I agree that you must work all muscles to get best definition, but talked with a buddy recently that gave me a pointer to help with the depth of your pecs, thereby making a more defined look in the center-he has been doing weights much longer than I and has a pretty good physique, so must know little about what he is doing-
Lie on your back on a bench, as when doing dumb-bell flys, but use only one dumb-bell of a moderate weight to begin with (8-15 lbs. is good). using both hands, with your palms upward, for a 'v' around the upper portion of the verticle dumb-bell. Make SURE you have a good grip about the weight..you don't want to drop it on you during the movement. Lift the verticle weight from your chest, clasping tightly as described in 'v'. place at about mid-ab level, with arms slightly bent at the elbows. Slowly bring the weight up and over your head and past horizontal toward the floor at the head of the bench. Hold for moment, then slowly, bring back up to the starting position. Repeat this movement 10-12 times to begin, building up to 2 sets of 25 reps. You can feel the "pull" in your chest...and abs..during this movement. I've only been doing this movement, in conjunction with the dumb-bell flys for a short period of time, but it seems that my chest is getting deeper with little better definition.
give it a shot, keeping good form, and moving slowly, to see if it helps in your development. And hold weight FIRMLY-
good luck, and let me know if it helps-Lynn
this movement probably has a name, but being fairly new to more serious weight training, unfamiliar with it-