There is no "format". The optic nerve transfers nerve impulses which are caused by changes in "trans-membrane" electrical charge gradient between the inside if the neuron and the surrounding tissues and fluids (iIRC the charge gradient is a consequence of the concetration of positively charged sodium and potassium ions).
If you really want to think of the nervous system as having a "format", then that format would just be the nerve imuplses themselves (and extending the analogy, you might say that the membrane of the neuron, the potassium ions, and the sodium ions constitute a lower level assembly language). Really though, I dont think any sort of syntactic or linguistic analogy is appropriate here. That sort of abstraction would be more appropriate for describing high level cognitive processes. For instance, it probably makes more sense to ask what "format" or language the brain uses to encode and identify Gestalt features of visual images once the signal gets to your visual cortex and the object recognition systems in the brain, but at the level if raw stimuli and the optic nerve I dont think any syntactic analogies apply.
