Models of Consciousness

in Split-brain Subjects

1. The split-brain phenomenon 2. Consciousness and conscious unity3. The conscious duality model4. 3 Unity models5. The partial unity model

Today 2

• The surgery• The experiments• Their design:

1. perceptual lateralization2. response control

1 The Split-brain Phenomenon

3

Sp

err

y, R

., V

og

el,

P.,

an

d

Bog

en

, J.

19

67.

Syn

dro

me o

f h

em

isp

here

d

eco

nn

ect

ion

.

4

Fro

m C

arl

son

, N

. 2

004.

Ph

ysio

log

y of

Beh

avi

or,

6

th E

d.

5

• The surgery• The experiments• Their design:

1. perceptual lateralization2. response control3. prevention of cross-cuing

1 The Split-brain Phenomenon

6

• The split-brain phenomenon has been taken to be a real-life “hard case” for accounts of conscious unity.

2 Consciousness and conscious unity 7

Perhaps a conscious mental state is…•one of which its subject is aware. •one that is not just used in local processing but which is available to a wide suite of reasoning systems simultaneously (or one that is “globally available”). •one for which there is something that it is like to have it.

Three notions of consciousness 8

Perhaps for any two simultaneous conscious experiences of a subject…•their subject is or can be aware of undergoing them at once (co-awareness)•their subject can reason about the pair, use their conjunction in reasoning (co-accessibility)•there is something it is like for the subject to undergo the two together, something different from what it is like to undergo one of them one day and the other the next (co-phenomenality)

Three notions of conscious unity 9

Split-brain results violate intuitions about the way consciousness is structured:

“Roughly, we assume that a single mind has sufficiently immediate access to its conscious states so that, for elements of experience or other mental events occurring simultaneously or in close temporal proximity, the mind which is their subject can also experience the simpler relations between them if it attends to the matter.” Nagel, 1971, “Brain bisection and the unity of consciousness”

The unity of consciousness 10

Experimental results violate intuitions about the way consciousness is structured.

“…. we assume that… for elements of experience or other mental events occurring simultaneously or in close temporal proximity, the mind which is their subject can also experience the simpler relations between them….”

… vs. standard cross-comparison test for “callosal disconnection syndrome”.

11The unity of consciousness

All kinds of divisions within and breakdowns of cognitive processing are now recognized.

But it is still commonly believed that consciousness is integrated. – E.g. Tononi’s (2004) “information

integration” theory– E.g. Baars’ (1988) “global workspace”

theory

12The unity of consciousness

How many streams of consciousness does a split-brain subject have……. where a stream of consciousness is a collection of conscious experiences that are co-conscious (co-aware, co-accessible, co-phenomenal) with each other?

13Co-consciousness and

streams of consciousness

Basic idea: A split-brain subject has two streams of consciousness, a RH stream and a LH stream.

143 Duality Model

Basic idea: A split-brain subject has two streams of consciousness, a RH stream and a LH stream.

Concerns: 1.A significant amount of conscious information appears shared between or undivided across the two hemispheres.2.Split-brain subjects act pretty normally, most or all of the time.

3 Duality Model 15

“Classic” (or “skeptical”) unity model

“Unity through duplication” model

“Switch” model

4 Unity Models 16

Basic idea: A split-brain subject has a single stream of consciousness, associated with only the left hemisphere.

Concerns: 1.RH controlled behavior seems the result of genuinely conscious experience and control. (E.g., Zaidel, Zaidel, and Sperry, 1981.)

Classic (skeptical) unity model 17

18

“Both patients did as well or nearly as well with their right hemispheres as with their left. Indeed, with his right hemisphere, LB scored at a level equivalent to an 11-year-old child. To perform correctly on this test, the relation between two items has to be abstracted and then extrapolated so as to infer the third item in a progression; finally, the result much be matched to one of a set of possible answers…. If this level of performance could be obtained unconsciously, then it would be really difficult to argue that consciousness is not an epiphenomenon.”(Shallice 1997, Modularity and Consciousness 264)

19

Basic idea: A split-brain subject has a single stream of consciousness, associated with only the left hemisphere.

Concerns: 1.RH controlled behavior seems the result of genuinely conscious experience and control. (E.g., Zaidel, Zaidel, and Sperry, 1981.)2.In some cases the RH learns to speak!

Classic (skeptical) unity model 20

Basic idea: common contents suffice for conscious unity; it doesn’t matter the routes or mechanisms by which contents are duplicated across the hemispheres.

Concerns: 1.Can the physical basis of conscious unity be partially behavioral?2.Isn’t it a little strange to think that simply e.g. plugging a nostril alters the structure of consciousness?

Unity through duplication model 21

Basic idea: A split-brain subject has a single stream of consciousness whose contents derive from both hemispheres in turn.

Switch model 22

Figure from Levy, Trevarthen, and Sperry, 1972, “Perception of bilateral chimeric figures following hemispheric deconnexion”, p. 68

23

3 Unity models

Figure from Levy, Trevarthen, and Sperry, 1972, “Perception of bilateral chimeric figures following hemispheric deconnexion”, p. 70

24

Manipulations

• In each standard trial, subject is asked to indicate the stimulus she’s just seen in one of two ways, by naming or by pointing to a match.

Basic results, non-split vs. split-brain subjects. (“Perceptual completion.”)

“An eye.”

25

• Certainly seems to show that, across trials, each hemisphere was subject to conscious experiences to which the other was not privy.

• But that’s compatible with the switch model as well.

Conscious duality or interhemispheric switching of conscious contents?

26

Basic idea: A split-brain subject has a single stream of consciousness whose contents derive from both hemispheres in turn.

Concerns: 1.2 hemispheres sometimes seem to be separately conscious simultaneously (e.g. Schiffer et al. 1997). 2.At most, means that (technically) no two streams at any single moment in time; still looks like two streams of consciousness across time.

Switch model 27

Basic idea: There are RH and LH experiences that are not consciously unified (co-conscious) with each other, even though they are all unified with a set of (subcortically transferred or sustained?) experiences.

5 Partial Unity Model 28

Basic idea: There are RH and LH experiences that are not consciously unified (co-conscious) with each other, even though they are all unified with a set of (subcortically transferred or sustained?) experiences.

Concerns: 1.Difficult to empirically distinguish between a partially unified stream of consciousness and two streams of consciousness with some common contents. 2.Appears to drop notion of unitary conscious perspective.

295 Partial Unity Model