2012年7月27日金曜日

Neural Correlates of Biased Competition in Premotor Cortex

Alexandre Pastor-Bernier and Paul Cisek
The Journal of Neuroscience, 11 May 2011, 31(19): 7083-7088

It has been proposed that whenever an animal faces several action choices, their neural representations are processed in parallel in frontoparietal cortex and compete in a manner biased by any factor relevant to the decision. We tested this hypothesis by recording single-unit activity in dorsal premotor cortex (PMd) while a monkey performed two delayed center-out reaching tasks. In the one-target task, a single target was presented and its border style indicated its reward value. The two-target task was the same except two targets were presented and the value of each was varied. During the delay period of the one-target task, directionally tuned PMd activity showed no modulation with value. In contrast, during the two-target task, the same neurons showed strong effects of the value associated with their preferred target, always in relation to the value of the other target. Furthermore, the competition between action choices was strongest when targets were furthest apart. This angular distance effect appeared in neural activity as soon as cells became tuned, while modulation by relative value appeared much later. All of these findings can be reproduced by a computational model which suggests that decisions between actions are made through a biased competition taking place within a sensorimotor map of potential actions.

Perceptual Learning and Decision-Making in Human Medial Frontal Cortex

Thorsten Kahnt, Marcus Grueschow, Oliver Speck and John-Dylan Haynes
Neuron, Volume 70, Issue 3, 549-559, 12 May 2011

知覚学習のfMRI研究。学習過程は強化学習で説明できて、前帯状回(初期視覚野ではなく!)に学習された変数が保持されている。 http://www.cell.com/neuron/abstract/S0896-6273(11)00296-0 おもしろそう。後でちゃんと読もう。

The dominant view that perceptual learning is accompanied by changes in early sensory representations has recently been challenged. Here we tested the idea that perceptual learning can be accounted for by reinforcement learning involving changes in higher decision-making areas. We trained subjects on an orientation discrimination task involving feedback over 4 days, acquiring fMRI data on the first and last day. Behavioral improvements were well explained by a reinforcement learning model in which learning leads to enhanced readout of sensory information, thereby establishing noise-robust representations of decision variables. We find stimulus orientation encoded in early visual and higher cortical regions such as lateral parietal cortex and anterior cingulate cortex (ACC). However, only activity patterns in the ACC tracked changes in decision variables during learning. These results provide strong evidence for perceptual learning-related changes in higher order areas and suggest that perceptual and reward learning are based on a common neurobiological mechanism.

Triangulating the Neural, Psychological, and Economic Bases of Guilt Aversion

Luke J. Chang, Alec Smith, Martin Dufwenberg and Alan G. Sanfey
Neuron, Volume 70, Issue 3, 560-572, 12 May 2011

人々は「Guilt-Aversion」に動機付けられて協力行動を行う。そして、その神経基盤をfMRIで…。fMRIの議論部分は「ホンマかいな?」という感じだけど… http://www.cell.com/neuron/abstract/S0896-6273(11)00299-6

Why do people often choose to cooperate when they can better serve their interests by acting selfishly? One potential mechanism is that the anticipation of guilt can motivate cooperative behavior. We utilize a formal model of this process in conjunction with fMRI to identify brain regions that mediate cooperative behavior while participants decided whether or not to honor a partner's trust. We observed increased activation in the insula, supplementary motor area, dorsolateral prefrontal cortex (PFC), and temporal parietal junction when participants were behaving consistent with our model, and found increased activity in the ventromedial PFC, dorsomedial PFC, and nucleus accumbens when they chose to abuse trust and maximize their financial reward. This study demonstrates that a neural system previously implicated in expectation processing plays a critical role in assessing moral sentiments that in turn can sustain human cooperation in the face of temptation.

Choosing Goals, Not Rules: Deciding among Rule-Based Action Plans

Christian Klaes, Stephanie Westendorff, Shubhodeep Chakrabarti, and Alexander Gail
Neuron, Volume 70, Issue 3, 536-548, 12 May 2011

In natural situations, movements are often directed toward locations different from that of the evoking sensory stimulus. Movement goals must then be inferred from the sensory cue based on rules. When there is uncertainty about the rule that applies for a given cue, planning a movement involves both choosing the relevant rule and computing the movement goal based on that rule. Under these conditions, it is not clear whether primates compute multiple movement goals based on all possible rules before choosing an action, or whether they first choose a rule and then only represent the movement goal associated with that rule. Supporting the former hypothesis, we show that neurons in the frontoparietal reach areas of monkeys simultaneously represent two different rule-based movement goals, which are biased by the monkeys' choice preferences. Apparently, primates choose between multiple behavioral options by weighing against each other the movement goals associated with each option.

トップジャーナルに掲載されるためには?


昨日聞いた話。

トップジャーナルに投稿される論文の80%は「Conceptual Advanceが(十分に)ない」という理由でリジェクトされる(残り10%はテクニカルな問題でリジェクト、10%がアクセプト)。聞くのは二回目だけど、身にしみる。

なので、
・「プロジェクトを始める前によく考える」ことが重要(データ採取、つまり、実験に闇雲に時間をかけてもしょうがない)。
・また、論文はMethodsやResultsからではなく、AbstractやIntroductionから書き始めるべき。
・データを"descriptive"に並べた論文はダメ。ストーリー(メカニズム/因果関係)を語らないといけない。

うーん。
分かっていてもなかなか難しいけどね。

2012年7月25日水曜日

Regionally Distinct Processing of Rewards and Punishments by the Primate Ventromedial Prefrontal Cortex


Ilya E. Monosov and Okihide Hikosaka
J. Neurosci. 2012;32 10318-10330
http://www.jneurosci.org/cgi/content/abstract/32/30/10318?etoc

彦坂研からvmPFC(前頭前野内腹側部)を対象としたサル電気生理。vmPFCの腹側/背側、後部/前部で、報酬に反応するのか、罰に反応するのか?報酬確率にphasicに反応するのか、tonicに反応するのか?がそれぞれ異なっている。

The ventromedial prefrontal cortex (vmPFC) is thought to be related to emotional experience and to the processing of stimulus and action values. However, little is known about how single vmPFC neurons process the prediction and reception of rewards and punishments. We recorded from monkey vmPFC neurons in an experimental situation with alternating blocks, one in which rewards were delivered and one in which punishments were delivered. Many vmPFC neurons changed their activity between blocks. Importantly, neurons in ventral vmPFC were persistently more active in the appetitive “reward” block, whereas neurons in dorsal vmPFC were persistently more active in the aversive “punishment” block. Furthermore, within ventral vmPFC, posterior neurons phasically encoded probability of reward, whereas anterior neurons tonically encoded possibility of reward. We found multiple distinct nonlinear valuation mechanisms within the primate prefrontal cortex. Our findings suggest that different subregions of vmPFC contribute differentially to the processing of valence. By conveying such multidimensional and nonlinear signals, the vmPFC may enable flexible control of decisions and emotions to adapt to complex environments.

Reward Prediction Error Signaling in Posterior Dorsomedial Striatum Is Action Specific


Thomas A. Stalnaker, Gwendolyn G. Calhoon, Masaaki Ogawa, Matthew R. Roesch, and Geoffrey Schoenbaum
J. Neurosci. 2012;32 10296-10305
http://www.jneurosci.org/cgi/content/abstract/32/30/10296?etoc

ドーパミン・ニューロンは「自分の行動選択に関わらず」報酬の予測誤差に反応するが、後部内背側・線条体ニューロンは「特定の行動から生み出される報酬予測誤差」にのみ反応する。線条体は「行動」の学習により直接的に効いている。

Neural correlates of reward prediction errors (RPEs) have been found in dorsal striatum. Such signals may be important for updating associative action representations within striatum. In order that the appropriate representations can be updated, it might be important for the RPE signal to be specific for the action that led to that error. However, RPEs signaled by midbrain dopamine neurons, which project heavily to striatum, are not action-specific. Here we tested whether RPE-like activity in dorsal striatum is action-specific; we recorded single-unit activity in posterior dorsomedial and dorsolateral striatum as rats performed a task in which the reward predictions associated with two different actions were repeatedly violated, thereby eliciting RPEs. We separately analyzed fast firing neurons (FFNs) and phasically firing neurons (total n = 1076). Only among FFNs recorded in posterior dorsomedial striatum did we find a population with RPE-like characteristics (19 of all 196 FFNs, 10%). This population showed a phasic increase in activity during unexpected rewards, a phasic decrease in activity during unexpected omission of rewards, and a phasic increase in activity during cues when they predicted high-value reward. However, unlike a classical RPE signal, this signal was linked to the action that elicited the prediction error, in that neurons tended to signal RPEs only after their anti-preferred action. This action-specific RPE-like signal could provide a mechanism for updating specific associative action representations in posterior dorsomedial striatum.

2012年7月18日水曜日

ミラーニューロンと他者行動の理解


サメジマさんに紹介してもらったPNAS論文。
「自分がモノを掴む」時と「他人がモノを掴む」時の両方に反応するミラーニューロンの活動は、その「モノ」の価値によってモジュレートされる。面白い。 http://www.pnas.org/content/early/2012/06/25/1205553109

著者らは「ミラーニューロンは他人の行動意図の理解に関わっている」としてるけど、この結果が「他者理解」を意味するのか「観察学習(他者の行動/結果から自分にとっての価値を学習)」を意味するのかはよく分からないと思う。著者らも特に議論してないし…(最後に一文だけ?)。

「自分にとっての価値」と「他人にとっての価値」が一致しない状況(例:他人はバナナ好きだけど、自分は嫌い)ではミラーニューロンはどちらにモジュレートされるのだろう?「他者にとっての価値」にモジュレートされるなら「他者理解」、「自分にとっての価値」なら観察学習と結論できるのかな?

まあ、もちろん「他者理解」はミラーニューロンだけではなく、色々なシステムの組み合わせで行われているはずなので、計算論モデルを書き下すことが重要で…
となって、自分の研究に繋がった(はず)?
ということで、他人の論文にゴチャゴチャ言うより自分の研究を頑張らないと…

追伸:
今週のJNSに観察学習の論文が出ています。
他人の行動/報酬から「刺激ー反応」連合を学習する場合は後部線条体(尾状核)、「行動ー結果」連合を学習する場合は前部線条体(尾状核)が関与する。
http://www.jneurosci.org/content/32/29/9878

Dissociable Brain Systems Mediate Vicarious Learning of Stimulus-Response and Action-Outcome Contingencies


Mimi Liljeholm, Ciara J. Molloy, and John P. O'Doherty
J. Neurosci. 2012;32 9878-9886
http://www.jneurosci.org/cgi/content/abstract/32/29/9878?etoc

Congratulations Mimi!

Two distinct strategies have been suggested to support action selection in humans and other animals on the basis of experiential learning: a goal-directed strategy that generates decisions based on the value and causal antecedents of action outcomes, and a habitual strategy that relies on the automatic elicitation of actions by environmental stimuli. In the present study, we investigated whether a similar dichotomy exists for actions that are acquired vicariously, through observation of other individuals rather than through direct experience, and assessed whether these strategies are mediated by distinct brain regions. We scanned participants with functional magnetic resonance imaging while they performed an observational learning task designed to encourage either goal-directed encoding of the consequences of observed actions, or a mapping of observed actions to conditional discriminative cues. Activity in different parts of the action observation network discriminated between the two conditions during observational learning and correlated with the degree of insensitivity to outcome devaluation in subsequent performance. Our findings suggest that, in striking parallel to experiential learning, neural systems mediating the observational acquisition of actions may be dissociated into distinct components: a goal-directed, outcome-sensitive component and a less flexible stimulus–response component.

Adaptation Paths to Novel Motor Tasks Are Shaped by Prior Structure Learning


Dmitry Kobak and Carsten Mehring
J. Neurosci. 2012;32 9898-9908
http://www.jneurosci.org/cgi/content/abstract/32/29/9898?etoc

After extensive practice with motor tasks sharing structural similarities (e.g., different dancing movements, or different sword techniques), new tasks of the same type can be learned faster. According to the recent “structure learning” hypothesis (Braun et al., 2009a), such rapid generalization of related motor skills relies on learning the dynamic and kinematic relationships shared by this set of skills. As a consequence, motor adaptation becomes constrained, effectively leading to a dimensionality reduction of the learning problem; at the same time, adaptation to tasks lying outside the structure becomes biased toward the structure. We tested these predictions by investigating how previously learned structures influence subsequent motor adaptation. Human subjects were making reaching movements in 3D virtual reality, experiencing perturbations either in the vertical or in the horizontal plane. Perturbations were either visuomotor rotations of varying angle or velocity-dependent forces of varying strength. We found that, after extensive training with both kinematic or dynamic perturbations, adaptation to unpracticed, diagonal, perturbations happened along the previously learned structure (vertical or horizontal), and resulting adaptation trajectories were curved. This effect is robust, can be observed on the single-subject level, and occurs during adaptation both within and across trials. Additionally, we demonstrate that structure learning changes involuntary visuomotor reflexes and therefore is not exclusively a high-level cognitive phenomenon.

2012年7月11日水曜日

Linking Brain Structure and Activation in Temporoparietal Junction to Explain the Neurobiology of Human Altruism


Yosuke Morishima, Daniel Schunk, Adrian Bruhin, Christian C. Ruff, Ernst Fehr
Neuron, Volume 75, Issue 1, 73-79, 12 July 2012

Human altruism shaped our evolutionary history and pervades social and political life. There are, however, enormous individual differences in altruism. Some people are almost completely selfish, while others display strong altruism, and the factors behind this heterogeneity are only poorly understood. We examine the neuroanatomical basis of these differences with voxel-based morphometry and show that gray matter (GM) volume in the right temporoparietal junction (TPJ) is strongly associated with both individuals' altruism and the individual-specific conditions under which this brain region is recruited during altruistic decision making. Thus, individual differences in GM volume in TPJ not only translate into individual differences in the general propensity to behave altruistically, but they also create a link between brain structure and brain function by indicating the conditions under which individuals are likely to recruit this region when they face a conflict between altruistic and selfish acts.

The Role of the Amygdala in Atypical Gaze on Emotional Faces in Autism Spectrum Disorders


Dorit Kliemann, Isabel Dziobek, Alexander Hatri, Jurgen Baudewig, and Hauke R. Heekeren
J. Neurosci. 2012;32 9469-9476

Reduced focus toward the eyes is a characteristic of atypical gaze on emotional faces in autism spectrum disorders (ASD). Along with the atypical gaze, aberrant amygdala activity during face processing compared with neurotypically developed (NT) participants has been repeatedly reported in ASD. It remains unclear whether the previously reported dysfunctional amygdalar response patterns in ASD support an active avoidance of direct eye contact or rather a lack of social attention. Using a recently introduced emotion classification task, we investigated eye movements and changes in blood oxygen level-dependent (BOLD) signal in the amygdala with a 3T MRI scanner in 16 autistic and 17 control adult human participants. By modulating the initial fixation position on faces, we investigated changes triggered by the eyes compared with the mouth. Between-group interaction effects revealed different patterns of gaze and amygdalar BOLD changes in ASD and NT: Individuals with ASD gazed more often away from than toward the eyes, compared with the NT group, which showed the reversed tendency. An interaction contrast of group and initial fixation position further yielded a significant cluster of amygdala activity. Extracted parameter estimates showed greater response to eyes fixation in ASD, whereas the NT group showed an increase for mouth fixation.

The differing patterns of amygdala activity in combination with differing patterns of gaze behavior between groups triggered by direct eye contact and mouth fixation, suggest a dysfunctional profile of the amygdala in ASD involving an interplay of both eye-avoidance processing and reduced orientation.