Question:
Can anyone tell me why don't you get apraxia in both arms with a broca'slesion? I can understand that broca's area is close to the premotor cortexso I have no prob's with actually getting apraxia, but why would you get itin the left arm and not the right.

My lecturer said something about "callosal fibers being damaged" or something

Any idea?

Answer: If I'm not mistaken, the traditional view is that the unilateralideomotor apraxia is caused by a disconnection of the left hemispherelanguage areas from the right hemisphere premotor cortex. The leftpremotor cortex need not be damaged, as long as the callosal fibresconnecting it to the right premotor area are. As far as I know, Brocasaphasia is not necessarily tied to ideomotor apraxia. If the anteriorcallosal fibres were involved but the left premotor area was spared,the syndrome you described would (theoretically) result. But if theleft premotor area was damaged, the apraxia would presumably bebilateral.

The disconnection of the language areas from the right premotor areameans that the left hand won't be able to respond to verbal commands.Furthermore, according to Liepmann's traditional view, the motorengrams necessary to carry out purposeful movement are also located inthe left hemisphere. So presumably, if the right premotor area wasdisconnected from these engrams, this would also interfere with theability to carry out purposeful movements with the left hand.

The key thing here is the disconnection, not the closeness of Brocasarea to the primary motor area. I assume that that's what yourlecturer was referring to with the reference to callosal fibres beingdamaged.

One thing to keep in mind: things are rarely as simple in a realpatient as these connectionist models imply.