One, not two, neural correlates of consciousness.

The authors critique Ned Block's distinction between phenomenological consciousness (what we experience) and access consciousness (information accessible via conscious experiences). They argue that evidence from cognitive neuroscience does not support this division. For instance, visual cortex activity is necessary but not sufficient for conscious visual qualities; without parietal and prefrontal activation, no correlation with consciousness is found. Similarly, when frontoparietal regions are impaired, sensory consciousness is absent despite sensory cortex activation. The authors contend that unconscious brain mechanisms interact with visual cortex to enable visual qualities, and that positing a separate 'access consciousness' is unnecessary. They suggest that what is needed is simply consciously-mediated access to brain capacities, most of which are unconscious.

Study at a glance

Abstract

Although we admire Ned Block's effort to include cognitive neuroscience in his philosophical work, we cannot agree with his interpretation of the evidence [1xTwo neural correlates of consciousness. Block, N. Trends Cogn. Sci. 2005; 9: 46–52Abstract | Full Text | Full Text PDF | PubMed | Scopus (252)See all References[1]. Block has long argued that there are two kinds of consciousness: ‘phenomenological consciousness’ (what we experience) and ‘access consciousness’ (roughly, the information we can access via conscious experiences). In Baars' theoretical work on Global Workspace Theory [2xA Cognitive Theory of Consciousness. Baars, B.J. See all References, 3xThe conscious access hypothesis: origins and recent evidence. Baars, B.J. Trends Cogn Sci. 2002; 6: 47–52Abstract | Full Text | Full Text PDF | PubMed | Scopus (406)See all References] that point is made more simply: the contents of visual consciousness, for example, clearly require visual cortex. But activity in visual cortex, although necessary, is not sufficient for conscious qualities, as the brain evidence so clearly shows. Without parietal and prefrontal activation, researchers such as Dehaene et al. [4xCerebral mechanisms of word masking and unconscious repetition priming. Dehaene, S. et al. Nat. Neurosci. 2001; 4: 752–758Crossref | PubMed | Scopus (688)See all References[4] find no correlation of visual cortical activity with consciousness. Similarly, Laureys et al. [5xImpaired effective cortical connectivity in vegetative state. Laureys, S. et al. Neuroimage. 1999; 9: 377–382Crossref | PubMed | Scopus (223)See all References[5] have shown that when frontoparietal regions are impaired, there is no evidence for sensory consciousness, despite activation of sensory cortices [6xAuditory processing in the vegetative state. Laureys, S. et al. Brain. 2000; 123: 1589–1601Crossref | PubMedSee all References, 7xCortical processing of noxious somatosensory stimuli in the persistent vegetative state. Laureys, S. et al. Neuroimage. 2002; 17: 732–741Crossref | PubMedSee all References]. The obvious inference is that unconscious brain mechanisms interact with visual cortex to make visual qualities possible. But why should such interactions be said to involve ‘access consciousness’?For example, we know that some parietal neurons act as egocentric visuospatial maps, and that without those neurons, visual objects disappear phenomenally from the contralateral visual field. But direct stimulation of these parietal egocentric map neurons does not evoke conscious experiences at all: they do not directly support visual qualia. That is, they are necessary for visual contents but not sufficient, just as brainstem arousal systems are necessary but not sufficient. Why then should they be thought to involve ‘access consciousness’?We are afraid that Block has fallen victim to his own theory. There is no need for ‘access consciousness’. All we need is consciously-mediated access to brain capacities, most of which are simply not conscious.