Influence of Word Class Proportion on Cerebral Asymmetries for High and Low Imagery Words Christine Chiarello 1, Connie Shears 2, Stella Liu 3, and Natalie A. Kacinik 4 University of California, Riverside 1, Chapman University 2 Northwestern University 3, & University of California, Davis 4 Results Introduction Acknowledgment The research was supported by NSF grant BCS Conclusions Method Robust RVF advantage for high or low image words, when presented in homogeneous contexts Asymmetries for low image words can be altered by varying the stimulus context in which they occur - the greater the proportion of low image words, the greater the LH processing advantage Asymmetries for high image words insensitive to word class proportion Differential hemispheric processing of high and low image words may depend on more than just the characteristics of the words themselves Overall stimulus context can play a role, suggesting that differential asymmetries for various word classes are more dynamic than previously acknowledged According to one view, the lexicon available to the right hemisphere (RH) is abridged, consisting primarily of high image nouns, but with less capacity for processing low image nouns, verbs, and function words; whereas the left hemisphere would represent all words in a person’s vocabulary (Zaidel, 1983). Hence, the typical RVF/LH advantage should be reduced or minimized for high imagery nouns, relative to other word types (e.g., Day, 1977). However, a recent literature review indicated that the LH asymmetry for high image nouns may be context- dependent, being robust in homogeneous contexts (only high image nouns presented), but smaller or absent in heterogeneous contexts (high and low image words intermixed in the experiment) (Chiarello, Liu, & Shears, 2001). Some support for this view was obtained in an experiment in which high and low image nouns were presented in either blocked (homogeneous) or mixed (heterogeneous) lists, suggesting a more dynamic context-sensitive view of hemisphere asymmetries in lexical processing (Chiarello, et al., 2001). We suggest that, when stimulus context is heterogeneous, the LH and RH may adopt complementary processing biases that optimize the processing of low and high image words, respectively. Furthermore, if the LH and RH are each responsive to different characteristics of the global stimulus context, then they should be sensitive to changes in the proportion of each word type in the stimulus list. That is, as the proportion of low image words increases, the LH, but not the RH, should be better able to optimize the processing of these words, leading to a larger RVF/LH advantage for low image items. Similarly, as the proportion of high image words increases, the RH, but not the LH, should increasingly optimize processing of these words, leading to a reduction or elimination of the RVF/LH advantage for high image words. The current experiment manipulated word-class proportion to test this hypothesis. References Stimuli presented for 135 ms to LVF or RVF (2.1 0 eccentricity) Lexical decision response Figure 1. Laterality index for reaction times to high and low image words across five stimulus proportion conditions. Chiarello, C., Liu, S., & Shears, C. (2001). Does global context modulate cerebral asymmetries? A review and new evidence on word imageability effects. Brain and Language, 79, Day, J. (1977). Right-hemisphere language processing in normal right-handers. Journal of Experimental Psychology: Human Perception and Performance, 3, Zaidel, E. (1983). On multiple representations of the lexicon in the brain - The case of the two hemispheres. In M. Studdert-Kennedy (Ed.), Psychobiology of language, pp Cambridge: MIT Press. Five (between subjects) Stimulus Proportion Conditions: 100% Low Image Words 75% Low Image/25% High Image Words 50% Low Image/50% High Image Words 25% Low Image/75% High Image Words 100% High Image Words Critical Stimuli: 40 high image nouns (mean image = 6.78) 40 low image nouns (mean image = 2.98) matched for length and frequency Additional high and low frequency ‘filler’ words used to adjust stimulus proportions across the five context conditions Pronounceable nonwords Stimuli randomly intermixed throughout experiment RT data converted to laterality index (LVF - RVF/LVF + RVF) and trend analyses computed High Image Words: no effect of stimulus proportion - RVF/LH advantage is unvarying Low Image Words: significant linear trend for stimulus proportion, F(1,126) = 4.28, p <.05. RVF/LH advantage increases as proportion of low image words increases