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Meeting Psychofysiologie.be
P300 From Practise to Guidelines
28/02/2008
Dr. G. Otte
Not the P300 again !!!
HELP HELP !!!!!!!!
The P300
• Born in 1965• 2 fathers: Sutton and Donchin• Off spring >> many 1000 articles and
publications• Star* of the ERP scene because
discrimination of external stimuli is a primary condition in many cognitive activities (attention, memory, action..)
• Evoked by odd-ball paradigma
What is the meaning of the P300
• Its is only a label for a concept that encompasses the Late positive Complex (LPC) +/-300 msec after a meaningfull stimulus.
P300 does not exist
• There are two different components– a fronto central P3a (“novelty P3a-NOGO
P3a) dopaminergic-indexes attentional network activation (concept of Posner)
– a temporoparietal P3b indexes memory and context updating and is noradrenergic driven.
P3aP3aP3bP3b
FrontalFrontal
DopaminergicDopaminergic
AttentionalAttentional
parietocentralparietocentral
noradrenergicnoradrenergic
Memory Memory encodingencoding
Guidelines
Use the paradigma that maximises these 2 components
General principle
Oddball paradigmaOddball paradigma
Three elements
Odd ball paradigma
• A rare target stimulus is presented with low probability (15-20%) in a series of frequent standard stimuli (modality independent)
• The patient has a primary task to perform on the target stimulus– Overt: pressing a button– Covert: counting silently the number of targets– Combined
Odd ball paradigma
• A rare target stimulus is presented with low probability (15-20%) in a series of frequent standard stimuli (modality independent)
• The patient has a primary task to perform on the target stimulus– Overt: pressing a button– Covert: counting silently the number of targets– Combined
Odd ball paradigma
• A rare target stimulus is presented with low probability (15-20%) in a series of frequent standard stimuli (modality independent)
• The patient has a primary task to perform on the target stimulus– Overt: pressing a button– Covert: counting silently the number of targets– Combination of both
Type 1: easy discrimination
S:Standard: 1000 Hz
T: Target: 2000 hz
(Classical type)
The amplitudo of the P300 on the target is inversely related to the probability of its occurence which can be expressed as a local probability : intertarget interval (ITI) N: number of standards between targets
The amplitudo increases on increasing ITI untill ~= around 8.
It fits a sec. order polynomial especially in visual modality where this effect is more pronounced.
Importance of Intertarget Interval ?A P300 (P3a) can be seen on simple stimuli independent on patient cooperation if the interval between stimuli is long enough (6-8 sec).
Every stimulus is then a target.
Can be very interesting to test P3a in mentally handicapped or other patients who cannot (will not) perform the primary task
Other factors influence amplitudo and latency
RESOURCERESOURCE ALLOCATIONALLOCATION
Arousal: tonic and phasicArousal: tonic and phasic
3 way stimulus
Target is difficult to discriminate from standard: attentional resources are allocated: large P3b
Infrequently and irregularely a strange more salient “distractor” stimulus is given (patient should not react on it): this “salient or novel” stimulus generates a large frontocentral P3a.
Novel
Standard
Target
Primary task
Examples from the clinic
Note a low voltage MMN area (adjacent to N1)
Oddball paradigma: 80 % standard 20% “target” easy discrimination (S:1000 Hz / T: 2000 Hz)
No primary task asked to patient
Boils down to MMN
Primary task:
“OvertOvert” Press RT button and
“covertcovert” count number of rare stimuli:
Result: Low voltage P300-non closure of CN in Cz and Pz
Count: correct
“Classical P300 odd-ball”: combined task
Task: P300: tap finger (no counting) on rare
stimulus: GO condition
GO conditionGO condition
Very low voltage P3a- delayed P3b and non closure CN
Task: tap finger on frequent only ( no no tappingtapping on rare: NO-GO).
NO-GO conditionNO-GO condition
Activates a large frontal P3a (“No GO” P3a)
Let us recapitulate
• Compare 4 different tasks and their influence on “the P300”
No P300No P300
FIRST: NO TASK at all
No task No attentionNo task No attention
Small preattentive MMN adjacent to N1
Oddball P300:overt (press RT button)Oddball P300:overt (press RT button) Oddball P300:overt + covert (press Oddball P300:overt + covert (press RT button and cound targets)RT button and cound targets)
<<Larger P300 <<Larger P300 (counting adds (counting adds attentional attentional resources to the resources to the task)task)
Larger P300Larger P300
Counting Counting mobilises more mobilises more
attentional attentional resourcesresources
NOGO conditionNOGO condition Overt + covertOvert + covert overtovert
No motor involvementNo motor involvement motor involvementmotor involvement
Influence of motor task
• It activates motor related potentials that cover the same time window as P3
• The medial BP can contaminate the P3
• Some motor potentials are lateralised (LRP) and in right handed people might obscure a lateral assymetry that is often seen in left temporal cortex of schidzophrenic patients.
Paradigma
• Single target: in patients unable to apply the primary task
• 3 way oddball: in all other patient– Use overt: (motor task) to obtain sequential reaction
times and after that a secund run but– Using covert ( count only-no motor task) to eliminate
contribution of motor related potentials (P3b will be higher)
– Use covert difficult task to appreciate attentional resources (TINNV D. de Batisse-Campanella-M.van de Velde) (but more clinical studies needed)
3 way paradigma
Conclusion
• In daily clinical routine a rigid standardised paradigm (fi 3 way) and task procedure is IMPERATIVE in order to lower outcome variance and allow multicentre comparison of results (and building normal databases).
• Freedom of testing is allowed but only in research situations.