Dynamics of potentials evoked in the auditory pathway and reticular ...

Kybernetik 16, 27--35 (1974) 9 by Springer-Verlag 1974

Dynamics of Potentials Evoked in the Auditory Pathway and Reticular Formation of the Cat. Studies during Waking and Sleep Stages* * * ft. Ozesmi and E. Ba~ar Institute of Biophysics, Brain Research Laboratory, Hacettepe University, Ankara, Turkey Received: November 13, 1973

Abstract Averaged evoked potentials in the inferior colliculus (IC), medial geniculate nucleus (MG) and reticular formation (RF) of chronically implanted and freely moving cats were measured using auditory step functions in the form of tone bursts of 2000 Hz. The most prominent components of the AEP of the inferior colliculus were a positive wave of 13 msec and a negative wave of 40-55 msec latency. The AEP of the medial geniculate nucleus was characterized by a large negative wave peaking at 35 40 msec. During spindle sleep and slow wave sleep stages changes in the AEPs of both nuclei occured. Transient evoked responses of the inferior colliculus, medial geniculate nucleus and reticular formation were transformed to the frequency domain using the Laplace transform (one sided Fourier transform) in order to obtain frequency characteristics of the systems under study. The amplitude characteristics of IC, MG, and RF obtained in this way revealed maxima in alpha (8 13 Hz), beta (18-35 Hz) and higher frequency (50-80 Hz) ranges. During spindle sleep stage a maximum in the theta frequency range (3-8 Hz) and during slow wave sleep maximum in the delta (1-3 Hz) frequency range appeared in the amplitude characteristics of these nuclei. The amplitude characteristics of the inferior colliculus and medial geniculate nucleus were compared with the amplitude characteristics of other brain structures. The comparison of AEPs and amplitude frequency characteristics obtained using these AEPs reveals that the existence of a number of peaks (waves) with different latencies in the time course does not necessarily indicate the existence of different functional structures or neural groups giving rise to these waves. The entire time course of evoked potentials and not the number and latencies of the waves, carries, the whole information concerning different activities and frequency selectivities of brain structures.

I. Introduction

The investigations presented in this study have been carried out in order to determine the systems characteristics of potentials evoked in some nuclei of the cat auditory pathway and the reticular formation during waking and sleep stages. * Supported by Turkish Scientific and Technical Research Council Grant TAG-266. ** Presented in Part at the VIIIth International Congress of Electroencephalography and Clinical Neurophysiology in Marseilles, September 1-7, 1973.

In previous studies we had determined the amplitude frequency characteristics of the cat acoustical cortex and of the hippocampus (Ba~ar, 1972 a, b; B a~ar and Ozesmi, 1972). In this study averaged evoked potentials (AEPs) of the inferior colliculus (IC), medial geniculate nucleus (MG) and mesencephalic reticular formation (RF) upon acoustical stimulation are measured during waking and sleep stages. The AEPs measured in the time domain were transformed to the frequency domain using a Laplace Transform (TRFCMethod described in Section II.1). The use of this method made it possible to obtain amplitude characteristics of the brain nuclei under study during sleep stages of short duration. Therefore the findings presented in this study allow a systems theoretical component analysis of sleep stages in a few brain nuclei.

II. Material and Method ILl. Mathematical Methods a) TRFC-Method. In order to obtain frequency characteristics of the systems under study (IC, MG, and RF nucleus) the step responses (transient AEPs) of these systems were transformed to the frequency domain with the Laplace transform. The general system theory states that any linear system can be fully described in the time domain or in the frequency domain (with frequency characteristics). All information concerning the frequency characteristics of a linear system is contained in the transient response of the system and conversely, all information concerning the time response of the system is contained in the frequency characteristics of the system. In other words, knowledge of the evoked potentials in the time domain can allow one to predict how this system would react to different stimulation frequencies, if the stimulating signal were sinusoidally modulated. If the step response of a system (time response of the system to a step function) is known, the transfer function, G0"m) of this system can be obtained with a Laplace Transform of the following form :

,1, or

o

G(jo) = ~ e -i~ d{c(t)}. o

(2)

G. Ozesmi and E. Ba~ar: Potentials Evoked in the Auditory Pathway of the Cat

28

G(je)) = Transfer function of the system; c(t) = step response of the system; co = 2n f, where f is the frequency of the input signal. The amplitude frequency characteristics IG(je))l and the phase angle q~ are obtained by numerical evaluation of the transform (2) with the help of a Burroughs 3500 digital Computer:

IG(/e))l=

+

(cose)t,).Ac(t, n

(sine)t,).Ac(t,

,

EEG signals were recorded with a Schwarzer Varioscript V 822 and averaged by means of a Fabri-Tek 1072 Instrument Computer. During the experiments the amplitude characteristics of the amplifier were flat between 0.2 and 1000 Hz.

(3)

III. Results

n

(p(e)) = arc tg

~ (sino~t.)-Ac(t.) , =1

] .

(4)

~ (cose)t,). Ac(t.) n=l

The use of the fast Fourier Algorithm for the evaluation of the integral (2) speeds the obtainment of frequency characteristics. More details concerning this method which we call the TRFCmethod (Transient Response-Frequency Characteristics Method) are given by Ba~ar and Weiss (1968). Although this method is valid only for linear systems it can also be applied to nonlinear systems as a first approach. The reliability of this method in the case of brain potentials was emphasized in our previous studies (Ba~ar and Ozesmi, 1972; Ba~ar and Ungan, 1973). b) Ideal Mathematical Filtering Method. This method consists of the following steps: 1. The frequency band limits of theoretical filters are choosen adequately according to the frequency and band width of the frequency components which should be selected or rejected. 2. The weighting function, gr(t) of the ideal filter characteristic Gv(jo~) is computed with the inverse Fourier Transform 1

gv(t) = 2~

+~

~ {IGv(Je))Je j,o~}e+i,o, de).

III.1. Transient Evoked Responses during Waking and Sleep Stages Typical averaged evoked potentials (AEPs) from the mesencephalic reticular formation (RF), medial geniculate nucleus (MG) and inferior colliculus (IC) during waking stage are illustrated in Fig. 1. The stimulation consisted of auditory step functions in the form of tone bursts of 2000 Hz. Our notations for different positive and negative waves are arbitrarily chosen and concern only this study. As we have shown in previous studies (Ba~ar and Ozesmi, 1972; Ba~ar and Ungan, 1973) the latencies and number of the waves in the evoked potentials do not allow any significant understanding of the underlying mechanisms. We will return to this very important point in Section IV.

During spindle sleep stage changes in the AEPs of the nuclei studied were observed. Figure 2 illustrates the AEPs of the RF, MG, and IC. When high voltage slow waves (waves of 1-3Hz) were seen in the

(5)

oo

3. The experimentally obtained transient evoked response, c(t) is filtered using the convolution integral: +~

RF

cF(t)= ~ gv(Z)ctt-z)dz, -

~o

where Cv(t) is the filtered response. Details concerning this method are given in references. (Ba~ar and Ungan, 1973).

,oo.v I

II.2. ExperimentalMethod Surgery. Our investigations were carried out using eleven cats with chronically implanted stainless steel electrodes of 0.2 mm diameter (IVM, NEX-100 electrodes) in the left inferior colliculus (Ft. P 2.5, L.5., H.3.5), in the left medial geniculate nucleus (Fr. A 3.5, L.9., H.I.5) and in the left mesencephalic reticular formation (Fr. A 3, L4., H.-1). The derivations were against a common reference which consisted of three stainless steel screws in different regions of the skull. A David Kopf 1404 Instrument was u~ed for stereotaxic surgery. During the surgery the cats were anaesthetized with 35 mg/kg Nembutal and they were used in experiments not sooner than 3 weeks after the surgery. During the experiments the cats were freely moving in an echofree and soundproof room. The cats were not previously trained for different tasks. Apparatus. The auditory step functions were generated with the help of HP 3300 and 3310 function generators. Usually step functions of 2000 Hz and - 8 0 dB were used. The auditory step functions lasted 3 sec.

MG I

I

IC

2000 Hz OHz[ I

0

100

200

300

400

tmsec Fig. 1. Typical averaged evoked potentials measured in the reticular formation (RF), medial geniculate nucleus (MG), and inferior colliculus (IC) of chronically implanted cats, elicited by an auditory stimulation in the form of step function (lowest curve). Negativity upwards

ft. Ozesmi and E. Ba~ar: Potentials Evoked in the Auditory Pathway of the Cat

i~

Spindlesleep

29

acoustical cortex (anterior ectosylvian gyrus), changes in the AEPs were more prominent. Figure 3 illustrates the AEPs of the RF, MG, and IC during the slow wave sleep stage. RF

lO0~V1 MG

111.2. Amplitude Frequency Characteristics during Wakin9 and Sleep Stages Figure 4 presents a m p l i t u d e frequency characteristics of the reticular f o r m a t i o n (RF), medial geniculate nucleus (MG) a n d inferior colliculus (IC) which were c o m p u t e d using t r a n s i e n t responses similar to those of Fig, 1 with the help of the m a t h e m a t i c a l m e t h o d ( T R F C ) described in " M a t e r i a l a n d Methods". A l o n g

IC

Waking stage 2000 Hz

2O

OHz ! I l

'

0

I

~

100

I

'

I

200

300

'

dB

I

400

t msec

Fig. 2. Typical averaged evoked potentials measured in the reticular formation (RF), medial geniculate nucleus (MG), and inferior colliculus (IC) of chronically implanted cats, elicited by an auditory stimulation in the form of step function (lowest curve). Negativity upwards

- 1 0 - I ........ I ........ I ........ I 0.1 2 4681 2 4610 2 46100

wave sleep

MG

20 3

d ~

1

10 dB

o0 4

0 -10 20 .......

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,

01 2 4681

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4610

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