Test link to anchor on pubs page: Addis and Kahana, 2004
Introduction to the Main Research Areas
Research in the CML revolves around the study of human memory, combining approaches from traditional experimental psychology, computational modeling, and experimental neuroscience. The laboratory study of human memory began more than a century ago, and although many behavioral phenomena have been discovered and well characterized, the development of predictive quantitative theories is still in its infancy. Our research aims to develop and test theories that address both behavioral and physiological data on human memory function.
Much of our recent work aims to understand the physiological underpinnings of human memory function. This work uses both invasive and non-invasive brain recordings. Invasive recordings (intracranial EEG, or iEEG) are taken from patients undergoing treatment for epilepsy or brain tumors to study the physiological correlates of memory function. Because iEEG recordings are taken for clinical purposes, and there is minimal risk associated with performing cognitive tasks, this approach provides a unique opportunity to study the neurobiology of human cognitive function. Non-invasive recordings of electrical activity at the scalp (scalp EEG) provides an excellent compliment to our iEEG research. Whereas intracranial EEG can only be recorded from clinical populations who may exhibit slightly different brain function, scalp EEG can be recorded from normal, healthy young adults. By comparing the highly localized, artifact-free intracranial potentials with those obtained at the scalp, we are able to learn more about the electrophysiological correlates of memory than we would using either method alone.
The following three pages describe research being conducted in the CML within three specific problem domains:
- ["Mechanisms of Episodic Memory"],
- ["Navigational Spatial Memory"], and
- ["Recognition Memory"].
Given the complexity of human memory, its reliance on a variety of brain structures and mechanisms, and its relevance to so many distinct, yet interrelated, facets of human experience, a multipronged approach to its study seems most profitable. Such an approach allows insights and methods developed within one domain to inform the others.
Current Research Projects
The following sections outline the major projects that are ongoing in the CML. A listing of recent publications, along with PDFs of the papers and raw data, can be found on the lab website. Major effort is currently directed toward the study of verbal episodic memory, as measured using free recall, serial recall and item recognition tasks, as well as both verbal and visual working memory, and spatial navigation. To increase our understanding of these aspects of human memory we employ a combination of electrophysiological, computational, and behavioral methods. The electrophysiological methods include iEEG recordings from neurosurgical patients and scalp EEG recordings from healthy young adults. For our scalp-recording studies at UPenn, we are using a 128 channel EGI system. Currently, our intracranial studies are being carried out at the following hospitals: Children's Hospital, Boston, Brigham & Woman's Hospital, Boston, Freiburg Epilepsy Center in Germany, UCLA medical center in Los Angeles, Children's Hospital of Philadelphia, the Hospital of the University of Pennsylvania (HUP) and the Mayo Clinic in Rochester, MN. At UCLA and the Mayo clinic we record both single cell responses and local-field potentials; at all of the other hospitals we record local field potentials. We are also setup to record single cell responses at HUP when the next appropriate patient becomes available.
iEEG and Scalp EEG Projects
Three main experimental paradigms are currently being run with both iEEG and scalp EEG recordings. Free Recall involves studying a list of words and then, after a brief filled delay, trying to recall them in any order. YellowCab II is our spatial navigation game, whereby we can study the behavioral and electrophysiological correlates of memory during spatial navigation. Finally, multi-modal Sternberg (MMS) involves the presentation of short lists consisting of verbal or non-verbal stimuli (e.g., words, letters, faces, or sinusoidal gratings); subjects then make recognition judgments on these lists, enabling us to compare working memory for both verbal and non-verbal stimuli. In addition to the three currently active paradigms, we have also amassed a large dataset on the Sternberg task with consonants as stimuli, and a small dataset on paired associate memory. The following is a brief description of each of the three main paradigms.
Free recall is the standard test of episodic memory. Subjects study a list of words and then try to recall as many words as they can in any order. Analyzing iEEG data, Sederberg et al (2003,2006) found that and gamma (> 32-80 Hz) oscillations increase, while alpha/beta (8-20 Hz) oscillations decrease during the encoding of a word that is later recalled. The increase in gamma oscillations during successful encoding is primarily seen in two brain regions: the left inferior prefrontal cortex and the lateral temporal cortex (Sederberg et al., 2006). In an experiment investigating temporal/semantic interactions as a guide for recall, Sederberg et al. (in preparation) have shown that ERP activity at time of encoding can predict whether items will later be recalled by way of temporal and/or semantic associations. Largest ERP increases are seen in the frontal regions when forming an association between nearby and semantically similar words, as well as in the posterior regions when forming an association between distant and semantically similar words. "Followup free recall studies are begin carried out by Dr. Sean Polyn, Ilana Jerud, Dov Kogen, Vadim Koshkin, Neal Morton"
Building on previous work by Dr. Jeremy Caplan, Dr. Arne Ekstrom, Ehren Newman, and Igor Korolev we are continuing our studies of spatial navigation using both iEEG and scalp EEG recording methods (for more information, see section 2.2). Josh Jacobs is analyzing YellowCab data to better understand the relation of cellular activity and brain oscillations during navigation. Matt Mollison and Igor Korolev are analyzing scalp EEG data collected during YellowCab. Dr. Christoph Weidemann is now beginning new analyses of our Yellow Cab datatsets. Jonathan Miller is working with Dr. Sean Polyn on a project designed to assess the role of temporal and spatial factors in memory retrieval using a new variant of the Yellow Cab task. We continue to work with Arne Ekstrom and Itzhak Fried at UCLA, where we are collecting single-unit recordings and local field potentials from patients with epilepsy. "Dr. Sean Polyn, Dr. Christoph Weidemann, Josh Jacobs, Matt Mollison, Jonathan Miller, Jeremy Manning"
Visual and Spatial Working Memory
Previous work in the lab has investigated the role of brain oscillations in short term memory for linguistic materials, such as letters (Raghavachari et al., 2001,2006; Rizzuto et al. 2003, 2005; Jacobs et al, 2006), and more recently, comparing the oscillatory correlates of working memory for verbal and nonverbal materials (see Hwang et al, 2005). Marieke van Vugt is furthering this line of research by examining the physiological correlates of summed-similarity and proactive interference in working memory for verbal and nonverbal stimuli, using both scalp EEG and iEEG data. Marieke's work is building on the NEMO model of Kahana and Sekuler (2002).
In this project, we examine whether human brain oscillations function as a broad neural timing signal. We find that the spiking of phase-locked neurons occurs at particular phases of neuronal oscillations at various frequencies. Phase-locked neurons activate at various phases of theta and delta oscillations (1-10 Hz), but spike primarily at the trough of gamma oscillations (30-100 Hz). These findings show that brain oscillations facilitate precise spike timing in humans, and suggest that gamma and theta oscillations play complementary roles in this system.
Temporal Context Model (TCM)
Building on the original model proposed by Howard & Kahana (2002), Dr. Per Sederberg has developed a new version of TCM (called TCM-A) that uses a more realistic retrieval process to fit a variety of data on free recall, including data on response latencies and amnesia. Sederberg's work is currently being written up for publication.
Strength Based Memory Models
We have recently developed an extension of the SAM model called eSAM, that accounts for the role of semantic and pre-experimental information on episodic memory (see Sirotin, Kimball, & Kahana, 2005). This work is a collaboration with Dr. Dan Kimball, assistant professor of psychology at the University of Texas, Arlington, and Gene Sirotin, a former undergraduate who is pursuing a Ph.D. in Neuroscience at Columbia. Our latest work uses eSAM to account for data on the false memory (DRM) effect (Kimball et al., in revision).
Noisy Exemplar Model (NEMO)
NEMO was presented initially in Kahana and Sekuler (2002), and follow up work has been carried out by Nosofsky (2005), Kahana et al. (in press) and Yotsumoto (in press). Marieke is working on extending NEMO to account for inter-list interference effects (proactive interference). In addition, she works on trying to understand how NEMO could inform our analysis of brain oscillations during this task.
Manning, Kahana & Sekuler (submitted) proposed a model of spatial navigation in Yellow Cab and showed how it fit learning curves and environment difficulty in the studies reported by Newman et al. (2006) and Korolev and Mollison (in preparation).