In a food processor, puree cheddar, cream cheese, pimiento peppers and cayenne. Slice the top quarter off a small round bread loaf and hollow it out, leaving a 3/4-inch-thick shell. Fill to top with the dip. Transfer remaining dip to a heavy-duty resealable plastic bag; snip 1/4 inch off one corner. Squeeze the dip on top, swirling to create a brain shape. Serve with crackers.