Zhurnal Vysshei Nervnoi DeyateI 'nost i imeni I. P. Pavlova, vol. 18, no. 3, pp. 500-506, 1968

D I F F E R E N C E S IN C O N F I G U R A T I O N OF E V O K E D R E S P O N S E S OF THE CAT

SENSORIMOTOR CORTEX

N. I. NEZLINA and A. D. VOROB'EVA

Insti tute of Higher Nervous Activity and Neurophysiology, USSR Academy of Sciences

Cor t ica l evoked potentials in unanesthet ized animals have a complex configurat ion consis t ing of a p r ima ry pos i t ive-nega t ive complex followed by additional posi t ive and negative waves of va r i ed duration and amplitude (3, 6, 18, 20, 22, 23)-. The shape of evoked potentials is known to differ in var ious par ts of the cor responding project ion a rea (4, 5, 25, 28). The configuration of the evoked response depends on s t rength and s teepness of i nc rease of the s t imulus (1), andmay also be affected by the shape and s ize of the record ing e lec t rode and i ts p r e s s u r e on the bra in t i s sue .

Evoked potentials recorded in man by e lec t rodes applied to the scalp are par t icu- l a r ly complex and var ied in configurat ion (9, 11, 12, 16, 24). Many inves t iga tors have noted that, despite s imi l a r i ty in shape of complex potentials a r i s ing in response to s t imu- lation of a given modali ty in the cor responding project ion co r t ex in all subjects , s ignif i - cant individual d i f ferences a re found in the configuration of the separa te components of potentials. These individual d i f fe rences , exp res sed as d i f fe rences in amplitude, latency, and duration of the separa te waves , a re p r e s e r v e d if repea ted record ings a re made f r o m the same individual over a long per iod of t ime (8, 13, 15, 26). Some authori t ies cons ider that the r ea son for these observed d i f ferences in potentials l i e s in d i f fe rences in th ick- ness of the bony skull and scalp, and d i f ferences in position of the record ing e lec t rode re l a t ive to the exci ted cor t i ca l point.

To what extent the fac to rs mentioned above are responsib le for d i f fe rences in con- f igurat ion of evoked potentials , and to what extent they r e f l ec t t rue individual var ia t ions in the sub jec t ' s nervous sys tem are difficult to decide f r o m invest igat ions on man. In animal exper iments the problem of individual d i f fe rences in configuration of evoked potentials has not yet a t t rac ted attention. If one or two e lec t rodes are inse r ted into a cor t i ca l project ion zone, d i f ferences in shape of the evoked potential in different animals can be attr ibuted to d i f ferences in posit ion of the record ing e lec t rode over the project ion area . Recordings f r o m large numbers of points in project ion zones have been made pr incipal ly in acute exper iments and near ly always under anesthesia , when individual d i f ferences are leas t v is ib le .

In the p resen t invest igat ion evoked potentials in the s enso r imo to r cor tex of un- anesthet ized animals were studied.

Exper iments were c a r r i e d out on cats in which 24-32 identical s i lve r ball e l ec - t rodes with a tip 1 mm in d iamete r were implanted epidural ly; the e l ec t rodes were located in the region of the an te r ior and pos te r io r sigmoid gyr i , and also in the upper par t of the anter ior suprasylvian and coronal gyri . Under these exper imenta l conditions i t was imposs ib le for thickness of bone, posit ion of e lec t rode and other f ac to r s which could be r e f l ec t ed in the configuration of the evoked potentials to have any effect . E l ec t r i ca l s t imuli , 0.2 msec pulses , were applied to the l a te ra l surface of the fo re leg , and potentials were recorded repeatedly for a long period -- f rom 2 weeks to 3 months. Recording was monopolar and the r e f e r e n c e e lec t rode was located on bone in the region of the external occipi ta l protuberance.

Potentials evoked in the cor responding p r im a ry somatosensory a rea by e l ec t r i ca l s t imulat ion of the con t ra la te ra l fo re l imb cons is t s of a s e r i e s of waves of di f ferent duration, in a definite o rder . At the focus of maximal act ivi ty, where responses to s t imulat ion of this par t icu la r par t of the skin have g rea t e s t amplitude (middle par t of the s igmoid gyrus) , the cha rac t e r i s t i c pos i t ive-negat ive p r i m a r y complex is followed

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INDIVIDUAL DIFFERENCES OF EVOKED RESPONSES

by a second, sl ightly s lower posi t ive wave, with i ts maximum 17-24 m s e e af ter the s t imulus ar tefact . Next follows a slow negative wave, the amplitude and duration of which v a r i e s cons iderably f r o m one animal to another. Bes ides these, in some cats another posi t ive wave of long duration (30-80 msec) is c l ea r ly v i s ib le , followed by a smal l , slow negative wave.

Fig. 1. S imi la r i ty in distr ibution of evoked potentials ove r senso r imoto r co r t ex to e l ec t r i ca l s t imulat ion of con t ra l a t e ra l fore l imb in two cats (A and B). Loca l i - zation of implanted e lec t rodes shown schemat ica l ly .

Responses of a different configuration are r ecorded in the an te r ior s igmoid gyrus (motor cortex) . The principal posi t ive wave is always preceded by a smal l pos i t ive- negative wave {with a slow sweep speed only the negative component of this wave is read i ly vis ible) , the negative wave of the main complex is l e ss well defined. Ad- ditionally, a smal l pos i t ive-negat ive wave is observed 30-40 m s e c af ter application of the s t imulus.

714

INDIVIDUAL DIFFERENCES OF EVOKED RESPONSES

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Fig. 2. Individual differences in configurat ion of evoked r e - sponses. A) Maximal responses of somatosensory cortex of five different cats; B) graph showing these responses plotted f rom mean pa rame te r s for three neighboring points of the maximal locus (five responses for each point); C) evoked responses in motor cortex of different cats .

The general pat tern of d is t r ibut ion of evoked activity over the sensor imotor cortex is s imi l a r in i ts main features for all the animals (Fig. 1A and B). However, dis t inct individual differences in configuration of evoked potentials are observed in identical points of the project ion zone, expressed as differences in amplitude and durat ion of par t icu la r components. Traces of maximal evoked responses in five different cats , obtained by superposi t ion of five responses , are given in Fig. 2A and the amplitude and duration of par t icu la r components of these responses are shown graphical ly (B). Clear ly the shape of the response differs in each of the five animals , pa r t i cu la r ly as regards configuration of the late components. Evoked potentials in the motor cortex also differ in shape in different an imals {C).

Although on frequent repet i t ion of the recording cer ta in var ia t ions can be observed in the amplitude and duration of par t icu la r components of the evoked responses in the same animal , the shape of the response r ema ins stable for each animal for a long period (Fig. 3).

However, in the same animal responses are identical over a re la t ive ly wide a rea of the somatosensory cortex, so that slight differences in a r rangement of the recording e lect rodes in this a rea are not very important . Mean va lues (for five responses} of

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INDIVIDUAL DIFFERENCES OF EVOKED RESPONSES

the amplitude of the evoked potential at var ious in terva ls after the s t imulus for each three points of the maximal ly responding area in two cats are compared in Fig. 4.

This shows that the ampli tude- t ime curve of the evoked potential at each of the three points is identical for the same animal but different for different animals .

No rela t ionship could be detected between configuration of the evoked potentials and strength of s t imulus. The s t imulus usual ly el ici ted only slight movement of the animal ' s l imb. With an increase f rom weak (not producing reflex movement of the s t imulated limb) to strong st imuli no significant change occurred in the general configuration of the evoked potential (Fig. 5).

Potentials ar is ing in a wide a rea of cortex, cor respond- ing to the p r imary projection area , in response to e lect r ical s t imulat ion of the fore l imb (7, 10, 21, 29) in cats, and recorded by implanted e lectrodes , thus possess a c h a r a c t e r - is t ic configuration for the somatosensory or motor area. They differ in different animals in amplitude, latency and duration of their components, especial ly of the la ter waves. These differences are evidently independent of d isplacement of the recording electrode over the projection zone and of s t rength of s t imulus applied. Uttal and Cook (26)also found in invest igat ions on man that changes in position of the recording electrode on the skull do not significantly change the shape of the response charac ter i s t ic of a par t icu- lar subject, while according to Becket et al. (9), for weak e lec t r ica l s t imulat ion of the upper l imb, not producing a

Fig. 3. Evoked r e - reflex response, and felt only as a weak tact i le s t imulus, sponse of somato- an evoked potential of typical shape is generated in the sensory cortex keeps somatosensory cortex. i ts shape when r e - Although there is as yet no general agreement regarding corded repea ted lydur - the nature and genesis of evoked potentials, it can be taken ing one month. In te r - that their components in the somatosensory cortex ref lect val between t races 1-4 the end resu l t of different processes and influences whose about one week. interact ion in t ime de termines the general configuration of

the potential. It is in te res t ing to note that individual ampl i tude- t ime indices of the evoked potential charac ter i s t ic of a par t icu lar animal are preserved for a long period. The amplitude of individual components of the evoked potential is known to vary in different types of condit ioned-reflex activity (2). Late components of the evoked potential are modified in man during concentrat ion or d is - t ract ion of attention (14, 17, 19, 27). The workers cited attr ibute this phenomenon to a change in physiological state of neural s t ruc tures taking par t in the genesis of these components. Differences appearing in the configuration of evoked potentials in different animals are perhaps associated with their typologic pecul iar i t ies , ref lect ing the re la t ionship between inhibition and excitation in the cortex. However, the influences of cer ta in individual anatomical differences between subjects , and also the influence of age on ampl i tude- t ime charac te r i s t i c s of evoked potentials, cannot be ruled out.

In this connection it is also important to note that if the configuration of evoked potentials recorded under chronic conditions in ma mma l s at different levels of phylo- genetic development is compared, increas ing complexity of configuration, especial ly of the late components of the potential, is seen in the more highly organized animals and, in par t icular , in man.

SUMMARY

1. Evoked potentials in the sensor imotor cortex of different cats show individual differences which pers i s t for a long period.

716

INDIVIDUAL DIFFERENCES OF EVOKED RESPONSES

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Fig. 4. Ampli tude-t ime curve of evoked responses recorded at three different points of the maximal ly responding a rea (mean data for five responses at each point) in two cats (No. 35 and 38).

Fig. 5. Evoked responses of s o m a t o s e n s o r y c o r t e x r e - corded during s t imulat ion of con t ra la te ra l forel imb at different in tensi t ies . 1) Response of somatosensory cortex, 2) EMG of flexor muscle of shoulder on s t imu- lated side. Numbers denote s t rength of s t imulus .

717

INDIVIDUAL DIFFERENCES OF EVOKED RESPONSES

2. Differences in configuration of evoked potentials are independent of recording conditions (thickness of bone, position of electrode, etc.), and also of strength of electri- cal stimulation of the skin.

3. Individual differences in evoked potentials probably reflect individual peculiari- ties in structural and functional organization of the central nervous system of particular animals, especially of their cerebral cortex.

Submitted 13 July 1966

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