Our second objective was to explore the nature of participants' spontaneous thoughts and to examine whether the individual variability underlying such thoughts was predicted by functional coupling between regions comprising the default network. Since prior studies have established that the MTL and distant cortical regions including the retrosplenial cortex (Rsp), posterior inferior parietal lobule (pIPL), and MPFC become engaged when participants remember their past and imagine their future, we investigated whether the tendency to experience spontaneous episodic thoughts about the past and future predicted functional coupling between the MTL and the default network. A significant relationship would strengthen support for the network's role in certain forms of spontaneous cognition.

Three different hypotheses about the expected results were entertained. If the default network plays a prominent role in external attention in general (i.e., watchfulness for upcoming stimuli; Gilbert et al. 2007 ), the contrast between the external attention conditions and the passive condition should isolate regions within the default network. If the default network plays a specific role in broad external attention, as suggested by a number of prior studies, the contrast between the broad and focal external attention conditions should elicit activity within the default network. If, on the other hand, the default network functions to support internal mentation, activity within the network should track reported measures of spontaneous cognition and should be weakest in the external attention conditions (both broad and focal). Of course an additional possibility is that distinct regions within the default network may enable broad attention and spontaneous cognition, respectively.

The first objective of the present set of experiments was to distinguish between two prominent, but opposing, hypotheses implicating the default network in external attention or spontaneous cognition. The distinct hypotheses have been difficult to disentangle because the two variables often track each other and because of the inherent challenge associated with measuring spontaneous cognition during unconstrained task states. Here we tested the two hypotheses by contrasting fMRI signal changes as an indirect measure of neural activity during fixation blocks that varied with respect to scope of external attention (broad and focal) and frequency of spontaneous thoughts as a measure of internal mentation. During conditions designed to encourage external attention, participants detected brief flickers occurring in either peripheral (broad) or central (focal) locations. In contrast, during a condition designed to encourage spontaneous cognition, participants passively fixated on a crosshair. To ensure that variables were manipulated as expected, an independent group of participants completed an identical paradigm in a mock MRI simulator. For these participants, randomly presented tones were placed throughout the tasks to gauge participants' frequency of spontaneous thoughts.

In all, 199 right-handed young adults participated in the study (22.2 yr, range: 18–35 yr, 97 male; ). Participants were native English speakers recruited from Harvard University and the greater Boston community. Exclusion criteria included a history of psychiatric or neurological illness as well as use of psychoactive medications. Procedures were carried out according to the guidelines put forth by either the Harvard University Committee on the Use of Human Subjects in Research (experiment 1: behavioral) or the Review Board of Partners Healthcare at Massachusetts General Hospital (experiment 1: fMRI and experiment 2).

Task paradigms and questionnaires

Experiment 1 (Overview). To test between the two prominent hypotheses about the function of the default network, three conditions were explored that varied the direction and scope of attention by manipulating expectancies: 1) broad attention, 2) focal attention, and 3) passive fixation. During critical task blocks used for analysis, stimuli were held constant and no responses were made in any condition ( ). Only expectancies and ancillary processes linked to focus of attention and spontaneous cognition differed. Open in a separate window A block-designed paradigm was used where blocks of fixation across the three conditions alternated with baseline blocks consisting of an abstract/concrete semantic classification task on visually presented words. Note that the baseline here is different from most prior neuroimaging studies; the active task block serves as a common baseline for the three critical fixation conditions that provide the test of the hypotheses. Participants completed a total of six task runs, each comprised of six 30-s fixation blocks flanked by seven 20-s blocks of the baseline semantic classification task. Participants performed one condition at a time across two consecutive runs and condition order was counterbalanced across participants. In the broad attention condition, participants were instructed to remain fixated on a central fixation crosshair for the duration of each fixation block and to concurrently press a button whenever a brief flicker was detected somewhere in the periphery of the visual display. A peripheral flicker was marked by a gray X that appeared for ~30 ms at one of three eccentricities (corresponding to visual angles: 1.4, 2.8, 4.2°) and eight angles (0, 45, 90, 135, 180, 225, 270, 315°), yielding 24 possible locations for each flicker. Importantly, unbeknown to the participants, flickers were omitted from four blocks and these blocks lacking flickers were the targets of analysis to compare across conditions. In the focal attention condition, participants were told that flickers would occur directly on top of the central fixation crosshair (marked by a gray X that appeared for 67 ms). Again, flickers were omitted from four unpredictable blocks. In the passive fixation condition, participants were instructed to fixate on the fixation crosshair but not to respond during these blocks because no flickers would occur. To encourage top-down modulation of attention, participants were told that flickers would occur infrequently, were informed of the importance of detecting each flicker as fast as possible, and were discouraged from detecting false positives. Flickers appeared only briefly and were difficult to detect. Four lists that varied in the total number of blocks containing flickers (some lists included flickers in the abbreviated fixation block inserted to allow for intensity stabilization of the blood oxygenation level dependent [BOLD] signal), the total number of flickers per run (three to six), and the onset of the flickers within the run were created and counterbalanced across participants to ensure that the broad and focal conditions did not systematically differ in the onset of flickers. As mentioned earlier, four fixation blocks within each run were matched across conditions with respect to stimuli since flickers did not appear in these blocks. These blocks were the targets of analysis. Thus for the purpose of the fMRI analysis, only expectations and focus of external attention differed between conditions. The same onsets used for fixation blocks extracted from the focal attention runs were also extracted from the passive runs. Blocks in which false-positive key-press responses occurred were removed from analysis in all conditions.

Experiment 1 (Behavioral Study). An underlying assumption of the study was that the passive condition would encourage more spontaneous cognition than the broad or focal external attention conditions. To test this assumption, we conducted a separate behavioral thought sampling study during which an independent group of participants (n = 30, 24.5 yr, range: 18–35 yr, 17 male) completed the same six task runs while spontaneous thoughts were sampled throughout each run. Five tones within different fixation blocks were sounded during each run. Lists were created that varied the placement of the tones within the run and these lists were counterbalanced across subjects to ensure that the broad and focal conditions did not systematically differ in tone placement. Participants were asked to classify their thoughts whenever a tone sounded by stating aloud the word “thought” if they experienced a spontaneous thought unrelated to the task at hand immediately prior to the tone or “nothing” if a task-unrelated thought was not experienced. The specific instructions are listed below: Now, there is one additional component to this experiment. At random times while the fixation crosshair is on the screen, a brief tone will occur that sounds like a ring. Here is an example: [sample tone played]. When you hear the tone, we would like you to classify what you were thinking about immediately before the tone sounded. If your mind was wandering and you were thinking about something unrelated to the task at hand, please say the word “thought” out loud. For example, you may have been thinking about something that you did last night or something you should do on your way home. Or you may have been thinking how tired or hungry you are. If you were thinking about either of these types of things or about something else that was not related to the task at hand, please say the word “thought” out loud. You should not press anything. On the other hand, if your mind was not wandering and you were thinking about the flickers, the fixation crosshair, or nothing at all, you should say the word “nothing” out loud. For example, you may have been wondering where or when the flicker is going to appear or thinking about the features of the fixation crosshair—for example, its color or size. Or you may have not been thinking about anything at all, meaning your mind was completely blank. What all these types of activities have in common is that they are not thoughts that are unrelated to the task. Therefore if you experience something that falls into this category, you should say the word “nothing” out loud. Again, you should not press anything. Flickers will not occur during or immediately after a tone—therefore you should not rush through your response to the tone for the sole purpose of being able to detect flickers afterward. Instead, think carefully about your response before you say it because you will be allowed to state only one of the two options, either “thought” or “nothing.” At the same time, you should remember that your primary task in this experiment is to determine whether words are abstract or concrete and to detect the flickers. An important aspect of the behavioral study was that participants performed the experiment while lying in a mock MRI simulator (Psychology Software Tools, Pittsburg, PA). A mock scanner ensured that a similar environment was present across the two experiments (including simulated scanner noise, earplugs, a button box, and a head coil) while simultaneously allowing us to sample spontaneous thoughts throughout each run. In addition to thought sampling using probes, participants completed a postexperimental questionnaire to assess subjective reports of mind-wandering frequencies for each condition as well as the self-relevancy and goal-oriented nature of such thoughts (see following text). Questions were answered using the scale 1 = never to 7 = always. In addition, participants were encouraged to list examples of any spontaneous thoughts that occurred during the experiment, providing further insight into the nature of their content. The questions were as follows. During the task that involved detecting flickers that occurred in the periphery, to what extent did your mind wander away from thoughts about the task at hand? During the task that involved detecting centrally located flickers, to what extent did your mind wander away from thoughts about the task at hand? During the task where you stared at the fixation crosshair for blocks of 30 s, to what extent did your mind wander away from thoughts about the task at hand? If your mind wandered away from the task at hand during any of the tasks, to what degree did your thoughts revolve around self-relevant concerns (i.e., things that are important to you)? If your mind wandered away from the task at hand during any of the tasks, to what degree were your thoughts directed toward a particular goal (e.g., trying to figure something out; planning something in your future)? Any examples of thoughts that occurred during any of the tasks would be extremely helpful to us. Please use the space below to describe a few of these thoughts. Administering a retrospective thought sampling questionnaire allowed us to assess frequencies of task-unrelated thoughts using a second method. Given the ambiguities in the validity of various thought sampling procedures, we chose to explore multiple convergent techniques. Although the on-line thought sampling method required less reliance on long-term memory compared with the retrospective questionnaire, it is possible that the on-line method caused participants to monitor the nature of their thoughts, bringing more thoughts into explicit awareness and leaving fewer thoughts as unaware (for discussion, see Smallwood and Schooler 2006). In addition, since participants may have experienced more thoughts relating to monitoring during the on-line thought probe study, they may have encountered difficultly determining whether these thoughts were task-unrelated. We reasoned that by introducing a small number of tones per run (n = 5) as well as emphasizing in the task instructions that the thought probe aspect of the study was secondary to the primary flicker/fixation task, participants would dedicate minimal time to monitoring their thoughts. Importantly, the retrospective thought sampling questionnaire was also given to the independent sample of 30 young adults who completed the experimental task in an MRI scanner (see following text).

Experiment 1 (fMRI Study). The fMRI study was conducted in an independent sample of participants matched on age and gender (n = 30, 21.7 yr, range: 18–31 yr, 17 male) using the identical experimental paradigm as outlined earlier, with the exception that the on-line thought probes were not present, nor were participants instructed to monitor the nature of their spontaneous thoughts so as not to interfere with performance on the primary task. As in the behavioral study, surprise postscanning questionnaires were distributed to gather subjective reports of spontaneous cognition for each condition.