It seems like the animal could be using additional information rather than just the visual input that an isolated retina is restricted to. In the text they show that the spike time code is significantly different from the animal results but I don’t see the test if it differs from the temporal correlation code. It seems directly comparing the decoders would be important in the case that some additional bias is influencing the whole animal.

Their reasoning certainly seems intuitive, but the challenge remains to measure the amount of information provided by various candidate codes in the activity of some animal’s nervous system during the performance of some task with measurable information requirements. In order to meet this challenge the authors performed an experiment with mice performing a visual discrimination task and measured the activity within the mice’s retinas. The retina provides a strategic advantage to the experimenters in that it’s exclusively feedforward and the only channel for visual information available to the animals. It follows that all visual information the animals appear to exploit in the task must be made available to the animal through whatever code is actually utilized in the retina.

The authors applied their strategy in their analysis of the mice’s visual discrimination task concluding that a temporal code that does not assume the independence of spiking events is the only code (in general terms, that is, where we compare it with a time-pooling rate code and the temporal code which makes the independence assumption) that encodes enough information to explain the animal’s performance. This is quite an exciting result. Such a code is both the most information-rich and the most mysterious in that it requires that detailed temporal relations between individual spikes be distinguished. In animals with large nervous systems, preserving detailed information about the temporal relations between individual spiking events seems difficult, so if such a code is utilized in human neocortex, for instance, then some interesting mechanisms for decoding such temporal relations must be at work.

The authors’ conclusions about the retina seem justified to me, but it is very much unclear what their findings concerning the retina imply about other parts of the nervous system, and particularly, the rest of the brain. It seems plausible that, as they suggest, multiple codes are used simultaneously or by different parts of the nervous system. Nevertheless, I appreciate the authors’ methodology and find their results and encouraging for prospects of a fine-grain neural code.

For code 3, in the supplementary information section they factor the joint distribution of spike time and previous spike interval as a product of two functions, fitted by cubic splines. Is this a natural thing to do, and is there any intuition behind using this particular form (product)?