EXPERIMENTAL
Interocular
ANDREW
53, 94-101 (1976)
NEUROLOGY
Equivalence C. LYNN
FRANCIS, Dcpartmelot
after Optic in Goldfish
ORNDWF
of Psychology, Stony Brook,
BENGSTON,
State New
Received May
Nerve
Regeneration
AND M. S. GAZZANIGA~
Usiversity of New York 11794
York,
3. 1976
Goldfish were tested for interocular transfer of a monocularly learned pattern discrimination after optic nerve regeneration. Transfer was comparable to normals for fish trained either on the regenerated side or on the normal side. The results suggest correct connections of the regenerated optic nerve with interhemispheric commissural systems.
INTRODUCTION The regenerative ability of the optic nerve in fish has been well documented (5, 14). After regeneration, fish show a normal learning acquisition on a color discrimination task (1) and a return to normal or near normal visual acuity (15). The map formed on the tectal neuropil by evoked potentials appears normal and retinotopic after optic nerve regeneration (5). There has been no attempt, however, to investigate the higher order visual connections after optic nerve regeneration in fish. Interocular transfer of a visual pattern discrimination can be used to test whether regenerated optic fibers project onto commissural systems in specific fashion, and whether there is equivalence between the normal and the regenerated projections. In the fish, interocular transfer is commissuredependent, because the primary optic fibers are completely crossed (13). Although there is disagreement as to which commissure mediates the transfer in fish (4, 6, 10, 16), transfer would necessarily involve a higher order (presumably tectofugal) projection in addition to the primary optic fibers. There have been some investigations of visual function in commissures of animals capable of optic nerve regeneration. Mark and Davidson (7) recorded visually driven units in the tectal commissure of the cichlid Astronotus ocellatus, but they did not investigate the regenerated case. In 1 Aided by USPHS
Grant No. MH25643-02Al 94
Copyright All rights
0 1976 by Academic Press,Inc.
of reproduction
in any
form
reserved.
awarded
to M. S. Gazzaniga.
OPTIC
KERVE
95
REGENERATION
frogs, the postoptic commissures mediate a multisynaptic ipsilateral evoked potential that is present and normal after regeneration of the optic nerve (3). The proposed intertectal pathway in the frog includes the primary optic projection to the tectum, an undetermined synapse or synapses in the tectum, and a tectal effere:rt pathway which crosses in the postoptic commissures. Similar pathways are seen histologically in fish (11)) but they have not been studied in detail. The present study utilizes interocular transfer of pattern discriminations as a test for the presence of intact higher order visual connections after optic nerve regeneration in goldfish. If transfer occurs in both directions (between the normal side and the regenerated side) then there is evidence for specificity of higher order connections after regeneration. METHODS Subjccfs. Sixteen goldfish (Curassi~s aerate) measuring 8 to 10 cm head to base of tail were used: eight normals, four with left optic regeneration, and four with right optic regeneration. The fish were housed in 4.5liter tanks before and after surgery and were fed Tetramin daily. During testing, the fish were kept in a 2.50-liter tank, where they received no food except that used in training. All tanks were maintained with constant aeration and filtration. Slhrgevy. The fish were anesthetized by immersion in dilute Tricaine (Finquel, Ayerst). Surgery was performed out of the water under a Zeiss operating microscope. A dorsolateral approach was used to transect the optic nerve in the orbit. The nerve was exposed and repeatedly pinched with a fine forceps, leaving a portion of the connective sheath intact. After recovery, the fish were periodically tested for return of vision by their
FIG.
1. Double
T-maze
used
to train
goldfish.
96
FRANCIS,
RENGSTON
AND
GAZZANIGA
response to food baiting. When their vision had returned, they were moved to the training tank. Apparatns. The fish were trained in a Plexiglas double T-maze (Fig. 1) which was installed just under the surface of the training tank. The four goal chambers each measured 20 X 23 cm, the runway connecting them was 60 x 2.5 cm, and the depth of the maze was 11.5 cm. White lights which could be individually activated were mounted on the far corner of each chamber just above the water line. Plexiglas stimulus cards (20 x 10 cm) could be affixed to the far wall of each goal chamber. The distance from the choice point at the end of the runway to the centers of the stimulus cards was 23 cm. The stripe stimuli consisted of vertical or horizontal black and white enamel 1.2-cm stripes. The cross stimulus (shown in Fig. 1) was a painted black cross (lo-cm length) centered on a white field. Its matching stimulus consisted of nine black squares (2 cm) arrayed in a 3 x 3 matrix with 2-cm spacing. Each stimulus pair was equated for brightness. The lenses for monocular blinding were molded from Kodak photographic film and painted with black enamel. They were inserted immediately before the daily training sessions and removed afterward. The food reward was a smalI bolus of a mixture of Gerber’s beef baby food and Tetramin, delivered manually at the end of a fine wire. Pretraining. Binocular training on a brightness problem (for all fish light was S+ ) was initiated when the fish would eat readily in the maze. Uncontrolled random orders of the position of Sf were used in pretraining. Twelve trials alternating from one end of the maze to the other were given each day until a criterion of ten out of 12 on one day was met. At the beginning of the next day’s trials the lens was inserted and training continued daily until a criterion of ten out of 12 was again met. No lens laterality interactions were noted in pilot work; therefore all lenses were placed on the right eye. Pattern Discrinzinution. The procedure remained the same except that the light cue was now paired with the positive pattern and 14 trials a day were given. After several days, the lights were discontinued, and training continued on the monocular pattern discrimination until a criterion of 12 out of 14 was met for four consecutive days. Four different Gellerman orders were alternated in daily discrimination training after the light cues were removed, with each order giving seven trials with S+ on the right, seven on the left, and a maximum of three trials running on the same side. The fish were assigned to patterns randomly, such that for each positive stimulus there were two normal subjects, one with optic regeneration trained on the normal side, and one with optic regeneration trained on the regenerated side.
OPTIC
NERVE
REGENERATION
97
Transfer Tests. Transfer tests were begun the first day following the final criterion day. To diminish the possibility of the fish failing to transfer due to unaccustomed use of the untrained eye alone in negotiating the maze, before the transfer trials a free-run period with the stimulus cards absent and the lens in the left eye was given (90 min first 2 days, 60 min third day, 45 min fourth day). Fourteen trials were given each of 4 days to test for interocular transfer of the discrimination. Correct performances were rewarded. The same daily sequences of S+ positions were used, except that the first transfer always began with the same one of these: RLLRLLRRLLRRLR. Histology. The brains and optic tracts of the fish with optic tract regeneration were fixed in alcoholic Bouin’s solution, embedded in paraffin, sectioned at 10 ,um, and stained with protargol. RESULTS One normal fish and one with optic nerve regeneration were discarded for failure to learn either the light or pattern discrimination. The results are based on the remaining 14 fish, Trials to criterion and learning curves were determined from the first day of discrimination training using only the stimulus cards. The results showing grouped median number correct for the first 4 days of training, final 4 days of training (criterion), and the four transfer days are shown in Fig. 2, for the normals and the two groups with regeneration. There were no significant differences in trials to complete criterion (Mann-Whitney U, P = 0.10).
FIG. 2. Grouped median correct scores for normal fish and fish with regenerated optic tracts. In the right panel, open circles represent data from fish trained on the regenerated side and closed circles fish trained on the normal side.
98
FRANCIS,
RENGSTON
AND
GAZZANIGA
FIG. 3. Photomicrographs of the regenerated optic nerves of a representative fish. Left-high power view of optic fiers on the normal side. Right-high power view of optic fibers in the region of transection. Protargol stain, lo-pm sections.
Transfer was assessed by comparing criterion performance to transfer differences scores and initial training scores. There were no significant between the groups of normal fish and of those with regeneration on transfer scores (Mann-Whitney U, P = 0.37) and the transfer scores for both groups were different from their corresponding initial training scores
OPTIC
NERVE
Frc.
REGENERATION
99
3 (continued)
(Wilcoxon signed-ranks, P = 0.05). As shown in Fig. 2, transfer was comparable for both groups with regeneration. Although the scores for all groups increased over the four transfer days, the initial transfer scores tended to be higher than the first 4 days’ training scores. Histological examination of the optic tracts of the seven operated fish showed in all cases the characteristic scar formation with its tangle of regenerated fibers at the site of transection (Fig, 3).
100
FRANCIS,
BENGSTON
AND
GAZZANIGA
DISCUSSION The present data confirm that a visual pattern discrimination can be learned or performed by fish with regenerated optic nerves (2, 15) and indicate that there is sufficient specificity of higher order connections after regeneration of the optic nerve to allow successful interhemispheric transfer of pattern discriminations. Prior studies have reported poor interocular transfer in goldfish on some tasks (9, 12). However, in the double T-maze with appetitive training, our results show reasonable interocular transfer in both normal and regenerated fish. Although it is agreed that interocular transfer in fish requires the use of commissures, there is by no means agreement as to which commissures are necessary. Mark and his co-workers (6, 7), using the cichlid Astronotus ocellatus, report lack of interocular transfer after transection of the tectal commissure. Ingle and Campbell’s data with goldfish (4) in a go-no go avoidance situation indicate that the postoptic commissures mediate transfer of visual discriminations. A recent report using an autonomic conditioning paradigm with goldfish supports this finding (16). Whichever commissure mediates the transfer of the pattern discrimination in our double T-maze, the transfer would presumably involve a tectofugal system to which the incoming optic fibers must relate in some orderly array. It is not known how many optic fibers are necessary to support a pattern discrimination in goldfish, nor is it known how many commissural fibers are necessary to mediate interocular transfer. Our results indicate that at least some portion of the regenerated fibers connect or are relayed specifically enough to allow for successful interhemispheric transfer of a pattern discrimination. Since transfer occurs equally well in either direction after a single optic nerve regeneration, it appears that the relation between the primary optic fibers and higher tectal efferents is restored after regeneration. REFERENCES 1. ARORA, H. L., and R. W. SPERRY. 1963. Color discrimination after optic nerve regeneration in the fish Astrortotus ocrllatus. Dcvel. Biol. I: 234-243. 2. CRONLY-DILLON, J. R., N. S. SUTKERLAND, and J. WOLFE. 1966. Intraretinal transfer of a learned visual shape discrimination in goldfish after section and regeneration of the optic nerve brachia. Erp. Newal. 1s: 455-462. 3. GAZE, R. M., and M. J. KEATING. 1970. The restoration of the ipsilateral visual projection following regeneration of the optic nerve in the frog. Brain Res. 21: 207-216. 4. INGLE, D., and A. CAMPBELL. 1976. Interocular transfer of visual discriminations in goldfish following selective commissure lesions. J. Conzp. Physiol. Psychol. (submitted).
OPTIC
7. 8. 9. 10. 11. 12. 13. 14. 15. 16.
REGENERATION
101
M., and R. M. GAZE. 1965. Selection of appropriate tectal connections by regenerating optic nerve fibers in adult goldfish. Exp. Neural. 13: 418-430. MARK, R. F. 1966. The tectal commissure and interocular transfer of pattern discriminations in cichlid fish. Exp. Newrol. 16: 4.26-463. MARK, R. F., and T. M. DAVIDSON. 1966. Unit responses from commissural fibers of optic lobes of fish. Science 152: 797-799. MARK, R. F., 0. PEER, and J. STEINER. 1973. Integrative functions in the midbrain commissures in fish. Exp. Ncwol. 39: 140-156. MCCLEARY, R. A. 1960. Type of response as a factor in interocular transfer in the fish. J. Camp. Physiol. Psychol. 53: 311-321. SAVAGE, G. E. 1969. Telencephalic lesions and avoidance behavior in the goldfish (Carussius albratzts) . A&l. Behov. 17 : 362-373. SCHNITZLEIN, H. N. 1962. The habenula and dorsal thalamus of some teleosts. J. Conzp. Nenrol. 118: 225-267. SHAPIRO, S. M. 1965. Interoxlar transfer of pattern discrimination in the goldfish. .41rzcr. J. Fsychol. 78: 21-38. SRARMA, S. C. 1972. The retinal projections in the goldfish : An experimental study. Braill Rcs. 39: 213-223. SPERRY, R. W. 1948. Patterning of central synapses in regeneration of the optic nerve in teleosts. Pltysiol. 2001. 21: 351-361. WEILER, I. J. 1966. Restoration of visual acuity after optic nerve section and regeneration in dstrorlotzu ocellatus. Exp. Neural. 15 : 377-386. YEO, C. H. 1975. Pathways of interocular transfer in the goldfish. Paper presented at the First European Neurosciences Meeting, Munich, 1975. Abstract in Exp. Brairt Rrs. 23 (Suppl.) : 223.
5. JACOBSON,
6.
NERVE
Interocular
ANDREW
53, 94-101 (1976)
NEUROLOGY
Equivalence C. LYNN
FRANCIS, Dcpartmelot
after Optic in Goldfish
ORNDWF
of Psychology, Stony Brook,
BENGSTON,
State New
Received May
Nerve
Regeneration
AND M. S. GAZZANIGA~
Usiversity of New York 11794
York,
3. 1976
Goldfish were tested for interocular transfer of a monocularly learned pattern discrimination after optic nerve regeneration. Transfer was comparable to normals for fish trained either on the regenerated side or on the normal side. The results suggest correct connections of the regenerated optic nerve with interhemispheric commissural systems.
INTRODUCTION The regenerative ability of the optic nerve in fish has been well documented (5, 14). After regeneration, fish show a normal learning acquisition on a color discrimination task (1) and a return to normal or near normal visual acuity (15). The map formed on the tectal neuropil by evoked potentials appears normal and retinotopic after optic nerve regeneration (5). There has been no attempt, however, to investigate the higher order visual connections after optic nerve regeneration in fish. Interocular transfer of a visual pattern discrimination can be used to test whether regenerated optic fibers project onto commissural systems in specific fashion, and whether there is equivalence between the normal and the regenerated projections. In the fish, interocular transfer is commissuredependent, because the primary optic fibers are completely crossed (13). Although there is disagreement as to which commissure mediates the transfer in fish (4, 6, 10, 16), transfer would necessarily involve a higher order (presumably tectofugal) projection in addition to the primary optic fibers. There have been some investigations of visual function in commissures of animals capable of optic nerve regeneration. Mark and Davidson (7) recorded visually driven units in the tectal commissure of the cichlid Astronotus ocellatus, but they did not investigate the regenerated case. In 1 Aided by USPHS
Grant No. MH25643-02Al 94
Copyright All rights
0 1976 by Academic Press,Inc.
of reproduction
in any
form
reserved.
awarded
to M. S. Gazzaniga.
OPTIC
KERVE
95
REGENERATION
frogs, the postoptic commissures mediate a multisynaptic ipsilateral evoked potential that is present and normal after regeneration of the optic nerve (3). The proposed intertectal pathway in the frog includes the primary optic projection to the tectum, an undetermined synapse or synapses in the tectum, and a tectal effere:rt pathway which crosses in the postoptic commissures. Similar pathways are seen histologically in fish (11)) but they have not been studied in detail. The present study utilizes interocular transfer of pattern discriminations as a test for the presence of intact higher order visual connections after optic nerve regeneration in goldfish. If transfer occurs in both directions (between the normal side and the regenerated side) then there is evidence for specificity of higher order connections after regeneration. METHODS Subjccfs. Sixteen goldfish (Curassi~s aerate) measuring 8 to 10 cm head to base of tail were used: eight normals, four with left optic regeneration, and four with right optic regeneration. The fish were housed in 4.5liter tanks before and after surgery and were fed Tetramin daily. During testing, the fish were kept in a 2.50-liter tank, where they received no food except that used in training. All tanks were maintained with constant aeration and filtration. Slhrgevy. The fish were anesthetized by immersion in dilute Tricaine (Finquel, Ayerst). Surgery was performed out of the water under a Zeiss operating microscope. A dorsolateral approach was used to transect the optic nerve in the orbit. The nerve was exposed and repeatedly pinched with a fine forceps, leaving a portion of the connective sheath intact. After recovery, the fish were periodically tested for return of vision by their
FIG.
1. Double
T-maze
used
to train
goldfish.
96
FRANCIS,
RENGSTON
AND
GAZZANIGA
response to food baiting. When their vision had returned, they were moved to the training tank. Apparatns. The fish were trained in a Plexiglas double T-maze (Fig. 1) which was installed just under the surface of the training tank. The four goal chambers each measured 20 X 23 cm, the runway connecting them was 60 x 2.5 cm, and the depth of the maze was 11.5 cm. White lights which could be individually activated were mounted on the far corner of each chamber just above the water line. Plexiglas stimulus cards (20 x 10 cm) could be affixed to the far wall of each goal chamber. The distance from the choice point at the end of the runway to the centers of the stimulus cards was 23 cm. The stripe stimuli consisted of vertical or horizontal black and white enamel 1.2-cm stripes. The cross stimulus (shown in Fig. 1) was a painted black cross (lo-cm length) centered on a white field. Its matching stimulus consisted of nine black squares (2 cm) arrayed in a 3 x 3 matrix with 2-cm spacing. Each stimulus pair was equated for brightness. The lenses for monocular blinding were molded from Kodak photographic film and painted with black enamel. They were inserted immediately before the daily training sessions and removed afterward. The food reward was a smalI bolus of a mixture of Gerber’s beef baby food and Tetramin, delivered manually at the end of a fine wire. Pretraining. Binocular training on a brightness problem (for all fish light was S+ ) was initiated when the fish would eat readily in the maze. Uncontrolled random orders of the position of Sf were used in pretraining. Twelve trials alternating from one end of the maze to the other were given each day until a criterion of ten out of 12 on one day was met. At the beginning of the next day’s trials the lens was inserted and training continued daily until a criterion of ten out of 12 was again met. No lens laterality interactions were noted in pilot work; therefore all lenses were placed on the right eye. Pattern Discrinzinution. The procedure remained the same except that the light cue was now paired with the positive pattern and 14 trials a day were given. After several days, the lights were discontinued, and training continued on the monocular pattern discrimination until a criterion of 12 out of 14 was met for four consecutive days. Four different Gellerman orders were alternated in daily discrimination training after the light cues were removed, with each order giving seven trials with S+ on the right, seven on the left, and a maximum of three trials running on the same side. The fish were assigned to patterns randomly, such that for each positive stimulus there were two normal subjects, one with optic regeneration trained on the normal side, and one with optic regeneration trained on the regenerated side.
OPTIC
NERVE
REGENERATION
97
Transfer Tests. Transfer tests were begun the first day following the final criterion day. To diminish the possibility of the fish failing to transfer due to unaccustomed use of the untrained eye alone in negotiating the maze, before the transfer trials a free-run period with the stimulus cards absent and the lens in the left eye was given (90 min first 2 days, 60 min third day, 45 min fourth day). Fourteen trials were given each of 4 days to test for interocular transfer of the discrimination. Correct performances were rewarded. The same daily sequences of S+ positions were used, except that the first transfer always began with the same one of these: RLLRLLRRLLRRLR. Histology. The brains and optic tracts of the fish with optic tract regeneration were fixed in alcoholic Bouin’s solution, embedded in paraffin, sectioned at 10 ,um, and stained with protargol. RESULTS One normal fish and one with optic nerve regeneration were discarded for failure to learn either the light or pattern discrimination. The results are based on the remaining 14 fish, Trials to criterion and learning curves were determined from the first day of discrimination training using only the stimulus cards. The results showing grouped median number correct for the first 4 days of training, final 4 days of training (criterion), and the four transfer days are shown in Fig. 2, for the normals and the two groups with regeneration. There were no significant differences in trials to complete criterion (Mann-Whitney U, P = 0.10).
FIG. 2. Grouped median correct scores for normal fish and fish with regenerated optic tracts. In the right panel, open circles represent data from fish trained on the regenerated side and closed circles fish trained on the normal side.
98
FRANCIS,
RENGSTON
AND
GAZZANIGA
FIG. 3. Photomicrographs of the regenerated optic nerves of a representative fish. Left-high power view of optic fiers on the normal side. Right-high power view of optic fibers in the region of transection. Protargol stain, lo-pm sections.
Transfer was assessed by comparing criterion performance to transfer differences scores and initial training scores. There were no significant between the groups of normal fish and of those with regeneration on transfer scores (Mann-Whitney U, P = 0.37) and the transfer scores for both groups were different from their corresponding initial training scores
OPTIC
NERVE
Frc.
REGENERATION
99
3 (continued)
(Wilcoxon signed-ranks, P = 0.05). As shown in Fig. 2, transfer was comparable for both groups with regeneration. Although the scores for all groups increased over the four transfer days, the initial transfer scores tended to be higher than the first 4 days’ training scores. Histological examination of the optic tracts of the seven operated fish showed in all cases the characteristic scar formation with its tangle of regenerated fibers at the site of transection (Fig, 3).
100
FRANCIS,
BENGSTON
AND
GAZZANIGA
DISCUSSION The present data confirm that a visual pattern discrimination can be learned or performed by fish with regenerated optic nerves (2, 15) and indicate that there is sufficient specificity of higher order connections after regeneration of the optic nerve to allow successful interhemispheric transfer of pattern discriminations. Prior studies have reported poor interocular transfer in goldfish on some tasks (9, 12). However, in the double T-maze with appetitive training, our results show reasonable interocular transfer in both normal and regenerated fish. Although it is agreed that interocular transfer in fish requires the use of commissures, there is by no means agreement as to which commissures are necessary. Mark and his co-workers (6, 7), using the cichlid Astronotus ocellatus, report lack of interocular transfer after transection of the tectal commissure. Ingle and Campbell’s data with goldfish (4) in a go-no go avoidance situation indicate that the postoptic commissures mediate transfer of visual discriminations. A recent report using an autonomic conditioning paradigm with goldfish supports this finding (16). Whichever commissure mediates the transfer of the pattern discrimination in our double T-maze, the transfer would presumably involve a tectofugal system to which the incoming optic fibers must relate in some orderly array. It is not known how many optic fibers are necessary to support a pattern discrimination in goldfish, nor is it known how many commissural fibers are necessary to mediate interocular transfer. Our results indicate that at least some portion of the regenerated fibers connect or are relayed specifically enough to allow for successful interhemispheric transfer of a pattern discrimination. Since transfer occurs equally well in either direction after a single optic nerve regeneration, it appears that the relation between the primary optic fibers and higher tectal efferents is restored after regeneration. REFERENCES 1. ARORA, H. L., and R. W. SPERRY. 1963. Color discrimination after optic nerve regeneration in the fish Astrortotus ocrllatus. Dcvel. Biol. I: 234-243. 2. CRONLY-DILLON, J. R., N. S. SUTKERLAND, and J. WOLFE. 1966. Intraretinal transfer of a learned visual shape discrimination in goldfish after section and regeneration of the optic nerve brachia. Erp. Newal. 1s: 455-462. 3. GAZE, R. M., and M. J. KEATING. 1970. The restoration of the ipsilateral visual projection following regeneration of the optic nerve in the frog. Brain Res. 21: 207-216. 4. INGLE, D., and A. CAMPBELL. 1976. Interocular transfer of visual discriminations in goldfish following selective commissure lesions. J. Conzp. Physiol. Psychol. (submitted).
OPTIC
7. 8. 9. 10. 11. 12. 13. 14. 15. 16.
REGENERATION
101
M., and R. M. GAZE. 1965. Selection of appropriate tectal connections by regenerating optic nerve fibers in adult goldfish. Exp. Neural. 13: 418-430. MARK, R. F. 1966. The tectal commissure and interocular transfer of pattern discriminations in cichlid fish. Exp. Newrol. 16: 4.26-463. MARK, R. F., and T. M. DAVIDSON. 1966. Unit responses from commissural fibers of optic lobes of fish. Science 152: 797-799. MARK, R. F., 0. PEER, and J. STEINER. 1973. Integrative functions in the midbrain commissures in fish. Exp. Ncwol. 39: 140-156. MCCLEARY, R. A. 1960. Type of response as a factor in interocular transfer in the fish. J. Camp. Physiol. Psychol. 53: 311-321. SAVAGE, G. E. 1969. Telencephalic lesions and avoidance behavior in the goldfish (Carussius albratzts) . A&l. Behov. 17 : 362-373. SCHNITZLEIN, H. N. 1962. The habenula and dorsal thalamus of some teleosts. J. Conzp. Nenrol. 118: 225-267. SHAPIRO, S. M. 1965. Interoxlar transfer of pattern discrimination in the goldfish. .41rzcr. J. Fsychol. 78: 21-38. SRARMA, S. C. 1972. The retinal projections in the goldfish : An experimental study. Braill Rcs. 39: 213-223. SPERRY, R. W. 1948. Patterning of central synapses in regeneration of the optic nerve in teleosts. Pltysiol. 2001. 21: 351-361. WEILER, I. J. 1966. Restoration of visual acuity after optic nerve section and regeneration in dstrorlotzu ocellatus. Exp. Neural. 15 : 377-386. YEO, C. H. 1975. Pathways of interocular transfer in the goldfish. Paper presented at the First European Neurosciences Meeting, Munich, 1975. Abstract in Exp. Brairt Rrs. 23 (Suppl.) : 223.
5. JACOBSON,
6.
NERVE
