The effects of phoneme discrimination & semantic therapies for speech perception deficits in aphasia

This study aimed to improve the treatment of speech perception impairments in aphasia.  Two approaches were tested: 'pure' phoneme discrimination therapy and semantic/phoneme discrimination therapy.  This three year project was funded by the Stroke Association and ran from 2005-2008.

Difficulties in understanding speech are common in aphasia, although they are often undetected or underestimated. These problems affect many aspects of an individual's life, including access to information, and participation in conversation, employment and leisure activities.   Despite this, relatively little attention has been paid to this area in aphasia therapy research.  Aphasic auditory comprehension problems arise from different processing impairments.  In some individuals there is an inability to discriminate speech sounds.  The discrimination problem may be general, or may affect only specific contrasts, such as voicing or place of articulation.  In either case there may be severe difficulties in recognising and comprehending spoken words.  Few well-designed experimental studies of explicit training in speech perception in aphasia have been attempted.  A recent systematic review for the Royal College of Speech & Language Therapists identified only three, with conflicting results.  The studies all used single case designs to evaluate phoneme discrimination therapy, which involves structured practice in distinguishing similar sounding words.  Two showed beneficial effects, while the third did not.  Even when positive results were achieved in discrimination tasks, this did not necessarily improve understanding of speech when discrimination was treated without also working on word-meanings. Replications with more participants are therefore needed to test discrimination therapy.  It is also unclear whether discrimination therapy is the most appropriate treatment.  There exists a complex relationship between the processing of speech-sounds and word meanings in aphasia.  Unlike controls, aphasic listeners are heavily influenced by context even in optimal listening conditions. Discrimination of meaningful words is often more successful than nonsense words, and judgements about words are heavily influenced by accompanying pictures. Aphasic listeners seem to make strategic use of meaning to compensate for their impaired speech perception.  It follows that therapies promoting this strategy might be advantageous, by helping individuals make further use of semantic context to support their understanding of speech.  This project aimed to compare such an approach with 'pure' phoneme discrimination therapy.

Twenty people with speech perception difficulties resulting from stroke were recruited.  All were at least 6 months post onset and were native speakers of English.  Only participants whose performance was stable after repeated baseline measurement (see outline of study below) were selected.  Hearing levels on pure tone audiometry must not exceed 40dB in the better ear at 1-4kHz, a level is sufficient for perception of test stimuli, whilst reflecting a realistic range of hearing abilities found in clinical populations. 

On admission to the study a range of experimental tests were carried out.  These included minimal pair discrimination using words and nonwords in both quiet and noise conditions, picture-word verification with phonological foils in quiet and noise, auditory lexical decision, spoken synonym judgement, non-speech auditory discrimination, and a novel telephone message task.  There was also a control task, written sentence to picture matching, which was not expected to improve through therapy.  Baselines were repeated after a six-week pause, during which no therapy was given. In order to attribute gains to therapy, results had to show that these occurred only after treatment, with a stable baseline period.  In addition, participants carried out a primed spoken to written word matching task to measure for change on a set of treated words.   

Pairs of participants were matched by performance on nonword discrimination, and randomly allocated to two treatment groups.  Group one received a phase of 'pure' phoneme discrimination therapy followed by a phase of semantic/phoneme discrimination therapy. Group two received a phase of semantic/phoneme discrimination therapy followed by a phase of 'pure' phoneme discrimination therapy.  Each phase of therapy comprised twelve sessions, administered twice a week by an experienced aphasia therapist.  Tests were re-administered after each phase to measure effects of therapy.  Testing was repeated six weeks after phase two to determine whether treatment gains were maintained.

Group and individual results will be analysed.  If therapy is effective, accuracy should improve in the phoneme discrimination tasks.  Response speeds on these tasks may also be more rapid.  Of interest is whether there are also improvements in non-speech auditory processing, word recognition, word comprehension and in a test of everyday comprehension using telephone messages.