Neural Mechanisms of Anesthesia (Contemporary Clinical Neuroscience)


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1. Introduction

Lamme holds that recurrent processing in Stage 3 is necessary and sufficient for consciousness. Thus, what it is for a visual state to be conscious is for a certain recurrent processing state to hold of the relevant visual circuitry. This identifies the crucial difference between the global neuronal workspace and recurrent processing theory: the former holds that recurrent processing at Stage 4 is necessary for consciousness while the latter holds that recurrent processing at Stage 3 is sufficient. Thus, recurrent processing theory affirms phenomenal consciousness without access by the global neuronal workspace.

In that sense, it is an overflow theory see section 2.

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Why think that Stage 3 processing is sufficient for consciousness? Given that Stage 3 processing is not accessible to introspective report, we lack introspective evidence for sufficiency. Lamme appeals to experiments with brief presentation of stimuli such as letters where subjects are said to report seeing more than they can identify in report Lamme It is not clear that this is strong motivation for recurrent processing, since the very fact that subjects can report seeing more letters shows that they have some access to them, just not access to letter identity.

Lamme also presents what he calls neuroscience arguments. This strategy compares two neural networks, one taken to be sufficient for consciousness, say the processing at Stage 4 as per Global Workspace theories, and one where sufficiency is in dispute, say recurrent activity in Stage 3.

Lamme argues that certain features found in Stage 4 are also found in Stage 3 and given this similarity, it is reasonable to hold that Stage 3 processing suffices for consciousness. For example, both stages exhibit recurrent processing. Global neuronal workspace theorists can allow that recurrent processing in stage 3 is correlated, even necessary, but deny that this activity is explanatory in the relevant sense of identifying sufficient conditions for consciousness.

It is worth reemphasizing the empirical challenge in testing whether access is necessary for phenomenal consciousness sections 2. The two theories return different answers, one requiring access, the other denying it. As we saw, the methodological challenge in testing for the presence of phenomenal consciousness independently of access remains a hurdle for both theories.

A long-standing approach to conscious states holds that one is in a conscious state if and only if one relevantly represents oneself as being in such a state.

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For example, one is in a conscious visual state of seeing a moving object if and only if one suitably represents oneself being in that visual state. The intuitive rationale for such theories is that if one were in a visual state but in no way aware of that state, then the visual state would not be conscious. Thus, to be in a conscious state, one must be aware of it, i. Higher-order theories merge with empirical work by tying high-order representations with activity in prefrontal cortex which is taken to be the neural substrate of the required higher-order representations.

On certain higher-order theories, one can be in a conscious visual state even if there is no visual system activity, so long as one represents oneself as being in that state. For example, on the higher-order theory, lesions to prefrontal cortex should affect consciousness Kozuch , testing the necessity of prefrontal cortex for consciousness.


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Against higher-order theories, some reports claim that patients with prefrontal cortex surgically removed maintain preserved perceptual consciousness Boly et al. This would lend support to recurrent processing theories that hold that prefrontal cortical activity is not necessary for consciousness. Bilateral suppression of prefrontal activity using transcranial magnetic stimulation also seems to selectively impair visibility as evidenced by metacognitive report Rounis et al.

IIT defines integrated information in terms of the effective information carried by the parts of the system in light of its causal profile. For example, we can focus on a part of the whole circuit, say two connected nodes, and compute the effective information that can be carried by this microcircuit.

The system carries integrated information if the effective informational content of the whole is greater than the sum of the informational content of the parts. Intuitively, the interaction of the parts adds more to the system than the parts do alone. On IIT, what matters is the presence of appropriate connections and not the number of neurons. A potential problem for IIT is that it treats many things to be conscious which are prima facie not in Other Internet Resources , see Aaronson a; for striking counterexamples and Aaronson b with a response from Tononi.

For certain higher-order thought theories, having a higher-order state, supported by prefrontal cortex, without corresponding sensory states can suffice for conscious states.

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In this case, the front of the brain would be sufficient for consciousness. Finally, the global neuronal workspace, drawing on workspace neurons that are present across brain areas to form the workspace, might be taken to straddle the difference, depending on the type of conscious state involved. They require entry into the global workspace such that neither sensory activity nor a higher order thought on its own is sufficient, i.


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What is clear is that once theories make concrete predictions of brain areas involved in generic consciousness, neuroscience can test them. Work on unconscious vision provides an informative example.

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In the past decades, scientists have argued for unconscious seeing and investigated its brain basis especially in neuropsychology , the study of subjects with brain damage. Interestingly, if there is unconscious seeing, then the intentional action inference must be restricted in scope since some intentional behaviors might be guided by unconscious perception section 2. That is, the existence of unconscious perception blocks a direct inference from perceptually guided intentional behavior to perceptual consciousness. The case study of unconscious vision promises to illuminate more specific studies of generic consciousness along with having repercussions for how we attribute conscious states.

Since the groundbreaking work of Leslie Ungerleider and Mortimer Mishkin , scientists divide primate cortical vision into two streams: dorsal and ventral for further dissection, see Kravitz et al. The dorsal stream projects into the parietal lobe while the ventral stream projects into the temporal lobe see Figure 1. Controversy surrounds the functions of the streams.

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Ungerleider and Mishkin originally argued that the streams were functionally divided in terms of what and where : the ventral stream for categorical perception and the dorsal stream for spatial perception. There continues to be debate surrounding the Milner and Goodale account Schenk and McIntosh but it has strongly influenced philosophers of mind. Lesions to the dorsal stream do not seem to affect conscious vision in that subjects are able to provide accurate reports of what they see but see Wu a.

Rather, dorsal lesions can affect visual-guidance of action with optic ataxia being a common result. Optic ataxic subjects perform inaccurate motor actions. Lesions in the ventral stream disrupt normal conscious vision, yielding visual agnosia, an inability to see visual form or to visually categorize objects Farah Dorsal stream processing is said to be unconscious.

Anesthesia and the dynamics of the unconscious mind

If the dorsal stream is critical in the visual guidance of many motor actions such as reaching and grasping, then those actions would be guided by unconscious visual states. The visual agnosic patient DF provides critical support for this claim. Like other visual agnosics with similar lesions, DF is at chance in reporting aspects of form, say the orientation of a line or the shape of objects. Nevertheless, she retains color and texture vision. Strikingly, DF can generate accurate visually guided action, say the manipulation of objects along specific parameters: putting an object through a slot or reaching for and grasping round stones in a way sensitive to their center of mass.

Simultaneously, DF denies seeing the relevant features and, if asked to verbally report them, she is at chance. What is uncontroversial is that there is a division in explanatory neural correlates of visually guided behavior with the dorsal stream weighted towards the visual guidance of motor movements and the ventral stream weighted towards the visual guidance of conceptual behavior such as report and reasoning see section 5.

A substantial further inference is that consciousness is segregated away from the dorsal stream to the ventral stream. How strong is this inference? Recall the intentional action inference. In performing the slot task, DF is doing something intentionally and in a visually guided way.

For control subjects performing the task, we conclude that this visually guided behavior is guided by conscious vision. Indeed, a folk-psychological assumption might be that consciousness informs mundane action Clark ; for a different perspective see Wallhagen Since DF shows similar performance on the same task, why not conclude that she is also visually conscious?

DF denies seeing features she is visually sensitive to in action. Should introspection then trump intentional action in attributing consciousness? Two issues are worth considering. The first is that introspective reports involve a specific type of intentional action guided by the experience at issue. One type of intentional behavior is being prioritized over another in adjudicating whether a subject is conscious. What is the empirical justification for this prioritization?

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The second issue is that DF is possibly unique among visual agnosics. It is a substantial inference to move from DF to a general claim about the dorsal stream being unconscious in neurotypical individuals see Mole for arguments that consciousness does not divide between the streams and Wu for an argument for unconscious visually guided action in normal subjects.

What this shows is that the methodological decisions that we make regarding how we track consciousness are substantial in theorizing about the neural bases of conscious and unconscious vision. A second neuropsychological phenomenon also highlighting putative unconscious vision is blindsight which results from lesions in primary visual cortex V1 typically leading to blindness over the part of visual space contralateral to the sight of the lesion Weiskrantz For example, left hemisphere V1 deals with right visual space, so lesions in left V1 lead to deficits in seeing the right side of space.

Subjects then report that they cannot see a visual stimulus in the affected visual space. For example, a blindsight patient with bilateral damage to V1 i. Blindsight patients see in the sense of visually discriminating the stimulus to act on it yet deny that they see it. Like DF, blindsighters show a dissociation between certain actions and report, but unlike DF, they do not spontaneously respond to relevant features but must be encouraged to generate behaviors towards them.

The neuroanatomical basis of blindsight capacities remains unclear. Certainly, the loss of V1 deprives later cortical visual areas of a normal source of visual information. Still, there are other ways that information from the eye bypasses V1 to provide inputs to later visual areas. Alternative pathways include the superior colliculus SC , the lateral geniculate nucleus LGN in the thalamus, and the pulvinar as likely sources.

Neural Mechanisms of Anesthesia (Contemporary Clinical Neuroscience) Neural Mechanisms of Anesthesia (Contemporary Clinical Neuroscience)
Neural Mechanisms of Anesthesia (Contemporary Clinical Neuroscience) Neural Mechanisms of Anesthesia (Contemporary Clinical Neuroscience)
Neural Mechanisms of Anesthesia (Contemporary Clinical Neuroscience) Neural Mechanisms of Anesthesia (Contemporary Clinical Neuroscience)
Neural Mechanisms of Anesthesia (Contemporary Clinical Neuroscience) Neural Mechanisms of Anesthesia (Contemporary Clinical Neuroscience)
Neural Mechanisms of Anesthesia (Contemporary Clinical Neuroscience) Neural Mechanisms of Anesthesia (Contemporary Clinical Neuroscience)
Neural Mechanisms of Anesthesia (Contemporary Clinical Neuroscience) Neural Mechanisms of Anesthesia (Contemporary Clinical Neuroscience)
Neural Mechanisms of Anesthesia (Contemporary Clinical Neuroscience) Neural Mechanisms of Anesthesia (Contemporary Clinical Neuroscience)
Neural Mechanisms of Anesthesia (Contemporary Clinical Neuroscience) Neural Mechanisms of Anesthesia (Contemporary Clinical Neuroscience)
Neural Mechanisms of Anesthesia (Contemporary Clinical Neuroscience)

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