This is garbage research. Nine participants took part in a study where they were prompted to do mental arithmetic adding or subtracting numbers between 0-5 while wired up with ultra-fine electrodes measuring the how frequently 585 individual neurons fired while completeing the task.

I presume at that point the data were mined for a publishable result. What they came up with was that by selecting a very small subset (~%5) of the neurons they measured they were able to tease out a result that certain neurons 'encode addition or subtraction'.

Is this real? Maybe. It's in now way explanatory. It doesn't offer any sort of model hypothesis or predictions worth testing. It's really a waste of time.