Differential cognitive effects of colloid cysts in the third ventricle that spare or compromise the fornix.

A series of twelve cases, all of whom had received surgery for the removal of a colloid cyst in the third ventricle, was examined on a series of memory tests. The only consistent predictor of poor memory performance that could be detected from MRIs was the presence of bilateral interruption of the fornix, which occurred in three of the subjects. Although these three cases were poor on tests of learning and recall, there was evidence that recognition was less impaired. The subjects were also tested on a set of recognition and concurrent discriminations that closely matched tests given to non-human primates. Clear parallels were found between the apparent effects of fornix damage in these clinical cases and those observed following more selective surgery in non-human primates. These findings not only indicate that fornix damage is sufficient to induce anterograde amnesia but also support the validity of recent animal tests that are thought to capture aspects of episodic memory.