bluuebunny/crossref_metadata_2025_split huggingface dataset:
Predicting Epistasis from Mathematical Models
Authors: Heckendorn Robert B. , Whitley Darrell 
Published: February 2008

Classically, epistasis is either computed exactly by Walsh coefficients or estimated by sampling. Exact computation is usually of theoretical interest since the computation typically grows exponentially with the number of bits in the domain. Given an evaluation function, epistasis also can be estimated by sampling. However this approach gives us little insight into the origin of the epistasis and is prone to sampling error. This paper presents theorems establishing the bounds of epistasis for problems that can be stated as mathematical expressions. This leads to substantial computational savings for bounding the difficulty of a problem. Furthermore, working with these theorems in a mathematical context, one can gain insight into the mathematical origins of epistasis and how a problem's epistasis might be reduced. We present several new measures for epistasis and give empirical evidence and examples to demonstrate the application of the theorems. In particular, we show that some functions display “parity” such that by picking a well-defined representation, all Walsh coefficients of either odd or even index become zero, thereby reducing the nonlinearity of the function.
Static and Dynamic Aspects of Cortical Somatotopy: A Critical Evaluation
Authors: Killackey Herbert P. 
Published: February 2008

Abstract
The demonstration that functional somatotopic maps within the adult neocortex undergo some degree of reorganization following peripheral injury has aroused considerable interest. The evidence for such reorganization in the rat and monkey is reviewed and it is concluded that in both species there is good evidence for limited functional map reorganization in the adult neocortex following peripheral injury. The significance of such functional map reorganization, particularly in terms of whether or not cortical maps are continuously modifiable throughout life, is discussed. It is concluded that the current evidence for map reorganization is best interpreted in terms of the unmasking of preexisting neuronal circuits rather than as evidence of dynamic cortical selection processes.
Is Word Recognition Automatic? A Cognitive-Anatomical Approach
Authors: Posner Michael I. , Sandson Jennifer , Dhawan Meena , Shulman Gordon L. 
Published: February 2008

Abstract
What are the implications of anatomical localization of component mental operations for cognitive models? In this paper we use the anatomical localizations of visual and auditory word processing that were previously reported from PET studies (Petersen, Fox, Posner, Mintun & Raichle, 1988. We hypothesize that two operations performed simultaneously by the same or heavily interconnected anatomical areas will produce specific interference. One task is repeating back (shadowing) auditory words as quickly as possible. This task is shown to interfere with shifts of visual attention in the direction of peripheral cues. Both tasks are known to require common attentional operations localized to the medial frontal lobe. The shadowing task does not interfere with operations involving priming of a visual word form. This kind of priming involves areas of the ventral occipital lobe not used during shadowing. Finally, both shadowing and semantic priming involve anterior semantic and intentional areas. Accordingly, they can interfere. The conditions under which they produce interference suggest that the interference involves operations performed by the anterior attention system. These experiments support the idea that words automatically activate visual word forms, but involve shared attentional systems for higher level processes.
Derivation of Encoding Characteristics of Layer II Cerebral Cortex
Authors: Granger Richard , Ambros-Ingerson José , Lynch Gary 
Published: February 2008

Abstract
Computer simulations of layers I and II of pirifonn (olfactory) cortex indicate that this biological network can generate a series of distinct output responses to individual stimuli, such that different responses encode different levels of information about a stimulus. In particular, after learning a set of stimuli modeled after distinct groups of odors, the simulated network's initial response to a cue indicates only its group or category, whereas subsequent responses to the same stimulus successively subdivide the group into increasingly specific encoding of the individual cue. These sequences of responses amount to an automated organization of perceptual memories according to both their similarities and differences, facilitating transfer of learned information to novel stimuli without loss of specific information about exceptions. Human recognition performance robustly exhibits such multiple levels: a given object can be identified as a vehicle, as an automobile, or as a Mustang. The findings reported here suggest that a function as apparently complex as hierarchical recognition memory, which seems suggestive of higher ‘cognitive’ processes, may be a fundamental intrinsic property of the operation of this single cortical cell layer in response to naturally-occurring inputs to the structure. We offer the hypothesis that the network function of superficial cerebral conical layers may simultaneously acquire and hierarchically organize information about the similarities and differences among perceived stimuli. Experimental manipulation of the simulation has generated hypotheses of direct links between the values of specific biological features and particular attributes of behavior, generating testable physiological and behavioral predictions.
Timing Functions of The Cerebellum
Authors: Ivry Richard B. , Keele Steven W. 
Published: February 2008

Abstract
This study investigated the effects of different types of neurological deficits on timing functions. The performance of Parkinson, cerebellar, cortical, and peripheral neuropathy patients was compared to age-matched control subjects on two separate measures of timing functions. The first task involved the production of timed intervals in which the subjects attempted to maintain a simple rhythm. The second task measured the subjects' perceptual ability to discriminate between small differences in the duration of two intervals.
The primacy of the cerebellum in timing functions was demonstrated by the finding that these were the only patients who showed a deficit in both the production and perception of timing tasks. The cerebellar group was found to have increased variability in performing rhythmic tapping and they were less accurate than the other groups in making perceptual discriminations regarding small differences in duration. Critically, this perceptual deficit appears to be specific to the perception of time since the cerebellar patients were unaffected in a control task measuring the perception of loudness.
It is argued that the operation of a timing mechanism can be conceptualized as an isolable component of the motor control system. Furthermore, the results suggest that the domain of the cerebellar timing process is not limited to the motor system, but is employed by other perceptual and cognitive systems when temporally predictive computations are needed.
Positron Emission Tomographic Studies of the Processing of Singe Words
Authors: Petersen Steven E. , Fox Peter T. , Posner Michael I. , Mintun Mark , Raichle Marcus E. 
Published: February 2008

Abstract
PET images of blood flow change that were averaged across individuals were used to identify brain areas related to lexical (single-word) processing, A small number of discrete areas were activated during several task conditions including: modality-specific (auditory or visual) areas activated by passive word input, primary motor and premotor areas during speech output, and yet further areas during tasks making semantic or intentional demands.
Why are “What” and “Where” Processed by Separate Cortical Visual Systems? A Computational Investigation
Authors: Rueckl Jay G. , Cave Kyle R. , Kosslyn Stephen M. 
Published: February 2008

Abstract
In the primate visual system, the identification of objects and the processing of spatial information are accomplished by different cortical pathways. The computational properties of this “two-systems” design were explored by constructing simplifying connectionist models. The models were designed to simultaneously classify and locate shapes that could appear in multiple positions in a matrix, and the ease of forming representations of the two kinds of information was measured. Some networks were designed so that all hidden nodes projected to all output nodes, whereas others had the hidden nodes split into two groups, with some projecting to the output nodes that registered shape identity and the remainder projecting to the output nodes that registered location. The simulations revealed that splitting processing into separate streams for identifying and locating a shape led to better performance only under some circumstances. Provided that enough computational resources were available in both streams, split networks were able to develop more efficient internal representations, as revealed by detailed analyses of the patterns of weights between connections.
Regulation of Synaptic Molecular Architecture in a Rat Sympathetic Ganglion and Hippocampus
Authors: Wu K. , Black I.B. 
Published: February 2008

AbstractExtensive evidence suggests that the synapse, the communicative organelle between neurons, plays a pivotal role in learning and memory. To begin defining epigenetic factors that potentially regulate molecular structure of the synapse, we have been studying a relatively simple model system, the rat sympathetic, superior cervical ganglion. Initially, we focused on the postsynaptic density (PSD), a disc-shaped structure in the postsynaptic neuron (see cover illustration). Previously, we found that trans-synaptic impulse activity regulates the predominant PSD protein molecule (PSDp). We now examine two related questions. Do other factors influence c synaptic structure independent of presynaptic innervation? Conversely, does denervation alter synaptic molecular structure in the hippocampus, as in the ganglion? Our studies indicate that the trophic protein, nerve growth factor, that governs sympathetic development and mature function, regulates the PSDp in normal and denervated ganglia. Consequently, synaptic structure in the periphery is, indeed, regulated by multiple factors. In the brain, fimbria-fornix lesions, which partially denervate the hippocampus, significantly reduce the hippocampal PSDp. We conclude that presynaptic innervation regulates synaptic structure in the hippocampus, as well as the periphery. More generally, epigenetic factors apparently regulate synaptic structure, potentially providing a molecular mechanism for information storage at the synapse.
A Simple Model of Prefrontal Cortex Function in Delayed-Response Tasks
Authors: Dehaene Stanislas , Changeux Jean-Pierre 
Published: February 2008

Abstract
Both psychologists and neurobiologists have used delayed response (DR), AB, and delayed matching-to-sample (DMS) tasks as tools to study the functions of prefrontal cortex in primates and humans. We describe a simulation model that relates behavioral and electrophysiological-data relevant to these tasks into a minimal neural network.
The inputs to the network are two visual objects and a positive or negative reinforcement signal. As the output, the network orients toward one of the two objects. We subdivide the architecture of the network into two levels, both of which embody constraints from neuroanatomy in a simplified form. Level 1 consists of a sensory-motor loop with modifiable synaptic weights and provides a capacity for grasping. Level 2 contains memory and rule-coding units and modulates the lower level 1. When level 1 only is simulated, the network fails to learn the tasks. The errors made by the network resemble those of young monkeys, infants, or adults with prefrontal lesions. In particular, the systematic AB error can be reproduced. With level 2 on top of level 1, the network acquires systematic rules of behavior by mere reinforcement and rapidly adapts to changes in the reinforcement schedule. Learning takes place by selection among a repertoire of possible rules. The properties of the model are discussed in terms of actual behavioral and physiological data, and several critical experimental predictions are presented. In particular, we address the issues of prefrontal functions, “systematicity” in neural networks, and “mental Darwinism.”
Temporal Constraints on Language Processing: Syntactic Priming in Broca's Aphasia
Authors: Friederici Angela D. , Kilborn Kerry 
Published: February 2008

Abstract
This study tests the hypothesis that agrammatic comprehension is due to a computational rather than a structural language deficit. It is claimed that grammatic Broca's aphasics do not meet the temporal constraints in the activation of different types of linguistic information necessary for normal parsing. These temporal constraints are investigated in two experiments using a crossmodal syntactic priming paradigm. Each experiment tests the effect on recognition of grammatical versus ungrammatical links between an auditory sentence fragment (the prime) and a visually presented word (the target). The experiments differ in the interstimulus interval (ISI i.e., the amount of time provided between the offset of the auditory prime and the onset of the visuai target. Experiment 1, with an ISI of 0 msec, reveals that Broca patients — much like the control groups — show a grammaticality effect. In contrast to normal students and age matched controls, however, these patients are in general much slower in making lexical decisions when auditory context is present as compared to when visual targets are presented in isolation. Experiment 2 with an ISI of 200 msec demonstrates that when additional time is given to process the syntactic context, Broca's aphasics in contrast to age matched controls show faster decision times on the target than when the ISI is 0 msec. The results are in accord with the view that agrammatic Broca's aphasics have not lost their syntactic knowledge, but that their a grammatic comprehension behavior is due to the inability to process the linguistic information within a given time frame. More generally, the findings suggest that language processing can break down once activation of different types of linguistic information does not follow the normal time pattern.
Comparisons between Active Properties of Distal Dendritic Branches and Spines: Implications for Neuronal Computations
Authors: Shepherd Gordon M. , Woolf Thomas B. , Carnevale Nicholas T. 
Published: February 2008

Abstract
The specific contributions of distal dendrites to the computational properties of cortical neurons are little understood and are completely ignored in most network simulations of higher brain functions. Compartmental models, based on realistic estimates of morphology and physiology, provide a means for exploring these contributions. We have pursued analysis of a model of synaptic integration in a distal dendrite bearing four spines, using a new general-purpose simulation program called SABER. We have analyzed this model under the assumption that the dendrite contains sites of impulse-generating membrane, and we have compared its responses to synaptic activation with the case of impulse-generating membrane located instead in the spine heads, as previously reported. Both types of models generate basic logic operations, such as AND, OR, and AND-NOT gates. Active spine heads require lower excitatory synaptic conductances, but active branch segments lead to larger responses in the soma. The transients recorded near the soma give no evidence of their origin in either active branch or active spines, indicating that the interpretation of experimental recordings with regard to sites of distal active responses must be viewed with caution. The results suggest the hypothesis that a hierarchy of logic operations is virtually inherent in the branching structure of dendritic trees of cortical pyramidal neurons. Inclusion of these properties in representations of cortical neurons would greatly enhance the computational power of neural networks aimed at simulating higher brain functions.
The Origins of Neocortex: Connections and Lamination as Distinct Events in Evolution
Authors: Karten Harvey J. , Shimizu Toru 
Published: February 2008

Abstract
In the nonmammalian telencephalon, there are neuronal populations corresponding to cell groups in the neocortex of mammals in terms of connections, single unit-responses, chemical content, and functions. Some of these populations in nonmammals, however, are organized in a nonlaminar, rather than laminar fashion. These observations may prompt a reassessment of the functional roles of lamination and the evolutionary origins of the mammalian neocortex. Thus, the role of neural circuits and laminar organization can be differentiated in order to understand the cognitive functions of the neocortex. Moreover, the origins of neocortex can be separable into the precursors of nonlaminar and laminar regions.
Microstimulation of a Neural-Network Model for Visually Guided Saccades
Authors: Goodman Sabrina J. , Andersen Richard A. 
Published: February 2008

Abstract
Microstimulation of many saccadic centers in the brain produces eye movements that are not consistent with either a strictly retinal or strictly head-centered coordinate coding of eye movements. Rather, stimulation produces some features of both types of coordinate coding. Recently we demonstrated a neural network model that was trained to localize the position of visual stimuli in head-centered coordinates at the output using inputs of eye and retinal position similar to those converging on area 7a of the posterior parietal cortex of monkeys (Zipser & Andersen 1988; Andersen & Zipser 1988). Here we show that microstimulation of this trained network, achieved by fully activating single units in the middle layer, produces “saccades” that are very much like the saccades produced by stimulating the brain. The activity of the middle-layer units can be considered to code the desired location of the eyes in head-centered coordinates; however, stimulation of these units does not produce the saccades predicted by a classical head-centered coordinate coding because the location in space appears to be coded in a distributed fashion among a population of units rather than explicitly at the level of single cells.
Transformation from Head- to Shoulder-Centered Representation of Target Direction in Arm Movements
Authors: Soechting J. F. , Tillery S. I. H. , Flanders M. 
Published: February 2008

Abstract
In a previous study (Soechting and Flanders 1990a) we suggested that subjects used a coordinate system centered at the shoulder while pointing to targets in extrapersonal space. In particular, we suggested that this coordinate system was used to define target location in terms of its distance and the direction from the shoulder. In this paper we examine this suggestion in more detail. We show that when subjects make errors in the distance of a pointing movement, the computed errors in direction will depend on the origin of the coordinate system chosen to measure direction. From an analysis of the computed error, we estimate the origin of each subject's coordinate system. We artificially induced large errors in pointing distance by asking subjects to point half-way to a target on a line from the shoulder or from the head, that is, in directions from two possible centers. The subjects' performance on both these tasks was comparable to the performance of subjects asked to point directly to the target. From this finding we argue that there exists both a head-centered and a shoulder-centered representation of target location within the central nervous system.
Genetics and Epigenetics of Neural Function: A Model
Authors: Kerszberg Michel 
Published: February 2008

Abstract
A neural model for one- or few-trial irreversible behavior learning such as occurs in imprinting is introduced. It is assumed that synaptic connections in the relevant parts of the central nervous system are initially set up in a largely, but not totally random fashion, as a result, for instance, of differential cell–cell adhesion. The behavior to be learned is then sometimes exhibited, but not in a reproducible, mature way. During early neural activity, active postsynaptic neurons may, however, deliver a putative retrograde trophic factor to some of their afferent synaptic boutons. This is taken to occur according to a Hebb-type rule. At a later stage, only those synapses that have accumulated enough trophic factor are stabilized selectively. We show explicitly how this process may lead to a perfectly wired circuit. The calculations indicate that if the connections were relatively well defined from the beginning, then random pulses at the inputs suffice for this refinement process to take place. This is analogous to the maturation of neural circuits under spontaneous electrical activity (unsupervised learning). If the initial connections are “fuzzy,” however, well-defined patterns of activation are needed at the inputs so that selective stabilization leads to a correct functional system (the model now behaves in an instructionist mode). Experiments suggested by the model are discussed, and involve the manipulation of afferent inputs, of the initial synapse distribution, or of the stabilization phase.
Effects of Ibotenic Lesions of Mammillary Bodies on Spontaneous and Rewarded Spatial Alternation in Mice
Authors: Beracochea Daniel J. , Jaffard Robert 
Published: February 2008

Abstract
Previous experiments in our laboratory have shown that lesions of the mammillary bodies (MB) in mice induce memory deficits in spontaneous spatial alternation (SA) tasks. However, it has also been shown that performance on memory tasks by both humans and animals depends on the degree of effort involved in a given task. Thus, we hypothesized that the involvement of MB-damaged subjects in tasks that require more effortful processing than does SA might reduce their memory deficits. Accordingly, in the first experiment, the effects of MB lesions were studied in both an SA and a rewarded alternation (RA) task. Daily test sessions consisted of six successive trials separated by a short intertrial interval (ITI) of 30 sec, which allowed us to study interference (i.e., the deleterious effect of the first trials of a session on the performance on the last trials of a session). Results of the first experiment showed that the MB-lesioned subjects were impaired on their performance on the SA (they were more sensitive to interference than were controls) but not RA task. This finding is consistent with our hypothesis. In the second experiment, the ITI in the RA task was increased from 50 sec to 3 min. At the longer ITI (3 min), we found that the MB-lesioned subjects had the same deficit in performance that the MB-lesioned subjects had on the SA task at an ITI of 30 sec. Control mice also had a similar deficit in performance at the ITI of 5 min. We suggest that even in the RA task, the performance of MB-damaged subjects remains more sensitive to time-dependent interference than does the performance of controls. The significance of our study to the understanding of amnesia (especially Korsakoff's syndrome and frontal lobe pathology) is discussed.