加齢に伴う「デフォルト・ネットワーク」の休息状態の脳活動の低下

Reduced resting-state brain activity in the "default network" in normal aging.

Damoiseaux JS, Beckmann CF, Arigita EJ, Barkhof F, Scheltens P, Stam CJ, Smith SM, Rombouts SA

Cereb Cortex. 2007 Dec 5;

Normal aging is associated with cognitive decline even in the absence of disease. Functions such as attention, information processing, and working memory are compromised. Based on the observation of age-related white matter degeneration, O'Sullivan et al. (2001) proposed the "disconnection" hypothesis, stating that decline in normal aging emerges from changes in connections between brain areas, in addition to dysfunction of specific areas. Here, we examined the functional properties of brain networks based on spontaneous fluctuations within brain systems using functional magnetic resonance imaging. We hypothesized that functional connectivity of intrinsic brain activity in the "default-mode" network (DMN) is affected by normal aging and that this relates to cognitive function. Ten younger and 22 older subjects were scanned at "rest," that is, lying awake with eyes closed. Our results show decreased activity in older versus younger subjects in 2 resting-state networks (RSNs) resembling the previously described DMN, containing the superior and middle frontal gyrus, posterior cingulate, middle temporal gyrus, and the superior parietal region. These results remain significant after correction for RSN-specific gray matter volume. The relevance of these findings is illustrated by the correlation between reduced activity of one of these RSNs and less effective executive functioning/processing speed in the older group.

　
O'Sullivan らの仮説 (2001)
「断絶」仮説 discconection hypothesis

• 加齢による認知機能の低下は、特定の領域の機能尾低下に加え、脳領域間の接続の変化による

　
⇒default-mode network (DMN) の自発的な変動が加齢によって影響を受け、認知機能に影響を与えるのでは？

結果

• 上前頭回、中前頭回、後帯状回、中側頭回、上頭頂部の休息時の活動が高齢者では低下
• 灰白質の容積変化を補正しても有意

これから読むところ
default-mode network と resting-state network を使い分けているのだが、違いは？

帰納的推論の脳機能：言語とは独立の分散したネットワーク

Functional neuroanatomy of deductive inference: a language-independent distributed network.
Monti MM, Osherson DN, Martinez MJ, Parsons LM.

Neuroimage. 2007 Sep 1;37(3):1005-16. Epub 2007 May 24.

Studies of brain areas supporting deductive reasoning show inconsistent results, possibly because of the variety of tasks and baselines used. In two event-related functional magnetic imaging studies we employed a cognitive load paradigm to isolate the neural correlates of deductive reasoning and address the role (if any) of language in deduction. Healthy participants evaluated the logical status of arguments varying in deductive complexity but matched in linguistic complexity. Arguments also varied in lexical content, involving blocks and pseudo-words in Experiment I and faces and houses in Experiment II. For each experiment, subtraction of simple from complex arguments (collapsing across contents) revealed a network of activations disjoint from regions traditionally associated with linguistic processing and also disjoint from regions recruited by mere reading. We speculate that this network is divided into "core" and "support" regions. The latter include left frontal (BA 6, 47) and parietal (BA 7, 40) cortices, which maintain the formal structure of arguments. Core regions, in the left rostral (BA 10p) and bilateral medial (BA 8) prefrontal cortex, perform deductive operations. Finally, restricting the complex-simple subtraction to each lexical content uncovered additional activations which may reflect the binding of logical variables to lexical items.

帰納的推論に関わる脳部位の研究の結果は課題やベースラインの違いから不一致が見られる。
言語学的複雑さを一定にして帰納的複雑さを変化させる

• 言語や読字とは異なる領域が帰納的複雑さに関わる
• Support regions: 左前頭(BA6, 47), 左頭頂(7, 40)
  • 項の形式的構造の維持
• Core regions: 左吻側 (BA 10p) 両内側前頭 (BA8)
  • 帰納操作

　
語彙の複雑さに関わる領域は論理的変項と語彙アイテムのバインディングを担っているかもしれない

人間とチンパンジーの休息状態の脳活動の比較

A comparison of resting-state brain activity in humans and chimpanzees.

Rilling JK, Barks SK, Parr LA, Preuss TM, Faber TL, Pagnoni G, Bremner JD, Votaw JR.

Proc Natl Acad Sci U S A. 2007 Oct 23;104(43):17146-51. Epub 2007 Oct 16.

In humans, the wakeful resting condition is characterized by a default mode of brain function involving high levels of activity within a functionally connected network of brain regions. This network has recently been implicated in mental self-projection into the past, the future, or another individual's perspective. Here we use [(18)F]-fluorodeoxyglucose positron emission tomography imaging to assess resting-state brain activity in our closest living relative, the chimpanzee, as a potential window onto their mental world and compare these results with those of a human sample. We find that, like humans, chimpanzees show high levels of activity within default mode areas, including medial prefrontal and medial parietal cortex. Chimpanzees differ from our human sample in showing higher levels of activity in ventromedial prefrontal cortex and lower levels of activity in left-sided cortical areas involved in language and conceptual processing in humans. Our results raise the possibility that the resting state of chimpanzees involves emotionally laden episodic memory retrieval and some level of mental self-projection, albeit in the absence of language and conceptual processing.

人間においては覚醒時における休息状態は高次領域のネットワークのデフォルトモードとして特徴付けられている。このネットワークは過去や未来、他者のパースペクティヴへの自己投影に関係していることが示されてきた。
休息状態をヒトとチンパンジーで比較 ([18F]-fluorodeoxyglucose PET)

• ヒト≒チンプ：内側前頭前野、内側頭頂葉
• ヒト＞チンプ：腹内側前頭前野
• ヒト＜チンプ：左半球の皮質 (言語や概念の処理に関係する部位)

　
情動を帯びたエピソード記憶の想起や何らかのレベルの自己投影を行っているが、言語や概念の処理は行われていない？

■

皮質の高解像度の被験者間平均と座標系

High-resolution intersubject averaging and a coordinate system for the cortical surface.

Fischl B, Sereno MI, Tootell RB, Dale AM.

Hum Brain Mapp. 1999;8(4):272-84.

皮質は細かく織り込まれたシート状の構造をなしている。

従来の3次元座直交標系に基づいた座標変換は脳溝や脳回といった構造を無視してしまう。

⇒皮質表面に基づいた球座標系での変換を提唱。

• 折りたたみ構造が保存される。
  
• 構造的、機能的な特性の被験者間平均をより高い解像度で実現
  

自然言語の教師無し学習

Unsupervised learning of natural languages (free)

Solan Z, Horn D, Ruppin E, Edelman S.

Proc Natl Acad Sci U S A. 2005 Aug 16;102(33):11629-34. Epub 2005 Aug 8.

ADIOS (automatic distillation of structure) というアルゴリズムを持ってすれば再帰的に階層構造を抽出できるとな。

人工の文脈自由言語、英語、中国語、タンパク質データの学習に成功。生成も可能。

マジか!?しかも二年前だった。Mclleland に直接提出された論文だからなあ。眉唾∧必読。

大脳皮質における神経代謝カップリングはスパイク活動ではなくシナプス活動を反映する

Neurometabolic coupling in cerebral cortex reflects synaptic more than spiking activity.

Viswanathan A, Freeman RD.

Nat Neurosci. 2007 Oct;10(10):1308-12. Epub 2007 Sep 9.

fMRI における BOLD シグナルが神経活動の何を反映しているか？

• 入力(シナプス)?
  
• 出力(スパイク)?
  

ネコの一次視覚野においてスパイクの無い状況で LFP (local field potential) と酸素濃度との相関

⇒BOLD はシナプス活動を反映。