Cortical and subcortical contributions to the representation of temporal information

Converging evidence suggests that temporal representations of brief durations are derived subcortically. We tested split-brain patient JW in order to investigate whether these representations project bilaterally or unilaterally to cortex. Using visual stimuli to signal time intervals, JW was asked to compare the duration of a pair of standard stimuli that were presented bilaterally with a comparison stimulus that was presented to either the left or right visual field. Assuming the hand of response is controlled by the contralateral cerebral hemisphere, a hand by visual field interaction was predicted if the representation of stimulus duration was restricted to the cerebral hemisphere receiving the lateralized stimulus. However, we failed to observe this interaction for two different ranges of stimulus durations, both in the hundred (Experiment 2) to hundreds (Experiment 1) of milliseconds range. Instead, there was a consistent right hemisphere advantage in task performance. When the task then required a discrimination based on the physical size of the stimuli rather than their duration, an interaction between response hand and visual field was obtained (Experiment 3). Taken together, these results suggest that (1) even though the comparison stimulus was presented unilaterally, the representation of its duration was available to both cerebral hemispheres, and (2) a right hemisphere advantage in psychophysical tasks requiring the comparison of successive stimuli is observed for temporal and non-temporal judgments.     

Associative priming in the hemispheres as a function of SOA

The representation of associative codes in the cerebral hemisphere was investigated in two priming experiments where associated prime and target words were independently projected to the left or right visual fields. The first experiment, using a stimulus onset asynchrony of 250 msec, found priming in all visual field conditions, except that in which both prime and target were projected to the right hemisphere. A second experiment was conducted to determine whether this absence of right hemisphere priming was due to an inadequate interval of time between prime and target. In this experiment, a stimulus onset asynchrony of 450 msec revealed significant priming in all visual field conditions. The results suggest that lexical representations are activated more slowly in the right hemisphere than in the left.     

Semantic category effects modulate visual priming in neglect patients

Previous studies indicate that extinguished stimuli can still be unconsciously processed, leading to implicit priming effects. Here we investigated whether these implicit effects might be modulated by the semantic nature of the stimuli. Five neglect patients and ten controls performed an identification task of items belonging to living and non-living categories. In the study phase photographs of animals and artifacts were presented either to the left visual field (LVF) or to the right visual field (RVF). In the identification phase, each stimulus was displayed centrally and was revealed in a sequence of frames where the item was represented by an increasingly less and less filtered image up to a complete version. The results showed that lateralized stimuli differentially affected controls’ and neglect patients’ memory retrieval. In controls memory traces from the study phase served as efficient primes, thereby reducing the amount of information necessary for the identification of both stimulus categories. Moreover, hemispheric differences emerged with an advantage of the RVF/left hemisphere for artifact items, while no difference was found for living things. Neglect patients showed a priming effect for artifact items presented either to the RVF/left hemisphere or LVF/right hemisphere, as well as for living items presented to the RVF/left hemisphere, but not for living items presented to the LVF/right hemisphere. The priming effect observed for extinguished artifacts is consistent with the evidence of the existence of a specific mechanism destined to analyze, in an automatic and implicit fashion, motor-relevant information of manipulable objects and tools, which are important for identification process. Results are discussed in relation to current models of organization of conceptual knowledge within the framework of different processes performed by the two hemispheres.     

Independent functioning of the two cerebral hemispheres for recognizing bilaterally presented tachistoscopic visual-half-field stimuli.

Bilateral tachistoscopic presentation of verbal stimuli produces a significantly larger right visual half-field (VHF) superiority than does unilateral presentation, when fixation is controlled by a center digit. This experiment tested whether the increased asymmetry was due to (a) subjects attending the right VHF and ignoring the left VHF or (b) interference between the hemispheres due to competition for the left hemisphere language areas. Words, shaptes, and pictures of faces were presented bilaterally to each VHF, with fixation controlled by a center digit. In three conditions, the same type of stimuli was presented in each VHF (e.g., a word in both the left and right VHF). In two conditions words were presented to one VHF and nonverbal stimuli to the opposite VHF (words paired with words and words paired with faces). It was found that the stimulus pairings did not affect VHF asymmetry for any stimulus. Words showed a large right VHF superiority in all conditions. Shapes showed a significantly smaller right VHF superiority in all conditions. Faces showed no VHF asymmetry in any condition. It was concluded that attentional factors were not important since shapes or faces could be recognized accurately from the left VHF without lowering verbal recognition from the right VHF. Thus the low recognition accuracy from the left VHF is specific for verbal stimuli rather than attentional. The interference hypothesis was also not supported since all the right VHF stimuli (words, shapes, or faces) were associated with low recognition of words from the left VHF. It was suggested instead that VHF asymmetry under unilateral and bilateral presentation reflect two different mechanisms. Under conditions of unilateral presentation, VHF asymmetries are caused by loss of information when any given stimulus must cross the callosum to reach the hemisphere specialized for its processing. However, with bilateral VHF presentation and fixation control, the two hemispheres act as independent channels for information processing. Under this condition, each hemisphere recognized the stimulus from its contralateral VHF. Thus the large right VHF superiority for words with bilateral presentation reflects the superior ability of the left hemisphere for verbal recognition.     

Clinical relevance of memory performance during Wada is stimulus type dependent

OBJECTIVES: This study aimed to investigate whether different types of memory stimulus provide different information during the Wada or intracarotid amytal procedure (IAP) in patients with refractory medial temporal lobe epilepsy (MTLE). METHODS: Eighty nine surgical candidates with documented MTLE and selected for left hemispheric language dominance underwent memory assessment with verbal and dually encodable stimuli during a presurgical IAP. RESULTS: The overall IAP memory performance with the left hemisphere is significantly better than with the right hemisphere regardless of lesion side. This can be explained by the left hemispheric advantage of encoding all stimuli, whereas the right hemisphere has only limited resources to encode verbal stimuli. More importantly, it appeared that dually encodable items remain more readily recognised following injection ipsilateral to the lesion, whereas verbal items are always better recognised following right hemisphere injection regardless of lesion side. CONCLUSIONS: Verbal IAP stimuli show left hemispheric sensitivity in left language dominant MTLE patients. The dually encodable items of the IAP appear lesion sensitive.     

Individual differences in the metacontrol of lateralization for recognizing facial expressions of emotion

Subjects judged whether two faces expressed same or different emotions. Subjects were alternately instructed to empathize with or label the first face which was shown for eight seconds. The second face was then exposed briefly in either the left or right visual field. Eighteen subjects had contrasting hemisphere superiorities in accuracy for making same/different judgments under the two instructions, whereas 12 subjects had consistent hemisphere superiorities. It is proposed that individuals may differ in the metacontrol of lateralization on a task, such that for some individuals, the hemisphere controlling cognitive operations can switch as a function of instructions, expectations, and strategies. For other individuals, the metacontrol of lateralization may dictate that the same hemisphere always controls processing on a particular task.     

Task requirements and hemispheric asymmetry for the processing of single letters

Subjects viewed letters to the left or right of fixation and were required to indicate whether or not each letter was a member of a target set. Half the subjects were told to base their responses on the name of each stimulus, while half were instructed to use stimulus configuration as the basis for their decision. An interaction of visual field and instructions was found for reaction time to target items. The role of task requirements in investigations of hemispheric asymmetry is discussed.     

Hemispheric differences for visual evoked potentials from checkerboard stimuli

Visual evoked potentials (VEPs) were elicited in right-handed male subjects with an alternating checkerboard pattern stimulus presented to either the left or right visual half-field. The sizes of the individual checks in different conditions were 0.25, 0.5, 2.0, or 4.0 cycles/degree of visual angle. The amplitude of the P100 VEP decreased while peak latency increased as check size decreased across both visual field conditions. Left hemisphere responses demonstrated significantly larger P100 amplitudes compared to the right hemisphere responses, although the interaction between hemisphere and stimulus size was not significant. No hemispheric effects of P100 latency were observed. The results suggest that the left hemisphere is engaged more than the right hemisphere for the sensory analysis of visual stimuli composed of straight edges over a wide range of spatial frequencies.     

Is judgement of line orientation selectively impaired in right brain damaged patients?

The judgement of line orientation test (JLOT) is widely used to assess visuo-spatial processing. Most neuropsychological studies have shown that on this task right hemisphere damaged (RHD) patients are significantly more impaired than left hemisphere damaged (LHD) patients, suggesting a dominant role of the right hemisphere in discriminating line orientation. To investigate whether other factors can affect performance on JLOT, a modified version of the test, consisting of the 30 original test items and their mirror images, was employed. In Experiment 1 normal participants were more accurate in discriminating the left lines of the original items, and the right lines of the mirror-reversed items, thus indicating that in original JLOT the stimulus arrays comprise lines on the left side that are easier to judge than lines on the right. In Experiment 2, RHD patients with visual neglect were significantly more impaired than patients without neglect, who performed similarly independently from the side of the lesion. Among patients without neglect, however, LHD patients were more accurate than RHD patients without neglect with the original items, but produced more errors than RHD patients without neglect when faced with the mirror-reversed items. Overall, the results of the present study suggest that the greater impairment on standard JLOT shown by RHD patients has to be interpreted as the by-product of the presence of visual neglect, which is more frequent following right hemisphere damage, and of the uneven distribution of the stimulus lines, which are easier to discriminate in the left space. The clinical and theoretical implications of the results are discussed.     

Asymmetry of flight and escape turning responses in horses

We investigated whether horses display greater reactivity to a novel stimulus presented in the left compared to the right monocular visual field, and whether a population bias exists for escape turning when the same stimulus was presented binocularly. Domestic horses (N=30) were tested on three occasions by a person opening an umbrella five metres away and then approaching. The distance each horse moved away before stopping was measured. Distance was greatest for approach on the left side, indicating right hemisphere control of flight behaviour, and thus followed the same pattern found previously in other species. When order of monocular presentation was considered, an asymmetry was detected. Horses tested initially on the left side exhibited greater reactivity for left approach, whereas horses tested on the right side first displayed no side difference in reactivity. Perhaps left hemisphere inhibition of flight response allowed horses to learn that the stimulus posed no threat and this information was transferred to the right hemisphere. No population bias existed for the direction of escape turning, but horses that turned to the right when approached from the front were found to exhibit longer flight distances than those that turned to the left.     

Sex differences and lateral specialization of hemispheric functioning

The study was designed to explore whether male-female differences in the EEG ratio of the left hemisphere to the right hemisphere could be found with general tasks which more closely approximate normal activities. Six males and six females performed tasks chosen to utilize one hemisphere of the brain more than the other. In all, there were four right hemispheric tasks and four left hemispheric tasks. The ratios of EEG power measured from the temporal lobes were statistically significant for the males between these tasks but not for the females. The results suggest that males and females process the same environmental event with different patterns of brain activity as reflected by EEG.     

Hemispheric asymmetry in use of semantic category information

When two pictures of common objects were presented sequentially, the second was named more quickly if both were members of the same semantic category. This semantic priming effect occurred only when both pictures went directly to the left hemisphere. If the target or prime stimulus was presented to the left visual field--right hemisphere, no priming effect was observed. These results suggest that semantic category information is activated and used by the left hemisphere of the brain.     

Lateral asymmetry: Hard or simple-minded?

A letter or a three-dimensional shape was presented in the center of the visual field. Following the off-set of this stimulus either a comparison letter or a three-dimensional shape was flashed briefly in either the right or left visual field. The subject's task was to respond SAME, or DIFFERENT. The stimuli could be in the same plane, rotated in two dimensions (letters) or in three dimensions (three-dimensional shapes). The left visual field presentations (right hemisphere) of same-pair matches for letters only produced faster reaction times and fewer errors. In all other conditions reaction time measures showed no hemisphere effects. By contrast, error score data indicated that the left hemisphere was overwhelmingly more accurate.     

Hemispheric asymmetry for trajectory perception

Broadly, the right hemisphere is known to be specialized for spatial processing whereas the left hemisphere is known to be specialized for temporal processing. However, it remains unclear how both hemispheres interact when processing spatio-temporal information. This study investigates, from a behavioral point of view, whether spatio-temporal processing involved in trajectory perception generates hemispheric asymmetries. An experimental task requiring the prediction of coincidence between ballistic trajectories and a stationary target was used. Reaction times were analyzed according to various interhemispheric conditions determined by the visual hemifield on which the stimulus was presented and the hand of response. There was shorter reaction time for the left hand than the right hand, and shorter reaction times for the left visual hemifield than the right visual hemifield for both hands. From these findings, it is inferred that there is likely to be right hemisphere specialization for trajectory perception and that this hemispheric asymmetry is independent of handedness.     

Task-related EEG asymmetries: a comparison of alpha blocking and beta enhancement

Alpha and beta content of spectrally analyzed EEG were used to assess differential hemispheric engagement during two linguistic tasks (semantic and phonetic) and one acoustic task involving detection of target items embedded within the same stimulus series. Beta content of the EEG increased reliably in the left hemisphere during the linguistic tasks, whereas the expected attenuation or blocking of alpha did not occur reliably. These data suggest that left hemisphere beta enhancement rather than alpha blocking is a more efficient index of differential hemispheric engagement during language processing.     

Visual field differences in semantic comparative judgments with digits and Kanji stimulus materials

Hemisphere specialization for two types of semantic comparative judgments were examined. In the first experiment a pair of Arabic numerals of different physical size were displayed and subjects were asked to judge if the physical size of the stimulus and the numerical magnitude were congruent or not by pressing the appropriate button. Reaction times were faster in the right visual field than in the left visual field. In the second experiment a pair of different physical sized Kanji representing concrete objects were displayed and subjects were required to judge if the physical size of the Kanji was congruent or not with the relative real life object size by pressing the appropriate button. Reaction times were faster in the left visual field. In the third experiment a pair of different physical sized Kanji representing numerical magnitude were displayed and subjects judged if the physical size of the Kanji and the numerical magnitude were congruent or not. Results showed a right visual field advantage and eliminated the possibility that Kanji complexity was the cause of the left visual field advantage in the second experiment. These results were interpreted as providing support for the proposal that there are two types of semantic comparative judgments, one relying on the verbal code and the other relying on the use of imagery, and suggesting that the former calls for stronger engagement of the left hemisphere while the latter calls for a stronger contribution from the right hemisphere.     

Imitation of gestures by disconnected hemispheres

PU's corpus callosum was severed as a sequel of bleeding from an arteriovenous malformation. The lesion affects the truncus and the splenium and caused somatosensory and visual disconnection of the hemispheres. On clinical testing PU's left hand was apraxic for pantomime of object use but not for imitation. By contrast, when stimuli for imitation of meaningless hand and finger postures were presented tachistocopically to either the left or the right visual field, both hemispheres turned out to be apraxic in different ways. Imitation of hand postures was perfect for right-handed imitation of stimuli presented to the left hemisphere but defective in all other conditions. Imitation of finger postures was below the normal range in all conditions initially, but improved to normality for right-hand imitation of stimuli presented to the left hemsiphere after repeated testing. After successful imitation of gestures presented to the left hemisphere PU commented that he imitated without really seeing the stimulus by ‘formulating the unseen’, whereas after presentation to the right hemisphere he felt that he saw the stimulus but could not imitate. We propose that imitation of meaningless gestures affords a coding of gestures with reference to knowledge about body parts which can be applied only by the left hemisphere. Imitation of finger postures puts additional demands on fine grained visuospatial discrimination which necessitates a contribution by the right hemisphere.     

The effects of study task on prestimulus subsequent memory effects in the hippocampus

Functional magnetic resonance imaging (fMRI) was employed to examine the effects of a study task manipulation on pre‐stimulus activity in the hippocampus predictive of later successful recollection. Eighteen young participants were scanned while making either animacy or syllable judgments on visually presented study words. Cues presented before each word denoted which judgment should be made. Following the study phase, a surprise recognition memory test was administered in which each test item had to be endorsed as “Remembered,” “Known,” or “New.” As expected, “deep” animacy judgments led to better memory for study items than did “shallow” syllable judgments. In both study tasks, pre‐stimulus subsequent recollection effects were evident in the interval between the cue and the study item in bilateral anterior hippocampus. However, the direction of the effects differed according to the study task: whereas pre‐stimulus hippocampal activity on animacy trials was greater for later recollected items than items judged old on the basis of familiarity (replicating prior findings), these effects reversed for syllable trials. We propose that the direction of pre‐stimulus hippocampal subsequent memory effects depends on whether an optimal pre‐stimulus task set facilitates study processing that is conducive or unconducive to the formation of contextually rich episodic memories. © 2015 Wiley Periodicals, Inc.     

Asymmetry of flight and escape turning responses in horses

We investigated whether horses display greater reactivity to a novel stimulus presented in the left compared to the right monocular visual field, and whether a population bias exists for escape turning when the same stimulus was presented binocularly. Domestic horses (N=30) were tested on three occasions by a person opening an umbrella five metres away and then approaching. The distance each horse moved away before stopping was measured. Distance was greatest for approach on the left side, indicating right hemisphere control of flight behaviour, and thus followed the same pattern found previously in other species. When order of monocular presentation was considered, an asymmetry was detected. Horses tested initially on the left side exhibited greater reactivity for left approach, whereas horses tested on the right side first displayed no side difference in reactivity. Perhaps left hemisphere inhibition of flight response allowed horses to learn that the stimulus posed no threat and this information was transferred to the right hemisphere. No population bias existed for the direction of escape turning, but horses that turned to the right when approached from the front were found to exhibit longer flight distances than those that turned to the left.     

Hemispheric processing of categorical and coordinate spatial relations in the absence of low spatial frequencies

Right-handed participants performed the categorical and coordinate spatial relation judgments on stimuli presented to either the left visual field - right hemisphere (LVF-RH) or the right visual field - left hemisphere (RVF-LH). The stimulus patterns were formulated either by bright dots or contrast-balanced dots. When the stimuli were bright, an RVF-LH advantage was observed for the categorical task, whereas an LVF-RH advantage was observed for the coordinate task. When the stimuli were contrast balanced, the RVF-LH advantage was observed for the categorical task, but the LVF-RH advantage was eliminated for the coordinate task. Because the contrast-balanced dots are largely devoid by of low spatial frequency content, these results suggest that processing of low spatial frequency is responsible for the right hemisphere advantage for the coordinate spatial processing.