It is known that all sensory information is input to the brain as neural spike sequences. Now, to distinguish between the spike sequences generated by retinal red/blue/green cone cells from each other, and these from the cochlear (inner ear) hair cells, and so on, some sort of encoding scheme must be used.
To further clarify, in the case of retinal cone cells, the rate of incident light quanta in a given energy range, is the primary information that is coded. But, if all 3 types of cone cells generated identical responses for a given rate-of-incidence, like the CCD pixels in our digital cameras do, then there would be no way for upstream neurons to tell what type of cone cell a spike sequence came from. Instead, my guess is that, each type of cone cell encodes the rate-of-incidence in its own characteristic way, similar to how different types of musical instruments sound differently, even when playing the same pitch at the same intensity, via timbre.
Is there evidence that each sensory neuron type has a characteristic spike sequence pattern?
Why message type must be encoded in the message itself
During the development of the visual system, the retina, the LGN and the visual cortex develop separately initially and sometime later, axons from the retinal ganglions grow into LGN, and optic radiations from the LGN grow and reach into the cortex. As far as we can tell it is not guaranteed that a specific ganglion will project its axon to a specific neuron in the LGN. All that is guaranteed by the growth process is that ganglions close together will project to LGN neurons that are also close together.
Given this development process, when a higher region say in the V1 receives a spike stream from a neuron somewhere lower, the question arises: how does it know that this spike stream means, red, blue or green? A simple idea that occurred to me from information theory is that the message type could somehow be encoded in the message itself.
Evidence for characteristic spike patterns for each sensory neuron type would take us one more step towards understanding qualia, the hard problem of consciousness. My speculation is that qualia are the neuronal analogs of timbre in musical instruments.
Erwin Schrödinger thought we'd never get there. He said, "The sensation of color cannot be accounted for by the physicist's objective picture of light [as] waves [or as quanta]. Could the physiologist account for it, if he had fuller knowledge than he has of the processes in the retina and the nervous processes set up by them in the optical nerve bundles and in the brain? I do not think so."
I guess he's right in the sense that we will never be able to fully wrap our minds around the mysterious and ineffable nature of qualia.
However, evidence of characteristic spike patterns would offer resolutions to qualia related thought experiments, such as Is there something about Mary?, that philosophers seem to be pulling their hair out over.