The Render Manager > mental ray (Global Renderer)
The Render Manager > (expand a pass) > mental ray
Set a current pass and choose Render > Render > Renderer Options from the Render toolbar.
Tab: Raytraced [Properties Reference]
Options: Ambient Occlusion [Properties Reference]
Select the Shadowmaps 
Motion Blur checkmark to activate the shadow mapping option for motion blur effects. Shadow maps are a scanline-based feature that produce
shadows faster than raytracing. Activating them with motion blur slows render times but matches the shadow to the motion blur
nicely. If you are running a quick render test, keep this option off.
For more information about shadow maps, see Creating Shadow-Mapped Shadows [Direct Illumination].
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