Semi-orthogonal subspaces for value mediate a tradeoff between binding and generalization

When choosing between options, we must associate their values with the action needed to select them. We hypothesize that the brain solves this binding problem through neural population subspaces. To test this hypothesis, we examined neuronal responses in five reward-sensitive regions in macaques performing a risky choice task with sequential offers. Surprisingly, in all areas, the neural population encoded the values of offers presented on the left and right in distinct subspaces. We show that the encoding we observe is sufficient to bind the values of the offers to their respective positions in space while preserving abstract value information, which may be important for rapid learning and generalization to novel contexts. Moreover, after both offers have been presented, all areas encode the value of the first and second offers in orthogonal subspaces. In this case as well, the orthogonalization provides binding. Our binding-by-subspace hypothesis makes two novel predictions borne out by the data. First, behavioral errors should correlate with putative spatial (but not temporal) misbinding in the neural representation. Second, the specific representational geometry that we observe across animals also indicates that behavioral errors should increase when offers have low or high values, compared to when they have medium values, even when controlling for value difference. Together, these results support the idea that the brain makes use of semi-orthogonal subspaces to bind features together.Comment: arXiv admin note: substantial text overlap with arXiv:2205.0676

Neural mechanisms of attending to items in working memory

Working memory, the ability to keep recently accessed information available for immediate manipulation, has been proposed to rely on two mechanisms that appear difficult to reconcile: self-sustained neural firing, or the opposite-activity-silent synaptic traces. Here we review and contrast models of these two mechanisms, and then show that both phenomena can co-exist within a unified system in which neurons hold information in both activity and synapses. Rapid plasticity in flexibly-coding neurons allows features to be bound together into objects, with an important emergent property being the focus of attention. One memory item is held by persistent activity in an attended or "focused" state, and is thus remembered better than other items. Other, previously attended items can remain in memory but in the background, encoded in activity-silent synaptic traces. This dual functional architecture provides a unified common mechanism accounting for a diversity of perplexing attention and memory effects that have been hitherto difficult to explain in a single theoretical framework

In search of the focus of attention in working memory: 13 years of the retro-cue effect

The concept of attention has a prominent place in cognitive psychology. Attention can be directed not only to perceptual information, but also to information in working memory (WM). Evidence for an internal focus of attention has come from the retro-cue effect: Performance in tests of visual WM is improved when attention is guided to the test-relevant contents of WM ahead of testing them. The retro-cue paradigm has served as a test bed to empirically investigate the functions and limits of the focus of attention in WM. In this article, we review the growing body of (behavioral) studies on the retro-cue effect. We evaluate the degrees of experimental support for six hypotheses about what causes the retro-cue effect: (1) Attention protects representations from decay, (2) attention prioritizes the selected WM contents for comparison with a probe display, (3) attended representations are strengthened in WM, (4) not-attended representations are removed from WM, (5) a retro-cue to the retrieval target provides a head start for its retrieval before decision making, and (6) attention protects the selected representation from perceptual interference. The extant evidence provides support for the last four of these hypotheses

Neural network mechanisms of working memory interference

[eng] Our ability to memorize is at the core of our cognitive abilities. How could we effectively make decisions without considering memories of previous experiences? Broadly, our memories can be divided in two categories: long-term and short-term memories. Sometimes, short-term memory is also called working memory and throughout this thesis I will use both terms interchangeably. As the names suggest, long-term memory is the memory you use when you remember concepts for a long time, such as your name or age, while short-term memory is the system you engage while choosing between different wines at the liquor store. As your attention jumps from one bottle to another, you need to hold in memory characteristics of previous ones to pick your favourite. By the time you pick your favourite bottle, you might remember the prices or grape types of the other bottles, but you are likely to forget all of those details an hour later at home, opening the wine in front of your guests. The overall goal of this thesis is to study the neural mechanisms that underlie working memory interference, as reflected in quantitative, systematic behavioral biases. Ultimately, the goal of each chapter, even when focused exclusively on behavioral experiments, is to nail down plausible neural mechanisms that can produce specific behavioral and neurophysiological findings. To this end, we use the bump-attractor model as our working hypothesis, with which we often contrast the synaptic working memory model. The work performed during this thesis is described here in 3 main chapters, encapsulation 5 broad goals: In Chapter 4.1, we aim at testing behavioral predictions of a bump-attractor (1) network when used to store multiple items. Moreover, we connected two of such networks aiming to model feature-binding through selectivity synchronization (2). In Chapter 4.2, we aim to clarify the mechanisms of working memory interference from previous memories (3), the so-called serial biases. These biases provide an excellent opportunity to contrast activity-based and activity-silent mechanisms because both mechanisms have been proposed to be the underlying cause of those biases. In Chapter 4.3, armed with the same techniques used to seek evidence for activity-silent mechanisms, we test a prediction of the bump-attractor model with short-term plasticity (4). Finally, in light of the results from aim 4 and simple computer simulations, we reinterpret previous studies claiming evidence for activity-silent mechanisms (5)

The role of distractor strength in visual working memory

U posljednjih desetak godina, spoznaje o funkcioniranju vidnog radnog pamćenja (VRP) iz temelja su promijenjene pojavom modela resursa VRP-a. Pretpostavka ove skupine modela jest kako je VRP ograničen skup resursa koji se kontinuirano dijeli između svih podražaja koji se pamte, bez pretpostavke o postojanju limita u broju podražaja na koje se resursi mogu podijeliti. Porastom broja podražaja koji se pamti smanjuje se količina resursa pridana svakom podražaju, a samim time i reprezentacijska snaga svakog podražaja. Ova pretpostavka potvrđena je nalazima kako preciznost dosjećanja kontinuirano pada s porastom broja podražaja koji se pamti. Premda modeli resursa imaju jasne pretpostavke o ulozi količine resursa kojom je neki podražaj kodiran u preciznosti dosjećanja, otpornost snažnijih reprezentacija na perceptivne distraktore gotovo je neistraženo pitanje. Intuitivno, mogućnost zadržavanja zapamćenih informacija uslijed distraktora ovisi o snazi reprezentacija tih informacija, pri čemu će snažnije reprezentacije u manjoj mjeri biti narušene distraktorima. Druga važna odrednica otpornosti reprezentacija VRP-a na perceptivne distraktore je snaga samih distraktora. U ovom istraživanju željeli smo sustavno ispitati ulogu ova dva čimbenika - snage distraktora i snage reprezentacija u VRP-u - na preciznost dosjećanja u zadatku kontinuirane procjene u VRP-u. U eksperimentu 1 snaga reprezentacije manipulirana je variranjem broja podražaja koji se pamtio i dostupnim vremenom kodiranja. U eksperimentu 2a nakon kodiranja podražaja jednom je podražaju dan prioritet za dosjećanje. U eksperimentu 2b tijekom faze zadržavanja pažnja je privremeno usmjerena na jedan od zapamćenih podražaja s ciljem osvježavanja njegove reprezentacije. U posljednjem, eksperimentu 3, distraktori su prikazivani tijekom različitih faza obrade podražaja (kodiranja, zadržavanja, dosjećanja) uz pretpostavku kako je snaga reprezentacije najslabija prije no što kodiranje završi. Snaga perceptivnih distraktora u svim eksperimentima manipulirana je kao vrijeme prikaza distraktora i to na tri razine (bez distraktora, slabi distraktor, snažni distraktor). Rezultati su pokazali kako snaga reprezentacije ima važnu ulogu u zaštiti reprezentacije od distraktora. Dosjećanje podražaja kojima je pridano više resursa uslijed pamćenja manjeg broja podražaja i dužeg vremena kodiranja (eksperiment 1), koji su označeni kao prioritetni za dosjećanje (eksperiment 2a), koji su osvježeni tijekom faze zadržavanja (eksperiment 2b), te kojima faza kodiranja nije ometena (eksperiment 3), bilo je preciznije i uslijed prikaza distraktora. Uloga snage distraktora je potvrđena, no bila je kompleksna i ovisila je o na cinu na koji su reprezentacije osvježene. Model mješovitih distribucija koji pretpostavlja kako se dosjećanje može opisati točnim dosjećanjima, pogreškama zamjene, intruzijama i pogađanjem, dobro je pristajao podacima. Najkonzistentnije promjene uočene su u vjerojatnosti točnog dosjećanja. Važno, intruzije su se pokazale sastavnom vrstom pogrešaka u svim eksperimentima.Introduction Our environment is overloaded with visual information, with only a fraction of them necessary for an ongoing task. What determines the success of performing everyday tasks in such an environment? Visual working memory (VWM) is considered a vital component of most complex behaviours, but previous studies provided evidence of its susceptibility to irrelevant visual information, i.e. distractors. When is VWM impervious to distractors? VWM is best described as a highly limited resource that is flexibly shared among items in a visual scene. As the number of items increases, the amount of resources allocated to each item decreases, leading to a decline in strength (i.e. quality) of memory representations, and consequently to less precise recall of each item. This finding is consistent with a resource model of VWM and has been highlighted as a hallmark observation in VWM studies. An alternative view is provided by the influential "slot" model of VWM which claims that VWM is limited with a fixed maximum number of items that can be held in memory at one time. Moreover, according to this model, an item is either represented in its entirety in a memory slot or not stored at all. Strength of memory representation is therefore almost completely neglected in this type of model. However, investigation of the role of representation strength, besides the well known set size effect, has been limited even in studies motivated by the resource model. It is an intuitive prediction that memorandum strength should influence task performance. For example, our ability to maintain relevant information in the presence of distracting visual input should depend on the strength of memoranda, with stronger representations suffering less from irrelevant visual input. Here we thoroughly investigated the roles of strength of representations in VWM and distractor strength. Method To this end, we conducted four experiments (N = 64) in which we systematically manipulated the strength of VWM representations and the strength of distractors. We employed a delayedestimation task with continuous report, wherein subjects memorized colour stimuli. We manipulated strength of representation by: manipulating set size and encoding time (experiment 1), prioritizing one item for recall (experiment 2a), refreshing a representation of a single item during maintenance (experiment 2b), or interrupting memory phases before and after a stable representation was formed (experiment 3). After showing a memory array but before recalling one of the memorized items, irrelevant visual stimuli were shown. Simultaneously with memoranda strength, we manipulated distractor strength (no distractors, weak distractors, strong distractors). Results In all experiments we consistently found evidence that strength of memoranda serves a protective role against visual distractors. Regardless of the method used to manipulate memoranda strength, recall of stronger memoranda was less vulnerable to distractors. On the other hand, the effect of distractor strength showed a complex pattern which differed between experiments and depended on the manipulation of VWM representation strength. Next, we fit the data with a mixture model which assumes that the recall error distribution is a mixture of target recall, swap errors, intrusions, and guesses. This model captured the data well and showed better fit than the alternative normal + uniform model. When analyzing parameters of this model the most consistent manipulation dependent changes were observed on the target recall parameter. Intrusions were observed in all experiments. Interestingly, they depended more on VWM representation strength than on distractor strength. Conclusion This study revealed that strength of memoranda in VWM serves a protective role against visual distractors, making any addition of mechanisms of memoranda protection or distractor inhibition unnecessary. However, we showed that distractors, regardless of their strength, are able to penetrate VWM and lead to a decrease in recall precision. These findings are consistent with a resource conceptualisation of VWM where representational strength (i.e. the amount of allocated resources) plays a crucial role in ability to perform a task

How does our ability to integrate information across space and time change as we age?

This thesis investigated the nature of changes in feature binding ability that occur as a function of healthy ageing. Under the premise that these changes may occur due to reduced attentional resources (Sylvain-Roy et al., 2005), or changes in the ability to use contextual information as cue for recall (Meulenbroek et al., 2010), two hypotheses were tested; the ageing-attention hypothesis, and the ageing-context hypothesis. These hypotheses were tested under intentional binding instructions (e.g. Allen et al., 2006), and incidental binding instructions (e.g. Campo et al., 2010) which also included tests of whether nearby contextual information or absolute location are used in location binding (e.g. Olson & Marshuetz, 2005). The thesis found no support for either the ageing-attention hypothesis or the ageing-context hypothesis. The most valuable findings were the effortful nature of younger adult incidental location binding, and perhaps more crucially, the demonstration that older adult binding deficits may be best explained in terms of inhibitory deficit and differences in processing style between older and younger adults