Visual masking in magnetic resonance imaging.

When a brief target picture is followed by another picture (mask), people often report that they are not consciously aware of the target. Thus, visual masking can be used to manipulate perceptual awareness of target pictures. To avoid interference with magnetic resonance imaging, pictures have been presented on liquid crystal device (LCD) and thin film transistor (TFT) projectors that were placed outside of the scanner room. However, we found that display devices with LCD/TFT technology exhibit poor accuracy in presenting pictures at brief durations [Wiens, S., Fransson, P., Dietrich, T., Lohmann, P., Ingvar, M., Ohman, A., 2004. Keeping it short: A comparison of methods for brief picture presentation. Psychological Science, 15, 282--285]. In this paper, we present a reliable and valid masking procedure involving two LCD/TFT projectors in combination with mechanical shutters. Because LCD/TFT projectors present pictures in steady state at longer durations (e.g., after 70 ms), picture presentation is more ecologically valid than for common cathode ray tube (CRT) monitors that present pictures in multiples of refresh cycles. Also, because picture presentation with mechanical shutters is instantaneous and reliable in terms of onset, rise time, and duration, shutters can be used to control picture durations precisely in steps of milliseconds. In this paper, we also discuss risks for confounding effects from unreliable picture presentations in masking. Our findings and arguments recommend the use of mechanical shutters in front of LCD/TFT projectors in imaging studies of visual masking.