東京大学学際科学科 2013年度 総合情報学特論 III 意識の科学的研究 - 盲視を起点に

6月26日(水) 3-4限 13:00-16:20, 駒場キャンパス15号館1階104講義室

吉田 正俊 (自然科学研究機構・生理学研究所・認知行動発達研究部門 助教)

要旨 Part 1: • The idea of two visual systems (cortical vs. subcortical) was confirmed in various animals. • The dorsal and ventral visual pathways may have different roles on vision for action and vision for

perception, respectively. • Feeling-of-something-happening in blindsight may be mediated by the dorsal pathway for saliency. 要旨 Part 2: • Neural correlates of awareness can be studied using bistable percepts. • Vision is not passive. Brain is predictive. • Sensorimotor contingency explains our perception to some extent. • The dorsal pathway for sensorimotor contingency and the ventral pathway for internal model? • Current scientific study of consciousness is the study of philosophical zombie. • Neurophenomenology tries to establish a first-person methodology but it is not successful yet.

PART 1

0. What is consciousness? -- Let’s start from examples

• Salient visual stimuli can disappear. • We are not conscious of all of the signals on the

retina.

1. What is Blindsight (盲視 )? • Q: What is blindsight? • A: “The visually evoked voluntary responses of

patients with striate cortical destruction that are demonstrated despite a phenomenal blindness” 1

• Phenomenal consciousness can be dissociated from visual information processing.

2. Two visual systems: Cortical vs. Subcortical

2-1. Two visual systems in monkeys • Bilateral lesion in V1 - first report of blindsight in

monkey 2 • Retained: visually guided reaching and obstacle

avoidance 3 2-2. Two visual systems in rodents • Double dissociation - lesion in visual cortex and in

the superior colliculus 4 • SC for orienting 5 • Visual cortex for pattern discrimination 6

2-3. Two visual systems in frogs • Two Visual Systems in the Frog 7 • Lesion in the optic tectum induces rewiring. • Neocortex for obstacle avoidance • Optic tectum for Response to preys

3. Two cortical visual systems: Dorsal vs. Ventral

3-1. What and where pathways (Mishkin & Ungerleider) • The neurons in the dorsal pathway are selective to

motion and binocular disparity. • The neurons in the ventral pathway are selective to

shape and color. 8 • Bilateral removal of area TE: Object discrimination -

Which is the unfamiliar object? • Bilateral removal of posterior parietal cortex:

Landmark discrimination - Which is near to the landmark? 9

3-2. Vision for perception and vision for action (Goodale and Milner) • Dorsal pathway: Vision for action • Ventral pathway: Vision for perception 10 • Optic ataxia (視覚性運動失調) • Damage in the posterior parietal cortex -

supramarginal gyrus and angular gyrus • Orientation error does not depend on hand but on

visual field. • Damage in the dorsal pathway affects vision for

action. 11 • Visual form agnosia (視覚失認): • Subject DF: Bilateral damage in ventral visual

pathway (Lateral occipital area: LO) • Very good performance in ‘posting’ task 12 • DF matched her card orientation to the slot during

the course of the movement, well before contacting the target. 13

• Functional double dissociation • "Visual phenomenology ... can arise only from

processing in the ventral stream ... visual-processing modules in the dorsal stream ... are not normally available to awareness." ("The visual brain in action" p.200-201) 14

3-3. Perceptual experience in visual agnosia • Retained color, texture perception • Degraded form perception • “a piece of kitchen equipment … it’s got a red part

to it, a red handle … it goes down into a silver corrugated part … the red part’s plastic and the other part’s metal.” (When she was handed it) “Oh, it’s a torch.” 15

• She had difficulty in describing her visual experience, only saying that objects tend to appear ‘blurred’ and that separate elements ‘run into each other’. 13

4. Blindsight in human

4-1. Case reports • G.Y. became blind in his right visual field due to

traffic accident in eight years old. He was diagnosed as homonymous hemianopia.

• Above-chance performance in forced-choice => blindsight 16

4-2. Blindsight and two-visual systems hypothesis • Access to the dorsal pathway in blindsight

5. Blindsight in monkey 5-1. Blindsight in monkeys • Blindsight after 2-3 months training 17 • Are the monkeys really ‘blind’ to the visual stimuli?

The monkeys behaved as if it is a No-target trial. The monkeys are ‘not able to see’, as in human blindsight.

5-2. Saliency in blindsight • Monkey without V1: Visually guided reaching • Frogs with rewired OT: Frogs with rewired OT • => Visual saliency?

• What is saliency? • Saliency computational model 18 • Salient stimuli attract gazes of blindsight monkeys

19 • The gaze positions have higher saliency, than

expected from random eye movement. • What it is like to be blindsight? • feeling ‘atmosphere’ = saliency without visual

consciousness • Dual system for conscious vision and saliency.

6. Summary

PART 2

1. How to study consciousness?

1-1 Definition of consciousness • ?? “Consciousness is not a subject of science

because we cannot define them.” • Let’s start from a common-sense definition, not

from an analytic definition. 20 • A common-sense definition of consciousness:

"consciousness refers to those states of sentience or awareness that typically begin when we wake from a dreamless sleep and continue through the day …”

1-2 Hard problem of consciousness • Philosophical zombie • The inverted spectrum • * How things that look green to you look red to me

and vice versa. • * The things we both call red look to you the way

the things we both call green look to me. • In his (Ned’s) class, ~2/3 of the students usually

say, ‘Oh yeah, I see what you’re talking about’ and some of them even say, ‘Oh yeah, I’ve wondered about that since I was a kid.’ ~1/3 of people say, ‘I don’t know what you’re talking about.’

• The hard problem of consciousness: • the problem of explaining how and why we have

qualia or phenomenal experiences — how sensations acquire characteristics, such as colors and tastes.

• Awareness: “a state wherein we have access to some information, and can use that information in the control of behavior.” “the psychological concept of mind”

1-3 Neural correlates of awareness • An experimental manipulation is required by which

a visual input is constant but perception of that visual stimulus varies.

• In binocular rivalry, the stimulus is stable but the content of awareness switches. Then, we can find the neural correlate of visual awareness by comparing two different perceptual reports.

• Activity of IT neurons reflects the monkeys’ perceptual report 21. This is strong evidence that IT neurons represent content of subjective experience.

• fMRI during binocular rivalry 22

2. Active vision

2-1 Corollary discharge and Remapping • We reconstruct images by constantly moving our

eyes. 23,24 • Corollary discharge • Helmholtz (1866) • why image motion caused by saccades passes

unnoticed • Why stability is maintained in spite of shifts in

image position • Efferent copy (Von Holst and Mittelstaedt 1954) • Keeping track of one's movements was based on

monitoring outputs to muscles (implication of exact copy)

• Corollary discharge (Sperry 1950) • The interaction of "motor patterns" with a

"sensorium" without specifying where the interaction occurs

• Why can’t you tickle yourself? - Cancellation of self-produced sensation

• Stability of vision during saccade • Pre-saccadic remapping in LIP

2-2 Bayesian surprise • How to evaluate temporal saliency? • Saliency model evaluates what is salient in an

image in term of spatial configuration. • How to evaluate what is salient in an image in term

of temporal changes? => Bayesian surprise • You have a belief about the world: “it is the CNN

news.” • We get surprised if we have to change our belief of

the world. • The world becomes boring again. • Bayesian surprise measures how much your belief

changed by the data. • Bayesian surprise is defined as the difference

between prior and posterior. =>KL divergence • Surprise is better predictor than saliency 2-3 Friston’s free energy principle • The free-energy principle 25 • An organism is able to minimize the free energy by

reducing bayesian surprise (internal state) or by changing sensation (action).

3. Enactive view

3-1 Sensorimotor contingency theory • Standard view: Seeing is making an internal

representation • New view: Seeing is knowing about things to do 26 • Alva Noeのエナクション説 27： • Sensorimotor dependence: 私たちが対象に向かって近づくと対象の姿が大きくなる。

• 私たちはこのような sensorimotor dependenceに

精通している。 • 私たちの知覚能力は、この種の感覚-運動的知識の所有によって構成されている。

• 感覚-運動的知識とは命題的なものではなくて、技能的なもの。(宣言的記憶と手続き記憶)

• 例：開眼手術、逆さメガネ 3-2 Brain or environment, which determines conscious experience? • Blindsight = Frog’s consciousness? • This is an ‘internalist’ view. Brain area determines

which kind of experience is evoked. • Hurley and Noë’s argument: 28 • Based on Enactive view, sensorimotor contingency,

rather than brain region, is the determinant of conscious experience.

• This is empirically testable. sensory input ? (=> Externalism, enactivism) or brain activity? (=> Internalism) • Phantom limb: the case for brain activity • Inverted glass: the case for sensory input 29

• Which occurs in blindsight? • Internalist view: ‘Feeling of something happening’

is a result of SC activity but it is overridden by ‘redness of red’ in normal subjects. After V1 lesion, it was unmasked.

• Enactive view: ‘Feeling of something happening’ can be a kind of conscious experience accompanied by functional recovery and expanded availability of sensorimotor skill.

• 盲視の例は externalist説を支持しているのではないか? • 1) Please remember blindsight is not available

just after the lesion. • 2) さらに empiricalにテストできる。Normal

subjectで V1を一時的に suppressしたらfeeling-of-somethingは起こるか? 盲視で SCを一時的に抑えたら何が起こるか?

4. Let’s combine everything

Dorsal: ‘Feeling-of-something-happening’ is shaped by sensorimotor contingency. Decision is not consciousness. Evidence accumulation and action are the same in term of “active inference”. Ventral: Conscious experience emerges as an internal model in predictive coding. Bottom up attention (or surprise) and consciousness is the same when they have no prediction error.

5. Hetero-phenomenology (ヘテロ現象学 ) • • Q: How to study consciousness scientifically? • A (by Dennett): Do it with heterophenomenology. • 火星から来た科学者(哲学的ゾンビ)が地球人の「意識経験」について調査しているとする。

• 火星人は行動データ、生理データを集める。 • 行動データのうち、言語報告やボタン押しについては信念や意図を表しているものとして解釈する (志向的態度)。30

• 火星人はこれらのデータから、地球人の意識経験をフィクションとして再構成する。

• 火星人は地球人の意識経験が実在するかどうかを問わなくてよいという意味で中立的であると言える。

• これは文化人類学者がある部族の宗教を研究するのに、その宗教を信じなくてもよいのと同じだ。

• この方法は科学の厳密さを失うことを最小限にした、意識の三人称的研究法であると言える。 31

• そしてこれこそが現在の実験心理学、認知神経科学が行っていることそのものである。

• 両眼視野闘争の例 22 • (a) 「顔を見たという意識経験そのもの」 • (b) その意識経験を持ったという信念 • (c) その信念を表出するためにボタンの右を選択する

• (d) 右ボタンを押す(=「顔を見た」と発声する) • ヘテロ現象学では(d)という一次データを解釈することで(b)という信念(志向的態度)に到達する。

• どうやって(b)という信念が生まれたかを解明することが意識の科学がするべきこと。

• (a)そのものは問わない。 • もし(a)=(b)でないのなら、(b)に「被験者は言い表せない信念を持っている」という信念を付け加えればよい。

6. Neurophenomenology (神経現象学 )

• 「意識のハードプロブレムに対する方法論的救済策」 32

• 意識経験を一人称的かつ誰でも同意できる形で説明するにはどうすればよいか?

• 三つの統合 • 1) 意識経験の(フッサール)現象学的な分析 • 2) 生物学的システムに関する経験的な実験 • 3) 力学系理論

• なんで力学系か?: • enactiveな認知観：認知は身体を持つ(embodied)エージェントによって行われ、感覚・運動的活動によって媒介される。

• => enactiveな認知は力学系的な道具立ての中に自然に収まる (<==> 計算論的) 33

• 現象学的な時間(「生きられた時間」=把持、原印象、予持)は内部発生的な力学系の中で協調している (=> trajectoryによって決まる)

• 神経現象学の実践例： 34 • 1) 被験者は「現象学的還元」によって、事項が経験される仕方に注目するように訓練する。 • 現象学的還元 = 「主観と客観」の二元論のような形而上学をいったん脇に置いて経験の構造を

反省的に捉えること • => 現象学的方法は内観主義ではない

• 1)「現象学的還元」の成果として、被験者は発見的に自分の準備状態について報告できるようになった。このカテゴリーが他の被験者と一貫していることを確認した。(「相互主観的な確証」)

• 2) 多点での EEG(脳波)の計測 • 3) 力学系的な方法での解析: 試行を平均化しない ガンマ帯の脳波の phase synchronization

• 現象学的に明らかにされた準備状態によって脳波の phase synchronyが変わる。

• My criticism: • 1) 意識状態について「発見的にカテゴリー分けをする」点以外はヘテロ現象学と違いはない。両眼視野闘争実験でだって、経験を整理していく際に「右、左、両方が混ざる」というカテゴリー分けが出来て、それは被験者に依らず共通。

• 2) 結局のところ「神経相関」であって、力学系的な「内的に区別可能なカテゴリーの創発」とはなっていない。

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