Word Retrieval in a Stem Completion Task: Influence of Number of Potential Responses Christine Chiarello 1, Laura K. Halderman 1, Cathy S. Robinson 1 & Natalie Kacinik 2 1 University of California, Riverside 2 University of California, Davis Introduction Conclusions The role of the right cerebral hemisphere (RH) has been well-studied in lexical comprehension tasks, but its role in word production is virtually unexplored. A variety of findings suggest that the normal right hemisphere may activate and maintain a wide variety of semantically related words (see Chiarello, 2003). In the current study we investigated lateralized stem completion in order to examine whether the right hemisphere might contribute to the availability of multiple potential responses when word production is required. Stems had either one prepotent response (BOD), two competing dominant responses (BAI), or many potential responses with none being prepotent (COM). In Experiment 1, the stems were centrally presented; in Experiment 2 they were presented to the LVF/RH or the RVF/LH. In general, we expect reaction time to increase with the number of alternative responses. However, if the right hemisphere is particularly involved with the activation of multiple potential responses (or is less impacted by inter-item competition), then the LVF should show a smaller effect of the number of response alternatives, than either the RVF or centrally presented items. Results Central Experiment - Reaction TimeLateral Experiment - Reaction Time Central Experiment - Percent Correct Lateral Experiment - Percent Correct The time taken to retrieve a word, given a three-letter stem, increases with the number of response alternatives. However, the data imply that this function may differ somewhat across the hemispheres. Two dominant, competing responses did not slow word retrieval, relative to a single dominant response, when the stems were presented to the LVF/RH, although both VFs evidenced additional slowing when there were many response alternatives. This provides only partial support for our hypothesis that the right hemisphere would be less impacted by number of response alternatives, relative to the left hemisphere. Central visual field performance was qualitatively similar to that observed in the RVF/LH, suggesting that the left hemisphere may dominate response output when stimulus information is not lateralized. It also appears that response competition between two equally dominant items may induce a mutual inhibition, reducing the ability to correctly retrieve either one. However, when there are multiple potential responses, the amount of cross-item inhibition any given response receives may be reduced. This could explain the greater accuracy for the Many, relative to the Two, response conditions. The correlational findings suggest that, at least some of the time, participants are covertly pronouncing the stems and utilizing this pronunciation to retrieve a compatible word response. Additional analyses will be needed to determine whether this small, but consistent, association is attributable to individual differences in task strategy. The stem completion task provides an interesting means to probe potential hemisphere differences in word retrieval for production. Follow-up investigations will explore repetition and semantic priming in this task as a means of assessing qualitative hemisphere differences in word retrieval. Method Stems were selected based on published norms (Shaw, 1997) and prior stem completion experiments conducted in our laboratory. Stems for the condition with a single prepotent response (henceforth ONE) had the dominant response given at least 75% of the time (mean = 87.9%) and the ratio of the top response frequency to the second response was at least 6.0 (mean = 25.4). Stems with two competing responses (TWO) had summed response frequencies for the top two responses of at least 64% (mean = 79.2%) and the ratio of the top response to the second response was no greater than 2.30 (mean = 1.55). Stems with many competing responses had the most frequent response given no more than 28% of the time (mean = 20.8%), and the ratio of the top response to the second response was no greater than 2.30 (mean = 1.34). Fifty-two stems were selected for each response condition. These were displayed for 150 ms, centrally presented in Exp. 1, and laterally to the RVF or LVF in Exp. 2. Participants were instructed to complete the stem with the first word that came to mind. Their responses were entered into the computer for later analyses. Thirty-two right-handed native English speakers participated in each experiment. References Chiarello, C. (2003). Parallel systems for processing language: Hemispheric complementarity in the normal brain. In M.T. Banich & M. Mack (Eds.), Mind, brain, and language: Multidisciplinary perspectives. Lawrence Erlbaum Associates. Shaw, R.J. (1997). Unprimed stem completion is only moderately predicted by word length and frequency. Behavior Research Methods, Instruments, & Computers, 29, This research was supported by NSF grant BCS We thank S. Welcome, J. Johnston, J. Julagay, & E. Cox for their assistance with this project. MAJOR FINDINGS: For centrally presented stimuli, RTs were fastest with One dominant response, with significant slowing when there were Two dominant responses (p <.001). Additional slowing occurred when there were Many competing responses (p <.0001). The central presentation pattern was also observed for RVF responses (nonsignificant interaction). However, LVF responses reliably differed from those observed for central stimuli (interaction p <.05) - no slowing was obtained in the LVF for the Two response vs the One response condition. However, for both LVF and RVF, responses were slower for the Many, relative to the Two, response condition (p <.01). The accuracy data revealed one surprisingly consistent result. The lowest accuracy was observed in the Two response condition, relative to either the One response (p <.001), or the Many response (p <.001), condition. This result was also obtained in an early pilot study. ADDITIONAL ANALYSES: To explore whether responses in this task were phonologically mediated, a third experiment was performed. Twenty participants were simply asked to pronounce each stem aloud, and their stem pronunciations were recorded. Unique pronunciation responses were identified for each stem, and we tallied how often each pronunciation was given. We then grouped the word responses made in the lateralized stem completion experiment (Exp. 2) by their pronunciations, and computed, by visual field, the proportion of responses that were made for each stem pronunciation. Positive correlations were obtained between isolated stem and stem-completion pronunciations for each VF: RVF r = +.22, p <.0001; LVF r = +.27, p <.0001.