What is the test for evidence of mental illness, defect, or mental disorder in a criminal case?

Section 28, subdivision (a) provides, "Evidence of mental disease, mental defect, or mental disorder shall not be admitted to show or negate the capacity to form any mental state, including, but not limited to, purpose, intent, . . . or malice aforethought, with which the accused committed the act. Evidence of mental disease, mental defect, or mental disorder is admissible solely on the issue of whether or not the accused actually formed a required specific intent, . . . when a specific intent crime is charged." Section 28 "precluded jury consideration of mental disease, defect, or disorder as evidence of a defendant's capacity to form a requisite criminal intent, but it did not preclude jury consideration of mental condition in deciding whether a defendant actually formed the requisite criminal intent." (People v. Williams (1997) 16 Cal.4th 635, 677.)