The default network constitutes a
collection of brain regions associated with and active during mind wandering.
Although studies on the cognitive foundations of mind wandering abound, this
week’s readings are the first (so far in class) to address the brain activity
associated with mind wandering in an in-depth manner. I find this very exciting because it allows
us to pair evidence from cognitive studies and with neurobiological data to
generate a more complete image of mind wandering. Interestingly, despite the
similarity in the neuroimaging methods used across all of this week’s studies,
their results are not entirely consistent and the authors sometimes reach
rather distinct conclusions based on the pattern of brain activation shown in a
resting state.
Summaries
and Comparative Analysis
Fox et. al. (2005)’s
study was motivated by the previously established finding attention-demanding
tasks routinely lead to the activation of some brain regions (task positive
regions such as frontal eye field and middle temporal region) and the deactivation of others (task negative
such as medial prefrontal cortex and posterior cingulate/prenucleus). The
authors were interested in investigating the degree to which this “task related dichotomy” is present intrinsically in the resting human
brain. To test this theory, Fox et. al.
collected fMRI data from ten normal subjects and analyzed the correlations and
anti-correlations in the spontaneous BOLD (blood oxygen level dependent signal)
associated with three task-positive brain regions and three task-negative brain
regions. From this analysis, the authors found the existence of two “diametrically opposed” anti-correlated
networks in the resting brain.
The task-positive network is composed of
regions activated in goal-directed tasks whereas the task-negative network
consists of regions with suppressed/decreased activity during task response.
The latter is usually referred to as the “default system.” The authors argue
that this data supports the concept of a
resting-state functionality in the brain.
Mason et. al. (2007)
expanded on Fox et. al.’s study by investigating the
role of the default network in mind wandering or the formation of
stimulus-independent thoughts. Specifically, he uses thought sampling and brain
imaging studies to demonstrate that mind
wandering is associated with activity in the default network, which
constitutes brain regions active during rest. fMRI studies were used to
demonstrate a correlation between high-incidence mind wandering periods and an
increase in default network activation and subject self-reports on individual
tendency to engage in mind wandering were also related to their default network
activation patterns. Furthermore, no default network area exhibited significant
activity during low-incidence mind wandering period. Taken together, these
results support the theory that the tonic activity present in the default
network during resting states is associated with mind wandering.
The authors offer
two possible explanations for the functional roles of mind wandering. For one,
it could aid in the maintenance of an optimal arousal level that enhances
performance on “mundane tasks.” Another possibility could be that SITs help
make sense of past, present, and future experiences. These results largely
agree with those of Fox et. al in
that they also support a resting state functionality. However, the results and
interpretations also differ from those of Fox et. al. in an important respect. Whereas the latter proposes that
the resting brain is characterized by representations of two anti-correlated
networks (the default network and the
task-positive network), Mason et. al.
proposes that resting-state mind wandering is attributable only to the default
network.
Andrews-Hanna et. al. (2010) was interested in distinguishing between
functional contributions of the default network to external attention shifts
and internal mentation. To this end, the authors essentially “decoupled” these
two processes by manipulating factors that specifically promote spontaneous
cognition. Specifically, the authors conducted a neuroimaging study in which
subjects were asked to detect a signal present under three fixation conditions:
broad attention, focal attention, and passive attention. The authors found that
broad attention had different effects than the default network. Furthermore, stimuli
and responses were held constant and only expectations differed across these
conditions, thus lending to the separation of spontaneous cognition from
factors that affect the scope of the external environment. Results indicate
that the default network is involved
in spontaneous cognition, but not on the level of broad attention. Thus, they
argue that the default network is not responsible for broad external attention
and thus is involved in internal mentation rather than strictly mind wandering.
These results and interpretations clearly differ from those of Mason et. al. and to some respect, also Fox et. al.
Stawarczyk
et. al. (2011) expands upon the previous studies’
results by providing a multi-dimensional scheme of consciousness as a function
of task-relatedness and stimulus-dependency. Specifically, the authors
introduce four classes of conscious experiences: complete task-directed focus,
districted focus by irrelevant sensations, interfering thought related to the
task, and mind wandering.
Subjects were probed for each of the four conditions
to investigate the brain activation patterns associated with each class of
conscious experience. Like previous studies, they found specific brain regions
highly conducive to mind wandering. The interesting part of this study lies in
the fact that they addressed the concern voiced in Andrews-Hanna’s 2010 study, namely the tangling mind wandering and
other processes. Specifically, the multi-dimensional scheme lends to the
possibility that the neurobiological mechanisms for mind wandering and external
shifts in attention may overlap and indeed coexist. Furthermore, the
consideration of both task-relatedness and stimulus dependency also introduces
a variable of complexity (that accounts for the complexity of the neural
networks) not present in the previous studies. This incorporation proved
essential, as evidenced by the authors’ finding of an additive effect along the
midline default mode network regions.
Questions and Comments
Although I find
this week’s readings particularly interesting, I still have some questions
about the methods used and result interpretation of some studies. For instance,
virtually all of the authors interpreted results from neuroimaging studies
without rigorous analysis of possible confounding factors. Fox et. al. and Mason et.
al.’s studies provides cases in point. Brain regions activated during
rest are classified as “task negative.” However, it is possible that these
brain regions are in fact engaged in essential tasks, and thus the term “task
negative” seems like an inappropriate description of these brain areas.
Furthermore, given the complex and interconnected nature of the brain, the
search for an engram or even a collection of engrams for mind wandering seems
to ignore the essential role of virtually every brain region in maintaining
coherent thought and understanding.
Furthermore, I
also took issue with some of the methods and experimental design in some of
these studies. For instance, Fox et. al. only tested ten subjects in
his fMRI study, and while the results were definitely interesting and holds a
lot of potential for future investigation, the low number of subjects increase
the probability that the results presented could just be an artifact of the
sample population chosen. In addition, the self-reporting technique utilized by
both Mason et. al. and Andrews-Hanna
et. al. present the same problem
discussed in early posts: inconsistency across subjects’ reports stemming from
the absence of a rigorous definition of stimulus-independent thought and
precise mechanism of evaluating its presence.
On the brighter
note, I found the dimensions of conscious experiences presented by Stawarczyk et. al. (2011) very enlightening and insightful. Although, like
previous authors, Stawarczyk and his team also found certain brain regions
highly conducive to mind wandering, I think this multi-dimensional scheme on
consciousness achieves what the three previous studies failed to achieve: an
integration of mind wandering with other cognitive experiences to provide an
overall image of conscious experiences and thought. Furthermore, this method
provides data on the relative activations
of brain regions in mind wandering compared to other conscious thought, and
thus provides a more physiological description of conscious thought and mind
wandering. Clearly, I find this study the most
compelling out of the four studies we read for this week.
Links to Neuroscience
Since this week’s reports were primarily
neuroimaging-based, the links to neuroscience are intrinsic and already
present. However, I think it would be interesting to expand on some of the
observations and interpretations offered in these studies. For example, I would
like to further the possibility offered by Mason et. al. that mind wandering contributes to the generation of a
coherency between past, present, and future experiences, almost like the glue
that links the pieces of experiences across the temporal space to create
meaning out of these experiences. This is extremely similar to memory and a key
function of memory. As neurobiologist Eric Kandel once said, memory is glue
that holds our lives together. Without memory, we are capable of living only in
a given moment in time, without any connection to our past experiences and
future hopes. I feel like mind wandering may have a similar function. Indeed,
it would be interesting to explore the role of mind wandering in memory
consolidation, which is the topic of my research proposal. I am very excited
about this topic, and I look forward to expanding and refining this idea.
Word
Count: 1490, not including references
References
Fox, M. D., A.
Z. Snyder, et al. (2005). "The human brain is
intrinsically organized into dynamic, anticorrelated functional networks."
Proc Natl Acad Sci U S A 102(27): 9673-9678.
Mason, M. F., M.
I. Norton, et al. (2007). "Wandering minds:
the default network and stimulus-independent thought." Science
315(5810): 393-395.
Andrews-Hanna,
J. R., J. S. Reidler, et al. (2010). "Evidence for the default
network's role in spontaneous cognition." J Neurophysiol 104(1):
322-335.
Stawarczyk, D.,
S. Majerus, et al. (2011). "Neural
correlates of ongoing conscious experience: both task-unrelatedness and
stimulus-independence are related to default network activity." PLoS
One 6(2): e16997
Image References:
Do You Ask Good
Questions? A Ragamuffin Voice. http://allenkleinedeters.wordpress.com/2011/11/28/do-you-ask-good-questions/
Accessed 03/27.
Persistence of
Memory. http://faculty.txwes.edu/csmeller/human-prospect/ProData09/02WW1CulMatrix/WW1PICs/Dali1904/Dali1931Memory444.htm.
Accessed 03/27.
