LPC subcomponents in a long-ISI dishabituation task
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.
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