Effects of attention, activation and stimulus regularity on short-term ‘habituation’ of the averaged evoked response

EFFECTS

OF ATTENTION,

REGULARITY AVERAGED

ACTIVATION

ON SHORT-TERM EVOKED

VLVIEN MACLEAN,

AND STIMULUS

‘HABITUATION’

OF THE

RESPONSE

ARNE OHMAN* and MALCOLM

LADERt

institute of Psychiatry, University of London, U.K.

Accepted for publication 19 September 1974

Three studies are reported in which the effects of direction of attention, level ofactivation and regularity of stimulation on the rate of amplitude decrement over time of the auditory evoked vertex responses in humans were examined. Short-term, stimulus-by-stimulus changes were assessed by averaging across trains each of 10 click stimuli. The effect of directing attention towards thestimuli was toenhance theN, - P,component, but usually only underco~dit~ons of high activation and with irregular stimulus presentation. Habituation rate was hardly affected by the experimental manipulations. The most clear-cut relationship between psychological influences and the AER was that between level of activation and the P2 - N2 component.

1. Introduction By presenting stimuli in discrete trains, short-term changes in the electroencephalographic evoked response (AER) can be studied by averaging the first, second, third stimuli, etc. across trains. With regular, relatively short interstimulus intervals (ISI) the amplitude of the N, - P2 component (IOO200 msec latency) rapidly decreases exponentially (Fruhstorfer, 197 I : Fruhstorfer, Soveri and J~rvilehto, 1970; ohman, Kaye and Lader, 1972; Ritter, Vaughan and Costa, 1968). The importance of such short-term ‘habituation’ (cf. &man et al., 1972) of the AER is that it implies a systematic change in the individual evoked responses with stimulus repetition, thus making the interpretation of changes in the averaged response more hazardous. To try to delineate some of the factors influencing the rate of decrement of the AER, ohman and Lader (1972) studied the effect of selective attention by means of a short-term habituation paradigm. Attention had quite clear effects on AER amplitude but not on AER habituation. __. . __ ‘Now at Uppsala, lAddress London,

Department of Psychology, University of Uppsala, Svartblcksgatan IO, S-753 30 Sweden. for correspondence: Dr. M. H. Lader, Institute of Psychiatry, De Crespigny Park, SE5 gAF, U.K. 57

To elucidate further the relationship between attention and habituation. the present study manipulated variables assumed to affect both phenomena. Two important moderator variables of the attention effect on the AER are regularity of IS1 (Naatanen, 1967) and task-induced activation (Eason, Harter and White, 1969; Lindsley, 1970): both variables might influence the rate of short-term decrement of the AER. When the sub.jects’ attention was directed away from the stimuli, ijhman et al. (1972) noted a pronounced exponential decrease with regular ISI, but only a slow linear decrease with irregular ISI. Activation,

however,

P, - N, component

had no effect on the h~~bitu~~tion function, of about 250 msec latency was significantly

but the smaller

during high activation. In the first experiment of the present series, the effect of regularity was assessed both with attention directed towards and away from the auditory stimuli: in the second experiment high and low activation tasks were used to manipulate both attention and activation. Since the attention etfect appeared quite elusive in these experiments, a third experiment was designed in order to maximize the likelihood of detecting an effect of attention on the AER. In all experiments, reaction time (RT) and skin conductance level (SCL) were monitored in order to provide additional estimates of the experimental manipulations. 2. General method The subjects were students or staff members at the Institute of Psychiatry. Their ages ranged from 21 to 40 yr, and they were paid for their participation. The experiments were carried out on-line using a PDP-l2A computer. The EEC was recorded from bipolar saline pad electrodes (C, -- r3 on the IO-20 system) by means of a Grass model 51 IC amplifier of bandpass 0.3-1000 Hz, and fed into an A-D converter of’the computer, The EEG was sampled every 2 msec after the stimulus for a 500 msec epoch. Tests for gross artefact in the background EEG and in the AER were included in the computer programs. SCL, measured as resistance through double-element electrodes (Lader and Wing, 1966), was sampled ilnmediateiy prior to each stimulus. Reaction times were measured in milliseconds by the computer. EEG epochs, skin resistance and RTs were stored in digital form on magnetic tape for later analysis. The subject was seated comfortably in an armchair in a sound-attenuated chamber separated from the experimenter and the recording equipment. The experimental procedure was outlined to the subject, and it was stressed that he should keep his eyes open and fixed on the timer (see below) and avoid moving during the trains of stimuli. To maintain the subject’s attention and alertness constant over the experimental session, a reaction-time task with feedback of information on performance was incorporated into the design.

attention,

actiuatiorz and er:oked resporrse decremetrt

59

Auditory evoked responses were elicited by click stimuli of about 70 db intensity and 1 msec duration, presented through a loudspeaker behind the subject. Each experimental condition incorporated 20 trains of 10 stimuli each. The start of each train was heralded by switching on a red lamp 2 m in front of the subject, the lamp remaining on throughout the train. The interval between the Iamp being switched on and the first stimulus corresponded to the ISI between the clicks for that particular condition. The intertrain interval (ITI) was irregular, varying between 24 and 36 set with a rectangular distribution. During the trains the computer 10 times set in motion a digital timer (Venner model TSA 6614) placed 1 m in front of the subject. In the ‘high activation’ condition, ‘attention’ was manipulated by an RT task. When instructed to ‘attend’ to the clicks, the subject had to press a microswitch as fast as he could in response to each click: after a short delay of at least I see the RT was displayed for 1 set on the digital timer. When instructed ‘not to attend’ to the clicks, the subject produced a visual RT. Each time the timer was activated the subject pressed the microswitch, thereby stopping the timer and enabhng him to read off his visual RT. In the visual RT condition, the interval between visual trials varied randomly between I and 5 set thus avoiding the alternation of visual and auditory stimuli. In the “low activation’ condition, the subject counted the stimuli in each train, but the general organization of visual and auditory stimulation remained the same: the subject counted the number of clicks in each train when ‘attending’ to the auditory modality, and counted the number of times the timer started during each train when visually attentive (‘non-attending’ to clicks). Occasional trains (randomly one in eight) where the number of stimuli was 11 were given, but the extra stimuli were not included in the analyses. Averages were computed across trains, stimulus-by-stimulus, by taking all the first stimuli in the trains, all the second, and so on. Thus, there were 20 constituent evoked potential epochs in each AER. Amplitudes and latencies were measured by setting cursors manually against the peaks of the AERs as displayed on the computer oscilloscope. Variances were also calculated and displayed as a check for artefacts. Four components were measured: a positive peak with a latency of 50-80 msec (PI>; a negative peak with a latency of 80130 msec (N,); a positive peak with a latency of IX-230 msec (PJ; and a negative peak with a latency of 230-280 msec (&). RTs and SCLs were obtained from the computer tape, the former being transformed to a speed measure (1000/x) and the latter to log conductance. Averages were computed stimulus by stimulus as for the AER. Trend analysis with tests of linear and quadratic components was performed on the amplitude data (Edwards, 1960). To assess the effects of attention during the various conditions, planned orthogonal t-tests were carried out (Kirk, 1968). Latencies, SCLs and RTs were subjected to routine analyses of variance.

I’. Mackorr, A. &maa

ho

anti M. Latb-

3. Experiment 1 The purpose of the first experiment was to study the effect of attention on habituation ofthe AER during conditions of regular and irregular stimulation.

Four females and eight males from the staff of the Institute of Psychiatry acted as subjects, seven of whom had participated previously in similar experiments. A 2 x 2 factorial design was used, with attention towards or away from the clicks as one factor, and regular versus irregular ISI for the clicks as the other. In the attending condition the subject was told to respond to the clicks, and that his RT would then be displayed on the timer. In the non-attending condition he was instructed to stop the timer from which he could directly read off his visual RT. In the regular IS1 conditions, the interval between clicks was 3 set, and in the irregular ones it varied between 2.4 and 3.6 set with a rectangular distribution. In the visual RT conditions the timer was activated at intervals varying between 1 and 5 sec. The subject attended the laboratory twice, two conditions being studied on each occasion. The order of the attention conditions was balanced over subjects b~~~~~~~~ sessions, and the order of the ISI conditions was balanced within sessions.

3.2.1. Amplitmk. The N, - P, habituation functions for the four experimental conditions are shown in fig. 1. There was an overall, slightly curved decrease

Fig.

I. Ekts

of stimults regularity and direction ofattention on the IV, - Pz component the auditory evoked response, averaged stimulus by stimulus.

of

Attention,

actiaalion

andecoked responsedecrement

61

over stimuli (F(9,99) = 9.15; p < 0.001; F(lin 1,99) = 45.16; p < 0.001, and F(quad 1,99) = 18.11; p < 0.001). None of the interactions between stimulus repetition and the other factors was significant. The t-tests revealed a significant effect of attention on overall amplitude in the irregular (t(l1) = 2.01; p < 0.05), but not in the regular ISI (t = 1.15). The main effect of attention in the analysis of variance, however, was not significant (F(l,ll) = 3.11) nor was the interaction attention x regularity (F(1 ,I 1) = 0.94). Similarly, the P2 - IV2 amplitude curved down with stimulus repetition, (F(9,99) = 4.23; p < 0.001, with significant linear, F(lin 1,99) = 18.18; p < 0.001, and quadratic, F(quad 1,99) = 12.86; p < 0.001, trend components). Neither the attention nor the regularity factors produced any effects on this measure. 3.2.2. Latencies. The latency of the stimulus repetition (F(9,99) = 3.42; attention was directed towards the N2 component appeared earlier with = 6.68; p < 0.05).

N1 component decreased gradually with p < O.Ol), and occurred earlier when stimuli (F(l,ll) = 4.89; p < 0.05). The regular than with irregular IS1 (F(1 ,l 1)

3.2.3. Reaction time. The RT decreased within trains and reached a minimum after about five stimuli (F(9,99) = 8.63; p < 0.001). This decrease was more pronounced for the auditory than for the visual RT, as indicated by the stimulus x attention interaction term (F(9,99) = 5.25;~ < 0.001). Furthermore the significant three-way interaction suggested that, whereas the nature of the IS1 did not influence visual RT trends, the speed of response to the clicks increased more for the regular than for the irregular IS1 (F(9,99) = 2.48; p < 0.05). Both auditory and visual responses were faster during the regular IS1 (F(l,ll) = 6.59; p < 0.05). 3.2.4. Skin conductance level. During the stimulus, then declined but no other effects were significant.

the SCL rose initially

4. Experiment 2 This experiment aimed at an assessment of the effect of attention tion during tasks assumed to induce high and low activation.

and

on habitua-

4.1. Method Five females and seven males (six psychology students and six staff members) were paid to participate. Again, seven had been subjects before in AER experiments. Two conditions of attention were combined with two levels of activation to form a 2 x 2 factorial design. The high activation auditory and visual conditions were identical to the irregular IS1 conditions in experiment 1. In the low activation conditions, the subjects counted the number of clicks in each

62

V. Maclean,

A. &man

and M. Lader

train while attending to the auditory modality, and the number of times the timer started during each train while attending to the visual modality. He was asked to report the number of non-standard trains at the end of each condition. Again the IS1 varied between 2.4 and 3.6 set for the clicks and between 1 and 5 set for the timer. Each subject was studied twice, two conditions to each occasion. The order of the attention conditions was balanced over subjects between sessions, and the order of the activation conditions was balanced within sessions. 4.2. Results 4.2.1. Amplitude. The mean amplitudes for the N, - P2 component are shown as a function of stimulus number in fig. 2. There was a clearcurved decrease in amplitude with stimulus repetition (F(9,99) = 13.67; p < 0.001, with significant linear, F(lin I ,99) = 74.77; p < 0.001, and quadratic, F(quad 1,99) = 30.13; p -=z0.001 trends). This decrease, furthermore, was more pronounced for the non-attending than for the attending conditions, as attested by the stimulus x attention interaction term (F(9,99) = 3.36;~ < 0.01, and F(lin 1,99) = 19.47; p < 0.001). However, whereas the mean values for the two attention conditions converged in the low activation condition, they diverged in the high activation one (F(lin 1,99) = 4.47; p < 0.05, and F(quad 1,99) = 4.21 ; p <: 0.05 for the three-way interaction). t-tests showed a significant attention effect on overall amplitude during high (t(l1) = 1.83; p < 0.05) but not during low (t = 1.22) activation. The main

Fig.

2.

Effects

of

activation and direction of attention on the N, --P, component of the auditory evoked response, averaged stimulus by stimulus.

Attention, activation and evoked response decrement

63

was not significant in the analysis of variance (F(I ,l 1) = 1.06) but the attention x activation interaction did approach significance (IJ(l,ll) = 3.33;p < 0.10). The response amplitudes tended to be greater during high activation (F(1 ,11) = 3.33; p < 0.10). The P, - Nz amplitudes are depicted in fig. 3. This measure decreased linearly as the stimuli were repeated (F(9,99) = 2.44; p < 0.05, and F(lin 1,99) = 10.58; p < 0.01). The decrease was more apparent for the nonattending than for the attending conditions: duri?g the former, the AER started larger and then approached the AER size during the latter condition (F(9,99) = 2.79; p < 0.01, and F(lin I ,99) = 10.62; p < 0.01, for the stimulus x attention interaction). The response amplitudes were consistentIy higher during low than during high activation (F(l,ll) = 35.81; p -=c0.001). effect of attention

4.2.2. Latencies. The latencies of all four peaks decreased with stimulus repetition (F(9,99) = 2.70; p < 0.01 for P,; F(9,99) = 4.72; p < 0.001 for N,; F(9,99) = 2.66; p < 0.01 for P,; and F(9,99) = 2.45; p < 0.05 for NJ. The Pz latency was shorter with high activation (F(l,ll) = 5.83; p < O.OS), and with attention directed towards the stimuli (F(1 ,l 1) = 5.05; p < 0.05). For the low activation condition, however, non-attending was associated with shorter latencies than attending conditions, whereas for the high activation condition the reverse occurred (F(l,l I) = 7.01; p < 0.05 for the activation x attention interaction). 4.2.3. Reaction time. The RT for the high activation 0

Attending.

conditions

decreased

high activation

Fig. 3. Effects of activation and direction of attention on the P2 - TV2component auditory evoked response, averaged stimulus by stimulus. 5”

of the

64

within-train p < 0.05).

V. Maclean,

to an asymptotic

A. &man

and M. Lader

level after about

three stimuli

(F(9,99) = 2.12;

4.2.4. Skin conductance level. Over the stimulus train the SCL showed an initial increase followed by a decrease (F(9,99) = 4.69; p < 0.001). The high activation conditions produced significantly higher SCLs (F(l,ll) = 23.19; p < 0.001). 5. Experiment 3 The effect of attention in these two experiments was much less than would have been predicted from our previous published report (ohman and Lader, 1972). This discrepancy could be attributable to the inclusion of two attending or two non-attending conditions on each occasion in the present experiments, whereas there was only one such condition per occasion in the previous study. Thus, in the present experiment, more stimuli were presented per occasion, which, because of ‘long-term’ habituation, would result in smaller and more variable AERs, less sensitive to psychological influences. Furthermore, as the previous study suggested that the effects of attention became less as the recording session continues (ohman and Lader, 1972), the longer sessions used here would also make it less likely to discern attention effects. Indeed, response amplitudes in general were smaller in the present experiments, especially for the attending conditions. Therefore, in this final experiment, attending and non-attending conditions were studied on separate occasions. In addition, na’ive subjects were used, with efforts made to maximize their motivation in the RT tasks. 5.1. Method Eight medical students, who had not previously participated in such experiments, were instructed that they would be paid according to their performance in the RT tasks. The conditions used were identical to the high activation, irregular ISI, attending and non-attending tasks used in the previous experiments. The order of the two conditions was balanced over subjects between occasions. There was an interval of at least three days between the two sessions for each subject. 5.2. Results 5.2.1. Amplitude. Figure 4 shows the habituation function for the N1 -P, component for the two conditions of attention. As before, there was a conspicuous curved decrease with stimulus repetition (F(9,63) = 12.93; p < 0.001, F(lin 1,63) = 97.49; p < 0.001, and F(quad 1,63) = 13.60; p < 0.001). As apparent in the figure, the attention effect was quite clear (F(I ,7) = 5.90; p < 0.05). The P, - N2 amplitude decreased linearly over stimuli (F(9,63) = 7.20; p < 0.001, F(lin 1,63) = 54.20; p < 0.001).

Attention, actiaationandevoked responsedecrement

65

l Attending

.

Non-attendtng

Fig. 4. Effects of direction of attention on the N, - P2 component response, averaged stimulus by stimulus. 5.2.2.

of the auditory evoked

Latency. None of the latencies

changed with stimulus repetition, but the complex tended to occur earlier when the subjects attended to the stimuli (F(1,7) = 15.55; p < 0.01 for P,; F(1,7) = 9.55, p < 0.05 for IV,; and F(l,7) = 3.87; p < 0.10 for Pz).

P, - N, -P,

5.2.3. Reaction time. The RT reached its minimum on trial 5 (F(9,63) = 5.28; p < 0.001) and this decrease was more apparent for the auditory than for the visual RTs, as indicated by the stimulus x attention interaction (F(9,63) = 4.36; p < 0.05). 5.2.4. Skin conductance level. As in the previous studies the SCL first increased and then decreased somewhat during the trains (F(9,63) = 6.20; p < 0.001). 6. Discussion In all three experiments the AER amplitude showed the type of gradual, slightly curved decrease with stimulus repetition, noted in previous studies (ohman and Lader, 1972; ohman et al., 1972). This general habituation function appears to be very stable, as it is little affected by different experimental conditions. Only in experiment 2 did direction of attention influence habituation, with a steeper decrease when attention was directed away from the stimuli. As fig. 2 illustrates, however, this effect pertained mainly to the low activation task, whereas only slightly diverging trends were apparent in the high activation condition. In agreement with the previous findings (ohman

66

V. Mac/earl, A. ijhmarr ami M. La&r

et al., 1972), activation did not affect rate of habituation for non-attending conditions nor was there any effect in attending conditions (experiment 2). The lack of effects associated with regularity of ISI were not consistent with our previous finding that this variable was critical for an exponentially curved habituation function during non-attending (ohman et al., 1972). As can be seen in fig. 1, there was some tendency, albeit non-significant, for a more rapid exponential decrease to occur with regular than with irregular stimulation (experiment I). The inconsistency was mainly due to an increase in response amplitude between stimuli 2 and 3 for the non-attending regular ISI condition which resulted in the alteration of a curved trend to an ill-fitting linear one. This feature was quite consistent in that it was present in seven out of I2 subjects, and its nature remains obscure. The duration of the IS.1 with irregular stimulus presentation hardly affects habituation function (ohman and Lader, 1972). If the ISI is regular, however, the habituation function is steeper with shorter intervals (Fruhstorfer et al., 1970; Ritter et al., 1968; Roth and Kopell, 1969). The interpolated visual stimuli might have altered the habituation function of auditory evoked responses by cross-model interactions. Such effects have been described with ISIS of less than 0.5 set (Davies, Osterhammel, Wier and Gjerdingen, 1972), but there seems no available information with longer ISIS. However, we decided that the complications arising from cross-modal interactions would be outweighed by the benefits from preventing decreases in attention and activation levels within-train. Systematic and consistent changes within trains were also found with the latency of some response components. The N, component in particular occurred progressively earlier during the train of stimuli. This phenomenon was observed in two of the three experiments of the present study, and it has been a consistent previous finding (ohman and Lader, 1972: ijhman et al., 1972). To summarize, the only variables that appreciably affect the short-term habituation of the vertex response seem to be length and regularity of the ISI. Less easily controlled psychological factors, such as attention and activation, are generally ineffective. Such findings support the interpretation of the habituation phenomenon in terms of basic neurophysiological processes, such as refractoriness in those neural mechanisms presumed to underly the vertex response (cf. Ritter et al., 1968; Roth and Kopell, 1969). In general, the effect of attention proved quite elusive in the present experiments, particularly in contrast to the clear effects earlier observed in the same experimental situation (ohman and Lader, 1972). In the previous study every subject’s individual data showed attention effects, whereas in the present experiments many subjects produced large error variances resulting in the insensitive F-tests. Furthermore, the error variances seemed to be somewhat heterogeneous, and this might account for the discrepancies between the

Atfent~#n, act~~at~onand ertoked response der~em~nt

67

results of the F- and t-tests. Whatever the explanation the results must be interpreted with caution. This caveat is particularly pertinent to experiment 1, where the inconsistency between the two types of statistical tests was most marked. The results . .. . from the t-tests are contrary to those predictable from Naatanen’s formulation (1967) as regular IS1 did not enhance the attention effect on the Ni -P, response, but instead attenuated it. One possible explanation is that the auditory stimuli when presented regularly could be accurately anticipated; the equivalent stimuli during our visual task were always presented at irregular intervals and were thus unpredictabie. Accordingly, the auditory task was facilitated by regular stimulus presentation, but the visual RT task remained the same for both IS1 conditions. Activation might therefore be reduced in the reguiar IS1 attending condition, in contrast to the other three conditions, and consequently dilute the attention effect. In experiment 2, an effect was observed when attention was manipulated with an RT task but not with a counting task. Similar results were reported by Spong, Haider and Lindsley (1965), who asserted that counting was an ineffective means of controlling attention to external stimuli, since it also involved attention to the internal activity of keeping track of the count. However, the clear differences in P2- NL amplitude and SCL emphasize the contrast between the two types of task with respect to the amount of activation produced (cf. Bostock and Jarvis, 1970; Wilkinson and Morlock, 1967). Such activation diflerences might be the decisive factor, as hypothesized by Lindsley (1970). Further support for this interpretation comes from the study of Eason et al. (1969). Manipulating activation by shock threat under the same task conditions, these workers discerned a clear effect of attention under high activation conditions. Experiment 3 shows that the attention effect is most apparent in a short experimental session. Thus, long-term habituation may interfere with the detection of any attention effect. In summary, then, the present results regarding attention suggest that its influence is most easily observed with a high activation task, a relatively short experimental session with the minimum of stimuli, and an irregular ISI. In general ample support is given to the conclusions reached byTecce(l97O)after an extensive review of the attention-AER relationship. First, the ‘relationship is a fragile one’ and second, ‘there is a striking interindividua1 variability in EP-attention relationships’. In contrast to the attention effect, the effect of activation on the P,- iV, component was pronounced. Such an effect was reported earlier in a similar situation (i)hman et al., 1972), and it has been repeatedly noted by other authors (Bostock and Jarvis, 1970; Wilkinson and Morlock, 1967). There was also a tendency towards an enhancement of the N, - P,component with high activation, as reported by Eason et al. (1969; 1970). Thus, the N, - P2wave seems

68

V. Maclean, A. ohman and M. Lader

to be directly related both to attention and activation manipulations, whereas the P, - N2 component is inversely related to the latter variable only. There are obvious problems in measuring peak-to-peak amplitudes, e.g. N, - P2, if the component peaks vary independently. For example, Hillyard, Hink, Schwent and Picton (1973), using stimulation rates more rapid than ours, reported an effect of attention N,, but not P2, measured baseline to peak. We have also analysed our data in this way but the results were entirely similar to those using peak-to-peak amplitude data. Finally, some co-variations between the various measures used are of interest. In general, the AER did not correlate with SCL and RT, as it followed a different time course during the trains. However, the NI - P, and P, - N2 waves and the SCL were affected by the activation manipulations in a consistent manner. Thus, we agree with the conclusion of Eason and Dudley (1970) that different physiological and behavioural measures do co-vary to some extent when the level of activation is altered.

Acknowledgements This research was supported by the Medical Research Council of Great Britain. The first author is a Roche Research assistant and the second author was supported by travel grants from the Wallenberg Foundation.

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