A comparison of egocentric and allocentric spatial memory in medial temporal lope and Korsakoff amnesics

Two patients with medial temporal lobe damage, seven Korsakoff amnesics and fourteen healthy control subjects were tested on three conditions of a spa tial memory test ('short delay', 'allocentric' and 'egocentric'). The task required subjects to recall the position of a single spot of light presente d on a board after various delays. The 'short delay' condition tested memor y over very short, unfilled intervals. The other two conditions used longer , filled delays. The allocentric condition required subjects to move to a d ifferent place around the board before recalling the position of the light. In the egocentric condition stimuli were presented in darkness, which elim inated allocentric cues. The Korsakoff amnesics were impaired at all delays of the short delay tasks, suggesting poor encoding. On the allocentric and egocentric tasks the Korsakoff amnesics showed a comparable impairment in the two conditions, which worsened with delay. This accelerated forgetting suggested that the Korsakoff amnesics also had impaired memory for allocent ric and egocentric information. The patients with medial temporal lobe dama ge were unimpaired in the 'short delay' condition suggesting intact encodin g and short-term memory of spatial information. However, they were impaired in the allocentric condition and showed accelerated loss of allocentric sp atial information. In the egocentric condition, while the performance of on e patient was impaired, the performance of the other was as good as control s. This result suggests that, in contrast to allocentric spatial memory, wh ich is sensitive to medial temporal lobe damage, an intact medial temporal lobe need not be necessary for successful performance on an egocentric spat ial memory task.