| Zaphod Beeblebrox — a character in Douglas Adams, comic sci-fi novel ‘‘The Hitchhiker’s Guide to the Galaxy [1]’’ — is in a rather confused state when his spaceship lands on planet ‘Vogtsphere’. Conveniently, he has at hand a ‘thinking cap’, a device that electrically stimulates the brain in order to improve cognitive function. This intuition that electrical stimulation modifies brain function is not only evident in various sci-fi novels, but actually has a long standing history in cognitive neuroscience. For instance, over half a century ago Wilder Penfield [2] pioneered the technique of pre-surgical mapping, whereby brain areas that underlie specific cognitive and motor functions are mapped by applying electrical pulses to the brain tissue. A given area is assumed to be functionally relevant if electrical stimulation interferes with the associated cognitive or motor function, as manifested, for example, by interruptions or difficulties in the naming of objects during the stimulation of language areas (Wernicke’s area, for example). But can this same stimulation technique be utilized in a way that does not disrupt but instead enhances cognitive performance? As they reported very recently in Current Biology, Ezzyat et al. [3] have developed a new approach to brain stimulation, obtaining results that show that brain stimulation is capable of improving memory, but only when applied during certain brain states. The relationship between brain activity and memory is commonly studied with so-called subsequent memory experiments [4], wherein a list of items, for example words, is presented sequentially to a participant who then has to recall the items during a later test. Based on the participant’s recall performance during the test phase, brain activity during the learning phase can be classified into ‘subsequent hit trials’ (items that were later recalled) or ‘subsequent miss trials’ (items that could not be recalled). Contrasting the internally generated brain activity between these two classes of items results in a so-called subsequent memory effect, which quantifies the difference between brain activity during subsequently recalled and forgotten items (Figure 1A). |