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. 2011 Dec;20(4):1899-915.
doi: 10.1016/j.concog.2011.03.008. Epub 2011 Apr 16.

The Neural Basis of Event-Time Introspection

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Free PMC article

The Neural Basis of Event-Time Introspection

Adrian G Guggisberg et al. Conscious Cogn. .
Free PMC article

Abstract

We explored the neural mechanisms allowing humans to report the subjective onset times of conscious events. Magnetoencephalographic recordings of neural oscillations were obtained while human subjects introspected the timing of sensory, intentional, and motor events during a forced choice task. Brain activity was reconstructed with high spatio-temporal resolution. Event-time introspection was associated with specific neural activity at the time of subjective event onset which was spatially distinct from activity induced by the event itself. Different brain regions were selectively recruited for introspection of different event types, e.g., the bilateral angular gyrus for introspection of intention. Our results suggest that event-time introspection engages specific neural networks to assess the contents of consciousness. Subjective event times should therefore be interpreted as the result of complex interactions between introspection and experience networks, rather than as direct reproduction of the individual's conscious state or as a mere post hoc interpretation.

Figures

Figure 1
Figure 1
A Paradigm. The participants introspectively assessed the timing of different events during an auditory two-alternative forced choice task. In each trial, subjects watched a rotating analogue clock, heard an auditory cue (megaphone), chose between the left or the right button either according to the cue or freely (mental event symbolized by a light bulb), and then pressed the chosen button with the index finger of the corresponding side (hand symbol). In different introspection conditions, subjects assessed the position of the clock at the time point of one of these three events (thought bubbles) and reported it after the clock stopped moving (speech bubble). B Epochs. Introspection epochs (continuous squares) were centred on the subjective times of phoneme perception (ip), decision (id), or movement (im). During control epochs (dashed squares), subjects made no introspection when they heard the phonemes (cp), made their decision (cd), or moved their fingers (cm). C Neural Processes of interest (black bars) in this study concerned the processing of choice-related events (phoneme perception, P; decision, D; movement execution, M) or their introspection (IP, ID, IM). They were isolated from other task-induced neural activations (grey bars) with subtraction procedures. Concurring neural processes may have occurred during the time window of introspection only (T, e.g., assessment of time and simultaneity, dual task interference). Other processes may have been present also before (E, e.g., expectancy of the task-relevant event) or after introspection (W, e.g., working memory), or may have both preceded and outlasted the time window of introspection (V, e.g., vision and motion processing; G, e.g., general attention and motor preparedness). Tables 1 and 2 demonstrate that all these concurring neural processes, if present, could be removed by the contrasts.
Figure 2
Figure 2. Average (± standard deviation) times of subjective onsets of the 3 events are shown relative to the times of physical phoneme presentation and physical button press
The asterisks indicate the level of statistical significance (* p<0.05, ** p<0.01, *** p< 0.001).
Figure 3
Figure 3. Brain regions with significant high-gamma band (65-175 Hz) activations induced by the Libet timing task
The colour overlays indicate significant contrasts between introspection and control epochs (first contrast in Table 1). The time window with the highest power difference is shown for each condition: 100 ms after the subjective onset time of phoneme perception for the PHONEME condition (top), 50 ms before the subjective onset time of the decision for the DECISION condition (middle), and 50 ms before the subjective onset time of the finger movement for the MOVEMENT condition (bottom). The temporal evolution of the contrasts across all time windows is shown in Supplementary Videos 4-6.
Figure 4
Figure 4. Common network recruited by the Libet timing task for all event types
The figure shows a conjunction analysis of the area under curve of all single contrasts shown in Figure 3. The marked regions were significantly more active when subjects had to determine the onset time of phoneme perception, intention, or movement as compared to the primary processing of these events. They are therefore associated with the assessment of time and simultaneity and possibly with other concurring processes. Abbreviations: V1, primary visual cortex; V3, associative visual area; Ang, right angular gyrus; Cb, cerebellum; MFG, right middle frontal gyrus; SFG, left superior frontal gyrus.
Figure 5
Figure 5. Spatiotemporal reconstruction of neural activations induced by the re-representation of specific conscious events during event-time introspection
First column Brain regions that showed significant double contrasts related to introspection of phoneme onset (IP), intention onset (ID) or movement onset (IM). Second column Mean (± standard error) activation time course from baseline of the brain areas marked in the first column, relative to the physical onset time of the cueing phonemes. The grey staircase plots indicate the cumulative distribution of average subjective event onset times of all subjects relative to physical phoneme onset. Third column Activation time course relative to subjective event onset time. Fourth column Time course of the contrast between introspection and control epochs relative to subjective event onset time. Last column Time course of double contrast relative to subjective event onset time. Abbreviations as in Table 3.
Figure 6
Figure 6. Spatiotemporal reconstruction of activations induced by the primary processing of perceptual, intentional, and motor events
First column Brain regions that showed significant contrasts related to the primary processing of phoneme perception (P), decision-making (D), movement execution (M). Second column Mean (± standard error) activation time course from baseline of the brain areas marked in the first column, relative to the physical onset time of the cueing phonemes. The grey staircase plots indicate the cumulative distribution of average subjective event onset times of all subjects relative to physical phoneme onset. Third column Activation time course relative to subjective event onset time. Fourth column Time course of the contrast isolating the choice-related processes from concurrent activity, relative to subjective event onset time. Abbreviations as in Table 3.

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