Here is the abstract of a paper by A. Shackman, J. Maxwell, B. McMenamin, L. Greischar and R. Davidson in The Journal of Neuroscience, Jan 2011, Stress Potentiates Early and Attenuates Late Stages of Visual Processing. The whole paper is not freely available unfortunately.

Stress can fundamentally alter neural responses to incoming information. Recent research suggests that stress and anxiety shift the balance of attention away from a task-directed mode, governed by prefrontal cortex, to a sensory-vigilance mode, governed by the amygdala and other threat-sensitive regions. A key untested prediction of this framework is that stress exerts dissociable effects on different stages of information processing. This study exploited the temporal resolution afforded by event-related potentials to disentangle the impact of stress on vigilance, indexed by early perceptual activity, from its impact on task-directed cognition, indexed by later postperceptual activity in humans. Results indicated that threat of shock amplified stress, measured using retrospective ratings and concurrent facial electromyography. Stress also double-dissociated early sensory-specific processing from later task-directed processing of emotionally neutral stimuli: stress amplified N1 (184–236 ms) and attenuated P3 (316–488 ms) activity. This demonstrates that stress can have strikingly different consequences at different processing stages. Consistent with recent suggestions, stress amplified earlier extrastriate activity in a manner consistent with vigilance for threat (N1), but disrupted later activity associated with the evaluation of task-relevant information (P3). These results provide a novel basis for understanding how stress can modulate information processing in everyday life and stress-sensitive disorders.

When involved in a task, the prefrontal cortex steers attention. Only surprising sensory input will usually overcome the task oriented focus of attention. It seems that stress overturns this situation and makes non-surprising sensory input override tasks in steering attention. This is probably important for the avoidance of danger – the environment requires careful monitoring. But I suppose this is part of why being upset can make it so hard to concentrate on what I’m trying to get done.