Memory-Guided Saccades in Psychosis: Effects of Medication and Stimulus Location.

The memory-guided saccade task requires the remembrance of a peripheral target location, whilst inhibiting the urge to make a saccade ahead of an auditory cue. The literature has explored the endophenotypic deficits associated with differences in target laterality, but less is known about target amplitude. The data presented came from Crawford et al. (1995), employing a memory-guided saccade task among neuroleptically medicated and non-medicated patients with schizophrenia (n = 31, n = 12), neuroleptically medicated and non-medicated bipolar affective disorder (n = 12, n = 17), and neurotypical controls (n = 30). The current analyses explore the relationships between memory-guided saccades toward targets with different eccentricities (7.5° and 15°), the discernible behaviour exhibited amongst diagnostic groups, and cohorts distinguished based on psychotic symptomatology. Saccade gain control and final eye position were reduced among medicated-schizophrenia patients. These metrics were reduced further among targets with greater amplitudes (15°), indicating greater deficit. The medicated cohort exhibited reduced gain control and final eye positions in both amplitudes compared to the non-medicated cohort, with deficits markedly observed for the furthest targets. No group differences in symptomatology (positive and negative) were reported, however, a greater deficit was observed toward the larger amplitude. This suggests that within the memory-guided saccade paradigm, diagnostic classification is more prominent in characterising disparities in saccade performance than symptomatology

The Hippocampus Promotes Effective Saccadic Information Gathering in Humans

It is well established that the hippocampus is critical for memory. Recent evidence suggests that one function of hippocampal memory processing is to optimize how people actively explore the world. Here we demonstrate that the link between the hippocampus and exploration extends even to the moment-to-moment use of eye movements during visuospatial memory encoding. In Experiment 1, we examined relationships between study-phase eye movements in healthy individuals and subsequent performance on a spatial reconstruction test. In addition to quantitative measures of viewing behaviors (e.g., how many fixations or saccades were deployed during study), we used the information-theoretic measure of entropy to assess the amount of randomness or disorganization in participants\u27 scanning behaviors. We found that the use of scanpaths during study that were lower in entropy (e.g., more organized, less random) predicted more accurate spatial reconstruction both within and between participants. Scanpath entropy was a better predictor of reconstruction accuracy than were the quantitative measures of viewing. In Experiment 2, we found that individuals with hippocampal amnesia tended to engage in viewing patterns that were higher in entropy (less organized) relative to healthy comparisons. These findings reveal a critical role of the hippocampus in guiding eye movement exploration to optimize visuospatial relational memory

Organization of spatiotemporal information and relational memory in the hippocampus

This work examines the role of the hippocampus and relational memory in organizing episodic memory during navigation and reconstruction. Navigation is a critical component in most organisms’ survival. Reconstruction, on the other hand, provides an incredibly rich method of evaluating the precise information remembered by an individual after attempting to learn and remember that information. Through validating the computational framework in this work on amnesic patients with hippocampal damage, an understanding of some of the specific types of relations which rely on the hippocampus can be established. Then, this framework can be applied to a much more complex, spatiotemporal navigation and reconstruction task in healthy individuals to gain a wider perspective on the organization of episodic memory, which is known to critically rely on the hippocampus. The first experiment and associated analysis framework presented in this document (Chapter 2) uses spatial reconstruction to establish that not all types of spatial relations are impaired in hippocampal damaged patients. In particular, the arbitrary, identity-location relations (i.e. those relationships where the element being bound could have just as easily been anything) are critically impaired in hippocampal damaged patients while location information, disregarding identity, is not. The use of reconstruction in this context allows for the establishment of a set of critical computational metrics which relate to hippocampal function in reconstruction which can then be applied to other reconstruction tasks in healthy individuals to learn more about the wider structure and organization of memory. In the second experiment (Chapters 3 and 4), the methodologies which were applied to hippocampal damaged patients in the first experiment are applied to a novel Spatiotemporal Navigation Task in healthy young adults. In this task, participants are not just asked to study and reconstruct items in space, but instead, participants are asked to, in Virtual Reality, navigate space and time (via normal movement and simulated Time Travel) and study, then reconstruct the locations of events in spacetime. The computational framework established in the previous chapter is then applied to show that relational memory errors in time are far more common in this task than in space, suggesting differences in representations between these two domains even when the navigation and exploration of the domains are put on a more equal footing. Additionally, in time, these relational memory errors are far more likely to occur within a shared contextual region than should occur by chance. In fact, this error (temporal relational memory error within a context) gets worse across the first 3 trials, suggesting a systematic bias due to context. Finally, a more traditional bias, the context boundary effect (i.e. a “squishing” of within context temporal locations and “stretching” of across context temporal locations) is observed even though participants are allowed to reexplore the contexts arbitrarily, multiple times. This suggests that the context boundaries are having a profound impact on both the distance judgements and relational memory structure associated with events in spacetime. Finally, in the fourth chapter, the navigation component of the previous Spatiotemporal Navigation Task is examined to determine if changes in study time navigation and exploration relate to changes in the various test metrics discussed in the previous chapter. More rapid improvements in spatial and temporal navigation are shown to relate to more rapid improvements in memory in those domains, separably, suggesting that spatial and temporal representations may in some way be separable in this task in both the relational representations and the navigation strategies supporting those representations. Relational memory improvements are shown to be uniquely tied to changes in navigation complexity and systematicity, pointing to an interplay between in-the-moment, memory-guided decision making and subsequent relational memory efficacy. Context boundaries are suggested to act as more of a discriminatory feature (at least in this task) than one used to strengthen within-context relational memory organization accuracy as there is a significant relationship between changes in context boundary crossing and both the context boundary effect and across-context temporal relational memory errors. Finally, a preference towards exploring an otherwise temporally-flexible environment in the implied, forward order with increasing contiguity is suggested to be a critical element in improving temporal, relational, and contextual memory organization. Taken together, this work shows the richness of spatiotemporal navigation and reconstruction in observing the complex interplay between navigation in space, navigation in time and how these ultimately may relate to navigation in memory. Through embracing principled approaches to analysis of behavioral data, and the inclusion of complex behavioral mechanics (such as simulated time travel), this work extends our understanding of the role of hippocampal relational memory and overall memory organization