Can a juror's affidavit be used to impeach a verdict?

Subject to the restrictions of Evidence Code section 1150, a juror's affidavit may be used to impeach a verdict. Evidence Code section 1150 states in relevant part: "(a) Upon an inquiry as to the validity of a verdict, any otherwise admissible evidence may be received as to statements made, or conduct, conditions, or events occurring, either within or without the jury room, of such a character as is likely to have influenced the verdict improperly. No evidence is admissible to show the effect of such statement, conduct, condition, or event upon a juror either in influencing him to assent to or dissent from the verdict or concerning the mental processes by which it was determined." Evidence Code "section 1150 properly distinguishes between 'proof of overt acts, objectively ascertainable, and proof of the subjective reasoning processes of the individual juror, which can be neither corroborated nor disproved, . . . ' " (Krouse v. Graham (1977) 19 Cal.3d 59, 80, italics added.)