Neuronal Patterns in the Cavity Wall of Lesions during Gait Cycle in

a Rat Model of Brain Lesion Cavities

Ioana Nica

, Marjolijn Deprez

, Frederik Ceyssens

, Kris van Kuyck

, Robert Puers

, Bart Nuttin

2,4

and Jean-Marie Aerts

Division Measure, Model & Manage Bioresponse (M3-BIORES), Department of Biosystems, KU Leuven, Leuven, Belgium

Research Group Experimental Neurosurgery and Neuroanatomy, KU Leuven, Leuven, Belgium

Division Microelectronics and Sensors(MICAS), Department of Electrical Engineering, KU Leuven, Leuven, Belgium

Department of Neurosurgery, University Hospitals Leuven, Leuven, Belgium

Keywords: Beta Oscillations, Frequency Modulations, Local Field Potential, Motor Cortex, Rehabilitation, Brain

Lesion, Gait.

Abstract: Oscillatory neural activity was reported to have various physiological roles in information processing of

brain functions. It is now established that extracellular activity in the motor cortex encodes aspects of

movement, involving planning and motor control. Oscillatory patterns have also been hypothesized to play a

role in brain recovery and functional remapping. In this study, we measured neural activity from within the

cavity wall of a motor cortex lesion, in a rat model, while the animals performed a skilled walking task. We

aim at providing a possible framework of analysis, focused on revealing oscillatory patterns in the cavity

wall and their correlation with motor deficits, by using a combination of spectral features, involving power

spectra and coherence estimates in the beta and gamma frequency bands.

1 INTRODUCTION

Synchronization phenomena have been shown to

play a significant role in processing information in

the nervous system, in the case of normal or

pathological brain activity. Strongly synchronized

oscillations occurring at the neuronal level can be

identified in measurements performed at the

population level, like local field potentials (LFPs).

Oscillations are reported to be related to the

planning and generation of movements in healthy

subjects. In a number of invasive and non-invasive

studies, in humans and animal models, prominent

10-30 Hz beta oscillations have been observed in

motor cortical and subcortical structures in idle

subjects (Jasper et al., 1949; Toma et al. 2000;

Neuper and Pfurtscheller, 2001; Alegre et al., 2002);

beta oscillations tend to get suppressed when the

subject begins planning a movement, while the 40-

90 Hz gamma band oscillations emerge (Bouyer et

al., 1981; Donogue et al., 1998; Hamada et al., 1999;

Aoki F. et al., 2001). On the other hand, oscillations

are also an expression of motor deficits. In

Parkinson's disease, increased synchronization in

beta oscillations in the basal ganglia has been

reported as probable underlying cause in

manifestation of resting tremor, bradykinesia, and

rigidity (Raz et al., 1996, 2000; Ruskin et al., 1999,

2003; Goldberg et al., 2004).

More recently, it has been hypothesized that

synchrony might play a key role in triggering self-

repair processes after brain injury. It is thought that

spontaneous modulation of extracellular fields and

emergence of synchronous activity act as a signal for

axonal sprouting (Carmichael et al., 2002) and may

influence Hebbian plasticity mechanisms, that favor

rewiring of neural pathways (Murphy and Corbett,

2009).

In this study, we aimed at developing a

methodology to quantify patterns of oscillatory

neural activity within the wall of cortical lesions.

The approach allows investigation of motor deficits

in relation with continuous neural input in the region

of lesions induced on the motor cortex of rats.

289

Nica I., Deprez M., Ceyssens F., van Kuyck K., Puers R., Nuttin B. and Aerts J..

Neuronal Patterns in the Cavity Wall of Lesions during Gait Cycle in a Rat Model of Brain Lesion Cavities.

DOI: 10.5220/0005275802890294

In Proceedings of the International Conference on Bio-inspired Systems and Signal Processing (BIOSIGNALS-2015), pages 289-294

ISBN: 978-989-758-069-7

 2015 SCITEPRESS (Science and Technology Publications, Lda.)

2 METHODS

We lesioned the contralateral forelimb area of the

primary motor cortex (M1) in nine rats that have

been trained to walk along a horizontal ladder with

unequally spaced rungs. We examined gait cycle

kinematics visible from video recordings of the rat

performing the task, and spectral features of the

simultaneously recorded LFPs.

The lesion caused a decrease in movement

precision, grasping strength and crossing speed in

the affected limb. However, the rats were still

capable of movement planning and control.

Oscillatory activity in electrophysiological signals

can be captured with a wide range of time-frequency

transforms. Since we recorded LFPs via a 4x4 multi-

electrode array that spans over the wall of the lesion,

we propose a space-time-frequency method of

analysis. We focus on power and inter-electrode

coherences in the beta and gamma bands, to

investigate dynamical relationships between electric

activity captured at the 16 electrode locations.

2.1 Animal Preparation and Surgical

Techniques

Before training started, the animals (nine male

Sprague-Dawley rats) were housed individually and

put on a restricted feeding schedule until the body

weight reduced with ± 10%. They were trained for

four consecutive days to walk over the horizontal

ladder in one direction. After training, a lesion was

created by aspiration of the forelimb area in the

primary motor cortex. The lesion was made in the

contralateral hemisphere to the preferred limb

(which was determined using the pasta matrix

reaching task). Lesions were made at the following

coordinates: 5 mm anterior to 1 mm posterior to

bregma, and 0.5 to 4.5 mm lateral to bregma. The

rats were allowed to recover for 3 weeks after which

the electrode array (Fig.1.a) was implanted against

the lesion cavity wall. After implantation, the lesion

cavity was filled with medical silicone (Kwik-Sil) to

keep the electrode array in place. Arrays were

implanted against the center and posterior cavity

wall, since the array was not large enough to cover

the entire lesion cavity wall (this placement was

determined based on the first rat, where it was not

possible to implant the electrode against the anterior

wall). The implant was secured with screws and

dental cement (Fig.1.b). Electrode arrays were

produced in house by the Electrotechnical

Engineering department (MICAS-ESAT) of the KU

Leuven. They consisted of 16 Platinum contacts

each with a diameter of 350 µm. Electrode contacts

were spaced ± 1.3 mm apart from each other.

a. b. c.

Figure 1: a. electrode array; b. view of the lesion site after

electrode and connector implantation; c. top view of the

rat head with headstage and connector.

2.2 Experimental Setup and

Recordings

Two weeks after electrode implantation, the animals

motor ability was tested by performing the

horizontal ladder walking they were trained for. We

performed three trials per rat, during which LFP and

video recordings were performed simultaneously.

Video recordings were taken at a sampling rate

of 40Hz with an iDS GigE camera (iDS Imaging

Development Systems GmbH, Obersulm, Germany).

To check for lost video frames, a LED flashing

protocol was implemented, so that a bursting flash

could be visible on the upper part of the frame at a

predetermined rate. Based on this, we implemented a

custom automatic video tracking algorithm in

Matlab to detect the flashes and synchronize camera

frames with LFP timestamps.

LFP recordings were performed at a sampling

rate of 10 kHz, wirelessly, using the W16 model

from Multichannel Systems (MCS GmbH,

Reutlingen, Germany), placed on top of the rat’s

head (Fig.2). The electrodes were connected to the

headstage via an Omnetics connector (npd-18-VV-

GS). The LFPs were preamplified in the range of

1Hz to 5 kHz and digitized within the headstage,

before the transfer to the receiver.

a. b.

Figure 2: a. side view during the ladder test; b. view of

the headstage location.

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2.3 Data Processing

The data processing was performed in Matlab,

version 2013b, using the available toolboxes and

custom algorithms.

2.3.1 Behaviour Labelling

The video recordings were analysed frame by frame,

to identify and describe quantitatively components

of the rat gait cycle according to a scoring system

presented elsewhere (Metz and Whishaw, 2009).

Extracted features are summarized in Table 1.

The initial group consisted of a number of 20

rats, 6 of which died before the experiment was

concluded. Based on the video analysis, 5 more rats

were excluded, since the high number of stops and

long delays while crossing the ladder would not

allow for a significant number of consecutive

completed gait cycles to be analysed. Three trials per

rat were analysed, the results presented in Table 1

represent the mean over all movements identified in

the three trials.

We analysed general features like latency (time

needed to cross the ladder) and number of times the

rat stops while being on the ladder, but also

quantitative features related to the accuracy of foot

placement and forepaw dexterity (according to

(Metz and Whishaw, 2009)).

Limb placement was scored on a scale from 0 to

6, and the individual scores were defined as follows:

midportion of the palm of the limb is placed on the

rung with full weight support (6 points); the limb is

partially placed on the rung with either wrist or

digits of the forelimb or heel or toes of a hind limb

(5 points); the limb aims for one rung but it is placed

on another rung without touching the first one or,

alternatively, the limb is placed on a rung but

quickly repositioned (4 points); the limb is placed on

a rung, but before it bears the weight, it is

repositioned on another rung (3 points); the limb is

placed on a rung, slips off slightly during weight

bearing without disrupting the gait cycle (2 points);

the limb is placed on the rung and slips off when

weight bearing which leads to a fall (1 point); the

limb completely misses the rung and falls (0

points). We analysed the movement of all four

limbs: left and right forelimbs (LFl, RFl) and left

and right hind-limbs (LHl, RHl).

Out of these, only the slips (score of 0-3) were

considered errors of foot placement.

Accuracy of forepaw placement and ability to

grasp were also considered, using a four-category

scale, as follows: digits completely flexed around

the rung (3 points), digits flexed in a 45˚ angle (2

points), digits placed in a 90˚ angle on the rung (1

point); forepaw partially placed on the rung, with

either wrist or tips of digits resulting in no grasp (0

points).

These scores provide an indication regarding the

position of the lesion and the functional area

affected; for instance, in some rats, the lesion

equally affects the contralateral fore- and the hind-

limbs.

2.3.2 LFP Analysis

We are interested in capturing oscillatory events and

synchrony, therefore, we focused on spectral

features of the local field potentials.

LFPs were down-pass filtered with a cut-off

frequency of 300 Hz (elliptic filter with passband

ripple of 0.5 dB and stopband attenuation of 60dB).

Since the recordings were performed wirelessly, no

powerline interference was visible and the data was

artefact-free.

Based on the video analysis, intervals of gait

cycles were identified and included in the analysis,

Table 1. Features used in scoring rat kinematics.

Rat (impaired

limb)

Latency

(sec)

Total

number

of stops

Limb fault scoring

(0 - 6)

Number of

slips

(Fl/Hl)

Forepaw

digit score

(0 - 3)

LFl RFl LHl RHl Left Right LFl RFl

2 (RFl) 9.57 6 5.1 5 5.8 4.7 0/1 2/4 1.3 1

9 (RFl) 11.9 5 5.3 5.7 5.2 5.5 2/2 0/1 0.9 1.8

11 (RFl) 8.93 5 5.6 5.4 5.2 3.9 0/3 3/6 1.6 1.6

4 (LFl) 8.07 2 5.2 5.8 4.7 5.6 1/2 0/0 0.8 1.4

6 (LFl) 9.6 0 4.6 5.3 4.6 4.8 4/3 0/3 0.9 0.9

8 (LFl) 9.64 4 5 5.2 5 4.6 2/2 0/3 0.5 0.7

10 (LFl) 9.54 6 4.9 5.8 5.8 5.4 1/3 0/0 0.8 1.3

13 (LFl) 10.2 4 4.8 4.7 4.7 5 4/3 2/2 1.4 1.5

14 (LFl) 9.17 6 5.3 5.7 5.6 5.1 0/0 0/2 1.6 1.5

NeuronalPatternsintheCavityWallofLesionsduringGaitCycleinaRatModelofBrainLesionCavities

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(only intervals comprising of consecutive steps were

taken into consideration, moments when the rat

stopped were excluded).

We estimated coherence between the 16 channel

recordings. Coherence - or, equivalently, magnitude-

squared coherence (C

x,y

) - provides a statistical

measure of the similarity between signals at a given

frequency and is defined in terms of the cross-

spectral density of two signals x(t) and y(t) (G

and normalized by their autospectra (G

, G

) as

follows:



,





,













(1)

Magnitude-squared coherence estimates were

performed in Matlab, on epochs of approximately

250 msec (LFPs and video recordings were

synchronized and the interval between 10

consecutive video frames was included in the

coherence calculations) by taking a window of 50

msec, with 80% overlap.

Figure 3: Example of patterns of coherences in the beta

band on 100 msec epoch prior to (left column) and

immediately after (right column) movement initiation. The

coherence patterns were generated based on averages of at

least 8 steps from the gait cycle of rat 2 such that in Figure

3.a.: epochs were selected for good steps; in Figure 3.b.:

epochs were selected for under-reached steps; in Figure

3.c.: epochs were selected for over-reached steps. The

orientation of the maps with respect to global coordinates

is North - anterior, South - posterior.

Coherence values were calculated for 120

possible pairs between the 16 electrodes, for

frequencies below 250 Hz. The resulting values

were averaged for the intervals of 15-30 Hz (beta

band), 30-45 Hz (low gamma) and 45-90 Hz (high

gamma).

Only coherence values beyond a threshold,

consisting of 125% of the mean over the 250 msec

epoch were taken into consideration and further

organized in a 16x16 adjacency matrix, used to

generate an undirected graph, that can be interpreted

as a map of significant coherences at a certain

frequency band (see Fig. 3).









1, 



0,

(2)

The vertices represent electrode locations and the

edges connecting them indicate the strength of the

coherence, according to the colormaps. This way of

estimating coherences over short time-windows can

be used to interpret data on-line, and it was

implemented so as to match the sampling rate of the

video, as a way of visualizing behavior and neural

activity simultaneously. It incorporates information

about frequency, time localization and spatial

location of most active connected sites within the

lesion in a bi-dimensional form.

3 RESULTS AND DISCUSSION

Based on preliminary analysis, our hypotheses are as

follows: (i) coherence maps could be used to better

understand the nature of an individual’s deficit, by

using as feature the location of the ‘maximally

connected’ electrode locations (these are vertices

with maximum number of significant coherence

pairs); (ii) the quality of a gait cycle could be

quantified and predicted, by determining the

structure of a specific coherence map per individual

rat, on a preliminary training set of steps (with

features such as the location of the significant

coherence pairs and their strength); (iii) a possible

feature in detecting movement onset could be the

number of significant coherence pairs (out of a

maximum of 120) between the 16 electrode contacts.

In Figure 4, a representative example for the

power content of the signals during one ladder run is

provided: as observed, the lower, 10-Hz frequency

power is consistently high and non-discriminative,

while there is a continuous power shift between beta

and the higher gamma-band, which was not

observed to be directly correlated to the movement

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onset, gait pattern or the specific type of errors in

movement coordination. However, it is this power

shift in power content in the higher frequency bands

that motivated the choice for beta and gamma bands

in the coherence investigation.

The behavior analysis revealed the deficit of

some rats to be more extensive than expected. For

instance, in Rat 2, the higher deficit is apparent in

the hind-limb. Therefore, in the corresponding

coherence maps illustrated in Figure 3, we expect

the region in the lower half to be also active, to

account for the involvement of the hind limb

functional area of the motor cortex and as shown,

the pattern is indeed concentrated in the left side of

the map. However, the main challenge in

interpreting these results is related to the question of

how observable the system is from our recordings,

since there is a strict dependence on the placement

of the electrode array within the lesion.

Regarding the second hypothesis, a recurrent

observation is that on correct steps,

there is a

focalized center of coherence, whereas on the onset

of an erroneous step, the connections are more

dispersed.

However, we investigated a very limited number

of examples from individual rats, a validation would

require an extensive longitudinal study.

The third hypothesis is sustained by the

observation that, on movement onset, the number of

significant beta coherences decreases, while the

number of gamma coherences increases, where a

significant level was defined as the value at least

equal to 125% of the mean coherence over all

coherence maps calculated within one ladder run.

An example is provided in Figure 5, where the

average number of significant coherence pairs is

approximated on a window equivalent to the interval

spanned by 10 video frames. This feature is then

plotted against time, for the course of a ladder run.

The vertical lines indicate the intervals of swing on

the affected limb (left fore-limb, in the example

provided, for rat 6). To be noted is that all the other

limbs are moving as part of a normal gait cycle

sequence. An interesting question would be whether

or not there exists a feature discriminating initiation

of swing on each limb, or if there is a rather more

global biomarker, specific to the initiation of gait. In

order to test this idea, the resting state condition

should be included in the experiment.

Figure 4: Example of power content and signal waveform for the 4x4 electrode array during one ladder run. Figure 4.a.:

mean power spectra content of the 16 LFP channels in the 10-90 Hz band during one ladder run (from rat 4, swing intervals

identified for the left forelimb; power spectrum calculated on windows of 20 msec, with 80% overlap); the intervals of gait

swing and the labels for each step are provided according to the color code. Figure 4.b.: Corresponding LFP traces from the

16 channels.

Step starts

Grasping

Correct step Under-reach Over-reach;

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293

Figure 5: Variation of number of connections during a ladder run test.

4 CONCLUSIONS AND FUTURE

WORK

In order to characterise motor deficits in a rat model

of brain lesion cavities, we developed a

methodology involving behaviour analysis along

with investigation of local field potentials

originating from the cavity wall of the cortical

lesion.

Based on preliminary results, coherence

measures of the neural activity within the cavity wall

may prove valid in discribing the nature of the

functional deficit, but also, the configuration of

coherence maps may constitute signatures for

movement initiation and coordination. An

immediate objective would be to design a more

extensive cross- and longitudinal study, to test the

two hypotheses.

The method we propose for generating coherence

maps cand be easily implemented in an on-line

configuration, to characterise signal similarity in

three dimensions: frequency, space and time.

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