Lexical and Environmental Processing and Recovery Patterns in Posterior Aphasia

Amy Ogburn Yeager, Ph.D., CCC-SLP
Auburn University Montgomery
Communication and Dramatic Arts
Post Office Box 244023
Montgomery, Alabama 36124-4023
Phone: (334) 244-3410
Fax: (334) 244-3409
ayeager@mail.aum.edu

Scott S. Rubin, Ph.D., CCC-SLP
LSU Health Sciences Center - New Orleans
School of Allied Health Professions
Department of Communication Disorders
1900 Gravier Street
New Orleans, Louisiana 70112
Phone: (504) 568-4346/4348
Fax: (504) 568-4352
srubin@lsuhsc.edu

Abstract:

Background: For decades, researchers have sought to determine whether language shifts to the right hemisphere after focal damage to the left hemisphere (Moore & Papanicolaou, 1992; Ogden, 1989). Related to a potential shift, there is currently a lack of information regarding how aphasic individuals process environmental sounds, what impact a possible shift of language would have on that environmental processing, and whether an interference effect would be present.

Aims: Two groups of individuals were studied to explore the effects of lexical and environmental sound processing. The primary measurements were the group's ear accuracy scores (i.e., ear advantage) and reaction time. It was hypothesized that the control group would demonstrate an REA (Right Ear Advantage) as they are left hemisphere dominant for language, while the aphasic group would demonstrate an LEA (Left Ear Advantage) due to a shift of language to the right hemisphere.

Methods & Procedures: Therefore, the groups were examined using two dichotic listening tasks, which evaluated lexical and environmental sound recognition and perceptual interference effects when the two were combined. Participants pressed the green button if the auditorily presented sound/word matched the picture/word, whereas if the auditory and visual stimuli were incongruent, the participant pressed the red button.

Outcomes & Results: The results suggested the recognition task was significantly less difficult than the perceptual interference task. There were significant reaction time effects with word recognition faster than environmental sound recognition. In the first task, the control group demonstrated an REA in both conditions. The perceptual interference effects were approximately equivalent between stimuli. The aphasic group demonstrated an LEA in both conditions in the first task and large interference effects.

Conclusions: The control group's REA in the first task may suggest the left hemisphere is responsible for both language processing and to some extent environmental sound processing. Environmental sounds could be bilaterally represented with a semantic encoding strategy facilitating the REA. Due to the equivocal nature of the perceptual interference effects, it seems that both types of stimuli affect each other. The LEA seen in the aphasic group may suggest the right hemisphere's role in language processing and that environmental processing was also affected following a stroke. This group demonstrated large interference effects with both stimuli types. Therefore, it appears that lexical information may take precedence over environmental stimuli. However, it also seems that environmental information may affect lexical processing, which may degrade the verbal code and result in impaired comprehension of the message.

Relative to aphasia recovery, three theories are predominant throughout the literature with regard to whether the right hemisphere actually gains language or if it just appears to be gaining strength relative to the weaker left hemisphere. The first theory suggests that a rightward shift of language is possible. According to some, in this theory the shift occurs in homologous areas (Gold & Kertesz, 2000; Heiss et al., 1999). The second theory states that non-brain damaged individuals have an inhibited right hemisphere and, when damage to the left hemisphere occurs, the right hemisphere becomes active due to a change in the inhibitory transcallosal pathways (Thomas et al., 1997). The final theory indicates that there is not a shift in language lateralization (i.e the left hemisphere maintains it's dominance for language) (Niccum & Speaks, 1991).