Neuropsychologia,1976, Vol. 14, pp. 247 to 250. PergamonPress.Printedin England.
NOTE DICHOTIC LISTENING PATTERN IN RELATION INTERHEMISPHERIC DISCONNEXION HANNA DAMASIO,* ANTONIO R. DAMASIO,* A. CASTRO-CALDAS and
J. M.
TO FERRO
Language Research Laboratory, Centro de Estudos Egas Moniz, Lisbon Faculty of Medicine, Lisbon, Portugal (Received
14 June 1975)
Abstract-A distinctive pattern of dichotic listening abnormality has been described in a small number of callosotomised patients and it has been suggested that the abnormality relates to the severing of pathways connecting both auditory cortices. The present paper supports those findings by presenting two cases of non-surgical disconnexion of interauditory pathways in which the pattern of disability conforms to the expectation raised by the previous reports.
INTRODUCTION THE ROLE of
the interhemispheric connections in the performance of dichotic listening tasks has been suggested by studies of split-brain patients in whom a distinctive pattern has been found. MILNER et al. [l] examined a group of 7 such patients with complete section of the callosum and established that there was a consistent “lesion effect” on the left channel in the sense that only an extremely small amount of information from the left ear was processed. SPARKS and GESCHWIND [2] obtained the same result after extensive investigation of a single callosotomised patient. The explanation advanced by both groups of authors was that: (1) the primary verbal input channel from the left ear to the left hemisphere, crosses first to the right temporal lobe and then travels from there to the left via the callosum (the secondary channel is ipsilateral and goes directly to the left auditory cortex); (2) the primary verbal input channel from the right ear also crosses and travels directly to the left temporal lobe; (3) this latter pathway inhibits the secondary ipsilateral channel from the left ear. Hence the callosal sectioning of pathways connecting the opposite auditory cortices prevents the left ear from using its stronger channel and makes it rely on the ipsilateral pathway which, in a competitive simultaneous process, is blocked by the more powerful channel from the right ear. If this explanation is correct, any case in which the auditory interhemispheric pathways are affected should exhibit the finding regardless of the point where the section occurs. The following cases exemplify such a situation. The first is a case of deep right hemisphere neoplasm sparing the temporal cortices and the callosum. The second is a syndrome of the left posterior cerebral artery a lesion of the caudal third of the callosum. Case I
J.A.F. was a right-handed 45 yr old illiterate farmer. Headache and gradual loss of vision brought the patient to the Neurological Unit in December 1973. At the time of his first observation he complained of intense headache and vomited frequently. He was alert and cooperative but in obvious distress. There was marked papilledema exudates and hemorrhages in both eyes and a left sixth cranial nerve palsy. Visual fields were normal as were the light reflexes. Tone was diminished on the left side and power was reduced in the left leg and the left arm. Deep reflexes were consistently brisker on the left. Plantar responses were extensor on both sides. Speech was fluent, with normal articulation, melody and syntax. Visual confrontation naming and verbal auditory comprehension were normal. There were no defects of body gnosis. Rightleft orientation and calculation were unimpaired. Constructional ability was much disturbed for both 2 and 3 dimensional tasks. Stereognosis, barognosis and two-point discrimination were impaired on the left hand and there was a left-side extinction on simultaneous tactile stimulation. There were moderate signs of left visual inattention. Carotid angiography disclosed signs compatible with a fronto-parietal malignant glioma. The patient underwent conservative radiological and pharmacological therapy and died 3 months later. Post-mortem examination revealed a swollen right hemisphere compressing the left across the midline. The tumor involved most of the parietal operculum and extended anteriorly deep in the frontal lobe. The callosum was spared but both its outflow to the temporal lobe and conversely, fibers travelling *Present address: The University of Iowa Hospitals and Clinics, Department Iowa 52242, U.S.A. 247
of Neurology,
Iowa City,
NOTE
248
from the temporal lobe, were cut by the tumor. The temporal lobe itself was spared including the first and second convolutions and the underlying white matter. Histological examination revealed an astrocytoma Grade III. E.P. is a right-handed 63 yr old physician who suddenly became ill in January 1973. He complained of an acute headache accompanied by visual loss for the right-side. A while later he was unable to read and his relatives noticed what they described as a state of mild confusion and loss of memory. During this episode language was fluent and well articulated. He was then hospitalised under the care of a neurologist. The confusion cleared, the reading and visual difficulties as well as the amnesic syndrome were confirmed and it became apparent that the patient had a right-sided hemiparesis involving proximal and distal segments in equal degree. On the right side there was also a well marked hypaesthesia for a superficial forms of sensibility. An EEG revealed a left temporo-occipital focus of slow waves. The patient suffered from diabetes and hypertension that had not been adequately treated. Therapy was aimed at controlling both disorders. In the following weeks he made a good recovery and both the hemiparesis and the sensibility loss improved. Gradually he started walking by himself. Six months later his only complaints concerned the visual difficulty and the reading impairment which remained unaltered. When first studied in our unit (October 1973) the patient was alert and cooperative. There was a rightsided hemianopia with bilateral macular sparing. Light reflexes on the left were normal and somewhat sluggish on the right. The fundi showed signs of atherosclerosis and were otherwise normal. Speech was fluent, with normal articulation, melody and syntax. Naming to confrontation was more or less impaired according to the nature of the item. Color naming was impossible and the naming of graphic representations of objec:s was deficient. Verbal auditory comprehension was normal for material of marked lexical and syntactic complexity. There was a pure alexia for material presented visually, which covered reading of !etters, words, phrases, figures and numbers (Arabian and Roman). Oral spelling was correct. Reading in the tactile modality was possible using both the right and the left hands. There was no agraphia in spontaneous writing or on dictation. However, the patient was unable to read what he had just written and he was practically unable to copy any sort of written production in manuscript or typescript. Rightleft orientation and finger recognition were normal. There was no extinction on simultaneous tactile stimulation. Non-verbal comprehension was normal. The patient could give an adequate account of the situation he was in, understand movies and television and follow a soccer match. The patient thus presented a complete syndrome of the left posterior cerebral artery due to total occlusion of this vessel early in its course. STUDY
OF DICHOTIC
LISTENING
Method The study comprised 2 dichotic listening tasks. The first consisted on the reporting of 18 pairs of different Portuguese words of 2 and 3 syllables. In the second there were 19 pairs of different digits. The presentation of tasks conformed to the standard procedures of the Lisbon Language Research Laboratory [3]. Before entering the test period, the tasks ar’: explained and trial runs are made. During the test period the patients have to report on the stimuli received in both ears and have unlimited time to do so, before the examiner proceeds to the next pair of items. The tapes used in the study are part of the battery of that facility, have been balanced for intensity and standardized in a control population consisting of several groups of normals with different age brackets and cultural levels. The procedure followed in the standardization is the same as the one used-in the two cases. Prior to the experiments hearing ability was tested with a Keeller type B pure tone audiometer and was symmetrical in both cases. Tapes were then played by means of KO-727B high fidelity earphones in a TEAC A-22 stereo cassette recorder. RESULTS The results are presented in Table 1, in which the score obtained in each channel is compared with the cut-off point of a group of normals corresponding in schooling and age bracket to the case in question, In this context cut-off point indicates the score under which performance was classified as abnormal in terms of standard deviation. Both cases exhibit a complete left channel extinction in the test of different words, which means that there was literally no reporting of material coming from that channel. In the test of digits, case 1 again shows a complete extinction while in case 2 there is simply a decrease of level with a consequent augmentation of the right-left ratio of reporting. This variance should not be surprising since the coding and decoding of words that stand for digits is, in many instances, not as lateralized a process as coding and decoding of words not representing digits [3] a phenomenon which probably corresponds to the peculiar way digits are learned. It is possible to postulate a redundancy of storage concerning digits that
NOTE
249
Table 1 Control
Case 2
Control
83.3
19.6
12.2
92.1
0
59.6
0
80.7
1.45
-
1.14
case Score of right
Different
channel
Score of left channel
1
worde Right/left
::core
Digits
ratio
of right
Score of left charnel Right/left
Score
100
93
94.7
99.4
0
78
78.9
94.1
ratio
Score of right OW?CLll Werage
chanr;el
1.19
channel
of left channel
91.9
0
1.20
1.05
86.5
83.7
96.1
19.9
40.5
87.9
result Right/left
ratio
1.17
2.06
1.09
may eventually ease any process concerned with their handling. It is also interesting to note, in this particular matter, that the amelioration of performance that SPARKS and GFSCHWIND [2] obtained in theh case after considerable training, concerned a test of digits and not a test of regular words. The overall average result may be summarized as complete extinction of left channel for case 1 and partial extinction of left channel for case 2. Accordingly the index of dichotic listening ability (percentage of correct double answers) was nil for case 1 and 37.8 ‘A for case 2. A complete extinction phenomenon is probably a result of very marked severing of an input channel. Using our test battery we have now seen more instances of this phenomenon, which seem to correlate with lesions entailing the destructions of either auditory path\vays or transcortical fibers.
DISCUSSION The fact that a very poor performance referred to the left channel may be obtained by (i) surgical midline split of the callosum, (ii) vascular sectioning of the callosum, or (iii) sectioning of fibres leading to the callosum in any of the hemispheres, supports the idea that the pathways connecting both temporal cortices are relevant for the performance of dichotic listening. Hence, we may say that stimuli arriving in the left ear of each patient can use only the ipsilateral pathway to the temporal cortex. This, we presume, is functionally suppressed by the crossed pathway coming from the opposite ear. In case 1; where the post-mortem examination showed a complete destruction of the pathways connecting the auditory cortices, the “extinction” is complete and the index of dichotic listening ability nil. It is possible that a pattern of left channel extinction also appears in other situations of deep hemispheral lesions that spare both the callosum and the auditory cortices. This should be taken into account if dichotic listening is to be used, as we have proposed it may be in the future, for diagnostic purposes [4]. Indeed, by and large, a left channel extinction correlates with predominantly cortical lesions of the right temporal lobe. In case 2 there was a partial but definite extinction of the left channel, similar in profile to the findings of MILNER er al. [l] and SPARKS and GESCHWIND [2] in the callosotomised patients. This suggests that partial severing of the corpus callosum produces the same pattern of lesion effect, provided it includes the portion where interauditory fibers cross. It is interesting to note that in case 2 damage of the callosum has presumably been circumscribed to the caudal third. But that sector includes, in all probability, the interauditory fibers which, as PANDYA et al. [5] have shown in the rhesus monkey, travel just in front of the intervisual connections and posteriorly to the ones that link the somesthetic areas. In point of fact, the distinctive pattern of dichotic listening ability demonstrated in our patient, probabJy completes, along with pure alexia without agraphia, the syndrome of the posterior cerebral artery. Acknowledgements-We from Case 1.
thank Professor Loso ANTUNESfor permission to use neuropathological
material
250
NOTE
REFERENCES 1. MILNER, B., TAYLOR,L. and SPERRY,R. W. Lateralized suppression of dichotically presented digits after commissural section in man. Science 161, 184-186, 1968. 2. SPARKS, R. and GESCHWINQ N. Dichotic listening in man after section of neocortical commissures. Cortex 4, 3-16, 1968. 3. DAKUIO, A. R. Neurologia da Linguagem. Livraria Buchholz, Lisboa, 1974. 4. DAMASIO,H. and DAMASIO,A. R. An appraisal of dichotic listening procedures. Excerpta Med. 296,119, 1973. 5. PANDYA,D. N., KAROL, E. A. and HEILBRONN,D. The topographical distribution of interhemispheric projections in the corpus callosum of the rhesus monkey. Brain Res. 32, 3143, 1971. RGsumt5 --
:
tique
dans
On a dkrit un petit
a BtiS propos6 les deux tions
cette
auditifs.
en pr&entant
interauditives form
que
cortex
dans
a ce qu'on
Deutschsprachige
un aspect nombre
dew
anomalie
des
ayant
depend
Ce travail
le type
attendre
anomalies
subi une
de l'atteinte
t6moiqnf
cas de disconnexions
lesquelles
pouvait
paiticulier
de malades
des voies
en faveur
les travaux
dichoet il
reliant
de ces constata-
non chirurqicale
de l'anomalie
d'apres
d'6coute
callosotomie
des voies
rencontr&est
con-
antGrieurs.
Zusammknfassung:
Bei einer kleinen Anzahl von Patienten mit Balkendurchtrennung wurde eine unterschiedliche Art dichotischer Harstijrung beschrieben und es wurde gesagt, daD diese Abnormitat auf die Trennung der Faserverbindungen zwischen beiden Hijrrindengebieten zuriickzufiihren ist. Die vorliegende Arbeit berichtet iiber solche Untersuchungen an zwei Fallen von nicht operativer Trennung der Hijrleitungsverbindungen, wobei die Art der Stiirungenden bisherigen Befunden entspricht.
NOTE DICHOTIC LISTENING PATTERN IN RELATION INTERHEMISPHERIC DISCONNEXION HANNA DAMASIO,* ANTONIO R. DAMASIO,* A. CASTRO-CALDAS and
J. M.
TO FERRO
Language Research Laboratory, Centro de Estudos Egas Moniz, Lisbon Faculty of Medicine, Lisbon, Portugal (Received
14 June 1975)
Abstract-A distinctive pattern of dichotic listening abnormality has been described in a small number of callosotomised patients and it has been suggested that the abnormality relates to the severing of pathways connecting both auditory cortices. The present paper supports those findings by presenting two cases of non-surgical disconnexion of interauditory pathways in which the pattern of disability conforms to the expectation raised by the previous reports.
INTRODUCTION THE ROLE of
the interhemispheric connections in the performance of dichotic listening tasks has been suggested by studies of split-brain patients in whom a distinctive pattern has been found. MILNER et al. [l] examined a group of 7 such patients with complete section of the callosum and established that there was a consistent “lesion effect” on the left channel in the sense that only an extremely small amount of information from the left ear was processed. SPARKS and GESCHWIND [2] obtained the same result after extensive investigation of a single callosotomised patient. The explanation advanced by both groups of authors was that: (1) the primary verbal input channel from the left ear to the left hemisphere, crosses first to the right temporal lobe and then travels from there to the left via the callosum (the secondary channel is ipsilateral and goes directly to the left auditory cortex); (2) the primary verbal input channel from the right ear also crosses and travels directly to the left temporal lobe; (3) this latter pathway inhibits the secondary ipsilateral channel from the left ear. Hence the callosal sectioning of pathways connecting the opposite auditory cortices prevents the left ear from using its stronger channel and makes it rely on the ipsilateral pathway which, in a competitive simultaneous process, is blocked by the more powerful channel from the right ear. If this explanation is correct, any case in which the auditory interhemispheric pathways are affected should exhibit the finding regardless of the point where the section occurs. The following cases exemplify such a situation. The first is a case of deep right hemisphere neoplasm sparing the temporal cortices and the callosum. The second is a syndrome of the left posterior cerebral artery a lesion of the caudal third of the callosum. Case I
J.A.F. was a right-handed 45 yr old illiterate farmer. Headache and gradual loss of vision brought the patient to the Neurological Unit in December 1973. At the time of his first observation he complained of intense headache and vomited frequently. He was alert and cooperative but in obvious distress. There was marked papilledema exudates and hemorrhages in both eyes and a left sixth cranial nerve palsy. Visual fields were normal as were the light reflexes. Tone was diminished on the left side and power was reduced in the left leg and the left arm. Deep reflexes were consistently brisker on the left. Plantar responses were extensor on both sides. Speech was fluent, with normal articulation, melody and syntax. Visual confrontation naming and verbal auditory comprehension were normal. There were no defects of body gnosis. Rightleft orientation and calculation were unimpaired. Constructional ability was much disturbed for both 2 and 3 dimensional tasks. Stereognosis, barognosis and two-point discrimination were impaired on the left hand and there was a left-side extinction on simultaneous tactile stimulation. There were moderate signs of left visual inattention. Carotid angiography disclosed signs compatible with a fronto-parietal malignant glioma. The patient underwent conservative radiological and pharmacological therapy and died 3 months later. Post-mortem examination revealed a swollen right hemisphere compressing the left across the midline. The tumor involved most of the parietal operculum and extended anteriorly deep in the frontal lobe. The callosum was spared but both its outflow to the temporal lobe and conversely, fibers travelling *Present address: The University of Iowa Hospitals and Clinics, Department Iowa 52242, U.S.A. 247
of Neurology,
Iowa City,
NOTE
248
from the temporal lobe, were cut by the tumor. The temporal lobe itself was spared including the first and second convolutions and the underlying white matter. Histological examination revealed an astrocytoma Grade III. E.P. is a right-handed 63 yr old physician who suddenly became ill in January 1973. He complained of an acute headache accompanied by visual loss for the right-side. A while later he was unable to read and his relatives noticed what they described as a state of mild confusion and loss of memory. During this episode language was fluent and well articulated. He was then hospitalised under the care of a neurologist. The confusion cleared, the reading and visual difficulties as well as the amnesic syndrome were confirmed and it became apparent that the patient had a right-sided hemiparesis involving proximal and distal segments in equal degree. On the right side there was also a well marked hypaesthesia for a superficial forms of sensibility. An EEG revealed a left temporo-occipital focus of slow waves. The patient suffered from diabetes and hypertension that had not been adequately treated. Therapy was aimed at controlling both disorders. In the following weeks he made a good recovery and both the hemiparesis and the sensibility loss improved. Gradually he started walking by himself. Six months later his only complaints concerned the visual difficulty and the reading impairment which remained unaltered. When first studied in our unit (October 1973) the patient was alert and cooperative. There was a rightsided hemianopia with bilateral macular sparing. Light reflexes on the left were normal and somewhat sluggish on the right. The fundi showed signs of atherosclerosis and were otherwise normal. Speech was fluent, with normal articulation, melody and syntax. Naming to confrontation was more or less impaired according to the nature of the item. Color naming was impossible and the naming of graphic representations of objec:s was deficient. Verbal auditory comprehension was normal for material of marked lexical and syntactic complexity. There was a pure alexia for material presented visually, which covered reading of !etters, words, phrases, figures and numbers (Arabian and Roman). Oral spelling was correct. Reading in the tactile modality was possible using both the right and the left hands. There was no agraphia in spontaneous writing or on dictation. However, the patient was unable to read what he had just written and he was practically unable to copy any sort of written production in manuscript or typescript. Rightleft orientation and finger recognition were normal. There was no extinction on simultaneous tactile stimulation. Non-verbal comprehension was normal. The patient could give an adequate account of the situation he was in, understand movies and television and follow a soccer match. The patient thus presented a complete syndrome of the left posterior cerebral artery due to total occlusion of this vessel early in its course. STUDY
OF DICHOTIC
LISTENING
Method The study comprised 2 dichotic listening tasks. The first consisted on the reporting of 18 pairs of different Portuguese words of 2 and 3 syllables. In the second there were 19 pairs of different digits. The presentation of tasks conformed to the standard procedures of the Lisbon Language Research Laboratory [3]. Before entering the test period, the tasks ar’: explained and trial runs are made. During the test period the patients have to report on the stimuli received in both ears and have unlimited time to do so, before the examiner proceeds to the next pair of items. The tapes used in the study are part of the battery of that facility, have been balanced for intensity and standardized in a control population consisting of several groups of normals with different age brackets and cultural levels. The procedure followed in the standardization is the same as the one used-in the two cases. Prior to the experiments hearing ability was tested with a Keeller type B pure tone audiometer and was symmetrical in both cases. Tapes were then played by means of KO-727B high fidelity earphones in a TEAC A-22 stereo cassette recorder. RESULTS The results are presented in Table 1, in which the score obtained in each channel is compared with the cut-off point of a group of normals corresponding in schooling and age bracket to the case in question, In this context cut-off point indicates the score under which performance was classified as abnormal in terms of standard deviation. Both cases exhibit a complete left channel extinction in the test of different words, which means that there was literally no reporting of material coming from that channel. In the test of digits, case 1 again shows a complete extinction while in case 2 there is simply a decrease of level with a consequent augmentation of the right-left ratio of reporting. This variance should not be surprising since the coding and decoding of words that stand for digits is, in many instances, not as lateralized a process as coding and decoding of words not representing digits [3] a phenomenon which probably corresponds to the peculiar way digits are learned. It is possible to postulate a redundancy of storage concerning digits that
NOTE
249
Table 1 Control
Case 2
Control
83.3
19.6
12.2
92.1
0
59.6
0
80.7
1.45
-
1.14
case Score of right
Different
channel
Score of left channel
1
worde Right/left
::core
Digits
ratio
of right
Score of left charnel Right/left
Score
100
93
94.7
99.4
0
78
78.9
94.1
ratio
Score of right OW?CLll Werage
chanr;el
1.19
channel
of left channel
91.9
0
1.20
1.05
86.5
83.7
96.1
19.9
40.5
87.9
result Right/left
ratio
1.17
2.06
1.09
may eventually ease any process concerned with their handling. It is also interesting to note, in this particular matter, that the amelioration of performance that SPARKS and GFSCHWIND [2] obtained in theh case after considerable training, concerned a test of digits and not a test of regular words. The overall average result may be summarized as complete extinction of left channel for case 1 and partial extinction of left channel for case 2. Accordingly the index of dichotic listening ability (percentage of correct double answers) was nil for case 1 and 37.8 ‘A for case 2. A complete extinction phenomenon is probably a result of very marked severing of an input channel. Using our test battery we have now seen more instances of this phenomenon, which seem to correlate with lesions entailing the destructions of either auditory path\vays or transcortical fibers.
DISCUSSION The fact that a very poor performance referred to the left channel may be obtained by (i) surgical midline split of the callosum, (ii) vascular sectioning of the callosum, or (iii) sectioning of fibres leading to the callosum in any of the hemispheres, supports the idea that the pathways connecting both temporal cortices are relevant for the performance of dichotic listening. Hence, we may say that stimuli arriving in the left ear of each patient can use only the ipsilateral pathway to the temporal cortex. This, we presume, is functionally suppressed by the crossed pathway coming from the opposite ear. In case 1; where the post-mortem examination showed a complete destruction of the pathways connecting the auditory cortices, the “extinction” is complete and the index of dichotic listening ability nil. It is possible that a pattern of left channel extinction also appears in other situations of deep hemispheral lesions that spare both the callosum and the auditory cortices. This should be taken into account if dichotic listening is to be used, as we have proposed it may be in the future, for diagnostic purposes [4]. Indeed, by and large, a left channel extinction correlates with predominantly cortical lesions of the right temporal lobe. In case 2 there was a partial but definite extinction of the left channel, similar in profile to the findings of MILNER er al. [l] and SPARKS and GESCHWIND [2] in the callosotomised patients. This suggests that partial severing of the corpus callosum produces the same pattern of lesion effect, provided it includes the portion where interauditory fibers cross. It is interesting to note that in case 2 damage of the callosum has presumably been circumscribed to the caudal third. But that sector includes, in all probability, the interauditory fibers which, as PANDYA et al. [5] have shown in the rhesus monkey, travel just in front of the intervisual connections and posteriorly to the ones that link the somesthetic areas. In point of fact, the distinctive pattern of dichotic listening ability demonstrated in our patient, probabJy completes, along with pure alexia without agraphia, the syndrome of the posterior cerebral artery. Acknowledgements-We from Case 1.
thank Professor Loso ANTUNESfor permission to use neuropathological
material
250
NOTE
REFERENCES 1. MILNER, B., TAYLOR,L. and SPERRY,R. W. Lateralized suppression of dichotically presented digits after commissural section in man. Science 161, 184-186, 1968. 2. SPARKS, R. and GESCHWINQ N. Dichotic listening in man after section of neocortical commissures. Cortex 4, 3-16, 1968. 3. DAKUIO, A. R. Neurologia da Linguagem. Livraria Buchholz, Lisboa, 1974. 4. DAMASIO,H. and DAMASIO,A. R. An appraisal of dichotic listening procedures. Excerpta Med. 296,119, 1973. 5. PANDYA,D. N., KAROL, E. A. and HEILBRONN,D. The topographical distribution of interhemispheric projections in the corpus callosum of the rhesus monkey. Brain Res. 32, 3143, 1971. RGsumt5 --
:
tique
dans
On a dkrit un petit
a BtiS propos6 les deux tions
cette
auditifs.
en pr&entant
interauditives form
que
cortex
dans
a ce qu'on
Deutschsprachige
un aspect nombre
dew
anomalie
des
ayant
depend
Ce travail
le type
attendre
anomalies
subi une
de l'atteinte
t6moiqnf
cas de disconnexions
lesquelles
pouvait
paiticulier
de malades
des voies
en faveur
les travaux
dichoet il
reliant
de ces constata-
non chirurqicale
de l'anomalie
d'apres
d'6coute
callosotomie
des voies
rencontr&est
con-
antGrieurs.
Zusammknfassung:
Bei einer kleinen Anzahl von Patienten mit Balkendurchtrennung wurde eine unterschiedliche Art dichotischer Harstijrung beschrieben und es wurde gesagt, daD diese Abnormitat auf die Trennung der Faserverbindungen zwischen beiden Hijrrindengebieten zuriickzufiihren ist. Die vorliegende Arbeit berichtet iiber solche Untersuchungen an zwei Fallen von nicht operativer Trennung der Hijrleitungsverbindungen, wobei die Art der Stiirungenden bisherigen Befunden entspricht.
