Mental training by meditation has been related to changes in high-level cognitive functions that involve top-down processing. The aim of this study was to investigate whether the practice of meditation is also related to alterations in low-level, bottom-up processing. Therefore, intersensory facilitation (IF) effects in a group of mindfulness meditators (MM) were compared to IF effects in an age- and gender-matched control group. Smaller and even absent IF effects were found in the MM group, which suggests that changes in bottom-up processing are associated with MM. Furthermore, reduced interference of a visual warning stimulus with the IF effects was found, which suggests an improved allocation of attentional resources in mindfulness meditators, even across modalities.
In recent years there has been a significant increase in research on the effects of meditation on brain and behavior (Davidson et al., 2003, Kabat-Zinn, 2003, Moore and Malinowski, 2009, Slagter et al., 2007 and van den Hurk et al., 2010). Among other findings, meditation has been associated with improvements in attentional processing (Lutz et al., 2009 and van den Hurk et al., 2010) and better resource allocation (Slagter et al., 2007). One of the most consistent findings has been better executive attention as shown by improved performance of meditators in Stroop (like) tasks (Chan and Woollacott, 2007, Moore and Malinowski, 2009 and van den Hurk et al., 2010). The improvement in executive attention points to a greater ability of meditators to inhibit incorrect responses and suggests a reduction in reactivity due to improved top-down control. Actually, many, if not all, research findings on meditation related effects have been limited to high-level, top-down processing. To our knowledge, only in a recent study by Vestergaard-Poulsen et al. (2009), have differences between meditators and non-meditators in low-level brain structures been investigated. An increase in gray matter density in the medulla oblongata, a brain region in the lower brain stem, was found in meditators. Thus, the aim of our study was to investigate whether also functional differences in low-level, bottom-up processing are associated with meditation.
One well-known phenomenon that is considered to reflect bottom-up processing is intersensory facilitation (IF) (Kirchner and Colonius, 2005). IF stands for the reduction in reaction time (RT) to a stimulus presented in one modality when it is accompanied, close in time, by the presentation of a stimulus in another modality (Keuss et al., 1990, Kirchner and Colonius, 2005, Schmidt et al., 1984 and Stoffels et al., 1985). For example, Keuss et al. (1990) demonstrated that non-informative sounds (auditory accessories) of low to moderate intensity facilitate RT to a visual stimulus. Even more, they showed that visual choice reactions become faster with increasing intensity of the auditory accessory, which is remarkable since the auditory stimulus does not provide any information about the correct response. Interestingly, the latter finding on IF effects by Keuss et al. (1990) bears a close resemblance to the StartReact Effect (SRE), i.e., the fact that high intensity, startle inducing auditory stimuli (high intensity accessory) can speed up simple as well as choice visual RT in comparison with auditory stimuli of moderate intensity (Oude Nijhuis et al., 2007 and Valls-Sole et al., 1995). In addition to RT facilitation as observed in IF, the SRE has been associated with increases in speed and acceleration of movement (Siegmund et al., 2001). Contrary to previous research findings, which suggested that the SRE is the result of startle, recent studies provided compelling evidence for the view that the SRE reflects stimulus-intensity facilitatory effects (Carlsen et al., 2007 and Lipp et al., 2006). As such, the SRE might be considered as a specific case of IF with auditory accessories of moderate and high intensity, of which the latter have high potential to elicit a startle response.
The research findings of reduced reactivity due to improved high-level, top-down processing led us to the hypothesis that meditation might be related to reduced (re)activity in low-level, bottom-up processing. In order to test this first hypothesis, a group with extensive mindfulness meditation (MM) experience was compared to an age- and gender-matched control group on IF effects in a visual choice reaction time task involving head rotations. The level of IF was indexed by the size of the SRE in the following outcome parameters: onset latency of head rotation, onset latency of EMG activity in neck muscles, peak angular velocity of head rotation and peak angular acceleration of head rotation. A reduction in (re)activity in bottom-up processing would be revealed by attenuated IF effects. In other words, for each of the outcome parameters a smaller SRE in the MM group was expected. In addition, since previous literature showed that a visual warning stimulus seems to draw attention away from the auditory modality and, as a corollary, reduces the influence of auditory stimuli (Lipp et al., 2000 and Schicatano and Blumenthal, 1998), we wanted to test the second hypothesis that, due to an improved deployment of attentional resources in the group of meditators (Slagter et al., 2007 and van den Hurk et al., 2010), the IF effects are less affected by an interfering visual warning stimulus in this group. For this reason, trials with and without a visual warning stimulus were presented and the condition without an interfering visual warning was considered the default condition in which to study the IF effects.
