1,217 research outputs found

    Functional specialization and generalization for grouping of stimuli based on colour and motion

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    This study was undertaken to learn whether the principle of functional specialization that is evident at the level of the prestriate visual cortex extends to areas that are involved in grouping visual stimuli according to attribute, and specifically according to colour and motion. Subjects viewed, in an fMRI scanner, visual stimuli composed of moving dots, which could be either coloured or achromatic; in some stimuli the moving coloured dots were randomly distributed or moved in random directions; in others, some of the moving dots were grouped together according to colour or to direction of motion, with the number of groupings varying from 1 to 3. Increased activation was observed in area V4 in response to colour grouping and in V5 in response to motion grouping while both groupings led to activity in separate though contiguous compartments within the intraparietal cortex. The activity in all the above areas was parametrically related to the number of groupings, as was the prominent activity in Crus I of the cerebellum where the activity resulting from the two types of grouping overlapped. This suggests (a) that, the specialized visual areas of the prestriate cortex have functions beyond the processing of visual signals according to attribute, namely that of grouping signals according to colour (V4) or motion (V5); (b) that the functional separation evident in visual cortical areas devoted to motion and colour, respectively, is maintained at the level of parietal cortex, at least as far as grouping according to attribute is concerned; and (c) that, by contrast, this grouping-related functional segregation is not maintained at the level of the cerebellum

    The Constructive Nature of Color Vision and Its Neural Basis

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    Our visual world is made up of colored surfaces. The color of a surface is physically determined by its reflectance, i.e., how much energy it reflects as a function of wavelength. Reflected light, however, provides only ambiguous information about the color of a surface as it depends on the spectral properties of both the surface and the illumination. Despite the confounding effects of illumination on the reflected light, the visual system is remarkably good at inferring the reflectance of a surface, enabling observers to perceive surface colors as stable across illumination changes. This capacity of the visual system is called color constancy and it highlights that color vision is a constructive process. The research presented here investigates the neural basis of some of the most relevant aspects of the constructive nature of human color vision using machine learning algorithms and functional neuroimaging. The experiments demonstrate that color-related prior knowledge influences neural signals already in the earliest area of visual processing in the cortex, area V1, whereas in object imagery, perceived color shared neural representations with the color of the imagined objects in human V4. A direct test for illumination-invariant surface color representation showed that neural coding in V1 as well as a region anterior to human V4 was robust against illumination changes. In sum, the present research shows how different aspects of the constructive nature of color vision can be mapped to different regions in the ventral visual pathway

    The role of parietal cortex in the formation of color and motion based concepts

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    Imaging evidence shows that separate subdivisions of parietal cortex, in and around the intraparietal sulcus (IPS), are engaged when stimuli are grouped according to color and to motion (Zeki and Stutters, 2013). Since grouping is an essential step in the formation of concepts, we wanted to learn whether parietal cortex is also engaged in the formation of concepts according to these two attributes. Using functional magnetic resonance imaging (fMRI), and choosing the recognition of concept-based color or motion stimuli as our paradigm, we found that there was strong concept-related activity in and around the IPS, a region whose homolog in the macaque monkey is known to receive direct but segregated anatomical inputs from V4 and V5. Parietal activity related to color concepts was juxtaposed but did not overlap with activity related to motion concepts, thus emphasizing the continuation of the segregation of color and motion into the conceptual system. Concurrent retinotopic mapping experiments showed that within the parietal cortex, concept-related activity increases within later stage IPS areas

    Change blindness: eradication of gestalt strategies

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    Arrays of eight, texture-defined rectangles were used as stimuli in a one-shot change blindness (CB) task where there was a 50% chance that one rectangle would change orientation between two successive presentations separated by an interval. CB was eliminated by cueing the target rectangle in the first stimulus, reduced by cueing in the interval and unaffected by cueing in the second presentation. This supports the idea that a representation was formed that persisted through the interval before being 'overwritten' by the second presentation (Landman et al, 2003 Vision Research 43149–164]. Another possibility is that participants used some kind of grouping or Gestalt strategy. To test this we changed the spatial position of the rectangles in the second presentation by shifting them along imaginary spokes (by ±1 degree) emanating from the central fixation point. There was no significant difference seen in performance between this and the standard task [F(1,4)=2.565, p=0.185]. This may suggest two things: (i) Gestalt grouping is not used as a strategy in these tasks, and (ii) it gives further weight to the argument that objects may be stored and retrieved from a pre-attentional store during this task

    Clive Bell's "Significant Form" and the neurobiology of aesthetics.

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    Though first published almost one century ago, and though its premise has been disputed, Clive Bell's essay on aesthetics in his book Art still provides fertile ground for discussing problems in aesthetics, especially as they relate to neuroesthetics. In this essay, I begin with a brief account of Bell's ideas on aesthetics, and describe how they focus on problems of importance to neuroesthetics. I also examine where his premise falls short, and where it provides significant insights, from a neuroesthetic and general neurobiological point of view

    NĂ€gemistaju automaatsete protsesside eksperimentaalne uurimine

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    VĂ€itekirja elektrooniline versioon ei sisalda publikatsiooneVĂ€itekiri keskendub nĂ€gemistaju protsesside eksperimentaalsele uurimisele, mis on suuremal vĂ”i vĂ€hemal mÀÀral automaatsed. Uurimistöös on kasutatud erinevaid eksperimentaalseid katseparadigmasid ja katsestiimuleid ning nii kĂ€itumuslikke- kui ka ajukuvamismeetodeid. Esimesed kolm empiirilist uurimust kĂ€sitlevad liikumisinformatsiooni töötlust, mis on evolutsiooni kĂ€igus kujunenud ĂŒheks olulisemaks baasprotsessiks nĂ€gemistajus. Esmalt huvitas meid, kuidas avastatakse liikuva objekti suunamuutusi, kui samal ajal toimub ka taustal liikumine (Uurimus I). NĂ€gemistaju uurijad on pikka aega arvanud, et liikumist arvutatakse alati mĂ”ne vĂ€lise objekti vĂ”i tausta suhtes. Meie uurimistulemused ei kinnitanud taolise suhtelise liikumise printsiibi paikapidavust ning toetavad pigem seisukohta, et eesmĂ€rkobjekti liikumisinformatsiooni töötlus on automaatne protsess, mis tuvastab silma pĂ”hjas toimuvaid nihkeid, ja taustal toimuv seda eriti ei mĂ”juta. Teise uurimuse tulemused (Uurimus II) nĂ€itasid, et nĂ€gemissĂŒsteem töötleb vĂ€ga edukalt ka seda liikumisinformatsiooni, millele vaatleja teadlikult tĂ€helepanu ei pööra. See tĂ€hendab, et samal ajal, kui inimene on mĂ”ne tĂ€helepanu hĂ”lmava tegevusega ametis, suudab tema aju taustal toimuvaid sĂŒndmusi automaatselt registreerida. IgapĂ€evaselt on inimese nĂ€gemisvĂ€ljas alati palju erinevaid objekte, millel on erinevad omadused, mistĂ”ttu jĂ€rgmiseks huvitas meid (Uurimus III), kuidas ĂŒhe tunnuse (antud juhul vĂ€rvimuutuse) töötlemist mĂ”jutab mĂ”ne teise tunnusega toimuv (antud juhul liikumiskiiruse) muutus. NĂ€itasime, et objekti liikumine parandas sama objekti vĂ€rvimuutuse avastamist, mis viitab, et nende kahe omaduse töötlemine ajus ei ole pĂ€ris eraldiseisev protsess. Samuti tĂ€hendab taoline tulemus, et hoolimata ĂŒhele tunnusele keskendumisest ei suuda inimene ignoreerida teist tĂ€helepanu tĂ”mbavat tunnust (liikumine), mis viitab taas kord automaatsetele töötlusprotsessidele. Neljas uurimus keskendus emotsionaalsete nĂ€ovĂ€ljenduste töötlusele, kuna need kannavad keskkonnas hakkamasaamiseks vajalikke sotsiaalseid signaale, mistĂ”ttu on alust arvata, et nende töötlus on kujunenud suuresti automaatseks protsessiks. NĂ€itasime, et emotsiooni vĂ€ljendavaid nĂ€gusid avastati kiiremini ja kergemini kui neutraalse ilmega nĂ€gusid ning et vihane nĂ€gu tĂ”mbas rohkem tĂ€helepanu kui rÔÔmus (Uurimus IV). VĂ€itekirja viimane osa puudutab visuaalset lahknevusnegatiivsust (ingl Visual Mismatch Negativity ehk vMMN), mis nĂ€itab aju vĂ”imet avastada automaatselt erinevusi enda loodud mudelist ĂŒmbritseva keskkonna kohta. Selle automaatse erinevuse avastamise mehhanismi uurimisse andsid oma panuse nii Uurimus II kui Uurimus IV, mis mĂ”lemad pakuvad vĂ€lja tĂ”endusi vMMN tekkimise kohta eri tingimustel ja katseparadigmades ning ka vajalikke metodoloogilisi tĂ€iendusi. Uurimus V on esimene kogu siiani ilmunud temaatilist teadustööd hĂ”lmav ĂŒlevaateartikkel ja metaanalĂŒĂŒs visuaalsest lahknevusnegatiivsusest psĂŒhhiaatriliste ja neuroloogiliste haiguste korral, mis panustab oluliselt visuaalse lahknevusnegatiivsuse valdkonna arengusse.The research presented and discussed in the thesis is an experimental exploration of processes in visual perception, which all display a considerable amount of automaticity. These processes are targeted from different angles using different experimental paradigms and stimuli, and by measuring both behavioural and brain responses. In the first three empirical studies, the focus is on motion detection that is regarded one of the most basic processes shaped by evolution. Study I investigated how motion information of an object is processed in the presence of background motion. Although it is widely believed that no motion can be perceived without establishing a frame of reference with other objects or motion on the background, our results found no support for relative motion principle. This finding speaks in favour of a simple and automatic process of detecting motion, which is largely insensitive to the surrounding context. Study II shows that the visual system is built to automatically process motion information that is outside of our attentional focus. This means that even if we are concentrating on some task, our brain constantly monitors the surrounding environment. Study III addressed the question of what happens when multiple stimulus qualities (motion and colour) are present and varied, which is the everyday reality of our visual input. We showed that velocity facilitated the detection of colour changes, which suggests that processing motion and colour is not entirely isolated. These results also indicate that it is hard to ignore motion information, and processing it is rather automatically initiated. The fourth empirical study focusses on another example of visual input that is processed in a rather automatic way and carries high survival value – emotional expressions. In Study IV, participants detected emotional facial expressions faster and more easily compared with neutral facial expressions, with a tendency towards more automatic attention to angry faces. In addition, we investigated the emergence of visual mismatch negativity (vMMN) that is one of the most objective and efficient methods for analysing automatic processes in the brain. Study II and Study IV proposed several methodological gains for registering this automatic change-detection mechanism. Study V is an important contribution to the vMMN research field as it is the first comprehensive review and meta-analysis of the vMMN studies in psychiatric and neurological disorders

    Picture recognition in animals and in humans : a review

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    The question of object–picture recognition has received relatively little attention in both human and comparative psychology; a paradoxical situation given the important use of image technology (e.g. slides, digitised pictures) made by neuroscientists in their experimental investigation of visual cognition. The present review examines the relevant literature pertaining to the question of the correspondence between and:or equivalence of real objects and their pictorial representations in animals and humans. Two classes of reactions towards pictures will be considered in turn: acquired responses in picture recognition experiments and spontaneous responses to pictures of biologically relevant objects (e.g. prey or conspecifics). Our survey will lead to the conclusion that humans show evidence of picture recognition from an early age; this recognition is, however, facilitated by prior exposure to pictures. This same exposure or training effect appears also to be necessary in nonhuman primates as well as in other mammals and in birds. Other factors are also identified as playing a role in the acquired responses to pictures: familiarity with and nature of the stimulus objects, presence of motion in the image, etc. Spontaneous and adapted reactions to pictures are a wide phenomenon present in different phyla including invertebrates but in most instances, this phenomenon is more likely to express confusion between objects and pictures than discrimination and active correspondence between the two. Finally, given the nature of a picture (e.g. bi-dimensionality, reduction of cues related to depth), it is suggested that object–picture recognition be envisioned in various levels, with true equivalence being a limited case, rarely observed in the behaviour of animals and even humans

    The form pathways in the visual brain.

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    The perception of visual forms is crucial for humans for successful interactions with the environment. This process occurs automatically, and its outcome is reflected in the inferences and decisions we constantly make. The focus of this thesis is on how the brain handles different aspects of the perception of forms. To study this in normal human individuals, experiments were performed using functional magnetic resonance imaging (fMRI), magnetoencephalography (MEG) and psychophysical methods. This thesis first discusses experiments designed to unravel the mechanisms of form construction, i.e. those from which all the component parts of a single form are assembled. Results suggest that the construction of very simple forms occurs in intermediate visual areas in a parallel and recursive process, with an increase in brain activity with increments in form complexity. A further experiment was performed to study how regularities or known characteristics of images, and the brain responses they elicit, will contribute to explain current percepts. Results from this experiment are consistent with a model where images with learnt attributes activate more strongly anterior visual areas and images with random patterns cause higher activations in earlier visual areas, probably due to top-down signals that reduce activity when it is possible to explain the causes of the sensory stimulation. Finally, it shows differences in the evoked neural activity when forms are either detected or classified, relating these processes to the activity generated in early visual areas. Based on the results of these experiments, a mechanism of top-down and bottom-up interactions between visual areas in the human brain is discussed in the context of the perception of forms

    Progress toward an understanding of cortical computation

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    The additional data, perspectives, questions, and criticisms contributed by the commentaries strengthen our view that local cortical processors coordinate their activity with the context in which it occurs using contextual fields and synchronized population codes. We therefore predict that whereas the specialization of function has been the keynote of this century the coordination of function will be the keynote of the next
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