Middle mouse wheel panning

When viewing a comp at 100% or larger, I'm pretty sure I was previously able to use my middle mouse wheel to pan around and inspect details. Rolling the wheel zoomed you in and out, while holding the button down enabled you to pan around when zoomed in. I've lost that functionality, and now I can't navigate around my comp when zoomed in. Is there another way to do this? I'm used to adjusting things at the pixel-level, but now I can only zoom in towards the dead center. Has anyone else experienced this? I've restarted a bunch of times... Thanks.