Cortical control of spatial memory in humans: the visuooculomotor model.

We review current knowledge of the cortical control of spatial memory, studied using visuooculomotor paradigms. Spatial memory is an essential cognitive process that can be involved in preparing motor responses. Our knowledge of spatial memory in humans recently has progressed thanks to the use of ocular saccades as a convenient model of motor behavior. Accuracy of memory-guided saccades, made to the remembered locations of visual targets, is a reflection of spatial memory. For the performance of memory-guided saccades with brief delays (up to 15-20 seconds), that is, involving short-term spatial memory, lesion studies have shown that the posterior parietal cortex, the dorsolateral prefrontal cortex, and the frontal eye field play significant roles. Studies of memory-guided saccades using transcranial magnetic stimulation have suggested that the right posterior parietal cortex is involved at the initial stage (<300 milliseconds) of visuospatial integration, whereas the dorsolateral prefrontal cortex in both hemispheres controls the following phase of short-term memorization, the frontal eye field mainly serving to trigger saccades. The new concept of a medium-term spatial memory has emerged from a behavioral study of memory-guided saccades in normal subjects, showing a paradoxical spontaneous improvement of spatial memory after delays of approximately 20 seconds. Lesion studies have shown that the parahippocampal cortex could specifically control this medium-term spatial memory. Last, different experimental and clinical arguments suggest that, after a few minutes, the hippocampal formation finally takes over the control of spatial memory for long-term spatial memorization. Therefore, spatial memory involved in the memorization of visual items could be successively controlled by the dorsolateral prefrontal cortex (short-term spatial memory), the parahippocampal cortex (medium-term spatial memory), and the hippocampal formation (long-term spatial memory), depending on specific periods of times. The applicability of this simple visuooculomotor model of spatial memory to other types of stimuli and general motoricity has yet to be confirmed.