臺灣博碩士論文加值系統

When a visual stimulus fails to reach awareness, what happens to its cortical representation? Previous work has shown that an unseen stimulus can still leave decodable traces in the occipital cortex, yet it remains unclear whether these traces track stimulus duration, whether they carry specific stimulus content, and how far they extend into the parietal and frontal cortex. The present study addressed these questions using magnetoencephalography. Twenty-one participants viewed sinusoidal gratings at an individually calibrated near-threshold contrast and reported on each trial whether the stimulus was seen and in which direction it was tilted. Source-level temporal generalization decoding was applied to ten cortical regions of interest spanning occipital, ventral temporal, parietal, and frontal cortex, at two stimulus durations of 500 ms and 1200 ms. The contrast between seen and unseen trials served as a control, confirming that the near-threshold manipulation produced behaviorally and neurally distinct awareness conditions before the same regions were tested for duration tracking, that is, short versus long, and for orientation content, that is, leftward versus rightward tilt.
As a control, the seen versus unseen contrast was carried by occipital and ventral temporal cortex as a robust and sustained distinction at both durations, strongest in lateral occipital cortex, while parietal cortex showed only a weak and transient awareness effect and frontal cortex was largely silent, reproducing the posterior dominant signature reported previously. Turning to the primary questions, duration was decodable in occipital and ventral temporal cortex across multiple regions under full visibility, narrowed to lateral occipital cortex at near-threshold, and persisted there regardless of whether the stimulus was seen, with no frontal region tracking duration in any condition. Orientation content was decodable only in the occipital cortex and only under limited conditions. These results reveal a posterior dominant gradient in how near-threshold stimuli are represented, in which the occipital cortex sustains the richest and most durable representation regardless of awareness, while the engagement of the parietal and frontal cortex is sparse and short-lived.

When a visual stimulus fails to reach awareness, what happens to its cortical representation? Previous work has shown that an unseen stimulus can still leave decodable traces in the occipital cortex, yet it remains unclear whether these traces track stimulus duration, whether they carry specific stimulus content, and how far they extend into the parietal and frontal cortex. The present study addressed these questions using magnetoencephalography. Twenty-one participants viewed sinusoidal gratings at an individually calibrated near-threshold contrast and reported on each trial whether the stimulus was seen and in which direction it was tilted. Source-level temporal generalization decoding was applied to ten cortical regions of interest spanning occipital, ventral temporal, parietal, and frontal cortex, at two stimulus durations of 500 ms and 1200 ms. The contrast between seen and unseen trials served as a control, confirming that the near-threshold manipulation produced behaviorally and neurally distinct awareness conditions before the same regions were tested for duration tracking, that is, short versus long, and for orientation content, that is, leftward versus rightward tilt.
As a control, the seen versus unseen contrast was carried by occipital and ventral temporal cortex as a robust and sustained distinction at both durations, strongest in lateral occipital cortex, while parietal cortex showed only a weak and transient awareness effect and frontal cortex was largely silent, reproducing the posterior dominant signature reported previously. Turning to the primary questions, duration was decodable in occipital and ventral temporal cortex across multiple regions under full visibility, narrowed to lateral occipital cortex at near-threshold, and persisted there regardless of whether the stimulus was seen, with no frontal region tracking duration in any condition. Orientation content was decodable only in the occipital cortex and only under limited conditions. These results reveal a posterior dominant gradient in how near-threshold stimuli are represented, in which the occipital cortex sustains the richest and most durable representation regardless of awareness, while the engagement of the parietal and frontal cortex is sparse and short-lived.

CONTENT
口試委員會審定書 #
ABSTRACT i
CONTENTS ii
LIST OF FIGURES iv
LIST OF TABLES vii
Chapter 1 Introduction 1
1.1 Encoding of Visual Stimulus in Occipital and Ventral Temporal Cortex 2
1.2 The Role of Prefrontal Cortex in Visual Awareness 3
1.3 Ongoing Debate on Prefrontal Cortex as a Neural Correlate of Consciousness 5
1.4 Encoding of Stimuli that Fail to Reach Awareness 6
1.5 Remaining Gap 9
1.6 Research Questions and Hypothesis 10
Chapter 2 Method 15
2.1 Participants 15
2.2 Stimuli and Experiment 15
2.2.1 Stimuli 15
2.2.2 Experiment 15
2.3 Behavioral Analysis 19
2.3.1 Quality Control 19
2.3.2 Behavioral Analysis 19
2.3.3 Signal-Detection Analysis 20
2.4 MEG Data Acquisition and Preprocessing 20
2.5 MEG Sources Reconstruction 21
2.6 Region-of-Interest 21
2.7 Label-Level Univariate Event-Related Field Analysis 23
2.8 Decoding 24
Chapter 3 Results 25
3.1 Behavioral Results 25
3.2 Category A — Pattern of Activity Difference for Near-threshold Seen and Unseen Trials 30
3.2.1 Prerequisite — Near-threshold Stimuli Evoke Above-Baseline Activity 30
3.2.2 Seen Versus Unseen in Occipital and Ventral Temporal Cortex 45
3.2.3 Seen Versus Unseen in Parietal and Frontal Cortex 50
3.3 Category B — Duration Tracking 54
3.3.1 Prerequisite — Duration is Decodable under Supra-threshold Condition 54
3.3.2 Duration Tracking When the Near-threshold Stimulus Was Seen 59
3.3.3 Duration Tracking When the Near-threshold Stimulus Was Unseen 63
3.4 Category C — Decoding of Stimulus Orientation 67
3.4.1 Prerequisite — Orientation is Decodable under Supra-threshold Viewing 67
3.4.2 Tilt Decoding When the Near-threshold Stimulus Was Seen 75
3.4.3 Orientation Decoding When the Near-threshold Stimulus Was Unseen 82
Chapter 4 Discussion 89
4.1 Overview of Findings 89
4.2 The Awareness Contrast Validates the Manipulation and Localizes Awareness to Posterior Cortex 89
4.3 Duration Tracking Dissociates from Awareness 90
4.4 Orientation Content at Threshold 91
4.5 A Posterior Dominant Gradient and Its Bearing on the Two Theories 91
4.6 Univariate and Multivariate Analyses 92
4.7 Limitations 93
4.8 Future Directions and Conclusion 93
REFERENCES 95

LIST OF FIGURES
Figure 2-1 A single trial of a QUEST staircase. 17
Figure 2-2 A single trial of a non-speeded target detection task. 17
Figure 2-3 Illustration of each ROI. 23
Figure 3-1 Detection rate by contrast level and duration. 26
Figure 3-2 Discrimination accuracy by contrast level and duration. 29
Figure 3-3 Discrimination accuracy for near-threshold trials. 29
Figure 3-4 ERF for Near-threshold seen vs. control at short duration. 32
Figure 3-5 Near-threshold seen vs. control decoding at short duration (occipital and ventral temporal ROIs). 33
Figure 3-6 Near-threshold seen vs. control decoding at short duration (parietal and frontal ROIs). 34
Figure 3-7 ERF for Near-threshold seen vs. control at long duration. 35
Figure 3-8 Near-threshold seen vs. control decoding at long duration (occipital and ventral temporal ROIs). 36
Figure 3-9 Near-threshold seen vs. control decoding at long duration (parietal and frontal ROIs). 37
Figure 3-10 ERF for near-threshold unseen vs. control at short duration. 39
Figure 3-11 Near-threshold unseen vs. control decoding at short duration (occipital and ventral temporal ROIs). 40
Figure 3-12 Near-threshold unseen vs. control decoding at short duration (parietal and frontal ROIs). 41
Figure 3-13 ERF for near-threshold unseen vs. control at long duration. 42
Figure 3-14 Near-threshold unseen vs. control decoding at long duration (occipital and ventral temporal ROIs). 43
Figure 3-15 Near-threshold unseen vs. control decoding at long duration (parietal and frontal ROIs). 44
Figure 3-16 ERF for near-threshold seen vs. unseen at short duration. 46
Figure 3-17 Near-threshold seen vs. unseen decoding at short duration (occipital and ventral temporal ROIs). 47
Figure 3-18 ERF for near-threshold seen vs. unseen at long duration. 48
Figure 3-19 Near-threshold seen vs. unseen decoding at long duration (occipital and ventral temporal ROIs). 49
Figure 3-20 Near-threshold seen vs. unseen decoding at short duration (parietal and frontal ROIs). 52
Figure 3-21 Near-threshold seen vs. unseen decoding at long duration (parietal and frontal ROIs). 53
Figure 3-22 ERF for short vs. long duration in the supra-threshold condition. 56
Figure 3-23 Short vs. long duration decoding in supra-threshold condition (occipital and ventral temporal ROIs). 57
Figure 3-24 Short vs. long duration decoding in supra-threshold condition (parietal and frontal ROIs). 58
Figure 3-25 ERF for short vs. long duration in the near-threshold seen condition. 60
Figure 3-26 Short vs. long duration decoding in near-threshold seen condition (occipital and ventral temporal ROIs). 61
Figure 3-27 Short vs. long duration decoding in near-threshold seen condition (parietal and frontal ROIs). 62
Figure 3-28 ERF for short vs. long duration in the near-threshold unseen condition. 64
Figure 3-29 Short vs. long duration decoding in near-threshold unseen condition (occipital and ventral temporal ROIs). 65
Figure 3-30 Short vs. long duration decoding in near-threshold unseen condition (parietal and frontal ROIs). 66
Figure 3-31 ERF for rightward vs. leftward tilt in the supra-threshold condition at short duration. 69
Figure 3-32 Rightward vs. leftward tilt decoding in supra-threshold condition at short duration (occipital and ventral temporal ROIs). 70
Figure 3-33 Rightward vs. leftward tilt decoding in supra-threshold condition at short duration (parietal and frontal ROIs). 71
Figure 3-34 ERF for rightward vs. leftward tilt in the supra-threshold condition at long duration. 72
Figure 3-35 Rightward vs. leftward tilt decoding in supra-threshold condition at long duration (occipital and ventral temporal ROIs). 73
Figure 3-36 Rightward vs. leftward tilt decoding in supra-threshold condition at long duration (parietal and frontal ROIs). 74
Figure 3-37 ERF for rightward vs. leftward tilt in the near-threshold seen condition at short duration. 76
Figure 3-38 Rightward vs. leftward tilt decoding in near-threshold seen condition at short duration (occipital and ventral temporal ROIs). 77
Figure 3-39 Rightward vs. leftward tilt decoding in near-threshold seen condition at short duration (parietal and frontal ROIs). 78
Figure 3-40 ERF for rightward vs. leftward tilt in the near-threshold seen condition at long duration. 79
Figure 3-41 Rightward vs. leftward tilt decoding in near-threshold seen condition at long duration (occipital and ventral temporal ROIs). 80
Figure 3-42 Rightward vs. leftward tilt decoding in near-threshold seen condition at long duration (parietal and frontal ROIs). 81
Figure 3-43 ERF for rightward vs. leftward tilt in the near-threshold unseen condition at short duration. 83
Figure 3-44 Rightward vs. leftward tilt decoding in near-threshold unseen condition at short duration (occipital and ventral temporal ROIs). 84
Figure 3-45 Rightward vs. leftward tilt decoding in near-threshold unseen condition at short duration (parietal and frontal ROIs). 85
Figure 3-46 ERF for rightward vs. leftward tilt in the near-threshold unseen condition at long duration. 86
Figure 3-47 Rightward vs. leftward tilt decoding in near-threshold unseen condition at long duration (occipital and ventral temporal ROIs). 87
Figure 3-48 Rightward vs. leftward tilt decoding in near-threshold unseen condition at long duration (parietal and frontal ROIs). 88

LIST OF TABLES
Table 1-1 Hypothesis Table for Category A. 12
Table 1-2 Hypothesis Table for Category B. 13
Table 1-3 Hypothesis Table for Category C. 14
Table 2-1 Stimulus contrast of a near-threshold trial. 18

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