Ted Kelly

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Multi Voxel Pattern Analysis in the Ventral Temporal Cortex

Recent Studies have shown that there are clearly defined regions within the ventral temporal cortex that preferentially respond to faces (fusiform face area), places (Parahippocampal Place Area) and objects. In particular, the FFA has been shown to undergo development from childhood throughout adolescence. This study uses multi voxel pattern analysis to confirm both the location and the growth of the FFA in the VTC.

Background

Multi Voxel Pattern Analysis vs. ROI analysis

Most of the current research on the VTC and the FFA has focused on region of interest (ROI) analyses. This analysis focuses on the highest activations of the voxels selective for faces. Usually, activations are thresholded so that only voxels responding over a certain level are looked at. Supporters of this type of analysis say that the highest activations are the most interesting and provide the clearest evidence for the age related growth of the FFA (in volume and in more well-defined areas selective for faces vs. non-face stimuli). However, multi voxel pattern analysis (MVPA) supporters insist that there is much more information in all the activations in the VTC, not just the highest ones. They say that the activations are distributed throughout the entire VTC and that the aforementioned growth of the FFA is actually just growth of the entire VTC. This study aims to use MVPA to support the initial analyses provided by ROIs.

Methods

Subjects

Multi-Voxel Pattern Analysis was done on 14 adolescents (ages 12-16yrs) and 11 adults (ages 18-40yrs)

MR Acquisition

Brain imaging was performed on a 3 tesla whole-body General Electric Signa MRI scanner. During fMRI subjects viewed gray-scale images of the following types: faces of male children and adolescents (ages 6–16 years), faces of male adults (ages 18–40 years), abstract sculptures, cars, indoor scenes, outdoor scenes, and scrambled images (created by randomly scrambling pictures into 225, 8 × 8 pixel squares). Stimuli were presented in 12 s blocks followed by 12 s of a blank screen with a fixation at a rate of 1 Hz. Subjects participated in two 396-s runs with different images.

MR Analysis

Below is an example of an initial MVPA analysis on the VTC. The MVP reflects differences in category selectivity in each voxel, rather than amplitude differences across voxels. For each stimulus type, MVPs were generated across the anatomical ROI of VTC, separately for data from run 1 and run 2. Corresponding graphs reflect the within-category reproducibility and across-category distinctness of MVPs to determine if there are between age group differences that extend beyond functionally defined ROIs.

Figure 1

From these initial MVP results we decided to create new ROIs by dividing the VTC in two. We created lateral and medial VTC ROIs for each subject using MrVista. We used the middle fusiform sulcus as the anatomical landmark to divide our ROIs.

Pre-processing

No thresholding or spatial smoothing was applied to the MVPA data. Using Z-scores to calculate correlations minimizes between-voxel effects

Results

Lateral VTC MVPs

The Lateral VTC MVPs show good reproducibility of patterns within category and show activation in the lateral part of the lateral VTC in response to faces where previous ROI analysis has shown the FFA is located. These MVPs are of a 14 yr old boy.

Figure 1

Lateral VTC correlation graphs

These graphs confirm the good reproducibility within category for all subjects both adolescents and adults. Reproducibility is better for faces and scenes than objects. Between category correlations suggest that there are no age related differences in age of face effects. Adults have better ability to distinguish between faces and cars, faces and absobj and faces and scenes. This also means that the areas of activation are more distinct in adults vs adolescents.

Figure 1

Medial VTC MVPs

Medial VTC scans show good reproducibility but no above average activation for faces. Highest activation in medial MVPs is for places and absobj> faces. MVPs provide further evidence that FFA is not in the medial side of the VTC.

Figure 1

Medial VTC correlation graphs

Medial VTC graphs again show good within category reproducibility better for faces and places than objects. There are no age related differences in age of face stimuli effects. Adult MVPs still show greater differentiation between faces and cars and faces and abstract objects although this differentiation is not as significant as in the lateral side of the VTC. Adolescents show less differentiation in the medial side as well for faces vs cars, faces vs abstract objects.

Figure 1

Conclusion

MVPs on both medial and lateral ROIs provide further evidence that previous ROI analyses has been correct in assuming that the FFA is located in the lateral part of the VTC. Adult MVPs show better distinctness for faces vs non-face stimuli than adolescents in both sides of the VTC although this distinctness is more pronounced and statistically significant in the lateral VTC. It appears as though the ROI hypothesis is holding true but we cannot rule out the pattern analysis hypothesis quite yet. More importantly, using pattern analysis in tandem with ROI analysis provides a more thorough investigation of the areas in question and neither technique should be ruled out when trying to localize novel brain regions.

ROIs may need to be redrawn due to the activation of the PPA in the medial part of the lateral VTC which we should not see. This could be accomplished by using the MVPs to redraw the division between the lateral and medial VTC in all the ROIs or the PPA could be subtracted out of the lateral VTC ROIs possibly saving time.

Implications

Using the evidence obtained in this analyses, further analyses is needed of the lateral part of VTC to better define location and growth of the FFA. One idea would be to create an ROI with further divisions in the lateral VTC and subtract out the highest activations in the lateral VTC (what we think is the FFA area) to look at activations in rest of the lateral VTC without them. Running a multi voxel pattern analyses on the lateral VTC without the FFA could reveal new findings. At some point we want to correlate findings to behavior outside the scanner (increased memory for faces, perceptual discrimination for faces)

References - Resources and related work

References

Golarai G, Liberman A, Yoon JM and Grill-Spector K (2010) Differential development of the ventral visual cortex extends through adolescence. Front. Hum. Neurosci. 3:80. doi:10.3389/neuro.09.080.2009