Wide-field conditioning effects on small-field neurons in the posterior sigmoid gyrus of domestic cats

EXPERIMESTAI,

NEUROLOGY

Wide-Field in the W.

R.

Dcpnrtwtc+tt

60, 603-613

(1978)

Conditioning Effects on Small-Field Posterior Sigmoid Gyrus of Domestic

SATTEKTHWAITE,

JANE

of Phjrsioloyjl and Medicim, Received

October

A.

BUKNHAM,

Biophysics, Utriversity Seattle, Waslrinyton

3, 1917;

rcvisiorr

rcccivcd

AND

A. L. TOWE 1

of Wnshitfytott 98195 January

Neurons Cats

School

of

9, 1978

Single neurons were isolated in four different recording sites in sensorimotor cortex of the domestic cat, all sites being topographically related to the contralateral forepaw. Conditioning-testing interactions were made on 409 small-field neurons, the testing input being the contralateral forepaw (also the ipsilateral forepaw in the case of bilateral-field neurons) and the conditioning input being any paw that was ineffective in exciting the neuron. About 29% of the neurons showed inhibitory interactions and 23% showed facilitatory interactions. Of these, about 5.5% showed both effects, according to the conditioning site. Nearly 54% of the neurons failed to show any interaction effects. Most of the latter were isolated in the two posterior recording sites, located in somatosensory area I; the interaction effects were found predominantly in the two anterior recording sites, located in agranular tissue. Neurons that responded only to contralateral forepaw stimulation (sa neurons) and showed facilitatory interactions from the other three paws had response properties characteristic of neurons that respond to ail four paws (nk neurons). The sa neurons that showed inhibitory interactions from the other three paws did not differ significantly in response properties from those that showed no interaction effects. These findings are relevant to current criteria for the classification of sensorimotor cortex neurons and lead to remarks on the possible thalamic routes that mediate the interaction effects. r Supported by U.S. Public Health Service Research Grants NS00396 and NS05136 from the National Institute for Neurological and Communicative Disorders and Stroke. Abbreviations : SSI, somatosensory area I ; o.d.-outside diameter ; ip, intraperitoneal; iv, intravenous; IF-, CH-, IH---‘ m h’b’t’ 1 I ion from all three nonexcitatory inputs ; IF’, CH’, IH+-facilitation from all three nonexcitatory inputs; IF-, CH”, IHo-inhibition from the ipsilateral forepaw only; IF+, CH”, IHo-facilitation from the ipsilateral forepaw only. 603 0014-4886/78/0603-0603$02.00/O Copyright @ 1978 411 rights of reproduction

by Academic Press, Inc. in any form reserved.

604

SATTERTHWAITE,

BURNHAM,

AND

TOWE

INTRODUCTION Single neuron studies in the posterior sigmoid gyrus of domestic cats have yielded conflicting ideas about the composition of that tissuelargely because of the different recording sites and anesthetics used. The posterior sigmoid gyrus is differentiated rostrocaudally into cytoarchitectonic fields 47, 3a, 3b, 1, and 2 (8), the latter three fields often being termed somatosensory area I, or SS I. Barbiturate anesthetics render most neurons of field 47 inexcitable and those of SS I “bursty.” Most neurons that remain responsive show small excitatory receptive fields arranged in relatively good topographic order (10, 12, 16). Chloralose anesthesia renders the neurons throughout the posterior sigmoid gyrus quite excitable, although their tonic activity is depressed. The neurons in SS I continue to show small excitatory receptive fields (4, 11, 21), whereas those of field 47 show a variety of receptive field sizes, from small, unilateral to large, bilateral fields, often involving the entire cutaneous receptor surface of the animal (3, 4, 19-21) . Recent studies in chronically implanted, waking prep.arations (2, 17, 18) yield a view intermediate between those obtained from barbiturate and chloralose preparations. They further emphasize that the sizes of some neuron receptive fields vary with the state of the animal and even with the posture of the animal. These different studies raise questions about the problem of classifying neurons into functional subsets, a procedure that is fundamental to unraveling the organization and mode(s) of operation of the tissue. In particular, it can be questioned whether or not there are any fundamental differences between the small-field and wide-field neurons-whether a small-field neuron is a less excitable, potential wide-field neuron ; whether chloralose anesthesia renders some “normally” weak or inoperative, potential pathways hyperexcitable. This paper shows that a wide-field facilitatory influence is detectable on some small-field neurons. In addition, wide-field inhibitory effects and mixtures of facilitatory and inhibitory effects were found on other small-field neurons. The data suggest that the criteria previously used for membership in the different functional sets should be changed to take account of these subthreshold wide-field effects. METHODS Domestic

cats weighing 2.8 to 3.8 kg were anesthetized with a-chloralose ip>, P ara1~zed with decamethonium bromide (1 to 3 mg/h, iv), and respired with a Palmer pump at 17 to 20 strokes/min and 28 to 40 ml/stroke. Core temperature was maintained at 38.5”C by an abdominal DC heating pad, servoregulated via a rectal thermoprobe. The animal’s head was fixed in a head holder, and the forepaw region of the right (50 mg/k,

U’IJ)E-FIELD

CONI)ITIONIKG

TABLE Ikfinitions

for Membership

605

EFFIX‘TS

1 in Three

Neuronal

Sets? _~--~-

Set

Forepaws Contralateral

CP, sa F, sb c, SC

means P 2 0.4 (spontaneous).

u Ii (response) variable latency

Ipsilateral

K 1c R --~ .-___

0 Ii 0 ^.. --~__ and

--

Hindpaws

0 (no

Contralateral

Ipsilatcral

0 0 I< response)

means

0 0 0 P 5 0.2,

plus

highly

cerebral hemisphere was then exposed. The dura mater was incised and reflected medially, and the exposed cerebral tissue was immediately covered with warmed mineral oil. Routinely, bilateral pneumothorax and cisternal drainage (via the atlanto-occipital nienil~rane) were accomplished to control movements of the brain due to respiratory and cardiac activity. Bipolar needle electrodes were inserted into the central footpad of each Square pulses of 0.1~nis duration and paw for electrical stimulation. variable amplitude (to 7.2 V) were produced via radio frequency isolation transformers. The hunting stimulus was a supramaximal shock delivered l/s to the left forepaw (contralateral to the recording site). Single neurons were isolated and recorded via 3 JI NaCl micropipets of l- to 2-~111 o.d. at the tip. Electrodes were driven through the tissue hy a hand-operated or machine-operated Trent I$Tells hydraulic microdrive system, according to whether the particular experiment was or was not automated (7). Signals were led through a cathode follower stage into a Grass P.5 preamplifier with a half-amplitude handwidth set at 30 Hz to 30 kHz. (Occasionally. the low-frequency end was raised to 500 Hz during photographic recording.) The signals were displayed on a Tektronix Type 502 dual-beam oscilloscope for visual monitoring and photographic recording and were also led to the computer during automated experinlents (7). TABLE Distribution Set CF. sa F, sb c, SC .~___...~ u Entries are shown

of Neurons

Precruciate

~-

41 (22) 37 (33) 0

show number in parentheses.

of neurons

2

by Sets at Each Postcruciate 118 (120) 22 (16) -c (1) stu(lie.1;

nunlber

Recording

Site<’

Midsigmoitl

I’recoronal

72 (57) 5 (5) 0

71 (60) 6 (5) 0

of neurons

\vith

complete

tlata

606

SATTERTHWAITE,

BURNHAM,

AND

TOWE

WIDE-FIELD

CONDITIONING

EFFECTS

607

FIG. ‘2. Records from an sa neuron isolated in the postcruciate site. showing responses to the testing stimulus alone (1) and 30 ms after a supramaximal conditioning shock to ipsilateral forepaw (2). contralateral hindpaw (3), and ipsilateral hindpaw (4). Time bases are 1 and 5 ms. This sa neuron was unusual in that it responded with many spikes per discharge to the contralateral forepaw testing stimulus.

The reqlonsiveness of each neuron to supramaximal electrical stimulation of each paw was tested, and-with few excel~tions-only neurons meeting the criteria specified in Tafjle 1 were studied. After isolation and classification, a condition-test interaction was done at 20- to 40-nis intervals for each no response/response combination possible. The interaction was carried out at supramaximal strength for the conditioning (no response) input and at either supramaximal or twice-threshold strength for the testing (response) input. Other condition-test intervals were also used, with complete curves being obtained in many cases. RESULTS A sample of 309 single neurons was obtained from four different forepaw sites : precruciate (A! = 81), postcruciate (N = 171). midsigmoid (Ai = 77), and precoronal (IV = 77). The location of these recording sites was shown in a previous paper (21) ; the first two are in cytoarchitectonic FIG. 1. Records from two sa neurons isolated in the precruciate site, showing inhibition (A) and facilitation (B) of the test response after a conditioning stimulus to the ipsilateral forepaw (2), contralateral hindpaw (4), and ipsitateral hindpaw (6), conditioning-testing intervals of 20 ms (2) and 30 ms (4 and 6). Traces 1, 3, 5, and 7 show the test responses alone, as obtained before and after each block of conditioning trials. All stimulus strengths were supramaximal. Records were digitized at 15 kHz. each being SO ms in duration and starting at the testing stimulus onset.

608

SATTERTHWAITE,

BURNHAM,

AND

TOWE

field 4y, the next in field 3a, and the last in field 3b (S), in what is often called primary somatosensory cortex. The latter three sites are in posterior sigmoid gyrus, and all yield large primary evoked responses to contralateral forepaw stimulation. Table 2 shows the distribution of neurons by sets at each recording site. Complete and satisfactory interaction data were obtained on 318 of these neurons (numbers shown in parentheses in Table 2). Because so much is known about the postcruciate forepaw site, twice as many neurons were gathered there as at the other recording sites. Odd-even, split-halves and pseudorandom halves analysis showed the postcruciate sample to be stable; hence, the three smaller samples can be taken as reasonably descriptive. Except where specifically stated, all calculations are confined to the set of 318 completely studied neurons. The two most frequently encountered patterns of effect-either inhibition or facilitation of the test response for about 7.5 ms after stimulation of any nonexcitatory 2 paw-are shown in Fig. 1. At supramaximal testing stimulus strengths, inhibition appeared as an increase in response latency and a decrease in spikes per response (Fig. lA), whereas facilitation either could not be detected or revealed itself through the opposite set of changes (Fig. 1B). At near-threshold testing stimulus strengths, inhibition appeared as a complete loss of responsiveness, whereas facilitation could readily be seen as a decrease in response latency and increase in spikes per response. Both interaction effects could be continuously graded by varying the strength of the conditioning stimulus or the test stimulus, or both. The magnitude of the conditioning effect often varied according to the conditioning stimulus site. A qualitative estimate was obtained by using the same conditioning stimulus strength and the same conditioning-testing interval for each nonexcitatory stimulus site. As shown in Fig. 2, the response to the contralateral forepaw testing stimulus decreased more after the ipsilateral forepaw conditioning stimulus than after either of the hindpaw conditioning stimuli. A similar estimate was obtained for the facilitatory effects. Because electrical stimuli were applied to equivalent sites in each animal, qualitative estimates of the magnitude of the conditioning effects could be made between neurons. All subsequent comparative statements are of this qualitative nature. The time course of inhibition was remarkedly uniform from one neuron to the next, attaining half-maximum in 10 ms, being maximal between 20 and 30 ms, and returning to normal within SO ms after the conditioning stimulus. The increase in response latency and decrease in spikes per response followed the same time course as response probability. No attempt was made to measure the time course of facilitation. 2Nonexcitatory here means failure to evoke (response probability low or zero, and response

a regular response from latency highly variable).

the

neuron

WIDE-FIELD

Percentage

of Neurons

CONI)ITIONING

at Each

I’rccruciate

Effect Inhibition Mixed Faci,itation No effect ‘I Percentages

for

Site Sho\ving

Interaction

Postcruciate

10.9 (33.3) 13.6 (9.1) 18.2 (15.2) 27.3 (12.1) sa nellrons

29.2 8.3 25.0 37.5 are

followed

609

EFFECTS

Effects,

1,). Sets”

Mitlsigmoitl

(18.0) (0) (37.5) (13.8)

8.8 1.8 10.5 79.9

in parentheses

Precoronal

(20.0) (0) (0) (80.0)

15.0 1.7 6.7 76.7

1)~. those

for

(0) (0) (20.0) (80.0)

sh neurons.

Disfd~ufiotl of Efffccfs. Overall, 28.27cm of the sa and 30.5%) of the sb neurons showed inhibitory interactions, whereas 22.8(? of the sa and 25.4% of the sh neurons showed facilitatory interactions. Of these, 5.8y of the sa and 5.17$ of the ~1) neurons showed lmth inhibitory ant1 facilitatory interactions, according to the nonexcitatory contlitioning site stimulatetl. The effects were not uniformly distrilmted among the recording sites, as shown in Table 3 ; rather, they were 1nuc11 more prevalent in the pericruciate sites (field 47) than in the two posterior sites (field 3). Puttem of Effects. \Vith the exception of the mixed effects, all possil)le patterns of interaction were ohserved. Of CcJLlrSej four patterns l~retloniinated among the sa neurons : (i) inhibition from all three nonexcitatory inputs. (ii) facilitation from all three nonexcitatory inputs. (iii) inhil)ition from the ipsilateral forepaw only, ant1 (iv) facilitation from the ilklateral forepaw only (no effect being detectalde from the hindpaw inputs in the latter two cases). These are hereafter symbolized as : (i ) (IF-,CH-,IH-), (ii) (IF+,CH+,IH+) , (iii) ( IF-,CH”,IH”), and (iv ) (IF+,CH”,IH” ) Table 4 gathers the data together in the ahove form, with the other patterns being gathered into “other” or mixed categories. The relative pt-oportions

TABLE Distribution

of sa Neurons

Pattern __--

by Patterns

Prccruciate

4 of Interaction

Postcruciatc

-~_ IF-,CH-,IHIF- >CH” 1IH” OtherMisetl-‘+ It;+,CH+,IEI+ IF+ >CH” 3IH” Other+ No effect -

U Entries

show

number

3 -c 2 3 2 1 1 6 of neurons.

21 6 8 10 14 9 7 1.5

at Each Midsigmoitl .._____ 0 2 3 1 I 0 2 15

Recording

SitetA

Prccoronal .3 3 3 1 0 2 2 16

610

SATTERTHWAITE,

BURNHAM,

TABLE Distribution

of sb Neurons

Pattern

Precruciate

CH-,IHOtherMixed-/+ CH+,IH+ Other+ No effect S Entries

show

by Patterns

TOWE

5 of Interaction

Postcruciate

8 3 3 4 1 14 number

AND

3 0 0 0 6 7

at Each Midsigmoid 0 1 0 0 0 4

Recording

Sitea

Precoronal 0 0 0 1 0 4

of neurons.

of neurons showing each of the different patterns were, with a few exceptions, similar among the four recording sites. Furthermore, ignoring the mixed category, the relative proportions were the same for inhibition and facilitation, viz., 0.466 and 0.455 for those affected from all the three nonexcitatory inputs, 0.259 and 0.273 for those affected from ipsilateral forepaw only, and 0.276 and 0.273 for those in the “other” category, respectively (see Table 4). Two patterns predominated among the sb neurons: (i) inhibition and (ii) facilitation from both hindpaws. Table 5 gathers the data together in the same manner as in Table 4. The interaction effects were the same, whether the ipsilateral or the contralateral forepaw served as the testing stimulus site, for all sb neurons so tested. Overall, about the same proportion of sb neurons (0.492) as sa neurons (0.548) failed to show any modulation of excitability after stimulation of any nonexcitatory conditioning site. The one SC neuron on which complete data were obtained was inhibited by ipsilateral forepaw and facilitated by ipsilateral hindpaw stimulation ; it showed a mixed pattern of effects. Among the sa neurons that were inhibited by all three nonexcitatory inputs, the ipsilateral forepaw usually had a stronger effect than either hindpaw, and the two hindpaws had equal effect. By contrast, five (3.4%) of the postcruciate sa neurons in the “other” category-the largest accumulation in any “other” categories-were clearly inhibited from both hindpaws, but were not measurably influenced by ipsilateral forepaw stimulation. For those neurons in the IF+,CH+,IH+ category, facilitation was usually equally strong from all three nonexcitatory inputs. DISCUSSION The presence of short-latency inhibitory effects from nonexcitatory input sites shows conclusively that neurons in sensorimotor cerebral cortex can be partitioned into at least two groups on the basis of receptive field

\VIDE-FIELD

CONDITIONIKG

EFFECTS

611

characteristics : (i) those that are excited or facilitated from all four paws and (ii) those that are excited by only one paw (the on-focus paw) and are inhibited from the others. These two groups correspond in a general sense to the ~12and s sets, respectively, that were distinguished in several studies (2, 5, 11, 14, 19-21) and that were also described under other terms (3, 4). The presence of mixed effects from nonexcitatory conditioning inputs-inhibition from some and facilitation from others-shows conclusively that at least one additional group exists. Thus, the widebilateral excitatory receptive fields that are so conspicuous in chloraloseanesthetized preparations and so rarely seen in Nembutal-anesthetized preparations, may potentially be more numerous than previously suspected. Observations in “chronic” preparations (2, 17, 18) lead to the idea that chloralose makes conspicuous the potential fields of facilitatory influence on certain cerebral neurons. The results of the present study suggest that some of these facilitatory influences were overlooked and that the defining criteria for membership in the different functional sets must be reassessed. ‘LIYyner (22) recently reviewed the general problem of neuronal classification and argued eloquently in favor of a polythetic scheme. Rowe and Stone (15) effectively reexamined the classification of retinal ganglion cells with such an idea in mind. The classification scheme currently followed in our laboratories depends on only two features: the size and the location of a neuron’s excitatory receptive field, as estimated by supramaximal shocks applied l/s to the central footpad of each paw. The criterion response probability was progressively lowered from the original 0.5 level (19, 21) to a low of 0.2 (5, 6) as circumstances changed. It now appears that the criteria might be altered to include facilitatory influences and might also be broadened to include inhibitory influences. Thus, the IF+,CH”,IHo and IF+,CH-,IHneurons might he reclassified from the sa to the sb set. Furthermore, the sa and sb neurons that show wide-field inhibitory interactions might be separated from those that show no interaction. In order to follow these more stringent classification criteria, it would be necessary to perform conditioning-testing interactions for selected nonexcitatory inputs for all neurons studied-a major undertaking, unless automated procedures can be applied (7). E ven so, the classification procedure falls far short of being polythetic in the sense that Tyner (22) proposes. A striking finding of this study was the strong concentration of widefield interaction effects on the neurons of the two anterior recording sites (precruciate and postcruciate) , both of which are in cytoarchitectonic field 47 (8 j . This region receives significant input from thalamic nucleus ventralis lateralis. However, cutaneous sensitivity is almost lacking in this nucleus (1, 13). On the other hand, if the cutaneous responsiveness and wide-field interaCtioilS seen 011 neurons in field 4y depend (Jll th&mic

612

SATTERTHWAITE,

BURNHAM,

AND

TOWE

nucleus ventralis posterolateralis, then that nucleus is complex, for it projects heavily to the field 3 recording sites, where wide-field interactions are nearly absent. Both sb and IJZ neurons were found intermingled with the sa neurons in the anterior half of this nucleus (9), but wide-field interactions have not been described. The posterior half or “lobe” of this “bi-lobed” nucleus may be less complex and may thus be the chief source of input to field 3. If the neurons of field 4y depend on those of field 3 for their cutaneous sensitivity, then the wide-field interactive effects come either via some other, as yet unidentified, thalamic nucleus or via an elaborate cerebral circuitry. The simplest model would have the anterior lobe of thalamic nucleus ventralis posterolateralis mediating all the effects seen at the two anterior recording sites, REFERENCES 1. ASANUMA, H., AND J. FERNANDEZ. 1974. Characteristics of projections from the nucleus ventralis lateralis to the motor cortex in the cats : An anatomical &id physiological study. Brain Res. 20 : 315-330. 2. BAKER, M. A., C. F. TYNER, AND A. L. TOWE. 1971. Observations on single neurons in the sigmoid gyri of awake nonparalyzed cats. Exp. Neural. 32: 388-403. 3. BROOKS, V. B., P. RUDOWN, AND C. L. SLAYMAN. 1961. Sensory activation of neurons in the cat’s cerebral cortex. J. Nrzbvophysd. 24: 286-301. 4. BUSER, P., AND M. IMBERT. 1961. Sensory projections to the motor cortex in cats: A microelectrode study. Pages 607-626 in W. A. ROSENBLITH, Ed., Sensory Cortlntzb1zicatioIt. M.I.T. Press, Cambridge, Mass. 5. DOETSCII, G. S., AND A. L. TOWE. 1976. Response properties of distinct neuronal subsets in hindlimb sensorimotor cerebral cortex of the domestic cat. Exp. Ncwol. 53: 520-547. 6. ENNEVER, J. A., AND A. L. TOWE. 1974. Response of somatosensory cerebral neurons to stimulation of dorsal and dorsolateral spinal funiculi. Exp. Neurol. 43: 124142. 7. HARDING, G. W., AND A. L. TOWE. 1976. An automated on-line, real-time laboratory for single neuron studies. Coarp. Bionrcd. Res. 9 : 471-501. 8. HASSLER, R., UND K. MUHS-CLEMENT. 1964. Architektonischer Aufbau des densomotorischen und parietalen Cortex der Katze. J. Wir~forsch. 6: 377-420. 9. JABBUR, S. J., M. A. BAKER, AND A. L. TOWE. 1972. Wide-field neurons in thalamic nucleus ventralis posterolateralis of the cat. Exp. Nczbrol. 36: 213-238. 10. LEVITT, M., AND J. LEVIS. 1968. Sensory hind-limb representation in SmI cortex of the cat. A unit analysis. Exp. Nctbrol. 22: 259-275. 11. MORSE, R. W., R. J. ADKINS, AND A. L. TOWE. 1965. Population and modality characteristics of neurons in the coronal region of somatosensory area I of the cat. Exp. Neural. 11 : 419-440. 12. MOUNTCASTLE, V. B. 19.57. Modality and topographic properties of single neurons of cat’s somatic sensory cortex. J. Neuroplzysiol. 20 : 408-434. 13. NYQUIST, J. K. 1975. Somatosensory properties of neurons of thalamic nucleus ventralis lateralis. Exp. Nczwol. 48 : 123-135.

14. NYQUIST, J. K., AND A. L. Towe. 1970. Neuronal activity evoked in cat precruciate cerebral cortex by cutaneous stimulation. Exp. Ncwol. 29: 494-512. 15. ROWE, M. H., AND J. STONE. 1977. Naming of neurones. Classification and naming of cat retinal ganglion cells. Brair~. Behaz~. Evol. 14: 185-216. 16. RUBEL, E. W. 1971. A comparison of somatotopic organization in sensory neocortex of newborn kittens and adult cats. J. Co~tp. Ncz~vol. 142: 447-480. 17. SLIMP, J. C., AND A. L. TOWE. 1977. Characteristics of somatic receptive fields of neurons in postcruciate cerebral cortex in awake-restrained and two anesthetic conditions in the same cat. Ncwosci. Abstr. 3 : 492. 18. TOWE, A. L., AND J. C. SLIMP. 1977. Organization of postcruciate neurons with respect to somatic modality and receptive field in an awake-restrained cat. Neurosci. Alvtr. 3 : 493. 19. TOWE, A. L., H. D. PATTON, AND T. T. KENNEDY. 1964. Response properties of neurons in the pericruciate cortex of the cat following electrical stimulation of the appendages. Escp. NCUYU~. 10 : 325-344. 20. TOWE, A. L., C. F. TYNEK, AND J. K. NYQUIST. 1976. Facilitatory and inhibitory modulation of wide-field neuron activity in postcruciate cerebral cortex of the domestic cat. Exp. Nrurol. 50: 734756. 21. TOWE, A. L., D. WHITEHORN, AND J. K. NYQUIST. 1968. Differential activity L among wide-field neurons of the cat postcruciate cerebral cortex. Exp. Ncllrol. 20 : 497-521. 22. TYNER, C. F. 197.5. The naming of neurons: Applications of taxonomic theory to the study of cellular populations. Bruilt Behm. Ez~ol. 12: 75-96.