bluuebunny/crossref_metadata_2025 huggingface dataset:
Conjugate Schema and Basis Representation of Crossover and Mutation Operators
Authors: Kazadi Sanza T.
Published: February 2008

In genetic search algorithms and optimization routines, the representation of the mutation and crossover operators are typically defaulted to the canonical basis. We show that this can be influential in the usefulness of the search algorithm. We then pose the question of how to find a basis for which the search algorithm is most useful. The conjugate schema is introduced as a general mathematical construct and is shown to separate a function into smaller dimensional functions whose sum is the original function. It is shown that conjugate schema, when used on a test suite of functions, improves the performance of the search algorithm on 10 out of 12 of these functions. Finally, a rigorous but abbreviated mathematical derivation is given in the appendices.
The Simple Genetic Algorithm and the Walsh Transform: Part II, The Inverse
Authors: Vose Michael D. , Wright Alden H.
Published: February 2008

This paper continues the development, begun in Part I, of the relationship between the simple genetic algorithm and the Walsh transform. The mixing scheme (comprised of crossover and mutation) is essentially “triangularized” when expressed in terms of the Walsh basis. This leads to a formulation of the inverse of the expected next generation operator. The fixed points of the mixing scheme are also determined, and a formula is obtained giving the fixed point corresponding to any starting population. Geiringer's theorem follows from these results in the special case corresponding to zero mutation.
Effects of Code Growth and Parsimony Pressure on Populations in Genetic Programming
Authors: Soule Terence , Foster James A.
Published: February 2008

Parsimony pressure, the explicit penalization of larger programs, has been increasingly used as a means of controlling code growth in genetic programming. However, in many cases parsimony pressure degrades the performance of the genetic program. In this paper we show that poor average results with parsimony pressure are a result of “failed” populations that overshadow the results of populations that incorporate parsimony pressure successfully. Additionally, we show that the effect of parsimony pressure can be measured by calculating the relationship between program size and performance within the population. This measure can be used as a partial indicator of success or failure for individual populations.
Collective Adaptation: The Exchange of Coding Segments
Authors: Haynes Thomas
Published: February 2008

Coding segments are those subsegments of the chromosome that contribute positively to the fitness evaluation of the chromosome. Clique detection is a NP-complete problem in which we can detect such coding segments. We extract coding segments from chromosomes, and we investigate the duplication of coding segments inside the chromosome and the collection of coding segments outside of the chromosome. We find that duplication of coding segments inside the chromosomes provides a back-up mechanism for the search heuristics. We further find local search in a collective memory of coding segments outside of the chromosome, collective adaptation, enables the search heuristic to represent partial solutions that are larger than realistic chromosomes lengths and to express the solution outside of the chromosome.
Scheduling of Water Distribution System Rehabilitation Using Structured Messy Genetic Algorithms
Authors: Halhal Driss , Walters Godfrey A. , Savic Dragan A. , Ouazar Driss
Published: February 2008

A methodology is presented for the optimal design and scheduling of investment for the rehabilitation of water distribution networks. Based on the evolutionary programming technique known as Structured Messy Genetic Algorithms, the methodology utilizes a multi-objective formulation which improves the evolutionary process and provides non-dominated optimal solutions over a range of costs and benefits. The model is applied to an example—a small artificial network of fifteen pipes. The effects on the optimal solutions of varying parameters such as interest rate and inflation rate are also investigated.
Scaling of Program Fitness Spaces
Authors: Langdon William B.
Published: February 2008

We investigate the distribution of fitness of programs concentrating on those represented as parse trees and, particularly, how such distributions scale with respect to changes in the size of the programs. By using a combination of enumeration and Monte Carlo sampling on a large number of problems from three very different areas, we suggest that, in general, once some minimum size threshold has been exceeded, the distribution of performance is approximately independent of program length. We proof this for both linear programs and simple side effect free parse trees. We give the density of solutions to the parity problems in program trees which are composed of XOR building blocks. Limited experiments with programs including side effects and iteration suggest a similar result may also hold for this wider class of programs.
Behavioral Indices of Multisensory Integration: Orientation to Visual Cues is Affected by Auditory Stimuli
Authors: Stein Barry E. , Meredith M. Alex , Huneycutt W. Scott , McDade Lawrence
Published: February 2008

Physiological studies have demonstrated that inputs from different sensory modalities converge on, and are integrated by, individual superior colliculus neurons and that this integration is governed by specific spatial rules. The present experiments were an attempt to relate these neural processes to overt behavior by determining if behaviors believed to involve the circuitry of the superior colliculus would show similar multisensory dependencies and be subject to the same rules of integration.

The neurophysiological-behavioral parallels proved to be striking. The effectiveness of a stimulus of one modality in eliciting attentive and orientation behaviors was dramatically affected by the presence of a stimulus from another modality in each of the three behavioral paradigms used here. Animals trained to approach a low intensity visual cue had their performance significantly enhanced when a brief, low intensity auditory stimulus was presented at the same location as the visual cue, but their performance was significantly depressed when the auditory stimulus was disparate to it. These effects were independent of the animals' experience with the modifying (i.e. auditory) stimulus and exceeded what might have been predicted statistically based on the animals' performance with each single-modality cue. The multiplicative nature of these multisensory interactions and their dependence on the relative positions and intensities of the two stimuli were all very similar to those observed physiologically for single cells. The few differences that were observed appeared to reflect the fact that understanding integration at the level of the single cell requires reference to the individual cell's multisensory receptive field properties, while at the behavioral level populations of receptive fields must be evaluated. These data illustrate that the rules governing multisensory integration at the level of the single cell also predict responses to these stimuli in the intact behaving organism.
The Effects of Focal Brain Damage on Sentence Processing: an examination of the neurological organization of a mental module
Authors: Swinney David , Zurif Edgar , Nicol Janet
Published: February 2008

The effects of prior semantic context upon lexical access during sentence processing were examined for three groups of subjects; nonfluent agrammatic (Broca's) aphasic patients; fluent (Wernicke's) aphasic patients; and neurologically intact control patients. Subjects were asked to comprehend auditorily presented, structurally simple sentences containing lexical ambiguities, which were in a context strongly biased toward just one interpretation of that ambiguity. While listening to each sentence, subjects also had to perform a lexical decision task upon a visually presented letter string. For the fluent Wernicke's patients, as for the controls, lexical decisions for visual words related to each of the meanings of the ambiguity were facilitated. By contrast, agrammatic Broca's patients showed significant facilitation only for visual words related to the a priori most frequent interpretation of the ambiguity. On the basis of these data, we suggest that normal form-based word retrieval processes are crucially reliant upon the cortical tissue implicated in agrammatism, but that even the focal brain damage yielding agrammatism does not destroy the normally encapsulated form of word access. That is, we propose that in agrammatism, the modularity of word access during sentence comprehension is rendered less efficient but not lost. Additionally, we consider a number of broader issues involved in the use of pathological material to infer characteristics of the neurological organization of cognitive architecture.
Brain Activity Underlying Mental Imagery: Event-related Potentials During Mental Image Generation
Authors: Farah Martha J. , Weisberg Lauren L. , Monheit Mark , Peronnet Franck
Published: February 2008

This article addresses two issues about the neural bases of mental imagery. The first issue concerns the modality-specificity of mental images, that is, whether or not they involve activity in visual areas of the brain. The second issue concerns hemispheric specialization for the generation of mental images. We compared event-related potentials recorded under two conditions: one in which subjects were shown words and asked to read them and one in which subjects were shown words and asked to read them and generate visual mental images of the words' referents. Imagery caused a slow, late positivity, maximal at the occipital and posterior temporal regions of the scalp, relative to the comparison condition, and consistent with the involvement of modality-specific visual cortex in mental imagery. Also noted was an asymmetry in the imagery-related ERP, consistent with left-hemisphere specialization for mental image generation. Similar results were obtained when subjects listened to auditorily presented words with and without instructions to generate mental images. To assess the specificity of the relation between these ERP effects and mental imagery, we compared the ERP changes brought about by imaging with those brought about by another effortful task using the same stimulus words: proofreading the words for occasional misspellings. This produced changes that differed in polarity, time course, and scalp distribution from the imagery-related changes.
The Temporal Dynamics of Event Memory: A Stage Analysis of Mnemonic Processing by Man and Macaque
Authors: Lawine Jeffrey David
Published: February 2008

Extensive anatomical and psychophysical data suggest that macaques can serve as surrogates for man in studies of the neural substrates of visual mnemonic processing. However, the extent to which mnemonic-mechanisms in macaques can be validly compared to those in man depends critically on the demonstration that the cognitive strategy and temporal dynamics of mnemonic retrieval are congruent For the two species.

Seven human and six macaque subjects were tested on identical versions of an item recognition task that required the classification of probe stimuli as positive or negative according to whether or not they were members of a previously defined set of target stimuli. For the human subjects, reaction time increased by an average of 24 msec/target. The macaques were able to respond as accurately as the human subjects, but each additional target resulted in a reaction time increment of only 7 msec. A detailed analysis of the statistical properties of the reaction-time distributions indicated that these data do not reflect a between-species difference in the efficiency of execution of otherwise comparable, serial-exhaustive, retrieval mechanisms. Rather, the data suggest that human subjects engage two memory-load dependent processes before generating a response, while macaques execute only one of these.

Additional data indicate that a reaction-time approach toward the analysis of the stages of mnemonic processing can provide new insights into the specific nature of mnemonic deficits induced by brain damage.
Spatially Distributed Cortical Excitation Patterns of Auditory Processing during Contralateral and Ipsilateral Stimulation
Authors: Rogers Robert L. , Papanicolaou Andrew C. , Baumann Stephen B. , Eisenberg Howard M. , Saydjari Christy
Published: February 2008

Utilizing the high spatial and temporal resolution of magnetoencephalography in conjunction with magnetic resonance images, the current study explored the underlying electrical patterns of cortical excitation during both contralateral and ipsilateral auditory stimulation. Instead of studying only the peaks of the N100 component of the evoked magnetic field, a 30-msec window was chosen about the area where the peaks occurred and the intracranial sources generating that component were estimated at successive 5-msec intervals. Results indicated that the sources for both contralateral and ipsilateral conditions were best represented as a continuous movement of activation in an anterior–inferior direction along the superior surface of the temporal lobe. Although the peak magnetic fields of the N100 to contralateral stimulation were of shorter latency and higher amplitude, the generating sources of both had very similar time-dependent movement patterns, and comparisons of source localizations were dependent on the latency at which they were contrasted.
The Bounded Brain: Toward Quantitative Neuroanatomy
Authors: Cherniak Christopher
Published: February 2008

An idea that human cognitive resources are virtually without limit turns up at all levels of mind/brain science. This tacit unbounded-resource assumption has paradoxical consequences in neuroscience, particularly involving the quantitative incoherence of some key anatomical studies of cortical connectivity resources: cortical sheet area, synaptic density there, and giant axonic arborizations in visual cortex. This inattention to quantitative consistency checking in neuroanatomy appears to stem from, as a notable instance, something of the nonspatial character of the Cartesian concept of mind being extended to the brain as physical structure.
Thalamic and Cortical Mechanisms of Attention Suggested by Recent Positron Emission Tomographic Experiments
Authors: LaBerge David
Published: February 2008

An attention mechanism in the pulvinar nucleus of the thalamus appears to be involved in the filtering of a location in a cluttered visual field, according to recent PET data. The mechanism in the pulvinar is assumed to be a particular type of circuit that reciprocally connects thalamic relay cells to cortical cells. This circuit, which is characteristic of thalamic nuclei in general, appears to embody an algorithm that enhances firing in a target set of cells while inhibiting firing in the surrounding set of cells. Recent PET studies have also identified specific areas of cortex that show increased blood flow when humans expect to process particular aspects of visual objects, such as location, shape, color, and movement velocity. An expectation for a particular feature can be regarded as a peaked activity distribution across cortical synaptic space that increases the effectiveness (accuracy and speed) of the perception of that feature and features similar to that feature. The expectation process is assumed to be generated by a thalamic enhancement circuit, which in turn is driven by a cognitive procedure, apparently located in the anterior cingulate area of the cortex. The selection of a particular cognitive procedure is assumed to depend on the momentary relative motivational value associated with it by means of connections to deeper limbic structures. Under usual conditions, the procedure having the dominant motivational value is the one selected.
Arm Trajectory Modifications During Reaching Towards Visual Targets
Authors: Flash Tamar , Henis Ealan
Published: February 2008

In this paper we study the question of how an aimed arm movement is modified in response to a sudden change in target location occurring during the reaction or movement time. Earlier monkey and human studies demonstrated that aimed arm movements can be elicited in quick succession, without appreciable delays in responding to the target displacement, beyond the normal reaction time. Nevertheless, it is not yet clear how this motor task is performed. A first guess is that when a new visual stimulus appears the old plan is aborted and a new one conceived. Upon analyzing human arm movements, however, we find that the observations can be well accounted for by a different movement modification scheme. It appears that a new plan is vectorially added to the original plan. Among the implications of this result is the possibility of parallel planning of elemental movements and further support for the idea that arm movements are internally represented in terms of hand motion through external space.
Differential Sensitivity to Errors of Agreement and Word Order in Broca's Aphasia
Authors: Wulfeck Beverly , Bates Elizabeth
Published: February 2008

We investigated the effect of morphosyntactic violation type on accuracy and processing time in Broca's aphasics engaged in an on-line error detection task. Five agrammatic Broca's aphasic subjects and 15 age-matched control subjects performed grammaticality judgments on auditorily presented grammatical and ungrammatical sentences. Both judgment accuracy and decision time were measured, so that the data revealed not only whether aphasics detected violations, but when they noticed them. The ungrammatical sentences were created by changing quantifiers and auxiliary verbs in one of two ways: substituting one quantifier or auxiliary for another to create agreement errors, or moving the quantifier or auxiliary “downstream” from its proper site to create word order errors. Also, the position of the violation in the sentence (early versus late) as well as the distance relationships among sentence elements involved in the violation (local versus global) were manipulated.

Results suggest that aphasic subjects retain some sensitivity to grammaticality, knowledge that they are able to use “online.” Performance was also affected by type of violation. Aphasic subjects were less sensitive to agreement violations than they were to violations created by moving the same elements to an illegal position—and this tended to be reflected in decision times as well as accuracy.

These results support two conclusions. First, although the performance of aphasic subjects was degraded relative to control subjects, the findings of overall grammaticality sensitivity and relatively rapid decision times suggest that the locus of grammatical impairment in these patients has more to do with the accessing of linguistic information than with loss of linguistic knowledge. Second, the difference between agreement and movement violations provides further evidence that morphological marking is relatively vulnerable in aphasia, compared with the principles that govern word and morpheme ordering.
Development of Spontaneous Alternation in Infancy
Authors: Vecera* Sham P. , Rothbart Mary K. , Posner Michael I.
Published: February 2008

Spontaneous alternation is a reduced tendency to return to the same location on successive trials. It is measured in rats in a T maze and is thought to depend on an intact hippocampus. In human infants, we measured alternation in the tendency to reach toward one of two identical toys placed in locations to the left and right of midline. Infants at 6 months returned to the same side as frequently as they alternated, but 18--month-old infants showed a significant alternation pattern. At 6 months, infants show inhibition of return, but do not show alternation in motor behavior; at 18 months, infants show both, but they are negatively related. These data suggest that preference for novelty may rest on different internal mechanisms even in quite similar tasks, and suggest that whereas inhibition of return is related to control by the posterior attention network, spontaneous alternation may be related to inhibitory control by the anterior attention network.
The Psychophysical Attributes of Heat-Induced Pain and Their Relationships to Neural Mechanisms
Authors: Price Donald D. , McHaffie John G. , Stein Barry E.
Published: February 2008

During the last two decades there has been a proliferation of studies evaluating the psychophysical and neural attributes of heat-induced pain. Experiments using radiant and contact heat-induced pain have produced different observations thereby broadening our appreciation of the importance of acknowledging how a noxious heat stimulus is delivered; moreover manipulations of stimulus parameters have now provided a foundation for understanding the underlying neural mechanisms of heat-induced pain and their biological significance. The psychophysical attributes of heat-induced pain include highly reliable thresholds for most body regions, minimal adaptation to maintained noxious stimuli, an exquisite sensitivity to small changes in stimulus intensity, slow temporal summation for some types of heat-induced pain (i.e., second pain) but not for others (i.e., first pain), spatial summation---especially for suprathreshold noxious temperatures---and the perceived spread of pain well beyond the actual body area stimulated (i.e., radiation). The present body of information indicates that the pain system is optimally adapted for conveying precise information about intensity, and is less concerned with other stimulus features, such as spatial patterns or boundaries.
Preserved Visual Imagery and Categorization in a Case of Associative Visual Agnosia
Authors: Jankowiak Janet , Kinsbourne Marcel , Shalev Ruth S. , Bachman David L.
Published: February 2008

A patient with associative visual agnosia secondary to a penetrating bitemporooccipital lesion remained able to draw complex objects from memory but could not subsequently recognize his sketches. His retained ability to copy and draw briefly exposed objects indicates that this is not a problem of visual perception. On tasks of categorization, mental imagery, drawing, and object decision, he demonstrates many instances of preserved visual semantic memories and imagery despite a sense of unfamiliarity with the visual stimuli. We infer a preserved ability to derive internal visual images from semantic memory. Cues may help him visualize the named object, which then serves as a model for comparison with the actual stimulus. However, his adequate visual perception and mental visual imagery, even when assisted by cues, are still insufficient to correct fully his difficulty in recognizing objects. Unique to his case is an inability to match the representation of stimulus objects with his intact internal image of the same object. Deficient lateral inhibition between neural representations of similar objects may be responsible.
Modes of Lexical Access in the Callosotomized Brain
Authors: Reuter-Lorenz Patricia A. , Baynes Kathleen
Published: February 2008

Left hemisphere processing is typically characterized as analytic and serial whereas the right hemisphere is characterized as wholistic and parallel. Word recognition may be an exception to this dichotomy if the letter-by-letter alexia produced by left hemisphere damage reflects the reading abilities of the right hemisphere. We investigated this possibility by studying prelexical and lexical processes in the separated hemispheres of callosotomy patient J. W. A word superiority effect demonstrated in each visual field suggests that both hemispheres have access to a visual lexicon. Error patterns, letter recognition thresholds, and lexical decision performance as a function of word length suggest that the left hemisphere tends to utilize a parallel access mode, whereas the right hemisphere mode is less efficient and may be serial. Furthermore, only J. W.'s left hemisphere showed letter priming, an outcome consistent with observations in letter-by-letter alexia. These findings suggest that the right hemisphere may have an independent visual lexicon and may provide an alternative although less efficient route to reading. We suggest that a serial encoding strategy results because the global processing mode for which the right hemisphere is specialized is largely ineffective for word reading.
Effects of Multiple Stimuli on Ocular Orientation by Cats
Authors: Kurylo Daniel D. , Pandey Vimal Ram L. , Hartline Peter H.
Published: February 2008

We investigated the characteristics of saccades made by cats in response to single and double stimuli. Stimuli were either visual, auditory, or bimodal. We initially trained cats to look toward the location of briefly presented single visual or single auditory targets that were extinguished before the initiation of eye movements. Following training, we monitored eye movements during and after the presentation of double targets, either two visual, two auditory, or bimodal, that were at disparate spatial locations. Cats made saccadic eye movements to positions that ranged between the location of the two targets. If the eye position at the start of a saccade was near the mid point of the targets, cats were less likely to initiate a saccade, and saccadic latencies were longer, compared to when starting eye position was at a distance from this location. These behavioral results are consistent with the hypothesis that the neural representations of briefly presented targets are combined and treated as a unitary, low resolution stimulus from which an orienting motor program is derived. The similarity of responses to double visual, double auditory, and bimodal stimuli suggests that a common sensorimotor mechanism applies within and between these sensory modalities.
Selective Interference with the Representation of Size in the Human by Direct Cortical Electrical Stimulation
Authors: Hart John , Lesser Ronald P. , Gordon Barry
Published: February 2008

A specific category in human cognition, size representation, was disrupted by the application of cortical electrical interference through a recently modified technique involving implantation of indwelling subdural electrode arrays. When subjected to electrical stimulation at a specific site, the subject was unable to access size information when questioned verbally, but showed no deficit if the size discrimination was presented visually. Verbal questions about size were answered correctly when the patient was not subjected to cortical interference. Other measures of verbal and visual comprehension for the categories of color, shape, orientation, movement, texture, and structure, tested under cortical interference, were normal. This clear-cut distinction between verbal and visual access to information about size, shown by a reversible block at a known and anatomically circumscribed site, provides further evidence that higher order neural processing is categorically represented.
Human Cerebral Cortex: Localization, Parcellation, and Morphometry with Magnetic Resonance Imaging
Authors: Rademacher J. , Galaburda A. M. , Kennedy D. N. , Filipek P. A. , Caviness V. S.
Published: February 2008

We describe a system of parcellation of the human brain that is based on the functional anatomy of the cerebral cortex and that is applied to the analysis of magnetic resonance images. This system is designed to support investigations of hemispheric asymmetries and quantitative lesion localization studies in cognitive neuroscience. The system of cortical subdivision is a neural systems oriented model that approximates subdivisions supported by previous architectonic and functional analyses. It is based primarily on boundaries determined by "limiting fissures." It is completed by a set of coronal planes, keyed to visible anatomic landmarks, which "close" the borders of the parcellation subdivisions. The method depends on computational reconstruction of the primary image data in multiple planes so as to allow the observed pattern of limiting fissures in a given brain to be digitized. In this presentation, the method is applied in order to define the surface anatomy of the cerebral hemispheres in a normal subject. Volumetric measurements of individual cortical regions are compared as hemispheric percentiles to areal perceniiles derived from the analysis of Jouandet et al. (1989), a conceptually related though methodologically different approach. We specifically address the approach to the study of interhemispheric differences and interindividual variations in cortical anatomy.
A Spurious Category-Specific Visual Agnosia for Living Things in Normal Human and Nonhuman Primates
Authors: Gaffan David , Heywood Charles A.
Published: February 2008

Patients with visual associative agnosia have a particular difficulty in identifying visually presented living things (plants and animals) as opposed to nonliving things. It has been claimed that this effect cannot be explained by differences in the inherent visual discriminability of living and nonliving things. To test this claim further, we performed two experiments with normal subjects. In Experiment 1 normal human observers were asked to identify objects in tachistoscopically presented line drawings. They made more errors with living things than with nonliving things. In Experiment 2 normal monkeys learned to discriminate among the same line drawings for food reward. They made many more errors in discriminating among living things than nonliving things. Agnosic patients' responses to the same line drawings were made available to us for correlative analysis with the subjects' responses to these drawings in Experiments 1 and 2. We conclude that a category-specific visual agnosia for living things can arise as a consequence of a modality-specific but not category-specific impairment in visual representation, since living things are more similar to each other visually than nonliving things are.
Differential Effects of Semantic and Identity Priming in Patients with Left and Right Hemisphere Lesions
Authors: Henik Avishai , Dronkers Nina F. , Knight Robert T. , Osimani Alicia
Published: February 2008

Patients with single brain lesions in the anterior or posterior left and right hemispheres and a group of controls were studied in two priming experiments. The first experiment employed associative pairs (DOCTOR-NURSE) and the second employed identical pairs (NURSE-nurse). Short and long prime-target stimulus onset asynchronies (SOAs) (i.e., 250 and 1850 msec) were manipulated within block in both experiments. In the first experiment, patients with left hemisphere injury showed a deficient priming effect while patients with right hemisphere injury and controls showed a normal pattern. In contrast, all groups showed an identity priming effect in the second experiment. These results indicate that while entries in the mental lexicon are available for the groups of patients studied, the spread of activation to related concepts in this lexicon is disrupted in the left hemisphere-damaged group.
Visual Mental Imagery Activates Topographically Organized Visual Cortex: PET Investigations
Authors: Kosslyn Stephen M. , Alpert Nathaniel M. , Thompson William L. , Maljkovic Vera , Weise Steven B. , Chabris Christopher F. , Hamilton Sania E. , Rauch Scott L. , Buonanno Ferdinando S.
Published: February 2008

Cerebral blood flow was measured using positron emission tomography (PET) in three experiments while subjects performed mental imagery or analogous perceptual tasks. In Experiment 1, the subjects either visualized letters in grids and decided whether an X mark would have fallen on each letter if it were actually in the grid, or they saw letters in grids and decided whether an X mark fell on each letter. A region identified as part of area 17 by the Talairach and Tournoux (1988) atlas, in addition to other areas involved in vision, was activated more in the mental imagery task than in the perception task. In Experiment 2, the identical stimuli were presented in imagery and baseline conditions, but subjects were asked to form images only in the imagery condition; the portion of area 17 that was more active in the imagery condition of Experiment 1 was also more activated in imagery than in the baseline condition, as was part of area 18. Subjects also were tested with degraded perceptual stimuli, which caused visual cortex to be activated to the same degree in imagery and perception. In both Experiments 1 and 2, however, imagery selectively activated the extreme anterior part of what was identified as area 17, which is inconsistent with the relatively small size of the imaged stimuli. These results, then, suggest that imagery may have activated another region just anterior to area 17. In Experiment 3, subjects were instructed to close their eyes and evaluate visual mental images of upper case letters that were formed at a small size or large size. The small mental images engendered more activation in the posterior portion of visual cortex, and the large mental images engendered more activation in anterior portions of visual cortex. This finding is strong evidence that imagery activates topographically mapped cortex. The activated regions were also consistent with their being localized in area 17. Finally, additional results were consistent with the existence of two types of imagery, one that rests on allocating attention to form a pattern and one that rests on activating stored visual memories.
Dissociation of Brain Activity Related to Syntactic and Semantic Aspects of Language
Authors: Münte Thomas F. , Heinze Hans-Jochen , Mangun George R
Published: February 2008

In psycholinguistic research, there has been considerable interest in understanding the interactions of difFerent types of linguistic information during language processing. For example, does syntactic information interact with semantic or pragmatic information at an early stage of language processing, or only at later stages in order to resolve ambiguities of language? Developing reliable measures of language processes such as syntax and semantics is important to address many of these theoretical issues in psycholinguistics. In the present study, event-related brain potentials (ERPs) were recorded from healthy young subjects while they read pairs of words presented one word at a time. The ERPs for the second word of each pair were compared as a function of whether the preceding word was or was not (1) semantically related (i.e., synonyms; “semantic condition”) or (2) grammatically correct (“syntactic condition”). In the semantic condition the ERPs obtained to words preceded by nonsemantically related words elicited an N400 component that was maximal over centroparietal scalp regions. In contrast, in the syntactic condition the ERPs obtained to words preceded by grammatically incorrect articles or pronouns yielded a negativity with a later onset, and a frontopolar, left hemisphere scalp maximum. This replicates our previous findings of a syntactic negativity in a word pair design that was performed in the German language. Further, the present data provide scalp distributional information, which suggests that the syntactic negativity represents brain processes that are dissociable from the centroparietal N400 component. Thus, these findings provide strong evidence for a separate negative polarity ERP component that indexes syntactic aspects of language processing.
The Human Auditory Sensory Memory Trace Persists about 10 sec: Neuromagnetic Evidence
Authors: Sams Mikko , Hari Riitta , Rif Josi , Knuutila Jukka
Published: February 2008

Neuromagnetic responses were recorded to frequent "standard tones of l000 Hz and to infrequent 1100-Hz "deviant" tones with a 24-channel planar SQUID gradiometer. Stimuli were presented at constant interstimulus intervals (ISIs) ranging from 0.75 to 12 sec. The standards evoked a prominent 100-msec response, N100m, which increased in amplitude with increasing ISI. N100m could be dissociated into two subcomponents with different source areas. The posterior component, N100m2, increased when the ISI grew up to 6 sec, whereas the more anterior component, N100m2, probably continued its growth beyond the 12-sec ISI. At ISIs from 0.75 to 9 sec, the deviants elicited additionally a mismatch field (MMF). The equivalent sources of both N100m and MMF were at the supra-temporal auditory cortex. We assume that auditory stimuli leave in the auditory system a trace that affects the processing of the subsequent stimuli. The decrement of the N100m amplitude as well as elicitation of MMF can be considered as indirect evidence of active traces. A behavioral estimate of the persistence of the sensory auditory memory was obtained in a separate experiment in which the subject compared, without attending to the stimuli, tones presented at the daerent ISIs. The subjects discriminated the stimuli better than merely by chance at ISIs of 0.75-9 sec. The ISI dependence of the behavioral estimate as well as of N100m2 and MMF are similar enough to suggest a common underlying mechanism that retains information for a period of about 10 sec.
Cognitive and Physiological Substrates of Impaired Sentence Processing in Parkinson's Disease
Authors: Grossman Murray , Carvell Susan , Gollomp Stephen , Stern Matthew B. , Reivich Martin , Morrison Donald , Alavi Abass , Hurtig Howard I.
Published: February 2008

Sentence comprehension is a complex process involving at least a grammatical processor and a procedural component that supports language computations. One type of cerebral architecture that may underlie sentence processing is a network of distributed brain regions. We report two experiments designed to evaluate the cognitive and physiological substrate of sentence processing diaculties in nondemented patients with Parkinson's disease (PD). In the first experiment, patients answered simple questions about sentences that varied in their computational demands. Group and individual patient analyses indicated that PD patients are significantly compromised on this task, and that their difficulties become more prominent as the computational demands of the sentences increase. We manipulated the set of sentences to stress performance aspects of sentence processing. PD patients were compromised in their ability to detect errors in the presence and nature of a sentence's grammatical morphemes, suggesting a deficit in selective attention, but their ability to answer questions about a sentence was not afFected by short-term memory factors. In the second experiment, positron emission tomography was used to correlate this pattern of sentence comprehension impairment with regional cerebral glucose metabolism (rCMRgl) obtained at rest in a representative subset of these PD patients. Grammatical comprehension and attention in sentence processing correlated significantly with mesial frontal rCMRgl. Regression analyses confirmed the central role of left mesial frontal cortex, and identified a subsidiary role for left caudate in overall sentence comprehension, for left dorsolateral frontal cortex in grammatical processing, and for bilateral dorsolateral frontal cortex in attending to the presence of grammatical features. We conclude that compromised mesial frontal functioning underlies in part the sentence processing deficit of these patients, and these data illustrate one method for mapping portions of a sentence processing mechanism onto a distributed cerebral architecture.
Object-Centered Neglect in Patients with Unilateral Neglect: Effects of Left-Right Coordinates of Objects
Authors: Behrmann Marlene , Moscovitch Morris
Published: February 2008

When patients with right-sided hemispheric lesions neglect information on the left side, with respect to what set of spatial coordinates is left defined? Two potential reference frames were examined in this study, one where left and right are defined with respect to the midline of the viewer and/or environment (viewer/env-centered) and the other where left and right are defined with respect to the midline of the object (object-centered). By rotating the stimulus 90° clockwise or counterclockwise, and instructing patients with neglect to report the colors appearing around the border of a stimulus, an independent measure was obtained for the number of colors reported from the left and right of the viewer/env- and from the object-based reference frame. Whereas significant object-centered neglect was observed only for upper case asymmetrical letters but not for symmetrical letters nor for drawings of familiar animals or objects, significant viewer/env-based neglect was observed with all the stimulus types. We present an account of the coexistence of neglect in more than one frame of reference and the presence of object-centered neglect under a restricted set of conditions.
Summation Priming and Coarse Semantic Coding in the Right Hemisphere
Authors: Beeman Mark , Friedman Rhonda B. , Grafman Jordan , Perez Enrique , Diamond Sherri , Lindsay Miriam Beadle
Published: February 2008

There are now numerous observations of subtle right hemisphere (RH) contributions to language comprehension. It has been suggested that these contributions reflect coarse semantic coding in the RH. That is, the RH weakly activates large semantic fields—including concepts distantly related to the input word—whereas the left hemisphere (LH) strongly activates small semantic fields—limited to concepts closely related to the input (Beeman, 1993a,b). This makes the RH less effective at interpreting single words, but more sensitive to semantic overlap of multiple words. To test this theory, subjects read target words preceded by either “Summation” primes (three words each weakly related to the target) or Unrelated primes (three unrelated words), and target exposure duration was manipulated so that subjects correctly named about half the target words in each hemifield. In Experiment 1, subjects benefited more from Summation primes when naming target words presented to the left visual field-RH (Ivf-RH) than when naming target words presented to the right visual field-LH (rvf-LH), suggesting a RH advantage in coarse semantic coding. In Experiment 2, with a low proportion of related prime-target trials, subjects benefited more from “Direct” primes (one strong associate flanked by two unrelated words) than from Summation primes for rvf-LH target words, indicating that the LH activates closely related information much more strongly than distantly related information. Subjects benefited equally from both prime types for Ivf-RH target words, indicating that the RH activates closely related information only slightly more strongly, at best, than distantly related information. This suggests that the RH processes words with relatively coarser coding than the LH, a conclusion consistent with a recent suggestion that the RH coarsely codes visual input (Kosslyn, Chabris, Mar-solek, & Koenig, 1992).
Learning and Maintaining Saccadic Accuracy: A Model of Brainstem–Cerebellar Interactions
Authors: Dean Paul , Mayhew John E. W. , Langdon Pat
Published: February 2008

Saccadic accuracy requires that the control signal sent to the motor neurons must be the right size to bring the fovea to the target, whatever the initial position of the eyes (and corresponding state of the eye muscles). Clinical and experimental evidence indicates that the basic machinery for generating saccadic eye movements, located in the brainstem, is not accurate: learning to make accurate saccades requires cerebellar circuitry located in the posterior vermis and fastigial nucleus. How do these two circuits interact to achieve adaptive control of saccades? A model of this interaction is described, based on Kawato's principle of feedback-error-learning. Its three components were (1) a simple controller with no knowledge of initial eye position, corresponding to the superior colliculus; (2) Robinson's internal feedback model of the saccadic burst generator, corresponding to preoculomotor areas in the brain-stem; and (3) Albus's Cerebellar Model Arithmetic Computer (CMK), a neural net model of the cerebellum. The connections between these components were (I) the simple feedback controller passed a (usually inaccurate) command to the pulse generator, and (2) a copy of this command to the CMAC; (3) the CMAC combined the copy with information about initial eye position to (4) alter the gain on the pulse generator's internal feedback loop, thereby adjusting the size of burst sent to the motor neurons. (5) If the saccade were inaccurate, an error signal from the feedback controller adjusted the weights in the CMAC. It was proposed that connection (2) corresponds to the mossy fiber projection from superior colliculus to oculomotor vermis via the nucleus reticularis tegmenti pontis, and connection (5) to the climbing fiber projection from superior colliculus to the oculomotor vermis via the inferior olive. Plausible initialization values were chosen so that the system produced hypometric saccades (as do human infants) at the start of learning, and position-dependent hypermetric saccades when the cerebellum was removed. Simulations for horizontal eye movements showed that accurate saccades from any starting position could be learned rapidly, even if the error signal conveyed only whether the initial saccade were too large or too small. In subsequent tests the model adapted realistically both to simulated weakening of the eye muscles, and to intrasaccadic displacement of the target, thereby mimicking saccadic plasticity in adults. The architecture of the model may therefore offer a functional explanation of hitherto mysterious tectocerebellar projections, and a framework for investigating in greater detail how the cerebellum adaptively controls saccadic accuracy.
Duration of EEG and MEG α Suppression Increases with Angle in a Mental Rotation Task
Authors: Michel Christoph M. , Kaufman Lloyd , Williamson Samuel J.
Published: February 2008

Electric and magnetic recordings of average power within the high a band (10–12 Hz) were made over the parietal and occipital areas of the scalp while subjects were engaged in the mental imagery task of Cooper and Shepard. The subject had to determine whether an abstract probe figure was identical to a memory figure presented earlier at a different orientation, or whether it was the mirror image of the memory figure. Alpha power was found to be suppressed while the subjects were engaged in the comparison, and the duration of suppression increased with the minimum rotation angle to achieve a match. Strong correlations between suppression duration and reaction time give further evidence that the visual cortex is engaged in the process of mental imagery. Moreover, for large rotation angles of the probe figures, where the task is markedly more difficult, a shift in the spatial pattern of suppression indicates some additional activity in left occipital areas.
Visual Attention in Children with Perinatal Brain Injury: Asymmetric Effects of Bilateral Lesions
Authors: Craft Suzanne , White Desiree A. , Park T. S. , Figiel Gary
Published: February 2008

The neural systems underlying visual attention have been well-documented in adults through studies examining the effects of brain lesions on specific attentional operations (Posner, Cohen, & Rafal, 1982). The questions of how this attentional system develops and how it is affected by disruption during development are only beginning to be addressed. In the present study, a covert orienting task (Posner et al., 1982) was administered to 33 children with bilateral perinatal injury to anterior, posterior, or diffuse brain regions and 36 normal children to determine the effects of such injury on visual attention. Children with bilateral anterior lesions showed lateralized impairment indicating compromise of left hemisphere early attentional processes. In contrast, children with posterior lesions that typically disrupt attention in adults showed only general slowing, with no differences in right or left visual field performance or deficits in specific attentional operations. These results suggest that anterior brain regions play an important role in the development of visual attention, and that left hemisphere attentional processes are particularly affected by disruption of anterior function.