Joe Lau's wiki: Main/Nagel On Split Brain

Split-brain

Readings

• Thomas Nagel (1991). Mortal Questions. Cambridge University Press. See chapter 11. Brain bisection and the unity of consciousness. Google bk preview intranet:nagel71.pdf
• Ch 5 of Tye (2003) 'Consciousness and Persons: Unity and Identity'' MIT Press. hkulib:b2663083~S6
• Daniel Dennett. 1992. The Self as a Center of Narrative Gravity. In F. Kessel, P. Cole and D. Johnson (eds.). Self and Consciousness: Multiple Perspectives. Hillsdale, NJ: Erlbaum.
• Charles E. Marks (1981) Commissurotomy, Consciousness, and Unity of Mind MIT Press.
• Tim Bayne (2008). The Unity of Consciousness and the Split-Brain Syndrome. Journal of Philosophy 105 (6).
• Gazzaniga (2005) Forty-five years of split-brain research and still going strong. Nat Rev Neurosci. 6(8):653-9.

Background

• Beginning: Roger Sperry operated on epileptic patients in the 1960s, cutting the corpus callosum to control seizures.

Video: http://www.youtube.com/v/ZMLzP1VCANo

Some examples of dissociation

• The word "pen" is shown to the left visual field, and "knife" to the right. Patient says that he sees only "knife", but left hand writes down "pen" slowly.
• Wada test
• Garlic smell presented to right nostril - left hand points to garlic, but verbally deny detecting any smell.
• The two hemispheres are still linked by sub-cortical pathways.
  • Carries information about touch around the head and the neck.
  • No problem in verbalizing the pointing to the location of tactile stimulus on the neck or the face.
  • When pornography is shown only to the right hemisphere, the subject reports feeling differently, and one patient says "That's quite a machine you have got there."

Some possible interpretations

1. Left side is a mind, right side is not. (Right side = automaton, unconscious system with isolated mental states ...)
2. Left is one mind, and right is another mind. (Sperry?)
3. Both sides make one mind with dissociated contents. (Bayne?)
4. Both sides make one mind, but it splits into two during experiments. (Tye)
5. It is indeterminate how many persons there are. (Nagel)

Comments on these possibilities

Case #1

• Argument for: Subject denies denies contents of right hemisphere.
• Why take the left-side subject as authoritative about the status of the right side?
• Should verbalization be a necessary condition?
• Seems inconsistent with right side's capacity to carry out coordinated responses (writing simple words, picking out an item from a group of objects) and to follow linguistic instructions.
  • We would not deny consciousness to unintelligent subjects who cannot speak.

Case #2

• Argument for: each hemisphere seems to have its own sphere of control.
  • Sperry: "two freewills in one cranial vault".
  • Roland Puccetti (1973) - Normal people also have two minds!
• Argument against: In normal situations, there is remarkable integration between the disconnected hemispheres and the subject is no different than a normal person. The two hemispheres do not operate independently of each other.

Case #3

• Argument against: If there is just one subject, how should we describe the beliefs and experiences of the subject? What is it like for the subject to perceive incompatible visual stimuli?

Case #4

• According to Nagel
  • Seems ad hoc - no internal change in the subject corresponding to the split.
  • No explanation of how a mind can come into or go out of existence.
  • Even during special situations, the subject largely behaves as a single integrated person, e.g. posture, eye gaze direction.

Nagel's conclusions

1. There is no number N such that the subject has N number of minds.
2. Attribution of mentality does not require a mental subject.
3. We should be skeptical of the concept of a single subject of consciousness applying to ourselves.
4. But: it is possible that we cannot abandon this idea "no matter what we discover".

Reply to Nagel - A self as emergent coherence

• The existence of a self depends on the coherence of mental states.
  • Vertical coherence - higher-level goals, action intentions, bodily movements
  • horizontal coherence - consistency of beliefs, coordination of actions
• Coherence is a matter of degree.
  • Whether the self exists is a matter of degree.
• Followup questions
  • Why does the existence of the self require coherence?
  • How can the existence of phenomenology be a matter of degree? Either there is a point of view or not.
  • A coherence theory of the self seems to rule out the possibility of a single self with multiple identities.

Personality disorder

• 1980 DSM III (Diagnostic and Statistical Manual of Mental Disorders)

@Multiple Personality. 1. The existence within an individual of two or more distinct personalities, each of which is dominant at a particular time. 2. The personality that is dominant at any particular time determines the individual's behavior. 3. Each individual personality is complex and integrated with its own unique behavior patterns and social relationships.@

• In 1994 DSM IV, MPD changed to Dissociative Identity Disorder (DID).

@A. The presence of two or more distinct identities or personality states (each with its own relatively enduring pattern of perceiving, relating to, and thinking about the environment and self).
B. At least two of these identities or personality states recurrently take control of the person's behavior.
C. Inability to recall important personal information that is too extensive to be explained by ordinary forgetfulness.
D. The disturbance is not due to the direct physiological effects of a substance (e.g., blackouts or chaotic behavior during Alcohol Intoxication) or a general medical condition (e.g., complex partial seizures).@

• Splitting-selves as a coping mechanism vs. therapy-induced artifact.
• DID might be a condition where a single subject fails to integrate behavioral dispositions and autobiographical memory.

Category.Mind