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Abstract:
Abstract: Attention to non-spatial features like color or
spatial frequency usually leads to effects in the event-related
potentials (ERP's) which are (a) late in time, and (b) not
modulations of the early, exogenous responses to the features per
se (for a review see Heslenfeld et al., Biol. Psych., 1997). One
reason for this may be the use of tasks in which subjects respond
to conjunctions of features, instead of to the presence or absence
of a single feature. This may lead to a conservative strategy, in
which the earliest moment of selective processing is postponed in
time. In the present study, just two stimuli were presented
equiprobably and unpredictably during each block of trials. Stimuli
were either foveal or extrafoveal gratings, whose spatial
frequencies differed by 1, 2, or 3 octaves. Task was to respond
manually to one of the two stimuli. ERP's were recorded from 64
scalp sites in 12 subjects. Data indicate that attention effects in
the ERP's occur indeed earlier in time under these conditions, and
that both onset and peak latencies decrease with increasing
discriminability (Frontal Selection Positivity: 180 to 120 ms;
Occipital Selection Negativity: 170 to 130 ms). In addition, there
were early occipital effects at 120 ms for foveal and at 90 ms for
extrafoveal gratings, which may point to a modulation of the
exogenous responses to these non-spatial features at a very early
level.
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