Serial position effects in spoken word production are determined by previous context: Evidence from aphasia

Abstract

Organising and producing a sequence of events is a basic human cognitive capacity. It occurs across a wide variety of domains including speech, writing, memory, planning and almost every type of skilled action. Errors involving sequences have been widely studied and often present two kinds of profiles: performance either declines across positions or it declines and then improves in the final positions (a U-shaped pattern). Studies of errors in aphasia have also reported these patterns with letters (in spelling) or phonemes (in speech). Another pattern, with more difficulty initiating speech, has been reported in apraxia of speech. Contrasting declines and increases in performance, however, have not been described in studies using the same methodology and evidence of performance linearly improving is very limited. We document all three patterns using statistical models in a case series of 23 people with aphasia (PwA) who make speech errors when repeating single words. We found that the declining pattern and the U-shape patterns occurred across patients, independent of whether their main impairment was a phonological impairment or apraxia of speech. Only people with apraxia of speech, however, showed the inverse pattern of linearly improving performance. Upward and downward patterns were not the consequence of a general factor like severity. Importantly, further exploration with statistical models revealed that phoneme position in the word was not, in fact, the dominant factor determining the visual patterns. Instead, performance was determined by either the number of previous errors (for declining performance) or the number of previous phonemes correct (for improving performance). Errors were almost never governed by serial position or word length per se. Our results support an important role for evolving context in the serial production mechanisms supporting single word production and we discuss implications for current models of speech production and, more generally, for models of serial performance. We suggest that temporary retention of novel sequences may rely more on an explicit representation of position, while stored articulatory representations may benefit from a contextual format (of the chaining type) where activation of previous units helps to support retrieval of units further along in the sequence.