bluuebunny/crossref_metadata_2025_split huggingface dataset:
Distributed Representation of Limb Motor Programs in Arrays of Adjustable Pattern Generators
Authors: Berthier N. E. , Singh S. P. , Barto A. G. , Houk J. C. 
Published: February 2008

Abstract
This paper describes the current state of our exploration of how motor program concepts may be related to neural mechanisms. We have proposed a model of sensorimotor networks with architectures inspired by the anatomy and physiology of the cerebellum and its interconnections with the red nucleus and the motor cortex. We proposed the concept of rubrocerebellar and corticocerebellar information processing modules that function as adjustable pattern generators (APGs) capable of the storage, recall, and execution of motor programs. The APG array model described in this paper extends the single APG model of Houk et al. (1990) to an array of APGs whose collective activity controls movement of a simple two degree-of-freedom simulated limb. Our objective was to examine the APG array theory in a simple computational framework with a plausible relationship to anatomy and physiology. Results of simulation experiments show that the APG array model is capable of learning how to control movement of the simulated limb by adjusting distributed motor programs. Although the model is based on many simplifying assumptions, and the simulated motor control task is much simpler than an actual reaching task, these results suggest that the APG array model may provide a useful step toward a more comprehensive understanding of how neural mechanisms may generate motor programs.
Perseverative and Semantic Influences on Visual Object Naming Errors in Optic Aphasia: A Connectionist Account
Authors: Plaut David C. , Shallice Tim 
Published: February 2008

Abstract
Although perseveration—the inappropriate repetition of previous responses—is quite common among patients with neurological damage, relatively few detailed computational accounts of its various forms have been put forth. A particularly well-documented variety involves the pattern of errors made by “optic aphasic” patients, who have a selective deficit in naming visually presented objects. Based on our previous work in modeling impaired reading via meaning in deep dyslexia, we develop a connectionist simulation of visual object naming. The major extension in the present work is the incorporation of short-term correlational weights that bias the network towards reproducing patterns of activity that have occurred on recently preceding trials. Under damage, the network replicates the complex semantic and perseverative effects found in the optic aphasic error pattern. Further analysis reveals that the perseverative effects are strongest when the lesions are near or within semantics, and are relatively mild when the preceding object evokes no response. Like optic aphasics, the network produces predominantly semantic rather than visual errors because, in contrast to reading, there is some structure in the mapping from visual to semantic representations for objects. Viewed together with the dyslexia simulations, the replication of complex empirical phenomena concerning impaired visual comprehension based on a small set of general connectionist principles strongly suggests that these principles provide important insights into the nature of semantic processing of visual information and its breakdown following brain damage.
Image Segmentation Cues in Motion Processing: Implications for Modularity in Vision
Authors: Stoner Gene R. , Albright Thomas D. 
Published: February 2008

Abstract
The problem of processing visual motion is underconstrained—many possible real world motions are compatible with any given dynamic retinal image. Recent psychophysical and neurophysiological experiments have shown that the primate visual system's normally veridical interpretation of moving patterns is attained through utilization of image segmentation cues unrelated to motion per se. These findings challenge notions of modularity in which it is assumed that the processing of specific scene properties, such as motion, can be studied in isolation from other visual processes. We discuss the implications of these findings with regard to both experimental and computational approaches to the study of visual motion.
Improved Localizadon of Cortical Activity by Combining EEG and MEG with MRI Cortical Surface Reconstruction: A Linear Approach
Authors: Dale Anders M. , Sereno Martin I. 
Published: February 2008

Abstract
We describe a comprehensive linear approach to the problem of imaging brain activity with high temporal as well as spatial resolution based on combining EEG and MEG data with anatomical constraints derived from MRI images. The "inverse problem" of estimating the distribution of dipole strengths over the cortical surface is highly underdetermined, even given closely spaced EEG and MEG recordings. We have obtained much better solutions to this problem by explicitly incorporating both local cortical orientation as well as spatial covariance of sources and sensors into our formulation. An explicit polygonal model of the cortical manifold is first constructed as follows: (1) slice data in three orthogonal planes of section (needle-shaped voxels) are combined with a linear deblurring technique to make a single high-resolution 3-D image (cubic voxels), (2) the image is recursively flood-filled to determine the topology of the gray-white matter border, and (3) the resulting continuous surface is refined by relaxing it against the original 3-D gray-scale image using a deformable template method, which is also used to computationally flatten the cortex for easier viewing. The explicit solution to an error minimization formulation of an optimal inverse linear operator (for a particular cortical manifold, sensor placement, noise and prior source covariance) gives rise to a compact expression that is practically computable for hundreds of sensors and thousands of sources. The inverse solution can then be weighted for a particular (averaged) event using the sensor covariance for that event. Model studies suggest that we may be able to localize multiple cortical sources with spatial resolution as good as PET with this technique, while retaining a much finer grained picture of activity over time.
Attention-Related Modulation of Sensory-Evoked Brain Activity in a Visual Search Task
Authors: Luck Steven J. , Fan Silu , Hillyard Steven A. 
Published: February 2008

Abstract
When subjects are explicitly cued to focus attention on a particular location in visual space, targets presented at that location have been shown to elicit enhanced sensory-evoked activity in recordings of event-related brain potentials (ERPs). The present study sought to determine if this type of sensory facilitation also occurs during visual search tasks in which a feature conjunction target must be identified, presumably by means of focal attention, within an array of distractor items. In this experiment, subjects were required to discriminate the shape of a distinctively colored target item within an array containing 15 distractor items, and ERPs were elicited by task-irrelevant probe stimuli that were presented at the location of the target item or at the location of a distractor item on the opposite side of the array. When the delay between search-array onset and probe onset was 250 msec, the sensory-evoked responses in the latency range 75-200 msec were larger for probes presented at the location of the target than for probes presented at the location of the irrelevant distractor. These results indicate that sensory processing is modulated in a spatially restricted manner during visual search, and that focusing attention on a feature conjunction target engages neural systems that are shared with other forms of visual-spatial attention.
Preattentive Processes Guide Visual Search: Evidence from Patients with Unilateral Visual Neglect
Authors: Grabowecky Marcia , Robertson Lynn C. , Treisman Anne 
Published: February 2008

Abstract
Preattentive processes such as perceptual grouping are thought to be important in the initial guidance of visual attention and may also operate in unilateral neglect by contributing to the definition of a task-appropriate reference frame. We explored this question with a visual search task in which patients with unilateral visual neglect (5 with right-, 2 with left-hemisphere damage) searched a diamond-shaped matrix for a conjunction target that shared one feature with each of two distractor elements. Additional grouping stimuli appeared as flanks either on the left, right, or both sides of the central matrix, and significantly changed performance in the search task. As expected, when flanks appeared only on the ipsilesional side a decrement in search performance was observed, but the further addition of contralesional flanks actually reduced the decrement and returned performance to near baseline levels. These data suggest that flanking stimuli on the neglected contralesional side of visual space can influence the reference frame by grouping with task-relevant stimuli, and that this preattentive influence can be preserved in patients with unilateral visual neglect.
Express Saccades and Smooth Pursuit Eye Movement Function in Schizophrenic, Affective Disorder, and Normal Subjects
Authors: Sereno Anne B. , Holzman Philip S. 
Published: February 2008

Abstract
Saccadic and smooth pursuit eye movements were recorded in three groups of subjects: a schizophrenic group, a non-schizophrenic psychotic patient comparison group, and a normal control group. Schizophrenic subjects demonstrated a greater decrease in saccadic response time than did normal controls in a gap task (when the fixation point was turned off 150 msec before the target appeared). The psychiatric comparison subjects did not differ from normal controls. Further, only schizophrenic subjects demonstrated a relation between smooth pursuit and saccadic eye movement performance, such that subjects with impaired smooth pursuit showed a larger decrease in saccadic response time in the gap task. The relation between performance on the gap task and quality of smooth pursuit and its relevance for a prefrontal deficit hypothesis of schizophrenia are discussed.
The Role of Repetition and Associative Interference in New Semantic Learning in Amnesia: A Case Experiment
Authors: Gordon Hayman C. A. , Macdonald Carol A. , Tulving Endel 
Published: February 2008

Abstract
The question of whether globally amnesic subjects can learn new semantic (factual) information is controversial. Some students of amnesia believe that they can, others that they cannot. In this article we report an extensive experiment conducted with the amnesic patient K.C. in which we examined the role of repetition and associative interference in his learning of new semantic information. In the course of 8 study sessions distributed over 4 weeks, we taught K.C. novel, amusing definitions of 96 target words (e.g., “a talkative featherbrain—PARAKEET”). We varied systematically the degree of both pre-experimental and intraexperimental associative interference, as well as the amount of study. The results of the experiment showed that K.C. can learn new semantic knowledge, and retain it over a period as long as 30 months indistinguishably from control subjects. The results further showed that the efficacy of such learning depends critically on both repetition of the material and the absence, or minimization, of pre-experimental and intraexperimental associative interference. These findings suggest that the extent to which at least some amnesic patients can acquire and retain new semantic knowledge depends on the conditions under which learning occurs, and that unqualified statements regarding the deficiency or absence of such learning in amnesia are not justified.
Development of Elementary Numerical Abilities: A Neuronal Model
Authors: Dehaene Stanislas , Changeux Jean-Pierre 
Published: February 2008

Abstract
Despite their lack of language, human infants and several animal species possess some elementary abilities for numerical processing. These include the ability to recognize that a given numerosity is being presented visually or auditorily, and, at a later stage of development, the ability to compare two nume-rosities and to decide which is larger. We propose a model for the development of these abilities in a formal neuronal network. Initially, the model is equipped only with unordered numerosity detectors. It can therefore detect the numerosity of an input set and can be conditioned to react accordingly. In a later stage, the addition of a short-term memory network is shown to be sufficient for number comparison abilities to develop. Our computer simulations account for several phenomena in the numerical domain, including the distance effect and Fechner's law for numbers. They also demonstrate that infants' numerosity detection abilities may be explained without assuming that infants can count. The neurobiological bases of the critical components of the model are discussed.
A Self-Organizing Neural Model of Motor Equivalent Reaching and Tool Use by a Multijoint Arm
Authors: Bullock Daniel , Grossberg Stephen , Guenther Frank H. 
Published: February 2008

Abstract
This paper describes a self-organizing neural model for eye-hand coordination. Called the DIRECT model, it embodies a solution of the classical motor equivalence problem. Motor equivalence computations allow humans and other animals to flexibly employ an arm with more degrees of freedom than the space in which it moves to carry out spatially defined tasks under conditions that may require novel joint configurations. During a motor babbling phase, the model endogenously generates movement commands that activate the correlated visual, spatial, and motor information that are used to learn its internal coordinate transformations. After learning occurs, the model is capable of controlling reaching movements of the arm to prescribed spatial targets using many different combinations of joints. When allowed visual feedback, the model can automatically perform, without additional learning, reaches with tools of variable lengths, with clamped joints, with distortions of visual input by a prism, and with unexpected perturbations. These compensatory computations occur within a single accurate reaching movement. No corrective movements are needed. Blind reaches using internal feedback have also been simulated. The model achieves its competence by transforming visual information about target position and end effector position in 3-D space into a body-centered spatial representation of the direction in 3-D space that the end effector must move to contact the target. The spatial direction vector is adaptively transformed into a motor direction vector, which represents the joint rotations that move the end effector in the desired spatial direction from the present arm configuration. Properties of the model are compared with psychophysical data on human reaching movements, neurophysiological data on the tuning curves of neurons in the monkey motor cortex, and alternative models of movement control.
The Nature and Limits of Orientation and Pattern Processing Supporting Visuomotor Control in a Visual Form Agnosic
Authors: Goodale M. A. , Jakobson L. S. , Milner A. D. , Perrett D. I. , Benson P. J. , Hietanen J. K. 
Published: February 2008

Abstract
We have previously reported that a patient (DF) with visual form agnosia shows accurate guidance of hand and finger movements with respect to the size, orientation, and shape of the objects to which her movements are directed. Despite this, she is unable to indicate any knowledge about these object properties. In the present study, we investigated the extent to which DF is able to use visual shape or pattern to guide her hand movements. In the first experiment, we found that when presented with a stimulus aperture cut in the shape of the letter T, DF was able to guide a T-shaped form into it on about half of the trials, across a range of different stimulus orientations. On the remaining trials, her responses were almost always perpendicular to the correct Orientation. Thus, the visual information guiding the rotation of DF's hand appears to be limited to a single orientation. In other words, the visuomotor transformations mediating her hand rotation appear to be unable to combine the orientations of the stem and the top of the T, although they are sensitive to the orientation of the element(s) that comprise the T. In a second experiment, we examined her ability to use different sources of visual information to guide her hand rotation. In this experiment, DF was required to guide the leading edge of a hand-held card onto a rectangular target positioned at dHerent orientations on a flat surface. Here the orientation of her hand was determined primarily by the predominant orientation of the luminance edge elements present in the stimulus, rather than by information about orientation that was conveyed by nonluminance boundaries. Little evidence was found for an ability to use contour boundaries defined by Gestalt principles of grouping (good continuation or similarity) or “nonaccidental” image properties (colinearity) to guide her movements. We have argued elsewhere that the dorsal visual pathway from occipital to parietal cortex may underlie these preserved visuomotor skills in DF. If so, the limitations in her ability to use different kinds of “pattern” information to guide her hand rotation suggest that such information may need to be transmitted from the ventral visual stream to these parietal areas to enable the full range of prehensive acts in the intact individual.
Global Semantic Expectancy and Language Comprehension
Authors: George Marie St. , Mannes Suzanne , Hoffinan James E. 
Published: February 2008

Abstract
Previous research on the N400 component of the event-related brain potential (ERP) has dealt primarily with measuring the degree of expectancy on the part of the reader as a result of the context within a sentence. Research has shown that when the final word in a sentence is unexpected or incoherent, a greater N400 amplitude is elicited than if the final word is expected or coherent within the context of the sentence. The present study investigated whether the N400 component is sensitive to global, as well as local, semantic expectancy. Global coherence refers to the ease with which subjects can relate the current proposition they are reading with theme-related ideas. In the present study, the effect of global coherence on event-related brain potentials was tested using four titled and untitled paragraphs (Bransford & Johnson, 1972; Dooling & Lachman, 1971), presented one word at a time. These paragraphs are noncoherent, and are made coherent only with the presentation of a title. The EEG was recorded in response to every word in all four paragraphs. We found an increase in N400 amplitude in response to the words in the Untitled paragraphs relative to the Titled paragraphs, indicating that global coherence does affect the N400. In addition, subjects in the Titled group showed an enhanced P1-N1 component relative to the Untitled group suggesting that the presence of global coherence allows greater attention to be allocated to early visual processing of words.
Independent Attentional Scanning in the Separated Hemispheres of Split-Brain Patients
Authors: Luck Steven J. , Hillyard Steven A. , Mangun George R. , Gazzaniga Michael S. 
Published: February 2008

Abstract
Previous studies of visuospatial attention indicated that the isolated cerebral hemispheres of split-brain patients maintain an integrated, unitary focus of attention, presumably due to subcortical attentional mechanisms. The present study examined whether a unitary attentional focus would also be observed during a visual search task in which subjects scanned stimulus arrays for a target item. In a group of four commis-surotomy patients, the search rate for bilateral stimulus arrays was found to be approximately twice as fast as the search rate for unilateral arrays, indicating that the separated hemispheres were able to scan their respective hemifields independently. In contrast, the search rates for unilateral and bilateral arrays were approximately equal in a group of six normal control subjects, suggesting that the intact corpus callosum in these subjects is responsible for maintaining a unitary attentional focus during visual search.
Lateralization of Categorical and Coordinate Spatial Functions: A Study of Unilateral Stroke Patients
Authors: Laeng Bruno 
Published: February 2008

Abstract
Sixty patients with unilateral stroke (half with left hemisphere damage and half with right hemisphere damage) and a control group (N = 15) matched for age and educational level were tested in two experiments. In one experiment they were first shown, on each trial, a sample drawing depicting one or more objects. Following a short delay, they were asked to identify the drawing when it was paired with a drawing in which the same object(s) was transformed in categorical or coordinate spatial relations. In the other experiment, the same subjects first were shown, on each trial, a sample drawing. They then judged which of two variants (each in one type of spatial relation) looked more similar to the sample drawing. Typically, patients with left-sided stroke mistakenly identified the categorical transformation for the sample drawing in the first task; in the second task, they judged the categorical transformation as more similar to the sample drawing. Patients with right-sided stroke mistakenly identified the coordinate transformations for the sample drawing in the first task, and, in the second task, typically judged the drawings transformed along coordinate spatial relations as more similar to the sample drawing. These findings provide evidence for complementary lateralization of the two types of spatial perception. It can therefore be inferred that separate functional subsystems process the two types of spatial relations.
Parietal Damage and Narrow “Spotlight” Spatial Attention
Authors: Townsend Jeanne , Courchesne Eric 
Published: February 2008

Abstract
Patients with parietal volume loss showed electrophysiological and behavioral signs of abnormally narrow regions of enhancement of sensory stimulation at an attended location. On a test of focused spatial attention, when compared to normal control subjects and patients without parietal abnormality, patients with abnormalities of parietal cortex demonstrated (1) faster button press RTs to targets, (2) earlier P3b event-related potential (ERP) latencies to targets, and (3) larger than normal P1 ERP attention effects (i.e., greater than normal enhancement of sensory responses at an attended location). These data are evidence for visual attention distributed as a spotlight at the attentional focus with little surrounding processing enhancement. This dysfunctional attentional map facilitates simple responses within the attentional beam quite well, but could hinder responses outside the beam. Severely gated sensory responses outside the immediate attentional focus are likely to result in severely delayed responses to information in those locations. This would be consistent with the response delays seen in patients with parietal damage following an incorrect spatial cue (extinction-like pattern), and also with clinical observations of inattention in such patients. The patterns of sensory enhancement seen in these data suggest a mechanism that may underlie the impairments in spatial attention that follow damage to parietal cortex, and help to specify the role of parietal cortex in spatial attention.