erp indices of undifferentiated visual attentional search strategies in 5 years old preterm children

ERP indices of undifferentiated visual attentional search

strategies in 5 years old preterm children.

Rémy Dupin*, Jean-Paul Laurent*, Johannes E. A. Stauder**, Eric Degiovani***,

Elie Saliba*,

* INSERM 316, Médecine Néonatale, Hôpital de Clocheville, 49 bd Béranger, 37044 Tours Cedex, France. ** Faculteit der Psychologie, Universiteit Maastricht, Postbus 616, 6200 MD Maastricht, Pays-Bas. *** Service d’exploration Fonctionelle, CHU Bretonneau, 2 bd Tonnelé, 37044 Tours Cedex 1, France

ERP indices of attentional dysfunction in 5 year old premature children 1

ABSTRACT

The assumption that preterm children have difficulties in maintaining their

selective attention was tested in a serial and automatic visual task. Ten 5 year old

premature children were compared to ten full-term children, matched for age and

IQ, using a search paradigm. Subjects had to detect a random target defined by a

feature (pop-out task) or a conjunction of two features (serial task) among 7

distractors. Accuracy and RT were analyzed and event-related potentials (ERP)

were recorded from 8 scalp sites, at F3, Fz, F4, T4, Cz, T5, Pz, Oz sites.

Behavioral and electrophysiological data were analyzed in repeated measure

Manovas. Results showed a significant group by task effect on N2b and P3

amplitude at anterior sites, and group by task by block effect on P3 amplitude at

posterior sites. There was no significant group effect on RT. These data support

the view that, in contrast to full-term children, 5 years old premature children are

not able to modulate visual search strategies according to task difficulty.

Key Words: Event-Related Potentials, attention, children, prematurity, cognition,

electrophysiology


INTRODUCTION

Preterm children have an increased incidence of cognitive deficits as

compared with healthy full-term children (Ornstein et al. 1991, Saigal et al. 1991).

These problems are of considerable importance as they can result in school failure

and many high-risk children ultimately require special education services. Even

within one standard deviation IQ mean scores, 19-23 % of 8-9 year old preterm

children cannot follow normal schooling compared to 6-14 % of age-paired

full-term children (Francis-William & Davies, 1974; Calame et al., 1986; Hille et

al., 1994; Roth et al., 1993). The reason for these scholastic difficulties is yet to be

adequately determined. According to some authors, learning disabilities could be

the consequence of visual processing abnormalities and attentional disorders in

preterm children. Attention is a fundamental prerequisite for learning, and more

serious form of attention deficits are often associated with learning disorders

(Cantwell et al., 1978). Children, whose primary complaints is poor learning, also

frequently have attentional problems (Ackerman et al.,1987). Damman et al.

(1996) investigated neurological and psychological sequelae of preterm birth at

the age of six years old. They found that a dysfunction of the visuomotor

integration was frequently related with attentional disorders and that visuomotor

development was clearly influenced by the neurological status. The Scottish low

birth weight study (1992) attempted to provide a picture of the performance of

premature children just before they entered school. One of their results indicated

that the visual recognition subscale, which assesses visual organizational skills,

was most affected, with 30% of the cohort performing below the 10th centile for

the test. They also found that a high proportion of the premature children


exhibited features of attention deficit. Teplin et al. (1991) showed, in 6 year old

premature group compared with a control group born at term, a significant group

difference for scores on visual-motor function, visual-perceptual abilities and

attention span. Klein et al. (1985, 1989) found that 5 year old premature children,

matched by race, sex and family background with classmate control children born

at full term, performed significantly less well on the Spatial Relations subtest of

the Woodcock-Johnson and on the Visual Motor Integration test, although no

significant differences in IQ were found between groups. Retesting these subjects

at 9 years of age, they found that the premature children scored significantly lower

than controls on reading and mathematics achievement. Saigal and al. (1991)

reported also that premature children, at 9 years old, did less well than matched

full term control group on the reading, spelling and mathematics test. Their visual-

motor integration performance were poorer as well (Beery developmental test of

visual-motor integration).

Event-related potentials (ERPs) have been found to provide a valuable

noninvasive method for studying human brain maturation associated with

perceptual and cognitive maturation (see review by Steinschneider et al ., 1992).

ERPs are scalp-recorded electrical potentials generated by neural activity

associated with specific sensory, cognitive and motor processes (Donchin et al.,

1986). Unlike behavioral data (such as reaction time and accuracy), ERPs

provide a continuous index of processing between the stimulus and the response,

allowing the study of mental chronometry. As a result, ERP data can be used to

isolate different processing stages (Hillyard & Munte, 1984; Renault et al., 1982).

In developmental research, Van de Stelt et al (1998) reported an age

related decrease in latencies of a frontal selection positivity, N2b and P3b that


paralleled decrease in reaction time in normal subjects ranging in age from 7 to 24

years using a visual (color) selective attention paradigm. The occipital selection

negativity amplitude increased with advancing age, whereas both N2b and P3b

showed changes in scalp topography. The authors concluded that their data

showed evidence that the efficiency of visual selective processes increases during

childhood and adolescence.

Several studies have investigated attention deficit in full term children

using “oddball” paradigm and recording ERP mainly in auditory modality. These

studies concluded that the P3 component was smaller and more frontally

distributed for Attention Deficit and Hyperactivity Disorders (ADHD) children

than for normal children. The ERP differences were unrelated to performance, as

both groups performed equally well. (Frank et al., 1994; Johnstone et al., 1996;

Kemner et al., 1996; Jonkan et al, 1997).

Visual studies have also been conducted to assess behavioral performance

and ERP components comparing a reading disability group to a control group.

Taylor et al. (1990) studied dyslexic compared to normal children in three visual

reading-related tasks. They found that N2-P3 components were usually longer in

latency and P3 was smaller in amplitude in the dyslexic than in the normal

children. The P3 also had a flatter, broader distribution in the dyslexics. Stelmack

et al (1995) outlined the current status of research on learning disabilities and

intelligence in children using ERP methods: Children with reading disabilities

(RD) exhibit longer P3 latency and smaller P3 amplitude than normal controls.

These group effects reflect an inadequate direction of attention in learning

disabilities and attentional problems, which must occur early in the information

processing sequence, and is related to both automatic and controlled processing .


With visual search paradigm, ADHD children were studied and compared to

normal children. Taylor et al. (1993) investigated ADHD children undergoing a

4 week double-blind medication assessment (methylphenidate) and normal

controls in a visual feature detection task. There were no significant differences in

RTs, but P3a and P3b latencies were significantly longer in the ADHD group. The

results of a visual-modality ERP study by Harter et al. (1998) are interesting from

an interpretive viewpoint. The results showed larger visual P3b amplitude in the

fronto-central region in children with ADHD (and comorbid reading disability).

These authors suggest that the increased fronto-central P3b amplitude may reflect

the clinical group’s attempt to compensate for their disorder as a group difference

was not found on performance. Unfortunately, the studies reviewed above are of

great diversity, and their results are not always consistent. One source of confound

may be the diversity of stimuli used in the different studies. Moreover, Stelmack

et al. (see above) pointed out that reading or learning disabled children do not

constitute a homogenous group.

At this time, few ERP studies have been conducted to investigate cognitive

disabilities in prematures. In an auditory oddball task, and without any specific

instruction, Tokioka et al.(1995) observed a reduction of P550 amplitude (possible

equivalent of P3 in the adult) for the rare sound in one year premature children,

With the same type of auditory task, but in 5 year old premature children, Lavoie

et al. (1997) did not find any significant difference of P240 (P3a) amplitude,

between the rare vs. frequent sound conditions. In a visual oddball task, the same

authors found differences in P280 and P650 amplitudes between non-targets and

targets, but no group effect.


Behavioral researches of selective attention have suggested that the

capacities to differentiate between relevant and irrelevant stimuli progressively

increase with age (Kail, 1991). Studying the modulation of ERP components

could offer major insights into the physiological basis for changes in processing

selectivity. In addition, insight could be provided into changes in the timing and

organization of different levels of selective information processing, which are

more difficult to infer from behavioral indices alone, such reaction time. To date,

no study has yet examined the effects of attentional manipulations on visual

ERPs in order to determine if attention deficit could contribute to learning

disabilities in premature children.

From the literature it appears that a lot of preterm children, with normal

IQ and without clinical attention deficit, may exhibit some subtle selective visual

attention difficulties which have implications later in their learning disabilities and

failure in school. Although the nature of this cognitive deficit is still matter of

debate we attempted to investigate these disabilities in visual search tasks. In these

tasks, an array of stimuli is presented and subjects must report the presence or

absence of a specified target item within this array. In a series of investigations

conducted in adults, Luck and colleagues (1990, 1993, 1994) have showed that

the amplitude and latency of N2 and P3 component can differentiate between two

classes of search tasks.

Based on the literature summarized above, the present study assessed

possible attentional strategy differences between premature and full-term children.

We suggest that premature children cannot engage controlled strategy to processes

complex visual stimuli. In this paper, we have approached this problem with the


following general strategies. (a) Reduce the heterogeneity of the premature

population by excluding subjects with neurological deficits, significant cerebral

ultrasonography anomalies or clinical attentional disorders, (b) Using a cognitive

task that has been explored in normal adult populations in order to highlight the

link between ERP modulations and the underlying cognitive process. (c) using a

two factor-within-subject design to identify a deficit as task specific, it is

necessary to isolate an independent variable (in our study pop-out and serial tasks)

that, when systematically manipulated, interacts with and increases the magnitude

of the deficit in the premature subjects. Consequently, using ERPs and RTs as

dependent variables we set out to test the assumption that only five year old full-

term children will show N2 and P3 amplitude modulation with task manipulation,

namely a N2 amplitude decrease and P3 increase in the serial task compared to

the pop-out task .

METHOD

Subjects

Ten 5 year old preterm children (between 26 and 32 completed weeks of

gestation; mean = 30.3; SD = 1.8), and ten same non corrected age controls born

at term (more than 37 weeks of gestation) participated in the experiment. The

premature children were born between July 1991 and march 1992, in Clocheville

Pediatric hospital (Tours). Psychological, pediatric, ophthalmologic and

audiometric assessments were performed by systematic exams at one, two, four

and five years: all records were appropriate (weight, cranial perimeter) for


gestational age (birth weight between 1380 and 2200g; mean = 1645; SD = 306.7

and birth cranial perimeter between 27 and 30; mean = 28.7; SD = 1.0). All

subjects had normal or corrected to normal eyesight. None of the premature

children had neuromotor impairment (hemiparesia, hemiplegia, cerebral palsy,

etc.) and the cerebral ultrasonography was evaluated normal for eight subjects and

grade 1 for two subjects.

The premature group was composed of 7 boys and 3 girls (7 were right

handed and 3 left handed), and the full-term group of 8 boys and 2 girls (8 were

right hands and 2 left handed). They all had a normal intellectual level, as tested

by the Kaufman Assessment Battery for Children (K-ABC) according the

standardized protocol (Kaufman & Kaufman, 1983). Their cognitive

competencies were within the normal range in the full K-ABC processing scale

(Mental Processing Composite, MPC). Furthermore, they scored between 92 and

127 (mean = 109.5) in the Simultaneous Processing tasks (visuo-spatial skills, in

which spatial stimuli are integrated at once), and from 85 to 139 (mean = 102.4)

in the Sequential Processing tasks (working memory and motor planning, in

which stimuli are integrate into temporally organized series). In addition, a

behavioral evaluation was performed using the Conners Parents Rating Scale to

exclude children with attention deficit disorder, hyperactivity or impulsivity from

this experiment. School performance was satisfactory for all participating children

(no school failure, special education or extra-school support). The same exclusion

criteria were used with the children in the full-term group.

Preterm children were matched with the control group according to

mother's educational years and the global socio-economic status.


-------------------------------------- Insert Table 1. Here -----------------------------------

Stimuli, apparatus and procedure

Each subject arrived at 9:30 am in the morning and the recording session

began at 10 am. The stimuli were presented foveally on a PC VGA screen that

was placed 80 cm in front of the subject's eyes. The fixation point was in the

center of their horizontal straight-ahead line of sight.

Pop-out task was a parallel processing task. The standard stimuli (n = 96)

comprised a group of eight small blue vertical rectangles. Within target stimuli (n

= 32), one of the rectangles was red. Non-target pop-out trials (n = 32) were also

presented in which one of the blue rectangles was horizontally oriented. Targets

and non-targets pop out occurred with a 20 % probability each. The order of

presentation was pseudo-randomized, so that two targets were never presented

successively to avoid ERP components overlapping. Subjects used their preferred

hand to press a keyboard space bar. The interval between trials (ISI) varied from

2000 to 2500 ms. The stimulus array was presented for 250 ms.

In serial task, deviant targets are defined by the association of color (red)

and size (thick size). The subjects had to identify the target (large red bar,

probability 20%) and press the spacing-bar (non-target stimuli can be either red

or thick). There were no deviant non-target stimuli. The others presentation

parameters were kept unchanged.


Recording and data analysis

EEG was recorded with 8 Ag-AgCl electrodes (Fz, Cz, Pz, Oz, F7, F8, T3

and T4), according to the 10-20 system. All electrodes were referenced to the

nose. Horizontal EOG was recorded bipolarly from electrodes at the outer canthus

of each eye, vertical EOG was recorded from electrodes above and beside the

right eye. Electrode impedance was kept below 5 Kohms. Eye movements were

removed using regression analysis in the frequency domain (R.GREEG, Dr JC.

Woesterburg, 1994). The EEG signal was amplified by a Medelec 1A93 device

with a bandpass between 0.1 and 30 Hz and was digitalized at 256 points per

channel. Digital filtering was done offline with a bandpass between 0.5 and 12

Hz. EEG and EOG were epoched off-line into periods of 1000 ms (50 ms prior to

the onset of the stimulus and 950 ms after).

Mean ERP amplitudes were computed as the maximum positive or

negative voltage between 90-180 ms (anterior N1, posterior P1), 150-300 ms

(anterior P2, posterior N2a), 275-525 ms (N2b), 525-750 ms (P3b in Pop-out

task) and 550-750 ms (P3b in serial task).

Statistical analyses

Statistical analysis were conducted using subject groups as a between-

subjects factor (two levels), and task conditions (two levels) and electrodes (four

levels at anterior sites FZ CZ F7 F8, two levels at posterior site Pz, Oz) as within

factors in repeated measure Manovas. Statsoft Statistica® software was used to


perform these analysis. Scheffé comparison test was also performed to assess

post-hoc significant effects.

RESULTS

Behavioral data

The tables 2 and 3 present the behavioral performances for each attentional

task and for each group. There were not significant interactions or group main

effects found. The task 2 (serial task) showed longer RTs and fewer correct hits

as compared to the task 1 (Pop-out task) (Rao’s R (2,17)= 32.82 p<.000, RTs:

F(1,18) = 25.78 p < .000, Correct hits: F(1,18)= 56,71, p <.000). This task effect

on correct response was significant mainly in the first half of the trials (Rao’s R

(5,17)= 5.08 p= .019, F(1.18)= 4.70 p= .044).

-----------------------------------Insert Tables 2 and 3 here--------------------------------

Electrophysiological data Fig. 1 and 2 illustrate ERPs elicited by the targets (solid line) and the

standards (dotted line) at each electrode site for task 1 and 2, respectively for

fullterm and preterm groups. The waveforms show that the N1 and P2

components are larger at midline than at lateral sites, and that these components

invert in polarity between anterior and posterior leads. Table 4 summarizes the


statistical significant results, as assessed by Anova procedures; F and p values for

the Group and Task factors are given.

----------------------------------Insert Table 4 here -----------------------------------------

Task effect

At posterior site N2b amplitude is larger and latency shorter for stimuli in

task 1 (-2.8 uV, 282 ms) than in task 2 (0.7 uV, 311 ms :Rao’s R(4,14)= 14.62 ;

p< .000). For standard stimuli task 2 elicited greater P3 amplitude then task 1

(10.8 uV vs. 3.8 uV: S x T interaction: Rao’s R(4,14)= 9.36 ; p= .001).

---------------------------------Insert Figure 1 and 2 here ----------------------------------

Group effect

In Table 5, N2b and P3b target amplitudes at anterior sites are presented

for each attentional task and according to preterm and full-term groups. In the

full-term group, N2b target amplitude is greater for task 1 than 2. In the preterm

P3b target amplitude is smaller in task 2 than 1. These effects appear in a

significant Group x Task x stimulus interaction: (Rao’s R(4,15)= 4.41 ; p= .015).

Moreover, only for the full-term group N2b amplitude is larger for target

(-5.56 uV) compared to standard stimuli at Oz (0.73 uV: Rao’s R(4,14)= 3.45 ; p=

.037).


----------------------------------Insert Table 5 here ----------------------------------------- In Table 6, P3b mean amplitude, for Cz and Pz pooled sites on task 2

between first of half trials (block1) and last of half trials (block2), is presented

according to preterm and full-term group. In block 2 the amplitude is larger in the

full-term compared to the preterm group (group by block interaction: F(18,1)=

6.76 p= .018). Scheffé post hoc test indicates that this difference is significant

between the groups in the second bloc (p= .034).

----------------------------------Insert Table 6 here -----------------------------------------

DISCUSSION

Behavioral Performance.

As in adult studies, this experiment shows that search for feature

conjunction targets among distractors is a more difficult task than single feature

search as indicated by a RT increase and detection decrease. Moreover our data

show that 5 year old children present learning effects in this serial task as assessed

by the performance improvement from the first to second half of the session.

Neurophysiological assessment of visual search strategy


Full-term group.

Our experimental data support the main hypothesis that only full-term 5

years old children demonstrate visual search strategy analogous to adult. Task and

stimulus effects do not affect early components but only N2 and P3. This N2 and

P3 amplitude modulation by the task indicates clearly that the subjects were

engaged in different types of search strategy, thus supporting the parallel/serial

distinction. As Luck et al. (1994) described in adults, pop-out targets elicited

enlarged anterior and posterior N2b components.

The P3 amplitude is sensitive to stimulus probability and task demand : the

amplitude is higher for targets than for standards and for serial than for pop-out

targets. As stimuli differ in probability (20% for targets and 60% for standards)

the P3 amplitude enhancement for targets is obvious. On the other hand, standard

amplitude increased between pop-out and serial tasks although the probabilities

have expanded from 60 % (pop-out) to 80 % (serial task). This indicates that

standards were processed at a more extensive level in serial than in the pop-out

task. This amplitude enhancement cannot be accounted for by a global probability

effect since it was increased in the serial task in comparison to the pop-out task..

As suggested by Luck et al. (1990), if the detection of conjunction targets implies

a serial search process, this will lead to an increase of P3 amplitude compared to

pop-out task. Indeed, P3 amplitude is highly sensitive to global (over an entire

block) and local (over the immediately preceding stimuli) probability. In serial

search both probabilities are low because this process represents a sequence of

decisions regarding stimulus identity, one for each item in the array. Compared to

pop-out task the global probability is higher but the local probability is smaller.

Otherwise, if the process requires a limited capacity, parallel process, this will


produce a reduction in P3 amplitude. This decrease will reflect the decreased

levels of confidence due to capacity limitations and the global probability

augmentation; previous studies have demonstrated this for less confident

perceptual decision (Johnson, 1986). In our 5 years old full-term group as in

adults, the experimental data favor a feature integration theory (Treisman et al.,

1980) which posits the existence of two different target detection processes.

Premature group.

In the pop-out task, ERP component modulation is in agreement with Luck

et al. studies: automatic feature detection results in a larger anterior and posterior

N2b followed by a large P3. In the preterm group the P3 component is widely

distributed from the frontal to the parietal lobe. This suggests a frontal P3a and a

parietal P3b overlap (Squires N.K et al. 1975). This may reflect that premature

subjects present an orienting reflex for all classes of stimuli that do not seem to

habituate across the experimental blocks. In the premature group, ERP

modulation does not differ in the two tasks (pop-out and serial condition). The

presence of an enhanced N2b component and a P3 amplitude decrease across the

serial task supports a single, attention demanding detection process for both

target types. Taken together the neurophysiological data advocate a parallel

process model for both tasks. This is supported by a P3 amplitude reduction, in

serial task, reflecting a channel capacity limitation. The same pattern of task

effect is found, by our group, in a 4 year old full-term group (Laurent et al. in

preparation). In contrast N2 and P3 latency is 80 ms delayed in the 4 year old

compared to the 5 year old group. Possibly this difference reflects neural

maturational improvement between 4 and 5 years. These results suggest that


premature attentional disabilities are more linked to psychological than

neurological developmental mechanisms.

Are these findings specific to premature? Children with attention

hyperactivity disorder (ADHD) exhibit problems in areas of attention,

distractibility, impulse control and overactivity (Douglas, 1983). It is a serious

disorder markedly affecting academic achievement (Barkley, 1990). There are a

number of studies that have investigated ERPs in children with ADHD (for review

see Taylor et al. 1993). The results have generally shown a lack of N2 and P3

modulation by instructions in the auditory or visual modality. But important

differences in experimental design between studies do not allow any decisive

conclusion. Future studies, comparing ADHD and premature groups, should

examine more specifically attentional deficit in each group.

In summary, the general pattern of results indicates that premature children

cannot engage in two classes of search strategy supporting the parallel/serial

distinction. When they have to pay attention to process complex stimuli they use

the same procedures as they do with an easy one. In addition the present results

show that their performances are reduced despite the subjects engaged more

capacities in the task. Longitudinal studies will have to be conducted in order to

determinate whether this lack of differentiation in attention strategies may

underly learning disabilities.


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Table. 1 Clinical data ========================================================== Prematures (n=10) Full-terms (n=10) ____________________________________ Ages (months) M 63.6 64.2 SD 1.5 2.2 Maternal education M 11.5 12.6 (Years) SD 2.5 3.5 MPC / K.ABC) M 110.9 103.9 SD 11.9 9.4 SIM / K-ABC M 111.5 107.4 SD 10.9 7.7 SEQ / K-ABC M 105.9 98.9 SD 13.7 14.9 ========================================================== M= mean; SD= standard deviation; K.ABC= Kauffman Assessment Battery of Children; MPC= Mental Processing Composite; SIM= Simultaneous Processing; SEQ= Sequential processing


Table 2. .Behavioral performance. ====================================================== Pop-out task Serial task Preterm Fullterm Preterm Fullterm ___________________________________________ |¯¯¯¯¯¯¯¯¯ ** ¯¯¯¯¯¯¯¯¯| RT (ms) M 693.6 659.8 803.6 772.1 |__________ ** _________| SD 117.6 83.5 106.2 84.8 |¯¯¯¯¯¯¯¯¯ *** ¯¯¯¯¯¯¯¯¯| Hit (%) M 91.8 94.0 77.2 79.3 |__________ *** ________| SD 7.5 4.8 11.3 11.5 ======================================================= M= mean, SD= standard deviation, ** p < .00 *** p < .000


Table 3. Behavioral performance. Analysis by block =========================================================== Pop-out task Serial task Preterm Fullterm Preterm Fullterm Block 1 Block2 Block 1 Block2 Block 1 Block2 Block 1 Block2 __________________________________________________ RT (ms) M 697.7 688.9 826.0 793.3 643.4 675.1 788.0 755.9 SD 130.2 111.1 112.4 166.6 75.5 104.7 82.8 103.7 Hit (%) M 93.7* 90.0 95.0 93.1 70.6* 84.4 79.4 80.0 SD 6.6 10.7 6.5 6.9 13.8 11.9 15.6 14.1 =========================================================== M= mean, SD= standard deviation, * p < .02


Table 4. ANOVAs: neurophysiological results. ========================================================== Vector Of dependant Factor df(1,2) F value p value variable _________________________________________________________________ Anterior sites Fz, Cz, F8,F7 N2b amplitude G x T x S 1,18 7.78 .012 P3b amplitude G x T x S 1,18 11.77 .003 _________________________________________________________________ Posterior sites Pz, Oz N2b amplitude Task 1,17 5.40 .033 G x S 1,17 7.94 .012 latency Task 1,17 43.46 .000 P3b amplitude T x S 1,17 12.74 .002

G: group; T: task ; S: stimulus


Table 5. Mean amplitude of N2b and P3b components in each group and for each task (pop-out and serial search). Full-term group Preterm group N2b Pop-out M - 5.1* - 2.5 SD 6.5 7.3 Serial M -1.0* - 5.5 SD 6.6 6.9 P3b Pop-out M +12.6 +22.9** SD 12.2 12.8 Serial M +16.7 +14.6** SD 14.7 15.5 M= Mean, SD= standard deviation, ** p < .005 ; * p < .05


Table 6 . P3 mean amplitude in uV for each Group for the serial task in blocks 1 and 2 ================================== Pre-term Full-term ______________________ Block1 M 37.2* 29.5* SD 19.9 16.2 Block2 M 27.0* 39.9* SD 11.8 17.4 =================================== M= mean SD= standard deviation, * p <.02


LEGEND

Figure 1: Electrode and stimulus effect. Task and group conditions are pooled.

Solid line represents waveform elicited by targets, dotted line waveform elicited

by standards. Positivities are displayed up.

Figure 2 : Group effect. Left side shows fullterm group effect on N2b between

pop-out task (solid line) and serial task (dotted line). Right side displays

premature group effect on P3b between pop-out task and serial task.

erp indices of undifferentiated visual attentional search strategies in 5 years old preterm children

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