(Institute of Space Medico-Engineering, Beijing 100094)

abstract：Objective To study the response characteristics of left and right brain to signals from left(LVF) and right(RVF) visual fields during cognitive activity. Method ERPs at 9 locations to LVF and RVF signals were compared in 23 normal subjects in 3 task conditions:looking forward only (VC); making switch response to target signals(T) only(SR); making switch response to T differentially(DR). Result (1)Significant difference in ERPs was found between that induced by LVF and RVF signals on left brain especially at frontal location(F5), which appeared as slow negative deflection induced by T and NT from RVF in SR and DR;(2) Condition-dependent unsymmetrical features between left and right brain were found: the slow potential at F5 was significantly more negative in T and contralateral spatial relation(i.e., left brain for RVF) but more positive in NT and ipsilateral relation than that at F6.Conclusion Signals with psychological meaning in RVF might cause more psychological load as suggested by the results obtained in this study.

Key words: cognition;visual field;brain function;selective attention;left brain;right brain

摘要： 目的 探讨在认知活动中左右脑对左右视场（LVF，RVF）信号的反应特性。 方法 对23名被试者在3种任务条件下9个部位的与LVF和RVF相应的事件关联电位(ERPs)进行了比较。刺激信号为位于LVF和RVF中的红、绿闪光，等概率随机出现，交叉地规定红或绿闪光为靶信号（T）。3种任务是：视对照（VC）；简单选择反应（SR）；选择区分反应（DR），对LVF和RVF的T分别向左和右拨开关。 结果 (1)除外源性的P1和N1成分外，与LVF和RVF相应的ERPs的差别突出地表现在左额区（F5），即在SR和DR条件下RVF的T－ERPs及NT－ERPs中出现显著的负慢波，而右脑对信号的方位不敏感；(2)左右脑对LVF和RVF信号反应的不对称性与认知状态有关：靶信号时与对侧视场相应的ERPs左脑比右脑显著偏负，而非靶信号则是与同侧视场相应的ERPs在左、右脑出现显著差别，且是右脑（F5，C5）偏负。 结论 左右脑对靶和非靶信号的反应不同提示二者在信号加工过程中的不同功能；左额区对RVF中靶和非靶信号的负慢电位反应提示右视场中具有心理意义的信号可能引起较大的心理负荷。

中图法分类号：B842.1文献标识码：A文章编号：1002-0837(2000)03-0157-05

The lateralization of brain response to signals from left and right visual field(LVF and RVF) has been revealed by divided visual field experiment, either by evoked potential^［1］ or by task related ERPs components^［2］. The main characteristics of the task-related lateralization was that the amplitude became greater for negative and positive potentials induced by signals from contralateral and ipsilateral visual field, respectively.

In view of the response features of the two sides of brain, it is interesting to know further whether this kind of lateralization is symmetrical or not. Although findings for unsymmetrical lateralization have been reported^［3～5］,the lateral attention was involved in those studies.To see the effect of visual field or the response characteristics of left and fight brain per se, lateral attention should be avoided. Thus, in the present study, the same flashing signals from LVF and RVF were used in different cognitive tasks, and the effects of ipsilateral and contralateral spatial relationship on ERPs components at the two sides of brain were compared. The special concern was the slow potential response as it related to the brain function state change^［6,7］.

Method

Subjects and experiment conditionThe subjects were 23 paid healthy right-handed volunteers aged 23.4±2.9 years.The experiment was conducted in an electrical shielded and sound-damped room with moderate background lighting. The subjects were in supine position during experiment.

Stimulation and taskThe stimulation signals were red(R) and green(G) flashes provided by two double-color LEDs with moderate brightness which were positioned 3cm apart from the center yellow LED on the left and right, respectively. While the center LED was lighted weakly all the time, the red and green flashes appeared randomly in LVF or RVF with equal probability and intervals of 1.5 to 2.3s so that four types of input signal were produced, i.e., LVF-R, LVF-G, RVF-R and RVF-G. The number of flashes in one run was 120. The plastic board carrying the LEDs was positioned 30 cm in the front of subject eyes.

While the subjects were asked to look at the center LED, 3 task conditions were assigned as follows: 1) looking at only(VC); 2)making switching response simply(SR) to target signals(T) and ignoring the non-target ones(NT); 3) making switch response differentially(DR) to target signals, i.e., switching to left or right for Ts from LVF or RVF, respectively. Taking red or green flashes as Ts was cross-balanced among subjects.

Signal recordingEEG signals were recorded at F5, F6, C5, C6, P5, P6, Fz, Cz and Pz according to the 10-10 EEG electrode system^［8］ referenced to the linked mastoids. EOG was recorded from left eye.The EEG, EOG and the event pulses representing the four types of signals were digitized on line at a rate of 256/s by a 12 bit A/D converter and stored on optical disk.

ProceduresAfter the subjects practiced all the 3 task operations, recordings were made in VC, SR and DR conditions, intervened by 3 min.

Data analysisERPs were derived by using the recorded 4 kinds of event pulses, representing red and green signals from LVF and RVF, respectively, as trigger signals. The resulted ERPs, with a time length of 2000 ms containing 500 ms before stimulus, in each subject were grouped into LT, RT(target from LVF and RVF respectively), and LN, RN(non-target from LVF and RVF respectively) according to the target assignment. Then it was digitally filtered(0～30 Hz) with no phase shift. The mean value of ERPs before stimulation(0.5 s) was used for baseline correction.

The ERPs were compared in terms of paired t-test between different conditions: LVF vs. RVF as well as left brain vs. right brain. Two-way ANOVA was used to evaluate the effect on ERPs of spatial relations and T and NT conditions.

Result

Features of ERPs induced by signals from LVF and RVF in VC, SR and DR conditionsThe ERPs induced by LVF and RVF signals in VC, SR and DR conditions were shown in Fig.1, Fig.2 and Fig.3, respectively. Two prominent points could be seen clearly in all the conditions. The P1(peaked around 160 ms) and N1(around 180 ms) components were evoked predominantly by the contralateral and ipsilateral signals, respectively; thereafter, significant difference appeared on left brain especially at F5 in a common way that the ERPs induced by contralateral signals were more negative than that induced by ipsilateral ones. The degree of this difference showed dependency upon mental condition, i.e., it became much more significant in SR and DR than in VC, and much more significant in T than in NT condition.

Fig.1ERPs induced by signals from LVF and RVF respectively in VC condition

LVF-thin line, RVF-heavy line. The vertical line between the two ERP curves indicates the difference at given point was statistically significant(P＜0.05). The presentation for target(A) and non-target(B) conditions in VC was only for the comparison with that in SR and DR as no selective attention was required in VC.

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