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. 2013 Jul 15:75:228-237.
doi: 10.1016/j.neuroimage.2013.02.073. Epub 2013 Mar 16.

Differential connectivity within the Parahippocampal Place Area

Christopher Baldassano  1 , Diane M Beck  2 , Li Fei-Fei  3
Affiliations

Differential connectivity within the Parahippocampal Place Area

Christopher Baldassano et al. Neuroimage. .

Abstract

The Parahippocampal Place Area (PPA) has traditionally been considered a homogeneous region of interest, but recent evidence from both human studies and animal models has suggested that PPA may be composed of functionally distinct subunits. To investigate this hypothesis, we utilize a functional connectivity measure for fMRI that can estimate connectivity differences at the voxel level. Applying this method to whole-brain data from two experiments, we provide the first direct evidence that anterior and posterior PPA exhibit distinct connectivity patterns, with anterior PPA more strongly connected to regions in the default mode network (including the parieto-medial temporal pathway) and posterior PPA more strongly connected to occipital visual regions. We show that object sensitivity in PPA also has an anterior-posterior gradient, with stronger responses to abstract objects in posterior PPA. These findings cast doubt on the traditional view of PPA as a single coherent region, and suggest that PPA is composed of one subregion specialized for the processing of low-level visual features and object shape, and a separate subregion more involved in memory and scene context.

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Figures

Figure 1
Figure 1. Sample stimuli used in our experiments
(a) Scene and object stimuli from the localizer experiment, which also included faces and scrambled objects. (b) Isolated object and object-in-scene stimuli from the object-in-scene experiment. (c) Beach and mountain stimuli from the scene category experiment, which also included cities and highways.
Figure 2
Figure 2. A comparison of the learned PPA weightmaps and the overall connectivity strength, for our four ROIs
(a) The timecourses of all four seed ROIs are better explained by a regularized voxel-level connectivity map in PPA, rather than a single connectivity weight for all of left and right PPA. Activity in LOC, TOS, and RSC is most closely related to PPA activity, while only a smaller amount of the cIPL timecourse is related to PPA activity. (b) To obtain a simple characterization of the learned maps, we compute the correlation between the connectivity weights and the anterior-posterior axis. This measure shows consistent differences between the four regions’ connectivity maps. LOC and TOS are preferentially connected to posterior PPA (since their corresponding PPA weightmaps increase along the anterior to posterior axis) while RSC and cIPL are preferentially connected to anterior PPA. Error bars represent s.e.m. across subjects, * p < 0.05,** p < 0.01.
Figure 3
Figure 3. Searchlight connectivity results
(a) Rendering of the group connectivity bias map on the left hemisphere of the Talairach 452 brain. Colored voxels are those that showed highly significant (FDR < 0.01, cluster size > 300mm3) bias in anterior-posterior connectivity to PPA, computed as the correlation between the learned PPA connectivity map and the anterior-posterior axis. Bilateral areas RSC and cIPL, as well as ventral PFC and lateral anterior temporal regions, exhibited connectivity with anterior PPA (blue voxels), while occipital visual areas (including LOC and TOS) exhibited connectivity with posterior PPA (orange-yellow voxels). The borders of the group ROIs are shown for reference (outlining the location where at least 3 subjects’ ROIs overlap). (b–d) The same connectivity map on an inflated surface and cortical flatmap.
Figure 4
Figure 4. Three slices of the group connectivity bias map
Seed voxels for which the PPA connectivity map has a strong anterior-posterior gradient (FDR < 0.01, cluster size > 300mm3) are shown in blue (preferential connectivity to anterior PPA) and yellow (preferential connectivity to posterior PPA). (a) In this coronal slice (y=−73mm), we identify bilateral cIPL regions that show a different connectivity pattern from adjacent area TOS. (b) At z=10mm, we observe anterior PPA connectivity in RSC, as well as posterior PPA connectivity in TOS and early visual visual areas. (c) At z=−5mm, ventral occipital areas including LOC show connectivity to posterior PPA. Additionally, anterior PPA connectivity can be seen in the frontal and anterior temporal lobes.
Figure 5
Figure 5. Functional gradients across PPA
The proportion of voxels responsive to scene and object stimuli, and the average t-statistic for the response to scene and object stimuli, were calculated in 10 bins along the anterior-posterior axis in each subject. The dotted line indicates the average t-statistic value corresponding to FDR=0.05 (across all subjects, for both stimulus categories). Scene sensitivity decreased from posterior to anterior PPA, but nearly all voxels across PPA responded significantly to scene stimuli. Object sensitivity substantially decreased from posterior to anterior PPA, with the majority of anterior PPA voxels failing to respond significantly to object stimuli. Error bars represent s.e.m. across subjects.
Figure 6
Figure 6. Regions throughout cortex showing connectivity differences similar to anterior and posterior PPA
In this sagittal slice (x=−26), colored voxels are those showing significantly (FDR < 0.05, cluster size > 1000mm3) different connectivity to LOC and TOS versus RSC and cIPL. The connectivity pattern in anterior PPA extends anteriorly along the parahippocampal gyrus and into the hippocampus. The connectivity patterns over the entire surface are shown in Supplementary Fig. 6.
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