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Corticofugal effects on the activity of thalamic taste cells
258
Braht Research, 193 (1980) 258 2~2 i~) Elsevier/North-Holland Biomedical Press
Corticofugal effects on the activity of thalamic taste cells
TAKASHI YAMAMOTO, RYUJI MATSUO and YOJ1RO KAWAMURA Department of Oral Physiology, Dental School, Osaka University, 4-3-48 Nakanoshima, Kitaku, Osaka 530 (Japan)
(Accepted March 6th, 1980) Key words: taste - - corticofugal feedback - - cortical gustatory area - - thalamus
To examine corticofugal influences on afferent thalamic cell responses in the gustatory system, effects of cortical conditioning stimuli on the responses of thalamic taste cells to peripheral stimulation were examined in the rat. Two types of excitability change of thalamic cells were observed, one is inhibitory (for about 60 msec) (20 ~) and the other is inhibitory (for about 10 msec)-facilitatory (for about 60 msec) (40 ~) to conditioning stimulus applied to the ventral gustatory area, and half of the cells belonging to the latter type also showed a similar pattern with more marked and prolonged excitability changes to conditioning stimulus applied to the dorsal gustatory area. It was suggested that some of the response characteristics of thalamic and cortical taste cells were attributed to the corticofugal feedback loop.
The existence o f corticofugal effects on sensory transmission through subcortical sensory relay stations is well k n o w n and t h o u g h t to be important for sensory discrimination within the somatosensory, visual and auditory afferent systems (for reviews, see refs. 8, 12 and 13). In the gustatory system, although the existence o f corticofugal fibers terminating subcortical structures have been suggested neuroanatomically 9,14, no electrophysiological studies have been reported on the effects of such corticofugal influences on the afferent neural activities. Therefore, the purpose o f this report is to outline the effects of cortical conditioning stimuli on the responses of thalamic taste cells to peripheral stimulation. Seven Wistar male rats ( 4 0 6 4 5 0 g/body weight) were used. Animals were anesthetized by i.p. injection o f a mixture of sodium pentobarbital (25 mg/kg) and urethane (1.0 g/kg), and additional urethane was supplied via a cannula placed in the femoral vein, if required. The trachea was cannulated. The hypoglossal nerves were cut bilaterally to avoid possible tongue movements. Animals were m o u n t e d in a headholder 7. The unilateral chorda tympani which innervates the taste buds in the anterior part o f the tongue was electrically stimulated with a pair o f platinum wire electrodes put to the whole chorda-lingual trunk. Since the chorda tympani, after leaving the tongue, travels in the lingual nerve before branching to eventually join the facial root, to stimulate the c h o r d a tympani the trigeminal c o m p o n e n t o f the lingual nerve was cut central to the branching o f the c h o r d a tympani. A single 0.05-0.1 msec, 4-12 V
259 square pulse was presented at 1.5 sec intervals. Through this operation, care was taken not to damage the chorda tympani and to maintain the connection between its receptors and the brain. To record unitary activities in the thalamic taste areal, TM,the top of the skull was removed to expose the cortex about 6 sq.mm around a point on the midline suture 4.5 mm posterior from the bregma, and a glass micropipette (1-3/~m in tip diameter) filled with 2 M NaC1 containing Fast Green FCF was inserted from the cortical surface. Drainage of cerebrospinal fluid through an opening in the dura over the foramen magnum reduced the brain pulsation and prevented swelling of the cerebral hemisphere. During the recording, each animal was paralyzed with gallamine triethiodide (Flaxedil, 50 mg/kg) and artificially ventilated. Local anesthetics (xylocaine-HC1) were periodically applied to wound margins and pressure points. To mark the electrode tip position where the unitary activity was recorded, an electrical current (5-10 #A) was applied for 5-10 min. Electrode tip marks were located by microscopic examination of serial brain sections. To stimulate the cortical gustatory area2,3A0,14,15,17,19, the overlying temporal muscle and skull were removed to permit free access to cortex adjacent to the rhinal sulcus. The exposed cortical face area, after the underlying dura was resected, was covered with warm liquid paraffin. The conditioning cortical stimulus was a brief square pulse (0.01-0.03 msec duration, 0.1-0.7 mA) applied through bipolar silver electrodes (100 #m in diameter, 1 mm interpolar distance). Our recent studies 17,19 have revealed that there are two separate cortical gustatory areas in the rat; the ventral gustatory area which is a thin band just dorsal to the rhinal sulcus and the dorsal gustatory area which is ovoid in its shape just posterior to the middle cerebral artery. Conditioning stimulus was applied to each of these areas. During the course of experiments, EEG was monitored, and the rectal temperature was kept at about 37 °C. Thalamic taste cells were identified by their responsiveness to electrical stimulation of the chorda tympani and to chemical stimulation (a mixture of 0.1 M NaC1, 0.01 M HC1 and 0.02 M quinine-HC1) of the anterior tongue. The principal criterion for antidromic activation of these thalamic cells by cortical stimulation was consistent all-or-none responses with little variation in latency following repetitive stimulation rates over 100 shocks/sec. Thus, 15 individual thalamic relay cells were obtained which satisfied this criterion. An example of thalamic responses is shown in Fig. 1. This unit was very sensitive to the search chemical stimulus and also responsive (two spikes are evoked in the figure) to a single shock applied to the chorda tympani. When a conditioning stimulus to the cortex was applied 14 msec prior to the test shock, there was a marked increase in the response rate (4 spikes in the figure). The upper graph (Fig. 1A) illustrates the time course of cortically evoked excitability change of this thalamic cell, when the conditioning electrodes were placed on the ventral gustatory area. There was a slight depression of activation (for about 10 msec) followed by a considerable facilitation (for about 60 msec) with subsequent gradual recovery to the control test firing level. Meanwhile, when the conditioning stimulus was applied to the dorsal gustatory area, the time course of excitability change of this same cell showed the similar pattern, but with
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Fig. 1. Time course of cortical conditioning effects on the spike discharge of a thalamic taste cell evoked by a test stimulus applied to the chorda tympani. Cortical conditioning stimuli applied to the ventral gustatory area (A) and the dorsal gustatory area (B) resulted in inhibition, then facilitation at longer conditioning-test (C-T) intervals. The actual recordings show that this cell is very sensitive to the search chemical stimulus (stim.), and the cortical conditioning applied to the ventral gustatory area 14 msec prior to the test stimulus increased the discharge rate (the middle record) over the control level (the leftmost record). Each solid circle in the graphs indicates the mean number of spikes evoked by 5 test stimuli in the presence of the conditioning stimulus at each conditioning-test interval, and a horizontal line with upper and lower dashed lines indicates the mean number of spikes ± S.D. evoked by 20 test stimuli.
more marked and prolonged excitability changes: inhibitory phase for about 30 msec and facilitatory phase for about 150 msec as indicated in the lower graph, Fig. 1B. Out of 15 thalamic cells examined, 6 (40 ~ ) belonged to this inhibition-facilitation type for cortical conditioning applied to the ventral gustatory area, and 3 out o f these 6 cells also showed this type of excitability change when the dorsal gustatory area was conditioned. The duration and the magnitude o f inhibition and facilitation for the remaining cells were within a similar range as shown in the graphs in Fig. 1. On the other hand, 3 cells (20 ~o) out o f 15 showed a considerable depression o f activation which lasted about 60 msec when the ventral gustatory area was conditioned. An example of this type is indicated in Fig. 2. N o n e o f these cells showed any significant excitability change when the dorsal gustatory area was stimulated. Excitability change of the remaining 6 cells (40 ~ ) was not significant within the range o f test firing level ± S.D. when both cortical gustatory areas were stimulated. There are a few anatomical and electrophysiological studies in rats which indicate the existence o f corticofugal connections with ceils in the thalamic taste area. W o l f 14 has suggested by means o f a degeneration method that corticofugal fibers terminate in the dorsal part o f the medial part o f the thalamic ventrobasal complex (thalamic taste area). More recently, Norgren and GrilP, using an autoradiography technique, have
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Fig. 2. Time course of cortical conditioning effects on the spike discharge of a thalamic taste cell evoked by a test stimulus applied to the chorda tympani. Cortical conditioning stimuli applied to the ventral gustatory area resulted in inhibition of the test response for almost 60 msec. Other further descriptions are in the legend for Fig. 1.
demonstrated the efferent distribution from the cortical gustatory area to the thalamic taste area as well as to other central afferent relays for taste and some subcortical structures. G a n c h r o w and Erickson 6 have recorded synaptic activation from the gustatory cortex o f some cells in the thalamic taste area, which might involve thalamic interneurons. A l t h o u g h explanation o f the mechanism o f the present results - - that a cortical stimulus influences the activity o f thalamic taste cells, either antidromically through axon collaterals of thalamic relay cells, or orthodromically by corticofugal fibers and either postsynaptically or presynaptically - - is beyond the scope o f the present study, it is relevant to say that some characteristic response patterns o f thalamic and cortical taste cells which are not observed at the peripheral level can be partly explained by the corticofugal feedback loop. Examples o f such response characteristics observed in thalamic and cortical cells are: (1) a decrease in the average evoked discharge rate11,1s,2°; (2) a tendency toward equalization o f effectiveness of stimuliU,20; (3) an uneven fluctuation o f the frequency of evoked discharge ratell,z0; (4) long-term or short-term inhibitions o f responsesS,11,1a, 20; (5) responses to distilled water for rinsing after stimulus applicationS,11,16,20; and (6) a relatively narrow breadth o f sensitivity to the 4 basic taste stimuliS, 20. This study was supported by Grant-in-Aid 457458 for Scientific Research f r o m the Ministry o f Education o f Japan.
1 Ables, M. F. and Benjamin, R. M., Thalamic relay nucleus for taste in albino rat, J. NeurophysioL, 23 (1960) 376-382. 2 Benjamin, R. M. and Akert, D., Cortical and thalamic areas involved in taste discrimination in the albino rat, J. comp. NeuroL, 111 (1959) 231-260. 3 Benjamin, R. M. and Pfaffman, C., Cortical localization oftastein albino rat, J. Neurophysiol., 18 (1955) 56-64. 4 Emmers, R., Benjamin, R. M. and Blomquist, A. J., Thalamic localization of afferents from the tongue in albino rat, J. comp. NeuroL, 118 (1962) 43-48. 5 Funakoshi, M., Kasahara, Y., Yamamoto, T. and Kawamura, Y., Taste coding and central perception. In D. Schneider (Ed.), Olfaction and Taste, VoL 4, Wissenschaftliche Verlagsgesellschaft, Stuttgart, 1972, pp. 336-342.
262 6 Ganchrow, D. and Erickson, R. P., Thalamocortica[ relations in gustation, Bra#t Research, 36 (1972) 289-305. 7 Hosko, M. J., An improved rat headclamp for neurosurgery or electrode implantation, Physiol. Behav., 8 (1972) 103-104. 8 Monnier, M., Functions of the Nervous System, Vol. 3, Sensory Functions and Perception, Elsevier, Amsterdam, 1975. 9 Norgren, R. and Grill, H. J. Efferent distribution from the cortical gustatory area in rats, Neurosci Abstr., 2 (1976) 124. 10 Norgren, R. and Wolf, G., Projections of thalamic gustatory and lingual areas in the rat, Brain Research, 92 (1975) 123-129. l I Scott, T. R. and Erickson, R. P., Synaptic processing of taste-quality information in thalamus of the rat, J. Neurophysiol., 34 (1971) 868-884. 12 Towe, A. L., Somatosensory cortex: descending influences on ascending systems. In A. lggo (Ed.), Handbook of Sensory Physiology, Vol. 2, Somatosensory System, Springer-Verlag, Berlin, 1973, pp. 701 718. 13 Wiesendanger, M., The pyramidal tract. Recent investigations on its morphology and function, Ergebn. Physiol., 61 (1969) 72-136. 14 Wolf, G., Projections of thalamic and cortical gustatory areas in the rat, J. comp. Neurol., 132 (1968) 519 530. 15 Yamamoto, T. and Kawamura, Y., Summated cerebral responses to taste stimuli in rat, Physiol. Behav., 9 (1972) 789-793. 16 Yamamoto, T. and Kawamura, Y., Cortical responses to electrical and gustatory stimuli in the rabbit, Brain Research, 94 (1975) 447-463. 17 Yamamoto, T. and Kawamura, Y., Physiological characteristics of cortical taste area. In J. Le Magnen and P. MacLeod (Eds.), Olfaction and Taste, Vol. 6, Information Retrieval, London, 1977, pp. 257 264. 18 Yamamoto, T. and Kawamura, Y., Response characteristics of cortical taste cells and chorda tympani fibers in the rabbit, Brain Research, 152 (1978) 586-590. 19 Yamamoto, T., Matsuo, R. and Kawamura, Y., Localization of the cortical gustatory area in rats and its role in taste discrimination, J. Neurophysiol., in press. 20 Yamamoto, T., Yuyama, N. and Kawamura, Y., Central processing of taste perception. In Brain Mechanisms of Sensation, John Wiley, New York, in press.
Braht Research, 193 (1980) 258 2~2 i~) Elsevier/North-Holland Biomedical Press
Corticofugal effects on the activity of thalamic taste cells
TAKASHI YAMAMOTO, RYUJI MATSUO and YOJ1RO KAWAMURA Department of Oral Physiology, Dental School, Osaka University, 4-3-48 Nakanoshima, Kitaku, Osaka 530 (Japan)
(Accepted March 6th, 1980) Key words: taste - - corticofugal feedback - - cortical gustatory area - - thalamus
To examine corticofugal influences on afferent thalamic cell responses in the gustatory system, effects of cortical conditioning stimuli on the responses of thalamic taste cells to peripheral stimulation were examined in the rat. Two types of excitability change of thalamic cells were observed, one is inhibitory (for about 60 msec) (20 ~) and the other is inhibitory (for about 10 msec)-facilitatory (for about 60 msec) (40 ~) to conditioning stimulus applied to the ventral gustatory area, and half of the cells belonging to the latter type also showed a similar pattern with more marked and prolonged excitability changes to conditioning stimulus applied to the dorsal gustatory area. It was suggested that some of the response characteristics of thalamic and cortical taste cells were attributed to the corticofugal feedback loop.
The existence o f corticofugal effects on sensory transmission through subcortical sensory relay stations is well k n o w n and t h o u g h t to be important for sensory discrimination within the somatosensory, visual and auditory afferent systems (for reviews, see refs. 8, 12 and 13). In the gustatory system, although the existence o f corticofugal fibers terminating subcortical structures have been suggested neuroanatomically 9,14, no electrophysiological studies have been reported on the effects of such corticofugal influences on the afferent neural activities. Therefore, the purpose o f this report is to outline the effects of cortical conditioning stimuli on the responses of thalamic taste cells to peripheral stimulation. Seven Wistar male rats ( 4 0 6 4 5 0 g/body weight) were used. Animals were anesthetized by i.p. injection o f a mixture of sodium pentobarbital (25 mg/kg) and urethane (1.0 g/kg), and additional urethane was supplied via a cannula placed in the femoral vein, if required. The trachea was cannulated. The hypoglossal nerves were cut bilaterally to avoid possible tongue movements. Animals were m o u n t e d in a headholder 7. The unilateral chorda tympani which innervates the taste buds in the anterior part o f the tongue was electrically stimulated with a pair o f platinum wire electrodes put to the whole chorda-lingual trunk. Since the chorda tympani, after leaving the tongue, travels in the lingual nerve before branching to eventually join the facial root, to stimulate the c h o r d a tympani the trigeminal c o m p o n e n t o f the lingual nerve was cut central to the branching o f the c h o r d a tympani. A single 0.05-0.1 msec, 4-12 V
259 square pulse was presented at 1.5 sec intervals. Through this operation, care was taken not to damage the chorda tympani and to maintain the connection between its receptors and the brain. To record unitary activities in the thalamic taste areal, TM,the top of the skull was removed to expose the cortex about 6 sq.mm around a point on the midline suture 4.5 mm posterior from the bregma, and a glass micropipette (1-3/~m in tip diameter) filled with 2 M NaC1 containing Fast Green FCF was inserted from the cortical surface. Drainage of cerebrospinal fluid through an opening in the dura over the foramen magnum reduced the brain pulsation and prevented swelling of the cerebral hemisphere. During the recording, each animal was paralyzed with gallamine triethiodide (Flaxedil, 50 mg/kg) and artificially ventilated. Local anesthetics (xylocaine-HC1) were periodically applied to wound margins and pressure points. To mark the electrode tip position where the unitary activity was recorded, an electrical current (5-10 #A) was applied for 5-10 min. Electrode tip marks were located by microscopic examination of serial brain sections. To stimulate the cortical gustatory area2,3A0,14,15,17,19, the overlying temporal muscle and skull were removed to permit free access to cortex adjacent to the rhinal sulcus. The exposed cortical face area, after the underlying dura was resected, was covered with warm liquid paraffin. The conditioning cortical stimulus was a brief square pulse (0.01-0.03 msec duration, 0.1-0.7 mA) applied through bipolar silver electrodes (100 #m in diameter, 1 mm interpolar distance). Our recent studies 17,19 have revealed that there are two separate cortical gustatory areas in the rat; the ventral gustatory area which is a thin band just dorsal to the rhinal sulcus and the dorsal gustatory area which is ovoid in its shape just posterior to the middle cerebral artery. Conditioning stimulus was applied to each of these areas. During the course of experiments, EEG was monitored, and the rectal temperature was kept at about 37 °C. Thalamic taste cells were identified by their responsiveness to electrical stimulation of the chorda tympani and to chemical stimulation (a mixture of 0.1 M NaC1, 0.01 M HC1 and 0.02 M quinine-HC1) of the anterior tongue. The principal criterion for antidromic activation of these thalamic cells by cortical stimulation was consistent all-or-none responses with little variation in latency following repetitive stimulation rates over 100 shocks/sec. Thus, 15 individual thalamic relay cells were obtained which satisfied this criterion. An example of thalamic responses is shown in Fig. 1. This unit was very sensitive to the search chemical stimulus and also responsive (two spikes are evoked in the figure) to a single shock applied to the chorda tympani. When a conditioning stimulus to the cortex was applied 14 msec prior to the test shock, there was a marked increase in the response rate (4 spikes in the figure). The upper graph (Fig. 1A) illustrates the time course of cortically evoked excitability change of this thalamic cell, when the conditioning electrodes were placed on the ventral gustatory area. There was a slight depression of activation (for about 10 msec) followed by a considerable facilitation (for about 60 msec) with subsequent gradual recovery to the control test firing level. Meanwhile, when the conditioning stimulus was applied to the dorsal gustatory area, the time course of excitability change of this same cell showed the similar pattern, but with
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Fig. 1. Time course of cortical conditioning effects on the spike discharge of a thalamic taste cell evoked by a test stimulus applied to the chorda tympani. Cortical conditioning stimuli applied to the ventral gustatory area (A) and the dorsal gustatory area (B) resulted in inhibition, then facilitation at longer conditioning-test (C-T) intervals. The actual recordings show that this cell is very sensitive to the search chemical stimulus (stim.), and the cortical conditioning applied to the ventral gustatory area 14 msec prior to the test stimulus increased the discharge rate (the middle record) over the control level (the leftmost record). Each solid circle in the graphs indicates the mean number of spikes evoked by 5 test stimuli in the presence of the conditioning stimulus at each conditioning-test interval, and a horizontal line with upper and lower dashed lines indicates the mean number of spikes ± S.D. evoked by 20 test stimuli.
more marked and prolonged excitability changes: inhibitory phase for about 30 msec and facilitatory phase for about 150 msec as indicated in the lower graph, Fig. 1B. Out of 15 thalamic cells examined, 6 (40 ~ ) belonged to this inhibition-facilitation type for cortical conditioning applied to the ventral gustatory area, and 3 out o f these 6 cells also showed this type of excitability change when the dorsal gustatory area was conditioned. The duration and the magnitude o f inhibition and facilitation for the remaining cells were within a similar range as shown in the graphs in Fig. 1. On the other hand, 3 cells (20 ~o) out o f 15 showed a considerable depression o f activation which lasted about 60 msec when the ventral gustatory area was conditioned. An example of this type is indicated in Fig. 2. N o n e o f these cells showed any significant excitability change when the dorsal gustatory area was stimulated. Excitability change of the remaining 6 cells (40 ~ ) was not significant within the range o f test firing level ± S.D. when both cortical gustatory areas were stimulated. There are a few anatomical and electrophysiological studies in rats which indicate the existence o f corticofugal connections with ceils in the thalamic taste area. W o l f 14 has suggested by means o f a degeneration method that corticofugal fibers terminate in the dorsal part o f the medial part o f the thalamic ventrobasal complex (thalamic taste area). More recently, Norgren and GrilP, using an autoradiography technique, have
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demonstrated the efferent distribution from the cortical gustatory area to the thalamic taste area as well as to other central afferent relays for taste and some subcortical structures. G a n c h r o w and Erickson 6 have recorded synaptic activation from the gustatory cortex o f some cells in the thalamic taste area, which might involve thalamic interneurons. A l t h o u g h explanation o f the mechanism o f the present results - - that a cortical stimulus influences the activity o f thalamic taste cells, either antidromically through axon collaterals of thalamic relay cells, or orthodromically by corticofugal fibers and either postsynaptically or presynaptically - - is beyond the scope o f the present study, it is relevant to say that some characteristic response patterns o f thalamic and cortical taste cells which are not observed at the peripheral level can be partly explained by the corticofugal feedback loop. Examples o f such response characteristics observed in thalamic and cortical cells are: (1) a decrease in the average evoked discharge rate11,1s,2°; (2) a tendency toward equalization o f effectiveness of stimuliU,20; (3) an uneven fluctuation o f the frequency of evoked discharge ratell,z0; (4) long-term or short-term inhibitions o f responsesS,11,1a, 20; (5) responses to distilled water for rinsing after stimulus applicationS,11,16,20; and (6) a relatively narrow breadth o f sensitivity to the 4 basic taste stimuliS, 20. This study was supported by Grant-in-Aid 457458 for Scientific Research f r o m the Ministry o f Education o f Japan.
1 Ables, M. F. and Benjamin, R. M., Thalamic relay nucleus for taste in albino rat, J. NeurophysioL, 23 (1960) 376-382. 2 Benjamin, R. M. and Akert, D., Cortical and thalamic areas involved in taste discrimination in the albino rat, J. comp. NeuroL, 111 (1959) 231-260. 3 Benjamin, R. M. and Pfaffman, C., Cortical localization oftastein albino rat, J. Neurophysiol., 18 (1955) 56-64. 4 Emmers, R., Benjamin, R. M. and Blomquist, A. J., Thalamic localization of afferents from the tongue in albino rat, J. comp. NeuroL, 118 (1962) 43-48. 5 Funakoshi, M., Kasahara, Y., Yamamoto, T. and Kawamura, Y., Taste coding and central perception. In D. Schneider (Ed.), Olfaction and Taste, VoL 4, Wissenschaftliche Verlagsgesellschaft, Stuttgart, 1972, pp. 336-342.
262 6 Ganchrow, D. and Erickson, R. P., Thalamocortica[ relations in gustation, Bra#t Research, 36 (1972) 289-305. 7 Hosko, M. J., An improved rat headclamp for neurosurgery or electrode implantation, Physiol. Behav., 8 (1972) 103-104. 8 Monnier, M., Functions of the Nervous System, Vol. 3, Sensory Functions and Perception, Elsevier, Amsterdam, 1975. 9 Norgren, R. and Grill, H. J. Efferent distribution from the cortical gustatory area in rats, Neurosci Abstr., 2 (1976) 124. 10 Norgren, R. and Wolf, G., Projections of thalamic gustatory and lingual areas in the rat, Brain Research, 92 (1975) 123-129. l I Scott, T. R. and Erickson, R. P., Synaptic processing of taste-quality information in thalamus of the rat, J. Neurophysiol., 34 (1971) 868-884. 12 Towe, A. L., Somatosensory cortex: descending influences on ascending systems. In A. lggo (Ed.), Handbook of Sensory Physiology, Vol. 2, Somatosensory System, Springer-Verlag, Berlin, 1973, pp. 701 718. 13 Wiesendanger, M., The pyramidal tract. Recent investigations on its morphology and function, Ergebn. Physiol., 61 (1969) 72-136. 14 Wolf, G., Projections of thalamic and cortical gustatory areas in the rat, J. comp. Neurol., 132 (1968) 519 530. 15 Yamamoto, T. and Kawamura, Y., Summated cerebral responses to taste stimuli in rat, Physiol. Behav., 9 (1972) 789-793. 16 Yamamoto, T. and Kawamura, Y., Cortical responses to electrical and gustatory stimuli in the rabbit, Brain Research, 94 (1975) 447-463. 17 Yamamoto, T. and Kawamura, Y., Physiological characteristics of cortical taste area. In J. Le Magnen and P. MacLeod (Eds.), Olfaction and Taste, Vol. 6, Information Retrieval, London, 1977, pp. 257 264. 18 Yamamoto, T. and Kawamura, Y., Response characteristics of cortical taste cells and chorda tympani fibers in the rabbit, Brain Research, 152 (1978) 586-590. 19 Yamamoto, T., Matsuo, R. and Kawamura, Y., Localization of the cortical gustatory area in rats and its role in taste discrimination, J. Neurophysiol., in press. 20 Yamamoto, T., Yuyama, N. and Kawamura, Y., Central processing of taste perception. In Brain Mechanisms of Sensation, John Wiley, New York, in press.