The authors review recent experimental evidence showing that in vivo, in the absence of any particular task (in particular learning task), synapses and functional properties of single neurons are not stable. For example, spines disappear and reappear; more significant in my view, motor tuning and place fields drift. Synaptic changes are still observed when ion channel activity is blocked. This might suggest that they are intrinsic as the authors point out,although in factit does not mean that in normal condition these changes are independent of activity; it could well be that the fluctuations are entrained by activity, in the same way as the response of an intrinsically noisy neuron is entrained by a time-varying current (Mainen and Sejnowski, 1995; see also Brette & Guigon, 2003 for some theory). The more significant point, I think, is that functional properties of neurons, e.g. tuning properties, seem to drift over time. This raises questions about the idea of a cell assembly as a memory engram. If a particular assembly encodes a particular memory, then after some time this same assembly should mean something completely different. Imagine, to take a caricatural example, that a memory of a red car is stored as a network of two connected neurons, the red neuron and the car neuron. After two weeks the red neuron becomes a green neuron. When cued with a car, I now remember a green car.

In theoretical neuroscience, one question which has been the subject of many studies is how can synaptic structure be stable enough to sustain memories while plastic enough to allow learning. Maybe this is not the right question; maybe the right question is: how can learning persists over a time scale longer than the functional dynamics of networks?