Simultaneous recordings from pairs of cat somatosensory cortical neurons with overlapping peripheral receptive fields

Brain Research, 341 (1985) 119-129

119

Elsevier BRE 10923

Simultaneous Recordings from Pairs of Cat Somatosensory Cortical Neurons with Overlapping Peripheral Receptive Fields R. METHERATE and R. W. DYKES

Departments of Physiology, Surgeryand Neurology and Neurosurgery, McGill University, Montreal, Que. (Canada) (Accepted November 20th, 1984)

Key words: primary somatosensory cortex - - cat - - cross-correlogram - - parallel processing - - neuron - - slowly adapting - rapidly adapting - - cutaneous mechanoreceptor

Fifty-three cell pairs in the somatosensory cerebral cortex were examined in pentobarbital-anesthetized cats for evidence of short latency interactions. Many neuronal pairs separated by distances of 150 to 500pm were observed to have temporal dependencies. In a subset of 19 pairs where the surrounding multiunit activity could be classified as rapidly adapting or slowly adapting, short latency interactions existed only between cell pairs sharing the same multiunit background activity. If one member of the pair was in a slowly adapting background and the other in a rapidly adapting background, the cells did not influence one another. This observation was taken as evidence for parallel and separate processing of afferent signals from rapidly and slowly adapting cutaneous mechanoreceptors in cat somatosensory cortex.

INTRODUCTION Many neurons in area 3b of the primary somatosensory cortex respond to tactile stimulation of the skin and hair. The responses of these n e u r o n s can be

ters separated by sparsely innervated cortex. They suggested that cells may skip over nearby cortex to communicate with more distant n e u r o n s sharing some functional property and/or projection target. In fact, T o y a m a et al. 29 have studied short latency synaptic interactions between cell pairs in the cat visual

classified as adapting slowly (SA) or rapidly (RA) to a maintained deformation of the skin. Cortical multiunit responses can also be classified in this manner 7,23. In the cat, multiunit mapping studies have

cortex using cross-correlation techniques and have found that only if both members of the pair were in the same ocular dominance column, was there evi-

shown that area 3b contains interdigitating R A and

dence of c o m m o n afferent input to the two cells or in-

SA regions that form submodality-specific mediolateral bands along the extent of the forelimb representation 7,26 and similar zones are found in primates27,28.

tracortical excitation or inhibition of one cell by the other. None of these effects were seen between adja-

Banding patterns have been seen in other cortical areas, notably the ocular dominance columns in the visual cortex 16. In that case the anatomical substrate is likely to be the clustered, patchy arborization of afferent thalamic fibers 10,15. Gilbert and Wiese112 filled cortical cells with horseradish peroxidase ( H R P ) and found very long (up to 4 mm), anisotropic horizontal processes that terminate in discrete, repeating clus-

cent columns. In the pericruciate cortex of the rat R e n a u d and K e l l y 25 used cross-correlation and joint scatter techniques to search for inhibitory influences between cells identified as pyramidal tract and non-pyramidal tract projecting neurons. They found negatively correlated activity between cells up to 920 p m apart. These interactions, which occurred more frequently and over greater distances in the anteroposterior orientation than in the mediolateral direction, may

Correspondence: R. Metherate, Microsurgical Research Laboratories, L4.67, Royal Victoria Hospital, 687 Pine/'.venue West, Montreal, Que. H3A 1A1, Canada.

120 be caused by basket cells which inhibit pyramidal cells and extend up to 1 mm in the antero-posterior direction 2~. The somatosensory cortex not only displays bands of functionally-distinct neurons but also has long, horizontally-oriented intracortical processes 14. Jones 17 has described two cells, his types 1 and 6, in the monkey cortex that have horizontally-oriented axonal arborizations running in the anteroposterior direction. The type 1 cell axon extends up to 1 mm from the cell body and may be inhibitory. The axon of the type 6 cell innervates the superficial layers 500-600 ,urn away. Excitatory horizontal influences may be conveyed via pyramidal cell axon collaterals as described by Landry et al. Is in the cat motor cortex. These collaterals can be short (up to 800#m) and extend in all directions, or as long as several millimeters and be preferentially-oriented. Afferent fibers from the ventrobasal thalamus terminate in two or more dense bushes, 300-600 #m in diameter each and are separated by an equal amount of sparsely innervated cortex 19. Based on this observation, Landry et al. 19 suggested that thalamic neurons of a given submodality (SA or RA) project to cortical regions of the same submodality. Thus, cortical bands may be related to the clustered arborization of thalamocortical axons in a manner similar to the ocular dominance bands in striate cortex, and intracortical elements may connect neurons within bands sharing functional properties or projection targets as suggested by Gilbert and Wiese112. Consequently, when examining the connectivity of neurons in this area, it is essential that their functional characteristics be noted. This report describes the results of simultaneous recordings from pairs of single cells in area 3b characterized by the nature of the afferent stimulus that excites them. Single unit activity was analyzed using cross-correlation 24 and joint peristimulus time scatter techniques u. The multiunit activity surrounding each of the two neurons under study was examined and used to assign the cells to an SA or an RA region. It was found that short latency interactions between cell pairs with overlapping receptive fields occurred only if both neurons were located within the same submodality band (RA or SA) but not if one member of the pair was located in each of these regions. These results lend support to the hypothesis that a significant degree of specificity exists in intracortical con-

nections and that SA and RA bands differ functionally2~, pharmacologically ~ and may receive functionally 4,5 and anatomically 19distinct afferent inputs. MATERIALS AND METHODS Twenty mongrel cats of either sex weighing between 2 and 3.5 kg were used. After an initial intraperitoneal (i.p.) injection of sodium pentobarbital (35 mg/kg) intravenous and tracheal catheters were installed and the animal mounted in a stereotaxic frame. Craniotomies over the somatosensory cortex and ventroposterior thalamus allowed the recording and stimulating electrodes to be inserted. Two concentric bipolar electrodes (ca. 1.0 Mg2) were positioned just above the ventroposterior lateral (VPL) nucleus of the thalamus (AP 10.2 and 8.7; ML 6.2) and then lowered to the point producing the maximum evoked potential in the somatosensory cortex. The cortical recording electrodes consisted of triple barrel pipettes with one barrel fiJled with a 7 ~m carbon fiber (impedance 0.5 to 5 MQ at 1 kHz). The carbon fiber provided a very low impedance with good signal-to-noise characteristics allowing both single unit and multiunit activity to be recorded 2. The other two barrels were filled with L-sodium glutamate (0.5 M, pH 8) for iontophoretic (0-100 nA) application to cells in the vicinity of the recording electrode using a custom-made iontophoretic device. The two multibarrel electrodes were mounted and controlled independently. Neuronal responses were amplified separately by standard electrophysiological amplifiers. The signals were subsequently observed on an oscilloscope and audio monitors. Wellisolated single unit responses were converted to pulses with voltage discriminating devices and all events were recorded on a laboratory computer. Somatic stimulation consisted of lightly touching or stroking the skin or hair or manipulating muscles and joints. Quantitative cutaneous stimuli were delivered via a computer-controlled electromechanical displacement transducer with a two mm diameter tip. The VPL was stimulated with 40 ~s pulses not exceeding 20/xA. To locate silent neurons and to facilitate data acquisition, L-glutamate was applied iontophoretically as the electrode was advanced. Each time a single unit was isolated, the same tests were applied; the VPL was stimulated, the sponta-

121 neous rate noted, and the body surface searched for a receptive field. If located, the receptive field was examined carefully to determine the nature of the stimuli that activated the neuron. These tests, described by Sretavan and Dykes 26 identify the general categories of sensory receptors capable of exciting the neuron. Both the responses of the cells and the responses of the multiunit activity were classified in this manner. First, the responses were identified as being derived from cutaneous or deep receptors. In cases when the response was not adequate to make this distinction it was classified as either no response or responsive only to tapping the body. The cutaneous responses were further divided in SA and R A categories based upon the presence or absence, respectively, of sustained activity during maintained pressure. When two cells with overlapping receptive fields and of known cell type and background characteristics were isolated, several different sets of data were collected. First, the spontaneous activity, if any, of both cells was recorded simultaneously. Then, epochs containing the response to thalamic or tactile stimulation were collected. Often these periods were obtained during glutamate ejection. Finally, the reaction of one cell to excitation of the other cell by glutamate was noted. Data analysis consisted of constructing cross-correlation histograms 24 and joint peristimulus time scatter diagrams 11. The cross-correlation histogram showed the activity of one cell (B) with respect to another (A). In all figures shown here, the action potential of cell A was shifted to the center of the histogram and the number of action potentials of cell B occurring within a predetermined time interval before and after the reference action potential was added to the appropriate bin of the histogram. The left hand side of the cross-correlogram shows the activity of cell B before cell A fired, but can also be viewed backwards as the activity of cell A after cell B fired 2j . The joint peristimulus time scatter technique enabled us to distinguish stimulus-related correlations from correlations in the firing patterns of the two cells that occurred in the absence of overt stimulation (hereafter referred to as neurally-related patterns) 11. To construct this figure, a 3 s data acquisition cycle, or trial, was initiated by the computer. Soon after the beginning of the trial, a stimulus was applied either electrically to the thalamus or mechanically to the

overlapping peripheral receptive fields. In the scatter diagram, stimulus-related responses occur at a fixed latency after the stimulus and result in horizontal or vertical areas of high point density. Correlated activity that is not caused by the stimulus occurs randomly during the trial but at similar times for each cell, thereby producing a high point density along the 45 ° diagonal. The cross-correlation technique cannot generally be used to differentiate between stimulus-related and neurally-related correlations, but produces a figure that gives a better visual impression of the strength of the correlation and its temporal relationships than does the scatter diagram. It is best used to examine correlations that are present in spontaneous or glutamate-driven activity. We have classified the crosscorrelograms by their amplitude, shape and position in time. The amplitude differences lead to inferences about the strength of the synaptic linkage, the shape allows inferences about the temporal precision or tightness of the correlation and the position with respect to the reference suggests a temporal sequence of activity. RESULTS Ninety-four units forming 53 cell pairs were studied in 20 cats. Of these, 30 pairs had overlapping receptive fields. However, for 9 pairs the background classification of one or the other cell could not be determined and in two other pairs one cell was lost before sufficient information was collected. This left 19 pairs that had a known background activity and were fully tested. Since the electrodes were moved independently, the isolated units were at variable distances one from the other and at different depths. Most of the neurons were isolated in the middle layers of cortex between 800 and 1200 ¢tm. The electrode separation varied from 150 ktm to 500 ,urn and averaged about 380/tin. In each experiment an electrode was placed in a region driven by cutaneous inputs and the second electrode nearby in a region identified by preliminary mapping penetrations to have neurons with overlapping receptive fields. While ejecting glutamate, the first electrode was advanced until a cell was isolated. Then the second electrode was advanced until both electrodes had isolated single neurons. A large hum-

122 ber of the neurons found by glutamate ejection did not have cutaneous receptive fields. Of the 94 cells

studied, 23 had no receptive fields and 15 others responded so poorly to somatic stimuli that the field

A I

I i1111 ]

I B

II .ss

lOms

I

I

l .25s

F

i

_..

,.

E

...

. .... , + '.

looms

, , , :,.

"~i.~.l;.~;".".,v:~f" :.: ,,,h ,,.~'.'~;" ',.~. ~ ,.,, I,~',ii;~,.',,,,.

I~~',

~ .

F~'I

~ . ~ i ; ~ _ ~ ' V ~

!',, Ii,~

'~I'

+

.,.

,

. + =:llLqt'

.4-

.~

,

,~

; , ':4 r" ',~£' , .=~,! , . + ~

, . ' . ~I~"

t' I*'

109

;' , ,

--

%

,,=, ,>.

~:',I e,';

i

65

,

,,=, V-- ~

~a~..,.,~.<;~.~?.-.

.= 22

2.0 TIME

(s)

-250

0 TIME(ms)

25o

Fig. 1. Analysis of two simultaneously recorded spike trains• A: oscilloscope traces showing spontaneous activity of cell A (top trace) and cell B (bottom trace). Two sweeps. B: response to stimulation of VPL thalamus (arrow) showing initial excitation, inhibition i rebound and ongoing activity of both cells. Five sweeps, C: stored record of spontaneous activity was plotted by computer. Upwardgoing lines are individual spikes from cell A, downward-going lines are bursts from cell B. D: computer-plotted record of activity following thalamic stimulus (arrow). E: scatter diagram showing responses of cells A, (horizontal axis)and B, (vertical axis) to thalamic stimulation (arrows). Diagram displays responses to stimuli as horizontal and vertical lines indicating excitation, inhibition and postinhibitory rebound activity. Correlated activity not related to the stimulus is manifest as a diagonal line: F: crOss-correlogram of glutamate-driven activity (no periodic stimulus). Diagram shows activity of cell B with respect to spikes from cell A (indicated~y mark at time 0). Cell B tended to fire before cell A and the discharge of both cells was followed by a period of reduced activity, See text for details.

123 could not be mapped. None of these cells was a member of any of the 19 pairs described below. Fig. 1 illustrates the data manipulation forming the basis for these analyses. This cell pair was recorded from two electrodes separated by 250 ~m in the mediolateral direction at the cortical surface. One cell was 600 and the other 1000 # m below the pia. Their respective action potentials are shown in Fig. 1A. Cell A (upper trace) always fired single action potentials while cell B (lower trace) tended to fire in bursts of 3 or 4 impulses. Fig. 1B shows the response of the same cell pair to stimuli delivered to the VPL. Each responded to the thalamic stimulus and each displayed a subsequent silent period followed by a rebound. After storing the spike trains in the computer they were replotted using the conventions shown in Fig. 1C. The upward going lines represent the action potentials of cell A and the downward going lines cell B. The thickness of the downward lines is due to the fact that cell B was firing in bursts. Although they cannot be resolved in the graphic display, the digital record contained the individual action potentials and had a resolution of 12 ~ts. Fig. 1D shows the plotted response to the thalamic stimulus. The scatter diagram for this pair is illustrated in Fig. 1E. The horizontal axis represents cell A and the vertical axis cell B. At the arrow the thalamus was stimulated. Within 2 - 3 ms an initial orthodromic response from both cells was followed by a silent period lasting for about 125 ms. Cell A showed a stronger rebound (vertical line) than cell B (horizontal line). These two lines indicate that the activity of each cell displayed stimulus-related changes. More relevant for this study is the diagonal line in Fig. 1E indicating that some related activity of the two cells was independent of their shared afferent volley. This line indicates that firing in one cell was related to an enhanced probability of firing by the other cell. Once they discharged, there was a mild reduction in firing probabilities lasting about 50 ms. This can be seen in Fig. 1E as a light band on each side of the diagonal line. To examine the nature of this interaction more closely, a cross-correlogram was produced from data obtained in the absence of stimulation. Fig. 1F illustrates a 0.5 s interval of the record from cell B centered on the response of cell A. The mean discharge rate for each neuron was raised to about 15 imp/s by

releasing glutamate; however, this rate was only an average of a discharge that fluctuated significantly. When plotted as a cross-correlogram it is clear that much of the fluctuation was related to the activity of cell A. Note that the response of cell B preceded the response of cell A and that the peak was about 50 ms wide and centered about 12 ms before the discharge of cell A. Immediately following the discharge of cell A, cell B had a reduced probability of firing. Since a cross-correlogram can be examined from right to left to determine the influence of the second cell upon the first it is possible to see that the discharge of cell A followed cell B and that there was a period of reduced firing for cell A after the action potential just as was the case for cell B. Since Fig. 1F shows a 12 ms mean latency between the activities of these two cells it seems unlikely that they share a strong monosynaptic linkage. The presence of a period of reduced activity suggests that an inhibitory process, initiated at the time of their discharges, lasted for about 40-50 ms. This can also be seen in the scatter diagram. In Fig. 2, three cross-correlograms are shown where each member of the pair was found in an R A zone. In the first two cases (Fig. 2A and B) cell B tended to fire before cell A with the peak of the responses occurring within 5 ms of cell A. There was a broader base to the correlogram showing an enhanced probability of firing for 15 ms before and after the discharge of cell A. Fig. 2C shows a tightly coupled relationship between two cells. The cross-correlogram shows a peak centered on the discharge of cell A with a spread of _+ 2 ms. Since there is no lead or lag it is possible that the two cells share a c o m m o n excitatory input. Fig. 3 illustrates 3 pairs of cells with one member in an R A zone and the other in an SA zone. These cells showed no neurally-related correlation of their activity despite having overlapping receptive fields. In Fig. 4 three cross-correlograms are shown where both members of the pair were in SA zones. There are two examples (Fig. 4A and C) of very tightly-coupled cells with cell B firing 5 ms or less before cell A. In Fig. 4C both cells of the pair were recorded by a single electrode implying that they were very close to each other. Interestingly, the coupling indicated by the cross-correlogram is similar to that of cells separated by much greater distances. In Fig. 4B cell B showed an enhanced probability of

124

21

A

f

13

~A

GO Z LU

-50

~J J. 0 n" &l 4 8 ~3

Z

29 ~

B

4e

i

0

50

C

28

1

-50

0

50

-50

TIME(MS)

0

50

TIME (MS)

Fig. 2. Cross-correlograms of activity from a cell pair showing the discharge of cell B with respect to celt A (action potential at time 0). Both electrode penetrations were made in a rapidly adapting region where the neurons encountered had overlapping peripheral receptive fields. In A and B, cell B tended to fire before cell A with the peak of the response occurring within 5 ms. An enhanced probability of firing occurred for 15 ms before and after 0. In C the response peak of cell B is centered on the discharge of cell A with a spread o f + 2ms.

firing for a b o u t 15 ms after cell A o n a t~me c o u r s e

c o r r e l a t i o n ( T a b l e I). I n 5 o u t of 14 cases t h e p e a k of

that was m u c h s l o w e r t h a n that s e e n for the o t h e r

the c o r r e l a t e d activity was c e n t e r e d a r o u n d

two. F r o m the 19 c o m p l e t e l y - s t u d i e d cell pairs with

TABLE [

o v e r l a p p i n g r e c e p t i v e fields t h e r e was n o e x c e p t i o n

The number of cell pairs showing neurally-related correlations in each experimental condition

to t h e o b s e r v a t i o n that i n t e r a c t i o n s b e t w e e n the cell pairs in the a b s e n c e of a f f e r e n t - d r i v e n activity w e r e c o r r e l a t e d only w h e n b o t h m e m b e r s of t h e pair w e r e in the s a m e s u b m o d a l i t y region. C o n v e r s e l y , across a s u b m o d a l i t y b o r d e r n o n e of the cell pairs s h o w e d a

correlated not correlated

time

Both RA

Both SA

One SA and one RA

6 0

8 0

0 5

125

~i i~¸¸ii ¸ oo IZ IJJ I..1.1 i1

o fr w rn

C

7O

o

o 50

o

50

TIME (MS)

-50

o

50

TIME (MS)

Fig. 3. When one electrode penetration was made in a slowly adapting region and the other in a rapidly adapting region, none of the cell pairs encountered displayed a neurally related correlation despite having overlapping receptive fields.

zero, suggesting that the two cells shared a common excitatory source. As expected, when two cells with overlapping receptive fields were subjected to cutaneous stimuli within the overlapping portions of these fields, a correlation was observed between their discharges. This correlation, due to the fact that the receptive fields of both cells were stimulated at the same time, was more temporally dispersed than the tight correlations illustrated here and could be seen between cells on both sides of the border. The cross-correlation of independent responses linked only by the simultaneous stimulation of overlapping receptive fields is the convolution of the distributions produced by each cell in response to the cutaneous stimulus. If the response latency of each cell forms a normal distribution, their cross-correlogram will have a variance proportional to the sum of the variances of the individual response distributions. Thus, the correlations between two cells due simply to the fact that they have overlapping receptive fields on the skin are much broader (20-40 ms) than the narrow peaks illustrated in Fig. 4A or C. Cell pairs with weak and temporally-dispersed interactions (wide central peaks) could be distinguished

from cells coupled by overlapping receptive fields with the scatter diagrams. The scatter diagrams provided useful insights into the temporal relationships of the discharges of two cells since it was possible to see the reflection of the response to a stimulus of each cell as well as the coupled activity. The 3 examples shown in Fig. 5 illustrate this and are interesting for several reasons. First, Fig. 5A shows a cell pair of unknown background where cell B (vertical axis) responded with a transient excitation followed by an inhibition lasting for about 100 ms at the onset (arrow) of the cutaneous stimulus. There was no change in the ongoing activity for the duration of the skin contact. Stimulus removal resulted in a slight reduction in activity for the ensuing 80 ms. In contrast, cell A displayed an initial and transient inhibition rather than excitation, both at the onset and removal of the stimulus. This reduction in activity was followed by a rebound excitation (much stronger at the onset). A second interesting phenomenon is that the very weak correlation between cell A and cell B that is shown by the diagonal in the diagram, is more complex than a simple facilitation. At the offset of the stimulus, during the

126 129

A

77

I.Z w > w 1.1_ 0 nW

26

0

7 5O

-50

rn

~ 29

-

B

171

Z

17 -

C

!

103

6

34

0

0 -50

0

50

TIME (MS)

-50

0 TIME

50 (MS)

Fig. 4. Cross-correlograms of ceil pairs located in slowly adapting regions. In A and C the activity was tightly correlated with cell B firing within 5 ms of cell A. The two cells in C were recorded by a single electrode. B: cell B had an enhanced probability of firing for about 15 ms after the discharge of cell A, resulting in a cross-correlogram with a wider main peak.

time that cell B was giving a r e b o u n d response to the removal, the activity of cell A was inhibited. This is seen as a diagonal light region in the off response of cell B. This weak interaction cannot be seen in the cross-correlogram. The scatter diagram in Fig. 5B illustrates the activity of two R A cells in an S A region driven by cutaneous stimulation of their receptive fields. T h e crosscorrelated activity of these same two cells (without the periodic stimulation) was shown in Fig. 4B. T h e weak interaction seen in the cross-correiogram is also evident in the scatter diagram despite the small num-

ber of d a t a points. B o t h cells were transiently excited by the stimulus onset and to a lesser degree by the stimulus removal. Cell B also had a long inhibitory period (250 ms) followed by a r e b o u n d increase in activity following the initial excitation. The ceils in Fig. 5C both r e s p o n d e d to thalamic stimulation with transient excitation followed b y 70 and 100 m s inhibitory periods for cells A and B, respectively. Cell A then displayed waves of r e b o u n d excitation with a period of 120 ms. This pair was also w e a k l y c o u p l e d as indicated by the diagonal line.

127

A 3.0"

B

~:~: ~._ .:'"~*.~..~,~*-~..'~.,~,.~.~.,,..':~:~.-:::,~.~..:!'.~:'.'l,~y~.-~.c*~3.0"

~4~ ~ ~ , ~,~::~,t~ ~ ~.~.,::~ ! ~

C

•".':: .'..'.'.'3: ".: ~.'.. . . . . . v ' . ' . . " :'" : ' - ." .. "" "..':3.0"

~.~

.'. :. 1",2. ¢;,":~.? : ,~ ,,."~."~'., .~';.'.."...~,.~~.,'4~"| ~,! .'

"f3L~g~is'~ -.'f' £..g: "

:~'~:'t~'

(~N ~

:. "

*':':~

-?..

?,"

' " i~ " " ~ ~"~

.3:;.> j[~÷: ,"

"~""'"~*"'":":";"':•:"

' i :.";'."'..,'.".:."

"'":'~u'.'.~"~" ":';::" '"" "~4

,'i"

i

-.~ * . ' : ' ' : , ' ~ ;

.'-"~J: " : ';' ~'". ; , : ! ~'~'2:,~:"~'~; i:~:~'

". :" .L",':::;;'22: ", :." "i", :?:. ~;' ?..~..,".%' '2 '."J ..',,: i, "..'.;...'~';,.,...*,

e',-'-..'~ . . . . .

...".' ..... "

,. ~.- ?.2-.' "-::..'..~ .'..'.:'. :...z,.', ;'-" : .,~"~.~ ',':"-~

.'":: ': ./i:.J~ .t '? ",, C:. : .;~".". "'7 " '...i ., V', ":" '" .":.',.~ 'i..... ."." .".' '.~::,:~. ' . , 1 4'

t~

30

TtME

"~ (S)

a!o

|

3.0

Fig. 5. Scatter diagrams showing stimulus-related and neurally related activity of cell pairs. In all 3 cases the activity of cell A is shown along the horizontal axis and that of cell B along the vertical axis. A: response to 1.5 s tactile stimulation of overlapping peripheral receptive fields. Onset and removal of stimulus marked by arrows. Cell A responded to onset with inhibition followed by rebound excitation and similarly, though less strongly, to stimulus removal. Cell B was excited by the stimulus then inhibited, and showed only slight inhibition of activity upon stimulus offset. A very weak neurally related correlation produced a faint 45 ° diagonal line in the diagram. The background submodalities could not be determined. B: response to 1.5 s tactile stimulation. The cross-correlation of this pair's glutamate-driven activity appears in Fig. 4B. Both cells were excited by probe onset. Cell B responded to probe onset with excitation followed by a 250 ms inhibitory period and rebound excitation. Both cells showed weak response to stimulus offset. Despite the low number of data points in the diagram, a diagonal line can be discerned indicating a weak correlation of activity that was not due to the periodic cutaneous stimulus. C: response of two cells to stimulation of the thalamus. Both cells were activated from the thalamus and showed an inhibitory period of 70 and 100 ms for cells A and B, respectively. Cell A then displayed oscillatory rebound excitation with a 120 ms period. A weak correlation was again seen along the diagonal. The background submodalities could not be determined.

DISCUSSION

s h a r e d input was a s p o n t a n e o u s l y active t h a l a m o c o r tical relay n e u r o n . T h e cells in the v e n t r o b a s a l thala-

T h e s e data i n d i c a t e that n e u r o n s in a r e a 3b receiv-

m u s often h a v e an o n g o i n g discharge~ and their ter-

ing input f r o m o v e r l a p p i n g r e c e p t i v e fields are synap-

minals c o n t a c t a large n u m b e r of cortical cells. It

tically r e l a t e d only if t h e y are f o u n d in the s a m e sub-

w o u l d m e a n , h o w e v e r , that the s p o n t a n e o u s l y active

modality

t h a l a m o c o r t i c a l n e u r o n t e r m i n a t e d in a s u b m o d a l i t y -

region.

The

observed

correlations

dis-

played t e m p o r a l r e l a t i o n s h i p s that w e r e consistent

specific p a t t e r n .

with e i t h e r m o n o - o r o l i g o s y n a p t i c c o n n e c t i o n s be-

T h e s e o b s e r v a t i o n s also p r o v i d e an i n t e r e s t i n g in-

t w e e n the two cells. T h e c o u p l i n g p r o d u c e d an en-

sight c o n c e r n i n g the o r g a n i z a t i o n of S A and R A re-

h a n c e m e n t or r e d u c t i o n in the p r o b a b i l i t y of firing

gions. M o u n t c a s t l e and Powel1:2 d e s c r i b e d S A and

for a few milliseconds b e f o r e or after the d i s c h a r g e of

R A c o l u m n s in the M a c a q u e m o n k e y , D y k e s et al. 7.

the o t h e r m e m b e r of a cell pair. A l t e r n a t i v e l y , in

D y k e s and G a b o r s and S r e t a v a n and D y k e s 26 de-

those cases w h e r e the e n h a n c e d firing was c e n t e r e d

scribed the s e g r e g a t e d and b a n d e d n a t u r e of t h e s e re-

on the discharge of the o t h e r cell, the c o r r e l a t i o n of

gions in cat p r i m a r y cortex. Sur et al. 27,2s d e s c r i b e d

discharges might h a v e b e e n p r o d u c e d by an e x c i t a t o -

t h e s e regions in owl m o n k e y s . T h e p r e s e n c e of segre-

ry input f r o m a third source. If this source was spont a n e o u s l y active, the resulting c o r r e l a t i o n w o u l d not

g a t e d S A and R A r e g i o n s in subcortical relay zones~,~ led to the suggestion that two parallel systems o f neu-

be r e l a t e d to the a f f e r e n t stimulus and w o u l d a p p e a r

rons r e l a y e d i n f o r m a t i o n f r o m rapidly and slowly

as a c o r r e l a t i o n of o n e cell with a n o t h e r in the ab-

a d a p t i n g c u t a n e o u s m e c h a n o r e c e p t o r s to the c o r t e x a l o n g s e p a r a t e p a t h w a y s 4. T h e o b s e r v a t i o n s r e p o r t -

sence of an a f f e r e n t stimulus. N o t e that such an obs e r v a t i o n does not rule out the possibility that the

ed h e r e suggesting that S A and R A z o n e s do not h a v e

128 synaptic interactions strengthens the idea that these regions are part of separate and parallel pathways. Since the thalamic and dorsal column nuclei appear to be divided into SA and RA submodality regions, one could argue that SA and R A neurons in the thalamus project only to SA and RA bands, respectively, in the cortex and contact only cells within these bands. The anatomic basis for such a segregated projection does exist. Landry et al. j'~ have studied the terminal arborizations of identified VPL neurons using HRP staining techniques. They found that both SA and RA neurons often terminated in two or more dense 'bushes', 300-600 #m in diameter, each separated by an equal amount of sparsely innervated cortex. They proposed that the thalamo-cortical axons from a given submodality region project only to cortical bands of the same submodality and avoid the interdigitating areas of the other submodality. The cortical SA and R A bands also differ pharmacologically. Hicks and Dykes 13 and Dykes et al. 6 showed that GABA-containing neurons control receptive field size for neurons in RA but not SA regions. Another line of evidence suggesting SA and RA bands are distinct regions of cells comes from the experiments of da Costa et al. 3 in the owl monkey showing that the cortical somatotopic order can be disrupted after median nerve section and repair without disruption of the SA and R A bands. This separation of sensory information requires a high degree of synaptic specificity but one that is not inconsistent with the degree of specificity already demonstrated in visual cortex by Toyama et al. 29. Using cross-correlation techniques to analyze the activity of pairs of single cells, they showed that common afferent input excited all combinations of functionally identified neurons except those pairs containing a hypercomplex iI cell. Intracortical excitation and inhibition, however, occurred between cells of specific response types. More importantly, all 3 of the above interactions were limited to neurons within the same ocular dominance band, a result bearing striking resemblance to the findings of this study. Figs. 2 and 4 suggest that if two cells were in the same submodality zone they tended to fire together. In some cases the cells fired with a lead or lag of about 2-5 ms relative to the other member of the pair. This tight time relationship suggests a mono- or oligosynaptic connection. In most cases there was

also an enhanced period of excitability that could extend as much as 15 ms around the discharge of cell A. This period of enhanced firing could be on one side of the zero time or on both sides. When it was only on one side of the middle it suggested that some polysynaptic input to cell B was coincident with the more tightly coupled activity represented by the peak at + 2 ms. When it extended on both sides of the midline it suggested that both cell A and cell B were subjected to an increase in excitability at a time near the more tightly coupled activity represented by the sharp peak. The broader peak could be interpreted as evidence for a generalized facilitation spreading among many members of the population being studied. The scatter diagrams proved useful for distinguishing features of neuronal activity that could not be observed with the cross-correlation technique alone. It is interesting to note that cells with very different firing patterns were correlated. The two cells in Fig. 5C responded in different ways to cutaneous and thalamic stimuli yet were weakly correlated. This display also allowed cells such as cell A in Fig. 5A which was inhibited by stimulus application and removal to be recognized. Such cells are seldom seen in the somatosensory cortex and their presence in this series of experiments is attributed to the fact that an otherwise silent cell was driven by iontophoretically applied glutamate to give an ongoing discharge that could then be modulated by transient inhibition and displayed in the scatter diagram. Since the data show no exceptions to the hypothesis of submodality-specific interactions (Table l), it can be suggested that many cells within the same submodality band have strong interactions that are lacking between cells across submodality borders. This observation should not be taken to mean that all cells are interconnected within a given cortical volume since our techniques which involved glutamate driving, positioning both electrodes at about the same depth and isolating large action potentials may have preferentially located synaptically coupled cells. However, it does mean that the activity of a significant proportion of cells is tightly interrelated. By contrast, across the border between SA and RA regions there were no exceptions to the lack of correlation between two cells. This observation is particularly relevant to ideas of cortical organization.

129 It s u g g e s t s t h a t a h i g h d e g r e e o f s y n a p t i c s p e c i f i c i t y

ACKNOWLEDGEMENTS

exists in this a r e a . E i t h e r s y n a p s e s d o n o t exist b e t w e e n S A a n d R A b a n d s o r t h e r e is a m e c h a n i s m t o

W e a r e g r a t e f u l to M r s . G e n e v i e v e H o l d i n g f o r

b l o c k o r r e m o v e a n y e f f e c t o f t h e a c t i v i t y o f o n e re-

typing and editorial work.

gion on the other.

graphist, prepared the illustrations. Research funds

Mr.

Maurice Murphy,

w e r e p r o v i d e d b y t h e M e d i c a l R e s e a r c h C o u n c i l of Canada.

REFERENCES 1 Anderson, P. and Anderssen, S. A., Physiological Basis of the Alpha Rhythm, A. Towe (Ed.), Appleton-CenturyCrofts, New York, 1968. 2 Armstrong-James, M. and Millar, J., Carbon fiber microelectrodes, J. Neurosci. Meth., 1 (1979) 279-288. 3 da Costa, D. C. N., Ruest, A., Diadori, P. and Dykes, R. W., Patterns of somatotopic organization within the hand representation of SI in the owl monkey, Canada Physiol., 12 (1981) 102. 4 Dykes, R. W., Parallel processing of somatosensory information: a theory, Brain Res. Rev., 6 (1983) 47-115. 5 Dykes, R. W. and Gabor, A., Magnification functions and receptive field sequences for submodality-specific bands in SI cortex of cats, J. comp. Neurol., 202 (1981) 597-620. 6 Dykes, R. W., Landry, P., Metherate, R. and Hicks, T. P., The functional role of G A B A in cat primary somatosensory cortex: shaping the receptive field of cortical neurons, J. Neuroph.ysiol., 52.(1984) 1066-1093. 7 Dykes, R. W., Rasmusson, D. D. and Hoeltzell, P., Organization of primary somatosensory cortex in the cat, J. Neurophysiol., 43 (1980) 1527-1546. 8 Dykes, R. W., Rasmusson, D. D., Sretavan, D. and Rehman, N. B., Submodality segregation and receptive field sequences in the cuneate, gracile, and the external cuneate nuclei of the cat, J. Neurophysiol., 47 (1982) 389-416. 9 Dykes, R. W., Sur, M., Merzenich, M. M., Kaas, J. H. and Nelson, R. J., Regional segregation of neurons responding to quickly adapting, slowly adapting, deep and Pacinian receptors within thalamic VPL and VPI nuclei in the squirrel monkey (Saimiri sciureus), J. Neurosci., 6 (1981) 1687-1692. 10 Ferster, D. and LeVay, S., The axonal arborization of lateral geniculate neurons in the striate cortex of the cat, J. comp. Neurol., 182 (1978) 923-944. 11 Gerstein, G. L. and Perkel, D. H., Mutual temporal relationships among neuronal spike trains. Statistical techniques for display and analysis, Biophys. J., 12 (1972) 453-473. 12 Gilbert, C. D. and Wiesel, T. N., Clustered intrinsic connections in cat visual cortex, J. Neurosci., 3 (1983) 1116-1133. 13 Hicks, T. P. and Dykes, R. W., Receptive field size for certain neurons in primary somatosensory cortex is determined by GABA-mediated intracortical inhibition, Brain Research, 274 (1983) 160-164. 14 Hoeltzell, P. B. and Dykes, R. W., Conductivity in the somatosensory cortex of the cat - - evidence for cortical anisotropy, Brain Research, 177 (1979) 61-82.

15 Hubel, D. H. and Wiesel, T. N., Laminar and columnar distribution of geniculo-cortical fibers in the macaque monkey, J. comp. Neurol., 146 (1972) 421-450. 16 Hubel, D, H. an d Wiesel, T. N., Functional architecture of macaque monkey visual cortex, Proc. roy. Soc. B, 198 (1977) 1-59. 17 Jones, E. G., Varieties and distribution of non-pyramidal cells in the somatic sensory cortex of the squirrel monkey, J. comp. Neurol., 160 (1975) 205-267. 18 Landry, P., Labelle, A. and Deschenes, M., Intracortical distribution of axonal collaterals of pyramidal tract cells in the cat motor cortex, Brain Research, 191 (1980) 327-336. 19 Landry, P., Villemure, J. and Deschenes, M., Geometry and orientation of thalamocortical arborizations in the cat somatosensory cortex as revealed by computer reconstruction, Brain Research, 237 (1982) 222-226. 20 Marin-Padilla, M., Origin of the pericellular baskets of the pyramidal cells of the human motor cortex: a golgi study, Brain Research, 14 (1969) 633-646. 21 Moore, G. P., Segundo, J. P., Perkel, D. H. and Levitan, H., Statistical signs of synaptic interaction in neurons, Biophys. J.. 10 (1970) 876-900. 22 Mountcastle, V. B. and Powell, T. P. S., Neural mechanisms subserving cutaneous sensibility, with special reference to the role of afferent inhibition in sensory perception and discrimination, Bull. Johns Hopk. Hosp., 105 (1959) 201-232. 23 Paul, R. L., Merzenich, M. and Goodman, H., Representation of slowly and rapidly adapting cutaneous mechanoreceptors of the hand in Brodmann's area 3 and 1 of Macaca mulatta, Brain Research, 36 (1972) 229-249. 24 Perkel. D. H., Gerstein, G. L. and Moore, G. P., Neuronal spike trains and stochastic point processes, lI. Simultaneous spike trains, Biophys. J., 7 (1967) 419-439. 25 Renaud, L. P. and Kelly, J. S., Identification of possible inhibitory neurons in the pericruciate cortex of the cat, Brain Research, 79 (1974) 9--28. 26 Sretavan, D. and Dykes, R. W., The organization of two cutaneous submodalities in the forearm region area 3b of cat somatosensory cortex, J. comp. Neurol., 213 (1983) 381-398. 27 Sur, M., Wall, J. T. and Kaas, J. H., Modular distribution of neurons with slowly adapting and rapidly adapting responses in area 3b of somatosensory cortex in monkeys, J. Neurophysiol., 51 (1984)724-744. 28 Sur, M., Wall, J. T. and Kass, J. H., Modular segregation of functional cell classes within the postcentral somatosensory cortex of monkeys, Science, 212 (1981) 1059-1061. 29 Tomaya, K., Kimura, M. and Tanaka. K., Organization of cat visual cortex as investigated by cross-correlation technique, J. Neurophysiol., 46 (1981) 202-214.