Neurolucida selects the lowest bit depth that includes the highest pixel value found within the image.
Example:
If the highest pixel value found in the image is 3965, then the minimum bit depth that could include this value is 12 since the range for a 12-bit image is 0-4096 (4096=212). Neurolucida will use 12 as the bit depth.
12 may not be the actual bit depth range of the image (i.e., the image could have been created in a 16-bit image space), but the maximal pixel value contained with that image is still 3965.