it was impossible to say which was which.
George Orwell Animal Farm Excitation, Sensation, Cognition Excitation: The pattern of neural responses elicited by a given stimulus. Sensation: An internal representation of the stimulus. Cognition: The interpretation of a
sensation on the basis of stored knowledge. Discrimination The ability to distinguish between two levels of a stimulus parameter (e.g., different wavelengths of light. Measured by the Just-NoticeableDifference threshold. Uses sensory representation.
Modality dependent Recognition The ability to distinguish categorize a stimulus as belonging to a particular class (e.g., colour or object type). Uses cognitive representation: needs to refer to stored knowledge. Representation dependent.
The relationship between discrimination and recognition Recognition relies on discrimination but does recognition also influence discrimination? Discriminability seems to be affected by category structures this is categorical perception.
Categorical perception: Discriminability across category boundaries is more sensitive than discriminability within categories. First example of categorical perception: the phoneme boundary effect.
Phonemes are the sounds that make up language: e.g., /b/ & /p/. The phonemes /b/ and /p/ differ in the time between the onset (stop) and voicing. Alvin Liberman (1917 2000) Liberman and colleagues (1957) showed a
phoneme boundary effect: A smaller change in delay was necessary to distinguish /b/ from /p/, than to distinguish two phonemes within these categories.
The phoneme boundary effect Motor theory of speech perception: The phoneme boundary effect is caused by activation of the motor program required to produce a phoneme. Category boundary effects in the colour domain Question: Is
the way we sense colour affected by the words for colours in our language? Benjamin Lee Whorf (1897-1941)
The question about colour perception can be operationalized: Color can be objectively measured in terms of its wavelength: 400nm 550nm Wavelength
700nm The question about colour perception can be operationalized: The number of basic color terms in a language can be measured. Basic color terms are: Single words.
Not subsumed by another term. Not restricted to a particular class of objects. Early research on color naming Different languages have a variation in the number of words for colour categories. Dani (New Guinea): Two basic colour
terms - mili (light), mola (dark). English: eleven basic color terms white, black, grey, red green, blue, yellow, orange, purple, pink, brown. Kay and Kempton (1984) Compared English and Tamahumara speakers. Tamahumara does not make a
distinction between blue and green. Kay and Kempton theorized that the perceptual distance between blue and green would be exaggerated in English speakers. Kay and Kempton (1984) 3 green
G G G 2 green, 1 blue
G G B 3 blue
B B B Kay and Kempton (1984) Tamahumara speakers were
equally likely to choose either extreme for all three types of triplet. Kay and Kempton (1984) English speakers were the same when all chips came from
the same category. When there was an odd one out, they were more likely to choose that one. Kay and Kempton (1984) Gives some support to the idea of linguistic relativism in color perception.
But other evidence suggests color perception is much more influenced by visual physiology than by language Also, notice that this experiment does not demonstrate a change in color discriminibility. How might categorical perception work?
The phoneme boundary effect has been explained in terms of the motor theory of speech perception. The colour boundary effect has been explained in terms of linguistic relativism. Is there an explanation that can cover any kind of categorical perception, regardless of the domain?
Harnads model Two levels of representation: Iconic Categorical Note: Both of these would be iconic by C.S. Pierces definition Iconic representation
Unbounded analog of excitatory information: mediates sensation. Categorical representation Bounded and context sensitive Based on information that is invariant within the category structure. Icon > Category filter
Extracts just the useful, invariant information from the iconic representation, so as to be able to construct the categorical representation. Categorical Representations Form the basis for symbolic language representations: Noun labels. Provide a method of grounding language
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