Knowledge of the cognitive processes underlying engineering design is essential for its comprehensive understanding and subsequent enhancement of the field. This paper contributes to building this knowledge by investigating the brain activity of engineering designers engaged in three visuospatial reasoning tasks of the Purdue Spatial Visualization Test (PSVT). These tasks assess three critical visuospatial factors for engineering design: spatial visualization, mental rotation, and spatial orientation. Brain activity is measured using electroencephalography (EEG) as a non-invasive neuroimaging method. The EEG results reveal significant differences in brain activity between the three tasks, considering three frequency bands (theta, alpha, and beta) and 14 electrodes spatially distributed across two hemispheres and seven cortical areas. Theta and beta task-related power (TRP) appear to be crucial in distinguishing among the visuospatial reasoning tasks at the neurocognitive level.