This research focuses on device design and process integration of high density nonvolatile memory devices. Research was carried out to improve scaling of floating gate memories by increasing charge density as well as spin-based memories by reducing critical switching current. This work demonstrates fabrication of CMOS-compatible nonvolatile hybrid memory device using fullerene molecules as a floating gate. Molecules have dimensions of several Angstroms resulting in an electron density of ~10¹³ cm⁻² or higher. In hybrid MOSCAPs, fullerenes were encapsulated between inorganic oxides, i.e. SiO₂ as a tunnel oxide and HfO₂ as a control oxide. Introduction of a high-k material as a control oxide improves capacitive coupling between control gate and floating gate as well as the program/erase efficiency. The MOS capacitors demonstrate nonvolatile memory operation at room temperature. The device data infers that program/erase mechanism in fullerene devices is Fowler-Nordheim tunneling; however, retention is determined by trap-assisted tunneling. The next part of the work focused on spin-transfer-torque (STT) based magnetic memory. Spin-based memory has the unique potential to be the universal memory because of its high density, fast switching, and nonvolatility. This work presents STT switching of perpendicular magnetic anisotropy (PMA) spin-valves with tilted magnetization using point contact measurement. The PMA materials have high coercivity resulting in good retention and tilted magnetization induces precessional switching resulting in a lower switching current density. First, micromagnetic simulations were performed for spin-valves with tilted magnetization and precessional switching was observed to reduce the switching current. Then, spin-valve structures were fabricated by e-beam evaporation. The structure consisted of Co/Pt and Co/Ni layers, where the thickness of the layers was optimized to obtain different amount of tilt in magnetization. Point contact measurements of tilted spin-valves show STT switching, where the switching field of the free layer varies with the magnitude and sign of the applied current. The observed STT effect is stronger in a 45° tilted spin-valve compared to a 12° tilted device presumably due to the tilted spin polarization. However, tilting introduces nonuniform effective field and canting of the domains which affect the STT. / text

Not available / text

published_or_final_version / Computer Science and Information Systems / Master / Master of Philosophy

Upon acquisition, memories undergo an initial stabilization, or consolidation, process after which they are generally resistant to interference. There is now an abundance of evidence that reactivation or retrieval of a consolidated memory opens up a window of time during which the memory can be strengthened, disrupted, or updated via a process of "reconsolidation". This dissertation is comprised of three experimental studies in rats aimed at investigating previously unexamined aspects of this dynamic memory process. The first study assessed whether spatial memories learned under positively-motivated conditions could be updated with new information following reactivation. Rats that learned a second spatial task in the same environmental context as a previously learned task intruded items from the second episode during recall of the first. This result suggests that the context reactivated the memory for the first task, triggering reconsolidation and updating of the memory. The second study used the memory updating effect obtained in the first study as a behavioral measure to investigate the effects of aging on reconsolidation. Unlike in the young rats, the context reminder did not lead to intrusions of the second learning episode during recall of the first. Older adult human participants in this study also showed a different pattern of results than what had been seen previously in young participants. Therefore, in humans as well as in rats, it appears that aging may lead to changes in spatial memory reconsolidation. The third study piloted an experiment to examine hippocampal network activity associated with the spatial memory reconsolidation task used in the first two studies. Preliminarily, we found that the context reminder manipulation was associated with more place field stability across some spatial tasks and that stability across certain tasks was positively related to our measure of memory updating. Additionally, we found evidence that the context reminder enhanced neural replay of some learning episodes. While preliminary, these results suggest that both place field stability and replay may play roles in this reconsolidation paradigm.

The study of developmental metacognition was originally proposed as a way to better understand memory, by elucidating the processes that act upon and therefore affect it. Much research has been conducted to examine the nature of metacognitive processes, and the interaction between metacognitive judgments and learning behavior in adults. Developmental research has demonstrated that there is a strong developmental trend, such that metacognitive abilities emerge at age four years at the earliest and mature until adulthood. However, this estimate raises a potential paradox, given young children's excellent learning abilities, if monitoring and differentially responding to changes in internal states of knowledge is an important component of learning. This dissertation proposes that metacognitive processes, like memory-monitoring, rather than being distinct from and externally operating on core cognitive process, may be intrinsically linked to basic cognitive functions, arising naturally as a result of processing. By this account, metacognitive abilities emerge in implicit form early in development along with other developing cognitive functions like memory, and what is observed later as the emergence of metacognition may rather be the transition from an implicit and undifferentiated process to an explicit and more readily testable process. This dissertation presents six experiments exploring the relationship between memory-monitoring in non-human animals, preschool children, and adults, using a non-verbal paradigm adapted from comparative literature. Participants learned a set of visual paired-associates, and at test were given the option to selectively accept or decline a memory trial for each item. Accuracy for accepted items was significantly higher than for declined in children and there was a similar tendency with adults, suggesting implicit memory-monitoring skills. Additionally, a relationship between memory-monitoring assessments and other cognitive processes was found, suggesting that memory-monitoring does not function independently of other cognitions. The results suggest that children may have implicit access to internal knowledge states at very young ages, providing an explanation for how they are able to guide learning, even as infants. Further the results suggest that the relationship between metacognitive and other cognitive skills may be rather more dynamic and complex than has typically been described.

Children with ADHD diagnosis often display working memory deficits, as well as reading and mathematical disabilities. Previous studies have demonstrated that computerized working memory training (WMT) is a promising intervention. The present study aimed at exploring the effects of WMT on working memory, scholastic skills and behavioral symptoms in children with ADHD. Thirty-two children, aged 6 to 11, were randomized to WMT or a control condition. WMT consisted of nine tasks taxing working memory with adaptive difficulty level. All children trained in their homes, with their parents acting as supervisors. Children who completed more than 20 days of training in 5-8 weeks (8 in the WMT condition and 13 in the control condition) were considered compliers. Assessments were conducted before and after intervention. Results indicated that WMT lead to significant gains of verbal working memory and arithmetic skills. More research is needed to further investigate the effects of WMT.

This experiment investigated memory changes across the adult lifespan and some factors that might be associated with these changes. Adult participants of all ages (16 to 83 years old) were asked to orally describe scenic color photographs, and then following a delay, to re-describe these pictures from memory. Given information is objective, physical objects and their attributes that are depicted in a target picture, whereas beyond information is subjective, personal experiences and inferences that are not depicted in a target picture per se but are associated with a target picture. Chapter 3 examined the content of these picture descriptions for the amount of given and beyond information that was encoded and retrieved about target pictures. The results indicated an age-related decline in memory for given information and preserved memory for beyond information. Chapter 4 examined the relationship between perceptual and verbal ability and memory for given and beyond information. Perceptual ability was assessed by self-report measures of auditory and visual ability and verbal ability was measured by a standardized test. The results indicated that an age-related improvement in verbal ability, but not an age-related decline in perceptual ability, was related to memory for given and beyond information. Chapter 5 explored age-related changes in memory for feminine and masculine information across the adult female lifespan. Feminine and masculine information is information that would be considered exclusively relevant to young women and men, respectively. The results indicated an age-related increase in memory for feminine information and no age-related change in memory for masculine information. The divergent age-related changes in memory for given and beyond information and for feminine and masculine information were interpreted in terms of a developmental approach to schema theory and the lifespan psychology notions of selective optimization with compensation and loss in the service of growth. The present study suggests an integration between the domains of personality and cognitive psychology as one avenue for future research that could lead to a more complete understanding of memory and aging.

The time required to identify a common object depends on several factors, especially pre-existing knowledge and episodic representations newly established as a result of a prior study. My research examined how these factors contribute to identification of objects (both studied and non-studied) and to performance on explicit memory tests. The overall goal was to explore the link between memory and object perception. One series of experiments examined influences due to object orientation in the plane of the page. Subjects were shown color photos of objects, and memory was assessed either with an old/new recognition test or with a test that required them to identify objects that were slowly faded in on a computer monitor. The critical variables were the type of photo — each showing either an object with a predominant or cardinal orientation (e.g., helicopter) or a non-cardinal object (e.g., pencil), and the orientation at which the photos were displayed at study and at test (e.g., rotated 0°, 120°, or 240°). For non-studied targets, identification test performance showed a large effect due to display orientation, but only for cardinal objects. For studied targets, study-to-test changes in orientation influenced priming for both non-cardinal and cardinal objects, but orientation specific priming effects (larger priming when study and test orientations matched rather than mismatched) were much larger with cardinal than non-cardinal objects, especially, when their display orientation, at test was unusual (i.e., 120°, 240°). A second series of experiments examined influences due to object size (size of an object presented alone) and relative size (size of an object relative to another object). Size manipulations had a large effect on identification of non-studied objects but study-to- test changes in size had only a minimal effect on priming. In contrast, study1to-test changes in relative size influenced recognition decision speed which is an index of priming. The combined findings suggest that both semantic and episodic representations behave as if they coded orientation but only for cardinal objects. They also suggest that episodic representations code relative size but not size information. The findings are explained by the instance views of memory.

No description available.

In an immediate serial recall task, participants are asked to recall lists of items in order. In the Hebb repeating-list variant of the task, subjects are read a series of lists, and every third list is repeated. Performance improves across repetitions but is stable for the non-repeated trials. The repetition advantage—the increased accuracy for the repeated list—is known as the Hebb effect. Several models have been advanced to explain how participants order successive items, but how participants take advantage of the repetition has largely been ignored. Although the task is usually discussed in terms of recall of verbal items, the Hebb effect has been observed with sequences of visuo-spatial positions. The present work assesses whether immediate serial recall of verbal material benefits from visuo-spatial context. Sequences of letters were presented in different spatial positions in a visual version of the Hebb task. Presenting lists in random spatial positions on the periphery of an imaginary circle did not boost performance, but if the sequence was predictable, overall accuracy increased. The spatial path of successive items influenced the Hebb effect. When the distance between successive positions was minimized, participants were able to exploit the repetition early in practice. The results deny an account based on item distinctiveness. I discuss the results in terms of contemporary models of ISR. / Thesis (Master, Psychology) -- Queen's University, 2007-10-03 13:01:00.716