Attention

Attention has been investigated in AD in four main areas: phasic, sustained, selective, and divided attention. Phasic attention involves maintaining a state of readiness to respond to a stimulus for very short periods. This appears to be normal in AD, as shown by the normal facilitatory effect of a warning signal prior to having to respond to a stimulus. For example, when a tone precedes having to press one of two keys, according to the position of a "square" on a computer screen, the speed advantage provided by the tone is normal in AD. Sustained attention or "vigilance" is the ability to attend to a particular sensory input or set of sensory inputs for long time periods. Preservation of vigilance in AD has been shown on various tasks, including that requiring a person to listen to trains of short tones.

Selective attention is the ability to shift attention at the perceptual stage. Within the visual domain this has been studied in relation to spatial attention, or moving the focus of attention between specific locations. Here, the processes of engaging or focusing attention on a location, disengaging, and shifting to another location can be studied. In AD, there is evidence that a deficit exists in disengaging and switching attention. Performance on the Posner selective attention task supports this conclusion. AD has been investigated by employing a task in which a particular response key has to be pressed according to whether a vowel or consonant appears to the left or the right of a central fixation stimulus. An arrow is shown just before the stimulus, pointing to either the left or right side of the screen, predicting the stimulus (valid cue) or pointing in the wrong direction (invalid). Most of the time the cue points in the right direction. In AD, there is a normal advantage when the valid cue is shown, indicating that engaging attention is normal. For the invalid cue, there is a greater increase in response time for AD, indicating problems with disengaging or switching attention away from the invalid cue. Impairments in selective attention have also been shown in relation to stimulus attributes. This has been investigated by showing digits constructed from arrays of smaller digits and then requiring attentional shifts between the larger digits (global) and the smaller ones (local). This requirement leads to much larger response times in AD. Auditory selective attention has been studied using the dichotic listening task, in which simultaneous presentation of two strings of digits is provided. The normal right ear advantage when recalling the two-digit strings is lost in AD. Additionally, when instructed to recall the left ear first, there is an inability to benefit from a left ear advantage. This is interpreted as indicating that attention cannot be switched at will in AD.

People with AD perform very poorly on tasks that involve divided attention. This has formally been studied by requiring patients to combine tracking a moving node on a visual display unit using a light pen and other distracter tasks, such as pressing a foot pad when a tone is heard or repeating a string of digits. This type of task results in very high levels of interference in AD patients when compared to controls, even if the difficulty level of the tasks is adjusted to ensure that overall the difficulty of each individual task matches the ability of the participant.

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