AND RENt MAROISVanderbilt University. Nashville, Tennessee Under conditions of rapid serial visual presentation, subjects display a reduced ability to report the second of two targets (Target 2; T2) in a stream of distractors if it appears within 200-500 msec of Target I (Tl). This effect, known as the attentional blink (AB), has been central in characterizing the limits of humans' ability to consciously perceive stimuli distributed across time. Here, we review theoretical accounts of the AB and examine how they explain key f"mdings in the literature. We conclude that the AB arises from attentional demands ofTl for selection, working memory encoding, episodic registration, and response selection, which prevents this high-level central resource from being applied to T2 at short T 1-T2 lags. TI processing also transiently impairs the redeployment of these attentional resources to subsequent targets and the inhibition of distractors that appear in close temporal proximity to T2. Although these f"mdings are consistent with a multifactorial account of the AB, they can also be largely explained by assuming that the activation of these multiple processes depends on a common capacity-limited attentional process for selecting behaviorally relevant events presented among temporally distributed distractors. Thus, at its core, the attentional blink may ultimately reveal the temporal limits of the deployment of selective attention.Our visual environment constantly changes across the dimensions of both time and space. Within the ftrst few hundred milliseconds of viewing a scene, the visual system is bombarded with much more sensory information than it is able to process up to awareness. To overcome this limitation, humans are equipped with fIlters at a number of different levels of information processing. For example, high-resolution vision is restricted to the fovea, with acuity drastically reduced at the periphery. Such front-end mechanisms reduce the initial input; however, they still leave the visual system with an overwhelming amount of information to analyze. To meet this challenge, the human attentionaI system prioritizes salient stimuli (targets) that are to undergo extended processing and discards stimuli that are less relevant for behavior after only limited analysis (Broad-

When humans attempt to perform two tasks at once, execution of the first task usually leads to postponement of the second one. This task delay is thought to result from a bottleneck occurring at a central, amodal stage of information processing that precludes two response selection or decision-making operations from being concurrently executed. Using time-resolved functional magnetic resonance imaging (fMRI), here we present a neural basis for such dual-task limitations, e.g. the inability of the posterior lateral prefrontal cortex, and possibly the superior medial frontal cortex, to process two decision-making operations at once. These results suggest that a neural network of frontal lobe areas acts as a central bottleneck of information processing that severely limits our ability to multitask.

In recent years there has been an exponential rise in the number of studies employing transcranial direct current stimulation (tDCS) as a means of gaining a systems-level understanding of the cortical substrates underlying behaviour. These advances have allowed inferences to be made regarding the neural operations that shape perception, cognition, and action. Here we summarise how tDCS works, and show how research using this technique is expanding our understanding of the neural basis of cognitive and motor training. We also explain how oscillatory tDCS can elucidate the role of fluctuations in neural activity, in both frequency and phase, in perception, learning, and memory. Finally, we highlight some key methodological issues for tDCS and suggest how these can be addressed.

Summary Our ability to multitask is severely limited: Task performance deteriorates when we attempt to undertake two or more tasks simultaneously. Remarkably, extensive training can greatly reduce such multitasking costs. While it is not known how training alters the brain to solve the multitasking problem, it likely involves the prefrontal cortex given this brain region’s purported role in limiting multitasking performance. Here we show that the reduction of multitasking interference with training is not achieved by diverting the flow of information processing away from the prefrontal cortex, or by segregating prefrontal cells into independent task-specific neuronal ensembles, but rather by increasing the speed of information processing in this brain region, thereby allowing multiple tasks to be processed in rapid succession. These results not only reveal how training leads to efficient multitasking, they also provide a mechanistic account of multitasking limitations, namely the poor speed of information processing in human prefrontal cortex.