Indian J Physiol Pharmacol 1999; 43(1) : 89-93

POST-TASK EVENT-RELATED-POTENTIAL (ERP) CORRELATESOF PSYCHOMETRIC INTELLIGENCE

SABYASACHI S. SIRCAR*, OM P. TANDON ANDANSHUMAN CHATURVEDI

Department of Physiology,University College of Medical Sciences &G.T.E Hospital, Shahdara,Delhi, - 110 095

(Received on February 12, 1998 )

Abstract : The standard psychometric scores show a fair degree ofcorrelation with event-r elate d evoked potential (ERP) latencies andamplitude. Since, both latency and amplitude are known to change duringmental activity, it becomes important to consider both pre- and post-taskERP parameters is such correlative studies. In the present study, we havecompared the psychometric scores obtained through WAIS-PR with evokedpotential latencies and amplitude recorded at 2 sessions, one before, andthe other immediately after a brief period of rigorous mental task. Anegative corrrelation (-0.4) was found between the IQ of a subject and hisP300 latency. Moreover, there was a positive correlation (+0.4) betweenthe IQ of a subject and the latency changes of his P 300 following mentalexercise (post-task P300 latency minus pre-task P300 latency). Hence, thereappears to be a reserve in 'latency increase' proportional to the IQ, whichis manifest only following a mental task.

Key words: event-related potentialsmental tasks

INTRODUCTION

Event-related potentials (ERP) reflectthe time course of information processingby the brain. Considerable literature hasaccumulated over the past few years on theERP correlates of performance ofintelligence tests (1). Much of the work hasbeen stimulated by the interest inalternatives to conventional IQ testingwhich place members of certain groups atan unfair advantage. Several paradigms ofevoked response testing as well as different

intelligence tests

P 300

analyses of the responses have beensuggested to obtain better correlations withintelligence quotient (IQ) scores. Forexample, Schafer (2) reported a highcorrelation between the scores of IQ testand ERP amplitude changes with stimulusexpectancy. Polich et. at. (3) found that thelatency of a positive wave (P300) evoked by atwo tone 'oddball' task was negativelycorrelated with digit span. Blackwood et. al(4) reported a correlation between this waveand the scores for expressive speech andverbal memory in Luria's neuropsychological

*Corresponding Author

90 Sircar et al

battery. Hendrickson and Hendrickson (5)have proposed 'string' measure of evokedresponses, claiming remarkably highcorrelations with IQ, of the order of 0.7-0.8. Pelosi et. at. (6) explored the correlationbetween amplitude and latency of the ERPsevoked by a memory scanning task test andtests of memory function and intelligence.Their results demonstrated that those withhigher scores on psychological tests asmeasured by the WAIS) have more negativeERPs. We propose a novel P300 measure -the 'post-cogitation increase in P300 latency'as a possible electro-physiological correlateof intelligence as we hypothesise that event-related potentials (ERP) recorded after asession of rigorous mental task could be abetter index of intelligence than the onesrecorded during mental relaxation.

METHODS

A total of 10 healthy subjects aged 18years (18.4 ± 0.26) years) drawn from themedical students of our college wererandomly selected for the study. Prior toselection, subjects had thorough ENT checkup to rule out any hearing disorder. Subjectswho had past history of conditionspredisposing to hearing disorders like otitismedia, meningitis, enteric fever, severejaundice, congenital abnormalities, diabetesmellitus etc., were also excluded from thestudy. The IQ of the 10 subjects wereevaluated using WAPIS-PR, a modifiedversion of the WAIS which has beenstandardized on Indian population and wasconstructed to allow for the multilingualityof Indians.

The subjects were briefed about the testprocedure. They were asked to lie down andrelax on a. bed in standard audiometric,

Indian J Physiol Pharmacol 1999; 43(1)

sound-proof and air-conditioned room. SMP-4100, Auditory/Visual stimulator and MEB-5200 Evoked Potential Recorder (NIHONKOHDEN, Japan) were used for this study.P3 was measured from vertex (Cz & Pz ) inresponse to random application of two typeof sound stimuli,presented bin aurallythrough head phones applied to subject ears.Standard auditory 'odd-ball' paradigm wasused. In this paradigm design the subjectwas presented with a sequence of twodistinguishable sound click stimuli, one ofwhich occurs frequently (the frequentstimulus - non-target) and the otherinfrequently (the rare stimulus - targetstimulus). The subject in asked to respondby pressing a button using his preferredhand, whenever a a target stimulus waspresented. The evoked responses so obtainedwere recorded on the screen of the evokedpotential recorder.

Ag/AgCI disc electrode, anchored withcolloidion were used for recording P300.Active electrodes (vve) were placed at Czand Pz with reference electrodes at earlobules (A1 + A2). The ground electrode wasplaced at Fpz. The input impedance waskept below 5K ohms. Alternating tone burstswith a starting condensation phase of 10msec rise/fall time, 100 msec duration(plateau time), intensity 70 dB nHL and rateof every 2 sec were used as target stimuli.80% of total (160) stimuli tones were 1 kHz(frequent, 128 in number) and 20% were 2kHz (rare, 32 in number). The stimulussequence was random. The signals were inphase at 2 ears. The MEB-5200 settingswere properly selected and evoked responsesto the rare stimuli were filtered with a bandpass of 5-30 Hz (Filter slope 12 dB/octave)and averaged simultaneously for 32responses. Data for 2 trials were obtained

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consequently and stored, analyzed andaveraged by the computer. The latency andamplitude of waves N2 and P3 for targetstimulus (rare) were calculated. Themethods used for recording P300 weresimilar to the ones reported earlier fromour laboratory (7).

During P300 recording session, subjectwas instructed to fix his eyes on a particularspot on the ceiling in order to avoid electro-oculographic artifacts due to eye movementsand improve his concentration and attentionto the stimuli presented. The equipment hadprovision for automatic rejection of any linedefect or power spectra. In case any trialcontained artifacts the entire trial wasrejected. In this manner, only artifact freedata were used in the final analysis.

The event-related potentials wererecored twice in quick succession, separatedonly by a period of 10 minutes during whicha mental task was administerded. Themental task comprised hearing and

Post-Task ERP Correlates of Intelligence 91

repeating strings of digits. The digits werespoken out clearly at a constant cadencewith the help of a metronome. The stringlength was varied according to theperformance of the subject: for 3 successivecorrect response, the string length wasincreased by one, while for each incorrectresponse, the string length was reduced byone. Most subjects could correctly respondto string lengths of 5 to 6 digits, and couldbe administered about 65 to 70 strings in aperiod of 10 minutes. The records of 2subjects which showed unacceptable degreesof artifacts were rejected, and the recordsof the remaining 8 subjects whichwere artifact free were used for furtheranalysis.

RESULTS

The results indicate that the latenciesof most of the evoked waveforms, viz., PI,Nl, P2, N2, P3, and N3 increase followingthe administration of mental task, althoughnone of them are statistically significant

TABLE I : Comparison of auditory ERP latencies before and after auditory task.

Variable Before task After task P value Significance"

PI 109.50 ± 35.58 103.00 ± 30.89 0.357 not significantN1 154.50 ± 38.06 153.30 ± 46.19 0.909 not significantP2 198.50 ± 33.46 217.40 ± 55.36 0.362 not significantN2 221.00 ± 35.7 243.40 ± 45.97 0.126 not significantP3 323.50 ± 21.53 334.50 ± 12.46 0.218 not significantN3 394.00 ± 49.22 410.50 ± 24.74 0.364 not significant

-at 5% level

TABLE II : Comparison of auditory ERP amplitudes before and after auditory task.

Variable Before task After task P value Significance *

PI 14.36 ± 6.28 12.60 ± 6.37 0.617 not significantN1 17.77 ± 10.12 15.65 ± 5.77 0.525 not significantP2 15.29 ± 9.48 20.84 ± 8.57 0.243 not significan tN2 9.95 ± 4.45 10.01 ± 4.37 0.973 not significantP3 38.69 ± 7.90 31.39 ± 13.19 0.055 not significantN3 23.03 ± 12.21 23.10 ± 10.65 0.983 not significant

-at 5% level

92 Sircar et al

(Table I). The amplitude of these peaks tendto show a decrease following the task, withthe exception of P2 (Table II). However, thechanges are not significant except in caseof P300, which showed a significant decrease(at nearly 5% confidence level) in amplitudefollowing the task. The latency andamplitude changes in P 300 were comparedwith the IQ scores of the subjects (TableIII). It was found that there was a negativecorrelation (-0.510) between the IQ of asubject and his P300 latency (Table IV). Also,there was a strong positive correlation(+0.789) between the IQ of a subject andthe latency changes of his P300 followingmental exercise (post-task P300 latencyminus pre-task P300 latency). The pre- andpost-task amplitudes or their differencecorrelated relatively poorly with the IQ.

TABLE III: IQ and the pre- and post-task P300latencies and amplitudes.

IQ P 300 latencies P 300 amplitudesBefore task After task Before task After task

137 300 332 25.70 10.90103 340 332 35.90 43.70117 368 348 32.00 14.80135 308 352 37.50 31.20121 312 340 41.40 40.60118 320 320 40.40 36.70123 316 336 51.50 46.7096 324 316 44.90 26.50

TABLE IV: Correlation coefficients of IQ withdifference P300 parameters.

(a) Pre-task P300 latency(b) Pre-task P300 amplitude(c) Post-post P 300 latency(d) Post-post P300 amplitude(e) (c) - (a)(f) (d) - (b)

- 0.510+ 0.574- 0.363- 0.268+0.789- 0.074

DISCUSSION

The correlations obtained in this studyare obviously not remarkable. However,

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several observations can be made aboutthese preliminary findings. The negativecorrelation between IQ and Paoo latency isconsonant with Hendrickson's theory thatproposes that higher intelligence is afunction of low error rates in the centralnervous system and as a result of the wayin which information is coded andtransmitted in the brain, the effect of erroris to reduce both the number and amplitudeof excursions of the trace (5). Several otherswho have reported similar findings includeBarry and Ertl (8), Chalke and Ertl (9) andErtl and Schafer (10). Part of the increasedpost-task latency that is more marked inhigh iQ groups could be due to the shorterpre-task P300 latency that leaves a greater'reserve' for increase. It is also possible thatinformation processing in more intelligentindividuals is associated with activation ofa wider neural circuitry, probably regardlessof the difficulty level of the task. It istempting to envisage a parallel processingsystem for problem-solving where some ofthe parallel circuits have longer latencies.Individuals with higher IQ are could beendowed with more number of these slower,multi-synaptic parallel neural circuits,presumably trans-hippocampal, that enablethem to process mental tasks moreeffectively though not necessarily, morespeedily. The whole machinery ofinformation processing is apparentlyactivated steeply when the challengeexceeds a threshold.

ACKNOWLEDGEMENTS

This study was financially supportedby a grant of the Department of Scienceand Technology, Government of India.

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