A couple of years ago, I ran an optogenetics experiment with bilateral light stimulation in the hypothalamus. Or rather, that was how I planned it. However, when it came to tethering and stimulating the mice, the fibres were too close to each other. I ended up doing unilateral stimulations, and realised that I would need to do angled opto fibres for any future bilateral studies.

Luckily, a stereotaxic frame comes with a pivot to allow exactly this kind of thing. There’s usually a pin holding it vertically; take that out and you can tilt the top half as needed to your desired angle. There’s an “angle scale” to do this accurately, and then you can clamp the whole thing in place.

So now comes the sticky question: how do you work out a new set of coordinates based on an angled implant? Turns out, all you need is a bit of trigonometry (Mr Turner did tell me it would be useful later in life, I just never believed him).

I drew a diagram to calculate the new coordinates based on my chosen angle of 10⁰. I used sine and cosine to derive the unknown lengths:

Now, I can tell you, these calculations were a faff, so I will avoid them in the future if possible. And, I will also say that I am the only person in the lab who has attempted angled fibres. Everyone else just does unilateral, and I thing a large part of their reticence is caused by ignorance of how to calculate the angled opto fibres.

With that in mind, I have developed a handy tool to do these calculations for you. Just input your starting lateral and dorsal coordinates, and the angle to tilt your fibres. I recommend 10⁰. Then press “Calculate” and the script will output your new lateral and dorsal lengths.

I hope people find my tool useful, and encourage them to use angled opto fibres in their studies.