Which colour reaches deepest into the brain?
A thin glass fibre (0.2 mm wide) shines 10 mW of light into brain tissue. The tissue scatters and absorbs it.
Pick a colour first.
How warm does the tissue get?
Light that is absorbed turns into heat. Here the light is blue (470 nm) and runs for 10 seconds. You can pulse it: the duty cycle is the share of time the light is on.
Type a guess first. You can change the settings and reveal again.
Can the neuron keep up?
Flash the light in short pulses (2 ms each). The cell keeps up if at least 95 % of the pulses each give exactly one spike. Different light switches close at different speeds.
Make a guess for all three switches first.
Neurons switched on per degree of warming
More light switches on more neurons, but it also warms the tissue. Blue or yellow: which colour gets more neurons for each degree of warming at the hottest point?
The trade-off at a glance
In this toy model, colour sets reach and heat; closing speed sets following.
Reach and heat, per colour
Volume lit above 3 mW/mm² and warming are per mW of light out of the fibre. Reach and heat depend only on colour here: every switch shares ChR2's light sensitivity and one 3 mW/mm² threshold. The 590 nm and 635 nm advantages come only from wavelength, and absorption outside blood is not modelled, so they look better than they are (635 nm: a toy upper bound).