Our Research

Research Summary:

The human brain is a large-scale, nonlinear dynamical system. In our lab, we study how large-scale brain dynamics give rise to perceptual awareness in humans. Every day, our brains cycle through different states of awareness: from the rich conscious experiences during wakefulness to dreamless sleep to the bizarre experiences during dreaming sleep. We are interested in what kind of brain activities underlie different contents of awareness. The neural basis of perceptual awareness has intrigued many scientists and has been intensely studied over the years. Yet, it remains one of the lasting mysteries in neuroscience. Because distorted perception of both external and internal events characterize many neuropsychiatric illnesses, research on this topic has potentially wide clinical and societal implications.


Sensory inputs from an environmental stimulus may be processed by the brain unconsciously (as in “subliminal priming”), or consciously – giving rise to our conscious awareness of surrounding environment (termed “perceptual awareness”).  We seek to elucidate the fundamental differences between conscious and unconscious processing in the brain, and how brain activity engenders various content of conscious awareness – questions that are uniquely addressable in human subjects given the availability of verbal report. Most of our work so far has concerned perception of external sensory stimuli. Recently, we have also studied perception of internal events, such as conscious movement intention. This work has revealed that perceptual awareness of external and internal events shares important common mechanisms. In our pursuit of neural mechanisms underlying perceptual awareness, we believe that simply mapping “where” and “when” is insufficient. Using empirical multimodal imaging research in humans, we aim to grasp the spatiotemporal brain dynamics giving rise to perceptual awareness. Through computational and theoretical pursuits, we hope to uncover the underlying principles governing such dynamics, and eventually connect our findings with neuronal-circuit-level mechanisms informed by animal research. In addition, we use causal manipulations of the brain, such as brain stimulation, to establish the causal role of the identified neural correlates in producing awareness.

Large-scale, nonlinear brain dynamics

Neurons in the brain form hierarchical, complex networks from microscopic, local circuit-level to macroscopic, large-scale network-level. Using magnetoenceophalography (MEG) and electroencephalography (EEG), we have uncovered that perceptual awareness of a visual stimulus is associated with long-lasting, relatively slow (< 5 Hz) brain activity distributed across widespread frontoparietal and temporal cortices (Li et al., 2014). By contrast, unconscious processing of the same visual stimulus is accompanied by more localized and transient neural activity.

Since the brain is a large-scale, nonlinear dynamical system, nonlinearities permeate brain activity at many different levels. In previous work, we have shown that human brain activity – recorded by either functional magnetic resonance imaging (fMRI) or electrophysiology – exhibit substantial nonlinear interaction between ongoing brain activity and sensory input. In other words, a certain stimulus input does not elicit a certain brain activity; instead, it merely modulates the ongoing brain activity that is always there (He, 2013; He & Zempel, 2013). It is the evolving cortical activity trajectory, shaped by ongoing activity and influenced by stimulus input, that determines the failure or success of perception and its content.

Our current work seeks to shed light on the large-scale network and dynamical mechanisms underlying such neural activity. We are also exploring the interaction between perceptual awareness and other high-level human cognition, such as attention and expectation.

Priors in perception

“Whilst part of what we perceive comes through our senses from the object before us, another part (and it may be the larger part) always comes out of our own head.” (James, 1890)

In other words, what we see is our brains’ reconstruction of the world. Increasing evidence over the past decade suggests that human perception is not simply a passive, feedforward process in which cortical areas relay progressively more abstract information to those higher up in the hierarchy, but rather an inferential process with internal priors in the brain actively guiding and shaping perception. Yet, how perceptual priors are established and represented in the brain remains largely unknown. In addition, how internal priors interact with bottom-up stimulus input to shape perception is also unclear.

We are currently investigating these questions using multimodal human brain imaging. We have discovered that when sensory stimuli contain ambiguity, perception is more reliant on internal models of the brain; this elicits dramatically enhanced bottom-up and top-down communications throughout the brain (Wang et al., 2013). In addition, perceptual content could be read out (i.e., decoded) from higher-order frontal cortical regions, which are traditionally thought to carry out executive control and reporting functions and not involved in the construction of perceptual content.

Conscious movement intention

What makes a movement feel voluntary, and what might make it feel involuntary? The sense of volition is fundamental to the human experience. It provides the foundation for an individual to attribute agency to the self, and for society to attribute responsibility to an individual. A distorted sense of volition is a hallmark of many neurological and psychiatric illnesses such as schizophrenia, alien hand syndrome, and psychogenic movement disorders. Thus, understanding the neurobiological basis of volition has paramount scientific, legal, and clinical value.

Recently, we demonstrated that enhancing baseline excitability in two specific brain regions – primary motor cortex and angular gyrus – significantly modulated healthy human volunteers’ perception of their conscious movement intention, such that the interval between intention and action onset during self-generated movements is prolonged (Douglas et al., 2015). We further established that slow brain activity from ~2-3 sec before to ~500 ms after movement carried this behavioral effect, suggesting that conscious movement intention is a type of perceptual awareness that develops in parallel with unconscious movement initiation, and is not fully solidified until hundreds of ms after movement commencement. We are currently pushing this work further to better delineate the neural circuit mechanisms underlying conscious movement intention and to explore potential application of these insights in developing better treatment for psychogenic movement disorders – an illness present in 2 – 40% of neurological patients that is marked by distorted perception of movement intention.

As of August 15, 2016 

Methodologies

We use multimodal imaging and stimulation methods in humans, including functional neuroimaging (e.g. fMRI), non-invasive and invasive electrophysiology (e.g. EEG, MEG, ECoG), brain stimulation (e.g. tDCS, TMS), combined with computational modeling to study the neural bases of human cognition, with a special emphasis on perceptual awareness.

Recording

Recording

Stimulation

Stimulation