In a small bowl, whisk together the sour cream, 1/4 cup of the mayonnaise, and the horseradish.
In a large bowl, combine the crabmeat, the remaining 1/4 cup mayonnaise, 1/4 cup of the bread crumbs, the scallions, parsley, cayenne, salt, and pepper. Shape the crab mixture into eight patties. Coat the patties with the remaining 3/4 cup bread crumbs and pat off the excess.
In a large nonstick frying pan, heat the oil over moderate heat. Working in batches if necessary, fry the cakes until golden brown and crisp, about 2 minutes. Turn and fry until golden brown on the other side, about 2 minutes longer. Drain on paper towels. Serve with the horseradish cream.
