Bilateral Stimulus Presentation Research Articles

Human olfactory lateralization requires trigeminal activation

Rats are able to lateralize odors. This ability involves specialized neurons in the orbitofrontal cortex which are able to process the left, right and bilateral presentation of stimuli. However, it is not clear whether this function is preserved in humans. Humans are in general not able to differentiate whether a selective olfactory stimulant has been applied to the left or right nostril; however exceptions have been reported.Following a screening of 152 individuals with an olfactory lateralization test, we identified 19 who could lateralize odors above chance level. 15 of these “lateralizers” underwent olfactory fMRI scanning in a block design and were compared to 15 controls matched for age and sex distribution. As a result, both groups showed comparable activation of olfactory eloquent brain areas. However, subjects with lateralization ability had a significantly enhanced activation of cerebral trigeminal processing areas (somatosensory cortex, intraparietal sulcus). In contrast to controls, lateralizers furthermore exhibited no suppression in the area of the trigeminal principal sensory nucleus. An exploratory study with an olfactory change detection paradigm furthermore showed that lateralizers oriented faster towards changes in the olfactory environment.Taken together, our study suggests that the trigeminal system is activated to a higher degree by the odorous stimuli in the group of “lateralizers”. We conclude that humans are not able to lateralize odors based on the olfactory input alone, but vary in the degree to which the trigeminal system is recruited.

Behavioral evidence for inter-hemispheric cooperation during a lexical decision task: a divided visual field experiment

HIGHLIGHTS The redundant bilateral visual presentation of verbal stimuli decreases asymmetry and increases the cooperation between the two hemispheres.The increased cooperation between the hemispheres is related to semantic information during lexical processing.The inter-hemispheric interaction is represented by both inhibition and cooperation.This study explores inter-hemispheric interaction (IHI) during a lexical decision task by using a behavioral approach, the bilateral presentation of stimuli within a divided visual field experiment. Previous studies have shown that compared to unilateral presentation, the bilateral redundant (BR) presentation decreases the inter-hemispheric asymmetry and facilitates the cooperation between hemispheres. However, it is still poorly understood which type of information facilitates this cooperation. In the present study, verbal stimuli were presented unilaterally (left or right visual hemi-field successively) and bilaterally (left and right visual hemi-field simultaneously). Moreover, during the bilateral presentation of stimuli, we manipulated the relationship between target and distractors in order to specify the type of information which modulates the IHI. Thus, three types of information were manipulated: perceptual, semantic, and decisional, respectively named pre-lexical, lexical and post-lexical processing. Our results revealed left hemisphere (LH) lateralization during the lexical decision task. In terms of inter-hemisphere interaction, the perceptual and decision-making information increased the inter-hemispheric asymmetry, suggesting the inhibition of one hemisphere upon the other. In contrast, semantic information decreased the inter-hemispheric asymmetry, suggesting cooperation between the hemispheres. We discussed our results according to current models of IHI and concluded that cerebral hemispheres interact and communicate according to various excitatory and inhibitory mechanisms, all which depend on specific processes and various levels of word processing.

Bilateral stimulus presentation in memory scanning.

Abstract Participants performed a memory-scanning task (Steinberg, 1966, 1969a) in which probe letters were displayed unilaterally or bilaterally after sets of two, four, or six letters were memorized. The mean response time (RT) to bilateral presentations was significantly longer than the mean RT to unilateral presentations, but the slope of the set-size function was not affected, suggesting that presenting stimuli bilaterally affected stages other than memory scanning. There were no significant visual-field effects in either the bilateral or unilateral conditions, suggesting that memory scanning is not a lateralized process. There was no evidence that bilateral presentation increased visual-field differences. This is not consistent with Boles' (1983, 1990) hypothesis that visual-field asymmetry effects are more pronounced with bilateral than with unilateral presentation of stimuli. Two methods are often used to assess visual-field differences in perception. One is the unilateral method, which involves presenting a single stimulus either in the left or the right visual field. A less frequently used method is bilateral presentation of stimuli, which involves presenting two different stimuli, one in each visual field, with an indication to the subject as to which stimulus to respond to. A response time (RT) or accuracy advantage for a visual field is then taken to indicate contralateral hemispheric specialization for the task. Boles (1983, 1990) has claimed that bilateral presentations of stimuli produce visual-field asymmetries larger than those of unilateral presentations. His explanation for this effect is that bilateral stimulation effectively disrupts communication between the two hemispheres, preventing information transfer. If information cannot be transferred between the hemispheres, each hemisphere's capability can be assessed more accurately, and any hemispheric differences would be accentuated. In the experiment reported here, subjects performed a memory-scanning task (Sternberg, 1966, 1969a) in which probe stimuli were presented both unilaterally and bilaterally. In a typical memory-scanning task, subjects memorize a set of between one and seven items and then decide whether a probe item is in the memorized list. Sternberg found that RT increased linearly with the number of items in the set and that the slope of this function was the same for Yes and No responses, which was taken to suggest that subjects perform an exhaustive, serial memory scan. Sternberg (1966) elaborated his memory-scanning theory by differentiating among four stages of processing: stimulus encoding, memory scanning, response selection, and response execution. The zero-intercept of the linear set size function is a measure of the total duration of all processes that occur just once, regardless of the size of the memory set -- that is, stimulus encoding, response selection, and execution. The slope, on the other hand, measures the rate at which members of the memory set are scanned. Introducing a new factor in a memory-scanning experiment could influence the RT function in two ways. It could influence the memory-scanning stage only, reflected in a change in slope and not in the zero-intercept. Alternatively, it might influence the stimulus encoding, response selection or execution stages, altering the zero-intercept and not the slope. The aim of the present experiment was to investigate the effect of bilateral stimulus presentation on memory scanning. Two different aspects were of interest. The first had to do with the effects that bilateral as opposed to unilateral presentation has on the different stages of a memory-scanning task. If bilateral and unilateral presentation have different effects on the memory-scanning stage, it would be reflected in a difference in slopes for unilateral and bilateral presentations. Alternatively, if presenting stimuli bilaterally only had an effect on stages other than memory scanning, there would be only an intercept difference between unilateral and bilateral presentations. …

An event-related potential examination of attended and unattended stimuli in visual selection using bilateral stimulus presentation

The experiment reported here examined an interference paradigm using a bilateral stimulus presentation in which stimuli were presented simultaneously in the left and right visual fields. The lateralization of the early visual components allowed an ERP examination of material presented in each field. Attention was directed to one field or the other on each trial by a 100% valid cue. Two letters were nominated as targets and the simultaneous presentation allowed presentation of material compatible, incompatible or neutral with reference to the target. A negative peak was observed at 230 ms post stimulus at occipital and temporal sites. There was a variation in this peak for unattended stimuli, with compatible and incompatible target letters being significantly different to non-target letters. Contrary to previous research, this finding suggests that material is not filtered out at an early stage as proposed by early selection. It was found that the response to unattended target material also varied according to the type of item presented at the attended location. These findings were discussed in relation to previous studies which found no difference in the processing of unattended target and non-target material, and also in relation to suggestions that automatic processing of unattended material occurs only when there is controlled processing occurring simultaneously.

Categorization versus distance: hemispheric differences for processing spatial information.

It has been hypothesized that the brain computes two different kinds of spatial-relation representations: one used to assign a spatial relation to a category and the other used to specify metric distance with precision. The present visual half-field experiment offers support for this distinction by showing that the left and right cerebral hemispheres make more effective use of the categorization and metric distance representations, respectively. Furthermore, the inclusion of a bilateral stimulus presentation condition permits the computation of a reversed association that offers additional support for the distinction between two types of spatial-relation representation.

Relation of binaural interaction and spectro-temporal characteristics in the auditory midbrain of the grassfrog.

The relation between binaural interaction type and spectro-temporal characteristics was studied for single units in the auditory midbrain of the grassfrog. Tonal and continuous wideband noise ensembles have been used as stimuli. Spectro-temporal sensitivities were determined for ipsi-, contra- and bilateral stimulus presentation by a closed sound system. Binaural interaction was classified in monaural EO (one ear excitatory), binaural EE (both ears excitatory) and EI (one ear excitatory, the other inhibitory) and purely inhibitory categories. Binaural interaction appeared to be rather invariant to alterations in stimulus intensity and type. A very clear correlation was observed between best frequency and binaural interaction type: EE units are predominantly of high best frequency, whereas EI units are predominantly of low best frequency. The correlation with latency was less significant: EE units tended to have somewhat shorter latencies that EI units. EO units take an intermediate position. Comparisons of ipsi-, contra- and bilateral spectro-temporal sensitivities, revealed differences in best frequency, latency and temporal discharge pattern. In some units a complex interplay of excitatory and inhibitory monaural influences was demonstrated. A number of units was recorded, which were characterized by multiple activation or suppression areas. The majority of these units exhibited frequency-dependent binaural interaction types. In some units it was noticed that binaural interaction type can be dependent on state of adaptation. A comparison of binaural interaction types of neighbouring units provided only weak evidence for a binaural organization in the anuran auditory midbrain, since simultaneously recorded pairs shared the same binaural interaction type only slightly more than expected by mere chance (chi 2-test, P less than 0.10).

Hemispheric dysfunction in schizophrenia: Assessment by visual perception tasks

Before and after a double-blind trial of haloperidol vs. mesoridazine, 24 hospitalized schizophrenics performed visual perception tasks designed to assess function of the cerebral hemispheres. Tasks involved identifying as “same” or “different” two images (either letters, digits, or unfamiliar shapes) projected tachistoscopically to the right or left visual field or to both together. Multivariate analysis of variance related response latency and accuracy to task type, hemisphere stimulated, and pre- vs. posttreatment testing. Both before and after treatment, subjects responded most slowly and least accurately to letter-matching. Bilateral presentation of stimuli resulted in faster and more accurate responses, except on shape- matching. Neuroleptic treatment improved speed and accuracy overall, though not under certain task conditions. Results accorded more with an impairment in verbal processing and interhemispheric coordination than with a specific left-hemispheric deficit in schizophrenia.

The interdependence of time and space in somesthesis: The Tau effect reexamined

The Tau effect, whereby the comparative perception of two spaces is altered by the relationship of the temporal intervals associated with them, was examined in the tactile sense. A bilateral stimulus presentation procedure was employed in which the standard and comparison spatial separations were presented alternately to either forearm. The temporal intervals separating the two stimuli delimiting the standard and comparison spatial separations were manipulated over a wide range. Subjects’ judgments of the comparison spatial separation, relative to the standard separation, were altered substantially by varying the temporal relations between the standard and comparison spaces, that is, the Tau effect was obtained. No systematic or substantial laterality effect was found, that is, varying the position of the standard and comparison spaces on the right or left forearm had only a negligible effect.

Recognition of bilaterally presented words varying in concreteness and frequency: Lateral dominance or sequential processing?

Words presented in the left visual field were more easily recognized than words presented in the right visual field. This effect was independent of frequency and concreteness. While these results contradict those reported by (Ellis and Shepherd (1974), they are in accord with the results of a majority of studies using bilateral stimulus presentation. The discussion stressed the importance of learned responses in perceptual processing and cautioned against attributing functional distinctions to the cerebral hemispheres.

Interhemispheric interaction during simultaneous bilateral presentation of letters or digits in commissurotomized patients

Interhemispheric interaction after forebrain commissurotomy was studied in six patients. Letters or digits were flashed for 0·1 sec either in the left or in the right visual field alone, or in both fields simultaneously. Patients were asked to identify the stimuli either verbally or by hand. Results showed generally better performance for each hemisphere during unilateral than during bilateral stimulus presentation. For both verbal and manual responses, identification of right-field stimuli was better than that of left-field stimuli, the latter often being completely ignored during bilateral presentations. Some evidence was obtained for right-hemisphere verbalization during unilateral presentation of left-field stimuli.