“Is there a default mode network in the rat brain?”. That was the title of a poster presented by H. LU and colleagues from the Neuroimaging Research Branch of the NIDA-IRP in Baltimore at SfN 10. Here’s the abstract:
The default mode network (DMN), a set of brain regions that exhibit higher basal blood flow, was first identified in humans based on PET measurements (Raichle et al., 2001), and later by resting state fMRI (Greicius et al., 2003). An anatomically similar network has been shown to be present in nonhuman primates (Vincent et al., 2007). From an evolutionary point of view, the discovery of the DMN raises an interesting question: Do rodents also have a DMN? In order to address this question, we performed resting state BOLD fMRI in Sprague-Dawley rats (n = 11) anesthetized with isoflurane in combination with Domitor on a 9.4T scanner. Each rat was scanned once a week for 2 consecutive weeks. Each scan day, four to six sessions of resting state data were acquired within a 2-hour period using single-shot gradient echo EPI. Data from individual animals were registered to a common space. Functional connectivity analyses were performed using both group independent component analysis (gICA) and seed-based correlation methods. The resulting resting state connectivity maps were overlaid onto a digital rat atlas for accurate identification of anatomical structures. The following highly significant bilateral networks were identified by both analytical methods: 1) a motor system; 2) a sensory network, including the whisker barrel cortex, S1FL, S1HL); 3) insular cortex; 4) striatum, which is further divided into ventral medial, ventral lateral and dorsal lateral networks. Some of the networks have been independently reported in previous publications (Lu et al, 2007; Zhao et al., 2008; Pawela et al., 2008; Majeed et al., 2009; Hutchison et al., 2010). Of particular interest, we found a network that includes the anterior cingulate cortex, retrosplenial cortex, bilateral orbitofrontal cortex, medial prefrontal cortex, hippocampus and bilateral temporal association cortex. The retrosplenial cortex is known to be the rat homologue of the human posterior cingulate cortex (Bussey et al., 1996), a key component of the human DMN. The above network appears to mirror that reported in humans (Raichle et al., 2001; Greicius et al., 2003) and nonhuman primates (Vincent et al., 2007). These data raise several interesting questions: What is the exact function of DMN? How did the DMN evolve and did its functional significance remain the same or take on other properties in primates? Our animal model, if further confirmed, could be used to address these questions
So not only DMN correlates with practically anything (see review here), it’s also present in rats? It’s also present in preterm human babies’ brains… What is the exact function of DMN? Probably not “mind wandering” – except if introspection is infants’ and rats’ favourite pastime..
A few days ago I came across a paper by Schupak and Rosenthal (2008). They report the case of a patient presenting with a long history of excessive daydreaming.
Their patient is a 36 year old successful, educated woman with a long history of excessive and persistent daydreaming, which according to her reports is a cause of distress. What’s really fascinating in this case is the lack of known injury, drug abuse or other disorder. After more than 10 years of therapy, she only found relief from her symptoms when she was prescribed fluvoxamine, an antidepressant drug believed to influence obsessiveness and/or compulsivity. Even though, she used to spend a big part of her free time engaged in daydreaming, she was never disconnected from reality. Growing older, she was forced to hide her excessive daydreaming from colleagues and friends. However, this behaviour didn’t interfere with her education and her career.
The authors do a short review of the existing literature in mind-wandering suggesting that the hypothesis by Mason et al. (2007), according to which “mind-wandering is associated with activity in the default mode network”, could explain their patient’s symptoms. See suggested readings for alternative theories/hypotheses.
Schupak and Rosenthal (2008) conclude:
The subject of the present case study claims to be adjusted to her high level of fantasy proneness. She manages to orchestrate a complex allocation of cognitive and emotional resources toward the competitive requirements of externally- versus internally-driven attentional demands on a daily basis, though at substantial psychological cost. Our question regards the extent to which this case may represent an unrecognized population, i.e., individuals whose mind wandering/daydreaming is experienced as a causative factor in producing psychological distress or functional impairment without meeting criteria for any DSM disorder. The fact that our patient reports a positive response to a medication that influences serotonergic tone may imply neurochemical irregularity
Unfortunately, there is no report on any neuroimaging studies on this specific patient.
Gilbert, S. J., Frith, C. D., and Burgess, P. W. (2005). Involvement of rostral prefrontal cortex in selection between stimulus-oriented and stimulus-independent thought. The European journal of neuroscience., 21(5):1423-1431.
Mason, M. F., Norton, M. I., Van Horn, J. D., Wegner, D. M., Grafton, S. T., and Macrae, C. N. (2007). Wandering minds: The default network and stimulus-independent thought. Science, 315(5810):393-395.
Raichle, M. E., MacLeod, A. M., Snyder, A. Z., Powers, W. J., Gusnard, D. A., and Shulman, G. L. (2001). A default mode of brain function. Proceedings of the National Academy of Sciences of the United States of America, 98(2):676-682.
(painting: girl-daydreaming by Anthony Staynes)