Brain and behavior: a task-dependent eye movement study

Recent electrophysiological and behavioral studies have found similarities in the neurology of pursuit and saccadic eye movements. In a previous study on eye movements using closely matched paradigms for pursuit and saccades, we revealed that both exhibit bimodal distributions of latency to predictable (PRD) and randomized (RND) stimuli; however, the latency to each type of stimulus was different, and there was more segregation of latencies in saccades than pursuit (Burke MR, Barnes GR. 2006. Quantitative differences in smooth pursuit and saccadic eye movements in humans. Exp Brain Res. 175(4):596-608). To investigate the brain areas involved in these tasks, and to search for correlates of behavior, we used functional magnetic resonance imaging during equivalent PRD and RND target presentations. In the contrast pursuit > saccades, which reflects velocity-dependent versus position-dependent activities, respectively, we found higher activation in the dorsolateral prefrontal cortex (DLPFC) for pursuit and in the frontopolar region for saccades. In the contrast RND > PRD, which principally reflects activation related to visually driven versus memory-driven responses, respectively, we found a higher sustained level of activation in the frontal eye fields during visually guided eye movements. The reverse contrast revealed higher activity for the memory-guided responses in the supplementary eye fields and the superior parietal lobe. In addition, we found learning-related activation during the PRD condition in visual area V5, the DLPFC, and the cerebellum.

Parvodicin, a novel glycopeptide from a new species, Actinomadura parvosata: discovery, taxonomy, activity and structure elucidation

An extensive taxonomic investigation identified strain SK&F-AAJ-271 as a new species, designated Actinomadura parvosata. Fermentations of this organism produce a complex of acidic, lipophilic glycopeptide antibiotics, the parvodicins. Structures for seven of the isolated components were derived from a combination of mass spectral, high-field NMR and chemical techniques. The O-acetyl functionality present in two of the isolated components is a structural feature unique among the known members of this class of antibiotics. The parvodicins are active in vitro against a range of Gram-positive bacteria. The most active parvodicin, C1, produces high serum levels in vivo and has the potential for a long duration of action.

Quantitative differences in smooth pursuit and saccadic eye movements

Recently it has been suggested that smooth pursuit (SP) and saccadic (SAC) eye movements share many common brain substrates in the planning and control of eye movements (Krauzlis in J Neurophysiol 91:591-603, 2004). Evidence is mounting that these two types of eye movements may also share similar mechanisms used to drive both reactive and predictive eye movement responses (Missal and Keller in J Neurophysiol 88:1880-1892, 2002, Keller and Missal in Ann NY Acad Sci 1004:29-39, 2003). The objective of this study was to quantify these similarities by establishing whether the behavioural response properties of human eye movements to predictive (PRD) and randomized (RND) conditions are quantitatively similar for both SP and SAC in directly comparable paradigms. Two previous studies have attempted to evaluate the coordination and motor preparation time of SP and saccadic eye movements (Erkelens in Vis Res 46:163-170, 2006; Joiner and Shelhamer in Exp Brain Res, Epub ahead of print, 2006). However, no previous study has quantitatively evaluated PRD and RND conditions to discretely presented SP and SAC tasks. We used simple SAC and SP paradigms in blocks of PRD and RND presentations, with eye movements monitored throughout using an IR-limbus eye-tracking system (Skalar). Twelve normal subjects (aged between 20 and 39 years) participated in the study which took place over two recording sessions, on two separate days. Data were analysed for two main comparable descriptive statistics: latency and eye velocity/displacement gain. The results presented here support the notion that SP and SAC share common brain substrates/mechanisms in the generation of responses to PRD and RND visual targets but differ in the movement preparation time.

Anticipatory eye movements evoked after active following versus passive observation of a predictable motion stimulus

We used passive and active following of a predictable smooth pursuit stimulus in order to establish if predictive eye movement responses are equivalent under both passive and active conditions. The smooth pursuit stimulus was presented in pairs that were either 'predictable' in which both presentations were matched in timing and velocity, or 'randomized' in which each presentation in the pair was varied in both timing and velocity. A visual cue signaled the type of response required from the subject; a green cue indicated the subject should follow both the target presentations (Go-Go), a pink cue indicated that the subject should passively observe the 1st target and follow the 2nd target (NoGo-Go), and finally a green cue with a black cross revealed a randomized (Rnd) trial in which the subject should follow both presentations. The results revealed better prediction in the Go-Go trials than in the NoGo-Go trials, as indicated by higher anticipatory velocity and earlier eye movement onset (latency). We conclude that velocity and timing information stored from passive observation of a moving target is diminished when compared to active following of the target. This study has significant consequences for understanding how visuomotor memory is generated, stored and subsequently released from short-term memory.

Dissociation of the rostral and dorsolateral prefrontal cortex during sequence learning in saccades: a TMS investigation

This experiment sought to find whether differences exist between the dorsolateral prefrontal cortex (DLPFC) and the medial rostral prefrontal cortex (MRPFC) for performing stimulus-independent and stimulus-oriented tasks, respectively. To find a causal relationship in these areas, we employed the use of trans-cranial magnetic stimulation (TMS). Prefrontal areas were stimulated whilst participants performed random or predictable sequence learning tasks at stimulus onset (1st presentation of the sequence only for both Random and Predictable), or during the inter-sequence interval. Overall, we found that during the predictable task a significant decrease in saccade latency, gain and duration was found when compared to the randomised conditions, as expected and observed previously. However, TMS stimulation in DLPFC during the delay in the predictive sequence learning task reduced this predictive ability by delaying the saccadic onset and generating abnormal reductions in saccadic gains during prediction. In contrast, we found that stimulation during a delay in MRPFC reversed the normal effects on peak velocity of the task with the predictive task revealing higher peak velocity than the randomised task. These findings provide causal evidence for independent functions of DLPFC and MRPFC in performing stimulus-independent processing during sequence learning in saccades.

Touch responses made to remembered and visual target locations in the dark: a human psychophysical study

Saccadic eye movements made to remembered locations in the dark show a distinct up-shift in macaque monkey, and slight upward bias in humans (Gnadt et al. 1991). This upward bias created in the visual spatial mapping of a saccade may be translated downstream in a hand/touch movement. This error could possibly reveal (a) information about the frames of reference used in each scenario and (b) the sources of this error within the brain. This would suggest an early planning stage if they are shared, or a later stage if the errors are distinct.

METHODS

Eight human subjects performed touch responses to a touch screen monitor to both visual and remembered target locations. The subjects used a high-resolution touch-screen monitor, a bite bar and chin-rest for restricting head movements during responses. All target locations were 20 degrees vectors from the central starting position in horizontal, vertical and oblique planes of motion.

RESULTS

Subjects were accurate to both visual and remembered target locations with little variance. Subject means showed no significant differences between control and memory trials; however, a distinct asymmetry was observed between cardinal and oblique planes during memory trials. Subjects consistently made errors to oblique locations during touches made to the remembered location that was not evident in control conditions. This error pattern revealed a strong hypermetric tendency for oblique planes of touches made to a remembered location.

The multiple roles of chronic stress and glucocorticoids in Alzheimer's disease pathogenesis

Chronic stress and the accompanying long-term elevation of glucocorticoids (GCs), the stress hormones of the body, increase the risk and accelerate the progression of Alzheimer's disease (AD). Signatures of AD include intracellular tau (MAPT) tangles, extracellular amyloid β (Aβ) plaques, and neuroinflammation. A growing body of work indicates that stress and GCs initiate cellular processes underlying these pathologies through dysregulation of protein homeostasis and trafficking, mitochondrial bioenergetics, and response to damage-associated stimuli. In this review, we integrate findings from mechanistic studies in rodent and cellular models, wherein defined chronic stress protocols or GC administration have been shown to elicit AD-related pathology. We specifically discuss the effects of chronic stress and GCs on tau pathogenesis, including hyperphosphorylation, aggregation, and spreading, amyloid precursor protein (APP) processing and trafficking culminating in Aβ production, immune priming by proinflammatory cytokines and disease-associated molecular patterns, and alterations to glial cell and blood-brain barrier (BBB) function.

The scientific rationale and development of an optimized stannous fluoride dentifrice, Part 1

Stannous fluoride has been recognized as an effective anticavity therapeutic agent since the early 1950s. There has recently been a resurgence in activity to discover ways to fully exploit its documented antimicrobial activity. Through the use of targeted in vitro methodology to predict in vivo efficacy, a highly optimized stannous fluoride dentifrice has been developed. Careful selection of stabilizing agents formulated into a unique system has resulted in a Colgate Optimized Stannous Fluoride (COSF) dentifrice that has been proven to simultaneously help control supragingival plaque, gingivitis, supragingival calculus, and caries. Furthermore, the COSF dentifrice has been clinically shown not to cause the traditional stannous fluoride staining of dentition.