Two neural correlates of consciousness

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Neuroscientists continue to search for ‘the’ neural correlate of consciousness (NCC). In this article, I argue that a framework in which there are at least two distinct NCCs is increasingly making more sense of empirical results than one in which there is a single NCC. I outline the distinction between phenomenal NCC and access NCC, and show how they can be distinguished by experimental approaches, in particular signal-detection theory approaches. Recent findings in cognitive neuroscience provide an empirical case for two different NCCs.

Introduction

I have previously proposed a conceptual distinction between phenomenal consciousness and access consciousness 1, 2, 3. Phenomenally conscious content is what differs between experiences as of red and green, whereas access-conscious content is content information about which is ‘broadcast’ in the global workspace. Some have accepted the distinction but held that phenomenal consciousness and access consciousness coincide in the real world (4, 5 but see [6]). Others have accepted something in the vicinity of the conceptual distinction but argued that only access consciousness can be studied experimentally [7]. Others have denied the conceptual distinction itself [8]. This article argues that the framework of phenomenal consciousness and access consciousness helps to make sense of recent results in cognitive neuroscience; we see a glimmer of an empirical case for thinking that they correspond to different NCCs.

Section snippets

Phenomenal NCC

Christof Koch defines ‘the’ NCC as ‘the minimal set of neuronal events and mechanisms jointly sufficient for a specific conscious percept’ ([9] p. 16). However, since there is more than one concept of consciousness, this definition allows that a given percept may have more than one NCC. In my proposed framework, the Phenomenal NCC is the minimal neural basis of the content of an experience, that which differs between the experience as of red and the experience as of green.

I will start with an

Access NCC

We can distinguish between phenomenal contents of experience and access-conscious contents – contents information about which is made available to the brain's ‘consumer’ systems: systems of memory, perceptual categorization, reasoning, planning, evaluation of alternatives, decision-making, voluntary direction of attention, and more generally, rational control of action. Wide availability motivates the idea that there is a ‘global workspace’ [29], and that information concerning conscious

But is the phenomenal NCC really the neural basis of a kind of consciousness?

You may ask, ‘If the Phenomenal NCC can perhaps occur without the Access NCC, how do we know that the Phenomenal NCC is really the neural basis of anything conscious?’ A quick answer is that, since the Phenomenal NCC determines the contents of experience, what it determines is ipso facto a kind of consciousness. The Phenomenal NCC for visual motion determines the experiential content of visual motion – as distinct from, say, the experiential content of seeing something as a face. That content

Signal detection theory (SDT) approaches

Suppose a subject is shown a series of stimuli at around threshold level and asked to press one button if a target is seen and another if not. SDT models the subject's behavior in terms of two factors: the extent to which the subject has an experience of seeing it and the criterion the subject implicitly sets for reporting seeing it. The criterion is famously influenceable by features of the experimental setup that affect the subject's expectations or motivation – such as the proportion of

Neural SDT

In a landmark series of experiments, Super et al. [37] recorded from V1 during a task in which monkeys were rewarded for saccading to a target if there was one or continuing to look at the fixation point if not. Super et al. manipulated whether the locations in V1 corresponded to figure or ground. When the monkey detected the target, there was an increased V1 response for figure as compared with ground (see Figure 3, in which this increased figure response is referred to as ‘modulation’).

Super

Can the phenomenal NCC be studied empirically?

Doubts about whether phenomenal consciousness (and hence its neural basis, the Phenomenal NCC) can be studied empirically are common (see also Box 2), and often based on the idea that ultimately, introspective reports, that is, reports about one's conscious experience, are the fundamental epistemological basis of theories of consciousness, the ‘gold standard’. 7, 41, 42. Reports are not supposed to be infallible, but any discounting of reports as reporting too much or too little, will

Conclusion

Where are we? I have proposed a distinction between a Phenomenal NCC and an Access NCC. The ‘single NCC’ framework does not do as well in making sense of the empirical data, in particular, signal detection theory data, as an account in which there are two NCCs. Of course both these NCCs are to be firmly distinguished from perceptual representations that are not conscious in any sense (as in the rightmost panel of Figure 3c). More generally, rather than asking ‘What is the direct evidence about

References (52)

  • A. Cowey et al.

    Visual detection in monkeys with blindsight

    Neuropsychologia

    (1997)
  • B.M. Esbensen

    Children's behavioral understanding of knowledge acquisition

    Cogn. Dev.

    (1997)
  • R. Nakamura et al.

    Blindness in monkeys following non-visual cortical lesions

    Brain Res.

    (1980)
  • N. Block

    Consciousness and accessibility

    Behav. Brain Sci.

    (1990)
  • N. Block

    Begging the question against phenomenal consciousness

    Behav. Brain Sci.

    (1992)
  • N. Block

    On a confusion about a function of consciousness

    Behav. Brain Sci.

    (1995)
  • D. Chalmers

    The Conscious Mind

    (1996)
  • D. Chalmers

    Availability: The cognitive basis of experience

    Behav. Brain Sci.

    (1997)
  • N. Block

    Biology versus computation in the study of consciousness

    Behav. Brain Sci.

    (1997)
  • Dehaene, S. and Changeux, J-P. (2004) Neural mechanisms for access to consciousness. In The Cognitive Neurosciences...
  • D. Dennett

    The path not taken

    Behav. Brain Sci.

    (1995)
  • C. Koch

    The Quest for Consciousness: A Neuroscientific Approach

    (2004)
  • D. Heeger

    Motion opponency in visual cortex

    J. Neurosci.

    (1999)
  • K. Britten

    The analysis of visual motion: A comparison of neuronal and psychophysical performance

    J. Neurosci.

    (1992)
  • J. Zihl

    Selective disturbance of movement vision after bilateral brain damage

    Brain

    (1983)
  • G. Rees

    Neural correlates of consciousness in humans

    Nat. Rev. Neurosci.

    (2002)
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