Coding strategies and cerebral laterality effects

In a short-term recognition memory task, Ss were given relational imagery and rehearsal coding strategies in different sessions, with probes presented to the left or right cerebral hemisphere. Consistent with a model of separate processing systems for verbally and visually coded information, Ss yielded significantly faster response latencies for probes to the left hemisphere than the right when employing the rehearsal strategy, and significantly faster latencies for probes to the right hemisphere than the left when using the imagery code. This suggests that cerebral laterality effects are functionally related to coding strategies, and argues for the inclusion of imagery, or generated visual information, as part of the visual processing system. As such, generated visual information may be viewed as a coding alternative to verbal mediation.     

Effects of a concurrent memory task on hemispheric asymmetries in categorization

Recent research on the division of processing between the two cerebral hemispheres has often employed two concurrent tasks to investigate the dynamic nature of hemispheric asymmetries. The experiment reported here explored the effects of two concurrent high-level cognitive tasks (memory retention and semantic categorization) on the direction and magnitude of hemispheric differences in the processing of words and pictures. Subjects were required to categorize words and pictures presented to either the left visual field-right hemisphere (LVF-RH) or the right visual field-left hemisphere (RVF-LH). The categorization could be performed while holding either verbal material in memory (digit span), pictorial material in memory (serial nonsense figure recognition), or with no concurrent memory task. The effects produced hemisphere-specific, material-nonspecific interference. The verbal task removed a RVF-LH advantage at word categorization and enhanced a LVF-RH advantage on picture categorization; the pictorial task interfered with picture categorization in the LVF-RH, while enhancing a RVF-LH advantage at word categorization. The results are discussed in terms of multiple resource models of hemisphere function, capacity limitations, and the functional locus of processing required to produce various dynamic hemispheric effects.     

Hemispheric processing asymmetries: implications for memory

Recent research has demonstrated that memory for words elicits left hemisphere activation, faces right hemisphere activation, and nameable objects bilateral activation. This pattern of results was attributed to dual coding of information, with the left hemisphere employing a verbal code and the right a nonverbal code. Nameable objects can be encoded either verbally or nonverbally and this accounts for their bilateral activation. We investigated this hypothesis in a callosotomy patient. Consistent with dual coding, the left hemisphere was superior to the right in memory for words, whereas the right was superior for faces. Contrary to prediction, performance on nameable pictures was not equivalent in the two hemispheres, but rather resulted in a right hemisphere superiority. In addition, memory for pictures was significantly better than for either words or faces. These findings suggest that the dual code hypothesis is an oversimplification of the processing capabilities of the two hemispheres.     

Writing induces a right hemisphere linguistic advantage in dysphonetic dyslexic children: implications for attention and capacity models of laterality

An experiment demonstrated a complete hemispheric processing reversal in 10 male, dysphonetic dyslexic children that occurred during a dichotic listening test of their verbal working memory. Requiring a written response to dichotic digits produced a right hemisphere/left ear superiority in the dysphonetic dyslexics whereas normal subjects and other dyslexics maintained a left hemisphere/right ear advantage. This reversal was unaffected by changes in task difficulty. A second experiment assessed the influence on producing the reversal of concurrent manual interference with left hemisphere verbal processing (responding orally vs. manually) and selective right hemisphere priming (Forward Writing vs. Backward Writing). The dysphonetic children reverted to a strong left hemisphere superiority when recalling the dichotic digits orally. Backward writing produced no ear advantage in either direction. The findings suggest that dysphonetic dyslexia may be related to (1) left hemisphere processing demands that exceed capacity, (2) easily activated right hemisphere processing strategies and (3) failure to coordinate linguistic processing interhemispherically. The results supported a novel hybrid conceptualization of dyslexia consisting of a synthesis of selective activation, and dual processor-limited capacity, theories.     

Effects of concurrent verbal memory on recognition of stimuli from the left and right visual fields.

Two experiments examined the effect of concurrently holding 0, 2, 4, or 6 nouns in memory on the recognition of visual stimuli briefly presented to the left or right visual fields. When stimuli to be visually recognized were complex visuospatial forms it was found that a relatively easy memroy load of 2 or 4 nouns improved visual recognition accuracy on right visual field (left-hemisphere) trials relative to the no-memory condition; however, a more difficult memory load of 6 nouns decreased visual recognition accuracy to a level slightly below the no-memory condition. There were no effects of concurrent verbal memroy on visual form recognition on left visual field (right-hemisphere) trials. When the stimuli to be visually recognized were words it was found that a relatively easy memroy load of 2 or 4 nouns improved visual recognition accuracy and a more difficult load of 6 nouns decreased visual recognition accuracy on both left and right visual field trials. The complete pattern of results indicates that several factors including cerebral hemisphere specialization, stimulus codability, selective perceptual orientation, and selective cerebral hemisphere interference interact in systematic ways to produce overall visual laterality effects.     

Effects of concurrent spatial and verbal memory loads on serial position functions of laterally presented letter strings

In this study the influence of visuospatial and verbal memory loads on the identification of laterally presented letter strings was investigated. In the asymmetry task without concurrent loads, a clear right visual field advantage for letter identification was obtained. Hemisphere-specific effects due to concurrent loads were particularly observed when the difficulty of the load tasks was increased. The effects of visuospatial loads were found to be sex related, suggesting that under heavy load conditions mental rotation selectively overloads the processing capacity of the right hemisphere in males, while in females capacity limitations were observed in both hemispheres. Concurrent visuospatial loads produced more facilitation (or less interference) for letters in the outermost positions of each visual field than for letters in the innermost positions of each visual field. The results of the verbal memory load tasks revealed that an easy verbal load task facilitated performance which was particularly manifest for the right-most letter of both the left visual field and the right visual field. A difficult verbal memory load task interfered with recognition accuracy of letters which was most marked for the center letter in the right visual field. Letter position effects obtained in this study were interpreted in terms of various processing mechanisms influencing the serial position curves.     

Recognition memory as a function of encoding strategy and stimulus codability.

The primary purpose of this research was to determine how imaginal and verbal encoding strategies intereact with various stimulus characteristics to either enhance or retard recognition; the secondary purpose of these studies was to test the conceptual coding hypothesis of Ellis, 1972. A between-groups multivariate factorial analysis of covariance experiment and a within-subjects multivariate factorial analysis of variance experiment were conducted. In Experiment 1 it was found that low-codability shapes were better recognized under the verbal encoding set rather than the imaginal encoding verbal encoding sets; and high-codability shapes were not better recognized than low-codability shapes. However, in Experiment 2, where instructional set was a within-subjects factor, it was found that low-codability shapes were not better recognized under the verbal encoding set than the imaginal encoding set.     

Hemispheric asymmetry in memory search for four-letter names and human faces

Two memory search experiments were conducted using vertically oriented four-letter names and human faces as stimuli. Subjects were required to indicate as quickly and as accurately as possible whether or not a single probe stimulus (presented for 150 msec to either the left or right visual field) was contained in a set of 2, 3, 4, or 5 items being held in short-term memory. The probe stimuli were presented alone (clear condition) or centrally embedded in a matrix of dots (degraded condition). In Experiment 1 (involving names), a right visual field/left hemisphere advantage was obtained and pinpointed at the encoding stage rather than at the memory comparison stage of the information-processing system. For Experiment 2 (involving human faces), no hemispheric advantage was readily observed. In each experiment, both the left hemisphere and the right hemisphere employed an abstract memory comparison operation from which the effects of probe degradation have been removed. These results are discussed in terms of their implications for various models of hemispheric asymmetry.     

Deficits in temporal sequencing of verbal material: The effect of laterality of lesion

Temporal sequencing of verbal materials (digits, words, and geometric forms) presented in two sensory modalities (auditory and visual) to three groups of subjects (Broca's with left anterior lesions, patients with right hemisphere lesions, and normals) was examined. Each subject was asked to point to a set of stimuli in the same sequence as presented by the examiner. Results indicated that patients with left hemisphere lesions were more impaired on all tasks than the right hemisphere lesioned patients who, in turn, were impaired compared to normal controls. Response to auditory presentation was superior to response to visual presentation. Also, digits were the easiest for all groups, and words were easier than geometric forms. Of special interest was the finding which suggested that right hemisphere lesions are associated with impairment of verbal temporal sequencing under either auditory or visual presentation.     

Memory deficits during electrical stimulation of the speech cortex in conscious man

Cerebral zones supporting language and recent memory were mapped by electrical stimulation during neurosurgical treatment of epileptic patients. Stimulation of sites within the left posterior temporo-parietal cortex produced transient dysphasia. Continued stimulation of this cortical region also produced a retrograde type of verbal memory disorder, indicating a failure in the mechanism responsible for retrieval of stored information. In contrast, stimulation of the anterior temporal neocortex did not produce anomia and, instead, resulted in an anterograde memory loss, apparently caused by a defect in the verbal storage mechanism. Comparable stimulation of homologous areas on the right hemisphere did not interfere with object naming or immediate verbal recall.     

Testing conclusions from functional imaging of working memory with data from acute stroke

Functional imaging studies indicate that the left hemisphere mediates verbal working memory, while the right hemisphere mediates both verbal and spatial working memory. We evaluated acute stroke patients with working memory tests and imaging to identify whether unilateral dysfunction causes deficits in spatial and/or verbal working memory deficits. While left cortical stroke patients had verbal working memory impairments (p<0.003), right cortical stroke patients had both verbal p<0.007) and spatial working memory (p<0.03) impairments, confirming functional imaging results. Patients with transient ischemic stroke and patients with non-cortical stroke did not have significant deficits in working memory in either modality.     

Emotional valence and arousal effects on memory and hemispheric asymmetries

This study examined predictions based upon the right hemisphere (RH) model, the valence–arousal model, and a recently proposed integrated model (Killgore & Yurgelun-Todd, 2007) of emotion processing by testing immediate recall and recognition memory for positive, negative, and neutral verbal stimuli among 35 right-handed women. Building upon methodologies of previous studies, we found that words presented to the right visual field/left hemisphere (RVF/LH) were recalled and recognized more accurately than words presented to the left visual field/right hemisphere (LVF/RH), and we found significant valence by visual field interactions. Some findings were consistent with one of the models evaluated whereas others were consistent with none of the models evaluated. Our findings suggest that an integration of the RH and valence–arousal models may best account for the findings with regard to hemispheric lateralization of memory for emotional stimuli.     

Right-hemispheric damage and verbal problem solving behavior.

Patients with right-hemisphere damage, who ostensibly have no linguistic impairment, are relatively incapable of solving two-term series problems in which comparative adjectives in the premise and question are antonymic. This finding suggests that such verbal reasoning depends, in part, upon nonlinguistic imaginal processes subserved by the right hemisphere. In this manner, the right hemisphere is often required for the full elaboration of linguistic input.     

An examination of relationship between subliminal perception, visual information processing, levels of processing, and hemispheric asymmetries

A sublimiinal letter was exposed to the left or right hemisphere for either 15 or 30 msec. Subjective guesses were more accurate for visuo-spatial positional recognition made to presentations in the right hemisphere whereas verbal recognition was more accurate to presentations in the left hemisphere. The 30-msec. exposure increased the accuracy of the guesses. These findings were discussed in terms of differential triggering mechanisms for levels of hemispheric processing.     

Task relevant effects on the assessment of cerebral specialization for facial emotion

Hemispheric specialization for processing different types of rapidly exposed stimuli was examined in a forced choice reaction time task. Four conditions of recognition were included: tacial emotion, neutral faces, emotional words, and neutral words. Only the facial emotion condition produced a significant visual field advantage (in favor of the left visual field), but this condition did not differ significantly from the neutral face condition's left visual field superiority. The verbal conditions produced significantly decreased latencies with RVF presentation, while the LVF presentation was associated with decreased latencies on the facial conditions. These results suggested that facial recognition and affective processing cannot be separated as independent factors generating right hemisphere superiority for facial emotion perception, and that task parameters (verbal vs. nonverbal) are important influences upon effects in studies of cerebral specialization.     

Processing persuasive arguments: recall and recognition as a function of agreement and manipulated activation asymmetry

Previous research has demonstrated greater attention toward disagree arguments and a greater persuasibility during relative right than during relative left hemisphere activation. This experiment was performed to conceptually replicate and extend these findings to memory stages in the processing of persuasive arguments. The experiment used induction of lateral orientation of attention as the means to selectively activate regions of the brain in normal right-handed subjects. The results showed the expected significant interaction for correct recall as a function of agreement with the argument and hemisphere activated. There was greater recall of agree arguments during left hemisphere activation and greater recall of disagree arguments during right hemisphere activation, as predicted. The effect for recognition memory was in the same direction, but was statistically nonsignificant. These findings support previous research on cerebral asymmetries in the processing of persuasive arguments. This phenomenon is discussed as a possible outcome of asymmetries of affect.     

Lateral specialization and social-verbal development in preschool children

Forty-two 2 1/2- to 5 1/2-year-old children's social and verbal behaviors were observed during free play in a preschool. A test measuring lateral specialization of verbal function and a standardized psychometric test of verbal ability were also administered. Analysis of variance indicated that the right ear (left hemisphere) is predominant in processing verbal stimuli in children as young as 2 1/2. Multiple regression analyses revealed significant relations between the right ear accuracy score for dichotically presented verbal stimuli and both psychometrically measured verbal ability and a social-verbal factor score derived from play behavior. After the increase related to age was statistically partialled out from both verbal ability and social-verbal scores, verbal expression, length of verbal utterances, time spent in conversation, and peer social interactions increased and parallel play decreased as a function of right ear (left hemisphere) accuracy for verbal stimuli. The relationship between left ear (right hemisphere) accuracy scores for verbal stimuli and social-verbal behavior, however, was not linear. Very high and very low levels of left ear recall predicted an increase in the frequency of parallel play and low social-verbal behavior while moderate levels of left ear accuracy scores predicted the reverse.     

Right hemisphere comprehension of verbs in patients with complete forebrain commissurotomy: Use of the dichotic method and manual performance

The paradigm of dichotic listening was used to investigate verbal comprehension in the right, so-called “nonverbal,” hemisphere. Verbal commands were presented to the right and left ears in the simultaneous (dichotic) paradigm. There were striking instances, especially when the left hemisphere was occupied with some extraneous task, in which the right hemisphere understood the verbal command and executed the appropriate motor responses. In those instances the left hemisphere gave no overt response. Although the left hemisphere was usually dominant, it can be nevertheless concluded that not only can the right hemisphere understand verbal commands but can also express itself manually by executing actions more complex than object retrieval or pointing. As has been known for some time, the blockage of the ipsilateral pathway seems so complete during dichotic listening in the commissurotomy patient that there is no report of the words in the left ear—only of those presented to the right. At the same time there is normal report when words are presented to the left ear alone. It was found in the present study, however, that this model is too simple and only applies to the verbal response paradigm of dichotic listening. Under circumstances of dichotic presentation where the stimulus in the left ear (ipsilateral pathway) is necessary or important to the left hemisphere for completing a task, words from both pathways are reported. One may conclude that there exists a gating mechanism in each hemisphere that controls the monitoring of each auditory pathway and the degree of ipsilateral suppression.     

Factor Analytic Studies Of Human Brain Damage: I. First And Second-Order Factors And Their Brain Correlates

This report is concerned with identifying the first and second-order cognitive factors underlying a battery of 49 measures taken from 22 brain damage tests. The test scores from 176 brain damaged patients between 16 and 65 years of age were intercorrelated and subjected to first-order alpha factoring followed by promax rotation to oblique simple structure. Ten of the 13 first-order factors extracted were interpretable, six of them being perceptual in nature and four being of a more conceptual nature. The perceptual factors include: perceptual organization, perceptual-motor speed, pattern recognition, temporal resolution, spatial orientation, and figure-ground identification. The conceptual factors include verbal comprehension, memory, and two abstraction factors. A second-order alpha factoring was performed on the matrix of correlations among the 13 primaries. Three of the five second-order factors extracted were interpretable. They were identified as perceptual integration (subsuming the first-order factors of perceptual organization, perceptual-motor speed, and temporal resolution), verbal memory (subsuming verbal comprehension and memory), and visualization (subsuming spatial orientation and figure-ground identification). Although factor interpretations were based primarily on the patterns of high loading variables, they were also influenced by lesion effects observed in this and related studies. About half the interpretable factors are relatively localized (i.e., confined to one or two lobes of one hemisphere), with the other half more diffuse (i.e., multi-lobed, combined with laterality or bilaterality). The more localized factors include the right hemisphere factors of perceptual-motor speed, temporal resolution, and spatial orientation, and the left hemisphere factors of verbal comprehension, memory, and verbal memory. The more neurally diffuse factors include the second-order factors and such broad gauged first-order factors as abstraction I and II, and pattern recognition. Furthermore, same lobe, bilaterally hemispheric effects were rare, and only four factors (memory, verbal memory, visualization, and abstraction I) were correlated with sub-cortical lesions.