A 10 Hz “alpha-like” rhythm in the visual cortex of the waking cat

Electroencephalography and clinical Neurophysiology, 83 (1992) 217-222

217'

© 1992 Elsevier Scientific Publishers Ireland, Ltd. 0013-4649/92/$05.00

EEG 91605

A 10 Hz "alpha-like" rhythm in the visual cortex of the waking cat M. Chatila, C. Milleret, P. Buser and A. Rougeul * Institut des Neurosciences, Ddpartement de Neurophysiologie Compar~e, CNRS-UPMC, 75005 Paris (France)

(Accepted for publication: 11 March 1992) Summary Rhythmsat about 10 Hz were recorded from the primary visual cortex of the cat (anterior part of area 18), with characteristics close to those of the alpha rhythm in man: frequencyband (7-14 Hz), localization and reactivity to visual stimulation. Coherence analysis of this activity with the "mu" rhythms on the somatosensorycortex showed that although both types develop in the same overall behavioural situations (quiet waking and/or expectancyof an event to occur), they are independent. Key words: Alpha rhythm; Mu rhythm; Visual cortex; (Cat) Since its discovery in man by Berger in 1930, the alpha rhythm, dominating in the posterior cortex, with its classical reactivity to visual stimulation, has been amply accepted by the scientific and clinical communities as one of the most reliable E E G manifestations. It is therefore surprising that so little consistent data exist so far regarding its origin and precise cortical localization. Even in animals, the findings remain scarce. It is true that alpha has been recorded from the visual cortex in cats (Lanoir and Cordeau 1970), from the occipital cortex in monkeys (Jurko and Andy 1967), from the visual cortex, lateral geniculate nucleus (LGN) and pulvinar (Pul) in dogs (Lopes da Silva et al. 1973, 1980). However, no further details have been available on the cortical localization with respect to the known contours of the visual areas. Our previous studies on waking cats have been concentrated so far on other subsets of rhythms, those developing in the anterior part of the cortex (sensorimotor and parietal areas). Two major conclusions were drawn from these investigations performed on behaving animals, implanted with multiple cortical electrodes. First, several types of rhythm could be described (Bouyer et al. 1981, 1983), that were markedly confined to given restricted cortical foci. Mu rhythms (average frequency 14 Hz) were localized in the hand area of the somatic cortex (areas 3, 2 and 1), whereas beta rhythms (average frequency 36 Hz) were found in two foci, one in the motor areas (areas 43, and 6a/3) and another one in the posterior parietal area 5a.

Second, their development occurred in specific behavioural situations. Fourteen Hz mu rhythms were observed when the animal displayed motionless "expectancy of an event to occur" and 36 Hz beta rhythms were seen during motionless "visual fixation on a specific target." Since both situations mimicked attention, we tended to conclude that mu as well as beta rhythms were characteristic of two distinct types of attentional states. We have now extended our study to the rhythmic activities in the posterior part of the cat cortex. So doing, we had in mind our previous findings on the anterior rhythms, showing their strict localization in well identified cortical zones; we felt that the now well accepted complex organization of the cat visual areas (Tusa et al. 1981)justified new investigations of rhythms in the posterior and postero-lateral parts of the cortex. Moreover, this investigation was prompted by the recent emphasis placed upon the rhythmic activities in the cat visual cortex as a support of cognitive processes (Eckhorn et al. 1988; Gray et al. 1989). In this search for a catalogue of the posterior rhythms, we first encountered those in the 10 Hz frequency band, with some characteristics corresponding rather closely to those classically accepted for the alpha rhythm in man. In this paper, we report on an exploration of some posterior and lateral cortical zones, with the main emphasis placed on areas 17 and 18. Parts of this paper have already been published in abstract form (Chatila et al. 1990).

Correspondence to: Dr. Arlette Rougeul-Buser, D~partement de Neurophysiologie Compar~e, Institut des Neurosciences, CNRSUPMC, 9 Quai St. Bernard, 75005 Paris (France).

Methods

* This study was supported by DRET (Contract No. 89-069) and Fondation pour la Recherche M~dicale.

Four adult cats were implanted under halothane anaesthesia with 12 or 22 cortical electrodes over one hemisphere. Each electrode consisted of a single wire

218

(0.5 m m in diameter) insulated except at its very tip, introduced into the cortex. The electrode array encompassed the presigmoid gyrus, the posterior parietal area 5a, the lateral postsigmoid area SI, parts of areas 17, 18 and 19 on the lateral gyrus and parts of areas 7 and 21 on the suprasylvian gyrus. One more electrode was a miniature screw fixed into the frontal sinus bone, which served as reference. Finally two additional screws were secured, one to the superior orbital bone, the other on the external canthus, for recording the electrooculogram (EOG). All electrodes were connected to a 15- or 25-pin socket fixed by acrylics at the top of the head. Two weeks after the operation, the animal was successively placed in each of the 3 standard situations used in our previous studies of the frontal and parietal rhythmic systems (Bouyer et al. 1981). These were: (a) a " n e u t r a l " one, during which the cat usually developed at least one sleep-waking cycle; (b) a condition of "expectancy" of a target to appear, where the cat was waiting near a small hole for a hidden mouse that he could hear moving and smell but could not see, except when it briefly popped out from time to time; (c) a situation with a mouse placed in a transparent box in front of him, in which he would usually display a behaviour suggesting focused attention. The cat's behaviour was recorded on video-tape, while the electrocorticogram (ECoG; active lead against reference) and eye movements ( E O G channel with a long time constant) were simultaneously registered on inkwriter and stored on computer. In this study, data processing was achieved by two distinct methods. One was based on our routine procedure of F F T spectral power analysis. Average spectra were automatically calculated for each channel over fifty 1 sec samples. Those were selected in the frequency band comprising both the visual alpha and the somatic mu rhythms, as evaluated in the raw inspection of each cat's record. The amplitude threshold of the spectral peak in each sample was arbitrarily fixed at 50% of the maximal power of the sample. From these spectral data, other values could be computed, in particular the autospectrum for two channels x and y (Sxx and Syy) and the cross-spectrum between x and y (S~y) at a given frequency f. Using S,=, Syy and S×y, the coherence function was established as G ~ (f) = ($2/S×× " Syy) 1/2, its value (in percent) providing an index of the degree of dependence between the two time series x(t) and y(t) at each frequency. It is usually accepted (Bendat and Piersol 1966) that a coherence value < 20% indicates a lack of correlation or dependence between the two time series, while one > 70% signals a strong relationship between the two activities. We also completed the spectral analysis with a noninstrumental visual inspection of the raw record. This

M. C H A T I L A ET AL.

procedure proved useful in this particular case, to study the "alpha" frequency band (7-14 Hz) with a better frequency resolution than that given by the automatic spectral analysis. In each cat, and for each site, 100 samples of 1 sec each were thus sorted out and the amount of rhythms at the various frequencies in the alpha band was calculated. So doing, we excluded activities in the 10 Hz range which were of very small amplitude, not exceeding that of the desynchronized background, and were most often of shorter duration than the large amplitude alpha trains. Based on these data, the mean and confidence limits of the dominating frequency were calculated for each subject and across the 4 subjects, as well as the probability of occurrence of each frequency, thus providing a first approach to the distribution of frequencies and its mode. For comparison, the same procedure was used

A EOG

a

v 18

Som

b

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on [

off L

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Fig. 1. Individual examples of raw records. EOG: electrooculogram; V18: visual cortex (rostral part of area 18); Som: somatic SI cortex (postcruciate gyrus); Mot: motor cortex (precruciate gyrus); VI7: visual cortex (caudal part of area 17); SL: signal of on and off switching of a light stimulus (bulb). A: independence of 10 Hz visual and 14 Hz SI somatic mu rhythms. At a, a saccade did not interrupt the ongoing mu rhythms at 14 Hz in the somatic cortex (indicated by c) on Som channel. At b, a sequence of 10 Hz Vis developed in visual area 18 (VI8 channel); during that period, no saccade could be detected. Notice the absence of any such rhythms in the precruciate gyrus (Mot channel) and in the posterior visual area 17 (V17). B: typical reactivity of the 10 Hz visual rhythms to light. Cat waiting motionless near a mouse hole; room in dim light. Switching on a bulb placed behind the hole stopped the rhythms. Notice that there was no overt saccade in the EOG.

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219

TABLE I Comparison of the mean frequency for 10 Hz visual rhythms and for mu rhythms in the 4 studied animals (means established from 100 segments of 1 sec). The visual rhythms ranged between 7 and 14 Hz and the mu rhythms between 10 and 18 Hz. C o m p a r e with Fig. 2. Subject

Rhythm 10 Hz Vis

Mu

Mean

Freq. bd

Mean

Freq. bd

• SAM • JAC * ELA • MrX

10.4+0.4 11.1+0.4 10.7_+0.4 10.4+0.4

7-13 8-14 8-14 8-14

Hz Hz Hz Hz

15.3_+0.4 13.4_+0.4 12.4+0.3 12.6_+0.4

12-18 10-18 10-15 10-17

Mean

10.6_+0.3

8 - 1 4 Hz

13.4_+ 1.4

10-18 Hz

0

10

20

30

40

50 Hz

0

10

20

30

40

50 Hz

0

10

20

30

40

50 Hz

0

10

20

3'0

40

50Hz

Hz Hz Hz Hz

with the mu rhythms (11-18 Hz) recorded from the somatic area SI during the same experimental periods. To investigate the reactivity of the visual rhythms to eye closure and opening, 2 TV cameras and a mixer were used for simultaneous observation of the animal's head and eyes, and of the polygraphic recording of the ECoG and EOG, with the possibility to observe them frame by frame. Histological controls: the tips of the cortical electrodes were identified on 100 # m Nissl stained slices of coronal serial brain sections. Their localizations were systematically verified using the histological documents provided by Otsuka and Hassler (1962) and Tusa eta[. (1981) for the cytoarchitectonic boundaries.

Results

Rhythms at about 10 Hz were easily recorded from the visual cortex (based on this localization, we shall designate them here as 10 Hz Vis, rather than alpha), and were composed of regular sequences (Fig. 1A: V18) of variable durations (0.5-11 sec). Nevertheless, as a rule, onset and termination of these trains were simultaneous on all active electrodes. Intersubject variations in frequency could be noticed, with means between 10.4 and 11.1 Hz (Table I), which gave an overall mean of 10.6 Hz. Fig. 4 shows curves (with black symbols) to illustrate probabilities of occurrence for the 4 subjects and the global curve, built from these figures. The respective modes were at 10, 11, 10 and 10 Hz. The spectral analysis also produced coherent results: Fig. 2 illustrates power spectra computed for each subject, with again a main peak always around 10 Hz. Taken together then, these figures justify calling these rhythms 10 Hz Vis. The 10 Hz Vis were only recorded from 2 electrodes, 6-8 mm apart, depending on the subject (see Fig. 3), in each of the 4 cats of our present series. In all 4 cats the active recording tips were located in area 18

Fig. 2. Individual power spectra of the visual alpha-like rhythms in the 4 subjects. Average spectra calculated from 50 samples of 1 sec each. Each subject is designated by a filled symbol (triangle, square, star and circle). Spectra correspond to the anterior ( • , * and e), and to the posterior ( B ) electrode locations indicated on Fig. 3. Peaks at 2 - 3 Hz correspond to movement artifacts.

(Fig. 3). One of them (posterior • ) was clearly situated in this area, while 4 electrodes (posterior * and e, and both • ) were at (or very close to) the border between areas 17 and 18, i.e., on the projection zone of the central vertical meridian of the visual field. Three other tips (anterior *, • , o) were found at (or very near to) the limit between areas 18 and 19 where the periphery of the visual field is represented. No electrode (©) located either more caudally in area 17 (Fig.

•

\

\ Fig. 3. Localization of the recording electrodes in the 4 studied cats. Black symbols designate those which recorded the 10 Hz Vis. Empty ones, those with no 10 Hz Vis. Circles with crosses ( ~ ) localize electrodes that recorded 14 Hz mu (from area SI). ANS, COR, CRU, LAT, SS respectively: sulcus ansatus, coronalis, cruciatus, lateralis and suprasylvius. Limits indicate the accepted cytoarchitectonic divisions of the cat parietal and posterior cortex (5, 7, 17, 18, 19, 20, 21). The location of the active electrodes with respect to these features were histologically controlled (see text). Horizontal metric scale indicates rostro-caudal frontal Horsley-Clarke coordinates in mm.

220

M. C H A T I L A ET AL.

T A B L E II Distribution of conditions of occurrence of 10 Hz Vis in % of trains of rhythms in the 4 subjects. F: flat EOG; AS: immediately after a saccade; SS: stopped by a saccade; PC: particular cases (e.g., persistence during pacing or licking). Subject

F (%)

AS (%)

SS (%)

PC (%)

• SAM IIII J A C ~, ELA • MrX

66 60 88 79

8 10 4 lI

23 19 8 8

3 11 0 2

1A: V17), more rostrally in the upper parietal associative 5 and 7 areas, more laterally in the suprasylvian gyrus (areas 7 and 21), or in the posterior part of area 19, showed any 10 Hz Vis distinct from the background noise. Pooling together all electrode locations in the 4 subjects (Fig. 3) indicates the existence of a focus in the lateral gyrus, extending at least 10 mm longitudinally. From our present series, we could not determine whether there existed 10 Hz Vis more laterally in area 19, or more medially in area 17, since these areas were not explored. On the other hand, no such rhythms could be recorded from the medial suprasylvian gyrus. The conditions in which the 10 Hz Vis displayed a reactivity to eye movements deserve comments. In most cases (60-88%, see Table II: F), these rhythms appeared with open eyes in the intervals between 2 ocular saccades, i.e., while the E O G record was flat (Fig. 1A). At other times (Table II, AS: 4-11%), they developed immediately after a saccade. Moreover, if a saccade occurred during a 10 Hz Vis train (e.g., when the cat's attention was drawn to some visible target), the train was immediately interrupted (Table II, SS: 8-23%). On some rare occasions (Table II, PC: 2 11%), it persisted despite some eye movements accompanying body activities like pacing or licking. Most important was of course the reactivity of these rhythms to light. 10 Hz Vis developed very rapidly when the experimenter closed the animal's eyes by hand. Moreover, the rhythms were not interrupted by eye movements as long as the eyes were closed. When removing the hand, the eyes usually remained closed for several seconds and 10 Hz Vis persisted, but as soon as the cat began to open his eyes, they disappeared (these results were clearly obtained through frame by frame observation of the tape recording). When the cat was placed in the "expectancy" situation (cf. Methods), with a small bulb placed in the mouse hole, turning the light on (Fig. 1B: SL) in front of the animal stopped the 10 Hz Vis (Fig. 1B: V18), although no saccade had occurred at that time, as indicated by the flat EOG. It was interesting to specify further the conditions in which we could notice a high density of 10 Hz Vis.

Three such situations could be identified. The first was, in our commonly used situation of expectancy, as the cat, in a "sphinx-like" position, was waiting for the hidden mouse to become visible. His eyes were open and his head was turned towards the mouse hole during most of the time. The second situation was, in more neutral conditions, usually designated as "quiet waking"; the animal was then also in a sphinx-like position, but often with his eyes closed. However in this case, the cat could soon pass into another stage, that of drowsiness, characterized by the development of slower visual rhythms at about 5 Hz, usually preceding the development of slow sleep with spindles (Rougeul et al. 1974). Finally, 10 Hz Vis could also be recorded during some stages of paradoxical sleep (PS) as we previously reported for the somaesthetic mu rhythms (Rougeul et al. 1972). During these episodes, the visual ECoG remained activated ("desynchronized") with at times trains of rhythms which displayed features of the 10 Hz Vis, since they were very similar in amplitude and frequency of those recorded from the same electrodes (and not from other ones) during the waking state. On the other hand, we never observed any 10 Hz Vis during periods of focused attentive fixation on a visible target (a situation that we knew to be accompanied by fronto-parietal 36 Hz beta activities (Bouyer et al. 1981). These 3 conditions for the appearance of 10 Hz Vis rhythms were in a way similar to those in which we could also record the 14 Hz mu rhythms in the somatic SI cortex (Rougeul ct al. 1972; Bouyer et al. 1981). The question could thus naturally be asked whether or not these two sets, the 10 Hz Vis and the mu rhythms, were distinct. The answer was unequivocally yes, for 4 reasons: (1) Although trains of 10 Hz Vis and of mu rhythms usually developed during the same overall periods, a more detailed inspection indicated that they were independent in time of occurrence, being sometimes simultaneous, sometimes only overlapping, sometimes not coinciding at all (Fig. IA: b and c). (2) While visual rhythms (and not mu rhythms) were interrupted by eye movements, as specified above, mu rhythms were stopped by the slightest body movement. (3) The frequencies of the two activities were clearly different in all subjects. The mean mu frequency was, depending on the cat, between 12.4 and 15.3 Hz (global mean 13.4 Hz, Table I), i.e., clearly higher than the 10 Hz Vis. In each subject, the envelopes of the occurrence curves overlapped, but were nevertheless distinct (Fig. 4); the modes for visual rhythm in the different cats were 10, 11, 10, 10 and the corresponding ones for the mu were 15, 14, 12, 12. (4) Finally, the coherence function, calculated for each cat between two closely lying active electrodes in the visual cortex (intra-alpha coherence) was from 56 to 82% (Table III: Vis-Vis). On the other hand, the coherence values computed in each subject

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221

A • SAM

*

........... B

~

l0

iI

•

~

1]

ELA

1

14 I~

]E

ll

IO 19HZ

JAC

• MrX

% i

....................................

ii

'r

B

%

50

i

B

9

10

11

t2

t3

t4

t5

t6

f7

IBHz

Fig. 4. Curves of probability of occurrence of 10 Hz Vis and mu rhythms in the frequency band 8 - 1 8 Hz taken from 100 samples of 1 sec each (see Methods). A: individual curves for each cat (filled symbols for alpha, open symbols for mu). Note that the abscissa scales are different for the 4 subjects. B: pooled data from the 4 cats, with SDs. Compare with Fig. 2. Notice that the two frequency distributions always overlap, but that their modes clearly differ (10 Hz for the visual ( , ) and 12 Hz for the somatic activities (~) for the global curves in B).

T A B L E 1II Coherence values in the 4 subjects. Vis-Vis: between two electrodes both recording 10 Hz Vis. Vis-Som: between a posterior electrode recording 10 Hz Vis and a rostral one recording m u rhythms. Subject

Vis-Vis (%)

Vis-Som (%)

• • * •

71 56 82 69

8 11 6 9

SAM JAC ELA MrX

between a 10 Hz Vis recording electrode and a somatic mu recording one, was always very low (from 6 to 11%, Table III: Vis-Som).

Discussion These rather preliminary observations deserve several brief comments.

They clearly indicate that some rhythms exist in cat that are very close to the human alpha rhythm: they belong to the same frequency band and display the same reactivity pattern to eye opening and to visual stimulation. Moreover, they are, like human rhythms, localized in the visual cortex. We were, however, able to add one original finding, in that we could localize them in a limited portion of the primary visual area. They were clearly located in area 18 and at 17/18 border. We could determine the anterior borderline of this focus, and its posterior one in area 17, although the lateral and particularly the medial limits (in area 17) remain to be investigated. Since we have previously concentrated our efforts on the anterior focal rhythms (which had indeed been previously described in the human E E G as mu and beta rhythms, but with only very limited details on their localization), we were indeed interested to extend our data to the posterior cortex and establish comparisons between mu and alpha rhythms. In this respect, our data support the contention that the two rhythms are distinct, by various criteria in addition to frequency, such as reactivity and localization, although they may develop in the same general conditions of quiet waking and of expectancy. Actually two aspects of our findings may seem contradictory since 10 Hz Vis could occur with eyes open during motionless expectancy, although in another situation, opening the eyes stopped these rhythms. Moreover, 10 Hz Vis were interrupted by eye movements with open eyes, and not during closed ones (as indicated by the EOG). These contradictory observations on reactivity to light had already been documented in classical studies of the alpha rhythms in man. It is well known and even trivial that alpha rhythm is highly facilitated by eye closure while it has also been reported that an unpatterned white surface with eyes open ("Ganzfeld," Avant 1965) can enhance its occurrence (Lehtonen and Lehtinen 1972). The key to this contradiction may well be found in the difference between passive and active vision. Alpha rhythm may be favoured by the passive one and suppressed by the active visual processing. Regarding the reactivity to eye movements, very few human findings are available so far, mainly because alpha is generally recorded in the "eye-closed" conditions with no specific control of eye movements. On the other hand, the cat's 10 Hz Vis being fairly easy to elicit with open eyes, we could show that saccades were always accompanied by blocking of these rhythms. In a way this observation reinforces the suggested hypothesis of a difference between passive and active vision. Our previous data on exploration of the rostral cortical areas had already emphasized the importance of attentive fixation in the development of at least one of the subsets of rhythms, namely the beta 36 Hz system, which became very dense when the animal was

222 " a t t e n t i v e " to a target p r e s e n t in its e x t r a p e r s o n a l space. T h e significance of the o t h e r a n t e r i o r rhythms, the 14 Hz mu, is less clearcut in that it was essentially seen in situations of " e x p e c t a n c y " or occasionally during " q u i e t waking." It is particularly striking that in a c o n d i t i o n i n g p a r a d i g m , w h e r e both situations can occur in succession, m u rhythms may first a p p e a r (as the a n i m a l waits) a n d be s u d d e n l y replaced by beta rhythms (as the a n i m a l watches a change in, for example, lumin a n c e of a fixation point, as in the now widely used attentive fixation paradigm). O n e would of course have expected to find similar p h e n o m e n a in the visual areas, d u r i n g visual a t t e n t i o n . F o r the time being, 10 Hz Vis a p p e a r in a situation of expectancy of a visual target (whether c o n d i t i o n s could be f o u n d which would specifically favour o n e of the rhythms, 10 Hz Vis or mu, r e m a i n s a topic for f u t u r e investigation). Conversely these rhythms d i s a p p e a r as soon as the visual i n f o r m a t i o n c o n c e r n i n g the expected target is available for processing. T h a t a n o t h e r set of rhythms, with m u c h higher frequency, can also be observed in the visual cortical areas, has in a way b e e n indicated by r e c e n t findings ( E c k h o r n et al. 1988; G r a y et al. 1989). U n f o r t u n a t e l y these data were mainly o b t a i n e d u n d e r N 2 0 anaesthesia. More e x p e r i m e n t s on the waking a n i m a l are n e e d e d to clarify this point. If it is true that both types of rhythm exist in the posterior cortex as they do in the a n t e r i o r areas, some i n t e r e s t i n g g e n e r a l i z a t i o n s could thus be possible r e g a r d i n g the substrate for cognitive o p e r a t i o n s in the various domains. It is now a well accepted view that cortical rhythms are u n d e r t h a l a m i c control (Steriade et al. 1990). W e previously showed that the a n t e r i o r rhythms were under the control of a " c o m m a n d " t h a l a m i c zone of restricted extent (situated in n u c l e u s ventralis posterior for the 14 Hz m u rhythms, a n d in the posterior group for the parietal 36 Hz rhythms). T h e same probably holds true for the posterior 10 Hz Vis. Lopes da Silva et al. (1973) f o u n d alpha rhythm in dog's lateral geniculate a n d pulvinar, while we also previously recorded 10 Hz rhythms with electrodes located close to the j u n c tion b e t w e e n lateral g e n i c u l a t e body, n u c l e u s lateralis posterior a n d p u l v i n a r ( L a c o m b e et al. 1974). Nevertheless, Lopes da Silva et al. (1973) claimed that, in a d d i t i o n to thalamo-cortical c o n n e c t i o n s , also corticocortical c o n n e c t i o n s are i m p o r t a n t . Clear-cut coherence data b e t w e e n activities of the cortex a n d of well identified sites of the t h a l a m u s will be n e e d e d to solve this question. Studies are c u r r e n t l y in progress along these lines. We acknowledge Dr. E• Ornitz for his kind collaboration in evaluating the data. We also wish to thank Mrs. C. Durand for her diligent and expert technical assistance.

M. CHAT1LA ET AL.

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