Shifting attention to different levels within global-local stimuli: A study of normal participants and a patient with temporal-parietal lobe damage.

Shifts of attention to different levels of global-local stimuli were examined in normal participants and a patient with right temporal-parietal lobe damage. Global-local stimuli were presented in sequential couplets and the target could either be at the same global-local level or the target could change levels within each couplet. Normal participants were faster to respond to the second stimulus when the target remained at the same level compared to when it changed levels. This level-shifting effect appeared to be independent of any perceptual- or identity/response-based priming, and did not appear to be due to the size of the stimuli per se. In contrast, the patient did not display any level-shifting effects when the target appeared at the global level, whereas he did display this effect when the target appeared at the local level. These results suggest that the right temporal-parietal lobe may be involved in activating attentional weights to the different levels of global-local stimuli. These results also indicate that intra-stimulus attentional shifts are mediated by different neurocognitive mechanisms than are spatial attentional shifts.

Altered brain response to verbal learning following sleep deprivation.

The effects of sleep deprivation on the neural substrates of cognition are poorly understood. Here we used functional magnetic resonance imaging to measure the effects of 35 hours of sleep deprivation on cerebral activation during verbal learning in normal young volunteers. On the basis of a previous hypothesis, we predicted that the prefrontal cortex (PFC) would be less responsive to cognitive demands following sleep deprivation. Contrary to our expectations, however, the PFC was more responsive after one night of sleep deprivation than after normal sleep. Increased subjective sleepiness in sleep-deprived subjects correlated significantly with activation of the PFC. The temporal lobe was activated after normal sleep but not after sleep deprivation; in contrast, the parietal lobes were not activated after normal sleep but were activated after sleep deprivation. Although sleep deprivation significantly impaired free recall compared with the rested state, better free recall in sleep-deprived subjects was associated with greater parietal lobe activation. These findings show that there are dynamic, compensatory changes in cerebral activation during verbal learning after sleep deprivation and implicate the PFC and parietal lobes in this compensation.

Sleep deprivation-induced reduction in cortical functional response to serial subtraction.

Thirteen normal volunteers were studied with fMRI during arithmetic performance after a normal night of sleep and following sleep deprivation (SD). Aims included determining whether the prefrontal cortex (PFC) and the parietal lobe arithmetic areas are vulnerable to the effects of SD. After a normal night of sleep, activation localized to the bilateral PFC, parietal lobes and premotor areas. Following SD, activity in these regions decreased markedly, especially in the PFC. Performance also dropped. Data from the serial subtraction task are consistent with Horne's PFC vulnerability hypothesis but, based on this and other studies, we suggest the localized, functional effects of SD in the brain may vary, in part, with the specific cognitive task.