MOVEMENT-RELATED POTENTIALS

(1) Readiness potentials – Lateralized Readiness Potential (LRP)

What is it?

Readiness potentials are a category of event-preceding potentials that are apparently related to the preparation for movement. For instance, prior to voluntary movement, a negative potential develops slowly, beginning some 800 msec before initiation of the movement. These potentials have been distinguished from those that follow a movement. These potentials seem to be maximal at electrodes located over motor areas of the cortex and some readiness potential components are larger at electrode locations contralateral to the responding limb. These potentials are known as lateralized readiness potentials – the readiness potential occurs prior to voluntary movements of the hand and is maximal at central sites contralateral to the responding hand. Additionally, these potentials have been observed in the foreperiods of warned reaction time tasks when subjects know in advance which hand to use in response to the imperative stimulus.

What can it tell us?

Response preference information

Transmission of information about a stimulus before it has been completely processed

Based on the idea that these potentials occur prior to voluntary movement and can be measured before a reaction time task, researchers have concluded that they may be able to infer whether and when subjects have preferentially prepared a response. To examine whether subjects have preferentially prepared a response (i.e. activity reflects motor-related asymmetries rather than other kinds of asymmetrical brain activity), a number of steps are followed: (1) potentials recorded from electrodes placed over the left and right motor cortices are subtracted – this is done separately for conditions where left-hand movements represent the correct response and for those where right-hand movements are correct. Thus, in each case, the potential ipsilateral to the side of the correct response is subtracted from the potential contralateral to the side of the correct response. (2) The asymmetry values for left-and right-hand movements are then averaged to yield a measure of the average lateralized activity as subjects prepare to move. This average measure is the LRP.

(2) Contingent Negative Variation (CNV)

What is it?

This component is characterized by a slow negative wave that occurs during the foreperiod of a reaction time task. For example, consider a task where a light flash signals that a tone will be presented and you must make some kind of motor response when the tone is presented. If you take a look at Figure 6 below, the CNV is the negative portion of the wave between the presentation of the warning and imperative stimuli.

What can it tell us?

The component has been variously described as related to expectancy, mental priming, association, and attention.

(3) Error-Related Negativity (ERN)

What is it?

This is a negative component of the ERP that occurs when subjects make errors in sensorimotor tasks. For instance, in the prototypical experiment, subjects perform a choice reaction time task in which they must respond to two different auditory (or visual) stimuli with their left or right hands. When they respond incorrectly – for example, by using the left hand to respond to a stimulus requiring the right-hand response – a negative potential is observed. The negativity peaks at around 150 msec after response onset (when one starts to make the response). The amplitude of the ERN depends on the degree to which experimental instructions stress accuracy over speed (larger amplitude when accuracy is stressed). Although errors in these tasks are sometimes followed immediately by correct responses, error correction does not appear to be a necessary condition for the appearance of an ERN (one may catch and stop the mistake or not even notice that one makes a mistake – regardless, an ERN can be observed).

SENSORY-RELATED POTENTIALS

What are they?

The presentation of stimuli in the visual, auditory, or somatosensory modality elicits a series of voltage oscillations that can be recorded from scalp electrodes. Sensory potentials can be elicited either by a train of relatively high-frequency stimuli or by transient stimuli. The waveforms driven by the periodic stimulation overlap and have quite fixed periodic characteristics and are referred to as ‘steady state’. Transient stimuli responses are separated in time. Both types of potentials appear to be obligatory responses of the nervous system to external stimulation. Activity is thought to represent activity of the sensory pathways that transmit the signal generated at peripheral receptors to central processing systems. Thus, these components are ‘modality specific – they differ both in waveshape and scalp distribution as a function of the sensory modality in which the eliciting stimulus is presented.

(1) The Early Negativities

What are they?

In addition to the sensory components which occur early (within 100 msec of the eliciting stimuli), there are also several negative components that have been described in the period between 100 msec and 300 msec after the presentation of an external stimulus. These are known as early negativities and have been associated with selective attention, elementary feature analysis, and auditory sensory memory.

What can they tell us?

Early negativities can be used to investigate where - in the sequence of electrophysiological responses that follow stimulation – the effect of selective attention begins to emerge. The ‘attention effect’ is usually defined as a larger response to stimuli when the subject’s attention is directed to some of the stimulus features than when the subject’s attention is directed elsewhere. Attended stimuli are associated with a more negative ERP between 100 and 200 msec, as illustrated by the figure below.

MIDDLE LATENCY COGNITIVE COMPONENTS

(1) The Mismatch Negativity (MMN)

What is it?

The MMN is a negative component with an onset latency as short as 50 msec and a peak latency of 100 – 200 msec. It is studied using a passive auditory ‘oddball’ paradigm; in this paradigm, subjects are presented with two auditory stimuli that occur in a sequence. The probability of one stimulus is generally less than that for the other, however the subject’s attention is not devoted to the series of tones but instead to another task such as reading a book. To derive the MMN, the average waveform elicit/p>

MMN: (1) may reflect the operation of a ‘mismatch detector’; (2) is related to automatic and preattentive processing of deviant features; (3) may be based on a type of memory that is transient in nature, as it is not recorded after long interstimulus intervals; and (4) as the presence of more than one deviant feature affects the amplitude of the MMN, the MMN may reflect the outcome of a comparison in which multiple features can be processed in parallel.

(2) The N200s

What are they?

The N200s usually refer to a family of negative components that are similar in latency and whose scalp distribution and functional significance vary according to modality and experimental manipulations. A N200 can be observed when the features in a second stimulus mismatch with a subject’s expectancies compared to when the stimulus features are consistent with these expectancies. They appear to reflect detection of some type of mismatch between stimulus features or between the stimulus and some previously formed template. N200s differ from the MMN in that a subject’s attention is usually engaged and the template for the comparison process may be actively generated by the subject.

THE LATE COGNITIVE ERPS

(1) The P300

What is it?

As mentioned earlier, deviant items in an oddball paradigm elicit early and middle latency negative ERP activity. Additionally, if the subject is attending the stimuli, deviant items also elicit various types of late positivities with a typical latency exceeding 300 msec; a P300 (P3) in other words. P300 amplitude can be sensitive to stimulus probability, provided stimuli are relevant to the subject’s task. Furthermore, a P300 can be elicited by stimuli in any modality.

What can it tell us?

It has been proposed that P300 latency may reflect stimulus evaluation or categorization time. Thus, P300 may be a manifestation of a process related to the updating of context in working memory. Additionally, several studies have demonstrated a relationship between the memorability of an event and the amplitude of the P300 response to the event at the time of initial presentation. Furthermore, the relationship between P300 amplitude and subsequent recall depends on the mnemonic strategy used by subjects and there is a relationship between a late positivity in the ERPs and the subsequent memory for items – increased P300 for items later memorized [see pg. 72 in Fabiani et. al. (2000)].

(2) The Frontal P3

What is it?

A Frontal P3 can be elicited by deviant stimuli that are exceedingly rare and unexpected within the context and for which there is no previously formed memory template. It occurs when deviant items are first presented, however subsequent repetitions will lead to more classical P300 activity.

(3) The N400

What is it?

The N400 was originally recorded in a sentence reading task. In this paradigm, words are presented serially and the subject is asked to read them silently in order to answer comprehension questions about the content of the sentence at the end of the experiment. Some sentences ended with a semantically incongruous (but syntactically correct) word. These incongruous words elicited a larger N400 component than words that were congruous with respect to the meaning of the sentence. Furthermore, the amplitude of the N400 was related to the degree of incongruity. In other words, moderately incongruous words had a smaller N400 than strongly incongruous words.

What can it tell us?

ERP/BESA Reading Group Research on the N400 generally suggests that this component is specifically sensitive to the violation of semantic expectancies.