Some evidence for 'speech' as an acoustic feature.

Under conditions of serial recall of auditorily presented lists of digits, recall of the last item has been shown to be adversely affected by the presence of a redundant item following the list. This is known as 'the suffix effect' (Crowder & Morton, 1969). In a series of experiments it is shown that the size of this effect is not influenced by the phonological complexity of the suffix. Non-speech sounds, on the other hand, produce no suffix effect even when the subjects are forced to process them. Certain speech sounds were also found to produce no effect. It is concluded that these sounds lacked properties which are characteristic of speech sounds and so were classified as 'non-speech' and that as a result, these sounds are processed by a separate system from the speech sounds.