DOI: 10.5061/dryad.qz612jm9x
title: Data from: Perturbation-evoked cortical responses associated with balance ability

description: Background: Reactive balance recovery evokes a negative peak of cortical
 electroencephalography (EEG) activity (N1) that is simultaneous to
 brainstem-mediated automatic balance-correcting muscle activity. This
 study follows up on an observation from a previous study, in which N1
 responses were larger in individuals who seemed to have greater difficulty
 responding to support-surface perturbations. Research Question: We
 hypothesized that people engage more cortical activity when balance
 recovery is more challenging. We predicted that people with lower balance
 ability would exhibit larger cortical N1 responses during balance
 perturbations. Methods: In 20 healthy young adults (11 female, ages 19-38)
 we measured the amplitude of the cortical N1 response evoked by 48
 backward translational support-surface perturbations of unpredictable
 timing and amplitude. Perturbations included a Small (8 cm) perturbation
 that was identical across participants, as well as Medium (13-15 cm) and
 Large (18-22 cm) perturbations scaled to participant height to control for
 height-related differences in perturbation difficulty. To assess
 individual differences in balance ability, we measured the distance
 traversed on a narrow (0.5-inch wide) 12-foot beam across 6 trials. We
 tested whether the cortical N1 response amplitude was correlated to
 balance ability across participants. Results: Cortical N1 amplitudes in
 response to standing balance perturbations (54±18 μV) were inversely
 correlated to the distance traveled in the difficult beam-walking task
 (R2=0.20, p=0.029). Further, there was a significant interaction between
 performance on the beam-walking task and the effect of perturbation
 magnitude on the cortical N1 response amplitude, whereby individuals who
 performed worse on the beam-walking task had greater increases in N1
 amplitudes with increases in perturbation magnitude. Significance:
 Cortical N1 response amplitudes may reflect greater cortical involvement
 in balance recovery when challenged. This increased cortical involvement
 may reflect cognitive processes such as greater perceived threat or
 attention to balance, which have the potential to influence subsequent
 motor control.   Note: This data set has also been
 appended to include additional data from the same data collection in the
 same participant cohort associated with the manuscript "Cortical beta
 oscillatory activity evoked during reactive balance recovery scales with
 perturbation difficulty and individual balance ability" submitted to
 NeuroImage
description: <b>Participants.
 </b>We recruited 20 young adults (11 female, ages 19-38) for a
 research study approved by Emory University’s Institutional Review Board.
 All participants reported no significant history of neurological or
 musculoskeletal disorders. Participants signed written informed consent
 before participation. Participants were 26 years old (SD 5), 168 cm tall
 (SD 8, range 156-185 cm), and 70 kg (SD 14).   <b>Perturbations.
 </b>Participants were given 48 backward translational
 support-surface perturbations of unpredictable timing and amplitude while
 barefoot (Figure 1A). Perturbations were delivered using a custom
 perturbation platform (Factory Automation Systems, Atlanta, GA).
 Perturbations were evenly divided between Small, Medium, and Large
 magnitudes to vary difficulty and maintain unpredictability of magnitude.
 The Small perturbation (7.7 cm, 16.0 cm/s, 0.23 g) was identical across
 participants. To control for height-related differences in perturbation
 difficulty, the Medium (12.6-15.0 cm, 26.6-31.5 cm/s, 0.38-0.45 g) and
 Large (18.4-21.9 cm, 38.7-42.3 cm/s, 0.54-0.64 g) perturbations were
 linearly scaled down from an upper bound (Medium: 15.8 cm, 34.1 cm/s, 0.49
 g; Large: 23.7 cm, 45.8 cm/s, 0.69 g) by multiplying by the participant’s
 height divided by 200 cm. The timing and duration of acceleration,
 velocity, and displacements were identical across perturbation magnitudes
 for the first 500 ms of perturbation (Figure 2). Participants were asked
 to execute a stepping response on half of perturbations and to resist
 stepping on the other half. Stepping and non-stepping responses are
 combined for analyses, except where explicitly stated for kinematic
 comparisons. <b>Electroencephalography
 (EEG) collection. </b>EEG data were collected during the
 perturbation series, and recording equipment was removed before the
 beam-walking task. Thirty-two active EEG electrodes (ActiCAP, Brain
 Products, Germany) were placed on the scalp according to the international
 10-20 system. Electrodes TP9 and TP10 were removed from standard locations
 and placed on the skin over the mastoid bones behind the ears for offline
 re-referencing. After the wired electrode cap was placed on the
 participant, the active electrode sites were prepared with conductive
 electrode gel (SuperVisc 100 gr. HighViscosity Electrolyte-Gel for active
 electrodes, Brain Products) using a blunt-tipped needle, which was
 simultaneously used to rub the scalp to improve electrode impedance.
 Mastoid sites were additionally scrubbed with an alcohol swab prior to
 placement. Impedances below 10 kOhm were obtained for Cz and mastoid
 electrodes before the start of data collection. To enable subtraction of eye
 movement and blink artifacts, electrooculography (EOG) data were collected
 with bipolar passive electrodes (E220x, Brain Products) vertically
 bisecting the right pupil with a reference electrode on the forehead.
 Before electrode placement, the skin was scrubbed with an alcohol swab,
 and electrodes were prepared with high-chloride abrasive gel (ABRALYT HiCl
 250 gr., High-chloride-10% abrasive electrolyte gel, Brain Products). EEG
 and EOG data were amplified on an ActiCHamp amplifier (Brain Products)
 sampling at 1000 Hz, with a 24-bit A/D converter and an online 20 kHz
 anti-aliasing low-pass filter. <b>EEG data
 preprocessing.</b> Raw EEG data were high-pass filtered offline at 1
 Hz with a third-order zero-lag Butterworth filter, mean-subtracted within
 channels, and then low-pass filtered at 25 Hz. Cz data were re-referenced
 to mastoids and epoched into 2.4 s segments beginning 400 ms before
 perturbation onset. Vertical EOG data were similarly filtered and
 segmented without re-referencing. Blinks and vertical eye movement
 artifacts were subtracted from the epoched data at Cz using the algorithm
 developed by Gratton and Coles, as described in Payne et al.. Single-trial
 epochs of Cz data were then baseline-corrected by subtracting the mean
 between 50-150 ms prior to perturbation onset. <b>EEG quantification.
 </b>Epoched cortical responses were averaged within each participant
 across trials, both within and across perturbation magnitudes. Cortical N1
 peak response amplitudes (μV) and latencies (ms) were then measured
 between 100-200 ms after perturbation onset in the averaged cortical
 responses. <b>Center of mass (CoM)
 position and trunk angle. </b>A 10-camera Vicon Nexus 2 motion
 capture system recorded body motion at 100 Hz during perturbations.
 Participants wore a reflective 25-marker set that enabled Vicon’s Plug-in
 Gait model to calculate positions and masses of the following body
 segments: head-arms-trunk, and bilateral thigh and shank-foot. CoM
 position was then calculated as a weighted sum of body segment positions
 and masses. CoM positions were baseline subtracted (-50 to 150 ms
 baseline) to obtain CoM displacements. Trunk angles relative to the
 vertical were calculated using a vector from the average position of hip
 markers to the average position of shoulder markers. One participant was
 excluded from CoM position and trunk angle calculations due to a missing
 marker that prevented calculation of one of the body segments
 (N=19).             <b>Quantification of CoM
 displacement. </b>CoM displacement along the axis of platform motion
 was averaged across non-stepping responses to Small perturbations and
 quantified as peak amplitude between 1-2 s after perturbation for each
 participant.             <b>Quantification of
 trunk angle. </b>Trunk angles were averaged across non-stepping
 responses to Large perturbations and quantified as the peak between 0.5-1
 s after perturbation. <b>Beam-walking task.
 </b>Balance ability was assessed after completion of the
 perturbation series. Participants were given 6 attempts to walk across a
 narrow beam (12 feet long, 0.5 inch wide, 1 inch high) while wearing
 standardized shoes and keeping their arms crossed (Figure 1B).
 Participants were not given instructions regarding speed or step length.
 Each trial ended when the participant (1) reached the end of the beam, (2)
 stepped off the beam, or (3) uncrossed their arms. Distance traversed was
 measured as the parallel distance from the start of the beam to the back
 of the heel on the forward foot when the trial ended. Balance ability is
 reported as the normalized distance traveled, with a maximum possible
 score of 1 if the end of the beam was reached on all 6 trials.
description: This is a matlab dataset. Matrix "Cz" contains the
 filtered single trial data from all subjects. Each row corresponds to a
 different trial. The "Flags" matrix has the same number of rows
 and contains the information relevant to each trial. The labels describing
 the columns of the "Flags" matrix are in the variable
 "FlagNames." Specifically, column 1 of "Flags"
 indicates the subject number for each trial, column 2 has the value 0 for
 trials in which subjects were asked not to step and the value 1 for trials
 in which subjects were asked to step, column 3 contains the value 270 for
 all trials (describing the backward motion of the moving platform), column
 4 has values 1, 2, or 3, corresponding to the different perturbation
 magnitudes, column 5 contains the value 1 on trials in which a step was
 taken, based on vertical forces on either force plate going below 5N
 within 1000ms of the onset of platform motion. The "time_erps"
 vector contains the time (in seconds) relative to the onset of platform
 motion for all trials. The vector "Y" is from an accelerometer
 placed on the platform and connected to the EEG recording system to
 identify the timing of platform motion, but the units are meaningless.
 Acceleration traces reported in the paper were collected separately,
 directly from the moving platform. Other subject measures mentioned in the
 paper are included in the matrix "Subject_data" with
 corresponding labels in "Subject_data_names".
 Additional data for the submission to NeuroImage include an
 additional column to the Flags matrix labeling steps within 2000ms of
 platform onset, and variables beta (single trial beta traces), MG and TA
 (single trial emg data), and their associated time vectors time_beta and
 time_emg.