Brain Research 938 (2002) 92–97 www.elsevier.com / locate / bres Short communication Visual cortex excitability increases during visual mental imagery—a TMS study in healthy human subjects a,b a c c Roland Sparing , Felix M. Mottaghy , Giorgio Ganis , William L. Thompson , b c a, * ¨ Rudolf Topper , Stephen M. Kosslyn , Alvaro Pascual-Leone a Laboratory for Magnetic Brain Stimulation, Department of Neurology, Beth Israel Deaconess Medical Center and Harvard Medical School, Boston, MA, USA b Department of Neurology, Technical University of Aachen, Aachen, Germany c Department of Psychology, Harvard University, Cambridge, MA, USA Accepted 21 February 2002 Abstract Previous neuroimaging studies provided evidence that visual mental imagery relies, in part, on the primary visual cortex. We hypothesized that, analogous to the finding that motor imagery increases the excitability of motor cortex, visual imagery should increase visual cortex excitability, as indexed by a decrease in the phosphene threshold (PT). In order to test visual cortex excitability, the primary visual cortex was stimulated with transcranial magnetic stimulation (TMS), so as to elicite phosphenes in the right lower visual quadrant. Subjects performed a visual imagery task and an auditory control task. We applied TMS with increasing intensity to determine the PT for each subject. Independent of the quadrant in which subjects placed their visual images, imagery decreased PT compared to baseline PT; in contrast, the auditory task did not change PT. These findings demonstrate for the first time a short-term, task-dependent modulation of PT. These results constitute evidence that early visual areas participate in visual imagery processing.  2002 Published by Elsevier Science B.V. Theme: Sensory systems Topic: Visual cortex: striate Keywords: Phosphene threshold; Primary visual cortex; Transcranial magnetic stimulation; Visual mental imagery The ability to form a visual representation in short-term fact performed meta-analyses of the entire neuroimaging memory in the absence of the appropriate sensory input is literature on visual mental imagery. Their results showed known as visual mental imagery; such representations are that early visual cortex (Areas 17 and / or 18) was found to typically accompanied by the experience of ‘seeing with be activated when three conditions were met: (1) the the mind’s eye’. The contribution of early visual areas, technique was more sensitive; (2) a resting baseline was specifically Brodmann Area 17 (BA17), to visual mental not used; and (3) the task required noting details of the imagery is still a matter of debate; previous neuroimaging imaged pattern with high resolution. Additional analyses studies (PET, fMRI, SPECT) have been controversial (for revealed that spatial imagery (e.g., of locations of objects) reviews, see Refs. [18,23,33]). Differences in methodology did not activate early visual cortex [18,33]. could be responsible for the disparities, and it is possible Barker et al. reported that transcranial magnetic stimula- that, at least in some circumstances, visual mental imagery tion (TMS) can produce phosphenes when it is applied to does not activate BA17. Members of our group have in the occipital cortex [4]. Phosphenes have subsequently been investigated in numerous studies [7,9,13,14,21,22,24,28]. Recent studies have shown that *Corresponding author. Behavioral Neurology, Beth Israel Deaconess the phosphene threshold (PT, which is the minimum TMS Medical Center, KS 454, 330 Brookline Avenue, Boston, MA 02214, intensity that evokes phosphenes) is a reliable measure of USA. Tel.: 11-617-667-0203; fax: 11-617-975-5322. E-mail address: apleone@caregroup.harvard.edu (A. Pascual-Leone). visual cortical excitability in patients with migraine [2,3] 0006-8993 / 02 / $ – see front matter  2002 Published by Elsevier Science B.V. PII: S0006-8993(02)02478-2