In Georgia, the procedure to be followed in making such a determination depends upon the date of trial. For those defendants tried after July 1, 1988, O.C.G.A. § 17-7-131 permits [*5] them to contend that they were mentally retarded at the time of the crime and to present evidence of such mental retardation to the fact finder. In capital cases, the fact finder is then required to determine during the guilt- innocence phase of trial whether the defendant is guilty but mentally retarded.