Which of the following is considered not a type of perceptual constancy in speech perception?

Categorical perception is not considered a type of perceptual constancy in speech perception. It refers to the phenomenon where humans perceive sounds as belonging to distinct categories, particularly in distinguishing phonemes. This process allows listeners to categorize speech sounds and recognize them as discrete entities, regardless of variations in acoustic signals due to factors like speaker or contextual change.

In contrast, vocal tract normalization and speech rate normalization are indeed types of perceptual constancy. Vocal tract normalization refers to the ability to recognize speech sounds despite variations in the vocal tract configuration of different speakers. Speech rate normalization allows listeners to understand speech at different speaking rates, ensuring that comprehension remains stable even when the tempo of speech changes.

Thus, while categorical perception deals with the categorization of speech sounds, it does not fall under the umbrella of perceptual constancy, which emphasizes a listener's ability to maintain consistent understanding against varying physical properties of speech input.