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ARTICLES: S. M. Bowyer, J. E. Moran, K. M. Mason, J. E. Constantinou, B. J. Smith, G. L. Barkley, and N. Tepley MEG localization of language-specific cortex utilizing MR-FOCUSS Neurology 2004; 62: 2247-2255 [Abstract] [Full text] [PDF]

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Reply to Papanicolaou et al

Susan M. Bowyer, J. E. Moran, K. M. Mason, J. E. Constantinou, B. J. Smith, G. L. Barkley, and N. Tepley   (19 October 2004)

MEG localization of language-specific cortex utilizing MR-FOCUSS

Andrew C Papanicolaou, Panagiotis G. Simos, Eduardo M. Castillo   (19 October 2004)

Reply to Papanicolaou et al 19 October 2004
Susan M. Bowyer, Henry Ford Hospital 2799 West Grand Blvd., Detroit, MI 48202, J. E. Moran, K. M. Mason, J. E. Constantinou, B. J. Smith, G. L. Barkley, and N. Tepley Send Correspondence to journal: Re: Reply to Papanicolaou et al drsusan{at}umich.edu Susan M. Bowyer, et al.	We thank Dr. Papanicolaou et al for recognizing the benefits of our current source density technique over the single equivalent current dipole (ECD) localization of the magnetic fields in the localization of language activity. As they indicated, the ECD has been the primary analytic methodology used in localizing magnetic source data. The ECD imaging technique is simple to use, easy to interpret, and valid when applied to isolated field patterns of a single compact neuronal source. These field patterns are often observed during short time windows of cortical evoked responses and during many epileptic spikes. However, magnetic fields from multiple sources overlap spatially during complex cognitive processes, such as the fields arising during language processing, so that it is not possible to isolate field patterns corresponding to all contributing sources. For imaging complex patterns of brain activity, other techniques including current source density mapping and beamforming are more appropriate than ECD. We agree that the results of these imaging techniques must be validated by comparison to physiological measurements of brain function. We are currently comparing our current density mapping methodology to the Wada test, direct cortical stimulation, and surgical outcomes. For these comparisons, it takes time to develop a sufficient sample size for a published study. Our manuscript is similar to the first papers on language mapping by the Papanicolaou group where results were not validated by comparison to intracranial mapping. Their paper was referenced in our article. [1] Dr. Papanicolaou correctly points out that current density mapping results also depend upon the threshold of activation used for determination of a positive result. This is a problem for our MEG imaging technique when attempting to image very low amplitude magnetic fields relative to the amplitude of included noise. However, all MEG imaging techniques including the ECD have difficulty with low signal-to-noise data. Our expectation is that our ongoing work, comparing MEG and other measures of language localization, will demonstrate the clinical value of our MEG imaging technique for detecting critical language cortex. We regret that the word ?epic? was substituted for ?epoch?. Reference 1. Simos PG, Breier JI, Zouridakis G, Papanicolaou AC. Identification of language-specific brain activity using magnetoencephalography. J Clin Exp Neuropsychol. 1998 Oct;20:706-722.
MEG localization of language-specific cortex utilizing MR-FOCUSS 19 October 2004
Andrew C Papanicolaou, Department of Neurosurgery, University of Texas, Health Science Center-Houston 1333 Moursund, Suite H114, Houston, TX 77030, Panagiotis G. Simos, Eduardo M. Castillo Send Correspondence to journal: Re: MEG localization of language-specific cortex utilizing MR-FOCUSS apapanicolaou{at}uth.tmc.edu Andrew C Papanicolaou, et al.	Bowyer et al present a novel mathematical approach for determining patterns of brain activity associated with performance of language tasks. [1] The software developed by the authors has two main advantages which could, in theory, ensure a level of confidence sufficient for presurgical planning applications. First, it takes into account the anatomy of the brain, constraining potential “sources” of magnetic activity based on physiological and physical considerations. Second, and perhaps most important, the software is designed to operate unsupervised, significantly reducing the impact of subjective input from the user, a limitation characteristic of the “standard” method of the iterative application of the single Equivalent Current Dipole (ECD) that the Bowyer et al method claims to surpass in efficiency. Incidentaly, the ECD method remains the “gold standard” in MEG studies on the cerebral mechanisms of basic sensory functions and of language, because it is the only one that has been validated against invasive brain mapping techniques and postoperative outcome [2-5] yet Bowyer et al do not acknowledge this. As with previous attempts to outline brain activity profiles using alternative mathematical approaches, the results reported by Bowyer et al are both interesting and promising. Clinical applications of MEG will be greatly facilitated by the use of user-independent analysis techniques, provided, however, that the key requirement of external validation of activation maps, that applies to every functional imaging method, is fully met. External validation is particularly crucial for techniques that model activity in terms of spatially extensive sources (like the Bowyer et al. technique does), given the inherent uncertainty surrounding threshold selection for displaying activation images. Adopting different image thresholding criteria may modify the extent of cortical regions that appear active, thereby seriously affecting the utility of the technique for presurgical mapping applications. Hopefully, the method Bowyer et al advocate will soon meet the validation requirement and emerging as a “more sensitive and useful technique” than the “standard” one which has been validated. Yet until this transpires, we believe it is advisable not to confound hope with fact as is, incidentally, also advisable not to confound literary genres (i.e. epic) with segments or durations of time-series (i.e. epochs) that the authors repeatedly do in their text. References 1) Bowyer SM, Moran JE, Mason KM, et al. MEG localization of language-specific cortex utilizing MR-FOCUSS Neurology 2004;62:2247-2255. 2) Breier JI, Simos PG,Papanicolaou AC, et al. Language dominance determined by magnetic source imaging: A comparison with the Wada Procedure. Neurology 1999;22:938-945. 3) Ganslandt O, Fahlbusch R, Nimsky C, et al. Functional neuronavigation with magnetoencephalography: outcome in 50 patients with lesions around the motor cortex. J Neurosurg 1999;91:73-79. 4) Maestu F, Ortiz T, Fernandez A, et al. Spanish language mapping using MEG: a validation study. Neuroimage 2002;17:1579-1586. 5) Papanicolaou AC, Simos PG, Castillo EM, et al. Magnetoencephalography: A non-invasive alternative to the Wada procedure. J. Neurosurg 2004;100:867-876.