Well, absent a reply from Steve, or someone more knowledgeable about psychopharmacology than I, here’s my anecdotal evidence from personal experience and chatting with other folks. For one, I’ve noticed degradation of control in the speech center — I’m likelier to stutter, lose track of what I’m saying, experience synaptic hang-fire, when I’ve had Aspartame than not. I’ve had others confirm experiencing speech center related anomalies.
For two, I find Aspartame puts me in a more agitated and ineffectual frame of mind, and seems to affect fine motor control so that I have trouble managing tasks like typing or operating a sewing machine without making errors at an increased rate. A rough and ready ladies’ description of the effect would be to say that I get a mild and unpleasant buzz from Aspartame. About two Diet Cokes in a row will do it.