Schaefer, A. T. and Margrie, T. W.
Trends Neurosci, 30:92–100, 2007
DOI, Google Scholar
A complete understanding of the mechanisms underlying any kind of sensory, motor or cognitive task requires analysis from the systems to the cellular level. In olfaction, new behavioural evidence in rodents has provided temporal limits on neural processing times that correspond to less than 150ms–the timescale of a single sniff. Recent in vivo data from the olfactory bulb indicate that, within each sniff, odour representation is not only spatially organized, but also temporally structured by odour-specific patterns of onset latencies. Thus, we propose that the spatial representation of odour is not a static one, but rather evolves across a sniff, whereby for difficult discriminations of similar odours, it is necessary for the olfactory system to “wait” for later-activated components. Based on such evidence, we have devised a working model to assess further the relevance of such spatiotemporal processes in odour representation.
They review evidence for temporal coding of odours in the olfactory bulb (and olfactory receptor neurons). Main finding is that with increasing intensity of an odour corresponding neurons fire more action potentials in a given time. However, this is achieved by an earlier onset of firing while inter-spike intervals stay roughly equal. The authors argue that this is a fast temporal code that can be used to discriminate odours. Especially, they suggest that this can explain why very different odours can be discriminated faster. The assumption there is that these differ mainly in high-intensity, i.e., fast subodours while similar odours differ mainly in low-intensity, i.e., slow subodours. But can it not be that similar odours differ only slightly in high-intensity subodours? My intuition says that the decision boundary is more determined by considerations of uncertainty rather than a temporal code of high- and low-intensity.
The authors ignore that there is an increased amount of action potentials for high-intensity odours and rely in their arguments entirely on the temporal aspect of earlier firing. If only the temporal code was important, this would be a huge energy waste by the brain. Stefan suggested that it might be related to subsequent checks and to cumulating evidence.