You can think of primary motor cortex as a "clickable map" of all the muscles in your body. The map can be "clicked" by either side of your brain, causing that muscle to move.

However, half of the map physically resides in one hemisphere, and the other half resides in the other hemisphere. If a hemisphere is lost, then the map is lost and a person will become hemiplegic. This is the same reason why some people are hemiplegic after a stroke. Note that in very young children, the brain can rebuild the map in the remaining hemisphere. But it is very difficult for adults to regain function.

On the other hand, a callosotomy severs some of the structures that the brain uses to reach across and click the map on the opposite hemisphere. It does not sever all of the structures and methods used to transmit information across hemispheres. For example, both hemispheres still have indirect visual and proprioceptive input regarding what is happening on the contralateral side.

So it's hard to predict what will happen, but generally it will be more difficult to coordinate movements.

Here's what happened to one patient:

> We examined bimanual coordination in a patient before and after each stage of callosotomy surgery. We tested how well the patient coordinated movement direction between the hands. The patient drew symmetrical or asymmetrical figures simultaneously with both hands. Before surgery, symmetrical figures were drawn well and asymmetrical figures were drawn poorly. Following anterior callosotomy, the drawings improved slightly. Symmetrical figures were still drawn well, and asymmetrical ones were still drawn poorly. Thus, spatial integration remained intact despite the loss of interhemispheric communication between frontal cortical sites. After posterior callosotomy, spatial coordination deteriorated significantly. Mirror-image drawings became less symmetrical, while asymmetrical drawings improved. These data indicate that the posterior callosum mediates the coordination of direction information between the hands during bimanual movements. Given the topographical organization of the corpus callosum, this integration is likely carried out by parietal cortex.