The concept of an illusory conjunction is not sufficiently explored in studies of perception and memory, and it is rarely discussed in philosophy. Yet this concept is of considerable importance to our understanding of perceptual and cognitive function. For example, when we hear a musical tone, we attribute a pitch, a loudness, a timbre, and we hear the tone as coming from a particular spatial location; so each perceived tone can be described as a bundle of attribute values. It is generally assumed that this bundle reflects the characteristics and location of the sound that is emitted. However, when multiple sequences of tones arise simultaneously from different regions of space, these bundles of attribute values sometimes fragment and recombine incorrectly, so that illusory conjunctions result.
This gives rise to several illusions of sound perception, such as the octave illusion and the scale illusion, in which the melodies we "hear" are quite different from those that are presented. The effect can even be found in live musical performances—for example in the final movement of Tchaikovsky’s 6th Symphony. Illusory conjunctions can also occur in vision. Under certain circumstances when people are shown several colored letters and asked to report what they saw, they sometimes combine the colors and shapes of the letters incorrectly—for example, when presented with a blue cross and a red circle, viewers sometimes report seeing a red cross and a blue circle instead.
Hallucinations—both auditory and visual—frequently involve illusory conjunctions. For example, in musical hallucinations many aspects of a piece of music may be heard accurately in detail, while some aspect is altered or appears corrupted. For example, a familiar piece of music may be "heard" as played by a different or even unknown musical instrument, as transposed to a different pitch range, or as played much faster or slower than it should be. In vision, hallucinated faces may be "seen" to have inappropriate components—in one report a woman’s face appeared with a long white Santa Claus beard attached.
Presumably, when we see and hear in the normal way we process the information in modules or circuits that are each specific to some attribute, and we combine the outputs of these circuits so as to obtain the final integrated percept. Usually this process leads to veridical perception, but under certain circumstances—such as in some orchestral music, or during hallucinations, this process breaks down—and our percepts are influenced by illusory conjunctions. An understanding of how this happens could shed valuable light on perceptual and cognitive processing in general.