In neuroscience, the lateralized readiness potential (LRP) is an event-related brain potential, or increase in electrical activity at the surface of the brain, that is thought to reflect the preparation of motor activity on a certain side of the body; in other words, it is a spike in the electrical activity of the brain that happens when a person gets ready to move one arm, leg, or foot. It is a special form of bereitschaftspotential (a general pre-motor potential). LRPs are recorded using electroencephalography (EEG) and have numerous applications in cognitive neuroscience.
History Kornhuber and Deecke's discovery of the Bereitschaftspotential (German for readiness potential) led to research on the now extensively used LRP, which has often been investigated in the context of the mental chronometry paradigm.Coles, M. G. H., 1988. Modern Mind-brain Reading: Psychophysiology, Physiology, and Cognition. 26, 251–269. In the basic chronometric paradigm, the subject experiences a warning stimulus, followed by an interval (foreperiod), and then an imperative stimulus that the subject must respond to (see chronometric paradigm). During this foreperiod, the subject may be able to prepare a unimanual response, based on information from the warning stimulus. Part of this preparation includes a slow negative wave bilaterally distributed over pre- and post-central sites, the readiness potential.Kornhuber, H.H.; Deecke, L. (1965). Hirnpotentialänderungen bei Willkürbewegungen und passiven Bewegungen des Menschen: Bereitschaftspotential und reafferente Potentiale. Pflügers Arch 284: 1–17 "Citation Classic" Vaughan, Costa, and Ritter (1968) noted that the readiness potential was larger contralateral to the side of the body where the muscle contraction occurred.Vaughan, H. G., Costa, L. D., Ritter, W., 1967. Topography of the Human Motor Potential. Electroencephalography and Clinical Neurophysiology. The only RPs that do not seem to be lateralized are face and tongue movements which have symmetrical distribution over both hemispheres with the maximum of the potential located in the lower half of the central sulcus. That the lateralized aspect of the readiness potential in general might be used to measure the amount of motor preparation for a direct specific action, termed "corrected motor asymmetry", was highlighted by De Jong and Gratton et al.Cacioppo John T., Tassinary, L. G., Gary G., 2000. Bernston Second edition of Handbook of Psychophysiology. 2, 522.
Present-day methodology The LRP is elicited whenever a subject initiates a voluntary movement with his/her hand (or feet). Typically, a subject may be given a task requiring a button press (or squeeze) response. The LRP is recorded from the ERP over part of the motor cortex associated with the body part used to initiate the movement.
The LRP is classically studied in response cuing paradigms (see cueing paradigm) and calculated by subtracting potentials recorded over the left and right side of the scalp in the motor cortex (Coles 1988). For example, if a subject were to move his or her left hand, the subsequent event related potential would be recorded over two scalp sites with the larger negativity over motor cortex on the right side of the scalp (C4) and the smaller potential over the left side of the scalp (C3). This voltage for the C3 is subtracted from C4 to yield a value that is then averaged over the course of all the subjects responses for left hand movement. The exact same procedure occurs for deriving right hand movement. The averaged potential is the LRP. The larger negativity (excluding face and tongue movements) is seen contralateral to the moving body part for all movements except foot movements which display a paradoxical ERP on the scalp (larger negativity is ipsilateral to moving body part).
LRPs may be stimulus-locked, meaning they are measured with respect to the moment the eliciting stimulus appeared, or response-locked, meaning they are measured with respect to the moment the subject performed the actual motor activity (as measured by the execution of the movement or by recording muscle activity in the effector). These two different kinds of analyses may reveal different kinds of effects.
If something in the experiment affects the amount of time it takes before the subject is able to make a decision about their response (for example, darkening the screen so it takes the subject longer to perceive the stimulus in the first place), a stimulus-locked analysis can show that the LRP itself starts later in that condition, but takes the same amount of time to "build up" to the real motor response. On the other hand, if the experiment does not change this sort of "premotor" processing but does affect the amount of time the motor process itself takes, a response-locked analysis can reveal that the LRP starts further ahead of the response and takes longer to build up.Mordkoff, J. T., & Gianaros, P. J. (2000). Detecting the onset of the lateralized readiness potential: A comparison of available methods and procedures. Psychophysiology, 37, 347–360
Main paradigms with examples of applications in cognitive psychology The LRP is a non-invasive brain measure that describes when someone starts preparing a motor response with either their right or left hand (note the measure would work for feet too, but it is most often applied for hand movements). That means it can be used to access whether the brain is simulating an action even when the action is never carried out and even if the participant is unaware of the ongoing simulation. This makes the LRP a powerful tool for investigating various questions in cognitive psychology.
There are three general types of inferences that the LRP can generate, including (1) whether a response has been preferentially activated, (2) the degree to which a response has been preferentially activated, and (3) when a response is preferentially activated. Experimental paradigms that interface nicely with these questions include cueing paradigms, the Go/No-Go paradigm and paradigms that induce conflict in the response system. Generally, cueing paradigms can be used to study factors that influence response preparation, the Go/No-Go paradigm is useful for asking questions about the temporal order of information processing, and conflict paradigms help answer questions about the types of information that reach the response system from other brain systems. Outside of these paradigms, studies have also used the LRP component to characterize the contribution of response processes in various cognitive processes and in characterizing individual differences in behavior. Below is a review of some examples from these general categories of LRP applications, from a range of cognitive disciplines.
Cueing paradigms for studying factors that influence response preparation In a basic cuing paradigm, for an LRP to occur there must be a cue presented that predicts a meaningful stimulus is about to be presented, to which the subject will have to respond. This creates a foreperiod when their response or some instructed behavior is contingent on some event they've just been warned will happen. The cue that predicts a future stimulus is usually called the warning stimulus, or cue, and the future stimulus to respond to is usually called the imperative stimulus, or target. Importantly, for the LRP to occur the imperative stimulus must be a cue that indicates which hand the subject should prepare to respond with, so that a period of response preparation occurs. For example, if a cue indicates a 50% chance of responding with the right or left hand, then no LRP is likely to occur. The amplitude of the lateralization effect is thought to represent the amount of differential response preparation elicited by the warning stimulus. The amplitude of the LRP also indicates how close one is to the response threshold—the point in the LRP just before response initiation occurs.
Cueing paradigms may even influence response preparation when the subject is unaware of the cue. In a special type of cuing paradigm the cue can be presented for a very short period of time (e.g., 40 ms) and preceded and followed by other visual stimuli that effectively "mask" the cue's presence. This type of paradigm, called "masked priming", has been used with the LRP to see whether a cue someone is unable to identify at all is still able to influence the response system. For example, one study showed that a masked prime that gave conflicting response information compared to the target reliably slowed subjects’ response times, even though the subjects reported never seeing the masked prime.Dehaene, S., Naccache, L., Le Clec'H, G., Koechlin, E., Mueller, M., Dehaene-Lambertz, G., van de Moortele, P.F., Le Bihan, D., 1998. Imaging unconscious semantic priming. Nature, 395, 597–600. They also showed that the conflicting masked prime induced an LRP such that the brain started preparing a response based on the semantic information in the masked prime. This suggests that a cue with newly learned meaningful implications for the motor system (i.e., arbitrary response-mappings) need not be consciously processed in order for response preparations to begin. Thus since the LRP can pick up signals for responses never actually initiated or perceived of, it can uncover information processing that happens without our awareness but that can still affect our overt behavior.
Go/No-Go paradigms for studying temporal order of information processing In a Go/No-Go paradigm participants are told to respond with their right or left hand according to a specific feature of a presented target. For example, subjects may be instructed to respond with their right hand if the target letter is red and with their left hand if the target letter is yellow. For the No-Go part, subjects are told to only respond to the hand-referenced feature based on some other feature of the target. For example, they may be instructed to not respond if the letter is a vowel. Trials consistent with instructions to respond are "Go" trials, and trials consistent with instructions to not respond are "No-Go" trials.
This paradigm helps answer questions about the order of information extraction by through comparison of LRPs (or lack of) to stimulus features in the Go versus No-Go conditions. Specifically, an LRP on No-Go trials would signify that whatever feature was driving hand selection was processed sometime before processing of the feature that indicated no response was necessary. To verify the order of information extraction it is important to flip the features that are mapped to hand selection and the No-Go instruction. If no LRP occurs in either condition of response and No-Go feature mapping, this suggests the stimulus features may be processed in parallel or at approximately the same time. Like the cueing paradigms, the LRP in the Go/No-Go paradigm can also occur at different time points and vary in magnitude, which gives additional information about the timing of information processing and the magnitude of differential order of processing.