Browsing by Author "Vanni, Simo"
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- Contextual modulation is related to efficiency in a spiking network model of visual cortex
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2016-01-19) Sharifian, Fariba; Heikkinen, Hanna; Vigário, Ricardo; Vanni, SimoIn the visual cortex, stimuli outside the classical receptive field (CRF) modulate the neural firing rate, without driving the neuron by themselves. In the primary visual cortex (V1), such contextual modulation can be parametrized with an area summation function (ASF): increasing stimulus size causes first an increase and then a decrease of firing rate before reaching an asymptote. Earlier work has reported increase of sparseness when CRF stimulation is extended to its surroundings. However, there has been no clear connection between the ASF and network efficiency. Here we aimed to investigate possible link between ASF and network efficiency. In this study, we simulated the responses of a biomimetic spiking neural network model of the visual cortex to a set of natural images. We varied the network parameters, and compared the V1 excitatory neuron spike responses to the corresponding responses predicted from earlier single neuron data from primate visual cortex. The network efficiency was quantified with firing rate (which has direct association to neural energy consumption), entropy per spike and population sparseness. All three measures together provided a clear association between the network efficiency and the ASF. The association was clear when varying the horizontal connectivity within V1, which influenced both the efficiency and the distance to ASF, DAS. Given the limitations of our biophysical model, this association is qualitative, but nevertheless suggests that an ASF-like receptive field structure can cause efficient population response. - Dynamic retrospective filtering of physiological noise in BOLD fMRI: DRIFTER
School of Electrical Engineering | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2012) Särkkä, Simo; Solin, Arno; Nummenmaa, Aapo; Vehtari, Aki; Auranen, Toni; Vanni, Simo; Lin, Fa-HsuanIn this article we introduce the DRIFTER algorithm, which is a new model based Bayesian method for retrospective elimination of physiological noise from functional magnetic resonance imaging (fMRI) data. In the method, we first estimate the frequency trajectories of the physiological signals with the interacting multiple models (IMM) filter algorithm. The frequency trajectories can be estimated from external reference signals, or if the temporal resolution is high enough, from the fMRI data. The estimated frequency trajectories are then used in a state space model in combination of a Kalman filter (KF) and Rauch–Tung–Striebel (RTS) smoother, which separates the signal into an activation related cleaned signal, physiological noise, and white measurement noise components. Using experimental data, we show that the method outperforms the RETROICOR algorithm if the shape and amplitude of the physiological signals change over time. - Fovea-Periphery Axis Symmetry of Surround Modulation in the Human Visual System
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2013) Nurminen, Lauri; Kilpelainen, Markku; Vanni, SimoA visual stimulus activates different sized cortical area depending on eccentricity of the stimulus. Here, our aim is to understand whether the visual field size of a stimulus or cortical size of the corresponding representation determines how strongly it interacts with other stimuli. We measured surround modulation of blood-oxygenation-level-dependent signal and perceived contrast with surrounds that extended either towards the periphery or the fovea from a center stimulus, centered at 6° eccentricity. This design compares the effects of two surrounds which are identical in visual field size, but differ in the sizes of their cortical representations. The surrounds produced equally strong suppression, which suggests that visual field size of the surround determines suppression strength. A modeled population of neuronal responses, in which all the parameters were experimentally fixed, captured the pattern of results both in psychophysics and functional magnetic resonance imaging. Although the fovea-periphery anisotropy affects nearly all aspects of spatial vision, our results suggest that in surround modulation the visual system compensates for it. - Mapping the retinotopy of object recognition areas with functional magnetic resonance imaging
School of Science | Master's thesis(2010) Karvonen, Juha MikaelThe human brain has several areas devoted to visual processing, and many of these areas are organized retinotopically, meaning that locations close to each other in the visual field are represented close to each other in the retinotopic area. Currently the most popular method of retinotopic mapping is the so-called traveling wave method, which applies the Fourier transform to functional magnetic resonance imaging (fMRI) data. Recent research using the method has shown that also the visual object processing areas are retinotopically organized. However, the traveling wave method is complex to use and requires special software. Multifocal fMRI (mffMRI) is a simpler retinotopic mapping method that is based on the general linear model and it has been successfully used to map the retinotopy of the visual areas V1, V2, V3 and V3A/B. This thesis presents a method that is similar to mffMRI and is able to map the retinotopy of the object processing areas. In the multifocal method a circle of 12_ in radius (with respect to the center of gaze) is divided into three rings and eight wedges, giving a total of 24 distinct regions. A region is active when it is displaying a flickering checkerboard pattern, and approximately half of the regions are active at a time. The pattern of active regions is designed so that there are approximately no correlations between the regions, making it easy to analyze the contribution of each active region using the general linear model. The earlier multifocal method was not able to generate enough observable activity in the object areas for retinotopic mapping purposes, and there are several likely reasons. First, the multifocal method uses flickering checkerboards, while the object processing areas sensitive to more complex stimuli such as images of plants, animals and manmade objects. Second, several visual field regions are stimulated simultaneously in the multifocal method, which may cause the stimuli to suppress the activity caused by one another. Third, the object processing areas are sensitive to relative large portions of the visual field and the visual field regions in the multifocal method have probably been too small. The new method addresses these problems. The new method has fewer visual field regions and uses images of man-made objects. It also uses a blocked design, meaning that no two stimuli are displayed simultaneously. These changes, along with some other smaller ones, enabled the new method to activate the object processing areas so that they may be retinotopically mapped. The method is easy enough to use for non-experts, and the mathematical tools for the analysis are readily available in fMRI data analysis packages. The new method has also already been adopted into use in the host laboratory. - Radial Frequency Analysis of Contour Shapes in the Visual Cortex
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2016-02-01) Salmela, Viljami R.; Henriksson, Linda; Vanni, SimoCumulative psychophysical evidence suggests that the shape of closed contours is analysed by means of their radial frequency components (RFC). However, neurophysiological evidence for RFC-based representations is still missing. We investigated the representation of radial frequency in the human visual cortex with functional magnetic resonance imaging. We parametrically varied the radial frequency, amplitude and local curvature of contour shapes. The stimuli evoked clear responses across visual areas in the univariate analysis, but the response magnitude did not depend on radial frequency or local curvature. Searchlight-based, multivariate representational similarity analysis revealed RFC specific response patterns in areas V2d, V3d, V3AB, and IPS0. Interestingly, RFC-specific representations were not found in hV4 or LO, traditionally associated with visual shape analysis. The modulation amplitude of the shapes did not affect the responses in any visual area. Local curvature, SF-spectrum and contrast energy related representations were found across visual areas but without similar specificity for visual area that was found for RFC. The results suggest that the radial frequency of a closed contour is one of the cortical shape analysis dimensions, represented in the early and mid-level visual areas. - Retinotopic Maps, Spatial Tuning, and Locations of Human Visual Areas in Surface Coordinates Characterized with Multifocal and Blocked fMRI Designs
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2012) Henriksson, Linda; Karvonen, Juha; Salminen-Vaparanta, Niina; Railo, Henry; Vanni, SimoThe localization of visual areas in the human cortex is typically based on mapping the retinotopic organization with functional magnetic resonance imaging (fMRI). The most common approach is to encode the response phase for a slowly moving visual stimulus and to present the result on an individual's reconstructed cortical surface. The main aims of this study were to develop complementary general linear model (GLM)-based retinotopic mapping methods and to characterize the inter-individual variability of the visual area positions on the cortical surface. We studied 15 subjects with two methods: a 24-region multifocal checkerboard stimulus and a blocked presentation of object stimuli at different visual field locations. The retinotopic maps were based on weighted averaging of the GLM parameter estimates for the stimulus regions. In addition to localizing visual areas, both methods could be used to localize multiple retinotopic regions-of-interest. The two methods yielded consistent retinotopic maps in the visual areas V1, V2, V3, hV4, and V3AB. In the higher-level areas IPS0, VO1, LO1, LO2, TO1, and TO2, retinotopy could only be mapped with the blocked stimulus presentation. The gradual widening of spatial tuning and an increase in the responses to stimuli in the ipsilateral visual field along the hierarchy of visual areas likely reflected the increase in the average receptive field size. Finally, after registration to Freesurfer's surface-based atlas of the human cerebral cortex, we calculated the mean and variability of the visual area positions in the spherical surface-based coordinate system and generated probability maps of the visual areas on the average cortical surface. The inter-individual variability in the area locations decreased when the midpoints were calculated along the spherical cortical surface compared with volumetric coordinates. These results can facilitate both analysis of individual functional anatomy and comparisons of visual cortex topology across studies. - Topographical and Quantitative Characterization of Visual Field Representation in Human Cortex with Multifocal fMRI
Helsinki University of Technology | Master's thesis(2004) Henriksson, LindaWith multifocal technique, responses from several visual field or retinal locations are characterized concurrently. Multifocal technique is used in electroretinograms and visually evoked potentials. In this thesis, multifocal fMRI (mffMRI) and tools for multifocal data analysis have been developed. In addition, human cortex has been analysed with mffMRI. Several visual cortical areas are organized retinotopically, i.e. neighbouring locations in visual field are mapped next to each other also in the cortex. Thus far, the retinotopic organization of visual cortex has been mapped in fMRI with so-called phase-encoded approach where the borders between visual areas can be mapped accurately with Fourier analysis. MffMRI responses are discrete, so compared to the phase-encoded response, the data-analysis is more straightforward and visual field representation is accurate also within visual areas. With our multifocal stimulus, visual field (1°-12°) was divided into 60 regions. Six subjects participated in the study. As reference, retinotopy was also mapped with phase-encoded approach. The results from mffMRI are promising; for all the subjects all 60 visual field regions evoked strong responses in VI and a subset also in V2 and V3. The statistical analysis of multifocal data was done with SPM2 Matlab toolbox and surface-oriented analysis with BALC toolbox. To illustrate the anisotropies in the visual cortex, an mffMRI grid was implemented. In addition, a function for the visualization of the retinotopic organization over an anatomical image has been developed and individual visual field representation in the cortex has been analysed with a mathematical model. MffMRI has potential for wide use in basic research and as clinical tool. One future aim is to develop multifocal MEG paradigm and to use mffMRI results as a priori models in MEG-analysis. For clinical use, it is essential that the statistical analysis of multifocal data has been automated. MffMRI provides rapid and accurate, e.g. preoperative, localization of VI, and objective perimetry to detect small lesions in retinocortical pathway. In addition, plastic changes of visual cortex could be followed with mffMRI. - Virtual Column Analysis: A novel technique for analyzing functional magnetic resonance imaging data
Sähkötekniikan korkeakoulu | Master's thesis(2015-03-30) Tolvanen, Tuomas