P161 Robot-guided approach to the correlation of TMS intensity and focality in man.

Introduction Strength and extension of the TMS-induced electromagnetic field increase with stimulation intensity at the expense of spatial accuracy.The concept of focality has not been investigated systematically in vivo yet. Objective To determine the focality of a given TMS coil we developed a linear mapping approach that assesses the correlation of intensity and spatial accuracy. Methods A line of 21 targets was placed in anterior–posterior direction through the center of gravity (COG) of the FDI hotspot with 2 mm spacing ( N = 8, Magstim 200 2 , 70 mm figure-8 coil).Recruitment curves were measured at each target with 15 pulses per intensity (threshold intensity + 1, 2 and 3% of maximum stimulator output (MSO)) with a randomized order over all targets and intensities.The probability was calculated as the ratio of pulses resulting in an MEP ⩾ 50 μV to the total number of pulses for each stimulation point and intensity.The intensities were categorized by the lowest intensity with a probability of 1 ( P 1) and lower ( P 1-1%, P 1-2% and P 1-3%).The bilateral data was centered on the COG, a logistic function ( P = A /(1+( x / x 0 ) p ) was fitted to the unilateral data to describe the relation of coil position, stimulation intensity and probability (MEP ⩾ 50 μV). The extent of FDI representation in relation to intensity was determined as two times the difference between the COG and the minimum of the 2nd derivative.Additionally, we calculated the slope width between the minimum and maximum of the 2nd derivative to determine the range at which the effective electric field leaves the hotspot, resulting in the highest decrease of probability.The extent of FDI and the slope width were plotted against the intensities to describe their correlation. Results A sigmoid relation between response rate and stimulation intensity was observed for all intensities.The plateau width (FDI extent) increased linear by 6.4 ± 1.3 mm/%MSO. In contrast, the slope width was 6.1 ± 0.2 mm across all intensities. Conclusion The results prove a linear relation of near threshold TMS intensities and focality with a decrease of spatial accuracy by 6.5 mm/%MSO when using a standard figure-8 coil.By calculating the slope widths of the intensity curves (6.1 mm) we propose a novel approach to determine the extent of a motor cortical representation that is not influenced by intensity. [ABSTRACT FROM AUTHOR]