Dynamics of contour, object and face processing in the human visual cortex

The neural basis of visual perception can be understood only when the sequence of cortical activity underlying successful recognition is known. The early steps in this processing chain, from retina to the primary visual cortex, are highly local, and the perception of more complex shapes requires integration of the local information. In Study I of this thesis, the progression from local to global visual analysis was assessed by recording cortical magnetoencephalographic (MEG) responses to arrays of elements that either did or did not form global contours. The results demonstrated two spatially and temporally distinct stages of processing: The first, emerging 70 ms after stimulus onset around the calcarine sulcus, was sensitive to local features only, whereas the second, starting at 130 ms across the occipital and posterior parietal cortices, reflected the global configuration. To explore the links between cortical activity and visual recognition, Studies II III presented subjects with recognition tasks of varying levels of difficulty. The occipito-temporal responses from 150 ms onwards were closely linked to recognition performance, in contrast to the 100-ms mid-occipital responses. The averaged responses increased gradually as a function of recognition performance, and further analysis (Study III) showed the single response strengths to be graded as well. Study IV addressed the attention dependence of the different processing stages: Occipito-temporal responses peaking around 150 ms depended on the content of the visual field (faces vs. houses), whereas the later and more sustained activity was strongly modulated by the observers attention. Hemodynamic responses paralleled the pattern of the more sustained electrophysiological responses. Study V assessed the temporal processing capacity of the human object recognition system. Above sufficient luminance, contrast and size of the object, the processing speed was not limited by such low-level factors. Taken together, these studies demonstrate several distinct stages in the cortical activation sequence underlying the object recognition chain, reflecting the level of feature integration, difficulty of recognition, and direction of attention.

Learning Nonlinear Visual Processing from Natural Images

The paradigm of computational vision hypothesizes that any visual function -- such as the recognition of your grandparent -- can be replicated by computational processing of the visual input. What are these computations that the brain performs? What should or could they be? Working on the latter question, this dissertation takes the statistical approach, where the suitable computations are attempted to be learned from the natural visual data itself. In particular, we empirically study the computational processing that emerges from the statistical properties of the visual world and the constraints and objectives specified for the learning process. This thesis consists of an introduction and 7 peer-reviewed publications, where the purpose of the introduction is to illustrate the area of study to a reader who is not familiar with computational vision research. In the scope of the introduction, we will briefly overview the primary challenges to visual processing, as well as recall some of the current opinions on visual processing in the early visual systems of animals. Next, we describe the methodology we have used in our research, and discuss the presented results. We have included some additional remarks, speculations and conclusions to this discussion that were not featured in the original publications. We present the following results in the publications of this thesis. First, we empirically demonstrate that luminance and contrast are strongly dependent in natural images, contradicting previous theories suggesting that luminance and contrast were processed separately in natural systems due to their independence in the visual data. Second, we show that simple cell -like receptive fields of the primary visual cortex can be learned in the nonlinear contrast domain by maximization of independence. Further, we provide first-time reports of the emergence of conjunctive (corner-detecting) and subtractive (opponent orientation) processing due to nonlinear projection pursuit with simple objective functions related to sparseness and response energy optimization. Then, we show that attempting to extract independent components of nonlinear histogram statistics of a biologically plausible representation leads to projection directions that appear to differentiate between visual contexts. Such processing might be applicable for priming, \ie the selection and tuning of later visual processing. We continue by showing that a different kind of thresholded low-frequency priming can be learned and used to make object detection faster with little loss in accuracy. Finally, we show that in a computational object detection setting, nonlinearly gain-controlled visual features of medium complexity can be acquired sequentially as images are encountered and discarded. We present two online algorithms to perform this feature selection, and propose the idea that for artificial systems, some processing mechanisms could be selectable from the environment without optimizing the mechanisms themselves. In summary, this thesis explores learning visual processing on several levels. The learning can be understood as interplay of input data, model structures, learning objectives, and estimation algorithms. The presented work adds to the growing body of evidence showing that statistical methods can be used to acquire intuitively meaningful visual processing mechanisms. The work also presents some predictions and ideas regarding biological visual processing.

Visual completion in an illusory figure

The visual systems of humans and animals represent physical reality in a modified way, depending on the specific demands that the species in question has for survival. The ability to perceive visual illusions is found in independently evolved visual systems, from honeybees to humans. In humans, the ability emerges early, at the age of four months. Thus the perception of illusion is likely to reflect visual processes of fundamental importance for object perception in natural vision. The experiments reported in this thesis employed various modifications of the Kanizsa triangle, a drawn configuration composed of three black disks with missing sectors on a white background. The sectors appear to form the tips of a triangle. The visual system completes the physically empty area between the disks, generally called inducers, with giving the perception of an illusory triangle. The illusory triangle consists of an illusory surface bounded by illusory contours; the triangle appears brighter than and to lie above the background. If the sectors are coloured, the colour fills the illusory area, a phenomenon known as neon colour spreading . We investigated spatial limitations on the perception of Kanizsa-type illusions and how other stimuli and viewing parameters affected these limitations. We also studied complex configurations thick, bent, mobile and chromatic inducers - to determine whether illusions combining several attributes can be perceived. The results suggest that the visual system is highly effective in completing a percept. The perception of an illusory figure is spatially scale invariant when perceived at threshold. The processing time and the number of fixations modify the percept, making the perception of the illusion more probable in various viewing conditions. Furthermore, the fact that the illusion can be perceived when only one inducer is physically present at any given moment indicates the potential of single inducers. Apparently, modelling illusory figure perception will require a combination of low-level, local processes and higher-level integrative processes. Our studies with stimuli combining several attributes relevant to object perception demonstrate that the perception of an illusory figure is flexible and is maintained also when it contains colour and volume and when shown in movement. All in all, the results confirm the assumed importance of the visual processes related with the perception of illusory figures in everyday viewing. This is indicated by the variety of inducer modifications that can be made without destroying the percept. Furthermore, the illusion can acquire additional attributes from such modifications. Due to individual differences in the perception of illusory figures, universal values for absolute performance are not always meaningful, but stable trends and general relations do exist.

Visual search and eye movements: Studies of perceptual span

In visual search one tries to find the currently relevant item among other, irrelevant items. In the present study, visual search performance for complex objects (characters, faces, computer icons and words) was investigated, and the contribution of different stimulus properties, such as luminance contrast between characters and background, set size, stimulus size, colour contrast, spatial frequency, and stimulus layout were investigated. Subjects were required to search for a target object among distracter objects in two-dimensional stimulus arrays. The outcome measure was threshold search time, that is, the presentation duration of the stimulus array required by the subject to find the target with a certain probability. It reflects the time used for visual processing separated from the time used for decision making and manual reactions. The duration of stimulus presentation was controlled by an adaptive staircase method. The number and duration of eye fixations, saccade amplitude, and perceptual span, i.e., the number of items that can be processed during a single fixation, were measured. It was found that search performance was correlated with the number of fixations needed to find the target. Search time and the number of fixations increased with increasing stimulus set size. On the other hand, several complex objects could be processed during a single fixation, i.e., within the perceptual span. Search time and the number of fixations depended on object type as well as luminance contrast. The size of the perceptual span was smaller for more complex objects, and decreased with decreasing luminance contrast within object type, especially for very low contrasts. In addition, the size and shape of perceptual span explained the changes in search performance for different stimulus layouts in word search. Perceptual span was scale invariant for a 16-fold range of stimulus sizes, i.e., the number of items processed during a single fixation was independent of retinal stimulus size or viewing distance. It is suggested that saccadic visual search consists of both serial (eye movements) and parallel (processing within perceptual span) components, and that the size of the perceptual span may explain the effectiveness of saccadic search in different stimulus conditions. Further, low-level visual factors, such as the anatomical structure of the retina, peripheral stimulus visibility and resolution requirements for the identification of different object types are proposed to constrain the size of the perceptual span, and thus, limit visual search performance. Similar methods were used in a clinical study to characterise the visual search performance and eye movements of neurological patients with chronic solvent-induced encephalopathy (CSE). In addition, the data about the effects of different stimulus properties on visual search in normal subjects were presented as simple practical guidelines, so that the limits of human visual perception could be taken into account in the design of user interfaces.

Temporal acuity in developmental dyslexia across the life span : Tactile, auditory, visual, and crossmodal estimations

Taustaa Kehityksellinen dysleksia (lukivaikeus) on erityinen lukemaan oppimisen vaikeus, johon liittyy usein myös vaikeuksia kirjoittamaan oppimisessa. Lukivaikeuden oletetaan useissa tapauksissa johtuvan vaikeudesta käsitellä kielen äännerakenteita (fonologinen prosessointi). Tämä poikkeavuus voi olla joko lukivaikeuden perimmäinen syy tai vaihtoehtoisesti ongelmat äänteiden käsittelyssä voivat heijastaa jotain vielä perustavamman tason vaikeutta. Eräs tällainen ehdotettu perustavan tasoin syy on poikkeavuus aistien toiminnoissa, erityisesti aistien aikatarkkuudessa. Aikatarkkuudella tarkoitetaan kykyä ja rajoja siinä, kuinka nopeasti esitettyä aistitiedon virtaa henkilö kykenee vastaanottamaan ja käsittelemään. Monet arjen toiminnot lukemisen rinnalla edellyttävät aistien erittäin tarkkaa ajallista erottelukykyä (esimerkiksi kuulo puheen ymmärtämisessä, tunto pintamateriaalin tunnistamisessa). Aikatarkkuusvaikeuksien esiintyvyyttä lukivaikeudessa on tutkittu aiemminkin, mutta yksimielisyyteen ei ole päästy siitä, onko kaikilla lukivaikeuksisilla näitä ongelmia tai mihin aisteihin vaikeudet mahdollisesti rajoittuvat. Myöskään ei tiedetä, havaitaanko aikatarkkuuden ongelmia kaiken ikäisillä lukivaikeuksisilla vai vaihteleeko mahdollinen ongelmien kuva iän mukana. Lisäksi on epäselvää, kuinka aikatarkkuuden ongelmat itseasiassa ovat yhteydessä kielen käsittelyn ja varsinaisen lukemisen vaikeuksiin. Tutkimussarjan aihe Tässä tutkimussarjassa aikatarkkuutta tutkittiin kolmessa yksittäisessä aistissa, joita olivat tunto, näkö ja kuulo, sekä kolmessa aistien välisessä yhdistelmässä, joita olivat audiotaktiilinen (kuulo-tunto), visuotaktiilinen (näkö-tunto) ja audiovisuaalinen (näkö-kuulo). Aikatarkkuutta arvioitiin kahdella eri menetelmällä, jotta saataisiin lisää tietoa siitä, missä tietyssä aikatarkkuuden osa-alueessa lukivaikeuksisilla mahdollisesti on vaikeuksia. Ensimmäisessä tehtävässä tutkittavan tuli arvioida, ovatko esitetyt ei-kielelliset ärsykkeet samanaikaisia vai eriaikaisia. Toisessa tehtävässä koehenkilön tuli arvioida esitettyjen ei-kielellisten ärsykkeiden esitysjärjestys. Molemmissa tehtävissä määriteltiin millisekuntitasolla (sekunnin tuhannesosa) se esitysnopeus, jolla koehenkilö kykeni arvioimaan ärsykkeiden ajalliset suhteet oikein. Englanninkielinen demonstraatio aikatarkkuustehtävistä löytyy internetistä (http://www.helsinki.fi/hum/ylpsy/neuropsy). Itse aikatarkkuustehtävien lisäksi tutkimussarjassa arvioitiin tutkimushenkilöiden päättelykykyä, kielellisiä toimintoja ja lukemista. Tutkimushenkilöt Tutkimuksiin osallistui 53 lukivaikeuksista ja 66 sujuvaa lukijaa, jotka oli jaettu kolmeen pääikäryhmään: lapset (8-12 vuotta), nuoret aikuiset (20-36 vuotta) ja ikääntyneemmät aikuiset (20-59 vuotta). Ikääntyneempien aikuisten ryhmä oli edelleen jaettu ikävuosikymmenluokkiin, mikä mahdollisti sen tutkimisen, vaikuttaako lisääntyvä aikuisikä lukivaikeuksisten aikatarkkuuteen (20-29, 30-39, 40-49 ja 50-59 -vuotiaat). Tutkimussarjan tulokset Aikatarkkuuden ongelmat lukivaikeuksisilla olivat yleistyneitä yli iän, aistien ja tehtävien Lukivaikeuksiset kaikissa pääikäryhmissä (lapset, nuoret aikuiset, ikääntyneemmän aikuiset) tarvitsivat samanikäisiä sujuvia lukijoita hitaamman esitystahdin, jotta he kykenivät arvioimaan ei-kielellisten ärsykkeiden ajallisen esitystavan oikein. Tämä aikatarkkuuden ongelma havaittiin lukivaikeuksisilla kaikissa aisteissa (tunto, kuulo, näkö) ja niiden yhdistelmissä (audiotaktiilinen, visuotaktiilinen, audiovisuaalinen). Lukivaikeuksisten aikatarkkuusongelmat ilmenivät edelleen molemmissa tehtävätyypeissä (samanaikaisuuden ja järjestyksen arvioinnissa). Aikatarkkuus ja sen ongelmat olivat yhteydessä äänteiden käsittelyyn Aikatarkkuus oli yhteydessä äänteiden käsittelykykyyn (fonologiseen prosessointiin), niin lapsilla kuin aikuisillakin, kaikissa aisteissa, niiden yhdistelmissä ja tehtävätyypeissä. Yhteys ei-kielellisen aikatarkkuuden ja kielellisten toimintojen välillä oli kuitenkin selkeämpi lukivaikeuksisilla kuin sujuvilla lukijoilla. Tämä tarkoittaa, että etenkin lukivaikeuksisilla ryhmätason huono aikatarkkuus oli yhteydessä huonoon äänteiden käsittelyyn (fonologiseen prosessointiin) ja päinvastoin. Suoraa yhteyttä lukemisen ja aikatarkkuuden välillä ei kuitenkaan havaittu. Lisääntyvä aikuisikä heikensi lukivaikeuksisten aikatarkkuutta suhteettoman paljon Tiedonkäsittelyn nopeuden on toistuvasti osoitettu hidastuvan normaalissa ikääntymisessä. Lisääntyvä aikuisikä (20-59 -vuotiailla) heikensikin sekä sujuvien että lukivaikeuksisten aikatarkkuutta. Toisin sanoen, mitä iäkkäämmästä aikuisesta oli kysymys, sitä hitaammin hänelle tuli esittää ärsykkeet, jotta hän kykeni arvioimaan niiden ajalliset suhteet oikein. Tämä ikään liittyvä tavanomainen hidastuminen oli kuitenkin yllättäen suhteettoman nopeaa lukivaikeuksisilla. Toisin sanoen, jo nuorilla lukivaikeuksisilla havaittu aikatarkkuuden vaikeus (ryhmäero verrattuna sujuviin lukijoihin) ei pysynyt saman suuruisena, vaan ryhmien ero kasvoi aikuisiän lisääntyessä. Tulosten merkitys Lukivaikeuden osoitettiin tässä tutkimussarjassa olevan yhteydessä yleistyneeseen vaikeuteen käsitellä ajassa nopeasti muuttuvaa ei-kielellistä aistitietoa (yli aistien ja niiden yhdistelmien, tehtävätyyppien, tutkittavien iän). Tämä osoittaa, että lukivaikeus ei ole ongelma, joka rajoittuu vain kielellisen materiaalin käsittelyn vaikeuksiin (äänteiden käsittely, lukeminen, kirjoittaminen). Nyt havaitut vaikeudet eivät myöskään rajoittuneet vain niihin aisteihin, jotka selkeimmin liittyvät lukemiseen (näkö) ja puhuttuun kieleen (kuulo); Ongelmia esiintyi myös muissa aisteissa (tunto). Lukivaikeuksisten lukijoiden ryhmätasolla havaittu aikatarkkuuden ongelma ei kuitenkaan heijastunut yksilötasolle; Jokainen lukivaikeuksinen ei ollut huono aikatarkkuustehtävissä. Näin ollen ei siis voida väittää, että kaikkien lukivaikeuksisten äänteiden käsittelyn tai lukemaan oppimisen vaikeudet voisivat selittyä aistien toimintojen poikkeavuudella. Aikatarkkuuden ongelmat eivät olleet yhteydessä varsinaiseen lukemiseen. Sekä lukivaikeuksisilla lapsilla että aikuisilla todettiin kuitenkin selkeä yhteys aikatarkkuuden ongelmien ja lukemaan oppimisen keskeisen ennakkoehdon, fonologisen prosessoinnin, välillä. Saattaa siis olla, että synnynnäinen aistien toimintojen poikkeavuus vaikuttaa yksilön suoriutumiseen jo ennen varsinaista lukemaan oppimista, kun ne taidot kehittyvät (fonologinen prosessointi), joille myöhempi lukemaan oppiminen perustuu. Ikäännyttäessä havaittu lukivaikeuksisten suhteettoman nopea aikatarkkuuden heikkeneminen osoittaa, että lukivaikeus ei voi olla ongelma, joka koskee vain lapsuusikää, tai vaikeus, joka johtuu vain kehityksen viivästymästä joka kurottaisiin iän myötä umpeen. Tulosten ymmärtämiseksi onkin muistettava kaksi seikkaa. Lukivaikeus on ensinnäkin yhdistetty synnynnäisiin, pieniin, poikkeavuuksiin aivojen rakenteissa ja toiminnoissa. Toisaalta tavanomaiseen ikääntymiseen liittyy se, että aivot kykenevät yhä huonommin korjaamaan ja kiertämään (kompensoimaan) pieniä vaurioita. Tämän perusteella tutkimussarjan tuloksista voidaan päätellä, että lukivaikeuksisten jo synnynnäisesti heikentyneet aivojen kompensointimahdollisuudet eivät ole yhtä tehokkaita puskuroimaan ikääntymisen tavanomaisia vaikutuksia kuin sujuvilla lukijoilla. Yllättävää kuitenkin on, että tämä korostunut heikkeneminen havaittiin jo suhteellisen nuorilla, työikäisillä, lukivaikeuksisilla, ennen 60 ikävuotta. Samanlaista ikäännyttäessä korostuvaa vaikeutta ei lukivaikeuksilla kuitenkaan havaittu päättelyssä, kielellisissä toiminnoissa tai itse lukemisessa. Vaikuttaakin siis siltä, että ne toiminnot, joita on harjaannutettu aktiivisesti, eivät heikkene kasvavan aikuisiän myötä yhtä suhteettomasti. Alkuperäiset artikkelit Laasonen M, Tomma-Halme J, Lahti-Nuuttila P, Service E, and Virsu V (2000) Rate of information segregation in developmentally dyslexic children, Brain and Language, 75(1), 66-81. Laasonen M, Service E, and Virsu V (2001) Temporal order and processing acuity of visual, auditory, and tactile perception in developmentally dyslexic young adults, Cognitive, Affective, and Behavioral Neuroscience, 1(4), 394-410. Laasonen M, Service E, and Virsu V (2002) Crossmodal temporal order and processing acuity in developmentally dyslexic young adults, Brain and Language, 80(3), 340-354. Laasonen M, Lahti-Nuuttila P, and Virsu V (2002) Developmentally impaired processing speed decreases more than normally with age, NeuroReport, 13(9), 1111-1113.

Feature integration in human vision

The earliest stages of human cortical visual processing can be conceived as extraction of local stimulus features. However, more complex visual functions, such as object recognition, require integration of multiple features. Recently, neural processes underlying feature integration in the visual system have been under intensive study. A specialized mid-level stage preceding the object recognition stage has been proposed to account for the processing of contours, surfaces and shapes as well as configuration. This thesis consists of four experimental, psychophysical studies on human visual feature integration. In two studies, classification image a recently developed psychophysical reverse correlation method was used. In this method visual noise is added to near-threshold stimuli. By investigating the relationship between random features in the noise and observer s perceptual decision in each trial, it is possible to estimate what features of the stimuli are critical for the task. The method allows visualizing the critical features that are used in a psychophysical task directly as a spatial correlation map, yielding an effective "behavioral receptive field". Visual context is known to modulate the perception of stimulus features. Some of these interactions are quite complex, and it is not known whether they reflect early or late stages of perceptual processing. The first study investigated the mechanisms of collinear facilitation, where nearby collinear Gabor flankers increase the detectability of a central Gabor. The behavioral receptive field of the mechanism mediating the detection of the central Gabor stimulus was measured by the classification image method. The results show that collinear flankers increase the extent of the behavioral receptive field for the central Gabor, in the direction of the flankers. The increased sensitivity at the ends of the receptive field suggests a low-level explanation for the facilitation. The second study investigated how visual features are integrated into percepts of surface brightness. A novel variant of the classification image method with brightness matching task was used. Many theories assume that perceived brightness is based on the analysis of luminance border features. Here, for the first time this assumption was directly tested. The classification images show that the perceived brightness of both an illusory Craik-O Brien-Cornsweet stimulus and a real uniform step stimulus depends solely on the border. Moreover, the spatial tuning of the features remains almost constant when the stimulus size is changed, suggesting that brightness perception is based on the output of a single spatial frequency channel. The third and fourth studies investigated global form integration in random-dot Glass patterns. In these patterns, a global form can be immediately perceived, if even a small proportion of random dots are paired to dipoles according to a geometrical rule. In the third study the discrimination of orientation structure in highly coherent concentric and Cartesian (straight) Glass patterns was measured. The results showed that the global form was more efficiently discriminated in concentric patterns. The fourth study investigated how form detectability depends on the global regularity of the Glass pattern. The local structure was either Cartesian or curved. It was shown that randomizing the local orientation deteriorated the performance only with the curved pattern. The results give support for the idea that curved and Cartesian patterns are processed in at least partially separate neural systems.

Auditory - Visual Matching in Learning Disabilities : Intervention Studies from Finland and Sweden

The present thesis discusses relevant issues in education: 1) learning disabilities including the role of comorbidity in LDs, and 2) the use of research-based interventions. This thesis consists of a series of four studies (three articles), which deepens the knowledge of the field of special education. Intervention studies (N=242) aimed to examine whether training using a nonverbal auditory-visual matching computer program had a remedial effect in different learning disabilities, such as developmental dyslexia, Attention Deficit Disorder (ADD) and Specific Language Impairment (SLI). These studies were conducted in both Finland and Sweden. The intervention’s non-verbal character made an international perspective possible. The results of the intervention studies confirmed, that the auditory-visual matching computer program, called Audilex had positive intervention effects. In Study I of children with developmental dyslexia there were also improvements in reading skills, specifically in reading nonsense words and reading speed. These improvements in tasks, which are thought to rely on phonological processing, suggest that such reading difficulties in dyslexia may stem in part from more basic perceptual difficulties, including those required to manage the visual and auditory components of the decoding task. In Study II the intervention had a positive effect on children with dyslexia; older students with dyslexia and surprisingly, students with ADD also benefited from this intervention. In conclusion, the role of comorbidity was apparent. An intervention effect was evident also in students’ school behavior. Study III showed that children with SLI experience difficulties very similar to those of children with dyslexia in auditory-visual matching. Children with language-based learning disabilities, such as dyslexia and SLI benefited from the auditory-visual matching intervention. Also comorbidity was evident among these children; in addition to formal diagnoses, comorbidity was explored with an assessment inventory, which was developed for this thesis. Interestingly, an overview of the data of this thesis shows positive intervention effects in all studies despite learning disability, language, gender or age. These findings have been described by a concept inter-modal transpose. Self-evidently these issues need further studies. In learning disabilities the aim in the future will also be to identify individuals at risk rather than by deficit; this aim can be achieved by using research-based interventions, intensified support in general education and inclusive special education. Keywords: learning disabilities, developmental dyslexia, attention deficit disorder, specific language impairment, language-based learning disabilities, comorbidity, auditory-visual matching, research-based interventions, inter-modal transpose

Developmental, functional brain imaging and electrophysiological evidence of visual and auditory working memory

Intact function of working memory (WM) is essential for children and adults to cope with every day life. Children with deficits in WM mechanisms have learning difficulties that are often accompanied by behavioral problems. The neural processes subserving WM, and brain structures underlying this system, continue to develop during childhood till adolescence and young adulthood. With functional magnetic resonance imaging (fMRI) it is possible to investigate the organization and development of WM. The present thesis aimed to investigate, using behavioral and neuroimaging methods, whether mnemonic processing of spatial and nonspatial visual information is segregated in the developing and mature human brain. A further aim in this research was to investigate the organization and development of audiospatial and visuospatial information processing in WM. The behavioral results showed that spatial and nonspatial visual WM processing is segregated in the adult brain. The fMRI result in children suggested that memory load related processing of spatial and nonspatial visual information engages common cortical networks, whereas selective attention to either type of stimuli recruits partially segregated areas in the frontal, parietal and occipital cortices. Deactivation mechanisms that are important in the performance of WM tasks in adults are already operational in healthy school-aged children. Electrophysiological evidence suggested segregated mnemonic processing of visual and auditory location information. The results of the development of audiospatial and visuospatial WM demonstrate that WM performance improves with age, suggesting functional maturation of underlying cognitive processes and brain areas. The development of the performance of spatial WM tasks follows a different time course in boys and girls indicating a larger degree of immaturity in the male than female WM systems. Furthermore, the differences in mastering auditory and visual WM tasks may indicate that visual WM reaches functional maturity earlier than the corresponding auditory system. Spatial WM deficits may underlie some learning difficulties and behavioral problems related to impulsivity, difficulties in concentration, and hyperactivity. Alternatively, anxiety or depressive symptoms may affect WM function and the ability to concentrate, being thus the primary cause of poor academic achievement in children.

A Probabilistic Approach to the Primary Visual Cortex

What can the statistical structure of natural images teach us about the human brain? Even though the visual cortex is one of the most studied parts of the brain, surprisingly little is known about how exactly images are processed to leave us with a coherent percept of the world around us, so we can recognize a friend or drive on a crowded street without any effort. By constructing probabilistic models of natural images, the goal of this thesis is to understand the structure of the stimulus that is the raison d etre for the visual system. Following the hypothesis that the optimal processing has to be matched to the structure of that stimulus, we attempt to derive computational principles, features that the visual system should compute, and properties that cells in the visual system should have. Starting from machine learning techniques such as principal component analysis and independent component analysis we construct a variety of sta- tistical models to discover structure in natural images that can be linked to receptive field properties of neurons in primary visual cortex such as simple and complex cells. We show that by representing images with phase invariant, complex cell-like units, a better statistical description of the vi- sual environment is obtained than with linear simple cell units, and that complex cell pooling can be learned by estimating both layers of a two-layer model of natural images. We investigate how a simplified model of the processing in the retina, where adaptation and contrast normalization take place, is connected to the nat- ural stimulus statistics. Analyzing the effect that retinal gain control has on later cortical processing, we propose a novel method to perform gain control in a data-driven way. Finally we show how models like those pre- sented here can be extended to capture whole visual scenes rather than just small image patches. By using a Markov random field approach we can model images of arbitrary size, while still being able to estimate the model parameters from the data.

Modeling Efficient Classification as a Process of Confidence Assessment and Delegation

In visual object detection and recognition, classifiers have two interesting characteristics: accuracy and speed. Accuracy depends on the complexity of the image features and classifier decision surfaces. Speed depends on the hardware and the computational effort required to use the features and decision surfaces. When attempts to increase accuracy lead to increases in complexity and effort, it is necessary to ask how much are we willing to pay for increased accuracy. For example, if increased computational effort implies quickly diminishing returns in accuracy, then those designing inexpensive surveillance applications cannot aim for maximum accuracy at any cost. It becomes necessary to find trade-offs between accuracy and effort. We study efficient classification of images depicting real-world objects and scenes. Classification is efficient when a classifier can be controlled so that the desired trade-off between accuracy and effort (speed) is achieved and unnecessary computations are avoided on a per input basis. A framework is proposed for understanding and modeling efficient classification of images. Classification is modeled as a tree-like process. In designing the framework, it is important to recognize what is essential and to avoid structures that are narrow in applicability. Earlier frameworks are lacking in this regard. The overall contribution is two-fold. First, the framework is presented, subjected to experiments, and shown to be satisfactory. Second, certain unconventional approaches are experimented with. This allows the separation of the essential from the conventional. To determine if the framework is satisfactory, three categories of questions are identified: trade-off optimization, classifier tree organization, and rules for delegation and confidence modeling. Questions and problems related to each category are addressed and empirical results are presented. For example, related to trade-off optimization, we address the problem of computational bottlenecks that limit the range of trade-offs. We also ask if accuracy versus effort trade-offs can be controlled after training. For another example, regarding classifier tree organization, we first consider the task of organizing a tree in a problem-specific manner. We then ask if problem-specific organization is necessary.

The visual preferences for forest regeneration and field afforestation - four case studies in Finland

The overall aim of this dissertation was to study the public's preferences for forest regeneration fellings and field afforestations, as well as to find out the relations of these preferences to landscape management instructions, to ecological healthiness, and to the contemporary theories for predicting landscape preferences. This dissertation includes four case studies in Finland, each based on the visualization of management options and surveys. Guidelines for improving the visual quality of forest regeneration and field afforestation are given based on the case studies. The results show that forest regeneration can be connected to positive images and memories when the regeneration area is small and some time has passed since the felling. Preferences may not depend only on the management alternative itself but also on the viewing distance, viewing point, and the scene in which the management options are implemented. The current Finnish forest landscape management guidelines as well as the ecological healthiness of the studied options are to a large extent compatible with the public's preferences. However, there are some discrepancies. For example, the landscape management instructions as well as ecological hypotheses suggest that the retention trees need to be left in groups, whereas people usually prefer individually located retention trees to those trees in groups. Information and psycho-evolutionary theories provide some possible explanations for people's preferences for forest regeneration and field afforestation, but the results cannot be consistently explained by these theories. The preferences of the different stakeholder groups were very similar. However, the preference ratings of the groups that make their living from forest - forest owners and forest professionals - slightly differed from those of the others. These results provide support for the assumptions that preferences are largely consistent at least within one nation, but that knowledge and a reference group may also influence preferences.

Spectral Tuning and Adaptation to Different Light Environments of Mysid Visual Pigments

In the present thesis, questions of spectral tuning, the relation of spectral and thermal properties of visual pigments, and evolutionary adaptation to different light environments were addressed using a group of small crustaceans of the genus Mysis as a model. The study was based on microspectrophotometric measurements of visual pigment absorbance spectra, electrophysiological measurements of spectral sensitivities of dark-adapted eyes, and sequencing of the opsin gene retrieved through PCR. The spectral properties were related to the spectral transmission of the respective light environments, as well as to the phylogentic histories of the species. The photoactivation energy (Ea) was estimated from temperature effects on spectral sensitivity in the long-wavelength range, and calculations were made for optimal quantum catch and optimal signal-to-noise ratio in the different light environments. The opsin amino acid sequences of spectrally characterized individuals were compared to find candidate residues for spectral tuning. The general purpose was to clarify to what extent and on what time scale adaptive evolution has driven the functional properties of (mysid) visual pigments towards optimal performance in different light environments. An ultimate goal was to find the molecular mechanisms underlying the spectral tuning and to understand the balance between evolutionary adaptation and molecular constraints. The totally consistent segregation of absorption maxima (λmax) into (shorter-wavelength) marine and (longer-wavelength) freshwater populations suggests that truly adaptive evolution is involved in tuning the visual pigment for optimal performance, driven by selection for high absolute visual sensitivity. On the other hand, the similarity in λmax and opsin sequence between several populations of freshwater M. relicta in spectrally different lakes highlights the limits to adaptation set by evolutionary history and time. A strong inverse correlation between Ea and λmax was found among all visual pigments studied in these respects, including those of M. relicta and 10 species of vertebrate pigments, and this was used to infer thermal noise. The conceptual signal-to-noise ratios thus calculated for pigments with different λmax in the Baltic Sea and Lake Pääjärvi light environments supported the notion that spectral adaptation works towards maximizing the signal-to-noise ratio rather than quantum catch as such. Judged by the shape of absorbance spectra, the visual pigments of all populations of M. relicta and M. salemaai used exclusively the A2 chromophore (3, 4-dehydroretinal). A comparison of amino acid substitutions between M. relicta and M. salemaai indicated that mysid shrimps have a small number of readily available tuning sites to shift between a shorter - and a longer -wavelength opsin. However, phylogenetic history seems to have prevented marine M. relicta from converting back to the (presumably) ancestral opsin form, and thus the more recent reinvention of marine spectral sensitivity has been accomplished by some other novel mechanism, yet to be found