Lateral brain resources and motor performance.

Speaking interferes with concurrent motor activity, the degree and direction of the interference being related to handedness. Dextrals have a decrement for right-hand performance, while sinisuals have left-hand performance decrements ( 2 ) . This is consistent with a limited resources model of brain function having verbal and spatial abilities localized in leftand right-brain, respectively, in dextrals, and the converse in lefthanders. The model predicts that concommitant tasks requiring similar (same hemisphere) resources compere and produce poorer dual-task performance ( 1 ). The present study was designed to test the model further and evaluated the effect of both verbal and spatial secondary tasks on motor performance. Twenty male subjects aged 18 to 21 yt., classified as strong dextrals o n a handedness questionnaire ( I ) , performed a hand-steadiness task under three task conditions, concurrently with a spatial task, concurrently with a verbal task, and a control condition having no secondary task. Subjects held a standard stylus for 3-min. periods inside a 9-mm. annulus fitted to the center of a rear-projection screen. Secondary task stimuli consisted of 35 common adjectives (verbal task) or 35 monochrome photographs of faces (spatial task) projected individually for 5 sec. duracion and centered above the annulus. Following each block of 35 trials having a secondary task, subjects were asked to identify the 35 faces from a matrix of 50 faces or identify 35 antonyms of the adjectives from a list of 50 words. Verbal and spatial secondary tasks were pretested for equal difficulty. Half of the subjects used the right hand and half the left for the steadiness task; and task condition order was counterbalanced. Mean errors on the secondary tasks in the four-hand/task conditions were almost identical: left/verbal = 21.4, left/spatial = 20.3, right/verbal = 19.4, and right/spatial = 20.1, indicating that secondary task difficulty was not influenced by response hand. Contact time, in sec., for the six hand/task conditions were: left/no task = 4.14 (SD = 2.28), left/verbal = 7.74 ( S D = 3.19). left/spatial = 11.01 ( S D = 6:15) , right/no task = 3.95 (SD = 2.77), right/verbal = 5.91 ( S D = 3.87), and r ~ g h t / spatial = 6.91 ( S D = 4.80). A repeated-measures analysis of variance produced significant effects for task (F2.30 = 23.85, p < .001) and for the response hand/cask interaction (F2 .m = 3.76, p = .032), but not for response hand. Both secondary tasks produced steadiness decrements in each response hand; left/verbal (to = 7.61), left/ spatial (to = 4.32), right/verbal (to = 3.55), and righdspatial ( t ~ = 3.59). The spatial task produced a larger deficit than the verbal task for right-hand performance ( to = 2.30), left-hand performance did not differ significantly between secondary task conditions. Both secondary tasks produced a performance decrement in both hands indicating that the steadiness task may be more sensitive to secondary task loading than the dowel balancing, critical instabiliry task reported elsewhere ( 1 ) . The larger decrement for left-hand performance with the spatial compared to the verbal secondary task is consistent with a lateralized, limited resources model of brain function. Right-hand performance did not support the model. This may be attributed to the absence of a motor component in the word recall versus a speaking secondary task (1). The steadiness task may draw upon both cognitive and manual motor resources in right brain but only motor resources in the left. Further study, using a more direct measure of brain activity, is needed to clarify this issue.