Iain McGilchrist’s theory of hemispheric brain function considers not what the hemispheres do but how they do it. The hemispheres construct different worlds and afford different ways of being and working. This paper contextualizes this theory in design by exploring hemispheric dominance during design ideation. Forty engineering professionals ideated solutions to design tasks. Functional near-infrared spectroscopy was used to measure change in oxygenated hemoglobin (oxy-Hb) in the prefrontal cortex (PFC). Shifts in the amount of oxy-Hb in the left and right hemispheres of the PFC were observed. The number of shifts between the left and right hemispheres was similar when designers were resting. Differences in rest and ideation were observed in the level of oxy-Hb in the left and right dorsolateral PFC. This subregion may provide a better indicator of the cognitive change that occurs during ideation. Implications for design research lie in the understanding of how different neurocognitive processes are engaged and how these processes are influenced by hemispheric functions. Brain imaging data can help corroborate theory, and provide new measures of cognitive function during design.