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Cat visual corticopontine cells project to the superior colliculus
Brain Research, 265 (1983) 227-232 Elsevier Biomedical Press
227
Cat Visual Corticopontine Cells Project to the Superior Colliculus JAMES BAKER*, ALAN GIBSON*, GEORGE MOWER**, FARREL ROBINSON* and MITCHELL GLICKSTEIN The Department of Psychology, Brown University, Providence, RI 02912 (U.S.A.) and MRC Unit on Neural Mechanisms of Behavior, 3 Malet Place, London I¥C1 ETJG ( U.K.J
(Accepted September 28th, 1982) Key words: pontine visual area - - superior colliculus - - axon collaterals - - bifurcation - - visual cortex - - antidromic stimulation - -
area 18 - - lateral suprasylvian
Antidromic stimulation was used to study corticopontine visual axons and their tectal collaterals in cats. Sixty-sevencortical cells were activated antidromicaUyby electrical stimulation of the rostral pontine visual area, 38 in area 18, and 29 in lateral suprasylvian cortex. Two thirds of these corticopontine cells (46 cells) could also be antidromicaily activated by stimulation of the superior colliculus, demonstrating that they gave rise to a tectal collateral. INTRODUCTION The visual cortex of cats sends fibers to a region of the rostral pontine nucleia,a,7,10, 29. Neurons in this same area of the pons respond exclusively to visual stimulation 3,14. The pontine visual cells have large receptive fields, are directionally selective over a wide range of stimulus velocities, and respond vigorously to moving spots or a large textured moving target. These response properties are probably derived largely from cells in area 18 and lateral suprasylvian (LS) visual cortex; cells in layer V of these areas have been identified as projecting to the rostral pons by retrograde horseradish peroxidase (HRP) transport2,1s, 19 and by antidromic invasion following electrical stimulation1,11. Corticopontine cells in area 18 and LS are directionally selective, respond to spots and textured stimuli, and their receptive fields, although smaller than pontine receptive fields, are larger than most cortical cell receptive fieldsX,9,11,17, al Corticotectal cells, like corticopontine cells, are
located in cortical layer V. Cells which project to the tectum respond to spots and are less dependent on oriented stimuli 26 than other visual cortex cells15,18. Gilbert and Kelly is showed by the retrograde transport of H R P that corticotectal cells are a distinct class containing all the largest pyramidal cells of layer V. Gibson et al. 1~ found that some large layer V pyramidal cells were labeled after H R P was injected into the visual area of the pons, suggesting that some large layer V cells project to both the tectum and pons. Kawamura and colleagues ls-20 studied the morphology of H R P labeled corticotectal and corticopontine neurons and concluded that the 'fibers originate from different, and independent sets of pyramidal neurons in layer V '19, but they also suggested that some of their results 'may reflect the existence of corticotectal neurons with axon collaterals supplying brain structures other than the superior colliculus'20. Here we report that many corticopontine cell axons bifurcate, with one branch projecting to the superior colliculus and the other to the pontine nuclei.
* Present address: PhysiologyDept., Ward 5, Northwestern University Medical School, 303 E. Chicago Ave., Chicago, IL 6061i, U.S.A. Present address: Dept. of Neurology, Childrens Hospital Medical Center, Boston, MA 02115, U.S.A. **
0006-8993/83/0000-0000/$03.00 © 1983 Elsevier Science Publishers
228 MATERIALS AND METHODS Ten cats were used under Nembutal anesthesia (sodium pentobarbital, 25 mg/kg, i.v., initially, supplemented as necessary). Many aspects of the animal preparation, visual stimulus presentation, electrical stimulation, and single neuron recording procedures have been reported previouslyZ,11,12. The animals were paralyzed with Flaxedil (80 mg IV q- 20 mg/h), ventilated, and end-tidal CO2 was maintained at 4.5 ~ . Body temperature was held at 38 °C. The eye was protected by a clear plastic contact lens and was corrected so that it was in focus on a tangent screen at a distance of 57 cm. A large craniotomy was made over visual cortex and the dura was carefully removed. The exposed cortex was covered with a viscous Vaseline-mineral oil mixture. The visual cortex projection area of pontine nuclei was located with a tungsten microelectrode lowered in a parasagittal plane at an anterior-posterior angle of 45 ° from the vertical. At this angle the microelectrode passed through the cerebellum and into the pons without damaging the visual cortex or superior colliculus. A variety of visual stimuli was used in the search for pontine visual cells 3. A 1.5 m m square array of 4 stimulating electrodes with 0.5 m m long exposed tips was lowered into the pontine visual area at coordinates determined from the microelectrode penetrations 11. A microelectrode was lowered in a frontal plane through the far lateral cortex to the superior colliculus, and small spot stimuli were used to determine the electrode location on a retinotopic map of the colliculus 23. A 1.5 m m array of stimulating electrodes was lowered 0.5 m m into the colliculus centered at about 15° eccentricity in the lower visual hemifield. When the collicular and pontine stimulating arrays were in place, a 1.5 ~ Agar solution at 40 °C was poured over the cortex to reduce pulsations. An electronically switched array of 4 tungsten microelectrodes was used in the search for single corticopontine neurons 11 in the lower field representations of area 18 and LS while stimulating through the pontine electrode array. Stimuli were 0.04 ms pulses delivered at 2 s intervals with a search current of 3-4 mA and a maximum current of 10 mA. Stimuli were delivered between pairs of electrodes in the array.
U p o n locating a driven cell, threshold stimuli were determined using the optimal stimulating electrode pair. Cortical neurons which responded to the pontine stimulus were considered to be antidromically activated when their responses had a sharply defined electrical current threshold, were of constant latency, followed high stimulus repetition rates with one impulse per stimulus, and showed orthodromic-antidromic impulse collision. Latency at threshold and at about 4 times threshold were determined for each corticopontine cell. Collision was tested by triggering the stimulator from a cell's spontaneous impulse after a variable 0-20 ms delay. The maximum delay at which collision occurred was determined for each cell. Antidromically activated cortical cells were tested for responses to superior colliculus stimulation, and cells which were activated antidromically were tested by successively stimulating the pontine nuclei and superior colliculus at varying delays. RESULTS Sixty-seven cortical cells were invaded antidromically by stimulation of the rostral pontine visual area, 38 in area 18, and 29 in LS. Fig. 1A shows antidromic responses in comparison with orthodromic responses, and Fig. 1B shows collision tests. The mean antidromic response latencies for area 18 and LS cells were not significantly different (area 18 mean = 3.02 ms, S.D. = 1.46 ms; LS mean = 3.44 ms, S.D. =- 2.10 ms), and the data were combined in the latency distribution shown in Fig. 2A. Sixty-nine percent of the corticopontine cells were also activated antidromically by stimulation of the superior colliculus. Examples are shown in Fig. 1A and B, and the combined latency distribution is shown in Fig. 2B (46 cells: area 18 mean = 2.16 ms, S.D. ~ 1.38 ms, n = 25; LS mean = 3.52 ms, S.D. = 2.91 ms, n = 21). Cells activated antidromically from the pontine visual area and the superior colliculus must project to both structures, provided that the same axon is not being stimulated in passage. Superior colliculus stimulation frequently evoked long latency ( ~ 5 ms) orthodromic responses from the recorded background activity or the isolated corticopontine cell. One corticopontine cell had a relatively short latency (3.5 ms) orthodromic re-
229
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•r,-,,::.sc Fig. 1. A: superimposed stimulus-triggered oscilloscope sweeps comparing one cell's orthodromic responses to pontine and collicular stimulation (left) with another cell's antidromic responses to pontine and collicular stimulation (right). Antidromic responses are rigidly time locked; orthodromic responses are not. B: single 10 ms sweeps demonstrating collision tests for one bifurcating area 18 corticopontine cell. In the upper 4 traces the sweep was initiated by a visually driven impulse and either the pons or the superior colliculus was stimulated after a delay. The slightly shorter delays in the traces on the right resulted in collision between the orthodromie and antidromic impulses. The bottom 2 traces show the ease where the impulses are electrically stimulated at the pontine branch and the collicular branch. Again, the interstimulus delay on the right is slightly shorter than on the left and results in collision between the electrically stimulated impulses. sponse to superior colliculus stimulation, possibly mediated by corticotectal axon collaterals. Successive stimulation of the two branches of a bifurcating axon will produce collision over a definite delay interval. Forty o f the cells which projected both to the pons and superior colliculus were tested with stimulation of the pontine visual area and superior colliculus in rapid succession with a variable delay between stimuli. An example is shown in Fig. 1B, bottom traces. In every case there was a range of delays over which only one antidromic impulse was recorded at the cell body. At longer delay intervals 2 antidromic impulses were recorded. The minimum delay between stimuli which resulted in 2 impulses reaching the cell body provides a measure of the impulse conduction time between the pontine and superior colliculus stimulating arrays. The delay interval includes the time for antidromic conduction from the site stimulated first to the axon branch point, plus the time in orthodromic conduction from the branch point to the axon terminals at the second site. (Refractory periods are considered
below.) At the same time as orthodromic conduction from the branch point occurs, an antidromic impulse is being conducted toward the cell body, where it is recorded by the cortical microelectrode. When the second stimulus is applied with insufficient delay, the antidromic impulse it generates collides with the impulse traveling orthodromically from the branch point and only one impulse is recorded at the cell body. A single response to two stimuli is always the impulse which reaches the branch point first. The conduction time between pontine and collicular stimulating arrays was always significantly greater than either of the antidromic latencies and less than their sum, which indicates that the branch points do not occur near the cell bodies or the axon terminals, but somewhere between. The mean conduction time between pontine and collicular stimulating arrays was 4.98 ms. Thirty-three of the 40 cells tested with double stimulation were tested for both pons-colliculus conduction time and colliculus-pons conduction time by presenting one or the other stimulus first.
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c u r r e n t for several cells, a n d the results for one c o r t i c o p o n t i n e cell are shown in Fig. 3A. As stimulus c u r r e n t was increased, plateaus a n d step decreases were observed in the p o n t i n e a n t i d r o m i c latency a n d r e f r a c t o r y p e r i o d o f several cells. The steps suggest t h a t s t r o n g e r stimuli invaded larger a x o n b r a n c h e s with s h o r t e r r e f r a c t o r y periods. Fig. 3B shows an e x a m p l e o f a step decrease in latency. The higher t h r e s h o l d o f the larger b r a n c h e s was p r e s u m a b l y a result o f their being at a greater distance f r o m the stimulus site. W h e n fast b r a n c h e s had low thresholds, the slower c o n d u c t i n g p a t h w a y s would n o t have been seen due to collision. The twin pulse d a t a gave estimates o f the a b s o l u t e r e f r a c t o r y p e r i o d s o f pontine a n d colliculus axon b r a n c h e s ; means were 0.68 ms (n = 25, S.D. = 0.16 ms) a n d 0.76 ms (n - - 17, S.D. = 0.22 ms), respectively. A b s o l u t e r e f r a c t o r y period also can be calculated by s u b t r a c t i n g the a n t i d r o m i c latency f r o m the value o f the m i n i m u m delay following a n o r t h o d r o m i c impulse at which a s u p r a t h r e s h o l d stimulus will initiate a n t i d r o m i c c o n d u c t i o n to the cell b o d y 4. The m e a n a b s o l u t e r e f r a c t o r y p e r i o d estimated in this w a y for 13 c o r t i c o p o n t i n e cells was 0.18 ms. Seven o f the cells h a d a tectal axon, a n d the m e a n
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Fig. 2. Latency distributions of corticopontine cell antidromic responses. A: responses to pontine stimulation. B: responses to superior colliculus stimulation. Data from area 18 and LS were combined. The two times should be n e a r l y identical for a given cell, a n y discrepancy resulting f r o m differences in r e f r a c t o r y p e r i o d at the colliculus a n d p o n t i n e axon terminals. The m e a n a b s o l u t e value o f the difference in c o n d u c t i o n time m e a s u r e d for the two directions was 0.33 ms. In a d d i t i o n to the b r a n c h i n g o f c o r t i c o p o n t i n e axons to the s u p e r i o r colliculus, we o b t a i n e d evidence o f b r a n c h e s within the p o n s b y delivering twin stimuli to the p o n t i n e terminals. T h e m i n i m u m twin stimulus s e p a r a t i o n for two impulses to be r e c o r d e d at the s o m a was tested as a f u n c t i o n o f stimulus
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Fig. 3. A: antidromic latency and minimum twin pontine stimulus separation which produced 2 recorded impulses, plotted for one corticopontine cell as a function of stimulus current. The minimum stimulus separation at high current is a measure of the pontine axon's absolute refractory period. B: superimposed oscilloscope sweeps showing a corticopontine cell's responses to pontine stimulation at 1.65 mA and 1.02 mA. The higher current elicited only 6.1 ms antidromic responses, while the lower current elicited a response at 6.1 or 10.9 ms, with no responses of intermediate latency. This result suggests that 2 pontine axon branches were being stimulated, and that 1.02 mA was at threshold for the shorter latency branch.
231 tectal refractory period was 0.26 ms. Such short refractory period estimates suggest that our cortical recordings were from a point delayed with respect to the initial axon segment. The delay may have been introduced by soma invasion time4, 5, but uncertainty about the exact recording site relative to the initial segment questions the use of this method as an estimate of refractory period. DISCUSSION Most corticopontine cells were activated by superior colliculus stimulation. Cells that we were unable to activate may have had no branch to the superior colliculus, or the stimulating electrodes may have been too distant from the branch. The failure, in some cases, to evoke orthodromic responses from background activity near a cell which gave no antidromic response suggests that stimulating electrode placement was sometimes at fault. Thus, our results probably underestimate the proportion of bifurcating corticopontine cells. Some corticopontine cells which did not respond antidromically to tectal stimulation were recorded in the midst of background activity which gave a vigorous response to the tectal stimulus; one such cell was itself orthodromically activated at short latency. These instances argue that tectal electrode placement was good, and that a few corticopontine cells may not have tectal collaterals. Although our data show that REFERENCES 1 Albus, K. and Donate-Oliver, F., Cells of origin of the occipito-pontine projection in the cat: Functional properties and intracortical location, Exp. Brain Res., 28 (1977) 167-174. 2 Albus, K., Donate-Oliver, F., Sanides, D. and Fries, W., The distribution of pontine projection cells in visual and association cortex of the cat: an experimental study with horseradish peroxidase, J. comp. NeuroL, 201 (1981) 175-189. 3 Baker, J., Gibson, A., Glickstein, M. and Stein, J., Visual cells in the pontine nuclei, J. PhysioL (Lond.), 255 (1976) 415-433. 4 Bishop, P. O., Burke, W. and Davis, R., Single-unit recording from antidromically activated optic radiation neurones, J. PhysioL (Lond.), 162 (1962) 432-450. 5 Bishop, P. O., Burke, W. and Davis, R., The interpretation of the extracellular response of single lateral geniculate cells, J. Physiol. (Lond.), 162 (1962) 451-472. 6 Brodal, P., The corticopontine projection from the visual cortex in the cat. I. The total projection and the projec-
most corticopontine cells send a branch to the tectum, apparently many corticotectal cells do not project an axon to the pons; retrograde transport studies show considerably fewer area 18 and LS neurons projecting to the pons than to the tectum. On the basis of differing soma sizes, Kawamura et al. 19 have concluded that the corticopontine and corticotectal pathways arise from different cell populations. However, 68 ~o of their corticotectal cell sizes fall within the range of corticopontine cell sizes (see Fig. 4, ref. 19), and this is consistent with our finding of bifurcated corticop~ntine cells. A quantitative estimate of overlap of the two populations might be obtained through the use of double label anatomical techniques. Why does the cortex send the same visual information to both the superior colliculus and the pons? Physiological properties of corticopontine cells suggest that they are providing information about velocity and direction of visual motion, which would be useful for controlling body movements. The colliculus appears to be mainly concerned with controlling eye and head movements 22,27,2s,30,32-34, and gives rise to a pontocerebellar pathway with different terminations from those of the corticopontine pathwayS,21,24, 25. Distribution o f motion information to other regions ofcerebellar cortex via the pontine branch may allow other body parts, such as the limbs, to be guided by the same visual input. tion from area 17, Brain Research, 39 (1972) 319-335. 7 Brodal, P., The corticopontine projection from the visual cortex in the cat. II. The projection from areas 18 and 19, Brain Research, 39 (1972) 319-335. 8 Brodal, A. and Jansen, J., The ponto-cerebellar projection in the rabbit and cat, J. comp. Neurol., 84 (1946) 31-118. 9 Camarda, R. and Rizzolati, G., Visual receptive fields in the lateral suprasylvian area (Clare-Bishop area) of the cat, Brain Research, 101 (1976) 427-443. l0 Fries, W. and Albus, K., Responses of pontine nuclei to electrical stimulation of the lateral and suprasylvian gyrus in the cat, Brain Research, 188 (1980) 255-260. l l Gibson, A., Baker, J., Mower, G. and Glickstein, M., Corticopontine cells in area 18 of the cat, J. Neurophysiol., 41 (1978) 484-.495. 12 Gibson, A., Mower, G., Baker, J. and Glickstein, M., Cortico-pontine visual cells in the cat, Soc. Neurosci., 0975). 13 Gilbert, C. D. and Kelly, J. P., The projections of cells in different layers of the cat's visual cortex, J'. comp. Neurol., 163 (1975) 81-105.
232 14 Glickstein, M., Stein, J. and King, R. A., Visual input to the pontine nuclei, Science, 178 (1972) 1110-1111. 15 Hubel, D. H. and Wiesel, T. N., Receptive fields binocular interaction and functional architecture in the cats visual cortex, J. Physiol. (Lond.), 160 (1962) 106-154. 16 Hubel, D. H. and Wiesel, T. N., Receptive fields and functional architecture in two non-striate visual areas (18 and 19) of the cat, J. Neurophysiol., 28 (1965) 229-289. 17 Hubel, D. H. and Wiesel, T. N., Visual area of the lateral suprasylvian gyrus (Clare-Bishop area) of the cat, J. Physiol. (Lond.), 202 (1969) 251-260. 18 Kawamura, K. and Chiba, C., Cortical neurons projecting to the pontine nuclei in the cat. An experimental study with the horseradish peroxidase technique, Exp. Brain Res., 35 (1979) 269-285. 19 Kawanmra, K., Konno, T. and Chiba, M., Cells of origin of corticopontine and corticotectal fibers in the medial and lateral banks of the suprasylvian sulcus in the cat. An experimental study with the horseradish method, Neurosci. Lett., 9 (1978) 129-135. 20 Kawamura, K. and Konno, T., Various types of corticotectal neurons of cats as demonstrated by means of retrograde axonal transport of horseradish peroxidase, Exp. Brain Res., 35 (1979) 161-175. 21 Kawamura, K. and Brodal, A., The tectopontine projection in the cat: an experimental anatomical study with comments on pathways for teleceptive impulses to the cerebellum, J. comp. NeuroL, 149 (1973) 371-390. 22 Mcllwain, J. T., Large receptive fields and spatial transformations in the visual system, Int. Rev. Physiol. Neurophysiol., II, 10 (1976) 223-248. 23 Mcllwain, J. T., Visual receptive fields and their images in the superior colliculus of the cat, J. lVeurophysiol., 38 (1975) 219-230. 24 Mower, G., Gibson, A. and Glickstein, M., Tectopontine
pathway in the cat: laminar distribution of cells of origin and visual properties of target cells in dorsolateral pontine nucleus, J. NeurophysioL, 42 (1979) 1-15. 25 Mower, G., Gibson, A., Robinson, F., Stein, J. and Glickstein, M., Visual pontocerebellar projections in the cat, J. Neurophysiol., 43 (t980) 355-365. 26 Palmer, L. A. and Rosenquist, A. C., Visual receptive fields of single striate cortical units projecting to the superior colliculus in the cat, Brain Research, 67 (1974) 27-42. 27 Robinson, D. A., Eye movements evoked by collicular stimulation in the alert monkey, Vision Res., 12 (1972) 1795-1808.
28 Robinson, D. L. and Jarvis, C. D., Superior colliculus neurons studied during head and eye movements of the behaving monkey, J. NeurophysioL, 37 (1974) 533-540. 29 Sanides, D., Fries, W. and Albus, K., The corticopontine projection from the visual cortex of the cat: an autoradiographic investigation, J. comp. NeuroL, 179 (1978) 77-88. 30 Schiller, P. H. and Stryker, M., Single unit recording and stimulation in the superior colliculus of the alert rhesus monkey, J. Neurophysiol., 35 (1972) 915-924. 31 Spear, P. D. and Baumann, T. P., Receptive field characteristics of single neurons in lateral suprasylvian visual area of the cat, J. NeurophysioL, 38 (1975) 1403-1419. 32 Straschill, M. and Rieger, P., Eye movements evoked by focal stimulation of the cat's superior colliculus. Brain Research, 59 (1973) 211-227. 33 Wurtz, R. H. and Goldberg, M. E., Activity of superior colliculus in behaving monkey III. Cells discharging before eye movements, J. Neurophysiol., 35 (1972) 575-586. 34 Wurtz, R. H. and Goldberg, M. E., Activity of superior colliculus in behaving monkey IV. Effects of lesions on eye movements, J. NeurophysioL, 35 (1972) 587-596.
227
Cat Visual Corticopontine Cells Project to the Superior Colliculus JAMES BAKER*, ALAN GIBSON*, GEORGE MOWER**, FARREL ROBINSON* and MITCHELL GLICKSTEIN The Department of Psychology, Brown University, Providence, RI 02912 (U.S.A.) and MRC Unit on Neural Mechanisms of Behavior, 3 Malet Place, London I¥C1 ETJG ( U.K.J
(Accepted September 28th, 1982) Key words: pontine visual area - - superior colliculus - - axon collaterals - - bifurcation - - visual cortex - - antidromic stimulation - -
area 18 - - lateral suprasylvian
Antidromic stimulation was used to study corticopontine visual axons and their tectal collaterals in cats. Sixty-sevencortical cells were activated antidromicaUyby electrical stimulation of the rostral pontine visual area, 38 in area 18, and 29 in lateral suprasylvian cortex. Two thirds of these corticopontine cells (46 cells) could also be antidromicaily activated by stimulation of the superior colliculus, demonstrating that they gave rise to a tectal collateral. INTRODUCTION The visual cortex of cats sends fibers to a region of the rostral pontine nucleia,a,7,10, 29. Neurons in this same area of the pons respond exclusively to visual stimulation 3,14. The pontine visual cells have large receptive fields, are directionally selective over a wide range of stimulus velocities, and respond vigorously to moving spots or a large textured moving target. These response properties are probably derived largely from cells in area 18 and lateral suprasylvian (LS) visual cortex; cells in layer V of these areas have been identified as projecting to the rostral pons by retrograde horseradish peroxidase (HRP) transport2,1s, 19 and by antidromic invasion following electrical stimulation1,11. Corticopontine cells in area 18 and LS are directionally selective, respond to spots and textured stimuli, and their receptive fields, although smaller than pontine receptive fields, are larger than most cortical cell receptive fieldsX,9,11,17, al Corticotectal cells, like corticopontine cells, are
located in cortical layer V. Cells which project to the tectum respond to spots and are less dependent on oriented stimuli 26 than other visual cortex cells15,18. Gilbert and Kelly is showed by the retrograde transport of H R P that corticotectal cells are a distinct class containing all the largest pyramidal cells of layer V. Gibson et al. 1~ found that some large layer V pyramidal cells were labeled after H R P was injected into the visual area of the pons, suggesting that some large layer V cells project to both the tectum and pons. Kawamura and colleagues ls-20 studied the morphology of H R P labeled corticotectal and corticopontine neurons and concluded that the 'fibers originate from different, and independent sets of pyramidal neurons in layer V '19, but they also suggested that some of their results 'may reflect the existence of corticotectal neurons with axon collaterals supplying brain structures other than the superior colliculus'20. Here we report that many corticopontine cell axons bifurcate, with one branch projecting to the superior colliculus and the other to the pontine nuclei.
* Present address: PhysiologyDept., Ward 5, Northwestern University Medical School, 303 E. Chicago Ave., Chicago, IL 6061i, U.S.A. Present address: Dept. of Neurology, Childrens Hospital Medical Center, Boston, MA 02115, U.S.A. **
0006-8993/83/0000-0000/$03.00 © 1983 Elsevier Science Publishers
228 MATERIALS AND METHODS Ten cats were used under Nembutal anesthesia (sodium pentobarbital, 25 mg/kg, i.v., initially, supplemented as necessary). Many aspects of the animal preparation, visual stimulus presentation, electrical stimulation, and single neuron recording procedures have been reported previouslyZ,11,12. The animals were paralyzed with Flaxedil (80 mg IV q- 20 mg/h), ventilated, and end-tidal CO2 was maintained at 4.5 ~ . Body temperature was held at 38 °C. The eye was protected by a clear plastic contact lens and was corrected so that it was in focus on a tangent screen at a distance of 57 cm. A large craniotomy was made over visual cortex and the dura was carefully removed. The exposed cortex was covered with a viscous Vaseline-mineral oil mixture. The visual cortex projection area of pontine nuclei was located with a tungsten microelectrode lowered in a parasagittal plane at an anterior-posterior angle of 45 ° from the vertical. At this angle the microelectrode passed through the cerebellum and into the pons without damaging the visual cortex or superior colliculus. A variety of visual stimuli was used in the search for pontine visual cells 3. A 1.5 m m square array of 4 stimulating electrodes with 0.5 m m long exposed tips was lowered into the pontine visual area at coordinates determined from the microelectrode penetrations 11. A microelectrode was lowered in a frontal plane through the far lateral cortex to the superior colliculus, and small spot stimuli were used to determine the electrode location on a retinotopic map of the colliculus 23. A 1.5 m m array of stimulating electrodes was lowered 0.5 m m into the colliculus centered at about 15° eccentricity in the lower visual hemifield. When the collicular and pontine stimulating arrays were in place, a 1.5 ~ Agar solution at 40 °C was poured over the cortex to reduce pulsations. An electronically switched array of 4 tungsten microelectrodes was used in the search for single corticopontine neurons 11 in the lower field representations of area 18 and LS while stimulating through the pontine electrode array. Stimuli were 0.04 ms pulses delivered at 2 s intervals with a search current of 3-4 mA and a maximum current of 10 mA. Stimuli were delivered between pairs of electrodes in the array.
U p o n locating a driven cell, threshold stimuli were determined using the optimal stimulating electrode pair. Cortical neurons which responded to the pontine stimulus were considered to be antidromically activated when their responses had a sharply defined electrical current threshold, were of constant latency, followed high stimulus repetition rates with one impulse per stimulus, and showed orthodromic-antidromic impulse collision. Latency at threshold and at about 4 times threshold were determined for each corticopontine cell. Collision was tested by triggering the stimulator from a cell's spontaneous impulse after a variable 0-20 ms delay. The maximum delay at which collision occurred was determined for each cell. Antidromically activated cortical cells were tested for responses to superior colliculus stimulation, and cells which were activated antidromically were tested by successively stimulating the pontine nuclei and superior colliculus at varying delays. RESULTS Sixty-seven cortical cells were invaded antidromically by stimulation of the rostral pontine visual area, 38 in area 18, and 29 in LS. Fig. 1A shows antidromic responses in comparison with orthodromic responses, and Fig. 1B shows collision tests. The mean antidromic response latencies for area 18 and LS cells were not significantly different (area 18 mean = 3.02 ms, S.D. = 1.46 ms; LS mean = 3.44 ms, S.D. =- 2.10 ms), and the data were combined in the latency distribution shown in Fig. 2A. Sixty-nine percent of the corticopontine cells were also activated antidromically by stimulation of the superior colliculus. Examples are shown in Fig. 1A and B, and the combined latency distribution is shown in Fig. 2B (46 cells: area 18 mean = 2.16 ms, S.D. ~ 1.38 ms, n = 25; LS mean = 3.52 ms, S.D. = 2.91 ms, n = 21). Cells activated antidromically from the pontine visual area and the superior colliculus must project to both structures, provided that the same axon is not being stimulated in passage. Superior colliculus stimulation frequently evoked long latency ( ~ 5 ms) orthodromic responses from the recorded background activity or the isolated corticopontine cell. One corticopontine cell had a relatively short latency (3.5 ms) orthodromic re-
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•r,-,,::.sc Fig. 1. A: superimposed stimulus-triggered oscilloscope sweeps comparing one cell's orthodromic responses to pontine and collicular stimulation (left) with another cell's antidromic responses to pontine and collicular stimulation (right). Antidromic responses are rigidly time locked; orthodromic responses are not. B: single 10 ms sweeps demonstrating collision tests for one bifurcating area 18 corticopontine cell. In the upper 4 traces the sweep was initiated by a visually driven impulse and either the pons or the superior colliculus was stimulated after a delay. The slightly shorter delays in the traces on the right resulted in collision between the orthodromie and antidromic impulses. The bottom 2 traces show the ease where the impulses are electrically stimulated at the pontine branch and the collicular branch. Again, the interstimulus delay on the right is slightly shorter than on the left and results in collision between the electrically stimulated impulses. sponse to superior colliculus stimulation, possibly mediated by corticotectal axon collaterals. Successive stimulation of the two branches of a bifurcating axon will produce collision over a definite delay interval. Forty o f the cells which projected both to the pons and superior colliculus were tested with stimulation of the pontine visual area and superior colliculus in rapid succession with a variable delay between stimuli. An example is shown in Fig. 1B, bottom traces. In every case there was a range of delays over which only one antidromic impulse was recorded at the cell body. At longer delay intervals 2 antidromic impulses were recorded. The minimum delay between stimuli which resulted in 2 impulses reaching the cell body provides a measure of the impulse conduction time between the pontine and superior colliculus stimulating arrays. The delay interval includes the time for antidromic conduction from the site stimulated first to the axon branch point, plus the time in orthodromic conduction from the branch point to the axon terminals at the second site. (Refractory periods are considered
below.) At the same time as orthodromic conduction from the branch point occurs, an antidromic impulse is being conducted toward the cell body, where it is recorded by the cortical microelectrode. When the second stimulus is applied with insufficient delay, the antidromic impulse it generates collides with the impulse traveling orthodromically from the branch point and only one impulse is recorded at the cell body. A single response to two stimuli is always the impulse which reaches the branch point first. The conduction time between pontine and collicular stimulating arrays was always significantly greater than either of the antidromic latencies and less than their sum, which indicates that the branch points do not occur near the cell bodies or the axon terminals, but somewhere between. The mean conduction time between pontine and collicular stimulating arrays was 4.98 ms. Thirty-three of the 40 cells tested with double stimulation were tested for both pons-colliculus conduction time and colliculus-pons conduction time by presenting one or the other stimulus first.
230
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c u r r e n t for several cells, a n d the results for one c o r t i c o p o n t i n e cell are shown in Fig. 3A. As stimulus c u r r e n t was increased, plateaus a n d step decreases were observed in the p o n t i n e a n t i d r o m i c latency a n d r e f r a c t o r y p e r i o d o f several cells. The steps suggest t h a t s t r o n g e r stimuli invaded larger a x o n b r a n c h e s with s h o r t e r r e f r a c t o r y periods. Fig. 3B shows an e x a m p l e o f a step decrease in latency. The higher t h r e s h o l d o f the larger b r a n c h e s was p r e s u m a b l y a result o f their being at a greater distance f r o m the stimulus site. W h e n fast b r a n c h e s had low thresholds, the slower c o n d u c t i n g p a t h w a y s would n o t have been seen due to collision. The twin pulse d a t a gave estimates o f the a b s o l u t e r e f r a c t o r y p e r i o d s o f pontine a n d colliculus axon b r a n c h e s ; means were 0.68 ms (n = 25, S.D. = 0.16 ms) a n d 0.76 ms (n - - 17, S.D. = 0.22 ms), respectively. A b s o l u t e r e f r a c t o r y period also can be calculated by s u b t r a c t i n g the a n t i d r o m i c latency f r o m the value o f the m i n i m u m delay following a n o r t h o d r o m i c impulse at which a s u p r a t h r e s h o l d stimulus will initiate a n t i d r o m i c c o n d u c t i o n to the cell b o d y 4. The m e a n a b s o l u t e r e f r a c t o r y p e r i o d estimated in this w a y for 13 c o r t i c o p o n t i n e cells was 0.18 ms. Seven o f the cells h a d a tectal axon, a n d the m e a n
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Fig. 2. Latency distributions of corticopontine cell antidromic responses. A: responses to pontine stimulation. B: responses to superior colliculus stimulation. Data from area 18 and LS were combined. The two times should be n e a r l y identical for a given cell, a n y discrepancy resulting f r o m differences in r e f r a c t o r y p e r i o d at the colliculus a n d p o n t i n e axon terminals. The m e a n a b s o l u t e value o f the difference in c o n d u c t i o n time m e a s u r e d for the two directions was 0.33 ms. In a d d i t i o n to the b r a n c h i n g o f c o r t i c o p o n t i n e axons to the s u p e r i o r colliculus, we o b t a i n e d evidence o f b r a n c h e s within the p o n s b y delivering twin stimuli to the p o n t i n e terminals. T h e m i n i m u m twin stimulus s e p a r a t i o n for two impulses to be r e c o r d e d at the s o m a was tested as a f u n c t i o n o f stimulus
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Fig. 3. A: antidromic latency and minimum twin pontine stimulus separation which produced 2 recorded impulses, plotted for one corticopontine cell as a function of stimulus current. The minimum stimulus separation at high current is a measure of the pontine axon's absolute refractory period. B: superimposed oscilloscope sweeps showing a corticopontine cell's responses to pontine stimulation at 1.65 mA and 1.02 mA. The higher current elicited only 6.1 ms antidromic responses, while the lower current elicited a response at 6.1 or 10.9 ms, with no responses of intermediate latency. This result suggests that 2 pontine axon branches were being stimulated, and that 1.02 mA was at threshold for the shorter latency branch.
231 tectal refractory period was 0.26 ms. Such short refractory period estimates suggest that our cortical recordings were from a point delayed with respect to the initial axon segment. The delay may have been introduced by soma invasion time4, 5, but uncertainty about the exact recording site relative to the initial segment questions the use of this method as an estimate of refractory period. DISCUSSION Most corticopontine cells were activated by superior colliculus stimulation. Cells that we were unable to activate may have had no branch to the superior colliculus, or the stimulating electrodes may have been too distant from the branch. The failure, in some cases, to evoke orthodromic responses from background activity near a cell which gave no antidromic response suggests that stimulating electrode placement was sometimes at fault. Thus, our results probably underestimate the proportion of bifurcating corticopontine cells. Some corticopontine cells which did not respond antidromically to tectal stimulation were recorded in the midst of background activity which gave a vigorous response to the tectal stimulus; one such cell was itself orthodromically activated at short latency. These instances argue that tectal electrode placement was good, and that a few corticopontine cells may not have tectal collaterals. Although our data show that REFERENCES 1 Albus, K. and Donate-Oliver, F., Cells of origin of the occipito-pontine projection in the cat: Functional properties and intracortical location, Exp. Brain Res., 28 (1977) 167-174. 2 Albus, K., Donate-Oliver, F., Sanides, D. and Fries, W., The distribution of pontine projection cells in visual and association cortex of the cat: an experimental study with horseradish peroxidase, J. comp. NeuroL, 201 (1981) 175-189. 3 Baker, J., Gibson, A., Glickstein, M. and Stein, J., Visual cells in the pontine nuclei, J. PhysioL (Lond.), 255 (1976) 415-433. 4 Bishop, P. O., Burke, W. and Davis, R., Single-unit recording from antidromically activated optic radiation neurones, J. PhysioL (Lond.), 162 (1962) 432-450. 5 Bishop, P. O., Burke, W. and Davis, R., The interpretation of the extracellular response of single lateral geniculate cells, J. Physiol. (Lond.), 162 (1962) 451-472. 6 Brodal, P., The corticopontine projection from the visual cortex in the cat. I. The total projection and the projec-
most corticopontine cells send a branch to the tectum, apparently many corticotectal cells do not project an axon to the pons; retrograde transport studies show considerably fewer area 18 and LS neurons projecting to the pons than to the tectum. On the basis of differing soma sizes, Kawamura et al. 19 have concluded that the corticopontine and corticotectal pathways arise from different cell populations. However, 68 ~o of their corticotectal cell sizes fall within the range of corticopontine cell sizes (see Fig. 4, ref. 19), and this is consistent with our finding of bifurcated corticop~ntine cells. A quantitative estimate of overlap of the two populations might be obtained through the use of double label anatomical techniques. Why does the cortex send the same visual information to both the superior colliculus and the pons? Physiological properties of corticopontine cells suggest that they are providing information about velocity and direction of visual motion, which would be useful for controlling body movements. The colliculus appears to be mainly concerned with controlling eye and head movements 22,27,2s,30,32-34, and gives rise to a pontocerebellar pathway with different terminations from those of the corticopontine pathwayS,21,24, 25. Distribution o f motion information to other regions ofcerebellar cortex via the pontine branch may allow other body parts, such as the limbs, to be guided by the same visual input. tion from area 17, Brain Research, 39 (1972) 319-335. 7 Brodal, P., The corticopontine projection from the visual cortex in the cat. II. The projection from areas 18 and 19, Brain Research, 39 (1972) 319-335. 8 Brodal, A. and Jansen, J., The ponto-cerebellar projection in the rabbit and cat, J. comp. Neurol., 84 (1946) 31-118. 9 Camarda, R. and Rizzolati, G., Visual receptive fields in the lateral suprasylvian area (Clare-Bishop area) of the cat, Brain Research, 101 (1976) 427-443. l0 Fries, W. and Albus, K., Responses of pontine nuclei to electrical stimulation of the lateral and suprasylvian gyrus in the cat, Brain Research, 188 (1980) 255-260. l l Gibson, A., Baker, J., Mower, G. and Glickstein, M., Corticopontine cells in area 18 of the cat, J. Neurophysiol., 41 (1978) 484-.495. 12 Gibson, A., Mower, G., Baker, J. and Glickstein, M., Cortico-pontine visual cells in the cat, Soc. Neurosci., 0975). 13 Gilbert, C. D. and Kelly, J. P., The projections of cells in different layers of the cat's visual cortex, J'. comp. Neurol., 163 (1975) 81-105.
232 14 Glickstein, M., Stein, J. and King, R. A., Visual input to the pontine nuclei, Science, 178 (1972) 1110-1111. 15 Hubel, D. H. and Wiesel, T. N., Receptive fields binocular interaction and functional architecture in the cats visual cortex, J. Physiol. (Lond.), 160 (1962) 106-154. 16 Hubel, D. H. and Wiesel, T. N., Receptive fields and functional architecture in two non-striate visual areas (18 and 19) of the cat, J. Neurophysiol., 28 (1965) 229-289. 17 Hubel, D. H. and Wiesel, T. N., Visual area of the lateral suprasylvian gyrus (Clare-Bishop area) of the cat, J. Physiol. (Lond.), 202 (1969) 251-260. 18 Kawamura, K. and Chiba, C., Cortical neurons projecting to the pontine nuclei in the cat. An experimental study with the horseradish peroxidase technique, Exp. Brain Res., 35 (1979) 269-285. 19 Kawanmra, K., Konno, T. and Chiba, M., Cells of origin of corticopontine and corticotectal fibers in the medial and lateral banks of the suprasylvian sulcus in the cat. An experimental study with the horseradish method, Neurosci. Lett., 9 (1978) 129-135. 20 Kawamura, K. and Konno, T., Various types of corticotectal neurons of cats as demonstrated by means of retrograde axonal transport of horseradish peroxidase, Exp. Brain Res., 35 (1979) 161-175. 21 Kawamura, K. and Brodal, A., The tectopontine projection in the cat: an experimental anatomical study with comments on pathways for teleceptive impulses to the cerebellum, J. comp. NeuroL, 149 (1973) 371-390. 22 Mcllwain, J. T., Large receptive fields and spatial transformations in the visual system, Int. Rev. Physiol. Neurophysiol., II, 10 (1976) 223-248. 23 Mcllwain, J. T., Visual receptive fields and their images in the superior colliculus of the cat, J. lVeurophysiol., 38 (1975) 219-230. 24 Mower, G., Gibson, A. and Glickstein, M., Tectopontine
pathway in the cat: laminar distribution of cells of origin and visual properties of target cells in dorsolateral pontine nucleus, J. NeurophysioL, 42 (1979) 1-15. 25 Mower, G., Gibson, A., Robinson, F., Stein, J. and Glickstein, M., Visual pontocerebellar projections in the cat, J. Neurophysiol., 43 (t980) 355-365. 26 Palmer, L. A. and Rosenquist, A. C., Visual receptive fields of single striate cortical units projecting to the superior colliculus in the cat, Brain Research, 67 (1974) 27-42. 27 Robinson, D. A., Eye movements evoked by collicular stimulation in the alert monkey, Vision Res., 12 (1972) 1795-1808.
28 Robinson, D. L. and Jarvis, C. D., Superior colliculus neurons studied during head and eye movements of the behaving monkey, J. NeurophysioL, 37 (1974) 533-540. 29 Sanides, D., Fries, W. and Albus, K., The corticopontine projection from the visual cortex of the cat: an autoradiographic investigation, J. comp. NeuroL, 179 (1978) 77-88. 30 Schiller, P. H. and Stryker, M., Single unit recording and stimulation in the superior colliculus of the alert rhesus monkey, J. Neurophysiol., 35 (1972) 915-924. 31 Spear, P. D. and Baumann, T. P., Receptive field characteristics of single neurons in lateral suprasylvian visual area of the cat, J. NeurophysioL, 38 (1975) 1403-1419. 32 Straschill, M. and Rieger, P., Eye movements evoked by focal stimulation of the cat's superior colliculus. Brain Research, 59 (1973) 211-227. 33 Wurtz, R. H. and Goldberg, M. E., Activity of superior colliculus in behaving monkey III. Cells discharging before eye movements, J. Neurophysiol., 35 (1972) 575-586. 34 Wurtz, R. H. and Goldberg, M. E., Activity of superior colliculus in behaving monkey IV. Effects of lesions on eye movements, J. NeurophysioL, 35 (1972) 587-596.