Functional anatomy of mental simulation of memorized routes

Topographical disorientation consequent to amnesia of spatial location in a patient with right parahippocampal damage

We describe a patient who selectively lost the ability to orient himself in the environment after a stroke involving the right parahippocampal gyrus. The neuropsychological assessment showed a specific pattern of impairment of topographical memory; the patient recognised and recalled environmental landmarks but was unable to recall their spatial location. This study provides evidence that different forms of topographical disorientation may be related to distinct mechanisms of cognitive dysfunction. Furthermore, neuroimaging data suggest that a lesion of the right parahippocampal gyrus is critically related to pure topographical disorientation.

Selective spatial memory impairment after right unilateral temporal lobectomy

Deficits in performance of both spatial and visual tasks are common following tissue loss in the right temporal lobe. Since spatial and visual attributes are frequently confounded in experimental tasks, we have studied patients following unilateral temporal lobectomy, in an attempt to determine which aspect mediates the observed deficits. Spatial and visual memory performance was compared in normal controls (n = 16), left temporal (LTL; n = 19) and right temporal (RTL; n = 19) lobectomy patients, by presentation of eight abstract designs in a spatial array for subsequent recall and recognition of the designs (visual memory) and recall of their spatial position (spatial memory). By varying the retention intervals for each group, all three groups were matched on both recall and recognition of the designs at sub-ceiling levels. In contrast, recall of the position of the designs (spatial memory), tested at equivalent delays to those of the visual memory tests, revealed a deficit in the RTL patients compared to both controls and LTL patients (p < 0.05). Magnetic resonance imaging (MRI) was used to quantify the extent of resection of the hippocampus and parahippocampal regions in the two patient groups and showed a significant correlation between hippocampal and parahippocampal removal and spatial memory in the RTL group only. These data support the notion of a disproportionately large involvement of the right hippocampus and adjacent regions in spatial memory.

Hippocampal involvement in human topographical memory: evidence from functional imaging

Functional brain imaging in humans is beginning to reveal a network of brain regions that subserve topographical learning: the medial parietal lobe, the posterior cingulate gyrus, occipitotemporal areas, the parahippocampal gyrus and the right hippocampus. These findings illuminate the patient lesion literature where all of these brain regions have been implicated at one time or another in cases of topographical disorientation. Once topographical information is acquired, the neuroanatomy that supports its use from either episodic or semantic memory is similar to that activated during encoding. The specific contributions of extrahippocampal regions within the topographical memory system are being revealed, such as the role of the right parahippocampal gyrus in object-in-place encoding. The right hippocampus is clearly involved in processing spatial layouts over long as well as short time-courses, and participates in both the encoding and the retrieval of topographical memory. The ventromedial orbitofrontal cortex is recruited when information in the topographical memory system is not sufficient to produce direct navigation to a goal place.

Parietal and hippocampal contribution to topokinetic and topographic memory

This paper reviews the involvement of the parietal cortex and the hippocampus in three kinds of spatial memory tasks which all require a memory of a previously experienced movement in space. The first task compared, by means of positron emission tomography (PET) scan techniques, the production, in darkness, of self-paced saccades (SAC) with the reproduction, in darkness, of a previously learned sequence of saccades to visual targets (SEQ). The results show that a bilateral increase of activity was seen in the depth of the intraparietal sulcus and the medial superior parietal cortex (superior parietal gyrus and precuneus) together with the frontal sulcus but only in the SEQ task, which involved memory of the previously seen targets and possibly also motor memory. The second task is the vestibular memory contingent task, which requires that the subject makes, in darkness, a saccade to the remembered position of a visual target after a passively imposed whole-body rotation. Deficits in this task, which involves vestibular memory, were found predominantly in patients with focal vascular lesions in the parieto-insular (vestibular) cortex, the supplementary motor area-supplementary eye field area, and the prefrontal cortex. The third task requires mental navigation from the memory of a previously learned route in a real environment (the city of Orsay in France). A PET scan study has revealed that when subjects were asked to remember visual landmarks there was a bilateral activation of the middle hippocampal regions, left inferior temporal gyrus, left hippocampal regions, precentral gyrus and posterior cingulate gyrus. If the subjects were asked to remember the route, and their movements along this route, bilateral activation of the dorsolateral cortex, posterior hippocampal areas, posterior cingulate gyrus, supplementary motor areas, right middle hippocampal areas, left precuneus, middle occipital gyrus, fusiform gyrus and lateral premotor area was found. Subtraction between the two conditions reduced the activated areas to the left hippocampus, precuneus and insula. These data suggest that the hippocampus and parietal cortex are both involved in the dynamic aspects of spatial memory, for which the name 'topokinetic memory' is proposed. These dynamic aspects could both overlap and be different from those involved in the cartographic and static aspects of 'topographic' memory.