The effects of closed head injury, senile dementia of the Alzheimer's type, and Parkinson's disease on covert orientation of visual attention

Semantic Priming from Uncued Distractors in Alzheimer's Disease

RESEARCH QUESTION

Are semantic impairments in Alzheimer's disease (AD) partially due to deficits in spatial attention?

METHODS AND RESULTS

In a target detection task, both older adults (OAs) and AD individuals were facilitated by valid spatial cues, but only OAs were impaired by invalid cues compared to neutral. In a reading task, spatial cues validly or invalidly cued the location of pictures, which were related or unrelated to subsequent, centrally presented, words. OAs showed semantic priming only after valid cues, whereas AD individuals showed priming after valid and invalid cues.

DISCUSSION

Failure to inhibit uncued locations results in processing of potentially distracting semantic information in AD.

Event-related potential assessment of information processing after closed head injury

We evaluated alterations in information processing after closed head injury as a function of task demands and stimulus modality. Visual and auditory discrimination tasks were administered to 11 survivors of a head injury and 16 matched healthy controls. In auditory tasks, compared with controls, the survivors had smaller N100s, smaller and later N200s, a more posterior scalp distribution of N200, and longer P300 and response latencies. Auditory N200 and P300 correlated highly with duration of unconsciousness. In contrast, in visual tasks, only a reduced N200 in the survivors differentiated the groups. Our results indicate that processing of auditory stimuli, including the perception and discrimination of stimulus features and the evaluation and categorization of stimuli, may be impaired after head trauma. Visual sensory processing may be spared, but higher-order visual processing involved in stimulus classification may be compromised.

The effects of nicotine on spatial and non-spatial expectancies in a covert orienting task

The present study examined how nicotine influences shifts of visuo spatial attention in casual smokers at each of three delays after smoking one cigarette: immediately, 1 h and 24 h. Informative peripheral cues were used to exogenously orient attention to the location where an increase or decrease in the size of a peripheral object was most likely to occur. One size change was more likely to occur than the other and the task was choice (expansion/contraction) reaction time. The performance decrement obtained when the target appeared at an uncued location was smallest in sessions run immediately after smoking (when nicotine levels were highest), suggesting that nicotine may increase the ease with which attention can be disengaged from a cued location. This finding confirms previous research which suggests a specific role for the basal forebrain cholinergic system in visual orienting. In contrast, nicotine was not found to affect non-spatial expectancies based on stimulus-response (expansion/contraction) probability. These findings, together with recent converging evidence, strongly support the proposition that different attentional operations are mediated by different neural subsystems.

Event related potentials during covert orientation of visual attention: effects of cue validity and directionality

Covert orientation of attention was studied in 30 adults who fixated warning cues and pressed a button at target onset. Directional cues (arrows) indicated the most probable (p = 0.8) side of target occurrence. Subjects responded fastest when validly cued, slowest to invalidly cued targets, and at an intermediate rate when the cue (a cross) was not directional. Directional cues took longer to evaluate (increased N1 and P2 latencies) and produced more focussed attention and greater response preparation (enhanced CNV and P3 amplitude) than non-directional cues. These findings indicate that the expectancy of a target can be manipulated by a spatial cue at three levels, sensory, attention, and response preparation, and lead to changes in the sensory perceptual processing of the target. Validly cued targets produced an increase in P1 amplitude reflecting attention enhanced sensory processing whereas invalidly cued targets increased N1 and P3 amplitudes reflecting the re-orientation of attention, and further processing and updating of information required of low probability stimuli respectively. P3 latency to invalidly cued targets was also delayed reflecting the additional processes required to shift attention to a new location. The P3 latency validity effect was smaller than that found for response time suggesting response execution may also be affected by spatial attention.