--upon touching chin to chest?

There is a name for this phenomenon--L'hermitte's sign--and it suggests damage to the dorsal cervical cord and or dorsal spinothalamic tracts. It is very common in multiple sclerosis, but also can be caused by cervical vertebrae or disc pressure on the spinal cord, B12 deficiency, and a host of other things. It can also be drug-induced--many chemotherapy patients have reported it.

See:

http://en.wikipedia.org/wiki/Lhermitte%27s_sign

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2447962/

http://www.hindawi.com/journals/ijol/2010/907960/