Stimulation Induced Changes in Ratio Scaling Between and Within Hemispheres

OBJECTIVES: This paper examines if ratio scaling, the principle behind the psychophysical Power Law, is similarly performed by the left and right cerebral hemispheres and how magnitude estimates derived in each hemisphere are integrated. METHOD: Three models of hemispheric integration were tested (dominance, summation, and inhibition) using a cross-modal matching procedure in right-handed, male subjects. Visual stimuli were presented to one or both hemispheres using a tachistoscopic method to test each model. Olfactory stimuli were also presented to one or both nares (hemispheres) to test the dominance and summation models. RESULTS: A dominance model was not supported as there was little difference in ratio scaling between hemispheres for either visual or olfactory stimuli. A summation model was supported for olfactory but not visual integration. Inter-hemispheric inhibition did not account for hemispheric integration. CONCLUSIONS: The most interesting findings stemmed from a comparison of experimental conditions within rather than between hemispheres. Ratio scaling parameters, the sizes of the exponents and constants, appeared to be driven by the amount of stimulation provided to a hemisphere - a greater amount being associated with higher exponents and lower constants. Variability in ratio scaling, how well data fit power functions, corresponded to whether the hemispheres received equal amounts of stimulation - equal stimulation producing a better fit than unequal stimulation. We conclude that stimulus induced cerebral activation influences the form of power functions; whereas equivalency of stimulation between hemispheres influenced the fit.