"Paradoxic" ear extinction in dichotic listening: possible anatomic significance

"Paradoxic" extinction in dichotic listening tasks has been attributed to lesions in the interhemispheric auditory pathways, the anatomic courses of which are unknown. The localization of cerebral lesions related to abnormal patterns of dichotic listening was studied in computerized tomographic (CT) scans with a method of anatomic templates. Paradoxic extinction was found consistently with lesions near the lateral wall of the lateral ventricles at the level of the trigone in both left and right hemispheres. On the basis of these findings, we propose that the pathway leaves the auditory cortex traveling backward and upward to arch around the lateral ventricles and join the callosum in its posterior region.     

Left-ear extinction in patients with MRI periventricular lesions

A dichotic listening test was administered to 22 elderly vascular patients with periventricular white matter changes on magnetic resonance imaging. We found four patients with a moderate-to-severe extent of these changes who showed a pattern of left-ear extinction. These findings suggest that periventricular lesions in patients with vascular risk factors may be associated with a functional disconnection of the interhemispheric auditory pathways.     

Auditory ear extinction in lacunar syndromes

A dichotic listening test was administered to 28 patients with lacunar syndromes in order to contribute to the investigation of the subcortical route of interhemispheric auditory pathways. Topographic study showed that lesions in the external capsule or in the anterior limb of the internal capsule in both hemispheres produced left ear extinction. The possibility that some of the auditory fibres travel through the external capsule, as well as the anterior limb of the internal capsule, before crossing the contralateral cerebral lobe is suggested.     

Left hemisphere contribution to motor programming of aphasic speech: a reaction time experiment in aphasic patients

Simple reaction time to lateralized visual dot stimuli was studied in 10 fluent and 10 nonfluent right-handed chronic aphasics with left hemisphere lesions. As well as the standard simple reaction time condition, the patients were given a concomitant verbal task, requiring overt articulation while reacting to the visual stimuli. Compared with the control condition, in both aphasic groups the verbal task produced an overall lengthening of latencies, with a significant slowing down of responses to the stimuli located in the right visual half-field. According to these results the verbal concurrent activity appears to involve the left hemisphere as in normal subjects, suggesting that the undamaged regions of the left hemisphere have a role to play in the motor programming of aphasic speech. As a collateral finding, the difference between latencies to stimuli ipsilateral and contralateral to the responding hand--a measure of interhemispheric transmission time--is greatly increased in patients with motor deficits. This is consistent with the view that, in simple visuo-motor reaction time, interhemispheric transfer takes place between anterior regions of the brain.     

Listening with one or two hemispheres: verbal dichotic testing after intracarotid barbiturate injection

A verbal dichotic test was given to 16 patients during right or left hemisphere narcosis induced by intracarotid barbiturate injection (Wada's procedure). During right hemisphere narcosis patients did not repeat left ear words. This demonstrates an intrahemispheric competition within the left hemisphere at the detriment of left ipsilateral ear words. After left dominant hemisphere narcosis, when aphasia had apparently disappeared, patients temporarily did not repeat right ear words. This suggests that, during this critical time period, two competitive processes took place. First the right hemisphere selected the left ear words by an intrahemispheric competition. Second it imposed them on the recovering left hemisphere at the detriment of right ear words, because of an interhemispheric competition.     

Right unilateral auditory agnosia following left lenticular hemorrhage

A 33-year old patient who had had left lenticular hemorrhage presented with an inability to understand with the right ear oral language and, in a less dramatic way, nonverbal sounds. This unilateral auditory agnosia was first associated with a right motor underutilization and right motor, sensitive, visual and auditive extinctions. Speech discrimination scores were 100% with the left ear and 15% with the right ear, even less in dichotic conditions. Tonal audiogram, as well as early and late components of the auditory evoked potentials were normal. Cerebral regional perfusion and metabolism were impaired over the left parietotemporal area. There was severe hypoactivation of the left hemisphere with right monaural verbal stimulations. Rehabilitation consisting of non-specific attention tasks and repetitions of words reaching only the right ear was undertaken 15 months after the stroke. The oral language comprehension improved, as did the left hemisphere activation, and the extinction phenomena disappeared, except for the auditory one. The unilaterality of the auditory agnosia could be due, in part, to a peculiar physiological processing in this patient, such as poor performance of his right ipsilateral auditory pathway which could be improved with practice. A striatal lesion could induce a spatial hemi-inattention as reflected by the multimodal extinction in this case. Besides, a lack of selective activation for verbal stimulation of the left hemisphere is suggested.     

Double disconnection effects resulting from infiltrating tumors

Patients with left hemisphere lesions deep in parietal or parietal-occipital regions close to the lateral ventricles have been reported to have impaired performance on left ear speech stimuli in dichotic listening tests. This loss has been termed "paradoxical" because it presents at the ear ipsilateral to the lesion. Two patients with infiltrating tumors which involved the corpus callosum demonstrated that effect, but also demonstrated right ear extinction on a complex-pitch discrimination test that required right hemisphere processing. Since the side at which the impairment will be demonstrated depends upon the type of test used, the term "paradoxical extinction" does not clearly describe this phenomena. It is suggested that the so-called paradoxical loss is better referred to as callosal extinction.     

Verbal visual extinction in right/left hemisphere lesion patients and the problem of lexical access

In order to explore perceptual processes preceding visual lexical access, we compared single words reading in both visual hemifields in right or left hemisphere damaged patients. Words were tachistoscopically presented in three conditions: (1) one word in one visual hemifield, (2) two words, one in each visual hemifield aside the fovea, (3) one longer word in both visual hemifields, centred on the fovea. In many patients, suffering from right as well as left hemisphere lesions, reading remains normal for unilateral presentation whereas bilateral presentation induced extinction in the hemifield contralateral to the lesion. Nevertheless, extinction was not frequently observed for the right or the left part of long centred words. Extinction proved to be most frequent in lesions centred on the occipito-temporo-parietal area. In addition, we found visual extinction not to be related to auditory extinction in verbal dichotic listening.     

Different mechanisms of ipsilateral and contralateral ear extinction in aphasic patients

Dichotic listening performance was studied in 35 aphasic patients with unilateral brain lesions. Within this group, 30 patients exhibited an abnormal ear discrepancy: 20 patients showed a left-ear advantage (right-ear extinction), and 10 patients showed a right-ear advantage (left-ear extinction). All subjects were given a number of verbal tasks in order to evaluate several aspects of their linguistic abilities. For the left-ear advantage group, verbal scores were positively related to left-ear scores, but not to right-ear scores. For the right-ear advantage group, verbal scores were positively related to both left- and right-ear scores. The present results lend partial support for the prediction that right-ear extinction is associated with damage to the temporal lobe involving the geniculo-temporal system. No support was found for the prediction that left-ear extinction is associated with lesions placed in the deep structures of the parietal or parieto-occipital lobe involving the corpus callosum system. Our data indicate that the initial severity of brain injury is a critical factor in determining the direction of ear extinction, the lesions being larger in the left-ear advantage group. Results are discussed in relationship to other explanations of ipsilateral and contralateral extinction in aphasic patients.     

Release from central auditory competition in the split-brain patient

We used dichotic digits (DD), staggered spondaic words (SSW), and frequency patterns (PATT) to study central auditory function before and after two-stage callosotomy. Preoperatively, the patient demonstrated reduced scores bilaterally on all these tests, consistent with documented bilateral hemisphere lesions. After the first operation (sectioning the posterior half of the corpus callosum), the dichotic tests (DD and SSW) revealed the expected decrease in left-ear scores, but there was improvement on the right, perhaps because there was release from central auditory competition. Our findings also suggest that the "auditory" portion of the corpus callosum may be in the posterior half of this structure.     

A case of acquired conduction aphasia in a child

A 10-year-old right-handed boy showed conduction aphasia with left-ear verbal extinction (paradoxical ipsilateral ear extinction) after removal of a arteriovenous malformation in the left parietal lobe. Buccofacial and ideomotor apraxia were not observed. Recovery from aphasia was dramatic. Postoperative computed tomography (CT) and magnetic resonance imaging scans demonstrated damage confined to the left supramarginal gyrus invading the arcuate fasciculus. Together with recently reported cases of acquired fluent aphasia in children with CT-verified left posterior lesions, this case seems to support the current view that both fluent and nonfluent aphasia that share many similarities with the symptoms and lesion localization associated with adult cases may exist in children.     

Relative processing demands influence cerebral laterality for verbal-motor integration in persons with Down syndrome

The study of cerebral specialization in the Down syndrome (DS) population has revealed an anomalous pattern of organization. In particular, dichotic-listening studies have suggested a left-ear/right hemisphere dominance for speech perception, whereas motor control research has revealed a left hemisphere dominance for executive-motor control. In the present investigation, we employed a recent adaptation of the dichotic listening procedure to examine interhemispheric integration during the performance of a lateralized verbal-motor task. Specifically, using the selective dichotic-listening procedure, participants were required to complete a rapid left or right hand pointing movement to one of two pictorial icons corresponding to the word presented to their precued ear. We observed that persons with DS (N = 17) and age-matched controls (N = 35) exhibited a right-ear advantage (REA) for our dichotic-aiming task. While these results appear to contradict previous dichotic listening studies, we propose that the manifestation of a lateral ear advantage in the DS population may have more to do with the response requirements of the task than with the characteristics or complexity of the stimulus material.     

Auditory extinction to nonverbal and verbal stimuli

The so-called auditory extinction phenomenon in a verbal dichotic listening test was investigated in seven brain damaged patients from the qualitative standpoint. As the result, it became evident that the phenomenon could result from competition between verbal materials presented to both ears in at least two different levels of auditory processing i.e., sound perception and linguistic recognition. From the present study, we emphasize the following: If Bender's definition is followed, it is advisable to use the term extinction only for the phenomenon at the acoustic level. If the use of the term extinction for the phenomenon is to be continued at the linguistic level, the presence of auditory extinction and/or obscuration to nonverbal stimuli should be noted.     

Dyslexic adolescents: evidence of impaired visual and auditory language processing associated with normal lateralization and visual responsivity.

The questions of whether chronically dyslexic adolescent suffer any deficits of simple language stimulus processing or are less left hemisphere dominant than normal reading controls were addressed. The dyslexics were chosen for clarity of their specific reading problem and were older than dyslexics previously studied with lateralizing tests. Tasks administered in Experiment I were unilateral and bilateral tachistoscopic work recognitions and a tachistoscopic recognition report-time task for single lateralized letter stimuli. Experiment II, conducted a year later, readministered these tasks with modifications, and added dichotic digits and motor reaction time-stimulus detection tasks. It was concluded that right handed, chronic dyslexics: (1) possess left hemisphere language specialization; (2) show normal interhemispheric processing delays for single letter stimuli; (3) are, unlike nondyslexis but equally poor-reading Ss, clearly impaired in their efficiency of visual and auditory processing of simple language stimuli; (4) possess clear auditory memory deficits for verbal material; and (5) may possess an additional deficit of left hemisphere visual association area function.     

Comparison of auditory deficits associated with neglect and auditory cortex lesions

In contrast to lesions of the visual and somatosensory cortex, lesions of the auditory cortex are not associated with self-evident contralesional deficits. Only when two or more stimuli are presented simultaneously to the left and right, contralesional extinction has been observed after unilateral lesions of the auditory cortex. Because auditory extinction is also considered a sign of neglect, clinical separation of auditory neglect from deficits caused by lesions of the auditory cortex is challenging. Here, we directly compared a number of tests previously used for either auditory-cortex lesions or neglect in 29 controls and 27 patients suffering from unilateral auditory-cortex lesions, neglect, or both. The results showed that a dichotic-speech test revealed similar amounts of extinction for both auditory cortex lesions and neglect. Similar results were obtained for words lateralized by inter-aural time differences. Consistent extinction after auditory cortex lesions was also observed in a dichotic detection task. Neglect patients showed more general problems with target detection but no consistent extinction in the dichotic detection task. In contrast, auditory lateralization perception was biased toward the right in neglect but showed considerably less disruption by auditory cortex lesions. Lateralization of auditory-evoked magnetic fields in auditory cortex was highly correlated with extinction in the dichotic target-detection task. Moreover, activity in the right primary auditory cortex was somewhat reduced in neglect patients. The results confirm that auditory extinction is observed with lesions of the auditory cortex and auditory neglect. A distinction can nevertheless be made with dichotic target-detection tasks, auditory-lateralization perception, and magnetoencephalography.     

Left ear suppression on verbal dichotic tests in patients with multiple sclerosis

Verbal dichotic listening tests employing nonsense consonant-vowel stimulus pairs were administered to 11 right-handed patients with chronic multiple sclerosis and to 10 right-handed normal controls. A highly significant reduction of left-ear scores was found in the multiple sclerosis group. We propose that the basis for this finding is a disconnection of the auditory callosal pathway and that dichotic listening tests may be of value in detecting the presence of lesions of the deep white matter of the cerebral hemispheres in multiple sclerosis.     

Long latency evoked potentials in a case of corpus callosum agenesia

Following monoaural stimulation, long latency auditory evoked potentials (LLAEPs) recorded from contralateral temporal areas have a shorter latency and larger amplitude than those recorded from the ipsilateral temporal areas. This observation agrees with the operational model drawn up in 1967 by Kimura, which assumes that only anatomically prevailing crossed auditory pathways are active during dichotic hearing, while direct pathways are inhibited. The inputs may then be conveyed to the contralateral cortex, from where they finally reach the ipsilateral temporal areas by means of interhemispheric commissures. It is this mechanism which may underline the right ear advantage for verbal stimuli and the left ear advantage for melodies observed when administering dichotic listening tasks. With the aim of verifying this hypothesis, we recorded temporal LLAEPs in a 21 year-old woman suffering from complex partial seizures, whose CT scan and MRI showed corpus callosum agenesia. Our data support the hypothesis that ipsilateral pathways are greatly inhibited by the contralateral pathways, and therefore auditory stimuli can be supposed to reach the contralateral auditory cortex from where they are transferred through the corpus callosum to the ipsilateral auditory cortex. This work was supported by a grant from the Italian Ministry of Education (ministero della Pubblica Istruzione).     

The effect of dichotic, monaural and diotic verbal stimuli on auditory evoked potentials

To clarify the nature of the right ear preference in dichotic listening to speech stimuli, comparisons were made between evoked responses from the two hemispheres of normals when verbal input (dichotic, monaural, diotic) varied and when response accuracy varied. The positive component of the evoked response was significantly larger in the right hemisphere in all conditions. No significant asymmetry was seen in the negative component.Negative and positive components did not vary with response accuracy. However, the negative component was significantly larger in dichotic tasks than in all others. This difference may be related to complexity of dichotic input or to inhibition of the ipsilateral auditory pathway during dichotic stimulation.     

Identification and discrimination disorders in auditory perception: a report on two cases

Auditory perception was investigated in two brain-damaged subjects. The first patient had a left temporoparietal ischaemic lesion. He presented a right-ear extinction in dichotic tasks, as well as difficulties in understanding and repeating verbal material and impaired identification of melodies. All discrimination tests were well performed. The second patient had a right capsulolenticular and frontal ischaemic lesion. He presented a left ear dichotic extinction and severe difficulties in discrimination of environmental sounds and melodies but no major difficulty in naming and identification. From these results, it is hypothesized that identification and discrimination involve distinct mechanisms within the processing of auditory stimuli, and that they may be selectively disrupted in brain-damaged subjects.     

Auditory extinction and dichotic listening CV task in cerebral infarction: preliminary report

Six stroke patients were studied using a dichotic listening CV task, 4 with left hemisphere infarction, 2 with right hemisphere infarction. It was observed a "lesion-effect", a shift of hemisphere prevalence to the side opposite a brain lesion. The authors suggest that the lesion-effect can be explained by the auditory extinction phenomenon at the linguistic level.