Was it a vision or a waking dream?
A commentary on Disrupting posterior cingulate connectivity disconnects consciousness from the external environment by Herbet, G., Lafargue, G., de Champfleur, N. M., Moritz-Gasser, S., le Bars, E., Bonnetblanc, F., et al. (2014). Neuropsychologia 56C, 239–244. doi: 10.1016/j.neuropsychologia.2014.01.020
Reminiscent of Wilder Penfield’s famous experiments, Neurologists in France have reported a remarkable case in which intra-operative electrical stimulations of the posterior cingulate cortex (PCC) in a conscious patient induced transient dreamlike states with vivid visual imagery (Herbet et al., 2014).
The implicated circuitry and nature of the experiences evoked comparisons with findings from our own neuroimaging research with the hallucinogen and putative “oneirogen” (dream-inducer) psilocybin, strengthening what can be inferred about the importance of the PCC in mediating the quality of consciousness. We were fascinated to read the case- report of a dreamlike experience evoked by direct electrical stimulation of the posterior cingulate cortex (PCC) in an epilepsy patient by Herbet et al. (2014). The PCC has attracted a lot of interest in recent years due to recognition of its high metabolic and vascular demand (Raichle et al., 2001) and importance as a cortical connector hub (Hagmann et al., 2008) and integration center (Leech et al., 2012). Perhaps due to its buffered location and rich vascular innervation, there is an absence of cases of focal PCC lesions (Leech and Sharp, 2014) and to our knowledge there are no reports on the effects of PCC stimulation in humans.
There are a few case-reports of impaired spatial navigation and related symptoms of Balint’s syndrome in patients with damage to the retrosplenial cortex (Leech and Sharp, 2014) but the stimulation site here was dorsal to the retrosplenial cortex, in white matter of the cingulum bundle, a major tract connecting the PCC with the medial prefrontal cortex (mPFC). This circuit constitutes the spine of the default-mode network (DMN), a system that has been associated with spontaneous cognition that is suspended or interrupted during periods of externally-directed attention (Raichle et al., 2001).