Impoverished descriptions of familiar routes in three cases of hippocampal/medial temporal lobe amnesia.

Recent research has challenged classic theories of hippocampal function in spatial memory with findings that the hippocampus may be necessary for detailed representations of environments learned long ago, but not for remembering the gist or schematic aspects that are sufficient for navigating within those environments (Rosenbaum et al., 2000; Rosenbaum, Winocur, Binns, & Moscovitch, 2012). We aimed to probe further distinctions between detailed and schematic representations of familiar environments in three cases of hippocampal/medial temporal lobe (MTL) amnesia by testing them on a route description task and mental navigation tasks that assess the identity and location of landmarks, and distances and directions between them. The amnesic cases could describe basic directions along known, imagined routes, estimate distance and direction between well-known landmarks, and produce sketch maps with accurate layouts, suggestive of intact schematic representations. However, findings that their route descriptions lack richness of detail, along with impoverished sketch maps and poor landmark recognition, substantiates previous findings that detailed representations are hippocampus-dependent.

Functional connectivity of hippocampal and prefrontal networks during episodic and spatial memory based on real-world environments.

Several recent studies have compared episodic and spatial memory in neuroimaging paradigms in order to understand better the contribution of the hippocampus to each of these tasks. In the present study, we build on previous findings showing common neural activation in default network areas during episodic and spatial memory tasks based on familiar, real-world environments (Hirshhorn et al. (2012) Neuropsychologia 50:3094-3106). Following previous demonstrations of the presence of functionally connected sub-networks within the default network, we performed seed-based functional connectivity analyses to determine how, depending on the task, the hippocampus and prefrontal cortex differentially couple with one another and with distinct whole-brain networks. We found evidence for a medial prefrontal-parietal network and a medial temporal lobe network, which were functionally connected to the prefrontal and hippocampal seeds, respectively, regardless of the nature of the memory task. However, these two networks were functionally connected with one another during the episodic memory task, but not during spatial memory tasks. Replicating previous reports of fractionation of the default network into stable sub-networks, this study also shows how these sub-networks may flexibly couple and uncouple with one another based on task demands. These findings support the hypothesis that episodic memory and spatial memory share a common medial temporal lobe-based neural substrate, with episodic memory recruiting additional prefrontal sub-networks.

Brain regions involved in the retrieval of spatial and episodic details associated with a familiar environment: an fMRI study.

Functional magnetic resonance imaging (fMRI) was used to compare brain activity during the retrieval of coarse- and fine-grained spatial details and episodic details associated with a familiar environment. Long-time Toronto residents compared pairs of landmarks based on their absolute geographic locations (requiring either coarse or fine discriminations) or based on previous visits to those landmarks (requiring episodic details). An ROI analysis of the hippocampus showed that all three conditions activated the hippocampus bilaterally. Fine-grained spatial judgments recruited an additional region of the right posterior hippocampus, while episodic judgments recruited an additional region of the right anterior hippocampus, and a more extensive region along the length of the left hippocampus. To examine whole-brain patterns of activity, Partial Least Squares (PLS) analysis was used to identify sets of brain regions whose activity covaried with the three conditions. All three comparison judgments recruited the default mode network including the posterior cingulate/retrosplenial cortex, middle frontal gyrus, hippocampus, and precuneus. Fine-grained spatial judgments also recruited additional regions of the precuneus, parahippocampal cortex and the supramarginal gyrus. Episodic judgments recruited the posterior cingulate and medial frontal lobes as well as the angular gyrus. These results are discussed in terms of their implications for theories of hippocampal function and spatial and episodic memory.

The hippocampus is involved in mental navigation for a recently learned, but not a highly familiar environment: a longitudinal fMRI study.

Functional magnetic resonance imaging (MRI) was used to investigate the hypothesis that memory for a large-scale environment is initially dependent on the hippocampus but is later supported by extra-hippocampal structures (e.g., precuneus, posterior parahippocampal cortex, and lingual gyrus) once the environment is well-learned. Participants were scanned during mental navigation tasks initially when they were newly arrived to the city of Toronto, and later after having lived and navigated within the city for 1 yr. In the first session, activation was observed in the right hippocampus, left precuneus, and postcentral gyrus. The second session revealed activation in the caudate and lateral temporal cortex, but not in the right hippocampus; additional activation was instead observed in the posterior parahippocampal cortex, lingual gyrus, and precuneus. These findings suggest that the right hippocampus is required for the acquisition of new spatial information but is not needed to represent this information when the environment is highly familiar.

Detailed descriptions of routes traveled, but not map-like knowledge, correlates with tests of hippocampal function in older adults.

We examined hippocampal contribution to remote spatial memory in older adults by correlating their performance on tests sensitive to hippocampal damage with their description of routes they traversed many times or only once, and with their map-like knowledge of downtown Toronto. We found that performance on table-top tests of spatial location (Smith and Milner (1981) Neuropsychologia 19:781-793) and on paired-associate learning, and the number of Internal Details on the Autobiographical Interview (Levine et al., (2002) Psychol Aging 17:677-689), all correlated significantly with the number and type of perceptual details used in describing routes one has traversed, but not with map-like knowledge of Toronto. No significant correlations were found with performance on tests of frontal function (WCST, phonemic fluency, and backward digit span). We conclude that the hippocampus is implicated in vivid re-experiencing of a familiar route, but not with map-like knowledge of a large-scale environment. These findings are interpreted as consistent with Multiple Trace Theory's prediction that it is the degree of detail of a retrieved memory that is crucially dependent on the hippocampus.

Examining the time course of facilitation and inhibition with simultaneous onset and offset cues.

The experiment conducted examined the effect of simultaneously presented onset and offset cues on the orienting of attention in the visual field. Subjects were presented with a display that consisted of four placeholder boxes around a central fixation point. An onset and an offset cue appeared simultaneously in two of the locations, and the other two locations provided a neutral baseline condition. Reaction times were measured in a simple target detection task with stimulus-onset asynchronies (SOAs) that ranged from 100 ms to 1,000 ms. As expected, the onset cue produced early facilitation and later occurring inhibition of return (IOR). The offset cue produced significant inhibition at all but the earliest SOA. These results suggest that simultaneously presented onset and offset cues both capture attention, but that attention is rapidly disengaged from the location of the offset cue, resulting in earlier occurring IOR. For the onset cues, attention is allocated for a longer period of time, producing the typical pattern of early facilitation and later occurring IOR. The differing time course of attention at each location may reflect separate facilitatory and inhibitory processes, and the priority given to the onset of a stimulus by the attentional system.