Spatial Remapping Mechanisms and their impairments in patients with right parietal lesions

We analysed location priming effects, and thus the ability to
integrate visual information across subsequent trials in the three
studies presented in the following chapters. Based on previous
findings in healthy subjects priming effects occur under conditions
that require no attentional shifts in-between subsequent trials,
i.e. with the subject’s fixation maintained and the search displays
presented at retinotopically equal locations with respect to the
fixation cross (Maljkovic & Nakayama, 1996; Geyer,
Krummenacher, & Müller, 2007). In study 1 we tested whether
location-based priming effects would outlive spatial shifts of
attention in healthy subjects. The basic assumption, derived from
Pisella and Mattingley´s (2004) account was they would, and that
the outliving would be based on intact spatial remapping mechanisms
operating in healthy subjects along with attention shifts. The aim
of study 1 in view of further investigations in patients, i.e. in
studies 2 and 3 was to test the suitability of the priming paradigm
to serve our purpose to investigate spatial remapping abilities,
and furthermore, in patients, purported deficits. To test whether
priming effects would outlive saccadic shifts of overt attention,
subjects had to re-fixate the fixation cross in turns on the left
and on the right side of the monitor between subsequent trials. In
two separate experiments priming effects were measured at
post-saccadically retinotopic and spatiotopic locations,
respectively, and compared to effects in a control experiment
requiring no attention shifts. The experiments were also applied in
study 2. To test whether priming effects would outlive covert
shifts of attention subjects´ attention was distracted between
subsequent trials while their fixation was maintained. In two
separate experiments priming effects were measured after exogenous
and endogenous distraction, respectively, and compared to a control
experiment without distraction. The experiments were also applied
in study 3. The presence of priming effects despite interfering
attention shifts could be taken as evidence that processes that
integrate visual information along with attention shifts, i.e.
spatial remapping mechanisms, operate flawlessly in healthy
subjects. The aim of study 2 was to test whether the parietal
cortex provides the neural correlate of spatial remapping as
assumed by Pisella and Mattingley (2004). To that end, we tested
whether spatial remapping deficits would occur after saccadic
shifts of overt attention in patients with right parietal lesions.
Deficits were expected to be indicated by altered or disturbed
priming effects in those patients as compared to priming effects in
healthy control subjects and furthermore, in patients with
right-hemispheric lesions not involving the parietal lobe. Latter
patients (without parietal lesions) were tested, to control for and
differentiate between deficits occurring more generally after
right-hemispheric damages and specific deficits occurring after
right parietal damages. Moreover, given the crucial role of priming
for visual search (Chun & Nakayama, 2000) disturbed priming as
an indicator of deficient spatial remapping would provide support
for the assumption made by Pisella and Mattingley (2004) that
spatial remapping deficits contribute to visual search impairments
as they are often observed in patients with right parietal lesions.
The aim of study 3 was to test the model’s assumption that spatial
remapping is required after spatial shifts of covert attention.
Again, we compared priming effects of patients with right parietal
lesions to those of patients with right-hemispheric lesions without
parietal involvement, and healthy subjects. We expected priming
effects to be disturbed in patients with right parietal lesions
provided that covert attention shifts induce spatial remapping
requirements, and that the anatomical site of spatial remapping
mechanisms operating to integrate visual information across spatial
shifts of covert attention is located in the parietal lobe.