On the geometric structure of fMRI searchlight-based information maps

Information mapping is a popular application of Multivoxel Pattern Analysis (MVPA) to fMRI. Information maps are constructed using the so called searchlight method, where the spherical multivoxel neighborhood of every voxel (i.e., a searchlight) in the brain is evaluated for the presence of task-relevant response patterns. Despite their widespread use, information maps present several challenges for interpretation. One such challenge has to do with inferring the size and shape of a multivoxel pattern from its signature on the information map. To address this issue, we formally examined the geometric basis of this mapping relationship. Based on geometric considerations, we show how and why small patterns (i.e., having smaller spatial extents) can produce a larger signature on the information map as compared to large patterns, independent of the size of the searchlight radius. Furthermore, we show that the number of informative searchlights over the brain increase as a function of searchlight radius, even in the complete absence of any multivariate response patterns. These properties are unrelated to the statistical capabilities of the pattern-analysis algorithms used but are obligatory geometric properties arising from using the searchlight procedure.Comment: 15 pages, 7 figure

Pallidotomy in advanced Parkinson's disease

Journal ArticleTo the Editor: We read with interest the comments published regarding our recent article on pallidotomy in patients with advanced Parkinson's disease (3). There seems to be confusion as to some of the technical methods used in creating the radiofrequency lesions in these patients. As noted in the Patients and Methods section, multiple lesions were created by moving the probe to achieve the ultimate overall lesion. The short radiofrequency times noted in the comments are used in performing thalamotomy by our group to titrate the size of the lesion to the clinical response. However, the lesions created in the medial pallidum in the present series of patients were large (lesioning times, 45-60 s for each probe position), as also noted by the dimensions of the lesions revealed by the follow-up MRI studies. The size and position of the lesions should target the globus pallidus internus and certainly some of the globus pallidus externus as well, given the relative size and proximity of these targets (see Plate 27 [coronal] and Plate 45 [sagittal] of the Atlas for Stereotaxy of the Human Brain [2)