Second somatosensory cortical area in macaque monkeys. I. Neuronal responses to controlled, punctate indentations of glabrous skin on the hand

Brain Research, 520 (1990) 262-271 Elsevier

262 BRES 15613

Second somatosensory cortical area in macaque monkeys. I. Neuronal responses to controlled, punctate indentations of glabrous skin on the hand Harold Burton and Robert J. Sinclair Department of Anatomy and Neurobiology, Washington University School of Medicine, St. Louis, MO 63110 (U.S.A.) (Accepted 19 December 1989)

Key words: Tactile stimulus; Primate; Parietal cortex

Responses of 492 single neurons from the second somatosensory cortical area of macaque monkeys were studied using computer controlled ramp indentations. Results were obtained using chronic recording techniques from lightly tranquilized or awake animals. Amongst those cells that were activated by punctate tactile stimuli, various subclasses of responses were identified that included neurons with phasic and sustained adaptation characteristics. In addition, several cells showed unusual firing patterns, such as delayed responses and reverberating afterdischarges. Latency measurements from 90 cells showed a modal latency to ramp stimuli of 34 ms and a second group of cells whose latency exceeded 75 ms. Measurements of response functions to different velocities of indentation revealed that some cells maintained relatively shallow ascending functions but that most cells were insensitive to the velocity variable. The response characteristics of these neurons in primates are discussed in reference to the hypothesis that in the somatosensory cortex, SII occupies a higher order, serially dependent region in a hierarchy from SI to other parts of the brain. INTRODUCTION The p r e d o m i n a n t description of physiology in the second somatic sensory cortical area (SII) has been based on studies of b o d y representations across the cortical surface of a variety of species 4. Most of these experiments have relied on recordings of mass activity either through recording e v o k e d potentials or multiple neuron discharges and less frequently on activity of single neuron responses. Even surveys based on single neuron responses have provided a limited, largely qualitative tabulation of responsiveness of SII neurons. These studies have shown that the somatotopic m a p in SII in all species is organized somewhat less precisely than in cutaneous representations in SI. Confirmation of these maps has also come from anatomical 2'~7 and 2 D G studies 21. In addition, and possibly because of decreased spatial resolution evident in SII maps, the somatotopography has a p p e a r e d to be m o r e consistently homologous across animal species 33'4°. Different distortions evident in maps of SI that derive from differential peripheral innervation densities and specializations are also found in SII but with some reduction in the m a r k e d distinctions seen in SI. F o r example, there are no barrel fields for the whisker representation in SII of rodents 12 nor is there a

clear proximal-distal specialization of a glabrous digit area as in SI of primates. Consequently, face or distal forelimb areas of SII in rodents, carnivores and primates a p p e a r m o r e alike despite some e x p e c t e d enlargements of cortical territories for the most heavily innervated peripheral structures. Similarities in these receptive field maps suggest the hypothesis that SII may serve a similar role across species. In contrast, the earliest comparative c o m m e n t s about physiology of SII in cats and primates noted important differences between responsiveness of these areas in the two species. Woolsey 44 restated his initial observations that e v o k e d responses in SII of primates had longer latencies and were m o r e suppressed by sodium pentobarbital anesthesia than in cats. Recently, several studies have indicated that SII in primates may require an intact ipsilateral SI to r e s p o n d to s o m a t o s e n s o r y inputs1°'35; a similar d e p e n d e n c e has not been noted in cats 9'24. These differences may arise from variations in relative densities of thalamocortical connections from the main somatosensory thalamic relay (ventroposterior nucleus) in different species 4'9"2° and serve to underscore the possibility that SII may not function equivalently across species. The nature of a possible role for SII has not been clearly defined. Bennett et al. 3 and Ferrington and

Correspondence: H. Burton, Department of Anatomy and Neurobiology, Washington University School of Medicine, 660 South Euclid Avenue, St. Louis, MO 63110, U.S.A. 0006-8993/90/$03.50 © 1990 Elsevier Science Publishers B.V. (Biomedical Division)

263 R o w e 15 h y p o t h e s i z e d t h a t SII m a y p r o v i d e a s p e c i a l i z e d parallel

somatosensory

channel

for processing

from pacinian corpuscle receptors because, neurons,

some

SII cells in cats r e t a i n ,

inputs

u n l i k e SI

with g r e a t e r

fidelity, t h e t e m p o r a l r e s p o n s e p a t t e r n c o n v e y e d by t h e s e r e c e p t o r s for high f r e q u e n c y m e c h a n i c a l v i b r a t i o n . In c o n t r a s t , M i s h k i n a n d c o l l e a g u e s 18'27'35 h a v e p r o p o s e d , on the

basis o f lesion studies

in m o n k e y s ,

that

SII

o c c u p i e s a serial, h i e r a r c h i c a l link for tactile l e a r n i n g . According processed memory

to

this

hypothesis,

signals f r o m centers

SII

serves

to

funnel

SI into h i g h e r c o g n i t i v e

in t h e brain.

Given

anatomical

and and

p h y s i o l o g i c a l d i f f e r e n c e s b e t w e e n SII in cats and prim a t e s , it is p o s s i b l e t h a t b o t h h y p o t h e s e s are c o r r e c t . However,

it is difficult to m a k e c o m p a r i s o n s b e t w e e n

c a p a b i l i t i e s o f SII n e u r o n s in cats a n d m o n k e y s w i t h o u t d a t a o n r e s p o n s e p r o p e r t i e s of SII n e u r o n s in p r i m a t e s to c o n t r o l l e d m e c h a n i c a l stimuli. T h e s e e x p e r i m e n t s s h o w t h e r a n g e o f r e s p o n s e s r e p r e s e n t e d in p r i m a t e SII to r a m p i n d e n t a t i o n s o f d i f f e r e n t a m p l i t u d e s , d u r a t i o n s and velocities. A s u b s e q u e n t p a p e r will c o n s i d e r r e s p o n s e s to m e c h a n i c a l oscillations o f d i f f e r e n t f r e q u e n c i e s .

MATERIALS AND METHODS Single neuron recordings were obtained from the parietal operculum in 11 hemispheres from 9 Macaca fascicularis monkeys. In all cases penetrations with glass coated Pt-Ir (resistance < 2 MI2) or varnish-insulated tungsten electrodes (Frederick Haer, 5-10 M~2) were made transdurally through chronically implanted chambers while animals were seated in a plastic restraint chair. The head was immobilized by attachments to inverted studs that were embedded in acrylic according to procedures modified for these experiments 3s. Chambers and head restraining bolts were attached using aseptic surgical procedures on animals that were deeply anesthetized (preanesthetic intramuscular injections of xylazine, 2 mg/kg and ketamine hydrochloride, 10 mg/kg followed by intravenous administration of sodium pentobarbital, 15-25 mg/kg). Preoperative injections of Bicillin (40,000 U/kg) were given for prophylactic protection against infection. Postoperatively, wound margins were cleaned as needed with hydrogen peroxide (3%) and topically treated with furamycin or Panalog. Stainless-steel chambers (1.8 cm diameter) were centered over the general vicinity of SII by using stereotactic coordinates from the atlas of Szabo and Cowan 41 plus adjustments according to the location of the frontoparietal suture in individual animals. The critical portion of the latter was obtained from extrapolating the distribution of the middle cerebral artery over the insula to the surface of the skull, These images were made with angiography and X-rays of the brain from an additional animal. Successful chamber placements were usually achieved by first locating AP +8 and sagittal +17 on the skull. The center of the chamber was re-aligned to this point after tilting the microdrive 30-60 ° laterally. The center of the chamber was then shifted 10 mm further lateral before marking and opening the calvarium. When all of these manipulations were done correctly, the lateral sulcus crossed diagonally through the bottom third of the chamber, generally from 7 to 2 o'clock, and the anterolateral tip of the intraparietal sulcus appeared within the center of the top-half of the opening. Electrode penetrations positioned close to the center of the chamber recorded from SI neurons with perioral receptive fields near the surface and neurons activated from distal forelimb receptive fields in SII at 5-9 mm below the surface.

Exposed dura was kept relatively free of infection by repeated flushing with sterile saline after every recording day and several times weekly and by keeping the chamber filled with chloramphenicol (30-50 mg/ml) in sterile saline when the animal was in its home cage. In addition, all instruments and probes used inside the chambers were kept aseptic. Chambers were filled with a sterile solution of 2% non-nutrient agar during recording sessions. Animals exhibited no symptoms of systemic infection from 1 to 12 months postoperation and did not need additional treatment with general antibiotics except for those used topically. Animals used in these studies had been acclimated to the restraining chair and laboratory prior to surgery and readily entered the experiment area as a result of positive conditioning procedures involving reward with food and fruit-flavored water. During training they also learned to adjust to contoured and padded orthoplastic splints that were used to hold the forelimbs. It was usually possible to search and isolate most cells while the monkeys were fully awake. In those cases where an animal appeared to be stressed or where stabilization of recordings was crucial for prolonged stimulusresponse analyses, tranquilizing doses of xylazine (1 mg/kg) plus ketamine (2-3 mg/kg) were injected intramuscularly or a mixture of 70% nitrous, 30% oxygen and 0.5-1.0% halothane was administered through an inhalation mask.

Cell identification Neurons responsive to touching, stroking or tapping glabrous skin of the contralateral hand and digits were sought in all penetrations into SII. Once an appropriate cell's action potentials were sufficiently isolated to be separated by an amplitude-time window (BAK Electronics, DIS-I), the most sensitive portion of the receptive field was determined using hand-held probes that included calibrated nylon filaments (Stoelting Co.). As noted previously37, SII contains a considerable variety of sensitivities including cells that respond consistently and crisply to cutaneous stimuli and those that respond only occasionally to high threshold, poorly defined gross manipulations and squeezing over large, ill-defined receptive fields. Responses to controlled sequences of ramp indentations were evaluated mostly for cells that initially demonstrated greater responsiveness to hand-held stimuli. Attempts to study less responsive cells generally were unsuccessful. Controlled mechanical stimulus procedures The computer controlled mechanical probe used to deliver ramp indentations has already been described z and was used without major modifications for these experiments. Various shaped probe tips contacted the skin. The distal end of these consisted of a 2 mm diameter cylinder of Delrin that was flat or conical with either a 30, 45 or 60° taper. All stimuli were delivered orthogonal to the skin surface. Generally only one point in a cell's receptive field was tested. Stimuli consisted of a ramp indentation at constant velocity to a plateau level, which was held for some time, followed by a constant velocity ramp withdrawal to the original position. Ramp indentation amplitudes were 1 mm from skin surface for all cells. The velocity of indentation was varied for some cells to cover the range from 1 mm/s to 50 mm/s. Most cells were tested with a single velocity of approximately 35 mm/s. The plateau phase of indentations was between 250 and 1000 ms. Some cells were studied during at least three different durations (250,500 and 750 ms). Nearly all cells were initially tested with 10 repetitions of a 1 mm ramp at 35 mm/s held for 1 s and retracted at 50 mm/s. Some cells that exhibited fairly robust responses to this test indentation were additionally studied with similar ramp indentations but at 2-4 different velocities. Data analyses Responses to a standardized punctate indentation used on most cells were collected as time intervals between action potentials with 100/~s resolution by a PDP 11/34 computer. Peri-event histograms were summed from repeated trials over the duration of indentations. Summations included 250 ms intervals before and following ramps.

264

Fig. 1. Photomicrograph showing representative localization of four electrode penetrations into SII. Microlesions visible as dark, wide spots mark the position of two penetrations closest to fundus. In the more superficial penetration, two lesions were placed above and below layer IV to border recordings from this layer. Two additional penetrations appear as thin black streaks to the right of the double lesion marks.

Histograms were inspected for the following features: phasic on and off discharges, sustained activity during the plateau, and afterdischarges following bursts of activity to stimulus changes. Patterns of activity for each cell were then tabulated. In order to determine whether response shifts were associated with different indentation vclocities, average firing rates were calculated by dividing the total number of impulses excited over 10 trials during the time interval from ramp initiation to the last bin for the peak of the phasic response at the top of ramp. Latency of responses to rapid indentations was measured for those cells that showed consistent activation to successive stimuli. Latencies were determined (with 1 ms resolution) from the time the computer initiated a ramp to the occurrence of first neural discharge. These values were averaged across number of stimulus trials. All latency measurements were taken from cells studied with the same ramp velocity.

Histology Following administration of an overdose of sodium pentobarbital, each animal was sacrificed by perfusion into the ascending aorta with 1 liter of 0.9% saline and 2 liters of fixative. The latter was 1% paraformaldehyde plus 1.5% glutaraldehyde in 0.1 M phosphate buffer for those cases receiving injections of HRP or 10% (by volume) neutral formalin. Serial frozen sections at 50 pm were collected through recording sites. These were mounted on chromealum, gelatin subbed slides and stained with thionin. Locations of electrode tracks were discovered and correlated with the distribution of penetrations made during recording experiments. Selected

sites were marked with microlesions (Fig. 1). These were recovered best from penetrations made closest to the date of sacrifice. Even where specific lesions were not found, it was still possible to note whether electrodes had passed into SII. All data presented below were obtained from penetrations that were in SII as defined previously36.

RESULTS In the c o u r s e of 102 p e n e t r a t i o n s into the f o r e l i m b r e p r e s e n t a t i o n in SII, action p o t e n t i a l s f r o m 492 n e u r o n s w e r e sufficiently r e s o l v e d for d e t a i l e d c h a r a c t e r i z a t i o n . Verification o f the localization of r e c o r d i n g sites was o b t a i n e d s u b s e q u e n t l y by n o t i n g positions of m i c r o l e s i o n s m a d e into s e l e c t e d p e n e t r a t i o n s (Fig. 1) and r e c o n s t r u c t ing m a n y e l e c t r o d e tracks in r e f e r e n c e to t h e s e m a r k e r s . A s n o t e d in T a b l e I, the s a m p l e was u n e v e n l y distributed and r e s p o n s e s to r a m p stimuli w e r e o b t a i n e d f r o m 7 out of 11 h e m i s p h e r e s . N e a r l y 4 1 % of i s o l a t e d cells w e r e not tested for their sensitivity to c o n t r o l l e d p u n c t a t e m e c h a n ical stimuli b e c a u s e

these

cells did n o t

respond

TABLE I

Sample of cells with cutaneous RF on the hand Mr29 Number of penetrations Response to ramps vibration No response Not tested Totals

Mf37

to

h a n d - h e l d application of such d i s c r e t e stimulation. Sim-

Mf44

Mf45

Mf47

Mf55L

Mf55R

Tota~

12

9

13

21

13

31

14

102

15 21 10 2 38

19 32 11 7 56

32 15 13 37 83

36 27 9 28 79

4 0 3 35 42

45 14 22 79 148

30 7 2 12 46

180 120 70 200 492

265

201

TABLE II

A 2s[

o

Distribution of major response types n

Type Phasic on 17 Phasic on-off 42 Sustained 14 Partial sustained 37 Offonly 12 Unclassified 24 Total 146 Distribution of subclassesof phasic and sustained types Phasic + afterdischarge 7 Sustain + afterdischarge 4 Sustain + off response 12 Subtotal 22

~. Li,d_.,a/,..u,,i.,t.,

%

11.6 28.8 11.6 22.6 8.2 16.4

Response patterns of skin indentation The behavior of 146 cells was relatively consistent over the course of repeated trials so that responses could be classified (Table II). As in previous studies of somatosensory neurons, distinctions between cells were made on the basis of responses to stimulus change (phasic or dynamic activity) and persistence (static or sustained activity during indentation). In addition, more than 14% of SII neurons also demonstrated secondary after-discharges that occurred subsequent to responses to inden-

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Fig. 2. Peri-event histograms summed from 10 indentation trials for 6 different cells with phasic responses. A-C: examples of cells that responded only to initiation of indentations. D-F: examples of responses to the beginning and end of indentations. Bin widths for all histograms is 10 ms. See text for further comments.

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ilar kinds of poorly activated, yet definably forelimb sensitive cells have been described in SI125'37. Many of these cells responded when the hand was squeezed, kneaded or tapped. A small number (70) of tested cells failed to respond even though preliminary observations with hand-held probes showed sensitivity to punctate stimuli. Responses to ramp stimuli were studied in 180 cells.

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Fig. 3. Peri-event histograms illustrating examples of sustained responses from 6 different cells. See text for further comments. tations. Some neurons (24) responded to ramp stimuli but the pattern of activity was too variable to permit classification. Phasic on responses. A small population of cells (17) briefly fired at approximately the same latency only at on-set of indentations. As shown by some examples (Fig. 2A,B), these responses generally were brief and tightly synchronized. Less commonly, some cells fired a longer duration burst of activity (Fig. 2C). Phasic on-off responses. A larger number of cells (42) responded phasically to indenting and retracting phases of the stimulus. Responses during both phases of the ramp were generally similar although most 'off' responses were smaller (Fig. 2D,E). As in phasic 'on responders', some cells fired a small number of action potentials that were tightly synchronized in latency on successive trials (Fig. 2D) while other cells displayed longer duration burst discharges (Fig. 2E,F). These differences in response duration were distributed in a continuum. Generally, cells with phasic responses were activated best by stimulus velocities in excess of 30 mm/s and showed fewer responses per bin to slower velocity ramps. More action potentials, however, were not observed with progressively faster ramps (see section on velocity sensitivity). These cells appeared, at best, to signal occurrence of a change in skin indentation. Their responses were equivalent to cells in other parts of the somatosensory system that have been defined as rapidly or quickly adapting. Sustained responses. Fifty-one cells exhibited some activity during the plateau phase of indentation that clearly exceeded baseline pre-stimulus levels. As shown in Fig. 3, the magnitude and degree of persistence of these responses varied considerably. In a small number of cases (14) responses were present throughout indentation (Fig. 3A,B). The remaining cells (37) all showed progressively greater decreases in activity during the plateau. Some of these cells (Fig. 3C) were still firing above

266 10

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Time (Seconds)

Fig. 4. Peri-event histograms illustrating examples of late responses to indentation from 4 different cells. In A - C a brief increase in activity occurred after the indentation phase of the stimulus had stopped; in D, a momentary decrease in activity appeared during the plateau. Note the long latencies for all of these responses.

background levels when the indentation ended. A larger proportion of the cells exhibited activity declines more quickly, but not totally to pre-stimulus levels (Fig. 3D). Dynamic responses (Fig. 3B-D,F) usually, but not always (Fig. 3A,E), preceded static activity. In the latter, an early phase of the response was broader and reached similar peaks over several bins before declining. On a few occasions dynamic and static phases of the response were separated by a period of inhibition (Fig. 3F). The secondary response during the plateau of indentation in this example may not, however, be similar to more unidimensional sustained responses described above (see results on after-discharges). Late responses. A dozen cells responded only after the indentation phase ended. As shown by examples in Fig. 4, most of these responses were minimal in comparison to the majority of recorded cells (note the reduction in the scale of the vertical axis). However small the total number of discharges, these cells distinctively increased their activity only when an indentation stopped moving. These cells may have reflected a rebound response to a phase of inhibition that developed during ramp stimulation. An example of apparently simple inhibition during ramps is shown by the histogram in Fig. 4D for a cell whose background firing was turned off by the ramp. When hand-held stimuli were applied to these cells, responses were detected only when a maintained contact with the skin was terminated. The proportion of 'late' responding cells in SII may be greater than indicated since it is likely that these sluggish, delayed responses may have been judged as being unlikely to respond to punctate stimuli when cells were initially screened for testing with the controlled stimulator. Afterdischarges. Another group of cells with more complex response patterns is shown in Fig. 5. These displayed multiple discharges following initial responses to indentation; the latter could be categorized in a variety of ways that matched types discussed above, e.g., phasic on-off (Fig. 5A,B,E,F), phasic late (Fig. 5C) or sustained

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Fig. 5. Peri-event histograms showing examples of after-discharge responses from 6 different neurons. Histograms in A - D obtained from trials with 100 (Aj, etc.) or 1000 ms (A> etc.) hold periods during indentation. See text for further comments.

(Fig. 5D). The number of cells showing these afterdischarges was not large enough to permit identification of predominant patterns. The responses described below represent types that were seen on several occasions. In some cells, sequences of discharges following the initial responses waxed and waned as though periods of excitation and inhibition dominated the cell's activity. The reverberating nature of these discharges was particularly evident when only brief (100 ms) indentation hold periods were used (Fig. 5A 1, B 1, C~). For example (Fig. 5 A l ) , secondary bursts of activity occurred 300 ms after the discharge to terminating the indentation followed by another small discharge - 4 0 0 ms later. Similarly (Fig. 5B1), in another cell, a second build-up of responses followed the phasic off-response and this afterdischarge was succeeded by another slight increase after - 7 0 ms. In most cells with afterdischarges, firing after the probe retracted gradually decreased over several hundred ms (Fig. 5At, B1, C], Dr). When responses of these cells were evaluated with longer duration indentations (Fig. 5A2, B2, C2, D2, E and possibly Fig. 3F), it was difficult to appreciate whether responses during indentation constituted slow adaptation to the stimulus or afterdischarges to the phasic response. In cases where patterns of activity following brief indentations resembled discharges during longer plateaus (Fig. 5A-D), it is likely that the activity was all due to afterdischarges. In other cases not tested

267 30~

!

1go

A

o ii

O tr iii an Z 0

10

20

30

40

50

(iO

70

80

go

100

110

LATENCY (ms)

10

20

30

4o

r~

B et ttJ

Fig. 6. Distribution of latencies to initiation of indentation in 90 cells. Cross-hatched bars mark cells whose mean latency was 34.27 ms; open bars show cells whose mean latency was 78.69 ms.

with brief stimuli (Fig. 3F, 5E,F), but where successively decreasing bursts during the hold phase appear more disjointed than was apparent for cells with sustained responses (see above), it is possible that the activity was also an after-discharge to the initial response.

Response latency Average latencies per trial to the first discharge were determined for 90 cells that had no activity during the pre-hold times prior to indentation. Time was measured from the computer marked start of ramp generation to the occurrence of the first action potential for each of 10-20 indentations. The distribution of latencies shown in Fig. 6 is unimodal but skewed to longer times. Positive kurtosis was due mainly to delayed responses seen in the class of cells defined as 'late responses' (see above). These cells had an average latency of 78.69 ms (+ 21.57) while the remaining cells had an average of 34.27 ms (+ 12.21). No differences were seen in average latencies between cells with phasic or sustained discharge patterns.

Velocity sensitivity Responses to indentations at 3 or more velocities were studied in 47 cells. Judgments of velocity sensitivity were based on determining total number of discharges per trial from the time of maximal firing per bin after start of indentation to the bin that contained background levels of activity and then dividing the count by the time occupied by the number of bins. In cells studied with at least 4 different velocities, this analysis showed progressively increasing dynamic firing to faster ramps (Fig. 7A). Slopes for 9 of these functions were relatively shallow (mean = 1.87 spikes/s/trial (s/s/t), S.E.M. = 0.3) and ranged from 0.91 to 3.26 s/s/t. These ascending functions were relatively monotonic with a mean linear regression

I

VELOCITY ram/see Fig. 7. Velocity intensity functions for two groups of cells. A: examples of response functions from 6 cells with ascending firing rates as a function of indentation velocity. B: examples of response functions from 4 cells whose firing rates did not change with velocity after threshold was passed.

correlation of 0.95. Consequently, these kinds of SII cells could provide some indication of different indentation ramp velocities during the dynamic phase of their responses. A greater number of cells showed very little relation between peak firing rates to slow and fast ramps. These cells generally asymptote at relatively low ramp velocities or had no differential activity as a function of velocity (Fig. 7B). Mean slope for 13 of these functions was 0.88 s/s/t (S.E.M. = 0.195) with a range of-0.08-2.4 s/s/t. The plateau shape of the curves also correlated poorly to a monotonic, linear function (mean r = 0.61). According to Mann-Whitney and t-test analyses, P = 0.05, mean slopes of the first group (Fig. 7A) were significantly greater than those with plateau functions (Fig. 7B). DISCUSSION

Identification of the second somatosensory area in macaque monkeys was obtained shortly after the original recognition of an additional cortical representation of the body in cats 1'a3. Despite repeated confirmation and elaboration of details concerning SII in a variety of primates in several subsequent s t u d i e s 4'5'12"22'23, there has been no systematic examination of response properties of SII neurons in monkeys prior to the present study. Previous work in SII of cats has investigated the

268 capacities of these cells to respond to controlled mechanical stimulation of the skin, but these investigators principally emphasized sensitivity to sinusoidal skin vibrations 3'15. In contrast, neurons in primary somatosensory cortex have repeatedly been studied in a variety of species with different parameters of controlled mechanical stimulation 13'3°. An influential hypothesis that has developed from information about response properties of neurons in SI is the notion of 'labeled lines' between specific classes of peripheral mechanoreceptor and the cerebral cortex. Thus, identification of response characteristics of different classes of cells in SI that match capabilities of only certain types of peripheral receptors supports the assumption that submodal specializations inherent in peripheral receptors is preserved centrally and that this conservation explains certain aspects of perceptual experience 3°. Concentrations of certain classes of cells in different subdivisions of SI has reinforced ideas about dedicated functional entities 13. Finding cells especially tuned to high frequency vibratory stimulation in SII of cats has prompted the suggestion that similar, modalityspecific, parallel 'labeled lines' of transformation exist for SII. However, recent lesion studies 1°'35 have indicated that, particularly in higher primates, SII responds to somatosensory inputs only when SI is intact. This has led to the suggestion that SII is not equivalent across species. In addition, these changes in SII following lesions of SI have tended to support the contention that transformations of somatosensory signals flow serially from SI to SII in primates x1'27'35. These results direct attention to the question of whether characteristics of peripheral inputs can still be recognized in responses of SII neurons. Corollary issues are whether SII neurons show evidence of convergence of inputs from different classes of peripheral receptors and whether these cells have other properties that possibly reflect serial processing of somatosensory signals. The following discussion will examine whether the present results provide evidence that response properties of SII neurons most likely arise from serial processing through SI.

Slowly adapting responses Distinctions between slowly and rapidly adapting responses to sustained pressure on skin have been cited repeatedly as a means of identifying the central representation of two readily identifiable classes of low threshold cutaneous mechanoreceptors 14'28'31'39. Slowly adapting responses in SII of any species have, however, been extremely rare 4. In the cat only 5% (9/164) of the cells were classified as slowly adapting and responses from just 1 neuron were illustrated 3. A larger number of cells in monkey SII had persistent activity during the

plateau phase of indentations. Many of these sustained responses appeared similar to slowly adapting responses noted elsewhere in the somatosensory system except for finding that most cells showed only fractional aspects of sustained responses on any one trial. We did not evaluate whether these relatively weak responses would have been improved by different amplitudes of indentation. Intensity functions for 3 slowly adapting cells recorded in cat indicated that indentation amplitudes with a 4 mm diameter probe generally had to exceed 500 /~m and maximum activity was seen at fairly large steps of >2000 /~m (ref. 3). In SI most cells with slowly adapting responses have thresholds (<50 pm) that are similar to those seen in peripheral n e u r o n s 31'39 and steps of 1000 ~m are closer to maximum sensitivity for these low threshold inputs. Consequently, most slowly adapting responses evaluated in SII in this study were not tested with large indentations. Although thresholds were not routinely determined, they were not similar to those seen in SI. This difference may not be significant since most cells were tested with much smaller probe tips (see Materials and Methods) than have been used in previous studies. Suspending for now the caveat introduced by differences in threshold and strength of responses, slowly adapting responses detected in SII probably can be attributed to central projections from slowly adapting cutaneous receptors. The question of whether these responses are projected to SI and SII in parallel from thalamus or serially from SI cannot be answered directly from current data. Several findings suggest, however, that thalamocortical connections may be the source of slowly adapting responses in SII. This hypothesis arises from observations in cat and monkey SI that most cells capable of demontrating sustained responses can only be located in or near layer IV 13'14"39, Cells above and below this layer, but within the same cortical column, demonstrate only rapidly adapting responses. As cells in supraand infragranular layers are the source of corticocortical connections from SI to SII in primates TM,they may not be capable of transmitting sustained responses. Some ambiguity enters this argument from imprecision in being able to localize all slowly adapting responses to layer IV local circuit neurons. Sur et a l . 39 stated that some cells were recorded within deep parts of layer III and some of these might project to SII. It could also be argued that slowly adapting responses have been so difficult to find in SII because serial processing through SI raises thresholds and creates generally weaker responses. Alternatively, if the thalamocortical source of these projections differs, i.e., SII slowly adapting inputs come through the posterior nucleus 6'8'16 where thresholds to pressure tend to be higher 34, then notions of parallel projections are still appropriate.

269 Rapidly adapting responses A substantial variety of rapidly adapting responses were noted in SII. This kind of response was most common in this and previous studies in several species. Finding low threshold rapidly adapting responses to ramp indentations into glabrous skin limits the range of peripheral receptors likely to be responsible for the activity. In SI these kinds of inputs are thought to arise from Meissner and Pacinian receptors. (A discussion of the separation between these two, which is based on sensitivity to vibratory frequencies of different frequencies 32, will be presented in a subsequent paper.) However, given that inputs from slowly adapting afferents may be reduced to just phasic responses even in SI outside layer IV 39, SII neurons with rapid adaptation characteristics and glabrous skin receptive fields could be activated by any one or more of at least three different peripheral receptor classes. Two features of the phasic responses may be relevant to the issue of serial and parallel processing through SII. First, latency to the first discharge was positively skewed to longer latencies that were delayed two or more fold from the modal point of the distribution. Except for the shortest latencies recorded (i.e. <20 ms), most cells were delayed considerably longer than has been reported previously with evoked potential recording methods 44. (It should be noted that latencies recorded in SI by Woolsey and Walz144 were still shorter.) Technical factors provide some explanation for this difference. For example, the earlier evoked potential study relied on a tap-like stimulus whose thrust against a larger portion of skin was more abrupt and intense than was applied with the punctate stimulator used in the present study. A delay was also caused by time taken to overcome inertia in our probe once it received a signal to advance. Second, evoked responses may have been dominated by inputs from the most rapidly conducting elements, whereas in this study each neuron was evaluated individually. Given these considerations, it is possible that many of the cells whose latency was less than 40 ms might have responded with much shorter delays to a large, tap-like stimulus. This still leaves a group of ceils whose latencies are quite long. Even longer latencies have been reported ( - 1 1 0 ms) using multiple neuron recordings in awake monkeys 19.

The simplest explanation for such long latencies is delays due to activation and propagation times through multiple synapses. As there are no known intrathalamic connections between thalamocortical relay cells, these delays can best be interpreted as arising from intracortical connections. These timing delays may be due to processing through multiple connections that involve serial processing from S119. They may also arise from

intracortical events within SII. It is also possible that projections from thalamus that are destined for SII are regulated from SI. A longer latency would arise if inputs to SII must await an enabling signal from SI corticothalamic projections. Although, such a circuit has not been demonstrated physiologically, anatomical evidence for it exists 2°. The purpose of this point is to indicate that longer latencies only suggest multiple connecting links; neither the polarity nor source of these connections are specified. Another feature of SII responses that points to the likely presence of a fairly complex circuit in activation of SII neurons was multiple, sometimes cyclical reverberations of discharges after a tactile stimulus was removed (see Fig. 5A 1, B1, C1). Similar kinds of complicated responses had been noted previously as 'off' responses 37. These after-discharges would not have been appropriately recognized if only long duration indentations had been used since slowly decaying cycles evoked after a stimulus stopped changing might be interpreted as a slowly adapting response. H/im~il~iinen et al. 19 have also noted more prolonged multiple neuron responses in SII compared to SI using identical recording conditions in both areas. A mechanism involving serial processing from SI to SII that would be responsible for persistence of discharges after a stimulus had ended seems unlikely since long reverberations have not been described in SI in comparison to SI119. Cycling of responses within SII might be responsible for persistent firing, perhaps through a circuit involving disinhibition. An alternative view is that thalamocortical projections to SII have been repetitively turned on and off after a stimulus arrives. A similar model has been proposed to explain the effects of SI lesions on responsiveness in SI11°. Do SII neurons code some feature o f a ramp stimulus? In comprison to SI, the recorded population of cells in SII did not convey much information about ramp indentations. Even cells with the steepest intensity function for indentation velocity exhibited relatively shallow functions. Most cells just responded to the occurrence without providing much information about the characteristics of a stimulus. Indeed, the largest number of cells could not be tested with punctate stimuli since they failed to respond to them. Ruling out those cells that were inactive because they were driven from 'deep' or high threshold receptors, the predominant conclusion is that SII is not processing punctate signals in a way that resembles capabilities so prominently displayed by SI. It was clear, however, that SII neurons have a variety of responses to tactile stimuli and that some of these responses can be very substantial. The absence of more elaborate stimulus response functions

270 cannot be attributed to anesthetics since many records were o b t a i n e d from monkeys that were either awake or only lightly tranquilized. Is the deficiency in SII neuronal responses to punctate indentations a consequence of serial processing of this information through SI? A l t h o u g h this question cannot be answered directly, evidence can be cited that bolsters opposing views. First, the degree to which the precision of responses in SI is absent in SII seems unusual if these responses came from SI. In comparison, it is possible, with hypotheses about serial processing in the visual cortex, to create schemes of connecting elements between p r i m a r y visual cortex and its targets that explain response features of the recipient areas 26'42. The important difference is that capacities of the primary region are not lost but emerge in some new feature. In contrast, responses of SII neurons a p p e a r less expressive of p e r i p h e r a l inputs than those seen in SI where the faithful replication of p e r i p h e r a l afferent response characteristics is so evident 3°. SII neurons do not r e s p o n d to velocity or indentation levels of a stimulus or some extraction of these p a r a m e t e r s . It may be said that the absence of peripheral-like responses in SII implies a complex transf o r m a t i o n of inputs from SI. The p r e s u m e d ' c o m p l e x ' part of the response actually refers to (1) the greater difficulty in driving SII cells to respond at all to punctate stimuli that are so effective in SI; (2) the presence of larger, less well-defined receptive fields that may be bilateral; (3) a greater sensitivity to anesthetics; (4) the occurrence of longer latency responses; and (5) the a p p e a r a n c e of p r o l o n g e d responses that can outlast the stimulus. N o n e of these characteristics can be exclusively inferred from the capabilities of converging inputs from

SI cells. In many ways they resemble responses of neurons in parts of thalamus (e.g., the posterior nucleus and its possible rostral extensions along the b o r d e r s of VPLc) that do not principally receive projections from the central core of the dorsal column nuclei 1°'34 and that provide a m a j o r source of thalamocortical connections to 8ii4-6,16,20. Before concluding that SII is not serially influenced from SI, and that its physiology is solely indicative of inputs from the thalamus, two unresolved issues have to be considered. (1) W h a t mechanism causes SII to b e c o m e less responsive when SI is lesioned particularly in certain primates? and (2) W h a t aspect(s) of somatosensory stimuli are the p r o v e n a n c e of SII? This last question addresses the possibility that SII's responses actually can be recognized as elaborations of features seen in SI if m o r e ' a p p r o p r i a t e ' stimuli were used. In the visual system, responses from neurons that e l a b o r a t e m o r e abstract features of visual stimuli, such as m o v e m e n t direction, also would show relatively weak, inconsistent responses to light flashes or even simple bars of light. Small punctate stimuli m a y r e p r e s e n t similarly inappropriate stimuli for SII and, therefore, it is p r o b a b l y p r e m a t u r e to consider the absence of robust responses to these stimuli as evidence that SII is not at a higher level in processing somatosensory stimuli.

Acknowledgements. Particular appreciation is extended to Dr. A.D. Craig, Jr. for his help in designing the overall structure of the software used in these studies. We also wish to thank Dr. Kevin Alloway for his editorial comments. Mike Morgan helped with photography and Susan Mantia provided secretarial assistance. Supported by funds from NIH NS09809 (NIDCD - 00096).

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