Correlates, Causes, and the Neurobiology of Consciousness
Presenter: Joeseph Neisser, Grinnel CollegeHere is some of the paper:
Correlates, causes and the neurobiology of consciousnessRead the whole paper.
2nd Consciousness Online Conference
Joe Neisser, Grinnell College
2/2010
For the past ten years it has been widely accepted among philosophers that neuroscientists are conducting a search for the neural correlates of consciousness (NCC).1 David Chalmers (2000) articulated a definition of the neural correlate concept which was meant to capture the observed relation between the contents of experience and activity in specific neural populations. But this now-standard definition is inadequate both to the phenomena of consciousness and to the actual research it is meant to illuminate. In what follows I first introduce the idea of an NCC as presented by Chalmers, and rehearse a criticism of it advanced by Noe & Thompson (2004). I then suggest that the neurobiological research in view should be understood as an attempt to localize the working parts of a complex causal mechanism rather than as an effort to identify the correlates of some discrete conscious content. Thus, the first ‘C’ in ‘NCC’ should stand for ‘causes’ rather than ‘correlates.’ I conclude by attempting a revised definition of the NCC.
1. The standard approach to the NCC
David Chalmers (2000) describes a research program in neuroscience dedicated to the search for the neural correlates of consciousness or NCC. Chalmers defined an NCC as follows:An NCC is a minimal neural system N such that there is a mapping from states of N to states of consciousness, where a given state of N is sufficient, under conditions C, for the corresponding state of consciousness. (Chalmers, 2000, p.31)Chalmers argues that this conception of the NCC is appropriate given what we know about consciousness and the brain, and that the goal of identifying NCCs can be attained in the foreseeable future (Chalmers, 2000, p.38).
Chalmers focuses on a particular kind of research aimed at a particular kind of consciousness, which he calls content consciousness. Content consciousness is primarily distinguished from creature consciousness, which is a property of a whole animal – that of being conscious, as opposed to being unconscious, as in a coma. The search for the NCC for content consciousness involves finding brain differences that correlate with consciousness of particular intentional objects, such as visual stimuli. This research compares states of the brain in which the subject is unconscious of the stimulus with states of the brain in which the stimulus is consciously experienced. The paradigmatic example of research that aims at discovering the NCC for content consciousness is a set of experiments conducted by Leopold & Logothetis (1996) and Sheinberg & Logothetis (1997).2 I pause to describe this research.
1.i The research paradigm
Logothetis’ primate laboratory was the site of a series of experiments that exploit the phenomenon of binocular rivalry. In binocular rivalry, competing stimuli are presented, one to each eye. The result in consciousness is an oscillation between perception of one stimulus and perception of the other. The subject does not perceive a hybrid composed of both stimuli, but instead experiences a flipping back and forth between the two. As the name ‘rivalry’ suggests, there is a sort of competition between neural processes, in which the ‘winner takes all’. Thus, as one or the other of the visual stimuli ‘dominates’ neural activity, we become conscious of it. With the use of electroencephalographic recordings, Sheinberg & Logothetis (1997) first identified neurons in the visual system of a rhesus macaque subject that respond to particular visual stimuli, such as a horizontal grate or a sunburst. They then trained the macaque subject to report its visual experience by pushing a button upon seeing the preferred stimulus pattern. The stimuli were then presented in a binocular rivalry situation, so that the monkey would respond when the preferred stimulus became dominant. Thus, the experimenters had a way to compare neural activation across very specific states of content consciousness - one in which the
preferred stimulus is not conscious because suppressed by the rivalry, and one in which it is the ‘winner.’
The elegance of the design lies in its ability to isolate the consciousness of a stimulus independently of the presentation of the stimulus. Logothetis et al were able to measure the difference in brain state that co-varies with conscious perception of the stimulus even while stimulus presentation is held constant. With this experimental design, these researchers have found a way to work directly in the field that Fechner called inner psychophysics. Inner psychophysics is the empirical study of the relation between the conscious mind and neurophysiology. Fechner distinguished inner psychophysics from outer psychophysics, which is the study of the relation between the mind and external stimuli. Historically, inner psychophysics had to be studied indirectly through outer psychophysics, because neuropsychologists could not precisely manipulate and measure neural activity independently from external stimulation (Hilgard, 1987, Boring, 1950). The binocular rivalry paradigm, together with improved microelectrical recording techniques, allows limited but quite direct experimental manipulation of the inner psychophysical relation. The results indicate that there is significant relation between the ‘winning’ visual perception and activation of particular neural populations in the inferior temporal cortex (IT) and the superior temporal sulcus (STS). Whenever the monkey reported seeing the preferred stimulus, 90% of the pre-identified neurons recorded in IT and STS were strongly active. However, this activity was almost extinguished when the monkey saw the ineffective stimulus (Sheinberg & Logothetis, 1997, p.3413). Thus, these seem to be the neurons that make the difference for consciousness of that visual content. Chalmers identified this work as being paradigmatic of the NCC research program, and attempted to derive his general definition of the neural correlate from it.
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