Analytic modeling of conductively anisotropic neural tissue

The abdominal ganglion of the Aplysia californica is an established in vitro model for studying neuroelectric behavior in the presence of an applied electrical current and recently used in studies of magnetic resonance electrical impedance tomography (MREIT) which allows for quantitative visualization of spatially distributed current and magnetic flux densities. Understanding the impact the Aplysia geometry and anisotropic conductivity have on applied electromagnetic fields is central to intepreting and refining MREIT data and protocols, respectively. Here we present a simplified bidomain model of an in vitro experimental preparation of the Aplysia abdominal ganglion, describing the tissue as a radially anisotropic sphere with equal anisotropy ratios, i.e., where radial conductivities in both intra- and extra-cellular regions are ten times that of their polar and azimuthal conductivities. The fully three dimensional problem is validated through comparisons with limiting examples of 2D isotropic analyses. Results may be useful in validating finite element models of MREIT experiments and have broader relevance to analysis of MREIT data obtained from complex neural architecture in the human brain.

Analytic Modeling of Neural Tissue: I. A Spherical Bidomain

Presented here is a model of neural tissue in a conductive medium stimulated by externally injected currents. The tissue is described as a conductively isotropic bidomain, i.e. comprised of intra and extracellular regions that occupy the same space, as well as the membrane that divides them, and the injection currents are described as a pair of source and sink points. The problem is solved in three spatial dimensions and defined in spherical coordinates [Formula: see text]. The system of coupled partial differential equations is solved by recasting the problem to be in terms of the membrane and a monodomain, interpreted as a weighted average of the intra and extracellular domains. The membrane and monodomain are defined by the scalar Helmholtz and Laplace equations, respectively, which are both separable in spherical coordinates. Product solutions are thus assumed and given through certain transcendental functions. From these electrical potentials, analytic expressions for current density are derived and from those fields the magnetic flux density is calculated. Numerical examples are considered wherein the interstitial conductivity is varied, as well as the limiting case of the problem simplifying to two dimensions due to azimuthal independence. Finally, future modeling work is discussed.

Scattering and Diffraction of Elastodynamic Waves in a Concentric Cylindrical Phantom for MR Elastography

AIM

The focus of this paper is to report on the design and construction of a multiply connected phantom for use in magnetic resonance elastography (MRE)-an imaging technique that allows for the noninvasive visualization of the displacement field throughout an object from externally driven harmonic motion-as well as its inverse modeling with a closed-form analytic solution which is derived herein from first principles.

METHODS

Mathematically, the phantom is described as two infinite concentric circular cylinders with unequal complex shear moduli, harmonically vibrated at the exterior surface in a direction along their common axis. Each concentric cylinder is made of a hydrocolloid with its own specific solute concentration. They are assembled in a multistep process for which custom scaffolding was designed and built. A customized spin-echo-based MR elastography sequence with a sinusoidal motion-sensitizing gradient was used for data acquisition on a 9.4 T Agilent small-animal MR scanner. Complex moduli obtained from the inverse model are used to solve the forward problem with a finite-element method.

RESULTS

Both complex shear moduli show a significant frequency dependence (p 0.001) in keeping with previous work.

CONCLUSION

The novel multiply connected phantom and mathematical model are validated as a viable tool for MRE studies.

SIGNIFICANCE

On a small enough scale much of physiology can be mathematically modeled with basic geometric shapes, e.g., a cylinder representing a blood vessel. This study demonstrates the possibility of elegant mathematical analysis of phantoms specifically designed and carefully constructed for biomedical MRE studies.

Analysis of bipolar external excitation of spherical tissue by spatially opposed current source and sink points

The recently increasing role in medical imaging that electrophysiology plays has spurned the need for its quantitative analysis at all scales-ions, cells, tissues, organs, etc.; so, here is presented a model of nerve tissue in a spherical volume excited by a point current source at one pole and a point current sink at the opposite pole. The sphere of tissue is described as an isotropic bidomain, consisting of the intra- and extra-cellular regions and the membrane that separates them, and is immersed in an infinite isotropic conductive bath. The system of coupled differential equations is solved by redefining the domains to be in terms of a monodomain and a membrane. The solution takes the form of an infinite sum of the product of certain transcendental functions. The study concludes with a numeric example in which the boundary conditions are shown to be satisfied, validating this analysis, paving the way for more sophisticated models of excitable tissue.

Mechanical analysis of an axially symmetric cylindrical phantom with a spherical heterogeneity for MR elastography

Cylindrical homogenous phantoms for magnetic resonance (MR) elastography in biomedical research provide one way to validate an imaging systems performance, but the simplified geometry and boundary conditions can cloak complexity arising at tissue interfaces. In an effort to develop a more realistic gel tissue phantom for MRE, we have constructed a heterogenous gel phantom (a sphere centrally embedded in a cylinder). The actuation comes from the phantom container, with the mechanical waves propagating toward the center, focusing the energy and thus allowing for the visualization of high-frequency waves that would otherwise be damped. The phantom was imaged and its stiffness determined using a 9.4 T horizontal MRI with a custom build piezo-elastic MRE actuator. The phantom was vibrated at three frequencies, 250, 500, and 750 Hz. The resulting shear wave images were first used to reconstruct material stiffness maps for thin (1 mm) axial slices at each frequency, from which the complex shear moduli μ were estimated, and then compared with forward modeling using a recently developed theoretical model which took μ as inputs. The overall accuracy of the measurement process was assessed by comparing theory with experiment for selected values of the shear modulus (real and imaginary parts). Close agreement is shown between the experimentally obtained and theoretically predicted wave fields.

Effects of central airway shunting on the mechanical impedance of the mouse lung

The mechanical properties of the lung are embodied in its mechanical input impedance, which it is interpreted in physiological terms by being fit with a mathematical model. The normal lung is extremely well described by a model consisting of a single uniformly ventilated compartment comprised of tissue having a constant-phase impedance, but to describe the abnormal lung it frequently becomes necessary to invoke additional compartments. To date, all evidence of regional mechanical heterogeneity in the mouse lung has been assumed to be of the parallel variety. We therefore investigated the use of a serial heterogeneity model, relative to parallel heterogeneity and homogeneous models, for describing impedance spectra in mice subjected to a variety of interventions designed to make their lungs heterogeneous. We found that functional evidence of the finite stiffness of the airway wall in mice with airways obstruction can sometimes be apparent in lung impedance below 20 Hz. The model estimates of airway stiffness were smaller than direct estimates obtained from micro-CT images of the lung in vivo, suggesting that the conducting airways alone are likely not the precise anatomical correlate of proximal functional stiffness in the lung. Nevertheless, we conclude that central airway shunting in mice can sometimes be an important physiological phenomenon.

Episodic memory in primates

Episodic memory refers to a system of memory with the capacity to recollect specific events from an individual's life. Some psychologists have suggested that episodic memory is a uniquely human phenomenon. We challenge that idea and present evidence that great apes and other primates may possess episodic-like memory. We review criteria developed to assess episodic-like memory in nonhumans, and how they apply to primates. In particular, we discuss the criteria of Clayton et al. [2001], who stated that episodic-like memory is based on the retrieval of multiple and integrated components of an event. We then review eight studies examining memory in great apes and apply the Clayton et al. criteria to each of them. We summarize the evidence that is compatible with the existence of episodic-like memory, although none of the data completely satisfy the Clayton et al. criteria. Morover, feelings of pastness and feelings of confidence, which mark episodic memory in humans, have not been empirically addressed in nonhuman primates. Future studies should be directed at these aspects of memory in primates. We speculate on the functional significance of episodic memory in nonhuman primates.

Topiramate improves deficit symptoms in a patient with schizophrenia when added to a stable regimen of antipsychotic medication

Topiramate was shown to attenuate the severity of negative symptoms (e.g., emotional withdrawal) in a patient with schizophrenia when added to his stable regimen of antipsychotic medication. Topiramate was administered for a period of 12 weeks; during the first 4 weeks, dosage was adjusted to the maximal tolerated dose ( i.e., 175 mg/d), and, thereafter, this dosage was maintained for 8 weeks. Topiramate was studied because of recent data and hypotheses suggesting that N-methyl-D-aspartate receptor hypofunction, dampened GABAergic inhibition, and excessive stimulation of the kainic acid (KA)/alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) class of glutamate receptors occur in at least some patients with schizophrenia, especially those with persistent negative symptoms and progressive psychosocial deterioration. Topiramate is a recently approved and marketed medication for the treatment of seizure disorders, whose mechanism of action includes potentiation of GABAergic neurotransmission and antagonism of KA/AMPA glutamate receptors. This case is presented because of the dramatic response of negative symptoms to the addition of topiramate. The severity of negative symptoms was assessed formally with the Negative Scale of the Positive and Negative Syndrome Scale. The negative symptoms of schizophrenia are usually resistant to most behavioral and pharmacologic interventions.

The relation of tip-of-the-tongue states and retrieval time

The tip-of-the-tongue state (TOT) is the phenomenological experience that a word is on the verge of being recalled. Participants rated TOTs as either emotional or nonemotional. In Experiment 1, given general-information questions, participants spent more time attempting retrieval during emotional TOTs than during nonemotional TOTs or n-TOTs (retrieval failures not accompanied by TOTs). Experiment 2 replicated the effect that TOTs show longer retrieval times than n-TOTs. In Experiment 3, with word definitions as stimuli, retrieval times were longer for emotional TOTs. Experiment 4 showed the same relation between retrieval times and TOTs even when participants made retrospective decisions about whether they had experienced a TOT before they retrieved the correct target. Valence of emotion was correlated with correct resolution of the TOT. These results are discussed in the context of a metacognitive model, in which TOTs serve to monitor and control cognition.

A revised excitotoxic hypothesis of schizophrenia: therapeutic implications

A revision of an "excitotoxic hypothesis" of schizophrenia is summarized. The hypothesis suggests that in, at least, a subtype of patients with schizophrenia, progressive excitotoxic neuronal cell death in hippocampal and cortical areas occurs via "disinhibition" of glutamatergic projections to these areas. Patients who have excitotoxic damage would be expected to have poor outcomes characterized, perhaps, by anatomic evidence of progressive neurodegeneration, pronounced negative symptoms and cognitive deficits, and profound psychosocial deterioration. Disinhibited glutamatergic activity could result from inhibition of N-methyl-D-aspartate (NMDA) receptor-mediated neurotransmission and a consequent failure to stimulate inhibitory gamma-aminobutyric acid (GABA)-ergic interneurons, and/or anatomic degeneration of inhibitory GABAergic interneurons. The result of these hypothesized mechanisms is excessive stimulation of the alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA)/kainate class of glutamate receptor complexes. In turn, this excessive stimulation of AMPA/kainate receptors could lead to disruption of ionic gradients, depletion of energy reserves expended in an attempt to restore and maintain the ionic disequilibrium across neuronal membranes, generation of reactive oxygen species, and cell death from apoptotic and other mechanisms. The postulated existence of disinhibited glutamatergic neurotransmission and the subsequent cascade of excitotoxic events resulting from NMDA receptor hypofunction (NRH), anatomic degeneration of inhibitory GABAergic interneurons, or a combination of the two has suggested a diverse variety of experimental therapeutic interventions for schizophrenia. These interventions include facilitation of NMDA receptor-mediated neurotransmission, potentiation of GABAergic neurotransmission, antagonism of AMPA/kainate receptors, and "quenching" of locally generated reactive oxygen species. In fact, several of these approaches have already been pursued or are proposed as part of a systematic clinical investigation of the revised excitotoxic hypothesis of schizophrenia.

Nefazodone in the adjunctive therapy of schizophrenia: an open-label exploratory study

Compared to conventional antipsychotic medications, atypical antipsychotic medications demonstrate greater central serotonin (5HT2) receptor antagonism than dopamine type 2 (D2) receptor antagonism. Nefazodone, an antidepressant medication, exhibits 5HT2 receptor antagonism; we therefore wondered if its addition to stable regimens of antipsychotic medication would increase antipsychotic efficacy, independently of a primary effect on mood, through the mechanism of augmented 5HT2 receptor antagonism. In a pilot investigation, we administered nefazodone (400 mg/d) for 6 weeks as an open-label adjunct to antipsychotic medication in 10 patients with chronic schizophrenia. The patients were moderately depressed at baseline but did not meet criteria for major depressive episode. The Brief Psychiatric Rating Scale (BPRS) and Montgomery-Asberg Depression Rating Scale scores showed statistically significant and clinically robust improvements with nefazodone treatment, which were maintained at follow-up evaluation 2 weeks after the end of nefazodone treatment. There were no adverse events. These results suggest that nefazodone may be a safe and effective adjunct to antipsychotic medications in schizophrenia and that augmentation of 5HT2 antagonism may prove to be a viable strategy for "boosting" antipsychotic efficacy and for treating depressive symptoms in schizophrenia.

The phenomenology of real and illusory tip-of-the-tongue states

The tip-of-the-tongue state (TOT) is the phenomenological experience that a word is on the verge of being recalled. Most research has been directed at TOT etiology and at retrieval processes occurring during a TOT. In this study, TOT phenomenology was examined. In Experiment 1, strong TOTs were more likely than weak TOTs to be followed by correct recognition, and resolution (later recall) of TOTs was higher for strong than for weak TOTs, but only for commission errors. In Experiment 2, emotional TOTs were more likely to be resolved and recognized than nonemotional TOTs. In Experiment 3, imminence was defined as the feeling that retrieval is about to occur. Imminent TOTs were more likely to be followed by resolution and recognition than were nonimminent TOTs. Illusory TOTs (TOTs for unanswerable questions) tended to be weaker, less emotional, and less imminent than TOTs for answerable questions.

Sparkling at the end of the tongue: the etiology of tip-of-the-tongue phenomenology

The tip-of-the-tongue experience (TOT) is the phenomenological experience that a currently inaccessible word is stored in memory and will be retrieved. TOTs appear to be a universal experience that occurs frequently in everyday life, making the TOT an ideal case study in human phenomenology. This paper considers TOTs in light of Tulving's (1989) challenge to the doctrine of concordance, which is the assumption that behavior, cognition, and phenomenology are correlated, if not caused by identical processes. Psycholinguistic and memory theories, consistent with concordance, argue for direct access, or the view that TOTs and word retrieval are caused by the same retrieval processes. The metacognition view challenges concordance and views TOTs as an inference based on nontarget information that is accessible to rememberers. Current data, reviewed here, suggest that TOTs are caused via direct access and through inferential processes. Dissociations between TOTs and retrieval suggest that the causes of TOT phenomenology and the processes of retrieval are not identical.

Visual scanning of facial expressions in schizophrenia

The authors previously observed that schizophrenic patients generated fewer fixations of < or = 50.1 ms in response to faces than did a clinical control group. This study examined whether deficits in short-duration eye movements were related to patients' problems in gestalt perception of faces. Faces were presented in upright and inverted orientations to examine the effects of distorting facial gestalts on eye movements. Normal subjects generated more saccades of < or = 50.1 ms to upright than to inverted faces. Patients' saccades of < or = 50.1 ms did not differ between orientations. Patterns of fixations and of saccades > 50.1 ms did not differ between groups. The results may indicate deficits in these patients in search strategies that underlie perception of facial gestalts.

Stress and a glycinergic intervention interact in the modulation of MK-801-elicited mouse popping behavior

The ability of D-cycloserine, a partial glycine agonist, to modulate mouse popping behavior elicited by MK-801, a noncompetitive NMDA receptor antagonist, was studied in unstressed and stressed mice. In unstressed animals, D-cycloserine (5.6 and 10 mg/kg) attenuated the ability of MK-801 (1.0 mg/kg) to elicit this behavior. However, the ability of D-cycloserine to attenuate MK-801-elicited mouse-popping behavior was not evident in stressed mice, 24 h after they were forced to swim for up to 10 min in cold water. Thus, the therapeutic value of glycinergic interventions may be limited by environmental factors, such as stress.

Effect of visual cues, vital signs, and protocols on triage: a prospective randomized crossover trial

STUDY OBJECTIVES

We sought to compare triage designations derived from in-person and telephone interviews and systematically examine the effect of visual cues, vital signs, and complaint-based protocols on the triage process.

METHODS

We conducted a 2-phase, prospective, observational study employing a randomized, crossover design in a university teaching hospital emergency department. In both phases, every eligible patient underwent sequential in-person and telephone triage interviews conducted by certified ED triage nurses. After taking a history, each nurse chose 1 of 5 hypothetical triage designations and, after being told the patient's vital signs, again selected a designation. Phase 1 designations were based solely on nurses' clinical expertise. In phase 2, both nurses used complaint-based protocols.

RESULTS

Agreement between telephone and in-person designations was poor (percent agreement, 43.1% to 48.8%; kappa,.19 to.26; taub,.34 to.45 for the 4 primary comparisons). Knowledge of vital signs and use of protocols did not improve agreement or increase identification of patients requiring admission to hospital.

CONCLUSION

These data establish that telephone and in-person triage are not equivalent and suggest that visual cues may play an important role in the triage process. It is unclear whether telephone triage is an adequate method of assigning patients to an appropriate level of care.

Illusory tip-of-the-tongue states

The tip-of-the-tongue state (TOT) is the phenomenological experience that a target word is on the verge of being recalled. An illusory TOT occurs when a person experiences a TOT, but the actual target is either unavailable, forgotten, or never learned. Illusory TOTs were induced by asking participants to answer questions that did not have correct answers. In Experiment 1, an episodic-memory paradigm, participants were shown fictional animals, some of which were accompanied by the animal's name (identified targets) and some of which were not (unidentified targets). Some participants experienced TOTs for unidentified targets. In Experiment 2, a semantic-memory paradigm, participants were asked general-information questions, some of which were questions with no correct answer. Every one of the 31 participants experienced at least one illusory TOT. The characteristics of illusory TOTs are discussed in light of inferential and direct-access views of TOTS.

Visual scanning of faces correlates with schizophrenia symptomatology

1. The purpose of this study was to examine whether quantitative measures of visual scanning of faces correlated with the severity of symptoms in patients with schizophrenia. 2. Preattentive visual fixations (fixations less than or equal to 50.1 ms in duration) were measured while 16 subjects with chronic schizophrenia and 38 comparison subjects scanned slides of human faces. 3. A significant inverse correlation was found between the number of preattentive fixations exhibited during 10 seconds of facial scanning and total Brief Psychiatric Rating Scale (BPRS) scores. 4. This study suggests that measures that probe preattentive processing during scanning of faces could represent a novel paradigm for studying the symptoms of schizophrenia.

The mismeasure of memory: when retrieval fluency is misleading as a metamnemonic index

The experiments address the degree to which retrieval fluency--the case with which information is accessed from long-term memory--guides and occasionally misleads metamnemonic judgments. In each of 3 experiments, participants' predictions of their own future recall performance were examined under conditions in which probability or speed of retrieval at one time or on one task is known to be negatively related to retrieval probability on a later task. Participants' predictions reflected retrieval fluency on the initial task in each case, which led to striking mismatches between their predicted and actual performance on the later tasks. The results suggest that retrieval fluency is a potent but not necessarily reliable source of information for metacognitive judgments. Aspects of the results suggest that a basis on which better and poorer rememberers differ is the degree to which certain memory dynamics are understood, such as the fleeting nature of recency effects and the consequences of an initial retrieval. The results have pedagogical as well as theoretical implications, particularly with respect to the education of subjective assessments of ongoing learning.

The relation of output order and commission errors in free recall and eyewitness accounts

We explored the relation between output order and the likelihood of a commission error in free recall under both laboratory and eyewitness conditions. In Experiment 1, participants studied a list of 20 unrelated words and, after a five-minute distractor task, were asked to recall those words. Whereas the items that participants recalled were mostly correct, commission errors were more likely to occur at the end of a participant's output. In Experiment 2, participants viewed a police film depicting an armed robbery. Participants described the perpetrators, their truck, and the sequence of events during the robbery. When describing the perpetrators or the truck, commission errors were more likely to occur at the end of the output. However, when describing the sequence of events, commission errors were more likely to occur in the middle of the output. In Experiment 3, we replicated the finding that commission errors are likely to occur at the end of the output order when participants are describing people. We speculate on the potential application of this finding and its theoretical underpinnings.

The mismeasure of memory: when retrieval fluency is misleading as a metamnemonic index

The experiments address the degree to which retrieval fluency--the case with which information is accessed from long-term memory--guides and occasionally misleads metamnemonic judgments. In each of 3 experiments, participants' predictions of their own future recall performance were examined under conditions in which probability or speed of retrieval at one time or on one task is known to be negatively related to retrieval probability on a later task. Participants' predictions reflected retrieval fluency on the initial task in each case, which led to striking mismatches between their predicted and actual performance on the later tasks. The results suggest that retrieval fluency is a potent but not necessarily reliable source of information for metacognitive judgments. Aspects of the results suggest that a basis on which better and poorer rememberers differ is the degree to which certain memory dynamics are understood, such as the fleeting nature of recency effects and the consequences of an initial retrieval. The results have pedagogical as well as theoretical implications, particularly with respect to the education of subjective assessments of ongoing learning.

Methylene blue adjuvant therapy of schizophrenia

There is growing interest in the role of the nitric oxide (NO) pathway in idiopathic psychotic disorders such as schizophrenia. In this preliminary study, we examined the therapeutic efficacy of methylene blue (MB), a "downstream" inhibitor of one of NO's actions, administered orally as an adjuvant to conventional neuroleptic medications. Specifically, MB blocks NO's activation of soluble guanylyl cyclase. MB has previously been reported to have therapeutic effects in the treatment of psychosis and mania. Preclinical data also suggest that MB might possess antipsychotic potential. Participants in the current study were eight patients with schizophrenia who had incomplete responses to conventional antipsychotics (as evidenced by a Brief Psychiatric Rating Scale [BPRS] total score of 35 or more). These patients completed a 4-week open-label study with a 1 week "off", 2 week "on", and one final week "off" design. Measures of treatment efficacy were the BPRS, Schedule for the Assessment of Negative Symptoms, and Clinical Global Improvement Scale administered weekly. Final scores for each outcome measure item were based on the consensus of at least two trained raters present during each rating interview. A statistically significant, albeit modest, decrease in the severity of psychopathology was observed while the subjects were taking MB, and psychopathology significantly worsened when MB was discontinued. The results suggest a need for further study with MB or perhaps other NO-dependent guanylyl cyclase-inhibiting medications.

Effects of nifedipine, a calcium channel antagonist, on cognitive function in schizophrenic patients with tardive dyskinesia

We examined whether nifedipine, a calcium channel antagonist, added to a stable regimen of neuroleptic medication would affect cognition in patients with chronic schizophrenia or schizoaffective disorder who had tardive dyskinesia. Fifteen patients with tardive dyskinesia were treated with nifedipine (60 mg daily) or matching placebo for 4 weeks and then were crossed over from nifedipine to placebo or from placebo to nifedipine for another 4 weeks. At the end of each 4-week phase of the study, the patients performed a rotary pursuit test of procedural learning and a dementia scale assessing general cognitive abilities. Nifedipine improved performance in the rotary pursuit test and conceptual abilities in the dementia scale compared with placebo, but only for patients who first were exposed to the tests during the placebo condition. These results provide preliminary evidence that calcium channel antagonists might enhance learning and memory in schizophrenic patients with tardive dyskinesia.

Frontal cortical atrophy and negative symptoms in patients with chronic alcohol dependence

It has been suggested that frontal lobe pathology is associated with negative symptoms in patients with chronic alcoholism. In his exploratory study, the authors examined 19 chronic alcoholic inpatients on an alcohol treatment unit and found a significant relationship between severity of frontal atrophy (as measured by CT) and negative symptoms (as measured by the Scale for the Assessment of Negative Symptoms).

Screening derivatized peptide libraries for tight binding inhibitors to carbonic anhydrase II by electrospray ionization-mass spectrometry

This paper describes the use of electrospray ionization-mass spectrometry (ESI-MS) to screen two libraries of soluble compounds to search for tight binding inhibitors for carbonic anhydrase II (EC 4.2.1.1). The two libraries, H2NO2SC6H4C(O)NH-AA1-AA2-C(O)NHCH2CH2CO2H where AA1 and AA2 are L-amino acids (library size: 289 compounds) or D-amino acids (256 compounds), were constructed by attaching tripeptides to the carboxyl group of 4-carboxybenzenesulfonamide. Screening of both libraries yielded, as the tightest binding inhibitor, compound 1 (AA1 = AA2 = L-Leu; binding constant Kb = 1.4 x 10(8) M-1). The ability of ESI-MS to estimate simultaneously the relative binding affinities of a protein to soluble ligands in a library, if general, should be useful in drug development.

Electrospray mass spectrometric investigation of the chaperone SecB

Electrospray ionization mass spectrometry was used to investigate the structure of the Escherichia coli chaperone protein SecB. It was determined that the N-terminal methionine of SecB has been removed and that more than half of all SecB monomers are additionally modified, most likely by acetylation of the N-terminus or a lysine. The use of gentle mass spectrometer interface conditions showed that the predominant, oligomeric form of SecB is a tetramer that is stable over a range of solution pH conditions and mass spectrometer interface heating (i.e., inlet capillary temperatures). At very high pH, SecB dimers are observed. SecB contains a region that is hypersensitive to cleavage by proteinase K and is thought to be involved in conformational changes that are crucial to the function of SecB. We identified the primary site of cleavage to be between Leu 141 and Gln 142. Fourteen amino acids are removed, but the truncated form remains a tetramer with stability similar to that of the intact form.

Impaired motor skill learning in schizophrenia: implications for corticostriatal dysfunction

We assessed skill learning in young and older schizophrenic patients using the rotary pursuit task. Schizophrenic patients displayed impaired learning on this task compared with normal control subjects, but older patients were not more impaired than young ones. The patients' rotary pursuit learning was not correlated to the severity of abnormal movements or to their treatment with medication, but it was associated to conceptual abilities assessed on the Dementia Rating Scale (Mattis 1988). An impairment in acquiring motor procedures in this task might reflect neuropsychological deficits associated with corticostriatal pathology.

d-Cycloserine enhances implicit memory in Alzheimer patients

We tested the ability of d-cycloserine, a partial glycine agonist acting at the N-methyl-D-aspartate (NMDA) receptor complex, to improve implicit memory in Alzheimer patients in a parallel-group, placebo-controlled, double-blind study. One-hundred eight patients with probable Alzheimer's disease of mild to moderate severity received d-cycloserine (5, 15, or 50 mg) or placebo twice daily for 10 weeks. We then evaluated their ability to identify perceptually degraded words, some of which were repeated over multiple trials across 3 days. Implicit memory performance of words repeated across trials was significantly enhanced for the patients who received 15 mg d-cycloserine compared with those who received placebo. These findings support development of NMDA receptor-mediated glutamatergic interventions for the treatment of Alzheimer-related memory disorders.

Engineered chimeric streptavidin tetramers as novel tools for bioseparations and drug delivery

We report the construction of chimeric streptavidin tetramers that are composed of subunits of both wild-type (WT) streptavidin and genetically-engineered streptavidin variants designed for enhanced bioseparation and drug delivery performance. Subunit mixing is accomplished by guanidine thiocyanateinduced denaturation of an equimolar mixture of WT streptavidin and the respective site-directed mutant, followed by renaturation and reassociation of mixed tetramers. In the first example, we demonstrate the mixing of WT subunits with an Asn49Cys (N49C) mutant. The WT/n49C tetramers can be used for site-specific and stoichiometric attachment of therapeutics/imaging agents or targeting proteins through the genetically-engineered thiol while retaining unhindered access to biotin-binding at the WT subunits. Second, we demonstrate that the His127Cys mutation (H127C) results in a streptavidin mutant that forms a disulfide-linked dimer under non-reducing conditions. Mixing of H127C and WT streptavidin subunits results in chimeric tetramers where both the stoichiometry (WT:H127C::1:1) and subunit architecture is controlled by the unique disulfide bridge engineered into H127C. In the third example, WT subunits were mixed with the subunits of a site-directed mutant, Trp120Ala (W120A), which displays a biotin dissociation constant that is enhanced by more than 10(4) compared to WT streptavidin. The W120 biotin-binding affinity is sufficiently high (Ka approximately equal to 10(7) M-1) to immobilize the mutant on a biotinagarose affinity chromatography column, but the engineered off-rate allows for facile elution with excess biotin at physiological pH, whereas WT streptavidin is irreversibly immobilized on the column. We demonstrate that the purified WT/W120A chimeric tetramers combine the advantages of both subunits, allowing for irreversible immobilization of biotinylated targets at the WT subunit, while retaining the reversible separation capabilities of the W120A subunits via biotin-agarose affinity chromatography.

Dissociation of tetrameric ions of noncovalent streptavidin complexes formed by electrospray ionization

The noncovalent tetrameric association of the protein streptavidin formed by electrospray ionization (ESI) mass spectrometry has been observed intact and dissociated in the gas phase. An extended mass-to-charge ratio range quadrupole mass spectrometer was employed to examine the effects of harsher conditions in the ESI atmosphere-vacuum interface region on the streptavidin tetramer. Thermally induced dissociation caused the mass spectra to exhibit a series of complementary monomer and trimer ions that correspond to decomposition of the tetrameric species. Similar results were obtained with tandem mass spectrometric experiments on a Fourier transform ion cyclotron resonance mass spectrometer by application of sustained off-resonance irradiation (SORI) on a selected tetrameric charge state. The technique of single-frequency quadrupole excitation was used to accomplish selected-ion accumulation of the 14 + charge state of the tetramer during ion injection. Subsequent low energy SORI combined with broadband quadrupole cooling produced the 7 + monomer and 7 + trimer species, as well as the 6 + monomer and 8 + trimer complementary ions. The observed asymmetric breakup of the tetramer is qualitatively explained by using physical models.

Bio-affinity characterization mass spectrometry

A new approach, bio-affinity characterization mass spectrometry (BACMS), aimed at providing a more rapid, sensitive and potentially more flexible alternative to techniques presently employed for the characterization of noncovalent interactions in mixtures, such as would be encountered in combinatorial chemistry, in presented. BACMS avoids some of the difficulties and potential artifacts associated with affinity chromatography since the noncovalent associations occur in solution; thus, BACMS avoids the requirement of solid support media and the development of non-interfering linker species. This paper describes the conceptual basis for the methodology and its potential use in applications which include the screening of high affinity ligands in support of new drug development. BACMS exploits new Fourier-transform ion cyclotron resonance (FTICR) mass spectrometry technologies which, when coupled to electrospray ionization (ESI), allow the investigation of specific noncovalent complexes formed in solution. BACMS utilizes the well-known attributes of FTICR, such as the high resolution mass analysis and (MS)n (n > or = 2) capabilities; however, it is even more directly a result of recently developed techniques involving quadrupolar excitation, such as selected-ion accumulation. These tools are demonstrated and the results illustrate the extraordinary sensitivity achievable (solution concentration of 1 x 10-9 M without the use of separations prior to ESI). Thus, the new capabilities demonstrated here, in conjunction with ESI, will be useful for the investigation of very low relative concentration noncovalent association directly from solution, and promote a faster alternative for combinatorial mixture screening and analysis.

Observation of noncovalent complexes to the avidin tetramer by electrospray ionization mass spectrometry

Intact avidin-biotin and avidin-biotin maleimide noncovalent complexes have been observed by electrospray ionization mass spectrometry (ESI-MS) by using an extended mass range quadrupole mass spectrometer. By utilizing mild ES1 interface conditions, the expected solution behavior of four biotin or biotin maleimide molecules noncovalently binding to each avidin tetramer can be preserved in the gas phase. The ESI-MS results show the appropriate mass additions of 973 ± 60 Da for biotin and 1802 ± 40 Da for biotin maleimide to the avidin tetramer species. These results support the hypothesis that substantial retention of higher order structure is possible in the gas phase by using gentle ESI conditions.

L-tyrosine pharmacotherapy of schizophrenia: preliminary data

The utility of L-tyrosine (10 g/day in four divided doses) as an adjuvant to molindone (150 mg/day) in the treatment of schizophrenia was investigated using a placebo-controlled, double-blind crossover design (3 weeks on L-tyrosine, 3 weeks on placebo). The objective of this inpatient study was to increase dopaminergic neural transmission along mesocortical projections in patients by increasing the precursor availability of L-tyrosine for dopamine biosynthesis. Theoretically, this approach might lessen both negative and positive symptoms of schizophrenia and improve frontal lobe-mediated neuropsychological performance. There was no evidence of statistically significant improvement conferred by L-tyrosine as measured by weekly Brief Psychiatric Rating Scale (BPRS), Schedule for the Assessment of Negative Symptoms (SANS), or Clinical Global Impressions (CGI) scales. The 12-h trough plasma level of L-tyrosine was significantly higher in all patients during the L-tyrosine phase of the study (t = -3.9, df = 20, p = 0.0009). At the end of each 3-week study period, no significant differences could be found in Wisconsin Card Sorting Test (WCST) or memory test performance. Smooth-pursuit eye movement (SPEM) performance had significantly more saccadic intrusions during the L-tyrosine supplementation phase compared to the placebo period. This increase in saccades during SPEM suggests that the tyrosine supplementation might have had some central effect.

The cue-familiarity heuristic in metacognition

Four experiments contrasted the cue-familiarity hypothesis of feeling-of-knowing judgments (FKJs) and tip-of-the-tongue feelings (TOTs) to the target-retrievability hypothesis. Familiarity of the cues was contrasted to memorability of the targets in a paired-associate design (e.g., A-B A-B, A-B A-B', A-B A-D, A-B C-D), in which the number of repetitions of the cue A terms was dissociated from the memorability of the target B terms. Little support was found for the target-retrievability hypothesis, because in none of the 4 experiments were FKJs related to target memorability. In one experiment, an omnibus retrieval hypothesis (which implicates total retrieval rather than just correct retrieval) and the cue-familiarity hypothesis produced isomorphic predictions that were borne out by the FKJ and TOT results. All 4 experiments supported the cue-familiarity hypothesis, because FKJs and TOTs were directly related to the number of presentations (and thereby the familiarity) of the cues.

The cue-familiarity heuristic in metacognition

Four experiments contrasted the cue-familiarity hypothesis of feeling-of-knowing judgments (FKJs) and tip-of-the-tongue feelings (TOTs) to the target-retrievability hypothesis. Familiarity of the cues was contrasted to memorability of the targets in a paired-associate design (e.g., A-B A-B, A-B A-B', A-B A-D, A-B C-D), in which the number of repetitions of the cue A terms was dissociated from the memorability of the target B terms. Little support was found for the target-retrievability hypothesis, because in none of the 4 experiments were FKJs related to target memorability. In one experiment, an omnibus retrieval hypothesis (which implicates total retrieval rather than just correct retrieval) and the cue-familiarity hypothesis produced isomorphic predictions that were borne out by the FKJ and TOT results. All 4 experiments supported the cue-familiarity hypothesis, because FKJs and TOTs were directly related to the number of presentations (and thereby the familiarity) of the cues.

Differentiation between selected pairs of tripeptide diastereomers by tandem mass spectrometry on a hybrid tandem mass spectrometer

Low-energy collisionally activated decomposition (CAD) and unimolecular decomposition of the [M+H]+ ions for X-L-Pro-L-Phe, where X is L-Ala, D-Ala, L-Asp, or D-Asp, allow easy differentiation between the LLL and DLL diastereomers. Tandem mass spectrometric (MS/MS) studies of the [M+H]+ ions formed by fast-atom bombardment (FAB) at various ion kinetic energies (Elab values) on a hybrid tandem instrument produced ions of different intensities for the diastereomers. The ratio of NH3 to H2O loss is 0.3 for the L-Ala peptide but 1.7 for the D-Ala isomer at 5 eV. In some L-Ala spectra, the [M+H-NH3]+ ion does not appear at all. The y2 ion is up to twice as abundant in the L-Ala spectra as in the D-Ala, while the b2 ion is somewhat more abundant for CAD of the D-Ala peptide for most collision energies investigated. The D-Asp peptide produces a b2 ion that is more than half-again as abundant as in the case of the L-Asp isomer, and an [M+H-H2O]+ ion that is up to twice as abundant in the D-Asp CAD spectra as in those of the L-Asp. The y1, a2, and phenylalanine immonium ions are each up to twice as abundant in the L-Asp spectra as in those of the D-Asp isomer. The major differences are correlated with force-field calculations on hydrogen-bonded tautomers.

Correlation between antisaccade and Wisconsin Card Sorting Test performance in schizophrenia

In 27 patients with chronic schizophrenia, there was a significant correlation between performance on an antisaccade eye movement task and on the Wisconsin Card Sorting Test. A significant correlation was not obtained between antisaccade task performance and scores on the modified Mini-Mental State examination or the Schedule for the Assessment of Negative Symptoms in Schizophrenia. In addition, patients' antisaccade task performance was impaired compared with that of 12 normal subjects.

Limits of the processing view in accounting for dissociations among memory measures in a clinical population

In three experiments, we examined the performance of patients with schizophrenia on implicit and explicit memory tests that have been shown to involve predominantly data-driven or predominantly conceptually driven processes. In Experiment 1, we compared the implicit tests of category production (conceptually driven) and word identification (data driven) and found that schizophrenic patients' performance on these tests did not differ from that of normal subjects. In Experiment 2, a comparison of the category-production and explicit cued-recall tests, both of which involve conceptual processes, showed that schizophrenic patients were impaired on the cued-recall test but not on the category-production test. In Experiment 3, a comparison of the word-identification and explicit graphemic cued-recall tests, both of which involve data-driven processes, showed that patients were impaired on the cued-recall test but not on the word-identification test. The results of both Experiments 2 and 3 revealed a dissociation between implicit and explicit test performance under conditions in which the two tests involve similar types of processes. These results support theoretical views that distinguish implicit from explicit modes of retrieval.

Effect of a Low-Tryptophan Diet as an Adjuvant to Conventional Neuroleptic Therapy in Schizophrenia

Eleven patients with DSM-III-R schizophrenia were entered into a 4-day tryptophan (TRP)-deficient diet. The diet lowered total plasma TRP levels in all patients; during the diet phase, there was a greater than 50% reduction in mean total plasma TRP levels from the pre-diet phase. The low-TRP diet improved performance on the Stroop Color and Word Test. These data are especially intriguing in view of the suggestion that a deficit in color-word naming is related to frontal lobe dysfunction and the possible occurrence of frontal lobe abnormalities in patients with schizophrenia. Interestingly, depressive symptomatology did not emerge on the TRP-deficient diet, despite the lowering of total plasma TRP levels. There were statistically significant improvements noted on objective ratings of the severity of psychotic symptomatology; however, these statistical improvements were without obvious clinical significance, as the magnitude of the changes on the behavioral ratings were minimal. The results of this study suggest that there might be some adjuvant potential for a low-TRP diet in the treatment of schizophrenia, and that schizophrenia or antipsychotic medications might offer some protection against the depressive effects of a TRP-deficient diet.

The effects of milacemide on item and source memory

Prior research has shown that memory for facts and items (item memory) and memory for where one has learned these items (source memory) can be dissociated under certain conditions. In this study, we examined the effects of milacemide, a derivative of glycine, on memory for item and source information in healthy young and older adults. These results pointed to a dissociation between source memory and item memory: Milacemide administration seemed to facilitate memory for source information but did not affect recognition memory.

Clock drawing in the screening assessment of cognitive impairment in an ambulatory care setting: a preliminary report

In an exploratory study to assess the utility of clock drawing as a screening test for cognitive impairment in medical/surgical outpatients, clock drawing and the 6-item Orientation-Memory-Concentration Test (OMCT) were administered to over 400 randomly selected ambulatory patients over the age of 55 in a busy inner-city hospital. The clock drawing test was completed by 431 patients, and 471 completed the OMCT. Clock drawing errors suggestive of moderate-to-severe cognitive impairment were found in 42.7% of patients; OMCT errors suggestive of moderate-to-severe cognitive impairment were found in 35.4% of the population tested. The clock drawing test might represent a quick-screen for cognitive impairment in an older general medical/surgical outpatient population, and might help identify patients not otherwise recognized as potentially unable to fully understand treatment recommendations.

Some proline substituent effects in the tandem mass spectrum of protonated pentaalanine

Tandem mass spectral effects of the replacement of alanine by proline in pentaalanine were studied. The principal difference when residue 3 or 4 is proline is the great enhancement of the y3 or y2 ion, respectively. This is precisely the principal difference which thermochemical arguments predict, since a y-type ion with N-terminal proline is estimated to be 32 kJ mol-1 more stable than with N-terminal alanine. When this proline effect and the effect of neutral diketopiperazine loss occur simultaneously, the y-type ion is not observed, as predicted from the additivity of substituent effects by the kinetic approach. No interpretation is offered for observations of b-type ion intensities, since the pattern is not obvious to us.

Cue familiarity but not target retrievability enhances feeling-of-knowing judgments

Two hypotheses concerning people's ability to predict later memory performance for unrecalled items were investigated. The target retrievability hypothesis states that feeling-of-knowing judgments (FKJs) are based on partial target information; and the cue familiarity hypothesis asserts that they are based on recognition of the cues. In Experiments 1 and 2, subjects either generated or read the targets of paired associates. Half of the cues had been primed in a pleasantness-rating task. The generation manipulation increased recall but had no effect on FKJs. Cue priming had no effect on recall but increased FKJs. In Experiment 3, using general information questions, primed after the initial recall attempt, both cue and target priming increased FKJs. Experiment 4, which remedied difficulties in Experiment 3, showed no effect of target priming whereas cue priming increased FKJs. The results favor the cue familiarity hypothesis.

Priming is independent of skill learning

This study examined the relation between repetition priming and skill learning. Priming refers to facilitation in processing a specific item as a result of previous exposure to that item. Skill learning refers to general improvement in task performance as a function of practice. In Experiment 1 (N = 60), skill acquisition occurred in partial-word identification and inverted reading tasks but not in a word-fragment completion task. However, the amount of priming was the same in all three tasks. In Experiment 2 (N = 52), priming effects in partial-word identification did not vary as a function of practice with degraded words. In Experiment 3 (N = 40), skill learning was greater with high- than with low-frequency words, whereas priming was unaffected by word frequency. Experiment 4 (N = 20) ruled out the possibility that explicit retrieval was involved in the implicit memory tasks. These results suggest that priming can be independent of skill learning.

Glycine prodrug facilitates memory retrieval in humans

Modulation of glycine concentrations in the brain influences learning and memory functions in experimental animals. We administered milacemide, a glycine prodrug, or placebo to young and older healthy adults, who performed a word-retrieval task. Milacemide administration increased the number of words retrieved and decreased the latency with which these words were retrieved for both young and older adults.

Subtype diagnosis in schizophrenia and its relation to neuropsychological and computerized tomography measures

We compared schizophrenic patients with a subtype diagnosis of paranoia (n = 14) to those with nonparanoid subtype diagnoses (n = 18) on the Wisconsin Card Sorting Test (WCST) and multiple computed tomography (CT) scan measures. The results showed that patients with nonparanoid diagnoses sorted fewer categories and made more perseverative errors on the WCST than did patients with the paranoid diagnosis. However, patients in the nonparanoid group could not be distinguished from those in the paranoid group on CT scan measures of brain structure. Additionally, a significant correlation was found between right frontal sulcal enlargement on CT scans and the number of perseverative errors made on the WCST.

An NMDA intervention strategy in schizophrenia with "low-dose" milacemide

Drugs (e.g., PCP) which interfere with glutamatergic transmission at the N-methyl D-aspartate (NMDA) subclass of glutamate receptors precipitate both positive and negative symptoms of psychosis in humans. Based on a proposed "glutamatergic deficiency" in schizophrenia, pharmacologic facilitation of NMDA-mediated neural transmission by direct stimulation of the strychine-insensitive glycine binding site was attempted with "low-dose" milacemide, an acylated "prodrug" of glycine that readily crosses the blood brain barrier and is converted into glycine in the brain. In a prior study, "high-dose" milacemide proved to have no therapeutic utility in schizophrenia. The failure was thought, possibly, to be related to higher doses of milacemide having antagonist actions at the NMDA receptor complex. In the current study, "low-dose" milacemide (400 mg/day), as the sole pharmacotherapeutic agent, was also without significant clinical benefit. Despite our negative findings for milacemide, other strategies for facilitating NMDA-mediated neural transmission in schizophrenia might be worth pursuing.

Memory for temporal order in schizophrenia

Memory for temporal order information was examined in patients with chronic schizophrenia using the recency discrimination task. In this task, subjects were shown a pair of previously studied words and were asked to choose which one of the two words they had seen more recently. In addition, subjects performed the Wisconsin Card Sorting Test (WCST). The results showed that schizophrenic patients differed from normal control subjects in their performance on the recency discrimination task. In addition, for schizophrenic patients, performance on the recency discrimination task was inversely related to the number of perseverative errors on the WCST. These results provide further evidence of prefrontal-type cognitive deficits in schizophrenia.

Effects of aging on priming and skill learning

This study examined the effects of aging on 2 kinds of implicit memory; repetition priming and skill learning. In Experiment 1, older adults showed less improvement in the skill of reading inverted words than did young adults, but priming performance did not differ for the 2 age groups. Similarly, in Experiment 2, in a partial-word identification task, skill learning was observed only for young adults, whereas there was no age difference in priming. Experiments 1a and 2a, however, showed that when older adults were presented with more perceptual information than were young adults, the age deficit in skill learning was eliminated. These results indicate that skill learning is impaired under data-limited conditions, whereas priming is unaffected under these conditions. It is proposed that the age deficit in skill learning is related to a deficit in perceptual organization and reorganization.