The asymmetric lateralization of tactile extinction in patients with unilateral cerebral dysfunction

Two hundred and thirty-four patients with unilateral cerebral pathology and 175 control subjects were examined with a sensitive test for tactile extinction. Damage to the right hemisphere was associated with extinction slightly (but not significantly) more often than damage to the left hemisphere; the asymmetry may be due to selective exclusion of aphasics with damage to the left hemisphere. Extinction of the left side of the body, however, was significantly more common than of the right side; this asymmetry could not be accounted for by exclusion of untestable aphasics, but was a consequence of frequent ipsilateral (left side) extinction among the group with damage to the left hemisphere while the group with damage to the right hemisphere extinguished the contralateral (left) side almost exclusively. Although the hemispheres as a whole did not differ in their association with extinction, lesions in the right parietal lobe were significantly more effective in producing extinction than lesions in the left; in both cases the contralateral side of the body was affected. By contrast, lesions in the left frontal lobe were moderately but not significantly more effective in producing extinction than right frontal damage; in almost all these cases the left side of the body was affected, regardless of which frontal lobe was damaged. A relationship between extinction and pathology in the vicinity of the anterior callosum, as determined from CT scan and angiography, was found among the frontal cases. We propose an anatomical model to explain tactile extinction and its asymetric characteristics in the human. During the extinction tests a response mechanism in the left (speech) hemisphere bases its perceptual output on the relative strengths of two simultaneous sensory inputs. Damage at any point in the channel from the periphery to the response mechanism weakens one signal in comparison to the other, resulting in a response bias favouring the stronger stimulus. Tactile information from the left hand, after reaching the somatosensory zone in the right hemisphere, is transmitted to the left hemisphere by a diffuse, widespread network including the frontal lobes and the anterior callosum. This anatomical arrangement renders left-hand information more vulnerable to chance lesions than right-hand information, which has direct access to the response mechanism via a more compact projection system.     

Lateralised repetition priming for featurally and configurally manipulated familiar faces: evidence for differentially lateralised processing mechanisms

Although early research suggested that the right hemisphere was dominant for processing faces, more recent studies have provided evidence for both hemispheres being involved, at least to some extent. In this experiment we examined hemispheric specialisations by using a lateralised repetition-priming paradigm with selectively degraded faces. Configurally degraded prime faces produced negative priming when presented to the left visual field (right hemisphere) and positive priming (facilitation) when presented to the right visual field (left hemisphere). Featurally degraded prime faces produced the opposite pattern of effects: positive priming when presented to the left visual field (right hemisphere) and negative priming when presented to the right visual field (left hemisphere). These results support the proposal that each hemisphere is differentially specialised for processing distinct forms of facial information: the right hemisphere for configural information and the left hemisphere for featural information.     

Age and hemispheric asymmetry in nonverbal tactual memory

The left and the right hand recognition of 120 age-differentiated volunteers (20 to 84 yr) were tested separately in a recurrent tactual recognition task using four non-meaningful wire shapes as targets among a 24-item distractor series. Signal detection measures of tactual memory were found to be progressively lower with age. Young persons of age range 20–29 and 30–39 yr showed a right hemisphere superiority which was considerably reduced in old persons (60–69, and over 70). The results suggested that loss in asymmetry can be ascribed to an age decrement in the ability of the right hemisphere to process tactual memory to which the left hemisphere has access.     

Right hemisphere reading mechanisms in a global alexic patient

We investigated the implicit, or covert, reading ability of a global alexic patient (EA) to help determine the contribution of the right hemisphere to reading. Previous studies of alexic patients with left hemisphere damage have suggested that the ability to derive meaning from printed words that cannot be read out loud may reflect right hemisphere reading mechanisms. Other investigators have argued that residual left hemisphere abilities are sufficient to account for implicit reading and moreover do not require the postulation of a right hemisphere system that has no role in normal reading processes. However, few studies have assessed covert reading in patients with lesions as extensive as the one in EA, which affected left medial, inferior temporal-occipital cortex, hippocampus, splenium, and dorsal white matter. EA was presented with lexical decision, semantic categorization, phonemic categorization, and letter matching tasks. Although EA was unable to access phonology and could not overtly name words or letters, she was nevertheless capable of making lexical and semantic decisions at above chance levels, with an advantage for concrete versus abstract words. Her oral and written spelling were relatively intact, suggesting that orthographic knowledge is retained, although inaccessible through the visual modality. Based on her ability to access lexical and semantic information without contacting phonological representations, we propose that EA's implicit reading emerges from, and is supported, by the right hemisphere. Finally, we conclude that her spelling and writing abilities are supported by left hemisphere mechanisms.     

Representations of facial identity in the left hemisphere require right hemisphere processing

A quintessential example of hemispheric specialization in the human brain is that the right hemisphere is specialized for face perception. However, because the visual system is organized contralaterally, what happens when faces appear in the right visual field and are projected to the nonspecialized left hemisphere? We used divided field presentation and fMRI adaptation to test the hypothesis that the left hemisphere can recognize faces, but only with support from the right hemisphere. Consistent with this hypothesis, facial identity adaptation was observed in the left fusiform face area when a face had previously been processed by the right hemisphere, but not when it had only been processed by the left hemisphere. These results imply that facial identity information is transferred from the right hemisphere to the left hemisphere, and that the left hemisphere can represent facial identity but is less efficient at extracting this information by itself.     

Lack of asymmetrical transfer for linguistic stimuli in schizophrenia: an ERP study.

OBJECTIVE: To assess the mechanisms underlying lack of speeded information transfer asymmetry (faster right to left) for verbal information in schizophrenia. METHODS: Interhemispheric transfer times (IHTT) between the hemispheres were assessed using a lateralized lexical-decision task in males with schizophrenia (N = 12) and matched controls (N = 12). Words were presented to the left visual field (LVF), right visual field (RVF), or bilaterally (BVF) while 128-channel EEG was recorded continuously. A direct measure of IHTT in each direction was obtained by comparing the latencies of the N160 evoked potential (EP) component in the hemispheres contralateral and ipsilateral to stimulation. RESULTS: Controls showed faster information transfer from the right to left hemisphere (R-to-L) for linguistic stimuli. The two groups did not differ for IHTTs L-to-R. Lack of IHTT asymmetry in the schizophrenia groups was associated with an overall concomitant decrease in the amplitude of the N160 in the right hemisphere. CONCLUSIONS: Differences in IHTT asymmetry may be attributed to lack of right hemisphere activation and not callosal dysfunction as has been previously suggested. SIGNIFICANCE: It is suggested that a relative excess of myelinated axons in the right hemisphere speeds IHTT faster R-to-L, findings are discussed with reference to differences in right hemisphere white matter connectivity in schizophrenia.     

The dominant direction of interhemispheric EEG changes in the linguistic process

Hemispheric specialization for the linguistic process was studied by the 'entropy analysis' method which distinguishes the direction of information flow for mutually coupled time series. Meaningful and non-meaningful stimuli were presented to 14 subjects. EEGs were recorded from homologous Wernicke's and occipital areas in the two hemispheres. The amount of information flow both from the left to right hemisphere and from the right to left hemisphere was obtained under the 3 conditions (i.e., meaningful, non-meaningful and resting baseline conditions) in each area. The information flow from the left to right hemisphere was significantly larger under the meaningful condition than from the right to left hemisphere in Wernicke's area, but no significant differences in the information flow could be found between the two directions under the non-meaningful condition in Wernicke's or the occipital area. Therefore, the dominant direction of the left to right hemisphere was observed under the meaningful condition in Wernicke's area but not in the occipital (non-speech) area, while no dominant direction was observed under the two control (i.e., non-meaningful and baseline) conditions in either area. These results may suggest that the dominant direction of information flow with the meaningful stimuli is related to linguistic meaningfulness in Wernicke's area.     

Auditory evoked potentials of adults who do or do not show a significant right ear advantage in dichotic listening

The Halwes Fused Dichotic Words Test was used to divide a sample of university students into a group having a statistically significant right ear advantage (REA) and a group having either a significant left ear advantage or a non-significant ear asymmetry (NREA). Of these participants, 30 (14 REA, 16 NREA) had electrical potentials measured from temporal, central, and frontal sites as series of brief tones were presented monaurally. No behavioural response was required. Group differences were found in the latency but not the amplitude of the averaged event-related responses. The REA group showed faster conduction to the right hemisphere than to the left hemisphere. In both groups the amplitude of left hemisphere responses was greater for right ear stimulation than for left ear stimulation. The results for amplitude indicate that the crossed auditory pathway is a superior conductor of information to the left hemisphere but not to the right hemisphere. Group differences, however, are related only to the speed with which information reaches the right hemisphere.     

Left ear advantages in detecting emotional tones using dichotic listening task in an Arabic sample

Advantages associated with the left ear (right brain hemisphere) have been reported in some studies. Of these, some have specifically suggested that the left ear has a more heightened ability to detect emotional tones. Meanwhile others have pointed to factors such as age and gender as potentially leading to manifestations of human laterality. This study investigates which brain hemisphere is more involved in emotional processing of auditory information in Arab participants. We aimed to replicate the previous studies because no single study has been done in the Arabic region previously. Additionally, people in this region prefer to use the right side of their body, e.g., hand, ear, foot, etc., for most daily tasks. To acquire data a dichotic listening task (DLT) was administered to 28 male and 23 female (Edinburgh, UK) university students aged 19 to 38; 13 were left-handed and 38 were right-handed. The results showed a significant left ear advantage in the auditory processing of emotional information. There was a significant negative correlation between ear preference and handedness. Left ear advantage related only to handedness. Thus right-handed participants were more likely than left-handers to have a left ear advantage. The relationship between ear preference and gender was non-significant. The conclusion that might be drawn from this study is that the left ear (right hemisphere) is more involved in emotional processing than the right ear (left hemisphere), especially for right-handed people.     

A corpus callosal deficit in sequential analysis by schizophrenics

This preliminary study evaluated interhemispheric (i.e., corpus callosal) information processing by chronic schizophrenic and normal subjects. In right-handed normals the left hemisphere has been reported to be superior in temporal sequential analysis. Consequently temporal information presented to the right hemisphere requires time to cross to the left hemisphere for analysis. Measurement of hemispheric laterality and corpus callosal transfer time were evaluated by a two-pulse temporal discrimination task. Two dot stimuli were presented with decreasing temporal separation during bilateral and unilateral conditions. Subjects were required to judge perceived simultaneity and nonsimultaneity of dot onset. The results indicate that left hemisphere function in chronic schizophrenic and normal controls is superior to the right hemisphere in temporal analysis. Corpus callosal transfer time was significantly slower for chronic schizophrenics than for normal controls.     

Hemispheric specialization in the detection of subjective objects

Three experiments were conducted to examine hemispheric specialization for the detection of subjective objects. In the first two experiments, observers searched for the presence of a square defined by subjective contours. The first experiment demonstrated that the left hemisphere made more errors for detecting these objects. The second experiment showed that the increased errors were due to the left hemisphere responding to the individual features of the objects and not the objects as a whole. In the second experiment, the right hemisphere was also faster for detecting the absence of a subjective object. A third experiment was conducted to determine if performance for the right hemisphere was due to object level processing. It was shown that the right hemisphere only makes illusory conjunctions for features within perceptual groups while the left hemisphere makes illusory conjunctions both within and across perceptual groups, providing converging evidence for object level processing in the right hemisphere. The results suggest that the right hemisphere conjoins feature information for the perception of objects.     

Right hemisphere contributions to the comprehension of low-imagery words

A priming experiment, with normal university students as subjects, was used to investigate whether the right cerebral hemisphere contributes to the comprehension of low-imagery words. Each hemisphere's access to semantic representations of low-imagery words was gauged by comparing responses to low-imagery targets preceded by associated low-imagery primes (e.g., BELIEF-IDEAL) with responses to the same targets when they were preceded by unrelated primes (e.g., FATE-IDEAL). All primes and targets were independently projected to the left or right visual fields (LVF or RVF), and temporally separated by a stimulus onset asynchrony of 250 ms. There was a clear RVF advantage in response speed and accuracy measures, confirming the left hemisphere's advantage in processing low-imagery words. Nonetheless, the priming effects provided evidence that the right hemisphere contributes to the comprehension of low-imagery words, as primes projected to the RVF equally facilitated responses to associated targets subsequently appearing in either visual field. In contrast, primes directed to the LVF did not facilitate responses to associated targets projected to the LVF or RVF. The results suggest that low-imagery words projected to the left hemisphere activated low-imagery associates in both hemispheres to an equivalent degree, whereas low-imagery primes directed to the right hemisphere failed to activate low-imagery associates in either hemisphere. Like Kounios and Holcomb's (1994) study of event-related response potentials evoked by abstract and concrete words, the findings indicate that while the left hemisphere is the primary processor of low-imagery/abstract words, the right hemisphere plays a subsidiary role in the comprehension of these words.     

Hemispheric asymmetries in memory processes as measured in a false recognition paradigm

Although memory differs in important ways between the left and right cerebral hemispheres, the nature of these differences remains controversial. We examined this issue in two experiments using a false memory paradigm that allowed novel tests of two theories that have not been assessed in a common paradigm previously. Lists of semantically related words (e.g., bed, rest, wake…), all highly associated to one “critical” word (e.g., sleep), were presented auditorily during a study phase. Memory for both the related words and the critical words was measured in a subsequent old/new recognition test using divided-visual- field word presentations. The most important results were that the ability to correctly reject previously unpresented words was greater when test items were presented to the right visual field/left hemisphere (RVF/LH) than to the left visual field/right hemisphere (LVF/RH) and that participants were more confident in correctly rejecting unpresented words when test items were presented to the RVF/LH than to the LVF/RH. Results were in line with the theory that associative activation of semantic information is restricted in the left hemisphere but diffuse in the right; however, these results contrasted with the theory that memory traces are interpretive in the left hemisphere but veridical in the right. A potential resolution to the seemingly contradictory theories of asymmetries in memory processing is briefly discussed.     

A deficit in perceptual matching in the left hemisphere of a callosotomy patient.

Decades of research have demonstrated dramatic differences between the hemispheres of the brain. While the most obvious asymmetries are in the areas of language and motor control, the visuospatial abilities of the left hemisphere are also known to differ from those of the right hemisphere. This hemispheric difference has been demonstrated empirically but its basis is thus far unclear. In this paper, we investigate the hypothesis that the left hemisphere is capable of sophisticated visual processing, but represents spatial information relatively crudely compared to the right hemisphere. The implication of this hypothesis is that pattern recognition is a function of both hemispheres but the right hemisphere is further specialized for processing spatial information. In a series of seven experiments we examined perceptual matching of mirror-reversed stimuli by the divided cerebral hemispheres of a callosotomy patient. In each experiment the left hemisphere's performance was impaired relative to the right hemisphere. This finding was independent of stimulus type, response bias and stimulus duration. These results are consistent with the idea that visual processing in the left hemisphere is directed towards pattern recognition at the expense of spatial information.     

Speech without conscious awareness

Following commissurotomy, it is usually the case that information presented to the left hemisphere can be named and described, while information presented to the mute, right hemisphere cannot be spoken about. In the present study, it was discovered that under special test conditions, an MRI-verified, callosally sectioned adult could name or write about information presented to the right hemisphere. Studies revealed this was not an instance of right hemisphere expression. Rather, the right hemisphere was somehow able to program the left hemisphere for a spoken or written response. Further, the studies also showed that the left hemisphere was not consciously aware that it possessed the information transmitted to it by the right half brain.     

Right words and left words: electrophysiological evidence for hemispheric differences in meaning processing.

Both cerebral hemispheres are involved in language processing, each playing a unique role that may derive from differences in knowledge organization and on-line meaning integration. Here, we examine lateralized differences in knowledge representation and retrieval using event-related potentials (ERPs) elicited by words in sentences. Volunteers read pairs of sentences ending with three target types: (1) expected words, (2) unexpected words from the expected semantic category, and (3) unexpected words from an unexpected category. Context was presented word by word at fixation while targets were presented two degrees to the right or left of fixation. ERPs to unexpected endings were more negative than those to expected endings in both visual fields. However, when presented to the right visual field (left hemisphere), unexpected items from the expected category elicited smaller N400s than those from an unexpected category. In contrast, when presented to the left visual field (right hemisphere) all unexpected endings elicited N400s of similar amplitude. Thus, while both hemispheres are sensitive to context, only the left hemisphere is sensitive to semantic similarity between an unexpected ending and the expected completion. The results suggest lateralized differences in how new information is integrated into sentences. We propose that right hemisphere processing is best characterized as 'integrative'; new information is compared directly with context information. In contrast, left hemisphere processing is better characterized as 'predictive'; the processing of context leads to an expectation about the semantic features of upcoming items and new information is compared with that expectation rather than directly with the context.     

Hemisphere memory of concrete and abstract information determined with the intracarotid Sodium Amytal test

An extended version of the bilateral intracarotid Sodium Amytal test was administered pre-operatively to 13 patients with intractable complex partial epileptic seizures, to determine cerebral hemisphere speech and memory. There were 6 patients with left temporal lobe lesions and 7 patients with right temporal lobe lesions. Amobarbital (175 mg, 10%), injected on 2 occasions, determined the left hemisphere to be speech dominant in all cases. Memory assessed with recall, cued recall, and recognition of concrete/abstract words and pictures, was studied on 3 occasions: in a baseline test considered to indicate the degree of patient cooperation and the bilateral hemisphere memory; in a right hemisphere Amytal test; and in a left hemisphere Amytal test. The specific data pattern obtained, that abstract pictorial information is most efficiently processed by an intact right hemisphere and that verbal information is processed best by an intact left hemisphere, demonstrates the reliability of the approach taken here to present abstract and concrete to be remembered information.     

The dynamic nature of language lateralization: effects of lexical and prosodic factors

In dichotic listening, a right ear advantage for linguistic tasks reflects left hemisphere specialization, and a left ear advantage for prosodic tasks reflects right hemisphere specialization. Three experiments used a response hand manipulation with a dichotic listening task to distinguish between direct access (relative specialization) and callosal relay (absolute specialization) explanations of perceptual asymmetries for linguistic and prosodic processing. Experiment 1 found evidence for direct access in linguistic processing and callosal relay in prosodic processing. Direct access for linguistic processing was found to depend on lexical status (Experiment 2) and affective prosody (Experiment 3). Results are interpreted in terms of a dynamic model of hemispheric specialization in which right hemisphere contributions to linguistic processing emerge when stimuli are words, and when they are spoken with affective prosody.     

Picture the difference: electrophysiological investigations of picture processing in the two cerebral hemispheres

The nature of semantic memory and the role of the two cerebral hemispheres in meaning processing were examined using event-related brain potentials (ERPs) elicited by pictures in sentences. Participants read sentence pairs ending with the lateralized presentation of three target types: (1) expected pictures, (2) unexpected pictures from the expected semantic category, and (3) unexpected pictures from an unexpected category. ERPs to contextually unexpected pictures were more negative 350-500ms (larger N400s) than those to expected pictures in both visual fields. However, while N400s to the two types of unexpected items did not differ with left visual field presentations, they were smaller to the unexpected items from the expected category with right visual field presentations. This pattern, previously observed to words [Brain Language 62 (1998) 149], suggests general differences in how the two hemispheres use context on-line. Other aspects of the N400 response-and effects on earlier ERP components-reveal differences between pictures and words, suggesting that semantic memory access is not modality-independent. The P2 component varied with ending type for right but not left visual field presentations, suggesting that the left hemisphere may use contextual information to prepare for the visual analysis of upcoming stimuli. Furthermore, there was clear evidence for an earlier negativity ("N300"), which varied with ending type but, unlike the N400, was unaffected by visual field of presentation. Overall, the results support our hypothesis that the left hemisphere actively uses top-down information to preactivate perceptual and semantic features of upcoming stimuli, while the right hemisphere adopts a "wait and see" integrative approach.     

Evidence of interhemispheric transmission in laterality effects

The study was aimed at testing various models that can explain visual lateral asymmetries due to hemispheric specialization. In Experiments 1-3 the subjects had to perform a lateralized "go-no go" discrimination of words (primary task) either alone or in association with secondary tasks that interfered with the processing of the left hemisphere (ordered tapping) or the right hemisphere (finger flexion). In Experiment 4 the primary task was one of lateralized "go-no go" discrimination of faces while the secondary tasks were again those of ordered tapping and finger flexion. The results showed that in the case of word discrimination the advantage in speed of response in favour of the right visual field/left hemisphere (RVF/LH), which was observed for the primary task alone, did not change when the secondary task was added. This held true irrespective of whether the secondary task loaded the left or right hemisphere. The advantage for the left visual field/right hemisphere (LVF/RH) observed for face discrimination alone, disappeared when the secondary task interfered with the processing of the right hemisphere and did not change when the secondary task concerned the left hemisphere. It was concluded that each hemisphere is able to elaborate in parallel the incoming information, but, in normal conditions, interhemispheric transmission is responsible for the lateral asymmetries in perception (conditional interhemispheric transmission model).