Simultaneous encoding of tactile information by three primate cortical areas

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1 rticles Simultneous encoding of tctile informtion by three primte corticl res Miguel A. L. Nicolelis 1, Asif A. Ghznfr 1, Christopher R. Stmbugh 1, Lur M. O. Oliveir 1, Mrk Lubch 1, John K. Chpin, Rndll J. Nelson 3 nd Jon H. Ks 1 Dept. of Neurobiology, Box 39, 11 Reserch Drive, Bryn Reserch Building, Duke University Medicl Center, Durhm, North Crolin 771, USA Dept. of Neurobiology nd Antomy, Allegheny University of the Helth Sciences, 3 Henry Avenue, Phildelphi, Pennsylvni Dept. of Antomy nd Neurobiology, University of Tennessee, 55 Monroe Avenue, Memphis, Tennessee 313 Dept. of Psychology, Vnderbilt University, 31 Wilson Hll, Nshville, Tennessee 37 Correspondence should be ddressed to M.A.L.N. (nicolelis@neuro.duke.edu) We used simultneous multi-site neurl ensemble recordings to investigte the representtion of tctile informtion in three res of the primte somtosensory cortex (res 3b, SII nd ). Smll neurl ensembles (3 neurons) of brodly tuned somtosensory neurons were ble to identify correctly the loction of single tctile stimulus on single tril, lmost simultneously. Furthermore, ech of these corticl res could use different combintions of encoding strtegies, such s men firing rte (res 3b nd ) or temporl ptterns of ensemble firing (re SII), to represent the loction of tctile stimulus. Bsed on these results, we propose tht ensembles of brodly tuned neurons, locted in three distinct res of the primte somtosensory cortex, obtin informtion bout the loction of tctile stimulus lmost concurrently. One of the most bsic functions of the nervous system is the locliztion of sensory stimuli. Informtion bout stimulus loction is, of course, vilble in the peripherl receptor rry, whose ctivity my be processed long divergent pths nd levels of centrl pthwys. For instnce, the trnsltion of this receptor ctivity through the thlmocorticl circuitry my produce senstion of locliztion, wheres trnsltion of the sme ctivity through the spinl circuitry my produce reflex movement towrd or wy from the stimulus loction. How might the brin ctully combine informtion vilble in mny neurl structures to produce perceptul indiction of the loction of tctile stimulus? In the complex somtosensory system of primtes, which includes mny reciproclly connected corticl res 1, fundmentl question is whether informtion bout tctile stimulus locliztion is preserved throughout these mny corticl res, or whether such informtion is conserved within prticulr corticl popultions tht then serve s generl reference. This second lterntive seems to be suggested by the degrdtion of topogrphic mps of body surfce s one proceeds from erly to lte stges of corticl processing 1. Clerly, informtion bout stimulus loction is well preserved in the precise topogrphic mps in the brinstem, thlmus nd primry somtosensory cortex. Yet, is this informtion necessrily lost in other corticl res where neurons hve lrge receptive fields? An lterntive possibility is tht informtion bout stimulus loction could be trnsformed from sptil code, esily recognized t single-neuron level, to distributed code, best recognized in the simultneous ctivities of lrge popultions of corticl neurons. It is lso conceivble tht t lte stges of corticl processing, the loction of stimulus could be represented in the temporl ptterns of neurl ensemble firing. In fct, reserch on rtificil neurl networks demonstrted tht temporl encoding strtegy emerges s signture of recurrent distributed systems tht cn resolve noisy ptterns. This temporl coding could mnifest itself through precise sequences of ction potentils, synchronous neuronl firing or time-dependent modultion of the neurl ensemble firing rte. Such issues re fundmentl to our understnding of how the primte cerebrl cortex opertes nd re not restricted to the specil functions of the somtosensory system. In the uditory cortex, for instnce, the lck of n obvious sptil mp for sound loction hs led some to propose tht popultions of uditory corticl neurons my encode loction in complex distributed fshion tht does not depend on precise sptil mpping t the single-neuron level 5. To dte, however, little ttention hs been pid to the possibility tht such nonsptil codes could effectively represent tctile stimulus loction in the somtosensory system. Indeed, the secondry somtosensory cortex nd fields of the posterior prietl cortex hve long been considered to be uninvolved in stimulus loction coding, becuse the lrge receptive fields of neurons in these regions 1 would seem to preclude the ccurte representtion of this informtion. This view could be chllenged by evidence tht ccurte stimulus loction informtion cn be extrcted from the collective responses of neurl ensembles in these corticl res, s proposed by distributed models of corticl function 1. To ddress this possibility, in this study we used lrge-scle, chronic, multi-site neurl ensemble electrophysiology 13,1 to simultneously record the sensory responses of up to 135 single neurons locted in three corticl somtosensory res (res 3b, nture neuroscience volume 1 no 7 november 199 1

2 rticles Monkey 1 Monkey Monkey 3 SMA M1 3 3b 1 SII Fig. 1. Loction of chroniclly implnted microwire rrys used to record neuronl ensemble ctivity in the somtosensory nd motor cortex. In this schemtic representtion of the somtosensory nd motor res of the neocortex of the owl monkey, ech colored rectngle depicts the ntomicl loction of chroniclly implnted rry contining 1 microwires (orgnized in two rows of eight). Note tht SII is buried within the lterl sulcus. SII nd ) in dult owl monkeys (Aotus trivirgtus) This species ws chosen becuse the corticl res of interest re ccessible on the dorsl surfce of reltively smooth neocortex, nd much is known bout the orgniztion nd connections of its cortex 15. Results In ech of the three monkeys used in this study, bout % of the microwires yielded t lest one discriminble single unit, nd up to four neurons per electrode could be discriminted in ech niml. Chroniclly implnted microwire rrys provided stble neurl popultion recordings for months in monkey 1, 19 months in monkey nd 5 months in monkey 3. During these recording periods, mximum of 135 (monkey 1, microwires), 1 (monkey, microwires) nd 77 (monkey 3, 3 microwires) corticl neurons were simultneously discriminted in single recording session. Different smples of neurons were obtined in ech recording session, lthough given smple of neurons ws often kept for mny dys. Histologicl nlysis showed tht the neurons recorded in this study were locted primrily in the infrgrnulr lyers of the following corticl res (Fig. 1): for monkey 1, primry motor cortex (MI), secondry somtosensory cortex (SII) nd posterior prietl re ; for monkey, supplementry motor re (SMA), re 3b, SII nd re ; nd for monkey 3, MI, SII nd re. Only results from the somtosensory res 3b, SII nd re discussed here. Brodly tuned neuronl responses to tctile stimultion were observed in the infrgrnulr lyers of the primte somtosensory cortex (Fig. ). Rster plots nd corresponding post-stimulus time histogrms (PSTHs) were used to depict the sensory responses of smple ( out of 3) of simultneously-recorded SII neurons following the stimultion of different hnd loctions. As rule, individul SII s well s re neurons responded to the stimultion of multiple digits nd regions of the dorsl surfce of the hnd (Fig. 3). Quntittive nlysis of PSTHs in both wke nd lightly ketmine-nesthetized (1 15 mg/ kg) nimls Rostrl Medil demonstrted tht virtully ll neurons in the infrgrnulr lyers of res SII nd showed lrge multi-digit receptive fields (RFs). These RFs were considerbly lrger thn those observed in lyer IV of re 3b, nd similr to the lrge neuronl RFs previously reported in rhesus monkeys 1,17. The rster plots (Fig. ) lso illustrte the difficulty of identifying which skin site ws stimulted when only single tril of single neuron s response ws considered (see neurons, 5 or 3, for instnce). In fct, for most neurons, the loction of the stimulus could not be identified by exmining either single-tril sensory responses or verged informtion from mny trils depicted in PSTHs (Fig. ). In ddition, the rel loction of corticl neuron could not be estblished by simple inspection of individul neuronl PSTHs (Fig. 3). This is becuse most somtosensory corticl neurons showed similr verged sensory responses tht could not be distinguished in terms of miniml ltency, durtion or frequency of responses. This similrity in intrinsic temporl response chrcteristics of somtosensory neurons is illustrted by the comprison of smple of simultneously recorded SII nd re neurons, which showed virtully identicl PSTHs in response to punctte tctile stimultion (Fig. 3). In generl, the multidigit RFs of SII nd re corticl neurons were chrcterized by the presence of center whose stimultion elicited the lrgest sensory response nd the shortest response ltency. Stimultion of surround regions of the RF induced smller but sttisticlly significnt excittory responses t longer response ltencies, s previously reported in rts 1. In ddition to hving multi-digit RFs, re neurons could be driven by stimultion of the fce nd the bck of the hnd (Fig. c nd d). By combining the sensory responses of individul neurons into popultion mps, we were ble to reconstruct the neurl ensemble representtions of punctte tctile stimuli cross the somtosensory corticl res (Fig. ) in the sme nimls. Punctte stimultion of different prts of the hnd nd/or fce produced distinct sptiotemporl ptterns of neuronl ctivtion in SII (Fig. nd b) nd re (Fig. c nd d). These distinct profiles were generted by vritions in the response mgnitude nd ltency of individul neuronl sensory responses. Note, however, tht mny neurons contributed to the popultion response resulting from given tctile stimulus, suggesting tht distributed representtions exist in res SII nd of the primte somtosensory cortex. Further nlysis of the popultion mps for ll three corticl res recorded in ech niml reveled considerble ltency overlp between the sensory responses of neurons (mesured with 1-ms ccurcy). In n exmple recording (Fig. 5), this ctivity overlp ws observed during stimultion of the tip of the D digit in monkey 3. This stimulus initilly triggered sensory response in neurons locted in SII (1 15 ms), nd couple of milliseconds lter mssive neurl firing ws observed in both SII nd re. Comprison of miniml response ltencies for ll neurons recorded in res 3b, SII nd (Fig. 5b) reveled significnt degree of overlp. ENSEMBLE CODING OF TACTILE STIMULUS LOCATION Our overll strtegy in this nlysis ws to mesure how well the firing ptterns of corticl ensembles could predict, on single tril, the loction of punctte tctile stimulus pplied to the niml s body. Becuse mny skin sites were stimulted in rndom order, multiple combintions involving 3, 5 or 1 sites were used for this nlysis. Ten-fold cross-vlidtion ws employed for ech nlysis to yield the highest stbility nd ccurcy of the results. For ech combintion of sites, 1% to 9% of the originl nture neuroscience volume 1 no 7 november 199

3 rticles Simultneously recorded SII neurons Stimulus site dsp1c dsp dsp5 dsp3 dsp3d dsp31d Spike counts per 1 ms bin Time (s) Fig.. Single-neuron responses to tctile stimultion t different sites on the hnd. Rster plots nd post-stimulus time histogrms were used to illustrte the sensory responses of subset of simultneously recorded SII neurons following the punctte tctile stimultion of seven neighboring regions of the monkey hnd. All single SII neurons (identified on top of ech column) tended to respond significntly to ll the stimulted skin sites. In ddition, ech neuron s verged responses (see histogrms) were similr cross these skin sites. Inspection of single tril responses for given single neuron did not llow identifiction of the site of stimultion, either becuse the response ws indistinguishble to tht obtined for other sites or becuse the neuron did not fire in tht tril. nture neuroscience volume 1 no 7 november 199 3

4 Spike Counts Per 1 ms Bin Spike Counts Per 1 ms Bin rticles Spike counts per 1 ms bin dsp17 dsp1c dsp19 dspc dsp5 dsp 3 stimulus trils obtined per skin site were used to trin different rtificil neurl networks or to derive discriminnt functions using liner discriminnt nlysis. Once the trining phse ws completed, the trined rtificil networks (or the discriminnt functions, in the cse of liner discriminnt nlysis) were used to identify which of the skin sites ws stimulted in ech of the remining testing trils. Testing trils were never presented 1 dsp1 dsp1b dsp3 dsp3b dsp dspb Are SII Are 3 dsp9b 1 dsp3 dsp3b dsp3d dsp31c dsp31d dsp5d dsp1 dsp1c 1 15 dsp1c 5 3 dsp dsp15b dspb dsp5 dsp1 3 dsp Time (sec) Time (sec) Time (sec) Time (sec) Time (s) Time (s) Time (s) Time (s) Dorsl hnd stimultion Fig. 3. Post-stimulus time histogrms depict the simultneously recorded sensory responses of ensembles of neurons in res SII nd following the punctte tctile stimultion of hir locted in the dorsl hnd of monkey. Notice tht ll neurons in these two res responded to this stimulus (s well s stimultion of mny other sites, not shown). Inspection of these nd other histogrms did not revel ny cler encoding strtegy. For instnce, no difference in the frequency of response cn be observed between SII nd re neurons. Moreover, both popultions showed overlpping miniml response ltencies. Finlly, the durtion of the singleneuron sensory responses in both res ws similr. Therefore, no difference in intrinsic temporl response chrcteristics between these two popultions of neurons ws observed in these post-stimulus time histogrms. during the trining phse of the nlysis. Both testing nd trining trils were presented in rndom sequence, nd equl numbers of trils per site were used. As wy to cross-vlidte our results, different nlysis prmeters were vried. First, three methods for sttisticl pttern recognition (liner discriminnt nlysis, lerning vector quntiztion nd bck-propgtion) were pplied to the sme dt Fig.. Distributed responses of re SII nd neurl ensembles. Popultion mps depict the simultneously recorded sensory responses of neurl ensembles locted in res SII nd of the owl monkey somtosensory cortex. (, b) Stimultion of the proximl phlnx of digit 1 nd of the dorsl surfce of the hnd elicit distinct sptiotemporl ptterns of response in the SII cortex. (c, d) Stimultion of the fce nd dorsl surfce of the hnd elicit distinct sptiotemporl ptterns of ensemble ctivity in re. In ech of the three-dimensionl plots, the x-xis depicts the simultneously recorded neurons, the y-xis is post-stimulus time in milliseconds, nd the z- xis is the response mgnitude in spikes per second. Spikes/s Spikes/s b Neurons Neurons SII Are digit 1p Time (ms) dorsl hnd Time (ms) c Spikes/s d Spikes/s Neurons Neurons chin Time (ms) dorsl hnd Time (ms) nture neuroscience volume 1 no 7 november 199

5 rticles b SII Are Are 3b s c rticl A Are SII Number of observtions v Miniml ltency (ms) Fig. 5. Concurrent ctivtion of somtosensory corticl res following punctte tctile stimultion. () Color-coded three-dimensionl mtrices depict the post-stimulus firing of simultneously recorded neurons in the SII cortex (top pnel) nd re (bottom pnel) following the stimultion of finger tip. These plots depict the response of ech neuron ccording to its position in the X electrode rrys implnted in the two corticl res. Dt were plotted t 5 millisecond post-stimulus time intervls. At ech time epoch, the two rectngulr pnels, seprted by verticl white line, were used to represent the sptil extent of neurl ctivity. The y-xis of ech rectngulr pnel represents the rostrocudl position of the microwires (top rostrl-most electrodes 1 nd 9; cudl-most wires nd 1), the left rectngle depicts the lterl component of the rry, nd the right pnel depicts the medil component of the rry. Becuse up to four neurons were recorded per microwire, the x-xis of ech rectngle contins up to four slots per microwire to represent the response of ech of the recorded neurons. The z-xis represents vritions in neuronl response mgnitude (drk red represents > stndrd devitions bove spontneous firing, nd blue represents spontneous firing, in spikes per second). Notice tht following smll ctivtion of the SII cortex t the 1 15-ms epoch, there is concurrent ctivtion of both corticl res (SII nd ). (b) Miniml ltency distributions for single neurons in ech of the three corticl res. The miniml ltencies clculted for dt obtined in two monkeys demonstrte tht following n initil ctivtion of both re 3b (miniml response ltency, 9. ±. ms) nd SII (1.5 ± 3.5 ms), concurrent ctivtion ws observed in these res nd re (miniml response ltency, 17. ± 5. ms). sets. Second, three unique formts of dt input were used for ech pttern recognition nlysis: rw spike trins, principl components 19 nd independent components derived from the rw spike trins. Finlly, different trining-set sizes (9%, 5% nd 5% of the totl number of trils) were used in the nlysis. Through ll of these vritions in nlyticl pproch, sttisticlly similr results were obtined (Fig. nd b), indicting tht neither the pttern recognition nlysis method (Fig., p <.77, MANOVA), nor the type of dt preprocessing, nor the trining schedule influenced the finl outcome of the popultion nlysis. The reltive proportion of trining to testing trils ws lso generlly unimportnt for these results, lthough significnt reductions in discrimintion performnce by the rtificil neurl networks were obtined when the number of trining trils ws reduced to 1% of the totl (p <.3, Fig. b). An rtificil neurl network bsed on lerning vector quntiztion ws used to evlute the discrimintion cpbilities of simultneously recorded corticl neurl ensembles locted in res 3b nd SII (monkey, Fig. 7 nd b) nd res SII nd (monkey 3, Fig. 7c nd d). A totl of (monkey 1), 1 (monkey ) nd 5 (monkey 3) unique combintions of 3 skin sites were used to quntify ech corticl ensemble s discrimintion cpbility. This nlysis reveled tht, when independently tested, sttisticlly significnt single-tril discrimintion of the loction of tctile ws chieved using corticl ensemble firing ptterns from ech of the three res investigted (3b, SII nd re ) nd vriety of skin site combintions. For exmple, the firing pttern of n ensemble of neurons locted in re 3b of monkey uniquely specified the correct identifiction of the stimulus loction in 7.5 ± 3.5% of the individul testing trils (Fig. 7 nd b). The sme nlysis reveled tht SII ensembles provided the correct identifiction of the stimulus loction in 9.5 ±.% of the single testing trils in monkey 1 ( neurons, not shown), 7. ± 1.9% of the trils in monkey ( neurons, Fig. 7 nd b) nd 73.3 ± 1.% in monkey 3 (3 neurons, Fig. 7c nd d). Above-chnce discrimintion ws lso obtined when re ensembles (. ± 1.%, 3 neurons, monkey 3, Fig. 7c nd d) were tested independently. The potentil encoding mechnisms used by ech corticl re were investigted by, first, sequentilly removing neurons ( neuron dropping ) from ech corticl ensemble to mesure the vrition in discrimintion cpbility s function of the ensemble size; second, independently shuffling the tril order for ech neuron, mneuver imed t testing the robustness of intertril neuronl correltions; nd third, vrying the integrtion time used to describe ech neuron s sensory response (from 3 to 5 ms), procedure tht ltered the temporl structure of ech neuron s sensory response ( bin clumping ). The neuron-dropping procedure further supported the existence of distributed representtions of tctile informtion in ech of the three corticl res, by demonstrting tht removl of individul best predictor neurons from the ensemble produced only smll nd grdul reductions in the correct clssifiction of single trils by the neurl ensemble, s shown for four neurl ensembles (Fig. nd b). SII functions (from two monkeys) were very similr to ech other but differed when compred to those obtined for res 3b nd ensembles. Even though re 3b ensembles outperformed re popultions of the sme sizes, nture neuroscience volume 1 no 7 november 199 5

6 rticles b RAW PCA ICA PCA & ICA Fig.. Multiple sttisticl pttern recognition pproches cross-vlidte results of neurl ensemble performnces. () Use of three pttern recognition lgorithms (lerning vector quntiztion, non-liner bck-propgtion network nd liner discrimintion nlysis) nd three types of dt input formt (rw spike dt, principl component, PCA, nd independent components, ICA) produced similr results (p <.77, MANOVA). (b) Similrly, no sttisticl difference ws observed when 5 9% of the trils were used to trin the rtificil networks (lerning vector quntiztion, LVQ, nd bck propgtion, NLB) nd derive the discrimintion functions (liner discriminnt nlysis, LDA). Becuse sttisticlly significnt reductions in discrimintion could occur when only 1% of the trils were used, this trining routine ws voided. Percentge of totl trils used in trining the two corticl res showed very similr grdul decys in single-tril discrimintion performnce s neurons were removed from the ensembles. Incresing the number of skin sites used in the nlysis (from 3 to 1 sites) reduced discrimintion performnce. However, bove-chnce discrimintion ws observed for ech combintion of sites. Overll, most individul neurons lone were poor discrimintors of the stimulus loction on single tril, s the vst mjority of the single 3b (9%), SII (9%) or re (1%) neurons tested could not be used to clssify more thn 5% of the trils correctly for combintions of 3 skin sites. Independent shuffling of tril order for ech single neuron hd little effect on the overll clssifiction of single trils by either rtificil neurl networks or liner discrimintion nlysis. This suggested tht the neuronl responses were tightly locked to the presenttion of the tctile stimuli throughout the durtion of ech recording session. In some cses, the performnce of rtificil neurl networks improved slightly for certin skin site combintions fter tril shuffling, which could suggest the existence of correlted biologicl noise embedded in the single-neuron responses on ech tril. Furthermore, this finding suggests tht effective single tril discrimintion of punctte tctile stimulus loction does not require precise (1 ms ccurcy) intr-tril neurl ensemble spiking sequences 1,, extensive neuronl synchroniztion 3 or covrince cross the popultion of simultneously recorded neurons. DIFFERENCES IN ENCODING STRATEGY BETWEEN CORTICAL AREAS Both temporl- nd rte-bsed codes hve been proposed to ccount for the bility of corticl neurl ensembles to represent sensory informtion. In the finl step of our nlysis, we inves- * tigted which of these encoding schemes could best explin the single-tril representtion of tctile stimuli in the somtosensory res 3b, SII nd. First, we studied whether the temporl structures of the sensory responses of neurons in ech corticl re were involved in the encoding of stimulus loction. To ddress this issue, we exmined the rtificil neurl network s bility to clssify correctly the loction of stimulus on single tril using bin clumping, by integrting neuronl firing dt into rnge of bin sizes (from 15 3-ms time bins to single 5-ms bin) used to represent the first 5 ms of post-stimulus period, s shown for pirs of corticl res recorded in two monkeys ( nd 3; Fig. 9). Bin clumping significntly reduced the discrimintion cpbility of SII neurl ensembles in ll three monkeys: from 9.5 ±.% to 9.3 ± 1.1% in monkey 1 (not shown), from 7. ± 1.9% to 1. ±.1% in monkey (p <.1; Fig. 9) nd from 73.3 ± 1.% to 59.5 ± 1.7% in monkey 3 (p <.1; Fig. 9b). Bin clumping, however, hd no effect on the discrimintion cpbility of the 3b ensemble in monkey (7.5 ± 3.5% using 3-ms bins nd 7. ± 3.7% using single 5-ms bin, p <.; Fig. 9c) or the re ensemble recorded in monkey 3 (. ± 1.% using 3- ms bins nd.7 ± 1.5% using single 5-ms bin, p <.7; Fig. 9d). Even though single SII neurons hve been reported to respond well to time-vrying stimuli, prticulrly high-frequency vibrotctile stimuli 5, no bsic differences in intrinsic temporl response chrcteristics were observed in the present study between SII neurons nd neurons recorded simultneously in either res 3b or. (See Fig. 3 for comprison between res SII nd.) A plusible interprettion of these bin-clumping results is tht the reduction in single tril discrimintion by SII ensembles resulted from the disruption of time-dependent ptterns of neur- nture neuroscience volume 1 no 7 november 199

7 rticles c Monkey Monkey 3 Sets of 3 stimulted skin sites Sets of 3 stimulted skin sites b d Averge over ll sets Averge over ll sets Fig. 7. Single-tril discrimintion by neurl ensembles locted in three somtosensory corticl res (3b, SII nd re ) s mesured by n rtificil neurl network trined with lerning vector quntiztion lgorithm. () Neurl ensembles in res 3b nd SII (monkey ) performed t higher-thn-chnce levels in discriminting the loction of punctte tctile stimuli from different combintions of three skin sites. (b) When the results obtined for ll skin site combintions were verged, 7.5% of the single trils were correctly clssified using re 3b neurons, wheres 7% of the single trils were correctly clssified with SII neurons. (c, d) Performnce of neurl ensembles locted in res SII nd in monkey 3. Although neurl ensembles in both res SII nd (monkey 3) showed higher-thnchnce performnce, SII ensemble showed much higher level of discrimintion (73.3%) on verge thn re (%, p <.1). l ensemble firing (on the order of 1 15 ms). Becuse bin clumping did not ffect the tril clssifiction ccomplished by using re 3b nd neurons, these results rise the hypothesis tht ensembles of corticl neurons my chieve the sme computtionl gol (in this cse identifying the stimulus loction) by using different encoding strtegies. Thus, coding of stimulus loction in the SII cortex could use the temporl structure of its neuronl popultion sensory responses, wheres res 3b nd b Number of neurons in ensemble Number of neurons in ensemble Fig.. For ll three corticl res nlyzed (3b, SII nd ), the single-tril discrimintion cpbility vried s function of the ensemble size. Shown here re the effects of sequentil removl of the best predictor neurons from the popultion (neuron dropping) on single-tril discrimintion of the stimulus loction. () Neuron dropping in two SII ensembles (from monkeys nd 3) resulted in similr trends of decy in single-tril discrimintion bility (percent correct). Notice tht when the SII ensembles were reduced to bout five neurons, the performnce ws close to chnce (33%). (b) Neuron dropping in res 3b nd. The curves depict the discrimintion bility s function of neurl ensemble size for these res. Grceful decy in discrimintion performnce lso occurred s result of decresing ensemble size. nture neuroscience volume 1 no 7 november 199 7

8 rticles SII Monkey b SII Monkey 3 c 3ms Bins 5ms Bins 15ms Bins 5ms Bins 3ms Bins 5ms Bins 15ms Bins 5ms Bins Are 3b Monkey Are Monkey 3 d Fig. 9. The effect of bin clumping on the discrimintion cpbility of corticl res. Wheres significnt reduction in discrimintion cpbility ws observed when bin clumping (tht is, incresing the size of the bin describing neuronl firing from 3 to 5 ms) ws pplied to SII neurl ensembles in monkeys () nd 3 (b), no effect ws observed either in re 3b (c, monkey ) or (d, monkey 3) neurl ensembles. These results suggest tht the temporl structure of popultion responses could be more fundmentl for encoding the loction of tctile stimulus in re SII thn in the other two somtosensory corticl res. could encode the sme stimulus loctions by simply using vritions in the men ensemble firing rte. Moreover, our results suggest tht SII contins the requisite informtion for coexistence of both temporl nd rte coding schemes becuse bove-chnce discrimintion ws observed for the SII cortex of ll three monkeys fter the temporl structure of neuronl sensory responses were eliminted by bin clumping. Therefore, residul rte code could still be sufficient for ensembles of SII neurons to compute the loction of tctile stimulus on single tril. As described bove (Fig. 5b), distributions depicting the miniml response ltency for neurons locted in three prietl corticl res showed gret del of overlp t bout 1 to 15 ms fter the stimulus delivery. However, neurons locted in ll corticl res lso exhibited long-ltency responses (15 5 ms). By independently testing the contribution of three consecutive 15 ms bins ( 15, 15 3 nd 3 5 ms) on the discrimintion cpbility of ech corticl re, we observed tht the highest single-tril discrimintion performnce ws obtined when we nlyzed neurl responses in the intervl from to 3 ms in res 3b nd SII nd in the intervl from 15 to 5 ms for re. These results provided further support for the notion tht the mny corticl res tht constitute the primte somtosensory cortex could operte lmost simultneously to compute the loction of given stimulus. Discussion The results described here demonstrte tht the sptiotemporl firing ptterns of neurl ensembles, formed by brodly-tuned neurons nd locted in three corticl res of the primte 3ms Bins 5ms Bins 15ms Bins 5ms Bins 3ms Bins 5ms Bins 15ms Bins 5ms Bins somtosensory cortex, contin enough informtion to specify the loction of punctte tctile stimuli on single tril. This study lso provides the first compelling evidence tht corticl res could crry out such computtions lmost simultneously, but using different representtionl strtegies (tht is, temporl versus rte coding or combintion of both schemes) to chieve the sme gol. In prticulr, the sptiotemporl chrcter of neuronl responses in the SII cortex ws shown to contin the requisite informtion for the encoding of stimulus loction using temporlly ptterned spike sequences, wheres the simultneously recorded neuronl responses in res 3b nd contined the requisite informtion for rte coding. Bsed on our results, we hypothesize tht informtion-processing strtegies my vry cross corticl res of the primte somtosensory system but tht these res my still ct collectively to encode vriety of stimulus ttributes. In support of this possibility, processing of sensory informtion in the primte brin involves interctions between dozens of corticl res, ech of which is interconnected with number of other res 1,. In the lst few yers, this distributed connectivity scheme hs provided the bsis for new theoreticl frmework for corticl function, which deprts from the notion tht ech corticl re is dedicted to one specific tsk nd contins highly specilized, feture-detector neurons. The lterntive notion suggests tht single corticl res, nd the neurons within them, prticipte in vriety of functions nd tht they interct extensively 1. This model of corticl function incorportes the benefits of popultion coding in distributed networks,1 nd implies tht lrge nture neuroscience volume 1 no 7 november 199

9 rticles number of neurons is involved in ny prticulr perceptul or motor tsk 9,11,1. Therefore, the fundmentl focus of corticl computtion shifts from the single neuron towrd interconnected neurl ensembles s originlly proposed by Hebb,9. Therefore we envision tht neurl popultions locted in different corticl res could either shre the sme type of informtion or cooperte to improve their individul performnce. Moreover, the finding tht informtion bout the loction of tctile stimulus could be redily extrcted from ll three corticl res suggests tht these fields my collborte in the definition of unified perceptul experience of the stimulus loction. Corticl res could lso use the informtion bout stimulus loction to produce unique computtions or to relte their outputs to precise representtion of the stimulus loction. In more generl sense, our results rise the possibility tht corticl res, lthough specilized to differing degrees, could process multiple ttributes of sensory stimulus simultneously. Thus, by using combintions of encoding mechnisms (temporl versus rte coding), popultions of neurons could multiplex vrious types of informtion in their firing ptterns 3,31. According to this scheme, single neurons would likely prticipte in multiple coexistent representtions within ech corticl re. Prdoxiclly, however, their individul firing ptterns would not relibly encode ny of the stimulus fetures on single tril. We cknowledge tht, in this model of corticl function, there is no strict requirement for specific binding mechnism, such s synchronous neuronl ctivity 3, lthough such mechnism my be involved in other spects of sensorimotor processing besides identifying stimulus loction. In our model, informtion regrding the tctile stimulus loction would rech ll somtosensory corticl res within reltively smll time intervl, nd consequently, there would be no need to bind this type of informtion together. Methods SURGICAL AND ELECTROPHYSIOLOGICAL PROCEDURES. In ech niml, multiple microelectrode rrys (NBLABS, Dennison, Texs), ech contining 1 teflon-coted, stinless steel microwires (5 µm in dimeter) forming X mtrix (totl smpling re, mm ), were chroniclly implnted in different corticl res under generl nesthesi (15 mg/kg ketmine, 5mg/kg xylzine nd 1% hlothne). During the surgicl procedure for microwire rry implnttion, the receptive fields of single nd multiple units were chrcterized to ensure correct plcement of ech rry. Five to seven dys fter the surgicl procedure, nimls were seted in recording chmber. A 9 chnnel mny neuron cquisition processor (MNAP, Plexon, Dlls, Texs) ws used to cquire nd discriminte single neurl unit ctivity from ech of the implnted microwires. Time-mplitude discrimintors nd modified version 1 of principl component lgorithm 3 were used to isolte single corticl units in rel time. Anlog smples of the wveforms of corticl potentils nd the time of occurrence of ech spike were stored. Off-line nlyses of interspike intervl histogrms, utocorrelogrms nd wveform shpes were used to crossvlidte the on-line discrimintion of single units. Once single units hd been discriminted, punctte tctile stimultion (step pulses with 1 ms durtion delivered t 1Hz) were delivered to different loctions of the hnd, forerm nd fce, using computer-controlled vibromechnicl probe driven by Grss S stimultor. A totl of 3 trils were delivered t ech skin site in rndom sequence. Up to 5 skin sites were stimulted in ech niml. Tctile stimultion ws delivered in both wke nd lightly nesthetized (ketmine i.m.) nimls. The results obtined were virtully identicl in both conditions. SINGLE-TRIAL ANALYSIS OF NEURAL ENSEMBLE DATA. Three pttern-recognition techniques were used to mesure the bility of neurl ensembles locted in three somtosensory corticl res (3b, SII nd re ) to encode stimulus loction on single tril: liner discriminnt nlysis (LDA), lerning vector quntiztion (LVQ) rtificil neurl network (ANN), nd bck-propgtion (BP) ANN. Our procedures for pplying LDA, clssicl multivrite sttisticl technique, hve been described 33. The BP ANN ws chosen becuse it is widely used for nonliner pttern recognition, not only in computer science nd engineering 3 but lso in neuroscience, where it hs been used in vriety of studies 5,35,3. The LVQ ANN ws chosen becuse it provides nonprmetric technique for clssifying lrge nd sprse non-liner pttern vectors 37. This mde the LVQ ANN n ttrctive option for single-tril clssifiction of input ptterns from simultneous multi-neuron recordings. A multiprocessor pentium-bsed worksttion running MATLAB (Mthworks, Boston, Msschusetts) nd its sttistics nd neurl network toolboxes ws used for ll pttern recognition nlysis. We used modified LVQ ANN to pply the Kohonen s optimized-lerning-vector quntiztion lgorithm, which is supervised version of Kohonen s selforgnizing feture mp 37. The network included two hidden units nd one output unit per clss (i.e. stimulus site) to be discriminted. Thus, when dt from three loctions were used, the networks hd hidden units nd 3 output units. The number of trining epochs used in these nlyses were the product of the number of hidden units nd the number of trining trils. The feedforwrd BP ANN ws configured s supervised lerning technique tht produced prmetric outputs in probbilistic rnges. It used two hyperbolic tngent hidden units per clss (skin site) nd one log sigmoid output unit per clss, nd trined using the resilient BP lerning rule. 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