2012年2月26日日曜日

Neural evidence for inequality-averse social preferences


Elizabeth Tricomi, Antonio Rangel, Colin F. Camerer & John P. O’Doherty
Nature 463:1089 (2010)

昨夜のNHKスペシャル「ヒューマン なぜ人間になれたのか 第4集 そしてお金が生まれた」で紹介されていたので、改めて読んでみた。

背景:
社会科学の世界では、人間が「不平等回避傾向」を持つ事が知られている。
しかし、その神経科学的基盤はよく分かっていない。
この研究では、食べ物・飲み物やお金などに反応する事が知られている報酬系脳部位の活動、ventral striatum(副側線条体)とvmPFC(ventromedial prefrontal cortex:前頭前野内腹側部)、が「不平等回避傾向」と一致するのか否かを確かめた。

実験:
まず、被験者は「Rich」と「Poor」のグループにランダムに分けられる。
Rich:無条件に$50のボーナスが貰える
Poor:何も貰えない
その後、MRIスキャナーの中で、被験者は「お金が自分か他人に支払われる」のを見て、その望ましさを答える(「Very appealing」から「Very unappealing」まで十段階)。

行動データ:
Richグループの被験者は「お金がPoorグループの被験者に支払われる」ことをポジティブに評価した。
一方、Poorグループの被験者は「お金がRichグループの被験者に支払われる」ことをネガティブに評価した。
(ちなみに、どちらのグループの被験者も「自分にお金が支払われる」ことはポジティブに評価した)
→これは「不平等回避傾向」に一致する

fMRIデータ:
報酬系、ventral striatum & vmPFC、の活動は、
Richグループでは「他の被験者(Poorグループ)にお金が支払われた」時の方が「自分(Rich)に支払われた」時より活発であり、Poorグループではその逆。
→これは「不平等回避傾向」に一致する

結論:
社会科学で知られていた「不平等回避傾向」の妥当性が神経科学的にも確かめられた。

感想:
「報酬系の活動が社会的効用(不平等回避)にモジュレートされるか?」という問いを立てる事で、Reverse InferenceやMultiple Comparisonといった厄介な問題を巧妙に避けているのが印象的。

2012年2月24日金曜日

Moderating supplementary data


Nature Neuroscience 15, 339 (2012) doi:10.1038/nn0312-339
Published online 24 February 2012

Nature Neuroscience announces changes to its supplementary information guidelines and expands the space allowed for methods.

2012年2月22日水曜日

Separate Prefrontal-Subcortical Circuits Mediate Different Components of Risk-Based Decision Making


Jennifer R. St. Onge, Colin M. Stopper, Daniel S. Zahm, and Stan B.
Floresco
J. Neurosci. 2012;32 2886-2899
http://www.jneurosci.org/cgi/content/abstract/32/8/2886?etoc

リスク下の意思決定(Risky/Large 対 Safe/Small reward;ラット損傷研究)。扁桃体と側坐核の結合を遮断するとRisky/Largeを選ぶ頻度が減少し、前頭前野から扁桃体の結合を遮断するとRisky/Largeを選ぶ頻度が増加する(一方、前頭前野と側坐核の結合を遮断すると反応時間が長くなる)。リスク下の意思決定は異なる「皮質・皮質下の回路」の競合によって行われている。http://www.jneurosci.org/content/32/16/5631

Choosing between smaller, assured rewards or larger, uncertain ones requires reconciliation of competing biases toward more certain or riskier options. We used disconnection and neuroanatomical techniques to reveal that separate, yet interconnected, neural pathways linking the medial prefrontal cortex (PFC), the basolateral amygdala (BLA), and nucleus accumbens (NAc) contribute to these different decision biases in rats. Disrupting communication between the BLA and NAc revealed that this subcortical circuit biases choice toward larger, uncertain rewards on a probabilistic discounting task. In contrast, disconnections between the BLA and PFC increased choice of the Large/Risky option. PFC–NAc disconnections did not affect choice but did increase choice latencies and trial omissions. Neuroanatomical studies confirmed that projection pathways carrying axons from BLA-to-PFC transverse a distinctly different route relative to PFC-to-BLA pathways (via the ventrolateral amydalofugal pathway and ventromedial internal capsule, respectively). We exploited these dissociable axonal pathways to selectively disrupt bottom-up and top-down communication between the BLA and PFC. Subsequent disconnection studies revealed that disruption of top-down (but not bottom-up) information transfer between the medial PFC and BLA increased choice of the larger, riskier option, suggesting that this circuit facilitates tracking of actions and outcomes to temper urges for riskier rewards as they become less profitable. These findings provide novel insight into the dynamic competition between these cortical/subcortical circuits that shape our decision biases and underlie conflicting urges when evaluating options that vary in terms of potential risks and rewards.

2012年2月21日火曜日

Optimal inference of sameness


Ronald van den Berg, Michael Vogel, Kresimir Josic, and Wei Ji Ma
PNAS February 21, 2012 vol. 109 no. 8 3178-3183

Deciding whether a set of objects are the same or different is a cornerstone of perception and cognition. Surprisingly, no principled quantitative model of sameness judgment exists. We tested whether human sameness judgment under sensory noise can be modeled as a form of probabilistically optimal inference. An optimal observer would compare the reliability-weighted variance of the sensory measurements with a set size-dependent criterion. We conducted two experiments, in which we varied set size and individual stimulus reliabilities. We found that the optimal-observer model accurately describes human behavior, outperforms plausible alternatives in a rigorous model comparison, and accounts for three key findings in the animal cognition literature. Our results provide a normative footing for the study of sameness judgment and indicate that the notion of perception as near-optimal inference extends to abstract relations.

2012年2月15日水曜日

納豆と私


たまには食べ物の話。

先日、某所で講演をした後、食事に招待していただきました。
比較的シニアな方々に囲まれやや緊張したものの、とても楽しい時間を過ごしました。

ただ、その食事会で手違いにより、15年ぶり(人生で二度目)に納豆を食べてしまいました。
ヤツは美味しそうな「マグロの山かけ」に埋もれていたため、気付かなかったんです…

初めてヤツに遭遇したのは15年前、中学3年の夏でした。
(あの夏も暑かった。覚えてないけど:笑)
親戚の家で出された太巻きに何の疑問も持たず食らいついた瞬間、なんとも言えない歯触りと悪臭が口の中に広がったのを昨日の事のように思い出せます。
せっかく生まれて16年間、食わず嫌いの純潔を貫いていたのに…

あれから15年、今回はとりあえず口に入れた分は吐き出さずに食べられました。
大事な席だったので惨劇にならず、本当に良かったです。
そのことに、「16年間の成長を喜ぶべきか」、「味覚・嗅覚の退化を悲しむべきか」、なんか複雑な気分でした。

p.s.
納豆はリスペクトできませんが、納豆を作っている方々、納豆の事が好きな方々のことはリスペクトしています。
(ただ、一緒に住むのはきついかも…冷蔵庫別々ならギリギリありかもですが…)

Role of Amygdala Central Nucleus in Aversive Learning Produced by Shock or by Unexpected Omission of Food


Robert J. Purgert, Daniel S. Wheeler, Michael A. McDannald, and Peter C. Holland

「罰(電気ショック)が与えられる」と「報酬が与えられない」。どちらも負の感情を惹起する出来事だが、その学習に共通して扁桃体が関与する? → 扁桃体、ただし、異なるニューロン群、が両方に関与する。 http://www.jneurosci.org/content/32/7/2461

Many psychological learning theories have noted commonalities between aversive states produced by presentation of negative reinforcers, such as electric shock, and the omission of expected positive reinforcers, such as food. Here, three groups of rats received training with one auditory cue paired with shock and another with the omission of expected food, a shock-paired cue and a food-omission control cue, or a food-omission cue and a shock control cue. Food-omission cues were established by contrast with food delivery; after extensive light–food pairings, the light was followed by the food-omission cue instead of food. Aversiveness of the food-omission cue was assessed with a conditioned punishment procedure, in which presentation of that cue was made contingent on performance of one previously trained instrumental response, whereas a second response had no consequences. We found that rats with lesions of amygdala central nucleus (CeA) showed impaired acquisition of freezing to the cue paired with shock and no evidence for acquisition of aversive properties by the cue that accompanied the omission of expected food. Furthermore, analyses of Arc and Homer1a mRNAs after rats were exposed to a two-epoch test procedure that allowed assessment of gene expression produced by two different test stimuli showed that both food-omission and shock-paired cues generated more neuronal activity in CeA than appropriate control cues. However, the number of neurons that were activated by both shock and food-omission cues was not significantly greater than expected by chance. Thus, under these test conditions, different subsets of CeA neurons represented these two aversive states.

Neuronal Activity in the Human Subthalamic Nucleus Encodes Decision Conflict during Action Selection


Kareem A. Zaghloul, Christoph T. Weidemann, Bradley C. Lega, Jurg L. Jaggi, Gordon H. Baltuch, and Michael J. Kahana

The subthalamic nucleus (STN), which receives excitatory inputs from the cortex and has direct connections with the inhibitory pathways of the basal ganglia, is well positioned to efficiently mediate action selection. Here, we use microelectrode recordings captured during deep brain stimulation surgery as participants engage in a decision task to examine the role of the human STN in action selection. We demonstrate that spiking activity in the STN increases when participants engage in a decision and that the level of spiking activity increases with the degree of decision conflict. These data implicate the STN as an important mediator of action selection during decision processes.

Bias in the Brain: A Diffusion Model Analysis of Prior Probability and Potential Payoff


Martijn J. Mulder, Eric-Jan Wagenmakers, Roger Ratcliff, Wouter Boekel, and Birte U. Forstmann

In perceptual decision-making, advance knowledge biases people toward choice alternatives that are more likely to be correct and more likely to be profitable. Accumulation-to-bound models provide two possible explanations for these effects: prior knowledge about the relative attractiveness of the alternatives at hand changes either the starting point of the decision process, or the rate of evidence accumulation. Here, we used model-based functional MRI to investigate whether these effects are similar for different types of prior knowledge, and whether there is a common neural substrate underlying bias in simple perceptual choices. We used two versions of the random-dot motion paradigm in which we manipulated bias by: (1) changing the prior likelihood of occurrence for two alternatives (“prior probability”) and (2) assigning a larger reward to one of two alternatives (“potential payoff”). Human subjects performed the task inside and outside a 3T MRI scanner. For each manipulation, bias was quantified by fitting the drift diffusion model to the behavioral data. Individual measurements of bias were then used in the imaging analyses to identify regions involved in biasing choice behavior. Behavioral results showed that subjects tended to make more and faster choices toward the alternative that was most probable or had the largest payoff. This effect was primarily due to a change in the starting point of the accumulation process. Imaging results showed that, at cue level, regions of the frontoparietal network are involved in changing the starting points in both manipulations, suggesting a common mechanism underlying the biasing effects of prior knowledge.

Deliberation in the Motor System: Reflex Gains Track Evolving Evidence Leading to a Decision


Luc P. J. Selen, Michael N. Shadlen, and Daniel M. Wolpert

Both decision making and sensorimotor control require real-time processing of noisy information streams. Historically these processes were thought to operate sequentially: cognitive processing leads to a decision, and the outcome is passed to the motor system to be converted into action. Recently, it has been suggested that the decision process may provide a continuous flow of information to the motor system, allowing it to prepare in a graded fashion for the probable outcome. Such continuous flow is supported by electrophysiology in nonhuman primates. Here we provide direct evidence for the continuous flow of an evolving decision variable to the motor system in humans. Subjects viewed a dynamic random dot display and were asked to indicate their decision about direction by moving a handle to one of two targets. We probed the state of the motor system by perturbing the arm at random times during decision formation. Reflex gains were modulated by the strength and duration of motion, reflecting the accumulated evidence in support of the evolving decision. The magnitude and variance of these gains tracked a decision variable that explained the subject's decision accuracy. The findings support a continuous process linking the evolving computations associated with decision making and sensorimotor control.

2012年2月8日水曜日

Ventral Striatum Encodes Past and Predicted Value Independent of Motor Contingencies


Brandon L. Goldstein, Brian R. Barnett, Gloria Vasquez, Steven C. Tobia, Vadim Kashtelyan, Amanda C. Burton, Daniel W. Bryden, and Matthew R. Roesch
J. Neurosci. 2012;32 2027-2036
http://www.jneurosci.org/cgi/content/abstract/32/6/2027?etoc

The ventral striatum (VS) is thought to signal the predicted value of expected outcomes. However, it is still unclear whether VS can encode value independently from variables often yoked to value such as response direction and latency. Expectations of high value reward are often associated with a particular action and faster latencies. To address this issue we trained rats to perform a task in which the size of the predicted reward was signaled before the instrumental response was instructed. Instrumental directional cues were presented briefly at a variable onset to reduce accuracy and increase reaction time. Rats were more accurate and slower when a large versus small reward was at stake. We found that activity in VS was high during odors that predicted large reward even though reaction times were slower under these conditions. In addition to these effects, we found that activity before the reward predicting cue reflected past and predicted reward. These results demonstrate that VS can encode value independent of motor contingencies and that the role of VS in goal-directed behavior is not just to increase vigor of specific actions when more is at stake.

2012年2月7日火曜日

Modulation of value representation by social context in the primate orbitofrontal cortex


João C. B. Azzi, Angela Sirigu, and Jean-René Duhamel
PNAS February 7, 2012 vol. 109 no. 6 2126-2131

サル電気生理。「正解したら、自分が報酬を得られる」場合と「自分だけでなく他者も報酬を得られる」場合の比較。後者の方が正解率が下がり、(前者では報酬をコードしていた)OFCの活動も低下する。色んな原因が考えられそうだけど興味深い。 http://bit.ly/AziC9F

Primates depend for their survival on their ability to understand their social environment, and their behavior is often shaped by social circumstances. We report that the orbitofrontal cortex, a brain region involved in motivation and reward, is tuned to social information. Macaque monkeys worked to collect rewards for themselves and two monkey partners. Behaviorally, monkeys discriminated between cues signaling small and large rewards, and between cues signaling rewards to self only and reward to both self and another monkey, with a preference for the former over the latter in both instances. Single neurons recorded during this task encoded the meaning of visual cues that predicted the magnitude of future rewards, as well as the motivational value of rewards obtained in a social context. Furthermore, neuronal activity was found to track momentary social preferences and partner's identity and social rank. The orbitofrontal cortex thus contains key neuronal mechanisms for the evaluation of social information.

Evidence for social working memory from a parametric functional MRI study


Meghan L. Meyer, Robert P. Spunt, Elliot T. Berkman, Shelley E. Taylor, and Matthew D. Lieberman
PNAS February 7, 2012 vol. 109 no. 6 1883-1888

Keeping track of various amounts of social cognitive information, including people's mental states, traits, and relationships, is fundamental to navigating social interactions. However, to date, no research has examined which brain regions support variable amounts of social information processing (“social load”). We developed a social working memory paradigm to examine the brain networks sensitive to social load. Two networks showed linear increases in activation as a function of increasing social load: the medial frontoparietal regions implicated in social cognition and the lateral frontoparietal system implicated in nonsocial forms of working memory. Of these networks, only load-dependent medial frontoparietal activity was associated with individual differences in social cognitive ability (trait perspective-taking). Although past studies of nonsocial load have uniformly found medial frontoparietal activity decreases with increasing task demands, the current study demonstrates these regions do support increasing mental effort when such effort engages social cognition. Implications for the etiology of clinical disorders that implicate social functioning and potential interventions are discussed.