Tokyo's key Nikkei index opened flat on Thursday in cautious trade ahead of the closely watched Jackson Hole conference of global financial authorities later in the day. The benchmark Nikkei 225 index was up just 0.02 percent or 4.27 points at 23,295.13 in early trade, while the broader Topix index was down 0.12 percent or 1.93 points at 1,622.55. kh/sah/jah