ELECTROENCEPILMJ3GRAPHY AND CLINICAL NEUROPHYSIOLOGY
SINGLE OF
UNIT
CORTEX-CAUDATE
237
ANALYSIS CONNECTIONS
C. E. ROCHA-MIRANDA,M.D. 1 Laboratory of Neurophysiology, National Institute of Neurological Diseases and Blindness, National Institutes of Health, Bethesda, Md. (U.S.A.) (Accepted for publication: February 2, 1965)
INTRODUCTION
Caudate-cortex and cortex-caudate pathways have been extensively analysed in neuroanatomical, electrophysiological and behavioural studies (for a review see Merrier 1942; RochaMiranda 1961; Laursen 1963) and the existence of cortex-caudate connections in particular, has been supported by recent experiments. Since the early observations based on strychnine neuronography (Dusser de Barenne et ai. 1942), a number of other investigations have provided evidence for these connections with electrophysiological techniques (see review in Rocha-Miranda 1961 and Laursen 1963). Neuro-anatomical studies have also yielded information on cortico-striate direct connections (Glees 1944; Nauta and Mehler 1961 ; Carman et al. 1963). The present study is concerned with an analysis of caudate neuron activity following stimulation of the neocortical surface and of the periphery. The presentation of the data will be subdivided into three groups: one dealing with the general functional characteristics of all the cells studied, the second with the features of those cells which seemed to be more directly related to the cortex and the third with observations on animals with chronic lesions. METHODS
Of a total of 44 experiments in adult cats, 11 were under chloralose anaesthesia (50-70 mg/kg of alpha chloralose, 1% aqueous solution, i.p.); 24 were under barbiturate anaesthesia (Nembutal 20-80 mg/kg). The other 9 were cerveau isold i Present address: Instituto de Biofisica, 458 av. Pasteur, Rio de Janeiro, Brasil.
preparations. Blood pressure (femoral), respiration and body temperature were systematically monitored. All experiments in ;~tact preparations were carried out under Flaxedil and artificial respiration after pneumothorax. With the head in a stereotaxic instrument, 2-3 rigid polyethylene tubings were introduced (through holes in the skull) with the aid of an orienting rod at the appropriate coordinates to serve as guides for the recording electrodes and when their distal end was at about H + 9 the tubes were fixed to the skull with dental cement and the rod withdrawn. Frontal and lateral planes for the penetrations were (A) 12.5-20 and (L) 3-6, respectively. Stimulating electrodes were placed through slightly larger holes and consisted of two Teflon coated stainless steel wires (34 gauge) with exposed tips I mm apart. These were inserted 2 mm deep in the cortex and secured to the bone. The actual site of stimulation was checked with macro- and microscopic examination. The cingulate gyrus was explored with electrodes consisting of two copper wires 1 mm apart, isolated and cemented together with varnish InsI-X. Unitary (extraceilular) activity from caudate cells was recorded by means of glass-coated platinum etched micro-electrodes (Wolbarsht et al. 1960) and, subsequently (and more successfully), with Woods aUoy-indium filled micropipettes (modified from Dowben and Rose 1953) double drawn from borosilicate Coming glass tubing (#7740). Recording was through a Unity Gain Bak Electrometer, its output leading to a Tektronix 122 pre-amplifier and a Tektronix 502 CRO. The animal was grounded by a saline-wick electrode Electroeneeph. clln. Neurophysiol., 19~, 19:237-247
238
C.E. R O C H A - M I R A N D A
placed on its skull and connected to the amplifier's ground through a low impedance SIE Microsource model K-I. Cortical stimulation was carried out with square pulses (Tektronix type 161) through a modified AEL stimulus isolator (mad. 112). Current was monitored at the output of the isolation unit. The following parameters were used: 1 mA (0.1-3.0); 0.5 msec (0.3-1.0) and 0.2-0.5/see. Bipolar stainless steel electrodes were also used for peripheral (ant. limb intradermic) stimulation (parameters: 1-10 V, 0.3-0.5 msec and 0.05-0.5/sec). For the placement of the micro-electrodes, the horizontal plane was determined by the first contact with the fluid filled tubing, whose upper extremity rested at plane H + I 2 . Subsequently, in Nissl stained sections tile relative depth at which each observation was made, was plotted along the prolonged longitudinal axis of the tube's track (one to four penetrations could be performed down each tube). The following criteria (derived from observations in the internal capsule) permitted a reliable differentiation between axonal and cell body spikes; the former being characterized by: (a) shorter observation period (never beyond 3 rain); (b) smaller potential field (i.e. below 15/~) and (c) shorter spike duration. Identification of any one element was based on the fulfillment of at least 2 out of the 3 preceding criteria. All isolated units which could be observed for a sufficient period of time were identified by a code similar to that used by Powell and Mountcastle (1959); their position in depth was noted, their duration and amplitude was measured and their firing frequency during the first and the last minute of observation v,,as ,,ou,ted. Highfrequency discharges were recorded on magnetic tape (Ampex model CPI00) and analysed by a mad;fled Pulse Height Analyser which fed into an Event Counter (model PHA-5 and Research Scaler Ratemeter, model RCR-3, Nucleonic Corporation of America), in parallel with a Tektronix 502 CRO. This system permitted observation and measurement of individual frequency of nearly-concurrent spikes. In addition, the pattern of discharge to supramaximal cortical as well as peripheral stimuli weJ'e analysed on film on the Behnson and Lehner's Oscar Record Reader model J.
TABLE I Conclusion of histologicalexamination Number of cells Localization insidecaudate (Cd) boundaries Localization within 1 mm of Cd boundaries Accepted cells Localization outside Cd (sigmoidor cingulate gyri) Lack of control Rejected cells Total number of cells
185 18 203 47 7 54 257
RESULTS 1. General characteristics Spikes to be analysed were selected on the basis of the following criteria: (a) good discrimination from electrode noise (usually below 50 ttV) and other spikes; (b) fulfillment of the above mentioned criteria for a "cell body spike"; (c) absence of obvious signs of injury: (d) response. Some cells which, at first, seemed responsive did not meet this criterium under later analysis. These were not excluded from the population. 1. Spike population. Activity was recorded from a total of 257 cells. Later histological examination indicated incorrect micro-electrode placements in the case of 54 of such cells (see Table I). Of the 203 remaining cell spikes, 18 which were recorded using potassium citrate electrodes under Nembutal, are not considered here. This leaves a working population of 185 cells of which 54 were from chloralose, 75 from Nembutal and 56 from cerveau isol# preparations. Wle
C~ralose N:34
27
49
71 g21mn
Nemtxttal --
Uecerebrat~l
N=62
"
~
N=4~)
~,
~
4~
7~
q~l,rm
Fig. 1 Frequency distribution of the spontaneous activity (spiker/rain) of caudate cells recorded under choralose, Nembutal and cerveau isold (decereL,rate,i) preparation. Histogramsare based on I rain epochs at the beginningof the observation. Electroenceph. clin. Neurophysiol., 1965, 19:237-247
I
239
CORTEX-CAUDATE CONNECTIONS
TABLE II Average probability of spike number in a response Preparation
Stimulus
N
I-Spike
Pgo
2-Rpike
Pg0
3-Spike
Pgo
Chloralose Nembutal Chloralose Nembutal Pentothal
paw paw cortex cortex cortex
58 27 102 145 !45
0.454 0. ! 21 0.425 0.354 0.344
0.86 0.27 0.96 0.96 0.96
0.039 0.022 0.020 0.027 0.034
0. !8 0.09 0.09 0.09 0.09
0.006 -0.002 0.002 0.002
0.05 -0.05 0.05 0.05
SA
2. Spontaneous activity. As shown in Fig. 1, the spontaneous activity o f caudate cells was predominantly characterized by low frequency in the three preparations and especially in the animals under Nembutal. This tendency towards low frequency firing would facilitate the study of excitatory effects during stimulation whereas the interpretation of possible "inhibitory" eff~ts would often be questionable. 3. Response pattern. Following a supramaximal stimulation, caudate cells tend to respond with one single spike rather than a train o f spikes as is commonly the case in other cerebral structures (Rose and Mountcastle 1954; Mountcastle et ai. 1957; Bishop et al. 1962a; M c C o m a s 1963). The average probability for one two or three spike discharges in response to supramaximal (peripheral and ¢¢¢tical) stimulation in the various preparations is shown in Table II. Here the cells were subjected to 22 trials of stimulation and the mean probabilities computed by adding the number of times a given response (I-spike, 2-spike, 3-spike) occurred and dividing by 22. The averages of the mean probabilities are represented with the 90 percentile o f the mean probability distribution.Cells with no spikes during 22 trials are not included in the number (N) of mean probabilities. Correction for spontaneous activity is not indicated. N o t e how a l-spike response is, on the average, from 5 to 21 times more likely to occur than a 2-spike response and up to 212 times more so than a 3-spike response. This is valid for the stimulus strength employed in these experiments (i.e. 0.5-1.0 mA and never exceeding 3 mA). A stimulus was judged supramaximal when no further increase in the probability of a response or in the n u m b e r o f spikes in the response occurred. Since responses with two or more spikes to
$P
L
AD-AS
:hlorolose N-43
0.50 i
T - 62
/
nembutal N=64
N=,,28 T==40
N =9 T=26
N=, 27 T=52
OecerebroteO
0.50
N-44 T,, 66
1
N-29 -
N-29 T,=49
Fig. 2 Frequency distribution of the probability of occurrence of l-spike responses to supramaximal stimulation of proteus and/or anterior sigmoid (SA), posterior sigmoid (SP), lateral (L) gyri and the anterior paws (AD-AS) in chloralosed, nembutalized and decerebrated animals. The abscissae have been divided in I i intervals (Roman numerals), I representing a probability of I or 2 spikes (P = 0.05 or 0.09) occurring in 22 trials under supramaximal stimulation, II, 3 or 4 spikes (P = 0.14 or 0.18) and so on, to 21 or 22 spikes (P = 0.96 or 1.00) at XI. Counts were taken from 200 msec epochs following stimulation. The ordinates are drawn in a proportion scale. N: numberof responsive cells from a total population (T). The similarity of probability distributions to stimulation of SA and SP is in marked contrast to those in response to L and AD-AS, wherein a shift to low probabilities predominates in the nembutalized and de,cerebrated preparations. (A cell may be represented once in two or more histograms or even twice in ¢he same histogram if it responded to more than one mode of stimulation or to the stimulation from 2 electrode placements in the same gyrus, respectively.)
Electroenceph. clin. Neurophysiol., 1965, 19:237-247
240
C.E. ROCHA-MIRANDA
either cortical or peripheral stimulation were relatively rare, the l-spike response was considered rather typical and was used to study the characteristics of the cell. In Fig. 2 several "response spectra" are represented in relation to the stimulated area of the cortex or to anterior limb stimulation and to the experimental condition. Each of these histograms represents the frequency distribution of the probability for l-spike response of individual cells. Data were derived from observations of l-spike incidences in 22 trials under supramaximal stimulation of the given site (only "responsive cells" are represented). Two main features stand out in Fig. 2: (a) Histograms from stimulation of proreus, anterior and posterior sigmoid gyrus are bimodal with very high and very low probabilities of l-spike response and only a slight difference in form with the preparation. (b) The spectra of response elicited by lateral gyrus and paw stimulation show that a fairly high probability of l-spike response becomes evident only under chloralose, being extremely low in the nembutaiized and (with lateral gyp'us stimulation) in decerebrated preparations. Stimulation of the cingulate and suprasylvian gyrus would yield response similar to those of the fir;t and second group respectively. (Chloralose preparations did not yield a large enough sample to draw satisfactory conclusions for these two areas.) 4. Response ratio. This represents the percentage of cells which respond to supramaximal stimulation of a given area. Before its determination it is necessary to take into consideration the spontaneous activity of the population and the probabiD:y of occurrence of different frequencies of spike discharges in order to differentiate between "response" and "no response". Table III, analogous to Table II, presents the average of the mean probability of a spontaneous discharge with one~ two and more spikes, corn-
puted from 22 epochs of 200 msec for each cell, under the three experimental conditions. Spontaneous discharges with 1-spike are the most frequent, particularly under Nembutal. Two or less discharges in 22 trials (P -- 0.090) have oeen considered as "no response". For a 2-spike discharge, even though the highest average was 0.023 for its spontaneous occurrence, we drew the line at one discharge in 22 trials (P = 0.050), for its low probability of occurrence under stimulation (slightly above spontaneous level, compare Table II and IIl) would make it haz5A
100t N=62
5P
N=66
N=96
N=41
L 100"
AD.A5 N=70
N=22
50
Fig. 3 Representation of responsive neurones (percentage) in the chloralose (black bar), Nembutal (gray bar) and decere. brated preparation (white bar) to supramaximal stimulation. The probability of response was estimated from 22 trials of 200 msec duration each, "no-response" being considered as a P = 0.09 for l-spike p~ttern or P ----0.05 for 2-spikes (see text). Whereas similar levels of responsiveness under different preparations are found with SA and SP stimulation, a drop from high level of responsiveness under chloralose, to low levels in other conditions accompany L and AD-AS stimulation.
TABLE III Average probability of spike number in a spontaneous discharge Preparation
N
l-Spike
Peo
2-Spike
Chloralose Nembutal Decerebrated
36 61 53
0,047 0.090 0.054
0.14 0.36 0.23
0.006 0.022 0.023
Pgo 0.09 0.18 0.32
3-Spike 0.000 0.001 0.009
Pgo -0.09 0. ! 8
4-Spike
Pgo
0.000 0.000 0.003
Electroenceph. clin. NeurophysioL, 1965, 19:237-247
241
CORTEX-CAUDATE CONNECTIONS
ardous to consider it as a "response" in the absence of other patterns. Discharges with higher number of spikes can be ignored due to their very low spontaneous and response probabilities. The relative amount of"drive" which different cortical and peripheral areas exert upon caudate cells in different preparations is shown in Fig. 3. On the basis of the above mentioned criteria, data were obtained from 156 cells subjected to supramaximal stimulation, their response patterns being analysed in 22 epochs of 200 msec duration. Also in this case anterior and posterior sigmoid gyri display a rather similar response ratio in the three preparations, the response level oscillating about the 50% level for SA and slightly lower for SP. On the other hand, responsiveness to stimulation of the lateral gyrus falls abruptly from a 77 O//olevel under chloralose to 18.5% and 24.4 % in the Nembutal and decerebrated preparations, respectively. Similarly, responses to stimulation of the paws drop from 72.9 % (chloralose) to 17.3 % (Nembutal). Stimulation of suprasylvian and ectosylvian gyrus both in the nembutalized and decerebrated animals exhibited a low response ratio; this tended to increase in the few experiments performed under chloralose. 5. Overlapping fields of influence. Two simple mechanisms could explain the increase in responsiveness of the chloralose preparation: (a) recording from a different population of otherwise unresponsive cells and (b) a facilitatory effect upon afferent pathways converging upon the same population. The first cannot be evaluated in the present experiment, since no record was taken of the relative number between responsive and unresponsive elements. The latter were studied only when doubt was expressed as to their unresponsiveness. Fig. 4, on the other hand, would suggest the possibility that the second mechanism be involved. These histograms were drawn from the behaviour of cells subjected to 22 trials of supramaximal stimulation from, at least, three different sites. The criteria adopted for a response were the same as those employed for Fig. 3 and 1, 2 and 3 or more cells were allocated to one of three groups, depending on the number of effective stimulating sites. Under chloralose, the majority (58°3 %) whereas in the other two preparations,
AD,A5 50-
Cortex
: 1 FIt ]3o,>1 1
2 3or> Chlof'OloSe
1
Nembuto!
Decerebratecl
Fig. 4 Relative frequency histograms of caudate ceils (submitted
to stimulation of at least three sites) according to the number of inputs ~.abscissae) in different preparations. The top histograms (AD-AS) represent the per cent distribution when supramaximalstimulation of the paws is included. The bottom histograms ate obtained from cells which were subjected to the stimulation of at least three cortical sites. The number of trials and the response level are those indicated in Fig. 3. Note how chioralose increasesconvergence.Between30 and 50 cells, whichmet the requirements,were included in each histogram. only 15.4 % and 29.3 % (for Nembutal and cerveau isole') responded to three or more different stimuli. It thus appears that irrespective of the stimulus origin, a caudate cell is under a great number of effective impingements under chloralose; this fact should be kept in mind in the interpretation of Fig. 3. 6. Latency values. These were obtained, for the first or only spike, in 107 cells, in each of which 20-30 responses were used to estimate the individual means. From these caudate cells, 165 mean latencies were obtained in their responses elicited by supramaximal stimulation of different cortical and peripheral areas. These values (which may be biased since they represent only cells with higher probability of evoked discharge) are shown in Table IV. Despite the considerable overlap of means with stimuli of different origin there is a trend on the averages and ranges for some of the points, which is probably not due to differences in sample number. The latency means of responses elicited from SA or SP have the same sampling distribution as those from L or AD-AS below 0.005 level (t test). Responses to stimulation of the paws and lateral gyrus exhibit the highest Eiectroenceph. clin. NeurophysioL, 1965, 19:237-247
242
C.E. ROCHA-MIRANDA TABLE IV Latency values of l-spike (msec) SA
Chloralose Nembutal Decerebrated Total
SP
Cells
,~
~
~
Range
Cells
18 35 26 80
17.8 22,1 16,5 19.2
7.0 10.6 7.1 9.1
2.4 3.4 2.3 2.8
9.1-30.4 8.4-47.7 8.5-30.8 8.4--47.7
3 11 10 24
~"
~z
CING ,~
. . . . 11.2 5.8 1.5 16.4 7.9 2.3 13.3 9.5 1.8
Range
Cells
X
~
4.3-20.3 5.7-33.1 4.3-33.1
1 8 7 16
_ 19.2 19.8 19.8
Range
,~
m
m m 8.4 3.6 5.7 3.9 10,3 3.5
4.1-29.0 13.1-31.1 4.1-31.1
AD-AS Chloralose Nembutal
10 .
D e c c r e b r a t e d
.
Total
10
35.4 . .
.
35.4
10.8 6.1 . .
.
13.5-57.4 .
27 .
.
.
.
4.5
19.7-59.2
4.5
19.7-59.2
10.1
.
.
10.8 6.1
41.0
~
13.5-57.4
~
27
~
41.0
10.1
,~ = weighted means of the individual mean latency values to the first spike. s~ = standard deviation of ~'. ----- mean standard deviation of the latencies. SA: anterior sigmoid; SP: posterior sigmoid; CING: anterior cingulate; L: lateral gyrus; AD-AS" right and left front paws.
average and an upper range of mean latency values. Conversely, responses from sigmoid and cingulate region display lower averages and the shortest latencies in the population. This same trend was observed when the comparison was drawn with the individual preparations instead of the total population. These differences would be enhanced if cells with lower probability of l-spike response for L and AD-AS were included, for in these high latency values were common. It should also be mentioned that in no instance were there mean latencies lower' than the ones represented in Table IV in responses to supramaximal paw (122 observations) and lateral gyrus (98 observations) stimulation. Latency variations (as expressed by the S.D.) proved to be a better indication as to the nature of a particular connection than the latency itself, as we will show later. These were minimal in responses elicited from SA, SP and CING and maximal in those from L and AD-AS.
II. Special characteristics The following tests, in view of their nature, could not be applied to the whole cell population. I. Recovery. Twin pulses (supramaximal and at 0. l-0.5/sec) were used to study the variation in excitability. Responses with long and responses with short recovery cycles were observed. These were quantified by estimating the average probability of l-spike response to the second (test)
POl-S
A
1.00-
®~
O* 0
@ 0
O ~r 0 - 2 0
0.50 -
O
0 C-24
e.
e •
,
!
,~T~
oe
1,00 -
•
I
'
~'~0
• b-28 • f-39 0 I .... ~
t
~0
*b-29 O,50-
0¢-29 eg-29 ,~ c - 3 0
h
'
'
I
5
....
I
10
'
'
'
I
50
....
I
'
'
100
I ~g30
Fig. 5 Recovery cycles of caudate cells as measured by the probability of l-spike discharge (P01-S) of the test response (ordinates) at intervals indicated in the abeissae (log scale). Each symbol represents a cell. A: Example of cells with long recovery cycles in response to supramaximal stimulation of proreus, SA, CING and SA, respectively (top to bottom of insert in right side of figure). B: Short recovery cycle as exhibited by caudate neurones in response to supramaximal stimulation of SP (upper 3 cells of insert) and SA (lower cell), respectively. The typeof preparation did not seem to influence appreciably the type of recovery. In B, cells b-29 and c-30 exhibited a cut-off of the test response at intervals too short to be precisely determined (due to stimulus artifact).
Electroenceph. clin. Neurophysioi., 1965, 19:237-247
mse¢
COlt~X-CAUDATE c o ~ c n o N s stimulus in 20 or 22 trials at several inter-stimuli intervals. The probability of l-spike response to the conditioning stimulus and to the test stimulus alone was used as a control of excitability. (a) Long recovery. This was characterized by a zero probability for inter-stimuli intervals shorter than l0 msec, with gradual return to the control situation, frequently followed by facilitatory effects. It was the most common type of recovery and could be observed in responses obtained in all three preparations, to stimulation of any cortical area, with high or low probability of l-spike discharge, long or short latency, to homologous (two stimuli to same locus) as well as heterologous (conditioning and test stimuli in different areas) stimulation (see Fig. 5, A). Nevertheless, cells with probability of 1-spike response lower than about 0.90, as well as those responding to heterologous stimuli, would exhibit exclusively this type of recovery cycle. This was true also for all the responses elicited by stimulation of the paws, lateral suprasylvian and ectosylvian gyri. (b) Short recovery. A few units exhibited identical probability of l-spike response to both stimuli even at very short intervals (down to 2 reset). Transition to no response to test stimulus would take place abruptly, with discrete reduction of the inter-stimuli interval (Fig. 5, B). These responses could only be obtained from stimulation of SA, SP and CING, and had in common a very high probability of I-spike d;scharge ( > 0.90) and a small dispersion of the mean latency ( S . D . = 0.07-1.50 msec). In some instances, a unit which responded to two different loci in the sigmoid gyrus with the above characteristics would display short recovery cycles to homologous but never to heterologous stimulation (see Fig. 5).
2. Changes in latency during recovery. A latency shift could be observed in the test response, its direction and magnitude being dependent on the lime interval and type of recovery cycle. The top two curves on Fig. 6 are taken from cells with long recovery cycles (see also Fig. 5, A) and illustrate the most common changes in latency during recovery phase; i.e., its lengthening with shortening of the inter-stimuli intervals and a progressively smaller probability of I-spike discharge down to the abolition of the test
243
response. At longer intervals (about 200 msec) facilitatory effects may be occasionally detected with shortening of latency and higher probability of firing. At shorter intervals, nevertheless, the behaviour is similar to that of cells b-28 and f-39 (see Fig. 6). Mean latency, in itself, was no criterium for predicting latency shifts (the cells in Fig. 6 were chosen with long latencies for display reasons). In cells with long recovery cycle, however, the direction of the shift, at intervals close to cut off values, was always towards an increase. The type of latency changes observed in the few cells with short recovery cycles is also illustrated in Fig. 6. Close to the cut off intervals (1-5 msec) an increase in latency was the general rule. At intervals of > 15 msec a small but definite faeilitatory effect could be observed which was maximal at 25-50 msec. It would then slowly decay to control values or reverse (slight latency increase).
LAT. rnsec 40-
..................Q ....................................................................
++ +
t
......... t .... ! .............. ,
.
.
.
.
.
.
.
.
.
.
f -39
.
c-29 g-29
~0-
50
100
1'30
2 0 0 mse¢ 6T
Fig. 6
Effect of recovery on latency of test responses. Each point represents the mean latency (ordinate) estimated from 20 to 22 test responses at various inter-stimuli intervals (abscissa). Cells t:39 and d-36 are in response to supramaximal stimulation of SA, all others to SP. Points at the 200 msec interval represent the mean latency of control test responses not preceded by the first shock. S.D. around the mean is represented by a vertical bar wherever feasible. The three lower curves are from cells with short recovery. Note the faeilitatory effect upon the latency with a maximum about the 25 m~c interval. Upper two curves are from long recovery cells.
Electroenceph. clin. NeurophysioL~196~. 19:237-247
244
C.E. ROCHA-MIRANDA
3. Fatigue. This term is used here to describe the failure on the part of the neurone to follow more than the few initial pulses of a train of stimuli. Caudate responses (both gross and unitary) are known to be quite sensitive to fatigue, especially under Nembutal and in the non-anaesthetized preparation (Albe Fessard et ai. 1960a, b). In the present work it was found that the caudate response to cortical stimulation would show no signs of fatigue with pulses separated by 2 sec. On the other hand, in the case of longer pathways (such as paw stimulation, or even lateral gyrus), intervals up to ten times this value had to be employed. Cells with a high probability of firing were the most resistant to fatigue, but even in most of these elements fatigue would appear with sigmoid gyrus stimulation at frequencies of 1-20 p/see. Also the shorter the recovery cycle of a cell, the more resistant this would usually be to fatigue, such cells being exceptionally able to follow stimulus frequencies up to 250/see. 4. Facilitation and inhibition. With twin pulse cortical stimulation it was also possible to demonstrate facilitatory effects upon caudate response to the second pulse at critical interstimuli intervals. From the data of Table V these effects would seem to begin at the 5-10 msec intervals. This may be seen in cells with low probability of response to supramaximal stimulation (i.e., a-23, i-31 and g-38) as well as in those with high probability under sub- and just-liminal stimuli (i.e., a-20 and c-24). In the latter case, as the first stimulus strength is raised with consequent increase in the response probability, a subnormal phase will generally appear (see section on Recovery). Inhibitory effects were occasionally observed:
3 cells had exceptionally high spontaneous activity (range: 215-550/min). In these, 22 successive trials of 500 msec epochs of activity were tape-recorded following stimulation of several cortical points, the number of spikes added and compared to the total number of spikes in an equal number of 500 msec epochs of spontaneous activity. A drop to 25-55% in spike number during these epochs was found for stimulat!on of SA, SP and CING which did not seem to induce any appreciable spike response. Stimulation of other areas in the cortex had no effect on the background firing frequency of the caudate elements. Reduction in the probability of firing of the test response in the absence of spike discharge to a conditioning subliminal pulse was never observed. IlL Observations on animals with chronic lesions In 5 of the 44 animals the experiments were performed a week after different types of lesions had been placed in an attempt to interrupt subcortico-cortical paths without damaging the head of the caudate. The lesions were performed with needle electrodes passing high frequency currents through their 1 mm exposed tips from a Wyss lesion-maker, every 2 mm in the dorsoventral axis and at four lateral planes on the right hemisphere at about frontal planes A 11-12. The lesions spared cortex and caudate. In all cases a continuous necrotic area about 10 mm in width and 4 mm in depth, with complete destruction of the internal capsule and anterior pole of the thalamus was demonstrated histologically. These lesions were made to evaluate the possible contribution, to the caudate responses of antidromic effects through collaterals from corticopetal fibers (Ram6n y Cajal 1952).
TABLE V FacUitatory effect of just-threshold stimuli upon probability of I-spike discharge (P01-S)of test responses at several intervals
Single
Test
(msec) Cell
St
P01 -S
a-20 a-23 c-24 i-31 g-3S
SA CING SA SA SS
0.23 0.15 0.15 0.09 0.23
I-3
4--6
0.05
0.77 1.00 0.36 0.55
0.27
7-9
10-12
13-15
16-18
0.96 0.68 1.00 0.45 1.00
0.32 ! .00 0.50 0.64
0.00 0.68
20
30
40
0.96
0.45
0.09
1.00 0.36 0.45
0.60 0.09
50
0.10
Electroenceph. olin. Neurophysiol., 1965, 19:237-247
CORTEX-CAUDATB CONNECTIONS TABLE VI Per cent of responsive neurones in animals with CNS lesions
N Response ~
SA
SP
CING
AD
AS
66 51.5
35 54.3
20 45.0
13 69.2
13 00.0
Three of these experiments were carried out under Nembutal ~nd 2 under chloralose and the behaviour of 35 caudate cells was studied in response to different stimuli. Sampling and criteria for response were the same as those employed in Fig. 3. The data are shown in Table VI (those for cingulate gyrus stimulation were only obtained under Nembutal and those for stimulation of the paws only under chloralose). The greater number of anterior sigmoid stimulations is due to the fact that the results of stimulation of two points in this area were grouped together. A comparison with Fig. 3 indicates that responses from both anterior an~ posterior sigmoid have not been altered by the lesion. Also the results obtained with cingulate gyrus stimulation did not show a significant change in the experiments with brain lesions. Curiously, ipsilateral paw stimulation in both animals under chloralose displayed a high response ratio in contrast with stimulation of the contralateral paw. This finding is not easily explained. It should be pointed out, however, that in these animals the lesion did not separate head and tail of the caudate nor did it affect the cortical mantle. No qualitative differences (firing probability, latency, recovery etc.) were observed in the behaviour of those cells which responded to cortical stimulation in the animals with lesions. This would suggest that as far as sigmoid and cingulate regions are concerned, the corticocaudate fiber system does not involve corticopetal fibers from thalamic and lower levels. DISCUSSION
These findings seem to indicate that there are cortical regions which, on stimulation, are capable of driving only a few caudate elements in the Nembutal or decerebrated preparations but greatly increase such capacity under chloralose.
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These regions include the lateral and possibly, the suprasylvian and ectosylvian gyri. On the other hand there are other cortical regions whose capacity of controlling caudate elements does not seem to vary appreciably in the three types of preparations. These latter regions are the proreus, anterior and posterior sigmoid and possibly the cingulate gyri, although as far as the latter is concerned it was difficult to rule out stimulation of buried portions of the pericruciate cortex or of its fibers. Several characteristics, among which the firing probability, response ratio, recovery pattern and latency, approach the activity evoked by peripheral stimulation to that elicited from the lateral and suprasylvi~l gyri of the cortex. It is thus probable that the pathway from these cortical regions to the caudate be multisynaptic as it is, obviously, the case for that from the periphery to the same structure. Against this last conclusion there are recent findings in the rabbit (Carman et al. 1963) suggesting a direct projection from the entire cortical mantle to the caudate in the rabbit. This discrepancy may be due to the fact that in the present study the survey was limited to head and body of the nucleus. (In the above work, direct projections from visual and auditory areas were very restricted, and exclusively, to the tail region of the caudate.) It is also possible that these direct projections be less dense or that they subserve a primarily inhibitory function and the~e be difficult to demonstrate in the present experimental situation. On the other hand our evidence seems to favour the existence of more direct connections between other cortical areas and caudate. This hypothesis is compatible even with the obtained latency values if one assumes that the pathway consists mainly of small fibers (a probable situation in view of the difficulty to demonstrate these connections by means of anatomical techniques). From the lower latency values, the conduction velocity would be approximately 2.0 m/see, assuming the shortest route between the stimulating and recording electrodes. Equally compatible with a direct pathway were the short recovery tinge values (Darian-Smit et ai. 1963) as well as the small variability in latency values (commonly with twice the S . D . - - 0.1 msec; see Darian-Smith 1960). Electroenceph. olin. Neurophysiol., 196~, 19:237-247
246
C. E. ROCHA-MIRANDA
From our group of units with a high probability of l-spike response following stimulation of sigmoid and cingulate regions, various values of frequency-following were noted, some units dropping out at 4/see, others being driven up to 250/see (the highest value tested). These findings, and in particular the occurrence of driving at relatively high frequency, are also compatible with (without providing, however, crucial evidence for) a direct pathway. Cortical inhibitory influences on caudate cells were not systematically investigated in the present work. Such influences were occasionally observed (e.g. drops of over 50 % of the spontaneous count in 200 msec epochs following cortical stimulation); but only with stimulation of sigmoid and cingulate gyri. Whereas some of the above mentioned findings have been considered to be compatible with, or to suggest the existence of a direct path to the caudate from certain cortical areas, it remains to be determined whether the related phenomena were the result of ort[ 3dromic or of antidromic activation. The latter would seem to be compatible with the following observations: (a) tendency for a single spike response; (b) small variability in latency values; (c) short recovery time and (d) capacity for the spike to follow relatively high freqrency of stimulation. All these features are essentially the same which have been used in support of a direct pathway and even though they are considered characteristic of antidromic responses, they can be present in cases of synaptic activation if the system consists of only one synaptic function. Actually, certain observations, such as e.g. the reduction of the mean latency either with increasing stimulus strength or with twin-pulse stimulation, would be more compatible with orthodromic phenomena. In analogy, the failure of observing a selective inhibition of the B component of the spike in the test response when twin pulses were used (a phenomenon described in a number of elements in different CNS levels ! see Brock el al. 1953; Fuortes et al. 1957; Phillips 1959; Bishop et al. 1962b; Phillips et aL 1963 and Towe et al. 1963), would make less likely the possibility of antidromic activation. Thus, in conclusion it is not possible to assess the true nature of most of these phenomena and
we can only note that a direct orthodromic nature of projections from sigmoid regions would be in agreement with neuroanatomical evidence whereas this is totally lacking (see Voneida 1960) to support the alternative possibility of their antidromic nature. SUMMARY
Extracellular recordings were obtained from 185 cells, sampled from head and body regions of the caudate nucleus in 44 cats under chloralose (54 cells), Nembutal (75 cells) and decerebrated (56 cells). 1. Stimulation of certain cortical regions may have an excitatory effect on caudate neurons. (Inhibitory effects were not investigated systematically and have been shown only with stimulation of sigmoid and cingulate gyri.) 2. The most common response consisted of one single spike, multispike discharges occurring infrequently. 3. Excitatory effects from anterior cortical regions (proteus, anterior and posterior sigmoid, anterior cingulate) differ from those observed from posterior regions (mainly lateral gyrus and possibly supra- and ectosylvian gyri) inasmuch as the former were characterized by (a) high probability of firing and ratio of responsive to unresponsive elements, irrespective of the preparation (posterior regions respond accordingly but only in the chloralose preparation); (b) responses with lower mean latency value and with smaller variations and (c) short recovery cycles and high following rate to repetitive cortical stimulation (up to 250/see). The posterior regions evoked responses which were akin to those obtained from the periphery, i.e., multisynaptic in nature, whereas the anterior cortical regions may evoke responses of a more direct nature. 4. The results have been interpreted as suggesting that a small fraction of our neurone population is directly (anti- or orthodromic monosynaptic) connected with the anterior cortical region by small diameter fibers which are not dependent on collaterals from corticopetal fibers from lower levels. This work was performed at Dr. Wade H. Marshall's laboratory to whom the author would like to express his gratitude, as well as to Mr. Anthony F. Bak, Mr. AI Electroenceph. clin. Neurophysiol., 1965, 19:237-247
CORTEX-CAUDATE CONNECTIONS Zyminski, Mr. William Buriss and Miss Moira Hurley for their technical help.
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Reference: ROCHA-MIRANDA,C. E. Single unit analysis of cortex-caudate connections. Electroenceph. clin. Neurophysiol., 1965, 19: 237-247.