Exp Brain Res (2011) 209:19–27 DOI 10.1007/s00221-010-2512-z R E S E A R C H A R T I C L E A chronometric exploration of high-resolution ‘sensitive TMS masking’ eVects on subjective and objective measures of vision Tom A. de Graaf · Jim Herring · Alexander T. Sack Received: 18 August 2010 / Accepted: 24 November 2010 / Published online: 15 December 2010 © The Author(s) 2010. This article is published with open access at Springerlink.com Abstract Transcranial magnetic stimulation (TMS) can induce masking by interfering with ongoing neural activity in early visual cortex. Previous work has explored the chro- nometry of occipital involvement in vision by using single pulses of TMS with high temporal resolution. However, conventionally TMS intensities have been high and the only measure used to evaluate masking was objective in nature. Recent studies have begun to incorporate subjective measures of vision, alongside objective ones. The current study goes beyond previous work in two regards. First, we explored both objective vision (an orientation discrimina- tion task) and subjective vision (a stimulus visibility rating on a four-point scale), across a wide range of time windows with high temporal resolution. Second, we used a very sen- sitive TMS-masking paradigm: stimulation was at rela- tively low TMS intensities, with a Wgure-8 coil, and the small stimulus was diYcult to discriminate already at base- line level. We hypothesized that this should increase the eVective temporal resolution of our paradigm. Perhaps for this reason, we are able to report a rather interesting mask- ing curve. Within the classical-masking time window, pre- viously reported to encompass broad SOAs anywhere between 60 and 120 ms, we report not one, but at least two dips in objective performance, with no masking in- between. The subjective measure of vision did not mirror this pattern. These preliminary data from our exploratory T. A. de Graaf (&) · J. Herring · A. T. Sack Faculty of Psychology and Neuroscience, Department of Cognitive Neuroscience, Maastricht University, P.O. Box 616, 6200 MD Maastricht, The Netherlands e-mail: tom.degraaf@maastrichtuniversity.nl T. A. de Graaf · A. T. Sack Maastricht Brain Imaging Center, Maastricht, The Netherlands design suggest that, with sensitive TMS masking, we might be able to reveal visual processes in early visual cortex pre- viously unreported. Keywords Multiple · Masking · Vision · Suppression · Transcranial · Occipital · Recurrent · Phase Introduction Studies into visual awareness often adopt masking para- digms. In such paradigms, a visual target stimulus is pre- sented brieXy, and a secondary visual stimulus (mask) is presented either before or after the target stimulus. By using diVerent stimulus onset asynchronies (SOAs) between target and mask, the level of processing of the visual target can be modulated (Breitmeyer and Ogmen 2006). This indicates that there is a temporal organization in visual cortex that underlies the establishment of con- scious vision. At certain points in time, this organization should be left unperturbed, or vision will be abolished. To target the organization in early visual regions speciW- cally, a brain interference method can be used. Transcranial magnetic stimulation (TMS) is ideal in this regard, because it aVords a high temporal resolution in brain interference. Using TMS at diVerent SOAs from a visual target stimulus has yielded diVerent time windows at which the organiza- tion of stimulated early visual cortex should be left unper- turbed (Kammer 2007a). Classically, a time window around 100 ms from visual target stimulus onset was revealed (Amassian et al. 1989). But in addition, earlier time windows have been reported (Corthout et al. 1999; Paulus et al. 1999; Laycock et al. 2007) and TMS eVects in later time windows as well (Heinen et al. 2005; Camprodon et al. 2010). When using longer-lasting motion stimuli and 123