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.^[7] 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.

For example, one study used the LRP component to characterize the temporal order with which grammatical and phonological information about a word is retrieved when preparing to speak.^[8] Like described above, the experiment used a Go/No-Go paradigm, such that grammatical and phonological features of a depicted word to be vocalized were mapped to either the "Go" response or the "No-Go" response instruction. The grammatical feature was the grammatical gender of the depicted noun; the phonological feature was phoneme that the noun label started with. Using the characteristic nature of the LRP, they showed that a response was prepared for grammatical features even when the phonological features of the word meant no response was necessary. Importantly, no LRP was evident on No-Go trials when grammatical gender determined whether a response was necessary and phonology determined response hand, suggesting that grammatical information is indeed retrieved before phonological information. Similarly, another study^[9] used the LRP in a Go/No-Go paradigm to show that conceptual information about nouns (e.g., is the depicted item heavier or lighter than 500g?) is retrieved approximately 80 ms before grammatical information. These and other studies have been seen as support for a serial model of speech production in which conceptual information about a word is retrieved first, followed by grammatical information and then by phonological information. However, more recent research using the Go/No-Go paradigm has challenged this model, showing that the relative order with which lexical features are retrieved may be modulated by attentional biases,^[10] and that retrieval difficulty can selectively delay the retrieval of semantic information without impacting the timing of phonological retrieval.^[11][12] Together, these studies show how the LRP has helped to map out the temporal dynamics of information processing during speech production.

Other studies have used the LRP in the Go/No-Go paradigm to study the temporal nature of information recalled about a person upon seeing their face. Think about when you see someone you know in the hallway, and immediately your brain starts to conjure up facts related to the person like their name or memories like their hobbies, their job, or what their personality is like. Studies have typically shown putting a name to a face is harder than remembering biographical memories about someone. Using the LRP, studies have tried to do precise mapping of different factors that affect the order of access to different types of information about someone, just by seeing their face.^[13][14]

Conflict paradigms for studying transmission of partial information

As described above, experiments have used the LRP to generate support for a continuous model of stimulus evaluation and response selection. This model predicts that partial information is continuously available from the environment and information can accumulate to an eventual response or near response that is never actually committed. This is in contrast to a discrete model that predicts full stimulus evaluation must be complete before response initiation can start. Thus results using the LRP suggest that partial information is accumulated in the sensory systems and is sent to the motor system before and during response preparation (Coles et al., 1988).

One classic cognitive "conflict" paradigm that illustrates these findings is the Eriksen flanker task. In this experiment participants must respond to a central target that is surrounded by distractors that either represent a response consistent with the target or a response inconsistent with the target (rather is consistent with the contralateral hand response). If partial information transmission occurs, then on trials where the target is surrounded by response-inconsistent distractors, there should be an LRP indicating response preparation to the incorrect hand even when the eventual response was correct, and there should be no LRP to the same target when response-consistent distractors surrounded it and the correct response was given. This pattern of results is traditionally shown. Importantly, the effect holds regardless of the response mappings (across hands).

The flankers task requires blocking out irrelevant distractors from the environment, but what if the relevant and irrelevant features are embedded in one target stimulus? This is often the case in the classic Stroop task, such as when one must inhibit their natural response to read a word by responding to only the ink color that the word is printed in. This requires focusing on the task-relevant features of a given stimulus while ignoring task-irrelevant features of the same stimulus. Is information about both features processed simultaneously? The LRP has been used to investigate transmission of partial information in this context. A nice example is in a paper co-authored by one of the first to discover the LRP, Dr. Gabriele Gratton.^[15] In this study, the subject performs a spatial stroop task, where they are cued to respond to an upcoming word that is either the word "ABOVE" or the word "BELOW" presented physically either above or below a central fixation cross. Subjects were cued (in random order) to respond to either the physical position of the word or to the conceptual meaning of the word. Responses are typically slower and less accurate when word position and meaning are inconsistent. For all conditions, the left and right hand button responses corresponded to the two response options. The research question was whether during the spatial stroop task conflict on position-inconsistent (or, incongruent) trials is represented in the motor response stage as can be indexed by the LRP. If an LRP was evident for incongruent trials, this suggests information about the irrelevant stimulus feature was processed at the response stage even on correct trials and this generated response conflict, again supporting a model of continuous information processing. Indeed, the results supported this hypothesis. The study also collected event-related optical signal (EROS) data, which has a spatial resolution for imaging cortical activity in-vivo that is somewhat more coarse than functional magnetic resonance imaging, but has a temporal precision similar to event-related potentials (ERPs). Using EROS they showed that at least one source of the LRP was the motor cortex ipsilateral to the response hand, supporting response conflict in the primary motor cortex as one source of conflict in the stroop task.

Assessing the contribution of response-system effects in cognitive processes

The study by DeSoto et al., 2001 is a nice example of not only demonstrating support for a continuous model of information processing, but also of using the LRP to characterize the contribution of response-based conflict in a cognitive process. This is also a type of application the LRP is useful for in cognitive psychology.

Clinical applications with the LRP

The LRP can also be used to characterize individual differences in aspects of information processing as described above. One example of this has been the use of the LRP to study cognitive aging.

For instance, the LRP has been used to specify whether age-associated slowed processing originates in motor or higher-level cognitive processes, or both.^[16][17][18] Yordanova et al., 2004 showed by using LRPs that stimulus processing and response selection were not affected by age. Rather there was slowing in response execution for older adults when there was increased response complexity (four response mappings) compared to simple stimulus-response mapping (one response mapping). In a follow-up study by the same group Kolev et al., 2006 used the LRP again to show that the effects from their 2004 study generalized to the auditory domain, and to extend further support that the effects of aging on slowed response time in a four choice reaction time task is in the response generation and execution stage and not in stimulus processing and selection.