A brain consists of many functionally specialized areas. Long-range synchronization is an important process to recruit relevant areas into a network for performing specific tasks.
Synchronization can be achieved with zero phase lag, even between two hemispheres.
The zero lag synchronization between distant areas is remarkable, considering that synaptic transmission and axon conduction will cause time delay.
It takes about 30 milliseconds (ms) for a nerve impulse to travel from one hemisphere to the other through myelinated axons in the corpus callosum,
and 150 - 300 ms through unmyelinated axons (Fields, 2008).
In comparison, the frequency of beta rhythms is about 20 Hz, corresponding to a period of 50 ms. Yet, the beta rhythms of subthalamic nuclei in both hemispheres can be synchronized
predominantly with zero phase lag (Figure 1).
Figure 1. Histogram of beta phase differences between subthalamic nuclei in both hemispheres for 23 human subjects. This demonstrates
predominance of zero phase lag. [Source: Little et al., 2013]
Synchronization among widely separated brain regions is prevalent. The following lists only a number of examples:
- In an awake cat, a sudden change of a visual pattern induces synchronization between areas of the visual and parietal cortex, and between areas of
the parietal and motor cortex with zero time lag (Roelfsema et al., 1997).
- In a rat during trace eyeblink conditioning, the lateral entorhinal cortex, hippocampus, and medial prefrontal cortex are synchronized
(Takehara-Nishiuchi et al., 2012).
- Light stimuli evoke synchronous responses in lateral geniculate nucleus of both hemispheres (Neuenschwander and Singer, 1996).
- In monkeys performing a visual short-term memory task, the visual area V4 and the lateral prefrontal cortex are synchronized
(Liebe et al., 2012).
- During fear memory retrieval, rhythmically synchronized activity at theta frequencies increased between the lateral amygdala and the hippocampal CA1
(Seidenbecher et al., 2003).
- During fear behavior, the amygdala also synchronized with the medial prefrontal cortex (mPFC) at 4-Hz oscillations
(Karalis et al., 2016).
- During the maintenance of visual working memory, alpha synchronization was observed in fronto-parietal, cingulate, and insular cortices concurrently with synchronization
in the beta- and gamma-band networks (Palva and Palva, 2011).
- During a verb generation task, the theta-modulated gamma-band synchronization was found among activated regions
(Doesburg et al., 2012).
- In a mouse during active exploration, mPFC is synchronized with ventral hippocampus (Harris and Gordon, 2015).
The mechanism of long-range synchronization is largely unknown. It cannot be mediated by gap junctions, interneurons
or ephaptic coupling described in previous chapters. The following chapters propose that the global synchronization among distant brain areas
could be mediated by radiative electromagnetic waves which propagate at the speed of light. The electromagnetic coupling is very different from the ephaptic coupling mediated by local electric fields,
as discussed in the next chapter.
Author: Frank Lee
First published: August, 2018