What is the legal test for pleading not guilty by reason of insanity?

Under California law, if a defendant pleads not guilty and joins it with a plea of not guilty by reason of insanity, the issues of guilt and sanity are tried separately. In such circumstance, "the defendant shall first be tried as if only such other plea or pleas had been entered, and in that trial the defendant shall be conclusively presumed to have been sane at the time the offense is alleged to have been committed." ( 1026, subd. (a).) Although the defense of diminished capacity has been abolished, diminished actuality survives, and "the jury may generally consider evidence of voluntary intoxication or mental condition in deciding whether defendant actually had the required mental states for the crime. [Citations.]" (People v. Steele (2002) 27 Cal.4th 1230, 1253.)