Paradigm Class

Acupuncture: Stimulation with filamentous needles to elicit cognitive, behavioral, and/or physiological responses. First: Han & Terenius, 1982 Most: Hui, et al. 2000

Affective Pictures: Stimuli utilize affective images to elicit a range of emotions. First: Cattell, Glascock & Washburn, 1918 Most: Cahill, et al. 2004

Affective Words: Stimuli utilize words that are concerned with or arouse feelings or emotions. First: Clore, Ortony & Foss, 1987 Most: Kensinger & Corkin, 2004

Anti-Saccades: Make a saccade (directed rapid eye movement) away from target. First: Hallett, 1978 Most: Paus, et al. 1993

Braille Reading: Read Braille words with their finger(s). First: Fertsch, 1946 Most: Sadato, et al. 1996

Chewing/Swallowing: Chew edible or non-edible items, and may be asked to swallow the food/nonfood item. First: Hollingworth, 1939 Most: Fraser, et al. 2002

Classical Conditioning: Respond to a previously neutral stimulus that has been repeatedly paired with an unconditioned (positive/negative) stimulus that elicits the desired response. First: Pavlov, 1927 Most: LaBar, et al. 1998

Competition/Cooperation: Work together with another participant to achieve a common goal, or strive to gain/win something by defeating another participant. First: Evans, 1965 Most: Rilling et al, 2002

Controlled Breathing: - a task in which the participant controls there own breathing or it is controlled mechanically; often used along with meditation, this task requires the subject manage the inhalation and exhalation of breath; can involve breathing through pursed lips, diaphragmatic breathing or deep breathing

Counting/Calculation: Count, add, subtract, multiply, or divide various stimuli (numbers, bars, dots, etc.) or solve numerical word problems. First: Piaget, 1953 Most: Dehaene, et al. 1999

Cued Explicit Recognition/Recall: A list of items (words, objects, textures, patterns, pictures, sounds) is presented and participants are subsequently tested with cues to recall previously presented material. First: Tulving & Osler, 1968 Most: Henson, et al. 1999

Deception: Perform a task and either lie or be truthful in his/her response. First: Yerkes & Berry, 1909 Most: Langleben, et al. 2001

Delay Discounting: Perform a reward task, and is asked to choose between a small immediate reward or large delayed reward. First: Loewenstein, 1987 Most: McClure, et al. 2004

Delayed Match to Sample: A stimulus that is followed by a probe item after a brief delay; subject is then asked to recall if the probe item was presented before the delay. First: Sternberg, 1969 Most: Smith, Jonides & Koeppe, 1996

Divided Auditory Attention: Respond to an auditory stimuli (tone or word) while performing an unrelated task. If the participants are presented with tones, then this is also often co-coded with Tone Monitor/Discrimination. First: Cherry, 1953 Most: Anderson, et al. 2000

Drawing: Draw lines, circles or more complex figures using a pen or stylus. First: Greenspoon & Foreman, 1956 Most: van Mier, et al. 1998

Driving: Control the operation and movement of a vehicle in a real or simulated driving environment. First: Gibson & Crooks, 1938 Most: Hsieh, et al. 2008

Drug Challenge: - a task in which the sole point of the task is to look at the effects of a certain drug

Emotion Induction: Stimuli with emotional valence (e.g., statements, films, music, pictures) to induce effect on mood. First: Velten, 1968 Most: Lieberman, et al. 2007

Emotional Body Language Perception: The recognition of emotions expressed by another’s body pose, posture or body movement. Tasks usually require participants to infer the emotional meaning of non-verbal stimuli by paying particular attention to head, hand and body cues in the judgment of emotionally valent criteria. Based on popular beliefs that certain body postures communicate things like deceit/lying, pain/discomfort and sexual interest and cognitive neuroscience models that the purport that the visual system analyzes human motion differently from object motion. First: Mattick, 1967 Most: Grèzes, Pinchon, & de Gelder, 2006

Encoding: Memorize stimuli such as words, pictures, letters, etc. Includes Incidental Encoding, which is a task in which the participant is creating new memories without purposely knowing that memorization is the task at hand. Participants’ memories are created through working in their environment and picking up information in the process. First: Tulving & Pearstone, 1966 Most: Henson, et al. 1999

Episodic Recall: Recall from episodic memory (autobiographical, long-term memories) in a guided/unguided manner. Episodic information can be personal experiences as well as the specific objects, people, and events experienced at a particular time and place and can involve, for example, items defined as constituting discrete story elements or personal memories such as those elicited from each participant during a pre-scanning interview and then recalled via recording/script in the scanner. Recall of memories to elicit emotions such as happiness or sadness, should be co-coded with Emotion Induction. Task does NOT probe semantic memory (memory of facts or concepts) in which participants are asked to recall stimuli that was memorized prior to scanning - those are coded as Cued Explicit Recognition/Recall. Recall of traumatic events are coded as Trauma Recall, not Episodic Recall. First: Tulving, 1972 Most: Mayberg, et al. 1999

Estimation: Calculate approximately the amount, extent, magnitude, position, or value of something. First: Kaufman & Lord, 1949 Most: Herwig, et al. 2011

Face Monitor/Discrimination: View face passively or discriminate human faces according to their order, gender, location, emotion, or appearance, etc. If the participants view the faces passively, then the experiment is co-coded with Passive Viewing. First: Langfeld, 1918 Most: Whalen, et al. 1998

Figurative Language: Respond to items/statements that convey non-literal/figurative language. Figurative language includes: similes, metaphors, personification, alliteration, onomatopoeia, hyperbole, idioms, proverbs, irony, sarcasm, metonymy, and cliché. First: Myers, 1940 Most: Aziz-Zadeh, et al. 2006

Film Viewing: View movies or film clips. First: Emery, 1959 Most: Buccino, et al. 2001

Finger Tapping/Button Press: Tap fingers or press a button in a cued/non-cued manner. Used for all experiments with a response type of Finger. First: Shimoyama, Ninchoji & Uemura, 1990 Most: Colebatch, et al. 1991

Fixation: Direct attention or gaze toward a visual stimulus, often a cross. First: Hackman & Guilford, 1936 Most: Corbetta, et al. 1991

Flanker: Respond to target stimulus that is surrounded by distracting or facilitating stimuli. First: Eriksen & Spencer, 1969 Most: Ullsperger & Cramon, 2001

Flashing Checkerboard: Flashing or reverse high contrast checkerboard. First: Spehlmann, 1965 Most: Moradi, et al. 2003

Flexion/Extension: Move (flex and extend) hands, arms, legs, feet, lips, tongue, etc. First: Dearborn, Spindler & Delabarre, 1897 Most: Dapretto, et al. 2005

Fluency Induction: Perform different fluency tasks in order to enhance the smoothness or flow with which sounds, syllables, words and phrases are joined together when speaking quickly. Tasks often including: verbal (semantic and/or phonological), perceptual (frequency, duration, font size, ease/difficulty to understand, and/or attractiveness), or speech production (masking, chorus reading, whispering, rhythmic stimulation, metronome pacing). First: Fletcher, 1914 Most: Watkins, et al. 2007

Free List Word Recall: Study a list of words presented in different order on successive trials; and then prompted to recall the items in any order. First: Waugh, 1961 Most: Fize, et al. 1996

Gambling: Make decisions about chance gambles. First: Tversky & Kahneman, 1974 Most: Izuma, Saito & Sadato, 2008

Go/No-Go: Perform a binary decision (go or no-go) on a continuous stream of stimuli. First: Drewe, 1975 Most: Konishi, et al. 1999

Grasping: Grasp or grip a presented stimulus with their hand or mimic grasping one that is not physically present (i.e., imaginary or presented as a picture or video). First: Jeannerod, 1981 Most: Stephan, et al. 1995

Hand-Eye Coordination: Coordinate eye movement with hand movement to process visual input and guide a behavioral response (i.e., spatial/orientation discrimination, motor planning, timing, monitoring, response selection, motor [sequence] learning). First: Korins, 1934 Most: Farrer, et al. 2004

Hunger/Satiety: The desire to eat, or to satisfy one's appetite. Measures often include pre-post hunger and satiety state, and/or hedonic value (reward/punishment) of food or solution. First: Matsuda, et al. 1999 Most: Small, et al. 2001

Hypercapnia/Air Hunger: The state of excessive carbon dioxide in the bloodstream, typically caused by inadequate respiration. Classically known as respiratory distress or air-hunger, it can be experimentally induced by having the subject perform a static apnea or breath-holding task; by training subjects to use a mechanical ventilator or using an experimenter-controlled air-supply device; or by having the subject inhale different mixtures of oxygen and carbon dioxide. First: Opie, Smith & Spalding, 1959 Most: Evans, et al. 2002

Imagined Movement: Subjects imagine performing some motor task (e.g., walking, reaching). First: Jacobson, 1932 Most: Grafton, et al. 1996

Imagined Objects/Scenes: Visualize patterns, objects, scenes, places, concepts, real/hypothetical events (not from personal memories), in a cued/non-cued manner. First: Overton & Jackson, 1973 Most: Ochsner, et al. 2002

Induced Panic: Experimentally induced panic created by injection or inhalation of chemicals, usually done to create trigger symptoms in individuals that resemble those of panic attacks or anticipatory anxiety. First: Reiman, et al. 1986 Most: Eser, et al. 2009

Isometric Force: Use hands or fingers to apply isometric force or complete a precision grip task. First: Hartree & Hill, 1921 Most: Ehrsson, Fagergren & Forssberg, 2001

Lexical Decision: Discriminate between words and pseudowords (orthographically and phonologically legal nonwords). First: Meyer & Schvaneveldt, 1971 Most: Farrer, et al. 2004

Lip Pursing/Tongue Movement: - a task which requires the participant to purse their lips or protrude their tongue or move their tongue side-to-side or up and down

Lower-body Negative Pressure (LBNP): - Lower-body Negative Pressure (LBNP)- is a task in which the blood from the upper body is redistributed to the pelvis and legs, thus reducing central venous pressure and venous return; the subject is placed in an air-tight metal tank, which is then sealed, and sub-atmospheric pressure is produced using a vacuum pump

Magnitude Comparison (Distance): Perform a task in which the distance between stimuli is estimated. First: Gilinsky, 1951 Most: Bartolo, et al, 2014

Magnitude Comparison (Luminance): Perform a task in which the luminance or intensity of light between stimuli is estimated. First: Herrick 1956 Most: Pinel et al, 2014

Magnitude Comparison (Non-symbolic/Numerical): Perform a task in which the differences between non-symbols or numbers is estimated (e.g., choose the more numerous array of dots). First: Buckley & Gillman, 1974 Most: Venkatraman et al, 2006

Magnitude Comparison (Physical Size): Perform a task in which the physical size between stimuli is estimated. First: Singer, 1952 Most: Lewis & Miall, 2003

Magnitude Comparison (Symbolic/Numerical): Perform a task in which the differences between symbols or numbers is estimated (e.g., choose the larger Arabic numeral or number). First: Moyer & Landaur, 1967 Most: Pesenti et al, 2000

Meditation: Conscious mental process that induces a relaxation response. Concominant measures often include cognitive, behavior, and or physiological activity. First: Anand, Chhina & Singh, 1961 Most: Lazar, et al. 2000

Mental Rotation: Mentally rotate visually presented 2D and 3D figures. First: Shepard & Metzler, 1971 Most: Kosslyn, et al. 2003

Micturition: Think about voiding urine, or provide urine samples. First: Tang & Ruck, 1956 Most: Blok, Willemsen & Holstege, 1997

Motor Learning: Repeatedly execute a motor task to improve performance. First: Sanderson, 1929 Most: Anguera, et al. 2007

Multi-Tasking: Perform simultaneous execution of one or more tasks that can be presented in different modalities. First: Shallice & Burgess, 1991 Most: Bowyer, 2009

Music Comprehension: Listen to tones, melodies, chords, dissonant/consonant music, rhythms, spoken lyrics, musical excerpts/notes. First: Brust, 1980 Most: Halpern & Zatorre, 1999

Music Production: Produce music by singing (overly/covertly), or by playing an instrument. First: Sergent, et al. 1992 Most: Halpern & Zatorre, 1999

Naming (Covert): View objects (pictures, line drawings, etc.) and name them silently. First: Martin, et al. 1996 Most: Same as First

Naming (Overt): View objects (pictures, line drawings, etc.) and name them aloud. First: Ligon, 1932 Most: Martin, et al. 1996

Object Manipulation/Discrimination: Physically interact with one or more objects, and/or discriminate between its physical (shape, color, texture) and/or semantic properties (use, previous experience, relationship to other objects). First: Tatarsky, 1974 Most: Stephan, et al. 1995

Oddball Discrimination: Detect the presence of an oddball (infrequent/distinct) stimulus from a continuous stream of stimuli. Examples include when participants listen to tones and indicate when they hear a target tone (oddball) or when participants view letters or objects and indicate when they see a target stimulus (oddball). First: Sutton, et al. 1965 Most: Braver, et al. 2001

Olfactory Monitor/Discrimination: Smell odors passively and/or discriminate according to some feature (pleasant/unpleasant, strong/weak, same/different, etc.). First: Adrian, 1950 Most: Wicker, et al. 2003

Orthographic Discrimination: View letters and discriminate according to some written/printed feature (i.e., upper–case/lower–case, alphabetic order, same/different spelling of words, vowel/consonant, font type/size). First: Reicher, 1969 Most: Oldrack, et al. 1999

Pain Monitor/Discrimination: Stimulation (i.e., thermal, electrical, tactile) at a painful threshold. First: Hardy, Wolff & Goodell, 1947 Most: Singer, et al. 2004

Paired Associate Recall: Encode paired items, and subsequently asked to recall the associated pair. First: Feldman & Underwood, 1957 Most: Paus, et al. 1993

Passive Listening: Listen to auditory stimuli (speech, noise, tones, etc.) and make no response. If the participants are presented with tones, then this is also often co-coded with Tone Monitor/Discrimination. If the participants are presented with music, i.e., a melody not just tones, then code as Music Comprehension. First: Nichols, 1948 Most: Mayberg, et al. 1999

Passive Viewing: View visual stimuli (objects, faces, letter strings, etc.) and make no response. If the presented stimuli are faces, the experiments are co-coded with Face Monitor/Discrimination. If presented stimuli are words, the experiments are not coded as passive viewing but rather as Reading (Covert). First: Alexander & Barrett, 1975 Most: Corbetta, et al. 1993

Phonological Discrimination: View or listen to phonemes, syllables, or words and discriminate according to some feature of their sounds (rhyming, number of syllables, homophones, pronounceable nonwords, etc.). First: Liberman, et al. 1957 Most: Demonet, et al. 1992

Pitch Monitor/Discrimination: Listen passively or discriminate stimuli (human speech and non-speech vocalizations, animal vocalization, mechanical noise, etc.) based on pitch (i.e., duration, familiarity, pleasantness, gender, frequency). If participants only listen passively, then also co-code with Passive Listening. First: Knudsen, 1923 Most: Royet, et al. 2000

Pointing: Point at a target mentally or physically (e.g., point with finger, or imagine pointing with finger). First: Peterson, 1965 Most: Simon, et al. 2002

Pursuit Rotor/Manual Tracking: Follow/pursue a small disc on a rotating turntable. Co-coded with Motor Learning, if participants are given practice trials to improve performance and accuracy. First: Ammons, 1955 Most: Grafton, et al. 1992

Reading (Covert): Silently read words, pseudo-words, characters, phrases, or sentences. First: Locke, 1971 Most: Petersen, et al., 1988

Reading (Overt): Read aloud words, pseudo-words, logograms, phrases, or sentences First: Haber & Haber, 1982 Most: Turkeltaub, 2002

Reasoning/Problem Solving: Make judgements, inferences, generalizations and/or conclusions through the use of logic. Tasks can include integration of different elements/information, probabilistic classification, text comprehension, falsification of a conditional rule, and word problems. First: Golding, 1981 Most: Aziz-Zadeh, et al. 2006

Recitation/Repetition (Covert): Silently repeat or recite phonemes, words, or well-known text (nursery rhymes, Pledge of Allegiance, months of the year, etc.). First: Gagné & Smith, 1962 Most: Warburton, et al. 1996

Recitation/Repetition (Overt): Repeat or recite aloud phonemes, words, or well-known text (nursery rhymes, Pledge of Allegiance, months of the year, etc.). First: Gagné & Smith, 1962 Most: Petersen, et al., 1988

Reward: A stimulus that serves the role of reinforcing a desired response. Participants perform a task in which correct performance is associated with reward, often monetary reward. First: Lorge, 1933 Most: O'Doherty, et al. 2001

Saccades: Perform a rapid eye movement or saccade to capture an object in the visual field. First: Stark, Vossius & Young, 1962 Most: Corbetta, et al. 1998

Self-Reflection: Meditate or think about one's character, actions, and motives. Co-code with the paradigm "Meditation" if this is part of a meditation task. First: Pollack, et al. 1962 Most: Modinos, Ormel & Aleman

Semantic Monitor/Discrimination: Discriminate between the meanings of individual lexical items or to indicate if target word is semantically related to the probe word. Stimuli in discrimination tasks can be words or pictures representing words. First: Xiang, et al. 2003 Most: Demonet, et al. 1992

Sequence Recall/Learning: Learn and/or perform a complex sequence of finger tapping, button pressing, pointing/clicking, or various other motor responses. First: Wilding & Mohindra, 1980 Most: Jenkins, et al. 1994

Sexual Arousal/Gratification: Induction of sexual readiness with or without sexual climax. First: Zuckerman, 1971 Most: Seftel, 2010

Sleep: Sleep with or without sleep stage monitoring. First: Johnson, Swan & Weigand, 1926 Most: Lerner, et al. 2007

Stroop-Color: Name the color of the ink for a list of words (color names) printed in congruent/incongruent colors, or determine if ink color and color name are congruent/incongruent. Color-congruent stimuli: ink color and color name are the same (e.g., the word "GREEN" printed in green ink). Color-incongruent stimuli: ink color and color name differ (e.g., the word "GREEN" printed in red ink). First: Stroop, 1935 Most: MacDonald, et al. 2000

Stroop-Counting: Report the number of words presented regardless of the word meaning. Congruent stimuli: semantic meaning and numerical value match (e.g., word "TWO" presented twice on the screen). Incongruent stimuli: semantic meaning and numerical value differ (e.g., word "TWO" presented four times). Neutral stimuli: semantic meaning and numerical value are unrelated (e.g., word "DOG" presented four times). First: Reisberg, Baron & Kemier, 1980 Most: Bush, et al. 1999

Stroop-Emotional: Name the ink color of emotionally charged words (i.e., positively or negatively valenced words). First: Whalen, et al. 1998 Most: Watts, et al. 2011

Stroop-Other: Allocate attentional resources to process a specific stimulus feature in a context with competing (interfering), typically more prepotent, information. First: Shor, 1975 Most: Huang, et al. 2011

Stroop-Spatial: The spatial Stroop effect demonstrates interference between the stimulus location with the location information in the stimuli. It is elicited when the direction of the arrow is incompatible with its spatial location. For example, subjects are presented with a word either to the right or left of a central fixation point. For congruent trials, subjects see either the word ‘right’ presented to the right of fixation, or the word ‘left’ presented to the left of fixation. For incongruent trials, subjects see either the word ‘right’ presented to the left of fixation or the word ‘left’ presented to the right of fixation. First: Shor, 1998 Most: Schmitz, et al. 2005

Syntactic Discrimination: Discriminate between grammatically correct and incorrect sentences. This class also includes morphosyntactic tasks such as gender discrimination of words. First: Dennis & Kohn, 1985 Most: Wartenburger, et al. 2003

Tactile Monitor/Discrimination: Manipulate, recognize and/or discriminate objects through touch (i.e., shape, texture, same-different). First: Lamb, 1983 Most: Paus, et al. 1993

Task Switching: Switch from one task or goal to another. First: Spector & Biederman, 1976 Most: Braver, Reynolds & Donaldson, 2003

Taste: Discriminate/rate flavor intensity, pleasantness, solution content (i.e., fat content) of an item, food, or solution with or without swallowing the item. Participants may be asked to evaluate the affective significance of other stimuli (i.e., pictures, words, objects) when paired with food. First: Gowers, 1882 Most: Hare, Camerer & Rangel, 2009

Theory of Mind: Understand another's personal beliefs and feelings or form hypotheses regarding the mental states of others. First: Gallagher & Frith, 2003 Most: Ochsner, et al. 2004

Thirst Induction: Inducing a sensation of dryness in the mouth and throat associated with a desire for liquids; dehydration. First: Giddan, 1966 Most: Egan, et al. 2003

Tone Monitor/Discrimination: Listen to tones passively or discriminate according to a sound property (e.g., order, timing, pitch, frequency, amplitude), and/or detect presence/absence of a tone. First: Strainer, et al. 1997 Most: Critchley, et al. 2004

Tower of London: Mentally or physically rearrange a set of three (or more) colored beads/discs arranged on pegs, in the fewest possible moves, to match a specified configuration. Also includes Tower of Hanoi tests for taxonomic parsimony. First: Shallice, 1982 Most: van den Heuvel, et al. 2005

Transcranial Magnetic Stimulation: Electromagnetic pulses are applied at the scalp to induce electrical currents in the brain (TMS). First: Barker, Jalinous & Freeston, 1985 Most: Fox, et al. 1997

Trauma Recall: Cued recall/re-experience of a previous traumatic event. Often achieved using a patient-generated script or set of cues calling to mind the trauma. First: Burgess & Holmstrom, 1979 Most: Bremner, et al. 2003

Urine Witholding: - a task that usually requires that the subject withhold urine with a partially full or full bladder, while controlling voiding

Vestibular Stimulation: Vestibular (labyrinthine) excitability under caloric, mechanic, or turning stimulation. Galvanic Vestibular Stimulation: uses direct current electricity stimulation to stimulate vestibular system. Also includes Caloric Vestibular Stimulation, a task in which cold and warm water to stimulate the vestibular system. First: Atkinson, 1939 Most: Bottini, et al. 1994

Vibrotactile Monitor/Discrimination: Vibrotactile stimulation (perception of vibration through touch) to skin or joints. First: Verrillo, 1963 Most: Coghill, et al. 1994

Video Games: Play video games often to measure emotional changes (e.g., aggression), perceptual learning/transfer of skills, or skilled vs. unskilled players. First: Jones, Kennedy & Bittner, 1981 Most: Mathaik & Weber, 2006

Visual Motion: Infer the speed and direction of elements in a scene based on visual input, or discriminate between coherent/incoherent biological motion. First: Ludvigh, 1949 Most: Deutschländer, et al. 2002

Visual Object Identification: Identify an object based on its visual attributes (e.g., shape, color, viewing angle), or detect/discriminate changes on the object's visual properties (e.g., size, illumination, position, relation between parts). First: Beauvois, 1982 Most: Lu, et al. 2009

Visual Pursuit/Tracking: Observe a moving target(s) and track its movement across the screen, or track visual changes on the moving target (e.g., velocity, trajectory, location in space). First: Barten, Birns & Ronch, 1971 Most: Corbetta, et al. 1991

Visuospatial Attention: Make cued/noncued shifts of visual attention to a particular spatial location in the visual field. Responses can be overt (with eye movement to target location) or covert (fixating on a central target while paying attention to spatial location changes of peripheral target). Also includes the Posner and Simon task. First: Eriksen & Hoffman, 1972 Most: Paus, et al. 1993

Wisconsin Card Sorting Test: Sort cards into groups based on some dimension (i.e., color, form, or number) that is changed intermittently, and requires participants to identify a new correct group dimension. First: Berg, 1947 Most: Monchi, et al. 2001

Word Generation (Covert): Semantic: Listen to or view nouns and silently generate an associated verb, or view a category and silently generate as many exemplars as possible. Orthographic: Listen to or view a letter and silently generate as many words as possible that start with that letter. Phonologic: Listen to or view a word and silently generate words that rhyme. First: Friston, et al. 1995 Most: Same as First

Word Generation (Overt): Semantic: Listen to or view nouns and overtly generate an associated verb, or view a category and overtly generate as many exemplars as possible. Orthographic: Listen to or view a letter and overtly generate as many words as possible that start with that letter. Phonologic: Listen to or view a word and overtly generate words that rhyme. First: Petersen, et al., 1988 Most: Same as First

Word Imageability: Participants are presented with a task that reflects how easy or difficult it is to imagine a word. First: Paivio, Yuille & Madigan, 1968 Most: Sabsevitz, et al. 2005

Word Stem Completion (Covert): Silently generate a word that completes the word stem. First: Graf, Mandler & Haden, 1982 Most: Buckner, et al. 2000

Word Stem Completion (Overt): Overtly generate a word that completes the word stem. First: Warrington & Weiskrantz, 1970 Most: Heckers, et al. 1998

Writing: Write letters or words with a pen, stylus, or their finger. First: Pennebaker, 1997 Most: Longcamp, et al. 2003

n-back: Indicate when the current stimulus matches the one from n steps earlier in the sequence. Load factor n can be adjusted to make the task more or less difficult. First: Kirchner, 1958 Most: Cohen, et al. 1997

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