Norbert J Fortin

University of California, Irvine | UCI · Department of Neurobiology and Behavior

Multivariate time series data appear often as realizations of non-stationary processes where the covariance matrix or spectral matrix smoothly evolve over time. Most of the current approaches estimate the time-varying spectral properties only retrospectively – that is, after the entire data has been observed. Retrospective estimation is a major lim...

The hippocampus is known to play a critical role in processing information about temporal context. However, it remains unclear how hippocampal oscillations are involved, and how their functional organization is influenced by connectivity gradients. We examined local field potential activity in CA1 as rats performed a nonspatial odor sequence memory...

The hippocampus is critical to the temporal organization of our experiences. Although this fundamental capacity is conserved across modalities and species, its underlying neuronal mechanisms remain unclear. Here we recorded hippocampal activity as rats remembered an extended sequence of nonspatial events unfolding over several seconds, as in daily...

The standard approach to analyzing brain electrical activity is to examine the spectral density function (SDF) and identify frequency bands, defined a priori, that have the most substantial relative contributions to the overall variance of the signal. However, a limitation of this approach is that the precise frequency and bandwidth of oscillations...

Multivariate time series data appear often as realizations of nonstationary processes where the covariance matrix or spectral matrix smoothly evolve over time. Most of the current approaches estimate the time-varying spectral properties only retrospectively - that is, after the entire data has been observed. Retrospective estimation is a major limi...

The standard approach to analyzing brain electrical activity is to examine the spectral density function (SDF) and identify predefined frequency bands that have the most substantial relative contributions to the overall variance of the signal. However, a limitation of this approach is that the precise frequency localization and bandwidth of oscilla...

We propose a novel regularized mixture model for clustering matrix‐valued data. The proposed method assumes a separable covariance structure for each cluster and imposes a sparsity structure (e.g., low rankness, spatial sparsity) for the mean signal of each cluster. We formulate the problem as a finite mixture model of matrix‐normal distributions w...

There is a strong interest in the neuroscience community to measure brain connectivity and develop methods that can differentiate connectivity across patient groups and across different experimental stimuli. The development of such statistical tools is critical to understand the dynamics of functional relationships among brain structures supporting...

The hippocampus is known to play a critical role in processing information about temporal context. However, it remains unclear how hippocampal oscillations are involved, and how their functional organization is influenced by connectivity gradients. We examined local field potential activity in CA1 as rats performed a complex odor sequence memory ta...

Dynamic functional connectivity, as measured by the time-varying covariance of neurological signals, is believed to play an important role in many aspects of cognition. While many methods have been proposed, reliably establishing the presence and characteristics of brain connectivity is challenging due to the high dimensionality and noisiness of ne...

The hippocampus is critical to the temporal organization of our experiences, including the ability to remember past event sequences and predict future ones. Although this fundamental capacity is conserved across modalities and species, its underlying neuronal mechanisms remain poorly understood. Here we recorded hippocampal ensemble activity as rat...

Considerable research in rodents and humans indicates the hippocampus and prefrontal cortex are essential for remembering temporal relationships among stimuli, and accumulating evidence suggests the perirhinal cortex may also be involved. However, experimental parameters differ substantially across studies, which limits our ability to fully underst...

It is well established that temporal organization is critical to memory, and that the ability to temporally organize information is fundamental to many perceptual, cognitive, and motor processes. While our understanding of how the brain processes the spatial context of memories has advanced considerably, our understanding of their temporal organiza...

Dynamic functional connectivity, as measured by the time-varying covariance of neurological signals, is believed to play an important role in many aspects of cognition. While many methods have been proposed, reliably establishing the presence and characteristics of brain connectivity is challenging due to the high dimensionality and noisiness of ne...

Modeling covariance (and correlation) matrices is a challenging problem due to the large dimensionality and positive-definiteness constraint. In this paper, we propose a novel Bayesian framework based on decomposing the covariance matrix into variance and correlation matrices. The highlight is that the correlations are represented as products of ve...

The hippocampus plays a critical role in the memory for sequences of events, a defining feature of episodic memory. To shed light on the fundamental mechanisms supporting this capacity, we recently recorded neural activity in CA1 as rats performed a nonspatial odor sequence memory task. Our main finding was that, while the animals' location and beh...

In this paper, our goal is to model functional and effective (directional) connectivity in network of multichannel brain physiological signals (e.g., electroencephalograms, local field potentials). The primary challenges here are twofold: first, there are major statistical and computational difficulties for modeling and analyzing high dimensional m...

Neuroscientists are increasingly collecting multimodal data during experiments and observational studies. Different data modalities—such as electroencephalogram, functional magnetic resonance imaging, local field potential (LFP) and spike trains—offer different views of the complex systems contributing to neural phenomena. Here, we focus on joint m...

Memory is one of the most studied cognitive abilities. Episodic memory, the capacity to remember personal experiences, has unquestionably increased the survival fitness of mammalian species, including humans. In fact, as animals live in a dynamic environment, the memory for unique experiences, organized in both space and time, has presumably evolve...

We propose an evolutionary state space model (E-SSM) for analyzing high dimensional brain signals whose statistical properties evolve over the course of a non-spatial memory experiment. Under E-SSM, brain signals are modeled as mixtures of components (e.g., AR(2) process) with oscillatory activity at pre-defined frequency bands. To account for the...

Unlabelled: The hippocampus is critical to the memory for sequences of events, a defining feature of episodic memory. However, the fundamental neuronal mechanisms underlying this capacity remain elusive. While considerable research indicates hippocampal neurons can represent sequences of locations, direct evidence of coding for the memory of seque...

Typical aging is associated with diminished episodic memory performance. To improve our understanding of the fundamental mechanisms underlying this age-related memory deficit, we previously developed an integrated, cross-species approach to link converging evidence from human and animal research. This novel approach focuses on the ability to rememb...

A critical feature of episodic memory is the ability to remember the order of events as they occurred in time, a capacity shared across species including humans, non-human primates and rodents. Accumulating evidence suggests this capacity depends on a network of structures including the hippocampus and the prefrontal cortex, but their respective co...

The overlap of neural circuits involved in episodic memory, relational learning, trace conditioning, and interval timing suggests the importance of hippocampal-dependent processes. Identifying the functional and neural mechanisms whereby the hippocampus plays a role in timing and decision-making, however, has been elusive. In this article we descri...

To shed light on the evolution of episodic memory, we recently integrated the behavioral evidence across species with a comparative analysis of the underlying neurobiology and neural mechanisms (1). We conclude that fundamental features of episodic memory are present in mammals and birds and that a similar neural circuit could support this capacity...

Episodic memory includes information about how long ago specific events occurred. Since most of our experiences have overlapping elements, remembering this temporal context is crucial for distinguishing individual episodes. The discovery of timing signals in hippocampal neurons, including evidence of “time cells” and of gradual changes in ensemble...

One prominent view holds that episodic memory emerged recently in humans and lacks a "(neo)Darwinian evolution" [Tulving E (2002) Annu Rev Psychol 53:1-25]. Here, we review evidence supporting the alternative perspective that episodic memory has a long evolutionary history. We show that fundamental features of episodic memory capacity are present i...

This chapter describes two different approaches to the comparative analysis of episodic memory. One approach begins with the literature on recognition memory in humans, where unique features of episodic recollection can be revealed in the dynamics of memory retrieval; it asks whether these features of episodic memory also exist in animal memory per...

The contributions of the hippocampus (HC) and perirhinal cortex (PER) to recognition memory are currently topics of debate in neuroscience. Here we used a rapidly-learned (seconds) spontaneous novel odor recognition paradigm to assess the effects of pre-training N-methyl-D-aspartate lesions to the HC or PER on odor recognition memory. We tested mem...

Here we describe a model of medial temporal lobe organization in which parallel "what" and "where" processing streams converge within the hippocampus to represent events in the spatio-temporal context in which they occurred; this circuitry also mediates the retrieval of context from event cues and vice versa, which are prototypes of episodic recall...

Here we review our development of an animal model of episodic memory and amnesia that employs a signal detection analyses to characterize recognition memory performance in rats. This approach aims to distinguish episodic recollection of studied items from mere familiarity for recently experienced stimuli, and then to examine the neural basis of the...

Recent findings indicate that, in humans, the hippocampal memory system is involved in the capacity to imagine the future as well as remember the past. Other studies have suggested that animals may also have the capacity to recall the past and plan for the future. Here, we will consider data that bridge between these sets of findings by assessing t...

The neurobiology of episodic memory is a particularly challenging problem. On the one hand, an understanding of the functional circuitry that underlies episodic memory depends crucially on experimental analyses using animals where the roles of candidate brain areas can be examined with highly selectively ablation and where memory representations ca...

In humans, recognition memory declines with aging, and this impairment is characterized by a selective loss in recollection of previously studied items contrasted with relative sparing of familiarity for items in the study list. Rodent models of cognitive aging have focused on water maze learning and have demonstrated an age-associated loss in spat...

A major controversy in memory research concerns whether recognition is subdivided into distinct cognitive mechanisms of recollection and familiarity that are supported by different neural substrates. Here we developed a new associative recognition protocol for rats that enabled us to show that recollection is reduced, whereas familiarity is increas...

The notion that non-human animals are capable of episodic memory is highly controversial. Here, we review recent behavioral work from our laboratory showing that the fundamental features of episodic memory can be observed in rats and that, as in humans, this capacity relies on the hippocampus. We also discuss electrophysiological evidence, from our...

In humans, episodic memory is most commonly defined as the subjective experience of recollection, presenting a major challenge to the identification of episodic memory in animals. Here we take the position that episodic memory also has several other distinctive qualities that can be assessed objectively in animals, as well as humans, and the examin...

Recognition memory may be supported by two independent types of retrieval, conscious recollection of a specific experience and a sense of familiarity gained from previous exposure to particular stimuli. In humans, signal detection techniques have been used to distinguish recollection and familiarity, respectively, in asymmetrical and curvilinear co...

Several recent studies have sought to develop animal models of episodic memory, the capacity to recollect unique personal experiences. However, these studies have not yet provided unambiguous evidence that this capacity is based on recollection of the learning episodes. A recent study that examined memory for the ordering of events within unique ex...

Recent models of hippocampal function emphasize its potential role in disambiguating sequences of events that compose distinct episodic memories. In this study, rats were trained to distinguish two overlapping sequences of odor choices. The capacity to disambiguate the sequences was measured by the critical odor choice after the overlapping element...

Recent models of hippocampal function emphasize the potential role of this brain structure in encoding and retrieving sequences of events that compose episodic memories. Here we show that hippocampal lesions produce a severe and selective impairment in the capacity of rats to remember the sequential ordering of a series of odors, despite an intact...