As discussed above mental phenomena must have spatio-temporal coordinates and thus should be able to be represented as spatio-temporal 4D structures. However, in most publications about brain networks, the issue of spatio-temporal maps does not appear prominent. Here, I will discuss and summarise next what the initial attempts to construct spatio-temporal maps of neural circuits have reported so far.
A common assumption is that in vivo and in conscious subjects, the different components of cerebral networks are functionally connected via synchronised neuronal firing1. However, because of the limited number of recording points, detailed spatio-temporal maps of brain activity have not been available. Instead, indirect measures of ‘synchrony’ have been proposed based on a temporal dimension with pairwise time series lags or latencies of spontaneous, correlated fluctuations in the fMRI BOLD signal among nodes within and between networks. These include the spectral power of brain oscillations and various indexes of complexity of brain activity such as statistical cross-correlations2. Not surprisingly, these indexes of ‘synchrony’ fail to capture the full spatio-temporal features of the underlying specific brain states.
In the developing mammalian CNS, spontaneous activity occurs as waves that propagate across large areas. Such waves have been observed in the cortex3, hippocampus4, retina5, midbrain6, hindbrain7 and spinal cord8. Waves initiated within a particular structure may spread to another structure, such as has been reported between midbrain and hindbrain 9. Interestingly during development, activity in slices of mouse cortex consists of travelling waves generated by localised pacemaker areas. Waves of activity propagate from the site of initiation at rates between 7 and 16 mm.s−1 10.
Wolf Singer describes well the generation of temporal patterns of brain activity by weakly coupled neural oscillators11. As a consequence, reaction-diffusion equations which describe well many phenomena in chemistry and biology also govern the spatio-temporal patterns of activity in neural circuits.
Many neural preparations isolated from the rest of the nervous system still contain intact functional circuits of neurons connected by synapses. Some such preparations maintain rhythmic spatio-temporal patterns of activity12.
I will start describing the spatio-temporal patterns of activity of these isolated preparations. Although lacking complete hierarchical connections, they can generate patterns of activity which may reveal some fundamental features of these reduced circuits.
- F Varela et al (2001): The brainweb: phase synchronization and large scale integration. Nature Reviews Neuroscence. 2, 229–239. ↩︎
- For example: A Mitra et al (2018): Spontaneous infra-slow brain activity has unique spatiotemporal dynamics and laminar structure. Neuron 98: 297-305;
A Mitra & M E Raichle (2016): How networks communicate: propagation patterns in spontaneous brain activity. Philosophical Transactions of the Royal Society of London, Series B, 371, 20150546;
A Mitra et al (2015): Lag threads organize the brain’s intrinsic activity. Proceedings of the National Academy of Sciences USA 112, E2235–E2224. ↩︎ - O Garaschuk et al (2000): Large-scale oscillatory calcium waves in the immature cortex. Nature Neuroscience 3, 452–459;
R Corlew et al. (2004): Spontaneous, synchronous electrical activity in neonatal mouse cortical neurons. Journal of Physiology 560, 377–390;
JW Lischalket al (2009): Bilaterally propagating waves of spontaneous activity arising from discrete pacemakers in the neonatal mouse cerebral cortex. Developmental Neurobiology 69, 407–414;
J Conhaimet al (2010): Bimodal septal and cortical triggering and complex propagation patterns of spontaneous waves of activity in the developing mouse cerebral cortex. Developmental Neurobiology 70, 679–692. ↩︎ - X Leinekugel et al. (1998): Giant depolarizing potentials: the septal pole of the hippocampus paces the activity of the developing intact septohippocampal complex in vitro. Journal of Neuroscience 18, 6349–6357. ↩︎
- M Meister et al. (1991). Synchronous bursts of action potentials in ganglion cells of the developing mammalian retina. Science 252, 939–943. ↩︎
- W Rockhill et al. (2009). Spontaneous activity in the developing mouse midbrain driven by an external pacemaker. Developmental Neurobiology 69, 689–704. ↩︎
- J Gust et al. (2003): Development of synchronized activity of cranial motor neurons in the segmented embryonic mouse hindbrain. Journal of Physiology 550, 123–133;
PN Hunt PN et al. (2005): Midline serotonergic neurons contribute to widespread synchronous activity in embryonic mouse hindbrain. Journal of Physiology 566, 807–819. ↩︎ - MJ O’Donovan et al (1998): Mechanisms of spontaneous activity in developing spinal networks. Journal of Neurobiology 37, 131–145. ↩︎
- J Conhaim et al. (2010): Bimodal septal and cortical triggering and complex propagation patterns of spontaneous waves of activity in the developing mouse cerebral cortex. Developmental Neurobiology 70, 679–692. ↩︎
- J Conhaim et al. (2011): Developmental changes in propagation patterns and transmitter dependence of waves of spontaneous activity in the mouse cerebral cortex. Journal of Physiology 589, 2529-2541. ↩︎
- W Singer (2013): Cortical dynamics revisited. Trends in Cognitive Science 17, 616-626. ↩︎
- A Arieli et al (1995): Wave-like neural activity occurs in various intact and semi-intact preparations of vertebrates. Jouirnal of Neurophysiology 73, 2072–2093;
Y-W L Lamet al (2000): Odors elicit three different oscillations in the turtle olfactory bulb. Journal of Neuroscience 20, 749–762;
RW Friedrich et al (2004): Multiplexing using synchrony in the zebrafish olfactory bulb. Nature Neuroscience 7, 862–871.
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