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., O¨ hman, 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.
2005. Vol. 27, no 2, 465-7 p.
Humans, Magnetic Resonance Imaging/instrumentation/*methods, Photic Stimulation/*methods, Reproducibility of Results, Transistors, Visual Perception/*physiology