Retinotopic maps, spatial tuning, and locations of human visual areas in surface coordinates characterized with multifocal and blocked FMRI designs - PubMed (original) (raw)
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Retinotopic maps, spatial tuning, and locations of human visual areas in surface coordinates characterized with multifocal and blocked FMRI designs
Linda Henriksson et al. PLoS One. 2012.
Abstract
The 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.
Conflict of interest statement
Competing Interests: The authors have declared that no competing interests exist.
Figures
Figure 1. Stimuli for the multifocal and object mapping.
The mapping tools can be obtained from the website
http://ltl.aalto.fi/wiki/RetinotopicMapping
. A) One frame of the 24-region multifocal stimulus. B) In the multifocal mapping, the visual field from 1° to 12° eccentricity was divided to 24 regions, which were stimulated in parallel with a contrast-reversing checkerboard pattern using temporally orthogonal stimulus sequences . The subjects fixated a point in the middle of the stimulus. For a video excerpt of the multifocal stimulus, see Video S1 in the Supporting Information. C) One frame of the object stimulus. D) In the object mapping, nine regions in the visual field (fovea and eight different polar angles at two different eccentricities) were stimulated with object images using a blocked fMRI design. For a video excerpt of the object stimulus, see Video S2 in the Supporting Information.
Figure 2. Representative retinotopic maps obtained with the multifocal and object mapping.
A) Medial view and B) unfolded patch of the subject's right occipital cortical surface show the polar angle map obtained with the 24-region multifocal stimulus. C) Medial view and D) unfolded patch of the subject's right occipital cortical surface show the polar angle map obtained with the 9-region object stimulus. See Supplementary Figures S2, S3, S4, S5 for the polar angle and eccentricity maps from both mapping measurements for all 15 subjects.
Figure 3. Retinotopic organization of the ventral visual cortex.
The top panel shows the retinotopic eccentricity maps and the middle panel the retinotopic polar angle maps obtained with the object mapping for representative subjects S4, S7, and S12. The bottom row shows the activation pattern for the foveal object stimulus. The distinct representations of the fovea in areas VO1 and VO2 are also denoted by asterisks on the eccentricity maps. See Supplementary Figures S6 and S7 for the maps for all subjects and for both hemispheres.
Figure 4. Retinotopic organization of the lateral visual cortex.
The top panel shows the retinotopic eccentricity maps and the middle panel the retinotopic polar angle maps obtained with the object stimuli for representative subjects S5, S6, and S14. The bottom panel shows the cortical areas sensitive to visual motion. See Supplementary Figures S8 and S9 for the maps for all subjects and for both hemispheres.
Figure 5. Group-average eccentricity and polar angle maps.
The retinotopic maps obtained with the object mapping were averaged across the 15 subjects using the cortical surface-based coordinate system. Nodes with data from less than three subjects were omitted from the maps. The visual area borders were defined based on group-averages of the individuals' visual area labels brought into the average surface.
Figure 6. Iso-polar region-of-interest (ROI) analysis of the retinotopic organization.
ROIs were drawn manually along lines of equal polar angle value on the group average retinotopic maps in the left and right hemispheres. The individual multifocal (gray markers) and the object (black markers) data were sampled by the same line ROIs drawn on the average surfaces. The data from the two hemispheres was averaged (UVM = upper vertical meridian; HM = horizontal meridian; LVM = lower vertical meridian). The error bars indicate the standard deviations of the polar angle values averaged across the subjects (N = 15).
Figure 7. Spatial tuning curves.
Representative single voxel tuning curves illustrate differences in the spatial tuning across visual areas in right hemisphere. The mean fMRI % signal changes are plotted as function of the polar angle of the object stimuli. The same data are also shown as polar plots, where the distance from the centre of the circle reflects the response amplitude at each polar angle and the colour codes the weighted average visual field position (see the colour wheel). The gray background highlights the responses for stimuli in the ipsilateral visual field. In the representative voxel within area V1, only the stimuli at the left horizontal meridian evoked a measurable response, whereas a range of stimulus positions produced measurable responses in the representative LO1 voxel. The L values are estimates for the strength of the spatial tuning (Eq. 4).
Figure 8. Polar plots of average spatial tuning curves obtained with the object mapping.
A) Voxels within each visual area in the right hemisphere were classified to eight different classes according to the polar angle they represent. The colour indicates the polar angle. The polar plots illustrate how much each of these classes represents also other visual field positions. For example, in V1 the voxels representing the lower vertical meridian (shown in red) did not respond to stimuli at any other polar angle, whereas the V3d voxels that represented the lower vertical meridian did respond to some amount also to the stimuli at neighbouring locations, and the TO1 voxels that represented the lower vertical meridian responded at some amount to any of the stimuli. The tuning curves were first averaged across voxels within a visual area and then across the subjects. The grey background indicates the hemifield ipsilateral to the studied hemisphere. B) Same as in A for the visual areas in the left hemisphere.
Figure 9. Strength of spatial tuning and amount of ipsilateral responses.
A) The mean strength of the spatial tuning in different visual areas was averaged across the 15 subjects. The error bars indicate the standard errors of the means (SEMs) across the subjects. Visual area had a significant effect on the tuning strength (***p<0.001, Page's L test for the trends). B) The mean amount of ipsilateral responses was defined as the sum of responses for stimuli in the ipsilateral visual field divided by the sum of responses for all of the stimuli. Negative responses were ignored. The results were averaged across the 15 subjects and the error bars indicate the SEMs across the subjects. Visual area had a significant effect on the amount of ipsilateral responses (***p<0.001, Page's L test for the trends).
Figure 10. Cortical maps of spatial tuning strength.
Lateral and ventral views of spatial tuning strength maps for two representative subjects (S4, S5) and a group-averaged tuning strength map. See Supplementary Figure S10 for data for all subjects and for both hemispheres.
Figure 11. Asymmetries in visual field representations.
The mean numbers of voxels that represented the lower (dark gray bars) and the upper (light gray bars) visual fields in different visual areas were averaged across the 15 subjects. The error bars indicate the SEMs across the subjects. **p<0.005, Wilcoxon Signed Rank-test; n.s., not significant (p>0.05, Wilcoxon Signed Rank-test).
Figure 12. A comparison of calculating the midpoint of an area in volumetric or spherical coordinate system.
A) An illustration of the clustering of the mean locations of area V1 for 15 subjects on the average cortical surface when the mean location of area V1 was calculated for each subject in the volumetric (cartesian) coordinate system. Note the spread of the points along the lips of the calcarine sulcus (CS = Calcarine Sulcus; POS = Parieto-Occipital Sulcus; IPS = Intra-Parietal Sulcus). B) An illustration of the clustering of the mean locations of area V1 for the 15 subjects on the average cortical surface when the mean location of area V1 was calculated for each subject along the cortical surface-based spherical coordinate system. Note the clustering of the points at the base of the calcarine sulcus. C) The mean locations of several visual areas for 15 subjects calculated either in the volumetric (left panel) or spherical (right panel) coordinate system. D) The group-average mean locations of the visual areas in left and right hemispheres. The average locations calculated in the volumetric coordinate system are marked with coloured squares and the white circles around the squares show the mean distance of the individual mean locations to the group average. The average locations calculated in the spherical coordinate system are marked with coloured circles and the mean distances with the black circles.
Figure 13. The mean locations of retinotopic visual areas on spherical cortical surface.
The mean locations of several visual areas (see Table 1 for the coordinates) were calculated along the spherical cortical surface and are shown on the average spherical cortical surface of the left and right hemispheres of the Freesurfer's surface-based atlas (CS = Calcarine Sulcus; POS = Parieto-Occipital Sulcus; IPS = Intra-Parietal Sulcus). The black circles represent the average standard distances of the individuals' visual area locations from their mean. In addition to the retinotopic visual areas localized with the object mapping, the mean locations of visual area V5 is shown in both hemispheres with black crosses and the average standard distances with dashed circles.
Figure 14. Surface-based probabilistic maps of the visual areas on Freesurfer's surface-based atlas of human cerebral cortex.
A) Spatial probability maps of visual areas V1, V2d, and V3d on the average cortical surface. B) Maximum probability atlas of visual areas. The probabilities of different visual areas are shown for representative vertices as examples from the surface-based probabilistic atlas which can be obtained from the website
http://ltl.aalto.fi/wiki/Atlas
. C) Iso-polar line ROI analysis of the progression of the visual area probabilities along the cortical surface.
References
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- Sereno MI, Dale AM, Reppas JB, Kwong KK, Belliveau JW, et al. Borders of multiple visual areas in humans revealed by functional magnetic resonance imaging. Science. 1995;268:889–893. -PubMed
- Warnking J, Dojat M, Guerin-Dugue A, Delon-Martin C, Olympieff S, et al. fMRI retinotopic mapping–step by step. NeuroImage. 2002;17:1665–1683. -PubMed
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