P300 guidelines

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.