The basic principles of ERD/ERS

Event-related potentials/fields vs. event-related EEG/ MEG changes(ERD/ERS)

event-related potentials (ERP)

1. The basic assumption is that the evoked activity, or signal of interest, has a more or less fixed time-delay to the stimulus, while the ongoing EEG/ MEG activity behaves as additive noise.

Phase-locked?
evoked potentials (EPS) can be considered to result from a reorganization of
the phases of the ongoing EEG signals.

2. Averaging techniques are commonly used

Not enough

3. ERPs represent the responses of cortical neurons due to changes in afferent activity

event-related desynchronization (ERD) and event-related synchronization (ERS)

1. ERD/ ERS phenomena can be viewed as generated by changes in one or more parameters that control oscillations in neuronal networks

   1. which are the general properties of such oscillations?

      1. the intrinsic membrane properties of the neurons and the dynamics of
         synaptic processes

      2. the strength and extent of the interconnections between the network
         elements, most often formed by feedback loops. Different kinds of
         feedback loops can be distinguished: involving thalamocortical or
         cortico-cortical either at short or at long distances

      3. The modulating influences from general or local neuro transmitter
         systems

   2. 

      Fig. 1. Schema for the generation of induced (ERD/ERS) and evoked (ERP)
      activity whereby the former is highly frequency-specific. TCR thalamic relay
      cells: RE reticular thalamic nucleus
      What do ‘induced’ and ‘evoked’ mean?

      1. Neuronal networks can display different states of synchrony, with oscillations at different frequencies.

2. ERD/ERS reflect changes in the activity of local interactions between main
   neurons and interneurons that control the frequency components of the
   ongoing EEG

   1. when referring to ERD/ERS of the EEG/MEG it is necessary to specify the
      frequency band.

   2. the term ERD is only meaningful if the baseline measured some seconds
      before the event represents a rhythmicity seen as a clear peak in the
      power spectrum Similarly, the term ERS only has a meaning if the event
      results in the appearance of a rhythmic component and therewith in a
      spectral peak that was initially not detectable.

Frequency-specificity of brain oscillations

In general, the frequency of brain oscillations is negatively correlated with their amplitude

1. Because the amplitude of oscillations is proportional to the number of synchronously active neural elements

   1. slowly oscillating cell assemblies comprise more neurons than fast oscillating cells

Quantification of ERD/ERS in time and space

Time course of ERD/ERS

1. the EEG/MEG power within identified frequency band is displayed relative (as percentage) to the power of the same EEG/MEG derivations recorded during the reference or baseline period a few seconds before the event occurs

   1. the interval between two consecutive events should last at least some seconds

      1. In the case of voluntary limb movement studies, an
         inter-event-interval of at least approximately 10 s is
         recommended

      2. in the case of a fore-period reaction time task, the
         interval should be even longer

2. The classical method to compute the time course of ERD

   1. bandpass filtering of all event-related trials

   2. squaring of the amplitude samples to obtain power samples

   3. averaging of power samples across all trials

   4. averaging over time samples to smooth the data and reduce the
      variability

   5. obtaining percentage values for ERD/ERS

      1. the power within the frequency band of interest in the
         period after the event is given by A whereas that of the
         preceding base line or reference period is given by R. ERD
         or ERS is defined as the percentage of power decrease or
         increase respectively, according to the expression ERD%=
         (A-R)/R×100.

3. The different methods used today for ERD/ERS quantification

   1. 

Spatial mapping of ERD/ERS

1. To convert the reference-dependent raw data in reference-free data.
   different methods are available

   1. Common average reference

   2. Laplacian reference

      1. Reference-free maps (e.g. surface Laplacian ) generally show
         a more focal pattern as compared to referential maps and are
         especially recommended when data corresponding to a movement
         task are analyzed.

   3. Local average reference

2. For spatial mapping of ERD/ERS, different methods are available

   1. surface Laplacian

   2. cortical imaging

   3. distributed source imaging

   4. Others

      1. 

Determination of subject-specific frequency bands

detection of the most reactive frequency band based on the comparison of

two short-term power spectra

1. One spectrum is calculated for the reference period (R)chosen some
   seconds before an event occurs

2. the other is calculated for the activity period (A)

3. The difference curve between the 2 logarithmic power spectra of
   periods A and R, together with 95% confidence interval. can be used
   to determine significant frequency components

4. 

   Fig. 5. (a)Superimposed logarithmic 1 s reference period (R) and the
   activity period (A) during cue-triggered finger movement as well as the
   difference between the two spectra with 95% confidence intervals indicated
   by the dotted lines. The frequency range displaying significant power
   increase is marked. (b) Band power time course calculated for the frequency
   band indicated in (a) triggered according to cue-onset (vertical line) and
   significance level (sign test P from 10\^-2 to 10\^-8) for power changes
   (step function). A power decrease indicates ERD and a power increase ERS.
   The horizontal line marks the band power in the reference period

continuous wavelet transform(CWT)

1. 

   1. The amount of dilation of the wavelet is represented by the scale a
      and the shifting of the wavelet by parameter b

      

2. 

3. 

   (c) Scalogram displaying the squared and over all trials averaged wavelet
   coefficients for the time interval 2 to 8 s (axis). Scale (left axis)
   running from 24 to 64 corresponds to a frequency range (right axis) from 12
   to 32 Hz. Color-scale from ’black’ (minimum) to red (maximum).

definition of frequency bands relative to the spectral peak frequency

1. The use of a mean alpha peak center of gravity (centroid) frequency f(i) as an anchor point to adjust frequency bands individually

ERD in memory and movement tasks

Lower alpha desynchronization (in the range of about 7-10 HZ) is obtained in response to almost any type of task

1. topographically widespread over wide areas of the scalp

2. probably reflects general task demands and attentional processes

Upper alpha( mu) desynchronization (in the range of about 10-12 Hz)

1. very often topographically restricted

2. develops during the processing of sensory-semantic information above
   parieto-occipital areas

memory

1. The degree of desynchronization is closely linked to semantic memory
   processes

Voluntary movement

1. Voluntary movement results in a circumscribed desynchronization in
   the upper alpha and lower beta bands, localized close to
   sensorimotor areas

2. This desynchronization starts about 2 s prior to movement-onset over
   the contralateral Rolandic region and becomes bilaterally
   symmetrical immediately before execution of movement

3. the topography of beta ERD was often more discrete and
   somatotopically specific than that of alpha ERD

4. The contralateral dominant pre-movement mu ERD is not only
   independent of movement duration, but also similar with index
   finger, thumb and hand movement

5. While a circumscribed hand area mu ERD can be found in nearly every
   subject, a foot area mu ERD localized close to the primary foot area
   between both hemi- spheres is less frequent

   1. 

      1. This can be interpreted as follows: the mu rhythm is
         generated mainly in the post-rolandic somatosensory area and
         the central beta rhythm (at least some components of it ) in
         the pre-rolandic motor area

6. a variety of Rolandic mu rhythms exist.

   1. mu rhythms not only selectively blocked with arm and leg
      movements, but also with face movement

Simultaneous occurrence of ERD and ERS in the alpha and lower beta bands

A visual input results not only in a desynchronization of occipital alpha rhythms but also in an enhancement or synchronization of central

mu rhythms

The opposite, an enhancement of occipital alpha rhythms and desynchronization of central mu rhythms, is found, e.g. during self-paced, voluntary hand movement

Post-movement beta ERS

These induced beta oscillations are found in the first second after termination of a voluntary movement, when the Rolandic mu rhythm still displays a desynchronized pattern of low amplitude

1. 

   Time evolution of band power changes calculated in the indicated frequency
   bands (right side). Brisk movement-offset is marked with a vertical line at
   second 7. In addition to the power changes (thick line), the significance
   levels of power changes (thin line) are indicated. The scale on the right
   indicates significance levels 10” to 10(sign test); the scale on the left
   gives percentage power changes

features

1. The post-movement beta ERS is a relatively robust phenomenon and is
   found in nearly every subject after finger, hand, arm and foot movement

   1. the beta ERS is significantly larger with hand as compared to finger
      movement

   2. For finger movement the largest beta ERS was found in the 16-21 Hz
      band, and for foot movement in the slightly higher 19-26 Hz band

2. the beta ERS has a somatotopic organization

   1. It is dominant over the contralateral primary sensorimotor area and
      has a m around 1000 ms after movement-offset

      1. the maximum of the beta ERS coincides with a reduced
         excitability of motor cortex neurons

         1. For the activation of a larger muscle mass, a relatively
            larger population of cortical is required

   2. 

   3. the beta ERS may be related to a deactivated state of the motor
      cortex

3. the beta ERS is found not only after a really executed but also after an
   imagined movement

Interpretation of ERD and ERS in the alpha and lower beta band

RED

1. Increased cellular excitability in thalamocortical systems results
   in a low amplitude desynchronized EEG

   1. ERD can be interpreted as an electrophysiological correlate of
      activated cortical areas involved in processing of sensory or
      cognitive information or production of motor behavior

2. An increased and/or more widespread ERD could be the result of the
   involvement of a larger neural network or more cell assemblies in
   information processing

   1. increased task complexity

      1. Once the movement sequence has been learned and the movement
         is performed more ‘automatically’, the ERD is reduced

         1. activity in primary sensorimotor areas Increases in
            association with learning a new motor task and decreases
            after the task has been learned

         2. The involvement of primary motor area in learning motor
            sequences was also suggested

   2. more efficient task performance

   3. more effort and attention as needed in patients

   4. elderly or lower IQ subjects

ERS

1. amplitude enhancement is based on the cooperative or synchronized
   behavior of a large number of neurons

   1. Large alpha or mu waves in the EEG/MEG need coherent activity of
      cell assemblies over at least several square centimeters

   2. When patches of neurons display coherent activity in the alpha
      band, an active processing of information is very unlikely and
      it may be assumed that the corresponding networks are in a
      deactivated state

      1. It is of interest to note that about 85% of cortical neurons
         are excitatory, with the other 15%o being inhibitory

2. occipital alpha rhythms can be considered as idling rhythms of the
   visual areas and mu rhythms as ‘idling rhythms’ of sensorimotor
   areas

3. there are also reports on induced (synchronized )alpha band rhythms
   before omitted stimuli

4. there are always scalp electrodes showing synchronized and
   desynchronized alpha (beta) band rhythms at the same moment of time

ERS in the gamma band (40Hz)

Difference

1. 

Separate foci of synchronized gamma activities occurring in cortical regions which are widely separated-often even in different lobes-can display high correlation during the performance of cognitive or motor tasks

1. an increase in gamma band coherence is functionally related to the performed task

ERD/ERS in neurological disorders

The quantification of movement-related desynchronization can improve the diagnosis of functional deficits in patients with cerebrovascular disorders and Parkinson’s disease ( PD)

1. Based on ERD measurements during voluntary hand movement it was, for
   example, possible to differentiate between superficial and deep
   vascular lesions

2. In case of PD. the pre movement ERD is less lateralized over the
   contralateral sensorimotor area and starts later than in control
   subjects

3. The beta ERS is of smaller magnitude and it is delayed as compared
   to controls

ERD changes in epilepsy with focal motor seizures were reported by Derambure et al

Mind Map of PDF version:
The basic princiles of ERD and ERS.pdf.pdf).

Reference:
Pfurtscheller G, Da Silva F H L. Event-related EEG/MEG synchronization and desynchronization: basic principles[J]. Clinical neurophysiology, 1999, 110(11): 1842-1857.