The Setup

Experiments make use of stimuli in which direction of F3 transitions distinguish [da] from [ga]. Without this transition, the rest of the stimulus pattern is ambiguous between [da] and [ga].

The critical formant transition (A below) is presented to one ear, and everything else (the ambiguous "base", B below) is presented to the other.

The two simultaneous percepts:

• Listeners hear a syllable in the ear that gets the base (B), but it is not ambiguous. Its identification is determined by which of the nine F3 transitions are presented to the other ear (A).
• Listeners also hear a non-speech "chirp" in the ear that gets the isolated transition (A).

Implications

The fact that the same stimulus is simultaneously part of two quite distinct types of percepts argues that the percepts are produced by separate mechanisms, or modules, that are both sensitive to the same range of stimuli.

In fact, the differing operational characteristics of the modules can be seen in how they process common stimuli.

• The discrimination functions for the isolated "chirp" and the speech percept are quite different, despite the fact that the acoustic event responsible for both is the same. The speech percept exhibits categorical perception, the chirp percept exhibits continuous perception.
• If the intensity of the isolated transition is lowered below the threshold of hearing, so that listeners cannot tell reliably whether or not it is there on a given trial, it is still capable of disambiguating the speech percept.