 Next, to figure out what these cells might do, the experimenter might express channelrhodopsin in each of these cells, by crossing the same GAL4 line with a channelrhodopsin reporter line.In the offspring the selected cells, and only those cells, will contain channelrhodopsin and can be triggered by a bright light.Now the scientist can trigger these particular cells at will, and examine the resulting behavior to see what these cells might do.