Taste laterality in the split brain

Functional topography of the corpus callosum as revealed by fMRI and behavioural studies of control subjects and patients with callosal resection

The concept of a topographical map of the corpus callosum (CC), the main interhemispheric commissure, has emerged from human lesion studies and from anatomical tracing investigations in other mammals. Over the last few years, a rising number of researchers have been reporting functional magnetic resonance imaging (fMRI) activation in also the CC. This short review summarizes the functional and behavioral studies performed in groups of healthy subjects and in patients undergone to partial or total callosal resection, and it is focused on the work conducted by the authors. Functional data have been collected by diffusion tensor imaging and tractography (DTI and DTT) and functional magnetic resonance imaging (fMRI), both techniques allowing to expand and refine our knowledge of the commissure. Neuropsychological test were also administered, and simple behavioral task, as imitation perspective and mental rotation ability, were analyzed. These researches added new insight on the topographic organization of the human CC. By combining DTT and fMRI it was possible to observe that the callosal crossing points of interhemispheric fibers connecting homologous primary sensory cortices, correspond to the CC sites where the fMRI activation elicited by peripheral stimulation was detected. In addition, CC activation during imitation and mental rotation performance was also reported. These studies demonstrated the presence of specific callosal fiber tracts that cross the commissure in the genu, body, and splenium, at sites showing fMRI activation, consistently with cortical activated areas. Altogether, these findings lend further support to the notion that the CC displays a functional topographic organization, also related to specific behavior.

The functional characterization of callosal connections

•

The functional characterization of callosal connections is informed by anatomical data.

•

Callosal connections play a conditional driving role depending on the brain state and behavioral demands.

•

Callosal connections play a modulatory function, in addition to a driving role.

•

The corpus callosum participates in learning and interhemispheric transfer of sensorimotor habits.

•

The corpus callosum contributes to language processing and cognitive functions.

The brain operates through the synaptic interaction of distant neurons within flexible, often heterogeneous, distributed systems. Histological studies have detailed the connections between distant neurons, but their functional characterization deserves further exploration. Studies performed on the corpus callosum in animals and humans are unique in that they capitalize on results obtained from several neuroscience disciplines. Such data inspire a new interpretation of the function of callosal connections and delineate a novel road map, thus paving the way toward a general theory of cortico-cortical connectivity. Here we suggest that callosal axons can drive their post-synaptic targets preferentially when coupled to other inputs endowing the cortical network with a high degree of conditionality. This might depend on several factors, such as their pattern of convergence-divergence, the excitatory and inhibitory operation mode, the range of conduction velocities, the variety of homotopic and heterotopic projections and, finally, the state-dependency of their firing. We propose that, in addition to direct stimulation of post-synaptic targets, callosal axons often play a conditional driving or modulatory role, which depends on task contingencies, as documented by several recent studies.

Functional MRI cortical activations from unilateral tactile-taste stimulations of the tongue

Functional magnetic resonance imaging (fMRI) was used for revealing activations in the human brain by lateralized tactile-gustatory stimulations of the tongue. Salt, a basic taste stimulus, and water, now recognized as an independent taste modality, were applied to either hemitongues with pads similar to the taste strips test for the clinical psychophysical evaluation of taste. With both stimuli, the observed cortical patterns of activations could be attributed to a combined somatosensory and gustatory stimulation of the tongue, with no significant differences between salt and water. Stimulation of each hemitongue evoked a bilateral activation of the anterior insula-frontal operculum, ascribable to the gustatory component of the stimulation, and a bilateral activation of the inferior part of the postcentral gyrus, ascribable to the tactile component of the stimulation. The results are in line with the notion that the representation of the tongue in the cerebral hemispheres in both the touch and the taste modalities is bilateral. Clinical and brain stimulation findings indicate that this bilaterality depends primarily on a partial crossing of the afferent pathways, perhaps with a predominance of the crossed pathway in the touch modality and the uncrossed pathway in the taste modality. Previous evidence suggests that the corpus callosum is not indispensible for this bilateral representation, but can contribute to it by interhemispheric transfer of information in both modalities.

Disentangling interoception: insights from focal strokes affecting the perception of external and internal milieus

Interoception is the moment-to-moment sensing of the physiological condition of the body. The multimodal sources of interoception can be classified into two different streams of afferents: an internal pathway of signals arising from core structures (i.e., heart, blood vessels, and bronchi) and an external pathway of body-mapped sensations (i.e., chemosensation and pain) arising from peripersonal space. This study examines differential processing along these streams within the insular cortex (IC) and their subcortical tracts connecting frontotemporal networks. Two rare patients presenting focal lesions of the IC (insular lesion, IL) or its subcortical tracts (subcortical lesion, SL) were tested. Internally generated interoceptive streams were assessed through a heartbeat detection (HBD) task, while those externally triggered were tapped via taste, smell, and pain recognition tasks. A differential pattern was observed. The IC patient showed impaired internal signal processing while the SL patient exhibited external perception deficits. Such selective deficits remained even when comparing each patient with a group of healthy controls and a group of brain-damaged patients. These outcomes suggest the existence of distinguishable interoceptive streams. Results are discussed in relation with neuroanatomical substrates, involving a fronto-insulo-temporal network for interoceptive and cognitive contextual integration.

Functional topography of the corpus callosum investigated by DTI and fMRI

This short review examines the most recent functional studies of the topographic organization of the human corpus callosum, the main interhemispheric commissure. After a brief description of its anatomy, development, microstructure, and function, it examines and discusses the latest findings obtained using diffusion tensor imaging (DTI) and tractography (DTT) and functional magnetic resonance imaging (fMRI), three recently developed imaging techniques that have significantly expanded and refined our knowledge of the commissure. While DTI and DTT have been providing insights into its microstructure, integrity and level of myelination, fMRI has been the key technique in documenting the activation of white matter fibers, particularly in the corpus callosum. By combining DTT and fMRI it has been possible to describe the trajectory of the callosal fibers interconnecting the primary olfactory, gustatory, motor, somatic sensory, auditory and visual cortices at sites where the activation elicited by peripheral stimulation was detected by fMRI. These studies have demonstrated the presence of callosal fiber tracts that cross the commissure at the level of the genu, body, and splenium, at sites showing fMRI activation. Altogether such findings lend further support to the notion that the corpus callosum displays a functional topographic organization that can be explored with fMRI.

Further evidence for the topography and connectivity of the corpus callosum: an FMRI study of patients with partial callosal resection

BACKGROUND AND PURPOSE

This functional MRI study was designed to describe activated fiber topography and trajectories in the corpus callosum (CC) of six patients carrying different degree of partial callosal resection.

METHODS

Patients receiving gustatory, tactile, and visual stimulation according to a block-design protocol were scanned in a 1.5 Tesla magnet. Diffusion tensor imaging (DTI) data were also acquired to visualize spared interhemispheric fibers.

RESULTS

Taste stimuli evoked bilateral activation of the primary gustatory area in all patients and foci in the anterior CC, when spared. Tactile stimuli to the hand evoked bilateral foci in the primary somatosensory area in patients with an intact posterior callosal body and only contralateral in the other patients. Callosal foci occurred in the CC body, if spared. In patients with an intact splenium central visual stimulation induced bilateral activation of the primary visual area as well as foci in the splenium itself.

CONCLUSION

Present data show that interhemispheric fibers linking sensory areas crossed through the CC at the sites where the different sensory stimuli evoked activation foci, and that topography of callosal foci evoked by sensory stimulation in spared CC portions is consistent with that previously observed in subjects with intact CC.

Functional topography of human corpus callosum: an FMRI mapping study

The concept of a topographical map of the corpus callosum (CC) has emerged from human lesion studies and from electrophysiological and anatomical tracing investigations in other mammals. Over the last few years a rising number of researchers have been reporting functional magnetic resonance imaging (fMRI) activation in white matter, particularly the CC. In this study the scope for describing CC topography with fMRI was explored by evoking activation through simple sensory stimulation and motor tasks. We reviewed our published and unpublished fMRI and diffusion tensor imaging data on the cortical representation of tactile, gustatory, auditory, and visual sensitivity and of motor activation, obtained in 36 normal volunteers and in 6 patients with partial callosotomy. Activation foci were consistently detected in discrete CC regions: anterior (taste stimuli), central (motor tasks), central and posterior (tactile stimuli), and splenium (auditory and visual stimuli). Reconstruction of callosal fibers connecting activated primary gustatory, motor, somatosensory, auditory, and visual cortices by diffusion tensor tracking showed bundles crossing, respectively, through the genu, anterior and posterior body, and splenium, at sites harboring fMRI foci. These data confirm that the CC commissure has a topographical organization and demonstrate that its functional topography can be explored with fMRI.

Topographical organization of human corpus callosum: an fMRI mapping study

The concept of a topographical map of the corpus callosum (CC) has emerged from human lesion studies and from anatomical tracing investigations in other mammals. Over the last few years, a rising number of researchers have been reporting functional magnetic resonance imaging (fMRI) activation in white matter, particularly the CC. In this study, the scope for describing CC topography with fMRI was explored by evoking activation through simple sensory stimulation and motor tasks. We reviewed our published and unpublished fMRI data on the cortical representation of tactile, gustatory, and visual sensitivity and of motor activation, obtained in 36 volunteers. Activation foci were consistently detected in discrete CC regions: anterior (taste stimuli), central (motor tasks), central and posterior (tactile stimuli), and splenium (visual stimuli). These findings demonstrate that the functional topography of the CC can be explored with fMRI.

Establishing a reliable protocol to measure tongue sensation

The relationship between tongue sensation and tongue function for speech, mastication and deglutition are growing areas of interest among rehabilitative professionals. To determine the potential effect that sensation has on function, it is imperative that, first, reliable and valid measures of tongue sensation be established. The aim of this study was to develop a protocol to test tongue sensation across a spectrum of sensory functions that included two-point discrimination, light-touch discrimination, thermal sensation, texture recognition, oral stereognosis and taste recognition. Materials tested within each domain respectively included: (i) the MacKinnon-Dellon Disk-criminator, paperclip and caliper; (ii) the Semmes-Weinstein monofilament and cotton wisp; (iii) dental mirrors and glass test tubes; (iv) spheres of textured acrylic resin on rods; (v) acrylic resin forms with differing shapes on rods and (vi) salty, sweet, sour, bitter and neutral solutions. Materials were tested on 40 healthy subjects between the ages of 20 and 55. The results from this study indicated that thermal, texture and taste sensations appear robust for accuracy and discrimination. Two-point discrimination and light touch seem to be influenced by location of stimulation on the tongue and force applied, whereas stereognosis was influenced by stimulus complexity. The results of this study indicate that clinicians may choose instruments as practical as paperclips and test tubes for testing two-point discrimination and thermal sensation, respectively. For the other sensations, it may be important to use more sophisticated instrumentation to control variables of force, surface area stimulated and assessing sensations in graded steps.

Interhemispheric transfer following callosotomy in humans: Role of the superior colliculus

It is now common knowledge that the total surgical section of the corpus callosum (CC) and of the other forebrain commissures prevents interhemispheric transfer (IT) of a host of mental functions. By contrast, IT of simple sensorimotor functions, although severely delayed, is not abolished, and an important question concerns the pathways subserving this residual IT. To answer this question we assessed visuomotor IT in split-brain patients using the Poffenberger paradigm (PP), that is, a behavioral paradigm in which simple reaction time (RT) to visual stimuli presented to the hemifield ipsilateral to the responding hand is compared to stimuli presented to the contralateral hemifield, a condition requiring an IT. We tested the possibility that the residual IT is mediated by the collicular commissure interconnecting the two sides of the superior colliculus (SC). To this purpose, we used short-wavelength visual stimuli, which in neurophysiological studies in non-human primates have been shown to be undetectable by collicular neurons. We found that, in both totally and partially callosotomised patients, IT was considerably longer with S-cone input than with L-cone input or with achromatic stimuli. This was not the case in healthy participants in whom IT was not affected by color. These data clearly show that the SC plays an important role in IT of sensorimotor information in the absence of the corpus callosum.

The appreciation of wine by sommeliers: a functional magnetic resonance study of sensory integration

We set out to investigate how the expertise of a sommelier is embodied in neural circuitry by comparing brain activity elicited by wine tasting with that found in naive drinkers of wine. We used fMRI to study 7 sommeliers and 7 age- and sex-matched control subjects to test the hypothesis that any difference in brain activity would reflect a learned ability to integrate information from gustatory and olfactory senses with past experience. A group analysis showed activation of a cerebral network involving the left insula and adjoining orbito-frontal cortex in sommeliers. Both these areas have been implicated in gustatory/olfactory integration in primates. In addition, activation was found bilaterally in the dorsolateral prefrontal cortex, which is implicated in high-level cognitive processes such as working memory and selection of behavioral strategies. Naive individuals activated the primary gustatory cortex and brain areas, including the amygdala, implicated in emotional processing.

Functional imaging of gustatory perception and imagery: “top-down” processing of gustatory signals

By recalling gustatory memories, it is possible to generate vivid gustatory perceptions in the absence of gustatory inputs. This gustatory image influences our gustatory processing. However, the mechanism of the "top-down" modulation of gustatory perception in the human is still unclear. Our findings propose a new perspective on the neural basis of gustatory processing. Although gustatory imagery and gustatory perception shared common parts of neural substrates, there was an asymmetrical topography of activation in the insula: the left insula was predominantly activated by gustatory imagery tasks. In addition, the middle and superior frontal gyri were not activated by gustatory perception but they participated in the generation of gustatory hallucinations. These regions in the frontal cortex may mediate the "top-down" control of retrieving gustatory information from the storage of long-term memories.

Dissociation between taste and tactile extinction on the tongue after right brain damage

In patients with right brain damage (RBD) or left brain damage (LBD) and healthy subjects, tactile and three basic gustatory stimuli (sour, salty, bitter) were applied to the left or right hemitongues or to both hemitongues simultaneously. Tactile stimuli were detected and localized by verbal report, whereas gustatory stimuli were identified by pointing to the corresponding name on cards bearing the names of the three tastes. In the tactile test, 9 of 18 RBD patients showed extinction of left hemitongue stimuli, whereas the remaining RBD patients, 9 LBD patients and 14 healthy subjects detected virtually all stimuli in all conditions. In the gustatory test, healthy subjects outperformed the two brain damaged groups which nevertheless responded well above chance and did not differ from one another. Unexpectedly, the nine RBD patients with left hemitongue tactile extinction showed no gustatory extinction, since performance did not differ significantly between the two hemitongues on both unilateral and bilateral stimulations. To account for these findings, some evidence suggests that the tongue representation is bilateral in both modalities, but predominantly ipsilateral in the gustatory modality and predominantly contralateral in the tactile modality. The RBD patients with left hemitongue tactile extinction were those with more marked symptoms of left-sided extinction in the visual and auditory modalities, making it likely that their brain damage was also responsible for left lingual tactile extinction. The absence of left gustatory extinction in those patients can be attributed to the predominant channelling of left hemitongue taste inputs into the intact left hemisphere.

Redundancy gain in simple reaction time following partial and complete callosotomy

Four subjects with partial or complete section of the corpus callosum were tested on simple reaction time (RT) to visual stimuli presented either singly in one or other visual field, or simultaneously in both visual fields. The subject with posterior callosal section showed evidence of redundancy gain with bilateral stimuli beyond that attributable to probability summation ("enhanced" redundancy gain), and prolonged interhemispheric transfer. One of the two subjects with anterior section, like normals, showed little evidence of enhanced redundancy gain, and no evidence of prolonged interhemispheric transfer. The other did show some enhanced redundancy gain at the fast end of the RT distribution. These and other results suggest that the posterior corpus callosum provides the principal route or routes of interhemispheric transfer of the information required for simple visuomotor responses, and is also responsible for the much reduced redundancy gain in normal subjects relative to that in split-brained subjects. The subject with complete callosal section was unusual in that he responded only very rarely to stimuli in the left visual field (LVF), yet he showed markedly reduced RTs to bilateral relative to right visual field (RVF) stimuli.

Some effects of cortical and callosal damage on conscious and unconscious processing of visual information and other sensory inputs

Although new methods of investigation from the molecular level to cognition are promoting major advances in the study of the functions of the human brain, the analysis of behavioral and psychological deficits following brain damage is still a major tool for the understanding of cerebral organization. The present paper reviews some aspects of work on functional losses and residual abilities following cortical damage that have allowed to distinguish conscious and unconscious levels of visual input processing. Attention is given to the possible contribution of residual conscious vision of color to unconscious form analysis in visual agnosia. The paper also reviews findings on temporary and permanent deficits that occur after selective lesions of a prominent input-output system of the cerebral cortex, the corpus callosum, with the aim of assessing the possibility of establishing a functional callosal topography.

Line Bisection in the Split Brain

The authors examined line bisection in 4 patients with resection of the corpus callosum and in 22 control participants. The control participants showed a leftward bias, especially with the left hand, implying right-hemispheric dominance in spatial attention. Two patients with anterior callosotomy showed similar biases, suggesting that the anterior callosum plays only a small role. A patient with complete callosotomy showed a strong right bias, regardless of hand use. A patient with posterior callosotomy showed the opposite pattern: a strong left bias, regardless of hand use. These data suggest that the posterior corpus callosum normally plays a role in line bisection and that the resection of the posterior corpus callosum produces consistent bias. The direction of the bias depends on which hemisphere assumes control.