Our a priori hypotheses were partly supported, but with important differences bearing further investigation. Although healthy controls did demonstrate leftward errors with left distraction, we only observed a trend toward leftward ‘where’ distraction when we compared left- and no-distraction conditions, which did not reach significance after correction for multiple comparisons. The small sample size could have also affected this. However, a highly significant three-way interaction between group, distraction side, and ‘where’/‘aiming’ bias type suggested that distraction affects different types of bias in subjects with dementia as compared with normal controls.
In a surprising finding, we did not note that subjects with dementia had increased errors in the presence of a novel distractor. When subjects with pAD were asked to bisect lines, they were not distracted by the presence of a person standing near the stimulus. This is similar to the effect reported by Drago et al. [26
], who implemented two types of distraction, a bottom-up meaningless condition and a top-down condition requiring the participant to name the color of the lateral distractor. Their meaningless, bottom-up condition, similar to our distractor condition, consisted of a light placed lateral to the line without specific response relevant to the participant. They observed that in the top-down condition, in which participants were required to name the color of the light, pAD participants made increased spatial errors in the direction of the stimulus (manifested distraction). This was attributed to difficulty with executive re-allocation of spatial attention.
However, our results, unlike those of Drago et al. [26
], support a difference in motor preparatory spatial system processing in pAD in response to distraction. In our experiment, subjects with pAD manifested rightward ‘aiming’ bias at baseline, which moved leftward under conditions of leftward distraction. Thus, although left-sided distraction in our pAD subjects was not associated with any increase in ‘where’ bias, it was associated with leftward increases in ‘aiming’ bias, a phenomenon not observed in controls.
In a normal individual, the predominant response to left-sided distraction may be reallocation of perceptual-attentional ‘where’ spatial resources toward the left-sided spatial region in which novel stimuli appear. Motor-intentional resources relevant to ‘aiming’ output systems may be unaffected in the normal brain, until top-down gating releases a response. We do not know why leftward distraction may affect ‘aiming’ spatial systems in pAD.
It is not clear why left-sided distraction might induce changes in ‘aiming’ bias in subjects with dementia and pAD, rather than inducing changes in ‘where’ bias; however, this may be consistent with exaggerated response to top-down spatial cueing observed by Drago et al. [26
]. It is also possible that subcortical activity induced by distracting stimuli via structures such as the superior colliculus may interact differently with spatial systems in dementia and pAD than it does in normal visual-motor function. Neurodegeneration affecting the superior colliculus in pAD [10
] may reduce perceptual-attentional ‘where’ responsivity to distraction. However, the deep superior colliculus, mediating rapid action decisions on novel environmental stimuli or unexpected events, relevant in ‘aiming’ bias, may be less affected [27
]. Thus, the deep, motor superior colliculus may still be responsive to input from premotor frontal lobe ‘aiming’ responses activated by distracting stimuli or by top-down spatial response sets [28
]. In pAD, this may result in a response to distraction which may be manifested more by an ‘aiming’ motor intentional abnormality than by a ‘where’ perceptual-attentional allocation asymmetry. Such an asymmetry of collicular activation could be small in magnitude and not normally measurable except in sensitive testing situations such as the video line bisection task.
If an ‘aiming’ spatial asymmetry in pAD in response to leftward distraction reflects a small, measurable spatial environmental dependency [13
], it could be a source of errors during complex spatial activities such as driving. It also might appear early in the course of pAD; thus, it is important to gain a better understanding of the mechanism of this phenomenon and how any impact on safety might be prevented, reduced, or eliminated. Drago et al.'s [26
] study suggests that leftward ‘aiming’ distraction in pAD might be increased by top-down motor cuing in the direction of a distractor, and reduced by top-down motor cuing in the opposite direction. If the deep, motor colliculus plays an important role in these errors, they might also be more marked in a body-centered than head-centered reference frame [27
]. Further experiments on these topics are indicated.