Categorical Perception

March 27, 2013

Don’t Forget!• Interim course project report #5 is due on Monday, April 1st!

• The earliest experiments on place perception were conducted in the 1950s, using a speech synthesizer known as the pattern playback.

Testing the Theory

Pattern Playback Picture

Haskins Formant Transitions• Testing the perception of two-formant stimuli, with varying F2 transitions, led to a phenomenon known as categorical perception.

Categorical Perception• Categorical perception =

• continuous physical distinctions are perceived in discrete categories.

• In the in-class experiment from last time:

• There were 11 different syllable stimuli

• They only differed in the locus of their F2 transition

• F2 Locus range = 726 - 2217 Hz

Source: http://www.ling.gu.se/~anders/KatPer/Applet/index.eng.html

Stimulus #1 Stimulus #6

Stimulus #11

Example stimuli from the in-class experiment.

Identification• In Categorical Perception:

• All stimuli within a category boundary should be labeled the same.

Discrimination• Original task: ABX discrimination

• Stimuli across category boundaries should be 100% discriminable.

• Stimuli within category boundaries should not be discriminable at all.

In practice, categorical perception means: the discrimination function can be determined from the

identification function.

Identification Discrimination• Let’s consider a case where the two sounds in a discrimination pair are the same.

• Example: the pair is stimulus 3 followed by stimulus 3

• Identification data--Stimulus 3 is identified as:

• [b] 95% of the time

• [d] 5% of the time

• The discrimination pair will be perceived as:

• [b] - [b] - .95 * .95 = .9025

• [d] - [d] - .05 * .05 = .0025

• Probability of same response is predicted to be:

• (.9025 + .0025) = .905 = 90.5%

Identification Discrimination• Let’s consider a case where the two sounds in a discrimination pair are different.

• Example: the pair is stimulus 9 followed by stimulus 11

• Identification data:

• Stimulus 9: [d] 80% of the time, [g] 20% of the time

• Stimulus 11: [d] 5% of the time, [g] 95% of the time

• The discrimination pair will be perceived as:

• [d] - [d] - .80 * .05 = .04

• [g] - [g] - .20 * .95 = .19

• Probability of same response is predicted to be:

• (.04 + .19) = 23%

Discrimination• In this discrimination graph--

• Solid line is the observed data

• Dashed line is the predicted data

(on the basis of the identification scores)

Note: the actual listeners did a little bit better than the predictions.

Categorical, Continued• Categorical Perception was also found for VOT distinctions.

• And for stop/glide/vowel distinctions:

10 ms transitions: [b] percept

60 ms transitions: [w] percept

200 ms transitions: [u] percept

Interpretation• Main idea: in categorical perception, the mind translates an acoustic stimulus into a phonemic label. (category)

• The acoustic details of the stimulus are discarded in favor of an abstract representation.

• A continuous acoustic signal:

• Is thus transformed into a series of linguistic units:

The Next Level• Interestingly, categorical perception is not found for non-speech stimuli.

• Miyawaki et al: tested perception of an F3 continuum between /r/ and /l/.

The Next Level• They also tested perception of the F3 transitions in isolation.

• Listeners did not perceive these transitions categorically.

The Implications• Interpretation: we do not perceive speech in the same way we perceive other sounds.

• “Speech is special”…

• and the perception of speech is modular.

• A module is a special processor in our minds/brains devoted to interpreting a particular kind of environmental stimuli.