The spatial resolutions of stereo and motion perception and their neural basis

PhD ThesisDepth perception requires finding matching features between the two eye’s images to estimate binocular disparity. This process has been successfully modelled using local cross-correlation. The model is based on the known physiology of primary visual cortex (V1) and has explained many aspects of stereo vision including why spatial stereoresolution is low compared to the resolution for luminance patterns, suggesting that the limit on spatial stereoresolution is set in V1. We predicted that this model would perform better at detecting square-wave disparity gratings, consisting of regions of locally constant disparity, than sine-waves which are slanted almost everywhere. We confirmed this through computational modelling and performed psychophysical experiments to test whether human performance followed the predictions of the model. We found that humans perform equally well with both waveforms. This contradicted the model’s predictions raising the question of whether spatial stereoresolution may not be limited in V1 after all or whether changing the model to include more of the known physiology may make it consistent with human performance. We incorporated the known size-disparity correlation into the model, giving disparity detectors with larger preferred disparities larger correlation windows, and found that this modified model explained the new human results. This provides further evidence that spatial stereoresolution is limited in V1. Based on previous evidence that MT neurons respond well to transparent motion in different depth planes we predicted that the spatial resolution of joint motion/disparity perception would be limited by the significantly larger MT receptive field sizes and therefore be much lower than the resolution for pure disparity. We tested this using a new joint motion/disparity grating, designed to require the detection of conjunctions between motion and disparity. We found little difference between the resolutions for disparity and joint gratings, contradicting our predictions and suggesting that a different area than MT was used

Temporära platser som dialog

Att arbeta med temporära platser blir allt vanligare i planeringskontext, och ett exempel på detta är den temporära gågatan Friisgatan i Malmö som under sommaren stängs av för biltrafik och utgör en sommargata. Sommargatan syftar till att tillfälligt förändra gaturummet och lyfta dess kvaliteter, uppfylla delar av stadens miljömål och etablera medborgardialog. Genom medborgardialog kan medborgare och andra aktörer få större inflytande och detta kan bidra till mer demokratiska planeringsprocesser. Syftet med denna uppsats är att, med avstamp i Friisgatan, belysa och diskutera samspelet mellan temporära platser och medborgardialog. Detta görs genom dokumentanalys och en semistrukturerad intervju. Resultatet pekar på att dessa teman på många sätt samverkar; medborgardialog kan bidra med legitimitet till det temporära användandet, och den temporära användningen ger en möjlighet att utvärdera hur konceptet/designen tas emot av brukarna. Temporär användning kan även involvera medborgare och fungera som en metod att nå marginaliserade grupper inom planering. Denna kombination kan på så sätt fungera som ett tillvägagångssätt för att nå det gemensamma målet – att skapa band mellan kommun, människor och den specifika platsen. Vidare redogör uppsatsen för de utmaningar som det innebär att arbeta med medborgardialog och temporära platser i praktiken. De mest betydande hinder som visat sig försvåra arbetet är att sommargatan snabbt blev väldigt resurskrävande samt en avsaknad av riktlinjer på en högre nivå.To work with temporary spaces in planning context has become more common the past years, an example is the temporary pedestrian street Friisgatan in Malmö which during the summer closes to car traffic to turn into a summer street. The summer street aims to temporarily change the street space and elevate its qualities, fulfill the city’s environmental strategies and establishing a citizen dialogue. Citizen dialogue allow citizens and other actors to participate, which in the long run makes the physical planning process more democratic. The aim of this thesis is to highlight and discuss the interplay between temporary spaces and citizen influence by evaluating the project surrounding Friisgatan. This has been done through document analyses and a semi-structured interview. The results indicate that these themes interact in several ways. Citizen dialogue can contribute with legitimacy of the temporary space, while the temporary space makes it possible to see how the concept/ design will be received by the users. The temporary space can also work to involve citizens, in particular citizens who generally are considered marginalized in discussions about urban planning. This combination aims to achieve the common goal – to create bonds between municipally, people and the place involved. Furthermore, the study describes the challenges involved with using citizen dialogue and temporary places in practice. The most considerable difficulties are lack of strategic policies on a larger scale and that the project became very resource-demanding when it developed

Room for children : a design proposal for Vaksalaskolan’s school yard with surroundings that creates space for children and youth

88% av Sveriges befolkning bor i tätorter, vilket endast utgör 1,3 % av Sveriges totala yta. Urbaniseringstrenden förväntas fortsätta och barns miljöer är de som försvinner först då konkurrensen om ytan är stor. Då barn spenderar majoriteten av sin vakna tid på skolan, får skolgården en allt viktigare roll i barns uppväxtmiljö. En skolgårds kvalitet har en stor effekt på barnens lek, utveckling samt hälsa och en skolgårds kvalitet är beroende av dess storlek. Detta till trots, finns det idag inga lagstadgade riktlinjer kring en skolgårds storlek, vilket har resulterat i att över 40 % av barn går i en skola vars skolgård understiger de forskningsbaserade rekommendationer som finns. Det här arbetet görs för att uppmärksamma vilken effekt marginaliseringen av barns miljöer har på barnen, men även på samhället i stort. Arbetet syftar till att föreslå en gestaltning som prioriterar barns jämlika uppväxtvillkor genom att främja lek samt utmana gaturummets traditionella hierarki. Vidare är syftet att exemplifiera hur en skolgård med svåra förutsättningar kan gestaltas med ökade miljöerbjudanden och rörelsefrihet. Vaksalaskolan, som arbetet appliceras på, har idag ett friytemått långt under rekommendationerna vilket resulterar i konflikter och omfattande slitage. Arbetet undersöker hur Vaksalaskolans skolgård samt omgivning kan gestaltas för ökade lekvärden med samnyttjande som strategi. Arbetet utgår från metoden Research by Design där den vetenskapliga förundersökningen pågår parallellt med gestaltningsarbetet. Metoden tillåter att nya frågor uppkommer under arbetets gång, vilka kan generera nya lösningar. Vidare baseras förslaget till stor del på de gåturer och semistrukturerade gruppintervjuer som anordnades med elever på Vaksalaskolan. Gestaltningsresultatet innebär samnyttjande med delar av Vaksala torg samt Salagatan vilket bidrar till att Vaksalaskolans skolgårds friyta ökar med över 30%. Skolgården får även en gestaltning med ökade miljöerbjudanden i form av flexibla lekelement, sociala platser och lösa föremål, baserat på en teoretisk grund. Resultatet visar exempel på hur barns behov kan prioriteras framför trafik och förtätning genom att låta barnens lek dominera det offentliga rummet. Sammanfattningsvis dras slutsatsen att samnyttjande inte bör användas som argument i en förtätningsstrategi då det tenderar att missgynna barns livsvillkor. Samnyttjande kan dock användas som en krishanteringsstrategi i de fall då en skolgårds förutsättningar inte uppnår de riktlinjer som finns, likt Vaksalaskolan, samt som en strategi för att prioritera barns plats i staden. I förslaget ligger friytan fortfarande under de forskningsbaserade rekommendationerna som finns, då den täta bebyggelsestrukturen förhindrar en ytterligare expandering av skolgårdens yta. Därför förespråkas en lagändring som säkerhetsställer en skolgårds minsta friytemått som ett sätt att skydda barns rätt till jämlika uppväxtvillkor.88 % of the population in Sweden lives in cities which only consists of 1,3 % of Sweden's area. The urbanisation is expected to increase, and as it escalates, children’s built environment will decrease as they are the first to be compromised. Since school is the one place where children spend most of their waking hours, the schoolyard plays an important part in a child’s life. The qualities of the schoolyard and therefore the quality of a child’s play is essential in the act of promoting a child’s development and health. Research has shown a connection between a schoolyard’s qualitative standards and its size. Thus, there are no regulations by law regarding a schoolyard's size or amount of open space per child. As a result, 40% of Sweden's school children go to schools where the schoolyards are in fact lower than what is recommended by research. The aim is to prioritise children’s need for an equal upbringing by creating a proposal that promotes play values as well as challenges the traditional hierarchy of the street. In addition, the aim was to exemplify how a schoolyard with difficult prerequisites can be designed to increase its affordances and the children’s independent mobility. The proposal is implemented at Vaksalaskolan, where the low amount of space has been proven to increase the risk for bullying as well as extensive wear and tear. This proposal investigates how Vaksalaskolan can be redesigned to increase its play values, by the strategy of shared space. The project is based on the method Research by Design, where research and design are intertwined by trying out design based solutions throughout the entire process. The method is used when a question has the possibility of multiple answers and is performed through three phases: pre-design, design, and post-design. The proposal was developed in a co-creation with the students at Vaksalaskolan, where student-led tours and semi structured interviews were arranged. The proposal increased the school yards' open space by over 30 % by sharing space with Vaksala torg and Salagatan. The play values were increased by research-based methods such as affordances, loose parts and social experiences. The design proposes an example of how to establish children’s rights to impact their own environment as well as show an example where children are prioritised in the public realm. In conclusion, we argue that shared space should not be an argument for densification as it tends to disadvantage children’s wellbeing. However, shared use can be a strategy where the lack of open space per child is critical, like in the case of Vaksalaskolan. The design proposes an increase of open space, but it is still lower than the recommended amount as the dense city structure will not allow for a further extension. To counteract the need for shared space, a sufficient amount of open space per child needs to be prescribed by law to protect children’s rights to an equal environment that promotes their health

The spatial resolutions of stereo and motion perception and their neural basis

Depth perception requires finding matching features between the two eye’s images to estimate binocular disparity. This process has been successfully modelled using local cross-correlation. The model is based on the known physiology of primary visual cortex (V1) and has explained many aspects of stereo vision including why spatial stereoresolution is low compared to the resolution for luminance patterns, suggesting that the limit on spatial stereoresolution is set in V1. We predicted that this model would perform better at detecting square-wave disparity gratings, consisting of regions of locally constant disparity, than sine-waves which are slanted almost everywhere. We confirmed this through computational modelling and performed psychophysical experiments to test whether human performance followed the predictions of the model. We found that humans perform equally well with both waveforms. This contradicted the model’s predictions raising the question of whether spatial stereoresolution may not be limited in V1 after all or whether changing the model to include more of the known physiology may make it consistent with human performance. We incorporated the known size-disparity correlation into the model, giving disparity detectors with larger preferred disparities larger correlation windows, and found that this modified model explained the new human results. This provides further evidence that spatial stereoresolution is limited in V1. Based on previous evidence that MT neurons respond well to transparent motion in different depth planes we predicted that the spatial resolution of joint motion/disparity perception would be limited by the significantly larger MT receptive field sizes and therefore be much lower than the resolution for pure disparity. We tested this using a new joint motion/disparity grating, designed to require the detection of conjunctions between motion and disparity. We found little difference between the resolutions for disparity and joint gratings, contradicting our predictions and suggesting that a different area than MT was used.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Distinguishing left- and right-handed molecules by two-step coherent pulses

Chiral molecules with broken parity symmetries can be modeled as quantum systems with cyclic-transition structures. By using these novel properties, we design two-step laser pulses to distinguish left- and right-handed molecules from the enantiomers. After the applied pulse drivings, one kind chiral molecules are trapped in coherent population trapping state, while the other ones are pumped to the highest states for ionizations. Then, different chiral molecules can be separated.Comment: 11 pages, 3 figures

Variation in the “coefficient of variation”

The coefficient of variation (CV), also known as relative standard deviation, has been used to measure the constancy of the Weber fraction, a key signature of efficient neural coding in time perception. It has long been debated whether or not duration judgments follow Weber's law, with arguments based on examinations of the CV. However, what has been largely ignored in this debate is that the observed CVs may be modulated by temporal context and decision uncertainty, thus questioning conclusions based on this measure. Here, we used a temporal reproduction paradigm to examine the variation of the CV with two types of temporal context: full-range mixed vs. sub-range blocked intervals, separately for intervals presented in the visual and auditory modalities. We found a strong contextual modulation of both interval-duration reproductions and the observed CVs. We then applied a two-stage Bayesian model to predict those variations. Without assuming a violation of the constancy of the Weber fraction, our model successfully predicted the central-tendency effect and the variation in the CV. Our findings and modeling results indicate that both the accuracy and precision of our timing behavior are highly dependent on the temporal context and decision uncertainty. And, critically, they advise caution with using variations of the CV to reject the constancy of the Weber fraction of duration estimation

Learning to suppress likely distractor locations in visual search is driven by the local distractor frequency

Salient but task-irrelevant distractors interfere less with visual search when they appear in a display region where distractors have appeared more frequently in the past (‘distractor-location probability cueing’). This effect could reflect the (re-)distribution of a global, limited attentional ‘inhibition resource’. Accordingly, changing the frequency of distractor appearance in one display region should also affect the magnitude of interference generated by distractors in a different region. Alternatively, distractor-location learning may reflect a local response (e.g., ‘habituation’) to distractors occurring at a particular location. In this case, the local distractor frequency in one display region should not affect distractor interference in a different region. To decide between these alternatives, we conducted three experiments in which participants searched for an orientation-defined target while ignoring a more salient orientation distractor that occurred more often in one vs. another display region. Experiment 1 varied the ratio of distractors appearing in the frequent vs. rare regions (60/40–90/10), with a fixed global distractor frequency. The results revealed the cueing effect to increase with increasing probability ratio. In Experiments 2 and 3, one (‘test’) region was assigned the same local distractor frequency as in one of the conditions of Experiment 1, but a different frequency in the other region – dissociating local from global distractor frequency. Together, the three experiments showed that distractor interference in the test region was not significantly influenced by the frequency in the other region, consistent with purely local learning. We discuss the implications for theories of statistical distractor-location learning

Duration reproduction under memory pressure: Modeling the roles of visual memory load in duration encoding and reproduction

Duration estimates are often biased by the sampled statistical context, yielding the classical central-tendency effect, i.e., short durations are over- and long duration underestimated. Most studies of the central-tendency bias have primarily focused on the integration of the sensory measure and the prior information, without considering any cognitive limits. Here, we investigated the impact of cognitive (visual working-memory) load on duration estimation in the duration encoding and reproduction stages. In four experiments, observers had to perform a dual, attention-sharing task: reproducing a given duration (primary) and memorizing a variable set of color patches (secondary). We found an increase in memory load (i.e., set size) during the duration-encoding stage to increase the central-tendency bias, while shortening the reproduced duration in general; in contrast, increasing the load during the reproduction stage prolonged the reproduced duration, without influencing the central tendency. By integrating an attentional-sharing account into a hierarchical Bayesian model, we were able to predict both the general over- and underestimation and the central-tendency effects observed in all four experiments. The model suggests that memory pressure during the encoding stage increases the sensory noise, which elevates the central-tendency effect. In contrast, memory pressure during the reproduction stage only influences the monitoring of elapsed time, leading to a general duration over-reproduction without impacting the central tendency.Competing Interest StatementThe authors have declared no competing interest

Long‐term (statistically learnt) and short‐term (inter‐trial) distractor‐location effects arise at different pre‐ and post‐selective processing stages

A salient distractor interferes less with visual search if it appears at a location where it is likely to occur, referred to as distractor-location probability cueing. Conversely, if the current target appears at the same location as a distractor on the preceding trial, search is impeded. While these two location-specific “suppression” effects reflect long-term, statistically learnt and short-term, inter-trial adaptations of the system to distractors, it is unclear at what stage(s) of processing they arise. Here, we adopted the additional-singleton paradigm and examined lateralized event-related potentials (L-ERPs) and lateralized alpha (8–12 Hz) power to track the temporal dynamics of these effects. Behaviorally, we confirmed both effects: reaction times (RTs) interference was reduced for distractors at frequent versus rare (distractor) locations, and RTs were delayed for targets that appeared at previous distractor versus non-distractor locations. Electrophysiologically, the statistical-learning effect was not associated with lateralized alpha power during the pre-stimulus period. Rather, it was seen in an early N1pc referenced to the frequent distractor location (whether or not a distractor or a target occurred there), indicative of a learnt top-down prioritization of this location. This early top-down influence was systematically modulated by (competing) target- and distractor-generated bottom-up saliency signals in the display. In contrast, the inter-trial effect was reflected in an enhanced SPCN when the target was preceded by a distractor at its location. This suggests that establishing that an attentionally selected item is a task-relevant target, rather than an irrelevant distractor, is more demanding at a previously “rejected” distractor location

Inter-trial effects in visual pop-out search: factorial comparison of Bayesian updating models

Many previous studies on visual search have reported inter-trial effects, that is, observers respond faster when some target property, such as a defining feature or dimension, or the response associated with the target repeats versus changes across consecutive trial episodes. However, what processes drive these inter-trial effects is still controversial. Here, we investigated this question using a combination of Bayesian modeling of belief updating and evidence accumulation modeling in perceptual decision-making. In three visual singleton (`pop-out') search experiments, we explored how the probability of the response critical states of the search display (e.g., target presence/absence) and the repetition/switch of the target-defining dimension (color/ orientation) affect reaction time distributions. The results replicated the mean reaction time (RT) inter-trial and dimension repetition/switch effects that have been reported in previous studies. Going beyond this, to uncover the underlying mechanisms, we used the Drift-Diffusion Model (DDM) and the Linear Approach to Threshold with Ergodic Rate (LATER) model to explain the RT distributions in terms of decision bias (starting point) and information processing speed (evidence accumulation rate). We further investigated how these different aspects of the decision-making process are affected by different properties of stimulus history, giving rise to dissociable inter-trial effects. We approached this question by (i) combining each perceptual decision making model (DDM or LATER) with different updating models, each specifying a plausible rule for updating of either the starting point or the rate, based on stimulus history, and (ii) comparing every possible combination of trial-wise updating mechanism and perceptual decision model in a factorial model comparison. Consistently across experiments, we found that the (recent) history of the response-critical property influences the initial decision bias, while repetition/ switch of the target-defining dimension affects the accumulation rate, likely reflecting an implicit `top-down' modulation process. This provides strong evidence of a disassociation between response- and dimension-based inter-trial effects