Creatures Caves

I'm not sure what version you're talking about (but I'm assuming the answer is similar for all of them). In the Creatures 3 genetics kit the "locus" combo box for receptor attachments to "brain organ" go from "Neuron(0) state(0)" to "Neuron(63) state(3)" without any regard to the actual number of neurons in the selected lobe.

The reason for this is that the genetics format and editor are extremely simple/stupid; it has only 1 byte (8bits) to store this information and (for the case of C3/DS) they choose to store the neuron number in the upper 6 bits (2^6 = 64, hence 0 to 63) and the state number in the lower 2 bits (2^2 = 4, hence 0 to 3). If you can only select 15 I'm guessing it's C1/C2 and they only used 4 bits instead of 6 for the neuron.

I'm not sure how to work around this in C1 or C2 (dendrites work differently there; a lobe can only have two input sources) but in C3/DS you could create a new lobe that you use to connect your receptor to and create a new brain tract that connects those neurons to whatever other neurons you want (e.g. new lobe "xxxx" with an "xxxx->noun" brain tract that connects neuron 0 to neuron 28 ). I can provide more information on this if required (but only for C3).