Figure 4. Histology at different depths as the fibers splay during insertion.

A bundle of approximately 1,125 optical microfibers implanted at a depth of 2.95 mm. Eight weeks after the implant, the animal was perfused and the brain sectioned perpendicularly to the insertion angle. At the surface, the bundle diameter was 1.03 mm. These 70 μm thick slices from depths (a) 2.76 mm, (b) 2.34 mm, (c) 1.57 mm and (d) 0.59 mm reveal a gradual spreading of the optical fibers during insertion as each fiber follows a path of least resistance.