Effect of the relevance and position of the target stimuli on P300 and reaction time

International Journal of Psychophysiology 41 Ž2001. 43᎐52

Effect of the relevance and position of the target stimuli on P300 and reaction time Antonio CastroU , Fernando Dıaz ´ Departamento de Psicoloxıa e Psicobioloxıa, ´ Clınica ´ ´ Facultade de Psicoloxıa, ´ Uni¨ ersidade de Santiago de Compostela, Campus Uni¨ ersitario s r n, 15706 Santiago de Compostela, A Coruna, ˜ Galicia, Spain Received 30 March 2000; received in revised form 15 September 2000; accepted 24 October 2000

Abstract The relationships among stimulus relevance and the position of target stimuli in a sequence with the P300 component of event related brain potentials and reaction time were investigated. An auditory oddball series was presented to 42 healthy, young, right-handed female participants. In the series, participants were to ignore the standard stimuli and count the deviants from 1 to 6. When reaching the count of 6, they had to restart their count from 1 again. While counting, half of the sample had to press a key after the deviants, numbers 1, 2 and 3, but not after the numbers 4, 5 and 6. The other half of the sample had to press the key when numbers 4, 5 and 6 rather than when numbers 1, 2, 3 appeared. The P300 amplitude of the more relevant Žcount and press. stimuli was higher than that of the P300 amplitude of the less relevant Žcount only. stimuli. This is associated with a greater allocation of cognitive resources to the more relevant stimuli. An interaction was also found between the position of the target stimuli in a sequence and the stimulus relevance, which is attributed to the cognitive preparation. Furthermore, the reaction time tended to increase along the sequence of the more relevant stimuli, due perhaps to an increase in physical fatigue. 䊚 2001 Elsevier Science B.V. All rights reserved. Keywords: Event related brain potentials; P300; Reaction time; Stimulus paradigm.

1. Introduction P300 is a component of the event related brain potential ŽERP., which was discovered by Sutton et al. Ž1965.. As a result of different attempts to functionally explain this potential, various theories have been arrived at. Donchin ŽDonchin, U

Corresponding author. Tel.:q 34-981-563100; fax:q 34 981-521581. E-mail address: [email protected] ŽA. Castro..

1981; Donchin and Coles, 1988a,b. maintains that P300 is a manifestation of a context updating process that takes place in working memory. Desmedt Ž1980. and Verleger Ž1988. propose that it is related to the end of processing periods. Ž1983. states that it reflects a controlled Rosler ¨ processing. Nevertheless, none of these theories are free from criticism Že.g. Nasman and Rosenfeld, 1990; Rasmusson and Allen, 1994; Verleger, 1998.. P300 latency has been considered as an index of the duration of the stimulus evaluation

0167-8760r01r$ - see front matter 䊚 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 8 7 6 0 Ž 0 0 . 0 0 1 8 2 - 3

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A. Castro, F. Dıaz ´ r International Journal of Psychophysiology 41 (2001) 43᎐52

process relatively independent of the time necessary to select and execute a response ŽJohnson and Donchin, 1980; Kutas et al., 1977; Magliero et al., 1984; McCarthy and Donchin, 1981.. However, more recently, Verleger Ž1997. has argued in a literature review that P300 is not a sensitive tool for separating between stimulus- and response-related processes. Its amplitude is related to the allocation of attentional and memory resources, increasing when the stimulus is infrequent and relevant to the task ŽSquires et al., 1977.. For example, to mentally count the target stimuli generates a higher P300 amplitude than to simply press a key after the stimuli ŽPolich, 1987.. Moreover, P300 amplitude varies inversely in relation to probability Že.g. Tueting et al., 1971; Katayama and Polich, 1996.. Specifically, as is shown in studies that analyse the effect of stimulus sequence on the ERPs Že.g. Squires et al., 1976; Johnson and Donchin, 1980. more than the objective probability, the expectation or subjective probability is the variable that influences P300 amplitude. Structurally, P300 amplitude may be considered as a function of three dimensions: subjective probability; stimulus meaning; and information transmission ŽJohnson, 1986, 1988.. Most important, P300 has been useful in the study of human cognitive resources. These resources, which underlie the concept of mental workload, are limited in their availability, are shareable among tasks and can be flexibly allocated to various assignments ŽKramer and Spinks, 1991.. A frequently applied technique for measuring them is the dual-task technique. Using this method, a subject performs two tasks concurrently, one of them is assigned primary emphasis and the other is designated as secondary Že.g. Isreal et al., 1980.. Increases in primary task difficulty are typically associated with decreases in secondary task P300 amplitudes and increases in primary task P300 amplitudes ŽSirevaag et al., 1989.. In this context, the degree to which the two tasks interfere with each other depends on the extent to which they compete for the common supply of resources ŽWickens et al., 1983.. Having in mind this notion of P300 as a sensitive measure of cognitive resources allocation, the relationships among the relevance of infrequent and physically

identical stimuli, and the position of the target stimuli in a sequence with the P300 amplitude, latency and reaction time ŽRT. were investigated in this study using a variation of the oddball paradigm. It was hypothesised that if two types of infrequent stimuli were presented, certain ones more relevant than others, and P300 was measured, then P300 amplitude would be higher in the former than in the latter case. In this sense, it was thought that P300 amplitude would depend on the allocation of attentional and memory resources, and it was supposed that this allocation would be in terms of the stimulus relevance.

2. Method 2.1. Participants The participants were 42 females, aged between 18 and 24 years, with a mean of age of 20. All of them were healthy, right-handed volunteers and university students. None of them were aware of the working hypothesis and their absolute auditory threshold was within normal range. Furthermore, subjects were grouped in two groups of equal size Ž21 cases each. which were assessed at the same time of the year. 2.2. Stimulation Two types of stimuli were diotically presented via headphones, once every 1000 ms. Each one lasted 50 ms Ž5 ms rise, 40 ms plateau, and 5 ms fall. and had an intensity of 80 dB SPL. They were differentiated by their frequency in lows Ž1000 Hz. and highs Ž2000 Hz.. Subjects were instructed to pay attention only to the high tones, counting them silently and continuously from 1 to 6 as they appeared. When reaching a count of 6, participants had to restart from 1 again. Half of the sample had to press a key after the first three stimuli Žfirst, second and third., but not after the last three Žfourth, fifth and sixth. and the other half of the sample had to do the same after the last three stimuli Žfourth, fifth and sixth., but not after the first three Žfirst, second and third.. Furthermore, subjects were told that when they were

A. Castro, F. Dıaz ´ r International Journal of Psychophysiology 41 (2001) 43᎐52

sure that they should make the response, they should do so quickly. Doing this, the target stimuli Žhigh tones. were converted into two types: those more relevant Žthe ones after which a key should have been pressed. and those less relevant Žthe ones after which no key should have been pressed.. Furthermore, both conditions were subdivided into three subtypes by order of the target stimuli within the particular conditions. All stimuli were administered in two identical blocks. Each block contained 240 tones, 168 Ž70%. of which were lows and 72 Ž30%. of which were highs Ž12 of each subtype.. Each block lasted for 4.20 min and between them there was a break of 3 min. The possible novelty difference of the infrequent stimuli was controlled by allowing only four or fewer frequent tones to appear between two infrequent stimuli and by making the sum of the frequent tones that preceded each type of infrequent stimulus the same for all of the infrequent stimuli. In addition, the first 100 stimuli of a block were presented for training purposes. 2.3. Recording and analysis of ERPs Recordings were carried out in a quiet, naturally and artificially illuminated laboratory. Participants, whilst awake and relaxed, sat in an armchair during the 1.5-h sessions. They kept their eyes open and, to avoid too many ocular movements and blinks, were requested to fix their gaze at a place located at a lower level with regard to their eyes and to try not to blink. Both the electro-oculogram ŽEOG. and the brain electrical activity were recorded using gold cup electrodes. Each channel’s impedance was kept below 10 k ⍀. The EOG was recorded using a bipolar montage. For the vertical electro-oculogram, two electrodes were placed near the right eye, one above and one below, and for the horizontal electro-oculogram an electrode was placed on each side of the external canthus of both eyes. In both cases, the signal was amplified by a gain of 10 K and band pass filtered a priori from 0.1 to 30 Hz. A posteriori, a low pass filter of 65 Hz with a 12 dBroctave rolloff was applied. The electrical activity of the brain was recorded using a monopolar montage. According to the 10᎐20 in-

45

ternational system ŽJasper, 1958., electrodes were placed at Fz, Cz and Pz. Fpz was used as ground and the reference was the tip of the nose. Filter values were the same as in the EOG but the amplifier gain was 20 K. The digitalisation rate was set at 284 pointsrs and the activity was recorded during the 100-ms prestimulus period and the following 800 ms after the presentation of each stimulus. P300 was identified as the maximum positive peak between 260 and 460 ms after the presentation of an infrequent stimulus. If two positive peaks appeared in this temporal interval, P300 was always considered to be the second peak Ži.e. P3b.. Both its amplitude, taking the reference as the 100-ms prestimulus of the baseline, and its latency were measured. The EEG was corrected for eye movements. The method used employed regression analysis in combination with artifact averaging ŽSemlitsch et al., 1986.. Firstly, a scan was made for maximum eye movement potentials, then an average artifact response was constructed, and finally the EOG was subtracted from the EEG channels in the following manner: corrected EEG s original EEG᎐␤U EOG, where ␤ s covariance ŽEOG, EEG.rvariance ŽEOG.. Based on the reliability of P300 ŽSegalowitz and Barnes, 1993., data from both blocks of each subject were pooled together yielding 24 trials for each type of infrequent stimulus for averaging. However, if the subject made an error in the execution of the motor response, or if the influence of the EOG over the EEG was subjectively high Žafter applying the method previously described., the epoch in question was rejected. Moreover, only those recordings acquired in response to infrequent stimuli were examined, obtaining, as a result, six averaged recordings for each subject. 2.4. Recording and analysis of RT For stimuli that required a motor response, RT was defined as the time that the subject took to press a key from the onset of the stimulus. The value, if be considered as correct, should be greater than 125 ms and lower than 770 ms. Scores outside this range were considered to be

A. Castro, F. Dıaz ´ r International Journal of Psychophysiology 41 (2001) 43᎐52

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due to casual right answers or to lack of attention for which they did not enter mathematical analysis. 2.5. Statistical analysis of the data Three main analyses of the variance in the repeated measures were carried out, one for each dependent variable. The first analysis, with values of P300 amplitude, was for three factors of fixed effects; stimulus relevance Žtwo levels. = position of the target stimuli in the sequence Ž3 levels. = electrode site Ž3 levels.. The second one, similar to the previous analysis, was done by taking P300 latency scores as data. Finally, the third one, with RT scores, was with one factor of fixed effects. Because only in the more relevant targets were RT scores provided, the stimulus relevance was no longer considered as a factor. The Huynh᎐Feldt ␧ correction was done with every effect where the within-subjects factor had more than two levels and with every interaction, where at least one of these effects was present. In addition, the Pearson product᎐moment correlation was calculated between P300 latency and RT. The significance level for all of these analyses was 0.05. 3. Results 3.1. Performance The percentage of errors after the stimuli that required a motor response, by omission or by the response being outside the acceptable time window, was relatively small Žmean s 1.85%, range 0᎐6.94%.. With the rest of the RT data, significant differences were found among the three levels of the factor ‘position of the target stimuli in a sequence’, F2,82 s 7.4, ␧ s 0.98, P- 0.001. Comparisons between positions of these target stimuli taking reaction times as values Ž as 343 ms, bs 361 ms, c s 365 ms.1 indicated that reaction times were shorter at position a than at

1

Due to counterbalancing, as 1 and 4, b s 2 and 5, c s 3 and 6.

positions b, P- 0.012, and c, P- 0.001. There were no significant differences between b and c. Further, a Pearson product᎐moment correlation was computed. Firstly, the mean of the RT corresponding to each sequence of stimuli, in which a motor response should be emitted, was calculated for each of both blocks and the resultant values among the two blocks were averaged. Secondly, time within the session corresponding to the presentation of these stimuli was also calculated. After later applying the correlation test to the resultant data, a significant correlation value of 0.69 was obtained, P- 0.013. Finally, within subject correlations between P300 latencies to each one of the three subtypes of the more relevant stimuli and corresponding RTs and between the amplitude and latency of this potential to each one of the six subtypes of target stimuli were not significant Žresults not shown.. 3.2. P300 amplitude ERPs grand mean recordings at Pz, Cz and Fz obtained after the target stimuli are shown in Fig. 1. Statistical analyses revealed significant P300 amplitude differences between those sites, F2,82 s 215.5, ␧ s 0.81, P- 0.001 and pairwise comparisons indicated that all three electrodes ŽPz s 15 ␮V, Czs 10.3 ␮V, and Fzs 5.1 ␮V. were different from each other, P- 0.001. Further, P300 amplitude obtained after the more relevant stimuli Žmean 11.0 ␮V. was significantly greater than the one produced after the less relevant stimuli Žmean 9.2 ␮V., F1,41 s 10.9, P- 0.002; see Table 1. The interaction of the variables ‘stimulus relevance’ and ‘position of the target stimuli in a sequence’ also showed significant results, F2,82 s 3.5, ␧ s 1.00, P- 0.036. Because of that, the position factor was tested for each relevance level at every electrode site. At Pz, results showed a significant position effect for the less relevant stimuli, F2,82 s 4.2, ␧ s 0.97, P- 0.020, but not for the more relevant ones, F2,82 s 1.0, ␧ s 1.00, n.s. In the former case, differences were found only between positions a Žmean s 11.9 ␮V. and c Žmean s 14.1 ␮V., P - 0.002. At Cz, results showed a significant position effect for the more

A. Castro, F. Dıaz ´ r International Journal of Psychophysiology 41 (2001) 43᎐52

relevant stimuli, F2,82 s 4.3, ␧ s 0.98, P- 0.018, but not for the less relevant ones, F2,82 s 1.97, ␧ s 1.00, n.s. In the former case, differences were found between positions a Žmean s 12.2 ␮V. and b Žmean s 10.4 ␮V., P- 0.010, and between positions a and c Žmean s 10.3 ␮V., P- 0.022. No differences were found at Fz. Another interaction effect was found between ‘stimulus relevance’ and ‘electrode site’, F2,82 s 28.6, ␧ s 0.87, P- 0.001; see Fig. 2. This effect remained significant after normalising the data ŽMcCarthy and Wood, 1985. for stimulus relevance, F2,82 s 22.06, ␧ s 0.87, P - 0.001. When, therefore, comparing the more relevant stimuli with the less relevant ones, separately for each electrode, differences were found at Pz, F1,41 s 31.9, P- 0.001, and Cz, F1,41 s 4.1, P- 0.050, but not at Fz, F1,41 s 0.0, P- 0.968. The interaction ‘stimulus relevance’= ‘position of the target stimuli in a sequence’= ‘electrode site’ was also significant, F4,164 s 3.5, ␧ s 0.83, P0.015, which was due to the fact that the relevance = position interaction was different at the electrodes. 3.3. P300 latency On P300 latency, there were differences among electrodes, F2,82 s 5.3, ␧ s 0.78, P- 0.012; see Table 2. Pairwise comparisons indicated that P300 latencies were longer at Pz Ž343 ms. than at Cz Ž339 ms., P- 0.012, and shorter at Cz than at Fz Ž342 ms., P- 0.001. No significant differences were found between Pz and Fz. In addition, P300 latency obtained after the more relevant stimuli Ž338 ms. was shorter than the one recorded after the less relevant stimuli Ž346 ms., F1,41 s 4.0, P0.053.

4. Discussion 4.1. Performance There were RT variations with regard to the stimuli that signalled a motor action. Specifically, RTs tended to increase in relation to the position of the target stimuli in a sequence which is possibly due, in a combined way, to the fact that the

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Table 1 Results of the analysis of variance of three factors: stimulus relevance Ž S ., position of the target stimuli in a sequence Ž P . and electrode site Ž E . a Source

F

d.f.

P

S P E S=P S=E P =E S=P=E

10.94 0.67 215.50 3.47 28.63 1.60 3.49

1.41 2.82 2.82 2.82 2.82 4.164 4.164

0.002 0.514 - 0.001 0.036 - 0.001 0.192 0.015

a

The dependent variable is P300 amplitude.

stimuli always occurred in the same order and that, as the task advanced, subjects tended to get tired. The relationship between fatigue and RT is broadly acknowledged Že.g. Welford, 1980.. It should also be noticed that RT showed differences to which P300 latency was not sensitive. This is one more piece of evidence that these two factors are different measures that focus on different phenomena. Fatigue affected the execution of the motor response but not the stimulus evaluation, which leads to the conclusion that it was physical and not cognitive fatigue. Further, there was no relationship between P300 latency and RT, given that experimentally as much importance was given to both the speed and the precision of the response. This result coincides with those of other investigations Že.g. Segalowitz et al., 1997.. Even though sometimes, and depending on the task characteristics, significant correlations between these two variables have been communicated ŽDuncan-Johnson and Donchin, 1982.. Kutas et al. Ž1977. point out that, when the variance of RT is mainly determined by stimulus evaluation, the correlation between P300 latency and RT will be considerable and positive and, on the contrary, when it is determined mainly by process of response selection, the correlation between the two variables will be low. In their results, these authors found that the correlation between P300 latency and RT was greater when subjects were given instructions in which the precision of the response was highlighted instead of the speed. However, not every investigation reached the same results Že.g. Pfefferbaum et al.,

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A. Castro, F. Dıaz ´ r International Journal of Psychophysiology 41 (2001) 43᎐52

Fig. 1. Grand mean recordings according to the position of the target stimuli in the sequence, and the stimulus relevance.

A. Castro, F. Dıaz ´ r International Journal of Psychophysiology 41 (2001) 43᎐52

49

Fig. 2. P300 amplitude as a function of electrode site and the stimulus relevance. Notice the interaction between stimulus relevance and electrode site. Dots indicate means and vertical lines show standard error means.

1983.. From this, the importance that the differences in the paradigm used may have on the final results of an investigation is underlined. 4.2. P300 amplitude P300 amplitude after the more relevant stimuli was greater than P300 amplitude after the less relevant stimuli. Thus, P300, as has been hypothesised, depends on the stimulus relevance. If it is assumed that the variations in the maximum voltage of an evoked potential reflect variations in Table 2 Results of the analysis of variance of three factors: stimulus relevance Ž S ., position of the target stimuli in a sequence Ž P . and electrode site Ž E . a Source

F

d.f.

P

S P E S=P S=E P= E S=P=E

3.96 0.87 5.33 0.83 0.60 0.13 2.51

1.41 2.82 2.82 2.82 2.82 4.164 4.164

0.053 0.423 0.012 0.438 0.490 0.882 0.076

a

The dependent variable is P300 latency.

the intensity of one or more cognitive processes, then when the amplitude is that of the P300, the cognitive processes are, in this instance, the ones of attention and memory. The influence of these two factors on this measure is globally recognised ŽColes et al., 1990; Naatanen, 1992; Picton and ¨¨ ¨ Hillyard, 1988.. This positive relationship between stimulus relevance and P300 amplitude is attributed to a difference in the resources associated with these two processes. Logically, an increase in the stimulus relevance implies a greater cognitive demand, which is associated with a greater allocation of attentional and memory resources, which is related to a higher P300 amplitude specifically at Pz. The possibility that this difference is due to the influence of brain electrical activity corresponding to the preparation and execution of the solicited movement is excluded. This is so because, according to the placement of the electrodes adopted in this investigation, both the readiness potential and the strictly motor potential would manifest themselves as negative polarities Že.g. Kutas and Donchin, 1980; Libet et al., 1983. and would show their maximum amplitudes at Cz. Further, P300 usually has a more central maxi-

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A. Castro, F. Dıaz ´ r International Journal of Psychophysiology 41 (2001) 43᎐52

mum on no-go- than on go-trials ŽJodo and Inoue, 1990; Pfefferbaum and Ford, 1988.. On the data presented here, because all infrequent stimuli were in fact go stimuli Ži.e. all of them required a response., the P300 elicited by them presented its maximum amplitude at Pz. Both the more relevant stimuli and the less relevant stimuli had to be evaluated and a response had to be selected. Falkenstein et al. Ž1993, 1995. have suggested a relation between stimuli evaluation and P-SR and between response selection and P-CR, two P300 subcomponents. In the present paper, the existence of two P300 subcomponents is indicated by the interaction between the stimulus relevance and the electrode. Furthermore, P300 amplitude scores increased in the last presentation of the less relevant stimulus compared to the first presentation of that stimulus and decreased in the second and third presentation of the more relevant stimulus compared to the first presentation of that stimulus. This significant interaction is attributed to a difference in the stimulus importance dependent on the proximity of a change in the stimulus relevance, which is associated with a difference in the allocation of cognitive resources Žcf. Johnson and Donchin, 1982.. A somewhat similar position effect was obtained by Verleger and Berg Ž1991. using what these authors called a ‘waltzing oddball’. The influence that the cognitive resources have on P300 has also been shown elsewhere ŽSirevaag et al., 1984; Garcıa-Larrea ´ and Cezanne-Bert, 1998.. Therefore, it can be ´ stated that P300 amplitude is sensitive to predictable future events, due to what is supposed is an effect of cognitive preparation. The brain takes the necessary steps to cope with more relevant stimuli while it is engaged in dealing with less relevant ones as shown in the parietal P300. Similarly, as reflected in the central P300, the brain also prepares itself to deal with less relevant stimuli while it is occupied in coping with more relevant ones. However, this preparation only affects the attentional and memory resources assigned to the stimuli and does not influence any temporal dimension. It has to be remembered that changes in P300 latency related to the position of the target stimuli in a sequence were not observed.

4.3. P300 latency P300 latency, which was longer at Pz and Fz than at Cz, was also longer after the stimuli after which no movement was required than after those which required a movement. This last finding coincides with the results of Pfefferbaum et al. Ž1985. who, instead of using an oddball paradigm, used a gorno-go paradigm, where the no-go implies an inhibition of the response. Moreover, in a different investigation published by some of these authors ŽPfefferbaum and Ford, 1988. the same result was reached. In the study presented here, this difference is indicative of more than one generator underlying P300 and a longer stimulus evaluation process in the case of the less relevant stimuli Žcf. Duncan-Johnson, 1981.. Furthermore the fact that there were no significant correlations between latency and amplitude scores excludes the possibility that this phenomenon is due to a lower allocation of attentional resources to the less relevant stimuli. This is in spite of the fact that, as has already been indicated, P300 amplitude was reduced after the less relevant stimuli in comparison with that of the more relevant stimuli. This absence of correlation differs from the findings of Polich Ž1986, 1992., who, using larger samples and a simple oddball paradigm, found correlations always lower than ᎐0.40 even though these were significant. Overall, the present paper clearly shows that P300 increases with cognitive load which was experimentally related to the stimulus relevance within one complex task. Similarly, the anticipation of a cognitive load change was also reflected on P300. References Coles, M.G., Gratton, G., Fabiani, M., 1990. Event-related brain potentials. In: Cacioppo, J., Tassinary, L. ŽEds.., Principles of Psychophysiology: Physical, Social, and Inferential Elements. Cambridge University Press, Cambridge, pp. 413᎐455. Desmedt, J., 1980. P300 in serial tasks: An essential post-decision closure mechanism. In: Kornhuber, H.H., Deecke, L. ŽEds.., Motivation, Motor, and Sensory Processes of the Brain. Progress in brain research. Vol 54. Elsevier, Amsterdam, pp. 682᎐686.

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