"It is well established that inhibitory interneuron networks have a prominent role in the generation of gamma-band synchronization"



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<from my paper Elk-2004
Some regions in our brain such as neocortex or hippocampus are said to be made up of two categories of neurons, that is, pyramidal cells and interneurons. Typically, the pyramidal cells communicate with each other via excitatory synapses (positive influences), while interneurons send signals to pyramidal cells via inhibitory synapses (negative influences).
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In the hippocampus, most pyramidal cells have so-called 'place fields'; that is, they fire preferentially when the animal moves through a certain region of its environment [51]. The discharges of place field neurons exhibit a prominent rhythmic synchronization in the theta-frequency band (8 Hz in this structure) (see Ref. [52] ...)


51 O�fKeefe, J. (1976) Place units in the hippocampus of the freely moving rat. Exp. Neurol. 51, 78.109

52 Burgess, N. and O�fKeefe, J. (2005) The theta rhythm. Hippocampus 15, 825.826


Similar to the above-described gamma-band synchronization, this theta-band synchronization entails rhythmic network inhibition at the theta frequency [53,54].

53 Harris, K.D. et al. (2002) Spike train dynamics predicts thetarelated phase precession in hippocampal pyramidal cells. Nature 417, 738.741

54 Mehta, M.R. et al. (2002) Role of experience and oscillations in transforming a rate code into a temporal code. Nature 417, 741.746

Theta-phase precession is a striking and well documented phenomenon that is likely to result from the interaction between excitatory drive onto pyramidal cells and rhythmically increasing and fading inhibition:whenthe animalmoves through the place field of a particular cell, the cell discharges rhythmically and phase-locked to the thetarhythm, whereby the discharges occur earlier and earlier in the theta phases as the animal moves into the place field. Several models of theta-phase precession explain this phenomenon as a consequence of the interaction between theta-modulated inhibition and place-field driven excitation [53,54]. When the animalmoves towards the place field of a
neuron, excitatory drive to this cell increases over the course of several theta cycles. At the same time, inhibition rhythmically increases and decreases within each theta cycle. At the periphery of the place field, the excitatory drive is weak and can elicit spikes only during the phase of the theta cycle when inhibition is minimal; that is, at the depolarizing peak of the theta-cycle. Howeverwhen excitation increases, it can overcome inhibition earlier and elicit spikes earlier in the theta-cycle [53,54] (Figure 2). Interestingly, this phase-precession is unidirectional and does not reverse when the firing rate decreases toward the end of the place field and several mechanisms, such as spike frequency adaptation [53] and synaptic plasticity [54], have been proposed to account for this asymmetry.

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As reviewed above, neuronal gamma-band synchronization is associated with rhythmic inhibition of pyramidal cells. Therefore, pyramidal cells drivenweakly will likely fire only late in the gamma cycle, whereas those driven strongly will fire earlier [31.33].

31 Buzsa�L ki, G. and Chrobak, J.J. (1995) Temporal structure in spatially
organized neuronal ensembles: a role for interneuronal networks.
Curr. Opin. Neurobiol. 5, 504.510

32 Olufsen, M.S. et al. (2003) New roles for the gamma rhythm: population
tuning and preprocessing for the beta rhythm. J. Comput. Neurosci. 14,
33.54

33 Bo�Nrgers, C. et al. (2005) Background gamma rhythmicity and attention
in cortical local circuits: a computational study. Proc. Natl. Acad. Sci.
U. S. A. 102, 7002.7007

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Thus, gamma-band synchronization has the effect that the stronger a pyramidal cell is driven, the earlier it spikes in the gamma cycle, similar to hippocampal theta-phase precession.

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However, in view of the increasing evidence that the precise timing of individual spikes matters, as, for example, in spike timing-dependent plasticity, gamma phase-dependent spike timing could be functionally relevant.

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Transforming an amplitude (rate) code into a temporal code could have a crucial role in object recognition: spike latency coding or spike rank order coding has been the core component in a class of computational models that perform object recognition with remarkable speed and computational efficiency [34,56-59].


56 Delorme, A. and Thorpe, S.J. (2003) SpikeNET: an event-driven
simulation package for modelling large networks of spiking neurons.
Network. 14, 613.627

57 Delorme, A. and Thorpe, S.J. (2001) Face identification using one spike
per neuron: resistance to image degradations. Neural Netw. 14, 795.
803

58 VanRullen, R. and Thorpe, S.J. (2001) Rate coding versus temporal
order coding: what the retinal ganglion cells tell the visual cortex.
Neural Comput. 13, 1255.1283

59 VanRullen, R. and Thorpe, S.J. (2002) Surfing a spike wave down the
ventral stream. Vision Res. 42, 2593.2615

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One important remaining issue is whether spike timing within the gamma cycle affects neuronal interactions, because only then could it have a functional role [66].

66 Buzsaki, G. (2006) Rhythms of the Brain, Oxford University Press

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As mentioned above, gamma oscillations are a ubiquitous phenomenon and if distributed but connected groups of neurons engage in gamma oscillations, precise spike timing


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matters: excitatory inputs that coincide with the period of fastest fall-off of the inhibition in the target group will be particularly effective because reduction of inhibition is by itself a potent trigger [43] and incoming excitation that coincides with falling inhibition will result in particularly steepmembrane potential slopes. It has beenshownthat the membrane potential threshold for eliciting spikes is inversely correlated to the membrane potential slope [67,68].

67 Azouz, R. and Gray, C.M. (2000) Dynamic spike threshold reveals a
mechanism for synaptic coincidence detection in cortical neurons in
vivo. Proc. Natl. Acad. Sci. U. S. A. 97, 8110.8115

68 Azouz, R. and Gray, C.M. (2003) Adaptive coincidence detection and
dynamic gain control in visual cortical neurons in vivo. Neuron 37, 513.
523

By contrast, excitatory inputs arriving at the beginning of the inhibitory period will have particularly little effect because they will be exposed to the shunting and the hyperpolarizing effect of IPSPs.

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This, in turn, can be used to support a variety of functions such as ...... attention-dependent selection of signals [15,25,29,30,33,74],

15 Fries, P. et al. (2001)
Modulation of oscillatory neuronal synchronization by selective visual attention.
Science 291, 1560.1563

25 Womelsdorf, T. et al. (2006)
Gamma-band synchronization in visual cortex predicts speed of change detection.
Nature 439, 733.736

29 Bichot, N.P. et al. (2005)
Parallel and serial neural mechanisms for visual search in macaque area V4.
Science 308, 529.534

30 Taylor, K. et al. (2005)
Coherent oscillatory activity in monkey area v4 predicts successful allocation of attention.
Cereb. Cortex 15, 1424.1437

33 Bo�Nrgers, C. et al. (2005)
Background gamma rhythmicity and attention in cortical local circuits: a computational study.
Proc. Natl. Acad. Sci. U.S.A. 102, 7002.7007

73 Mishra, J. et al. (2007)
Early cross-modal interactions in auditory and visual cortex underlie a sound-induced visual illusion.
J. Neurosci. 27, 4120.4131