Abstract
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The Late Positive Complex (LPC) of the event-related potential
(ERP) has long been considered a central nervous system index of the
orienting reflex (OR). Previous studies have found that when
examined as a single ERP component, the LPC demonstrates habituation,
and reflects stimulus characteristics that are associated with the
OR (novelty, intensity, significance). However, a growing body of
evidence suggests that the LPC is not simply a unitary complex. It has
been shown that the LPC is composed of several distinct subcomponents,
each with different topographic distributions and sensitivities to
a variety of stimulus characteristics. Our previous work has aimed at
systematically disentangling these subcomponents of the LPC in the
OR context by employing a dishabituation paradigm with an 8 s
stimulus onset asynchrony (SOA). In the present study, we aimed to
extend our previous findings by exploring LPC subcomponents in a
long-interstimulus interval (ISI) dishabituation task. Twenty four
university students completed an auditory dishabituation task whilst
their electrodermal and electroencephalographic activity was recorded.
The task consisted of two counterbalanced blocks: indifferent,
in which there were no task requirements; and significant, in which
participants were instructed to count the stimuli. Stimuli consisted of
1000 and 1500 Hz counterbalanced (standard, deviant) 60 dB tones
that were separated by a random SOA of 12–15 s and delivered in the
following sequence: 10 standards, 1 deviant, 2–4 standards. Skin
conductance responses (SCRs) were extracted for each subject and
each trial. Single-trial EOG-corrected ERPs from 32-sites were
submitted to principal components analysis, from which five identifiable
LPC subcomponentswere extracted:Novelty P3, P3a, P3b, and two
SlowWaves (SW1 and SW2). As expected, SCR behaved as an OR index,
demonstrating habituation (decrement, recovery, dishabituation) and
larger magnitude responses to significant compared to indifferent
stimuli. Novelty P3 evidenced topography-specific decrement over
trials, a main effect of recovery, but little evidence of dishabituation.
P3a and P3b also showed topographic decrement, but topographyspecific
recovery and dishabituation were only apparent for P3a.
Neither of the two SWs demonstrated habituation, but SW2 did
evidence a main effect of condition. The LPC subcomponents identified
here did not demonstrate a uniform response-pattern across trials and
conditions. These findings suggest that the LPC represents a variety of
distinct neural processes, and support the notion that the LPC should
be examined further as a series of subcomponents, rather than a global
complex. Importantly, our LPC findings differed from that of our
previous work with shorter ISIs, with weaker trials effects for longer
ISIs. Taken together, these findings suggest that the mechanisms
underlying habituation of ERPs may be less functional when repetitive
stimuli are separated by longer ISIs.