A neural model of binocular integration and rivalry based on the coordination of action-potential timing in primary visual cortex

In normal vision, the inputs from the two eyes are integrated into a single percept. When dissimilar images are presented to the two eyes, however, they compete for perceptual dominance, so that one eye's view suppresses that of the other. Recent evidence suggests that this phenomenon, known as binocular rivalry, arises through competition between alternative stimulus interpretations in extrastriate cortex. Because eye-specific information appears to be lost at this stage, it remains unclear how the stimulus conditions that yield binocular rivalry are distinguished from those that produce stable single vision. Using a neural network that models the mammalian early visual system, I investigate here the hypothesis that congruent and conflicting stimuli are distinguished by their different effects on the relative timing of action potentials in primary visual cortex (V1), where monocular inputs are first combined. In the model, congruent stimulation of both eyes results in synchronization of discharges among binocular neurons in V1. By contrast, conflicting stimulation of the two eyes results in neuronal asynchrony in this area. This asynchrony then produces rivalrous response suppression at later stages in the visual pathway. Synchronization of firing in V1, however, prevents such competition, thereby ensuring non-rivalrous responses. These novel effects of spike timing on competition emerge naturally from the network dynamics. The results suggest that input-related differences in relative spike timing at an early stage of visual processing may play an important part in the phenomena both of binocular integration and rivalry; furthermore, they indicate that the temporal patterning of cortical activity may be a fundamental mechanism of selection among competing stimulus representations.      

Amiodarone reduces procedures and costs related to atrial fibrillation in a controlled clinical trial.

BACKGROUND: Atrial fibrillation is the most common sustained cardiac arrhythmia, and engenders significant health care costs. The impact of various treatment options for atrial fibrillation on hospital costs has not been evaluated in a randomized trial. METHODS: We analysed 1-year follow-up data on 392 patients randomized to low dose amiodarone (200 mg. day(-1)) or alternative first-line therapy (sotalol or propafenone) in a multicentre trial (Canadian Trial of Atrial Fibrillation, CTAF). RESULTS: Patients in the amiodarone group had fewer electrical cardioversions (65 vs 109 for patients in the sotalol/propafenone group, P<0.0001), and pacemaker insertions (4 vs 11, P=0.07). The average amiodarone patient spent fewer days in hospital (0.47 vs 0.97, P=0.01), and incurred lower costs ($532 vs $898, P=0.03), for admissions where atrial fibrillation was the admitting diagnosis. Average total hospital costs per patient for all admissions, as well as average combined hospital and physician costs per patient, showed wide variations within the treatment arms and were not significantly different between groups. CONCLUSION: For patients in whom antiarrhythmic drug therapy is indicated, low dose amiodarone significantly reduces atrial fibrillation-related costs by reducing the number of atrial fibrillation-related procedures.     

Zero-lag synchronous dynamics in triplets of interconnected cortical areas.

Oscillatory and synchronized activities involving widespread populations of neurons in neocortex are associated with the execution of complex sensorimotor tasks and have been proposed to participate in the 'binding' of sensory attributes during perceptual synthesis. How the brain constructs these coherent firing patterns remains largely unknown. Several mechanisms of intracortical synchronization have been considered, in particular mutual inhibition and reciprocal excitation. These mechanisms fail to account for the zero-lag correlations observed among areas located at different levels in the visual hierarchy because the asymmetric laminar organization of ascending and descending connections in this hierarchy would predict systematic inter-areal phase lags. Here we show through detailed computer simulations that, when triplets rather than pairs of reciprocally connected areas in a cortical hierarchy are considered, zero-lag synchronization emerges naturally from their three-way interactions. These simulations were motivated by the observation that most areas in the cat and macaque monkey visual cortex are organized in such triplets. Our results suggest that patterns of anatomical connections in the mammalian neocortex provide a structural basis for the multi-level synchronization of neuronal activity.     

Covariation of activity in visual and prefrontal cortex associated with subjective visual perception

Visual areas of the occipitotemporal pathway are thought to be essential for the conscious perception of objects, but the contribution of other cortical regions and the neural mechanisms leading to the awareness of a visual stimulus remain unclear. By using functional MRI in humans exposed to bistable viewing conditions, subjective visual perception was related to covariation of activity in multiple extrastriate ventral, parietal, and prefrontal cortical areas. The coordination of activity among these regions was not linked to external sensory or motor events; rather, it reflected internal changes in perception and varied in strength with the frequency of perceptual events, suggesting that functional interactions between visual and prefrontal cortex may contribute to conscious vision. Because similar cortical systems have been implicated in short-term memory and motor planning, the results also imply that related neural processes may underlie visual awareness and the organization of voluntary behavior contingent on sensory cues.     

Cost Evaluation of Rhythm Control Methods for Atrial Fibrillation: Evidence from CTAF

Atrial fibrillation (AF) is a highly prevalent arrhythmia that is difficult to treat and generates important health care costs. One consideration in the selection of various therapeutic options is the cost of a given treatment compared to that of alternatives. The Canadian Trial of Atrial Fibrillation (CTAF) evaluated the effectiveness of sinus rhythm maintenance with amiodarone compared to propafenone or sotalol in a prospective, randomized fashion. A subsequent CTAF substudy of the medical costs associated with amiodarone vs. propafenone/sotalol found that amiodarone decreased AF-related costs. This paper reviews the results of the CTAF cost-analysis substudy in the context of other analyses in the literature of the cost effectiveness of amiodarone in AF. The costs associated with amiodarone therapy are no greater than for other sinus rhythm maintenance drugs, and for some cost categories and some patient subgroups are likely to be less, despite amiodarone's greater therapeutic efficacy. However, additional considerations are important in evaluating the clinical place of amiodarone, including its adverse effect and pharmacokinetic profile. As well, the results of recent randomized clinical trials have highlighted the limitations of sinus rhythm maintenance as a primary therapeutic objective in AF. The decision about whether and at what point to use amiodarone in a given patient requires a careful analysis of the individual case, in terms of symptomatology during AF, the response to previous treatment regimes, and risk factors for various forms of adverse drug reactions.     

Parental divorce and young adult children's romantic relationships: resolution of the divorce experience

Fifty-one romantically involved young Israeli adults, whose parents were divorced, were questioned about their romantic relationship, parents' conflict, and current feelings about and reconstruction of the divorce. An integrative perception of the divorce was found to be related to fewer problems and to higher levels of friendship, enjoyment, and intimacy in the relationship. Implications for research and intervention with young adults are discussed.