Parallel visual search is not always effortless.

Serial visual search can become parallel with practice. We tested whether this parallelisation is accompanied by a transition from an attentive to a pre-attentive and thus effortless mode of visual perception. While psychophysiological indicators of attentional effort (galvanic skin conductance, muscle tonus) reflected the distinction between serial and parallel search modes, they did not follow the perceptual changes during learning. Despite the perceptual parallelisation with practice, the attentional load remained high for initially serial tasks.