Elsevier

Brain and Cognition

Volume 69, Issue 1, February 2009, Pages 170-175
Brain and Cognition

The extent of active processing of a long-duration stimulus modulates the scalp-recorded sustained potential

https://doi.org/10.1016/j.bandc.2008.07.004Get rights and content

Abstract

A long-duration stimulus will elicit a negative sustained potential (SP) that is maximum in amplitude over fronto-central areas of the scalp. This study examines how the duration of active attentional processing of the stimulus might also elicit a nonsensory contingent negative variation (CNV) that overlaps and summates to the SP. Subjects were presented with either a low- or high-pitched 1.4 s duration stimulus. In different conditions, a 20 ms gap occurred either 300 or 1300 ms after stimulus onset in half the stimuli. The subject’s task was to button press upon detection of the gap in the low-pitched stimulus. The subject was not required to respond to the gap occurring in the high-pitched tone. In a separate Ignore condition, subjects ignored all stimuli and read a book. In the Ignore condition, a SP was apparent to the long-duration stimuli. In the Attend condition, a large amplitude slow wave, probably the CNV, overlapped and summated with the SP. The extent of the overlapping CNV was dependent on whether the gap occurred early or late. The CNV was not apparent following presentation of the high-pitched tone, when the gap did not need to be detected. Consistent with previous studies, a SP was thus elicited even when active processing of the stimulus was not required. The morphology of the scalp-recorded SP was, nevertheless, much modified by an overlapping and summating CNV, depending on the extent to which active processing of the long-duration stimulus was required.

Introduction

The processing of a long-duration auditory stimulus involves both transient and sustained activity. The onset and the offset of the stimulus elicit transient negative-going event-related potentials (ERPs), N1-on and N1-off, each peaking at about 100 ms following the change in transient energy. The continued presentation of the stimulus also elicits a negative sustained potential (SP), maximum over fronto-central areas of the scalp, lasting for the entire duration of the stimulus. The scalp distribution of the SP has been explained by source activity in the auditory cortex (Gutschalk et al., 2007, Pantev et al., 1994, Scherg et al., 1989, Sieroka et al., 2003) although there may be variance depending on the nature of the stimulus (Gutschalk et al., 2004, Seither-Preisler et al., 2006).

The elegant Picton et al., 1978a, Picton et al., 1978b studies indicated convincingly that the SP is affected by manipulation of a number of physical features of the stimulus including its duration, intensity, rise-time and location. Recent studies have confirmed that the SP, or its magnetic equivalent, the sustained field (SF), are also affected by more subtle physical attributes of the stimulus including its regularity and pitch (Gutschalk et al., 2004, Gutschalk et al., 2007, Seither-Preisler et al., 2006). There is less agreement about whether the SP is also affected by nonsensory factors, such as attention. Picton et al. (1978a) noted that when subjects were asked to actively discriminate the duration of two long-duration stimuli, one slightly longer than the other, the SP was larger compared to when subjects were asked to ignore the stimuli. They speculated that this might be because of an influence of attention on the actual SP or alternatively, the superimposition of an expectancy-related contingent negative variation (CNV) on the SP. More recently, Sieroka et al. (2003) also asked subjects to discriminate the duration of two long-lasting stimuli. Consistent with Picton et al. (1978a), a higher amplitude magnetic sustained field (SF) was observed in an attend compared to an ignore condition. Dipole modeling of the data revealed sources in the auditory cortices probably related to sensory processing of the auditory stimulus and an additional source, in the precuneus or posterior cingulate regions, probably related to nonsensory, attentional processing. Sieroka et al. also suggested that the apparent increase in the scalp-recorded SF during attention may have been a result of the superimposition of an expectation wave, the CNV.

When Picton et al. (1978a) asked subjects to actively discriminate the intensity of two long-duration stimuli, the amplitude of the SP did not however vary compared to when subjects ignored the stimuli. An overlapping CNV was not apparent. Similarly, the amplitude of the SP did not vary if the subjects were asked to discriminate the pitch-warble of the stimuli. In these conditions, attention to the long-duration stimuli did not, therefore, appear to modulate the amplitude of the SP.

The difference in results might be explained by how long active processing needed to be maintained in the different conditions in order for the relevant feature to be extracted from the stimulus. The discrimination of the intensity or the pitch of the long-duration stimulus could be made rapidly, allowing active processing to cease shortly after stimulus onset. Because active processing did not need to be maintained for the entire duration of the stimulus, the superimposition of the CNV on the SP was not apparent. On the other hand, the discrimination of another stimulus feature, its duration, required active processing to be maintained until stimulus offset, hence the superimposition of the CNV.

The present study tested the hypothesis that the extent of the CNV overlap is dependent on the duration that active processing needed to be maintained. The duration of active processing was manipulated in three ways: (1) The subject either attended or ignored the auditory stimulus; the Attend condition required active processing to be maintained for almost the entire duration of the stimulus while in the Ignore condition, active processing of the auditory stimulus was not required. This is thus essentially a replication of the Picton et al. and Sieroka et al. stimulus duration discrimination conditions. A problem with the introduction of separate attend and ignore conditions is that differences in processing could be explained by factors other than attention, such as differential arousal or motivation, although this is unlikely. This problem was overcome by varying processing demands within the same condition. (2) Within the Attend condition, the pitch of the long-duration stimulus signaled whether active processing need continue or not; the pitch of the stimulus could presumably be identified shortly after stimulus onset, and subsequent active processing might continue or could cease. (3) A physical gap (a break in the stimulus) might or might not have occurred within the long-duration stimulus. If it did occur, it might have occurred either very early or late after stimulus onset, defining the temporal period active processing needed to be maintained.

Section snippets

Subjects

Ten healthy young adults (3 men, 7 women) aged 19–24 years (Mean = 20.3 years) volunteered to participate in this study. None reported a history of neurological or psychiatric disorders. All reported normal hearing. Written informed consent was obtained prior to the study and an honourarium was offered as compensation. This study was conducted following the Canadian Tri-Council (Natural, Health, and Social Sciences) ethical guidelines.

Stimuli

An 80 dB SPL auditory stimulus having a duration of 1400 ms and

Effects of attention

In the both the late-Attend condition and Ignore conditions, a gap might have occurred near the end of the long-duration stimulus. Fig. 1 presents the ERPs from all scalp sites following presentation of the low-pitched stimulus in both Attend and Ignore conditions. Only trials in which a gap did not occur are illustrated. As may be observed, in both conditions, the onset of the long-duration stimulus elicited a transient N1-on about 100 ms following its onset and a transient N1-off about 100 ms

Discussion

The purpose of this study was to determine whether the sustained potential elicited by a long-duration stimulus was affected by nonsensory processes such as attention. The extent that active processing was required to make a task-relevant decision about a specific feature of the long-duration stimulus was manipulated. In the Attend condition, active processing needed to be maintained in order to detect a possible physical gap in the auditory stimulus. In the Ignore condition, subjects read a

Conclusion

In summary, active processing of a long-duration stimulus will result in the elicitation of a nonsensory CNV that that will summate both spatially and temporally with the sensory-related SP. The extent of this CNV overlap is highly dependent on the extent to which active processing of the long-duration stimulus needs to be maintained. If subjects ignore the auditory stimuli while engaged in another task (reading in this experiment), the CNV will not be elicited. The SP will however continue to

Acknowledgments

Financial support for this research was provided by the Natural Sciences and Engineering Research Council (NSERC) of Canada.

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