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ECVPの感想

European Conference on Visual Perception(略してECVP)という学会が、懐かしのユトレヒトであった。ユトレヒトに行ったのは久しぶりで、行っても実験したりしてたからなかなか学会という感じにはならなかった。それから、ロンドンに来てから自分がやってる研究が脳刺激ばかりで、なかなか心理物理学から離れてしまっていて、今回の学会はちょっと別の角度から接する感じになった。

それでも、Binocular RivalryなどのPerceptual Rivalry関連とTime Perception関連では面白い発表がいくつかあった。Rivalryで一番気になったのは、Jan Brascampの見えない刺激に対するafterimageの実験だ。

Invisible images can leave stronger afterimages than visible images
J W Brascamp, J J A van Boxtel, T H J Knapen
After removal of a visual stimulus observers may experience a negative afterimage, an illusory image with identical contours as the previously viewed image, but of opposite polarity. Afterimages arguably arise due to adaptation of cells activated during presentation of the original stimulus. Here we study afterimages of stimuli that are perceptually suppressed during binocular rivalry. Surprisingly, our results indicate that images that do not reach awareness due to rivalry suppression can leave a stronger afterimage than the same images when they are consciously perceived. We account for our findings by positing that perceptually suppressed stimuli cause stronger adaptation in polarity sensitive cells than in polarity insensitive cells. This can explain our findings in a way that is analogous to previous accounts of the influence of attention on afterimage formation. Finally, we show that the effect of rivalry suppression on afterimage formation depends strongly on the spatial frequency of the inducing stimulus, which reconciles our results with previous studies that reported weakening of afterimages due to perceptual suppression.

Tsuchiya & Koch (2005)では、continuous flash suppressionで見えない刺激に対する残像(afterimage)が弱くなるが、一方で注意(attention)を向けると残像は弱くなる(Suzuki & Grabowecky (2003))。だから、意識と注意の効果が逆方向になるという議論だったのが、残像に関しては空間周波数の問題で、Tsuchiya論文ではlow spatial frequencyの刺激にadaptしていて、Suzuki論文ではhigh spatial frequencyの刺激にadaptしていることが決定的な違いなのではないかという話だった。Brascampはさらに見える刺激に対する注意の効果が空間周波数によって変わるということをもう少しで示すところまできている。少なくとも、低い周波数を使えば、注意を向けることでafterimageを強くすることはできている。ただ、もとのSuzuki論文をhigh spatial frequencyでreplicateするのにもう少し時間がかかりそうだ。

この例が、最終的に注意と意識が別物だという議論のもとにならなくても、ほかにもたくさん注意と意識が別だという例はいくつもあるから、それ自体が覆ることはないだろう。

Jan Brascampは長年共同研究者だけど、とくにperceptual stabilizationの研究をやっていたときに、難しい実験をかなり執念深くやって、最終的にちゃんと理解できるような話にたどり着くという印象だ。彼のおかげで、stabilizationがちょっと盛り上がったときの浮かれた論文が実は間違っていたり、小さなパラメタースペースの中だけの話でうまくまとめたにすぎないものばかりたと気づかされたし、本当はどうなっているのかを誠実に研究していることが尊敬できる。本人的には、そのやり方だとインパクトファクターの高い雑誌とかになかなかいけないから、悩むらしい。Ken Nakayamaが「新しい研究を始める最初の人になるのもいいけど、最後の人になるのが重要だ」といっていたのを思い出した。その点、Janは最後の人になりそうな研究の仕方だ。

PsychophysicsにImpact factorが7とか8ぐらいの、physiologyにとってのjournal of neuroscienceのような雑誌があれば、しっかりしたpsychophysicsをやることを目標にするincentiveができるんじゃないだろうかという話を何人の人かとした。本来は、それぞれ個人がポリシーをもってやるしかないのだろうけど、中堅の雑誌の存在というのはきっとフィールドにとって意味があるだろう。

それから、自分がspatial attentionはunconsciousなsignalには効かないという論文を出した直後に、Bahador Bahramiたちは、spatial attentionが効くケースの論文を出した(Bahrami et al. 2008と今度Perceptionにでる論文)。Bahadorとはしょっちゅう会うから、話をするけれど、おそらくこのdiscrepancyには考えられる原因が2つある。Bahador説は、刺激のコントラストの違いだろうということだ。かつてSheng Heがcrowdingで見えないときに、adaptationは同じだけ起きると主張したのが、コントラストを下げればcrowdingでadaptationが低下しているのが見えるとうBlakeの議論と同じだ。で、Kanai説では、spatial attentionはcontrast adaptationには効くが、orientation specificityがないからtilt aftereffectには効果がないのだろうという、測り方の違いだと考えている。

これと関連したdecodingのトークがあった。

Attention improves orientation selectivity in human visual cortex
J F M Jehee, D K Brady, F Tong
Does attending a spatial location only strengthen neural responses in corresponding regions of the human visual cortex, or does it also result in additional information processing about the attended features at that location? We used fMRI in combination with pattern-based analyses to address this question. Subjects performed an attention-demanding orientation discrimination task on either one of two laterally presented gratings. Analyses revealed much stronger BOLD responses for attended stimulus locations than unattended locations in early visual areas. To test whether this also improved information available about the attended features at these locations, we used a pattern-based analysis to decode the presented stimulus orientation from cortical activity in these regions (Kamitani and Tong, 2005 Nature Neuroscience 8 679 - 685). We found significantly better decoding performance in the attended condition as compared to the unattended condition in areas V1, V2, and V3. We conclude that attention not only results in enhanced cortical responses in early visual areas, but also improves the orientation selectivity of these responses when analysed at a population level.

このアブストには書いてないが、実際の話の内容はspatial attentionはorientationのdecoding performanceをあげないが、feature-based attentionはあげるという話だった。これは、spatial attentionがvisual areasの活動をあげても、specificityをあげないという話と合致したから、自分のなかでつじつまがあった。

それから、すごい謎なポスターがあった。

Acute local changes in cortical anatomy during visual activity
A Hernowo, R Renken, F Cornelissen
Studies have indicated the presence of volumetric changes both in glial cells and the extracellular space during neuronal activity (Sykova and Chvatal, 2000 Neurochemistry International 36 397 - 409; Dietzel et al, 1980 Experimental Brain Research 40 432 - 439). In this study, we examined whether such volume changes at the cellular level are translated into changes that can be observed at the scale of the cerebral cortex. Two subsequent T1-weighted anatomical MRI scans were obtained in eight subjects. During the first scan, the subject was viewing a 6.5 min movie of natural scenery; during the second scan, the subject had his eyes closed (rest). The acquired cortical scans were subsequently analyzed using voxel-based morphometry. Comparing cortical anatomy during viewing vs rest revealed no detectable volumetric changes in the grey matter of visual cortex. However, such volumetric changes were observed in the anterior cingulate cortex and bilaterally in the insulas. These results suggest that acute volumetric changes at the cellular level related to neuronal activity can result in macroscopically detectable changes in cortical anatomy.

映画を見ているときと、目をつぶっているときで、T1でgray matterの違いがinsulaとanterior cingulateででるとう話だった。Visual cortexとかはまったくなにもなかった。まず、artifactじゃないかと思うのだけど、どういうartifactだったらこのspecificな効果を説明できるのかわからない。もしかしたら、dopamineの量の違いとかがT1で見えることがあるのかもしれないけれど、それは自分のまったくしらない分野だからなんとも判断がつかない。もしこれが意味のあるものをみていることが証明できれば、MRIの新しい使い方になるかもしれないとう希望もあるけれど、けっこう実践で使うまでには何ステップか乗り越える課題がありそうだ。でも、心に残るインパクトはでかかった。

それからロンドンのラボの同じオフィスにいるひとのトークを初めて聞いた。

Posthypnotic suggestion induces synaesthetic crossmodal sensation in the absence of abnormal neuronal connections
R Cohen Kadosh, A Henik, A Catena, V Walsh, L J Fuentes
Abnormal crossmodal interactions can occur following brain damage, or in participants with additional experiences, such as synaesthetes, who otherwise can be considered neurologically normal. Grapheme-colour synaesthetes, for example, will experience certain achromatic graphemes in specific colours. A better understanding of the causes of synaesthesia and, in turn, of the causes of the abnormal crossmodal interactions is fundamental for our understanding of crossmodal connectivity and inter-aerial interactions in the normal brain, as well as other phenomena such as perceptual awareness, feature binding, and automaticity. The question of whether the kinds of abnormal crossmodal interactions seen in synaesthesia or following brain damage occur due to hyperconnectivity between or within brain areas or whether they are a result of lack in inhibition is highly contested. We will show that under posthypnotic suggestion non-synaesthetes can be induced to have synaesthetic experiences. Due to the short time-frame of the experiments, it is unlikely that new cortical connections were established, leading to the conclusion that synaesthesia can result from disinhibition between brain areas.

どうもロンドンのボスと催眠術を習いにいったり、あやしげなことをやっているなと思ったら、催眠術で共感覚を引き起こす実験をやっていたらしい。この論文は近々Psychological Scienceにでる。

これが、その催眠術で共感覚を引き起こしている場面のビデオで、なかなか面白い。

ただ、トークのあとの質問で、元々共感覚のない人が、共感覚がないとできないことができるようになることを示すべきじゃないかという意見があった。まったく、その通りだと思う。そこが一番気になる。

あと、これも面白かった。

Visual field maps in a subject without an optic chiasm
S O Dumoulin, Y Masuda, H Horiguchi, R F Dougherty, S Prakash, Y J Liao, B A Wandell
Signals from the two eyes cross at the optic chiasm, producing the characteristic hemifield maps in primary visual cortex (V1). We report the V1 map in a subject born without an optic chiasm. The achiasmic subject had normal visual fields but a 1 - 4 Hz nystagmus of about 10deg in amplitude. We measured fMRI responses. To verify the achiasma, we presented full-field checkerboards to each eye, singly. We reconstructed his visual field maps using the population receptive field (pRF) method (Dumoulin and Wandell, 2008 Neuroimage 39 647 - 660). Results. Stimulation of either eye resulted in ipsilateral fMRI signals only, confirming functional achiasma. The pRF method showed that both hemispheres contained a full map of the visual field; ipsi- and contra-lateral visual field maps were topographically preserved but intermixed in V1. In conclusions we can confirm the anatomical diagnosis of achiasma and we found atypical V1 maps. This subject made effective use of his vision, including sports activities and reading. Hence, these highly atypical visual field maps did not prevent basic visual functions.

Optic Chiasmが交差してない人でretinotopyがどうなっているかという話で、片目が片半球に対応するようだということだけど、binocular rivalryがどう見えるのか気になる。

それから、これも印象的だった。

The effect of repetition on subjective duration at brief timescales
V Pariyadath, D M Eagleman
Events that are repeated are sometimes perceived to last shorter in duration than novel events of equal length. We hypothesize that these subjective duration distortions are a consequence of differential neural responses to repeated and novel stimuli. Generally, the neural response to a stimulus diminishes with repeated presentation, an effect known as repetition suppression. We have leveraged these findings to develop a new psychophysical paradigm. When a single letter of the alphabet was serially flashed onto different locations on the screen, several letters appeared to be present on the screen simultaneously due to persistence of vision. Participants' estimates of how many letters they perceived on the screen at any instant of time were significantly lower when the same letter was flashed repeatedly than when different letters were used for each flash. This result suggests that visual persistence is briefer for repeated than for novel stimuli. fMRI experiments are under way in our laboratory to further test the relationship between neural activity and perceived duration using this novel paradigm.

同じ刺激を繰り返しだすと、visible persistenceが短いが、新しい刺激だと長くなる。かなりlow levelだと思われるpersistenceが顔のfamiliarityなんかにも影響されていることに驚かされた。

それから、rivalryがawarenessなしで起きるかという問題が前から気になっているが、少なくともattentionがないと起きにくい(あるいは起きない場合がある)ということも確実になりつつある。

Binocular rivalry from asynchronously flickering images: the temporal limit depends on trial duration and attention
J J A van Boxtel, C J Erkelens, R van Ee
Binocular rivalry may ensue between stimuli that are presented intermittently and asynchronously, as long as each of the rivalrous patterns are presented at >3 Hz. We call this limit the temporal limit to rivalry. We show that the 3Hz limit is not reached instantaneously. At the beginning of the trial stimuli need to be presented at >6 Hz to induce rivalry, only after >8 s the 3 Hz limit is reached. This finding suggests that a rather slow process underlies the buildup of the temporal limit to binocular rivalry. In a second set of experiments we show that this build-up of the temporal limit is dependent on the deployment of attention to the stimulus. When attention is diverted during the initial part of a trial with a secondary task, the maximum temporal limit is reached later. Therefore the slow process that underlies the build-up of the temporal limit is gated by attention, suggesting that the build-up of the temporal limit is not just due to autonomous adaptation of early visual neurons.

The number of perceptual alternations occurring during binocular rivalry is limited by attention
C L E Paffen, I T C Hooge
Paffen and Alais (2006) recently showed that drawing away attention from a target engaged in binocular rivalry reduced the number of alternations reported. Their finding suggests that the number of alternations is highest when attention is fully available. Based on this, we hypothesize that increasing the number of rival targets in a single display does not increase the number of alternations. We used a display containing either 1, 2 or 3 rival targets. Targets consisted of Gabors with orthogonal orientations placed around the fixation point. Observers were instructed to press a button whenever a perceptual alternation was perceived. Results showed that the number of alternations reported increased only slightly with increasing number of rival targets. Control experiments ruled out the possibility that observers were unable to report alternations occurring at high frequency. We conclude that, as hypothesized, the number of alternations is independent of the number of rival targets in the display. We suggest that the number of perceived alternations is limited by the amount of attention paid to each rival target.

アテンションなしでrivalryが起きないのであれば、awarenessなしでrivalryが起きていないことも非常にありそうだ。いろいろあって、今はrivalryはawarenessなしでは起きていないという確信が持ててきたから、今までの実験をまとめて論文を書こうという気になった。

ほかにもいろいろ、久しぶりにオランダに帰ってきて自分が歳をとったと感じたとかいろいろあったけど、長くなったのでこのへんで。


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