Hakwan Lau

Relative blindsight in normal observers and the neural correlate of visual consciousness.

By using a paradigm based on metacontrast masking, we created experimental conditions in which the subjective report of consciousness differs but the objectively measured ability to discriminate visual targets does not. This approach allowed us to study the neural correlate of consciousness while having performance levels carefully matched in healthy human subjects. A comparison of the neural activity associated with these conditions as measured by functional MRI showed that conscious perception is associated with spatially specific activity in the mid-dorsolateral prefrontal cortex (area 46). Further analysis confirms that this activation is not only free from any performance confound, but is also not driven by differences in the timing of the physical stimuli. Our results suggest that the prefrontal cortex is important for the essentially subjective aspects of conscious perception.

Unconscious Activation of the Cognitive Control System in the Human Prefrontal Cortex

Using functional magnetic resonance imaging, we tested whether unconscious information can influence the cognitive control system in the human prefrontal cortex. Volunteers had to prepare to perform either a phonological judgment or a semantic judgment on an upcoming word, based on the instruction given at the beginning of each trial. However, in some trials they were visually primed to prepare for the alternative (i.e., “wrong”) task, and this impaired their performance. This priming effect is taken to depend on unconscious processes because the effect was present even when the volunteers could only discriminate the identity of the primes at chance level. Furthermore, the effect was stronger when the visibility of the prime was near zero than when the visibility of the prime was significantly higher. When volunteers were unconsciously primed to perform the alternative task, there was also decreased neural activity in the brain areas relevant to the instructed task and increased neural activity in the brain areas relevant to the alternative task, which shows that the volunteers were actually engaged in the wrong task, instead of simply being distracted. Activity in the mid-dorsolateral prefrontal cortex was also found to be associated with this unconscious priming effect. These results suggest that the cognitive control system in the prefrontal cortex is not exclusively driven by conscious information, as has been believed previously.

Empirical support for higher-order theories of conscious awareness

Higher-order theories of consciousness argue that conscious awareness crucially depends on higher-order mental representations that represent oneself as being in particular mental states. These theories have featured prominently in recent debates on conscious awareness. We provide new leverage on these debates by reviewing the empirical evidence in support of the higher-order view. We focus on evidence that distinguishes the higher-order view from its alternatives, such as the first-order, global workspace and recurrent visual processing theories. We defend the higher-order view against several major criticisms, such as prefrontal activity reflects attention but not awareness, and prefrontal lesion does not abolish awareness. Although the higher-order approach originated in philosophical discussions, we show that it is testable and has received substantial empirical support.

How to measure metacognition.

The ability to recognize ones own successful cognitive processing, in e.g., perceptual or memory tasks, is often referred to as metacognition. How should we quantitatively measure such ability? Here we focus on a class of measures that assess the correspondence between trial-by-trial accuracy and ones own confidence. In general, for healthy subjects endowed with metacognitive sensitivity, when one is confident, one is more likely to be correct. Thus, the degree of association between accuracy and confidence can be taken as a quantitative measure of metacognition. However, many studies use a statistical correlation coefficient (e.g., Pearsons r) or its variant to assess this degree of association, and such measures are susceptible to undesirable influences from factors such as response biases. Here we review other measures based on signal detection theory and receiver operating characteristics (ROC) analysis that are “bias free,” and relate these quantities to the calibration and discrimination measures developed in the probability estimation literature. We go on to distinguish between the related concepts of metacognitive bias (a difference in subjective confidence despite basic task performance remaining constant), metacognitive sensitivity (how good one is at distinguishing between ones own correct and incorrect judgments) and metacognitive efficiency (a subjects level of metacognitive sensitivity given a certain level of task performance). Finally, we discuss how these three concepts pose interesting questions for the study of metacognition and conscious awareness.

Medial frontal cortex: from self-generated action to reflection on one's own performance.

It was suggested over 20 years ago that the supplementary motor cortex is involved in self-generated behaviour. Since then, there have been many studies using electrophysiology and brain imaging of the role of the supplementary motor cortex and anterior cingulate cortex. In light of the findings, the proposal that these regions are crucial for self-generated action has recently been challenged. Here, we review the recent literature and argue that the proposal survives the findings. We further argue that it can be generalised to cover reflection on mental states. Finally, we suggest that the pattern of anatomical connections is consistent with the proposal that the medial frontal cortex is crucially involved in self-generated action and self-reflection.

Anatomical coupling between distinct metacognitive systems for memory and visual perception.

A recent study found that, across individuals, gray matter volume in the frontal polar region was correlated with visual metacognition capacity (i.e., how well ones confidence ratings distinguish between correct and incorrect judgments). A question arises as to whether the putative metacognitive mechanisms in this region are also used in other metacognitive tasks involving, for example, memory. A novel psychophysical measure allowed us to assess metacognitive efficiency separately in a visual and a memory task, while taking variations in basic task performance capacity into account. We found that, across individuals, metacognitive efficiencies positively correlated between the two tasks. However, voxel-based morphometry analysis revealed distinct brain structures for the two kinds of metacognition. Replicating a previous finding, variation in visual metacognitive efficiency was correlated with volume of frontal polar regions. However, variation in memory metacognitive efficiency was correlated with volume of the precuneus. There was also a weak correlation between visual metacognitive efficiency and precuneus volume, which may account for the behavioral correlation between visual and memory metacognition (i.e., the precuneus may contain common mechanisms for both types of metacognition). However, we also found that gray matter volumes of the frontal polar and precuneus regions themselves correlated across individuals, and a formal model comparison analysis suggested that this structural covariation was sufficient to account for the behavioral correlation of metacognition in the two tasks. These results highlight the importance of the precuneus in higher-order memory processing and suggest that there may be functionally distinct metacognitive systems in the human brain.

Dopaminergic stimulation enhances confidence and accuracy in seeing rapidly presented words

Liberal acceptance, overconfidence, and increased activity of the neurotransmitter dopamine have been proposed to account for abnormal sensory experiences, for instance, hallucinations in schizophrenia. In normal subjects, increased sensory experience in Yoga Nidra meditation is linked to striatal dopamine release. We therefore hypothesize that the neurotransmitter dopamine may function as a regulator of subjective confidence of visual perception in the normal brain. Although much is known about the effect of stimulation by neurotransmitters on cognitive functions, their effect on subjective confidence of perception has never been recorded experimentally before. In a controlled study of 24 normal, healthy female university students with the dopamine agonist pergolide given orally, we show that dopaminergic activation increases confidence in seeing rapidly presented words. It also improves performance in a forced-choice word recognition task. These results demonstrate neurotransmitter regulation of subjective conscious experience of perception and provide evidence for a crucial role of dopamine.

A detection theoretic explanation of blindsight suggests a link between conscious perception and metacognition

Blindsight refers to the rare ability of V1-damaged patients to perform visual tasks such as forced-choice discrimination, even though these patients claim not to consciously see the relevant stimuli. This striking phenomenon can be described in the formal terms of signal detection theory. (i) Blindsight patients use an unusually conservative criterion to detect targets. (ii) In discrimination tasks, their confidence ratings are low and (iii) such confidence ratings poorly predict task accuracy on a trial-by-trial basis. (iv) Their detection capacity (d′) is lower than expected based on their performance in forced-choice tasks. We propose a unifying explanation that accounts for these features: that blindsight is due to a failure to represent and update the statistical information regarding the internal visual neural response, i.e. a failure in metacognition. We provide computational simulation data to demonstrate that this model can qualitatively account for the detection theoretic features of blindsight. Because such metacognitive mechanisms are likely to depend on the prefrontal cortex, this suggests that although blindsight is typically due to damage to the primary visual cortex, distal influence to the prefrontal cortex by such damage may be critical. Recent brain imaging evidence supports this view.

Action-Specific Disruption of Perceptual Confidence

Theoretical models of perception assume that confidence is related to the quality or strength of sensory processing. Counter to this intuitive view, we showed in the present research that the motor system also contributes to judgments of perceptual confidence. In two experiments, we used transcranial magnetic stimulation (TMS) to manipulate response-specific representations in the premotor cortex, selectively disrupting postresponse confidence in visual discrimination judgments. Specifically, stimulation of the motor representation associated with the unchosen response reduced confidence in correct responses, thereby reducing metacognitive capacity without changing visual discrimination performance. Effects of TMS on confidence were observed when stimulation was applied both before and after the response occurred, which suggests that confidence depends on late-stage metacognitive processes. These results place constraints on models of perceptual confidence and metacognition by revealing that action-specific information in the premotor cortex contributes to perceptual confidence.

Direct injection of noise to the visual cortex decreases accuracy but increases decision confidence.

The relationship between accuracy and confidence in psychophysical tasks traditionally has been assumed to be mainly positive, i.e., the two typically increase or decrease together. However, recent studies have reported examples of exceptions, where confidence and accuracy dissociate from each other. Explanations for such dissociations often involve dual-channel models, in which a cortical channel contributes to both accuracy and confidence, whereas a subcortical channel only contributes to accuracy. Here, we show that a single-channel model derived from signal detection theory (SDT) can also account for such dissociations. We applied transcranial magnetic stimulation (TMS) to the occipital cortex to disrupt the internal representation of a visual stimulus. The results showed that consistent with previous research, occipital TMS decreased accuracy. However, counterintuitively, it also led to an increase in confidence ratings. The data were predicted well by a single-channel SDT model, which posits that occipital TMS increased the variance of the internal stimulus distributions. A formal model comparison analysis that used information theoretic methods confirmed that this model was preferred over single-channel models, in which occipital TMS changed the signal strength or dual-channel models, which assume two different processing routes. Thus our results show that dissociations between accuracy and confidence can, at least in some cases, be accounted for by a single-channel model.