Suzanna Becker

Remembering the past and imagining the future: A neural model of spatial memory and imagery

The authors model the neural mechanisms underlying spatial cognition, integrating neuronal systems and behavioral data, and address the relationships between long-term memory, short-term memory, and imagery, and between egocentric and allocentric and visual and ideothetic representations. Long-term spatial memory is modeled as attractor dynamics within medial-temporal allocentric representations, and short-term memory is modeled as egocentric parietal representations driven by perception, retrieval, and imagery and modulated by directed attention. Both encoding and retrieval/imagery require translation between egocentric and allocentric representations, which are mediated by posterior parietal and retrosplenial areas and the use of head direction representations in Papezs circuit. Thus, the hippocampus effectively indexes information by real or imagined location, whereas Papezs circuit translates to imagery or from perception according to the direction of view. Modulation of this translation by motor efference allows spatial updating of representations, whereas prefrontal simulated motor efference allows mental exploration. The alternating temporal-parietal flows of information are organized by the theta rhythm. Simulations demonstrate the retrieval and updating of familiar spatial scenes, hemispatial neglect in memory, and the effects on hippocampal place cell firing of lesioned head direction representations and of conflicting visual and ideothetic inputs.

Long-Term Semantic Priming: A Computational Account and Empirical Evidence

Semantic priming is traditionally viewed as an effect that rapidly decays. A new view of long-term word priming in attractor neural networks is proposed. The model predicts long-term semantic priming under certain conditions. That is, the task must engage semantic-level processing to a sufficient degree. The predictions were confirmed in computer simulations and in 3 experiments. Experiment 1 showed that when target words are each preceded by multiple semantically related primes, there is long-lag priming on a semantic-decision task but not on a lexical-decision task. Experiment 2 replicated the long-term semantic priming effect for semantic decisions with only one prime per target. Experiment 3 demonstrated semantic priming with much longer word lists at lags of 0, 4, and 8 items. These are the first experiments to demonstrate a semantic priming effect spanning many intervening items and lasting much longer than a few seconds.

A model of hippocampal neurogenesis in memory and mood disorders.

The mounting evidence for neurogenesis in the adult hippocampus has fundamentally challenged the traditional view of brain development. The intense search for clues as to the functional significance of the new neurons has uncovered a surprising connection between neurogenesis and depression. In animal models of depression, neurogenesis is reduced, whereas many treatments for depression promote neurogenesis. We speculate on why the hippocampus, traditionally viewed as a memory structure, might be involved in mood disorders, and what specific role the new neurons might have in the pathogenesis of and recovery from depression. The proposed role of neurogenesis in contextual-memory formation predicts a specific pattern of cognitive deficits in depression and has important implications for treatment of this highly prevalent and debilitating disorder.

Adult hippocampal neurogenesis reduces memory interference in humans: opposing effects of aerobic exercise and depression

Since the remarkable discovery of adult neurogenesis in the mammalian hippocampus, considerable effort has been devoted to unraveling the functional significance of these new neurons. Our group has proposed that a continual turnover of neurons in the DG could contribute to the development of event-unique memory traces that act to reduce interference between highly similar inputs. To test this theory, we implemented a recognition task containing some objects that were repeated across trials as well as some objects that were highly similar, but not identical, to ones previously observed. The similar objects, termed lures, overlap substantially with previously viewed stimuli, and thus, may require hippocampal neurogenesis in order to avoid catastrophic interference. Lifestyle factors such as aerobic exercise and stress have been shown to impact the local neurogenic microenvironment, leading to enhanced and reduced levels of DG neurogenesis, respectively. Accordingly, we hypothesized that healthy young adults who take part in a long-term aerobic exercise regime would demonstrate enhanced performance on the visual pattern separation task, specifically at correctly categorizing lures as “similar.” Indeed, those who experienced a proportionally large change in fitness demonstrated a significantly greater improvement in their ability to correctly identify lure stimuli as “similar.” Conversely, we expected that those who score high on depression scales, an indicator of chronic stress, would exhibit selective deficits at appropriately categorizing lures. As expected, those who scored high on the Beck Depression Inventory (BDI) were significantly worse than those with relatively lower BDI scores at correctly identifying lures as “similar,” while performance on novel and repeated stimuli was identical. Taken together, our results support the hypothesis that adult-born neurons in the DG contribute to the orthogonalization of incoming information.

UNSUPERVISED LEARNING PROCEDURES FOR NEURAL NETWORKS

Supervised learning procedures for neural networks have recently met with considerable success in learning difficult mappings. However, their range of applicability is limited by their poor scaling behavior, lack of biological plausibility, and restriction to problems for which an external teacher is available. A promising alternative is to develop unsupervised learning algorithms which can adaptively learn to encode the statistical regularities of the input patterns, without being told explicitly the correct response for each pattern. In this paper, we describe the major approaches that have been taken to model unsupervised learning, and give an in-depth review of several examples of each approach.

A Computational Model of Prefrontal Control in Free Recall: Strategic Memory Use in the California Verbal Learning Task

Several decades of research into the function of the frontal lobes in brain-damaged patients, and more recently in intact individuals using function brain imaging, has delineated the complex executive functions of the frontal cortex. And yet, the mechanisms by which the brain achieves these functions remain poorly understood. Here, we present a computational model of the role of the prefrontal cortex (PFC) in controlled memory use that may help to shed light on the mechanisms underlying one aspect of frontal control: the development and deployment of recall strategies. The model accounts for interactions between the PFC and medial temporal lobe in strategic memory use. The PFC self-organizes its own mnemonic codes using internally derived performance measures. These mnemonic codes serve as retrieval cues by biasing retrieval in the medial temporal lobe memory system. We present data from three simulation experiments that demonstrate strategic encoding and retrieval in the free recall of categorized lists of words. Experiment 1 compares the performance of the model with two control networks to evaluate the contribution of various components of the model. Experiment 2 compares the performance of normal and frontally lesioned models to data from several studies using frontally intact and frontally lesioned individuals, as well as normal, healthy individuals under conditions of divided attention. Experiment 3 compares the models performance on the recall of blocked and unblocked categorized lists of words to data from Stuss et al. (1994) for individuals with control and frontal lobe lesions. Overall, our model captures a number of aspects of human performance on free recall tasks: an increase in total words recalled and in semantic clustering scores across trials, superiority on blocked lists of related items compared to unblocked lists of related items, and similar patterns of performance across trials in the normal and frontally lesioned models, with poorer overall performance of the lesioned models on all measures. The model also has a number of shortcomings, in light of which we suggest extensions to the model that would enable more sophisticated forms of strategic control.

Computational modeling and empirical studies of hippocampal neurogenesis-dependent memory: Effects of interference, stress and depression

Prolonged stress causes dysregulation in the hypothalamic-pituitary-adrenal axis and may contribute to the pathogenesis of major depressive disorder (MDD). MDD is associated with pathological changes in several brain regions, particularly the prefrontal cortex and hippocampus. Evidence from animal research suggests that one of the earliest signs of pathological change after exposure to stress is a reduction in hippocampal neurogenesis. We therefore sought to test the prediction that people in the earliest stages of a first episode of depression would show selective memory deficits on neurogenesis-dependent tasks. Our computational model predicts that new neurons are important for representing distinct contexts; thus, when overlapping memories are learned over an interval of several days, during which time some neuronal turnover has taken place, the neurogenesis should reduce the potential for interference between the overlapping memories. At much shorter time scales, within the span of a single memory episode, rather than contributing to pattern separation, neurogenesis might play more of an integrative role in mediating contextual associative learning. Consistent with this, empirical evidence from animal studies suggests a role for the new neurons in forming complex event memories that bridge across time delays. This leads us to predict selective memory deficits on putative neurogenesis-dependent tasks in the earliest pre-clinical stages of a first episode of depression, before a clinical diagnosis has been made and prior to the development of more serious pathological brain changes. We present the results of new simulations with the model, lending further support to the prediction that neurogenesis reduces interference when memory events are separated by several days. We also report findings from an empirical study in which we tested a large number of undergraduates on a set of cognitive and memory tests from the CANTAB battery, and also administered neuropsychological inventories for stress, depression and anxiety. One of the subtests in the CANTAB battery, the delayed match to sample (DMS) task, was of particular interest as delayed non-match to sample has been found in animal studies to be dependent upon neurogenesis. Our empirical results indicate that as predicted, participants scoring high on the Beck Depression Inventory show a selective deficit on the DMS at long delays while performing on par with non-depressed participants on all other tasks. The potential to detect very early signs of major depression using simple neurogenesis-dependent cognitive tests could have important implications for the diagnosis and treatment of this debilitating and highly prevalent disorder.

Unsupervised Neural Network Learning Procedures for Feature Extraction and Classification

In this article, we review unsupervised neural network learning procedures which can be applied to the task of preprocessing raw data to extract useful features for subsequent classification. The learning algorithms reviewed here are grouped into three sections: information-preserving methods, density estimation methods, and feature extraction methods. Each of these major sections concludes with a discussion of successful applications of the methods to real-world problems.

A spiking neuron model of cortical correlates of sensorineural hearing loss: spontaneous firing, synchrony, and tinnitus

Hearing loss due to peripheral damage is associated with cochlear hair cell damage or loss and some retrograde degeneration of auditory nerve fibers. Surviving auditory nerve fibers in the impaired region exhibit elevated and broadened frequency tuning, and the cochleotopic representation of broadband stimuli such as speech is distorted. In impaired cortical regions, increased tuning to frequencies near the edge of the hearing loss coupled with increased spontaneous and synchronous firing is observed. Tinnitus, an auditory percept in the absence of sensory input, may arise under these circumstances as a result of plastic reorganization in the auditory cortex. We present a spiking neuron model of auditory cortex that captures several key features of cortical organization. A key assumption in the model is that in response to reduced afferent excitatory input in the damaged region, a compensatory change in the connection strengths of lateral excitatory and inhibitory connections occurs. These changes allow the model to capture some of the cortical correlates of sensorineural hearing loss, including changes in spontaneous firing and synchrony; these phenomena may explain central tinnitus. This model may also be useful for evaluating procedures designed to segregate synchronous activity underlying tinnitus and for evaluating adaptive hearing devices that compensate for selective hearing loss.

Adult hippocampal neurogenesis and memory interference

Rats, subjected to low-dose irradiation that suppressed hippocampal neurogenesis, or a sham treatment, were administered a visual discrimination task under conditions of high, or low interference. Half of the rats engaged in running activity and the other half did not. In the non-runners, there was no effect of irradiation on learning, or remembering the discrimination response under low interference, but irradiation treatment increased their susceptibility to interference, resulting in loss of memory for the previously learned discrimination. Irradiated rats that engaged in running activity exhibited increased neuronal growth and protection from memory impairment. The results, which show that hippocampal cells generated in adulthood play a role in differentiating between conflicting, context-dependent memories, provide further evidence of the importance of neurogenesis in hippocampus-sensitive memory tasks. The results are consistent with computational models of hippocampal function that specify a central role for neurogenesis in the modulation of interfering influences during learning and memory.