Symposium: Neural correlates of consciousness

Organizer: Peter Tse

List of confirmed speakers in alphabetical order:

Sheng He, "Conscious and unconscious visual processing of faces, bodies, and tools"

Humans are experts in extracting important visual information from faces (identity, expression) and body movements (intention, direction of motion). We are also very efficient in processing visual information of tools for action. In addition, erotic body images are of high arousal values interests to human observers. In a series of psychophysical and neuroimaging experiments using an interocular suppression technique to render images invisible, we show that 1) Face orientation and facial expression information could be processed automatically in the absence of awareness; 2) Invisible images of familiar tools could activate the dorsal cortical regions; 3) Invisible erotic images could influence the distribution of spatial attention; 4) Humans have sensitive and automatic mechanisms for detecting local biological motion. These results also demonstrate that rendering stimuli invisible through interocular suppression is an effective way to isolate selective neural mechanisms.
Stephen L. Macknik, "The role of feedback in visual attention and awareness"

The mammalian visual system includes numerous brain areas that are profusely interconnected. With few exceptions, these connections are reciprocal. Anatomical feedback connections in general outnumber feedforward connections, leading to widespread speculation that feedback connections play a critical role in visual awareness. However, evidence from physiological experiments suggests that feedback plays a modulatory role, rather than a driving role. I will discuss theoretical constraints on the significance of feedback’s anatomical numerical advantage, and describe theoretical limits on feedback’s potential physiological impact. These restrictions confine the potential role of feedback in visual awareness and rule out some extant models of visual awareness that require a fundamental role of feedback. I will propose that the central role of feedback is to maintain visuospatial attention, rather than visual awareness. These conclusions highlight the critical need for experiments and models of visual awareness that control for the effects of attention.
Rafael Malach, "Localized neuronal “Ignitions” underlying human visual perception"

It is remarkable that despite the large influx of information regarding neuronal mechanisms underlying human perception, we are still in the dark with regards to the very basic question- how much of the human brain must be engaged in the generation of a conscious visual percept?
A large body of brain imaging work appears to support the notion that such percepts result from a wide spread activation pattern that includes not only visual areas but also fronto-parietal networks as well. However, it is difficult to assess how much of this wide-spread activity is associated with visual perception and how much is engaged during auxiliary functions that occur subsequent to the perceptual event- such as memory, action planning, shifting attention etc.
Here I will present data from fMRI and Intra-cranial recordings of the human brain indicating that a conscious visual percept is associated with an early and highly non-linear "ignition" of neuronal activity which outlasts the visual stimulus. These "ignitions" are localized to content-specific neuronal groups in high order visual areas. Neuronal activity in non-visual areas indeed emerges in association with conscious percepts, however this activity begins ~100 msec after the onset of activity in visual areas. These results are consistent with the hypothesis that a localized, non-linear, "ignition" of neuronal activity in high order visual areas is sufficient for the emergence of a visual percept, while the spread of activity to non-visual areas is associated with cognitive processes subsequent to the percept itself. Supported by the Minerva and Israeli Science Foundations.
Alexander Maier, "Visual awareness correlates with layer-specific activity in primary visual cortex"

Whether or not activity in the primary visual cortex (V1) is directly related to the visibility of a stimulus is a long-standing debate. To investigate the basis of existing discrepancies in the literature, we measured the BOLD response, along with electrophysiological signals, in area V1 of two behaving monkeys, and correlated responses there with the perceived visibility of a salient stimulus. We show that stimulus visibility can be reliably derived from the fMRI signal, but not from neuronal spiking activity. We further demonstrate by laminar sampling of V1 local field potentials (LFP) that there is an uneven distribution of percept-related current changes between the different cortical laminae. Thus, we show that fMRI and neurophysiological signals, while generally in good agreement, become uncoupled during perceptual suppression.
Furthermore, our data reveals that if a visual stimulus goes unperceived, there is a drop in the membrane currents in the upper layers of V1.
Peter Tse, "Is the fronto-parietal network necessary for the experience of qualia?"

The field of consciousness researchers is divided. Indeed, some members of this symposium fall into one or the other of the two camps. Some argue that the frontoparietal network is necessary to have conscious contents, while others argue that this is not necessarily so, and that there can be conscious contents even in the absence of frontoparietal network activity. I will discuss a conceptual framework that can encompass evidence for both of these camps. I conclude that the frontoparietal network allows one to experience qualia in the particular sense of an attended object that is tracked through some representational space. However, even in the absence of attentional engagement, qualia can exist as 'feature soup' (i.e. unbound and untracked) in the perceptual background, and these experiences can be reported when probed. Such unattended qualia can be realized in the absence of the frontoparietal network's engagement, but are thereby fleeting and easily flushed from short-term perceptual buffers. In addition, I will present related psychophysical and fMRI evidence that allows a characterization of the representational structure of qualia, with particular focus on perceived color in the context of filling-in.