Unraveling inner experiences during resting state

The investigation of resting state activity has become an extremely busy topic in neurocognitive research. A recent pubmed search revealed more than 2000 entries for the terms “resting state” AND “fMRI.” Several putative clinical applications have been suggested, such as the detection of early signs of Alzheimer’s disease (e.g., Lustig et al., 2003) or risk of developing psychosis (e.g., Jukuri et al., 2013). However, the usefulness of resting state data is hampered by the fact, that up to now only very few investigations have tried to elucidate the mental processes occurring during these experiments. Without knowledge of these processes the purely physiologically based findings of altered activations and functional connectivities during resting state lack a meaningful neurocognitive interpretation. Today, more and more knowledge accumulates that our minds frequently wander during daytime activities and create thoughts which are not directly associated with the situation at hand (e.g., Schooler et al., 2011). These thoughts are, for instance, related to autobiographical memories, future planning, social interactions, and problem-solving. By exploiting the technique of so-called experience sampling, it has been demonstrated that such mind wandering occupies up to 50% of our daytime experiences (Killingsworth and Gilbert, 2010). During times when no task performance is required, such mind wandering is even more prevalent. Thus, resting state activity may veridically be described as reflecting not an idling state, but rather a “free thinking” state (Everding, 2003). Accordingly, the patterns of resting state activity, such as activations of so-called default-mode regions, are probably strongly related to the contents of these thoughts (e.g., Mason et al., 2007). Individual differences in the frequency and contents of mind wanderingmay thus correspond to discrepant patterns of resting state activity and even may lead to systematic biases, for instance, caused by age (e.g., Maillet and Rajah, 2013) or population (e.g., Song and Wang, 2012) related differences. Recent studies have moreover shed light on the more general question, which kinds of inner experiences are prevalent in humans, i.e., experiences that occur in addition to the permanent sensory monitoring of the external environment (e.g., Heavey and Hurlburt, 2007; Hurlburt, 2011). Based on the technique of socalled descriptive experience sampling and investigations of several hundreds of subjects, a catalog of these inner experiences has been suggested (Hurlburt and Heavey, 2006). According to these studies (e.g., Heavey and Hurlburt, 2007) the most common forms of inner experiences are “inner seeing” (i.e., experiencing inner images), “feelings,” “inner speech” (constituting a certain kind of expression of thought), “unsymbolized thinking” (i.e., thinking which is not conveyed in words, images or other symbols) and “sensory awareness” (i.e., paying attention to a particular sensory aspect of the environment). With the exception of the latter category (sensory awareness) these inner experiences again are closely related to mind wandering. A particularly interesting outcome of these studies is, that the distribution of the categories of inner experiences has an extremely high interindividual variability across individuals (e.g., Heavey and Hurlburt, 2007)—even though we humans experience the external environment in a rather similar fashion. There are, for instance, subjects, who mainly experience inner seeing, but very rarely inner speech, whereas for other individuals the opposite is true. Accordingly, the individual patterns of resting state activity may strongly depend on the categories of inner experiences, which are prevalent in a certain subject. Alterations of a subject’s resting state patterns as compared to an average collective sample may thus be largely determined by her/his individual characteristics of inner experiences. This interindividual variability, of course, seriously impairs the clinical usefulness of such data, for instance, in the detection of early Alzheimer’s disease. Following on from these considerations, it is of paramount importance to access the categories and contents of inner experiences occurring during resting state experiments. A fundamental methodological difficulty complicating this endeavor is that we are typically unaware of our mind wandering and inner experiences. In other words, we commonly lack metaawareness of these experiences (Schooler, 2002). This is the case even, for instance, when subjects know their mind wandering during a sustained attention task will be sampled (Christoff et al., 2009). Interestingly, mind wandering related activations are stronger when subjects are unaware as compared to when they are aware of these experiences (Christoff et al., 2009). Accordingly, the frequency distribution of absent versus present meta-awareness represents another individual factor determining resting state patterns. Related to the common lack of metaawareness for inner experiences is the prevailing inability to remember these experiences (research addressing this interrelation is yet also lacking), which makes them difficult to reconstruct by later inquisition. Actually, it has been suggested to inquire about inner experiences during resting state fMRI using a post-hoc questionnaire (Delamillieure et al., 2009). This questionnaire comprised of categories such as “visual imagery,” “inner language,” “somatosensory awareness,” “inner musical experiences,” and “mental processing of numbers,” as well as several items addressing the contents of these experiences. Based on this questionnaire, for