Hemispheric differences for visual search: Serial vs parallel processing revisited

Subjects were tachistoscopically presented with arrays of two, three or four stimuli to the right or left hemisphere and judged whether all of the items were the same or whether one was physically different from the rest. Separate groups of right-handed subjects viewed letters of featurally similar symbols as stimulus items. Faster and more accurate responding was obtained for left hemisphere presentations for both same and different response judgments. Response time was independent of array size, with same judgments made faster than different judgments for both visual field conditions. Extensive practice shortened reaction time and decreased error rate, but did not change the pattern of hemispheric or judgment effects. Virtually identical results were observed for both stimulus conditions. These findings suggest that the left hemisphere can process information in parallel when the task situation requires featural analysis of stimulus materials.     

Event related potentials (ERP) and behavioral measurements to verbal stimulation of visual fields

Event related potentials (ERP) to visually presented linguistic stimuli were examined using a lexical-decision task and an oddball paradigm. Stimuli were presented to the central, right or left visual fields (CVF, RVF and LVF) and generated ERP with very clear N100-P300 components. The question addressed was whether there is ERP evidence for left hemisphere (LH) superiority in linguistic discrimination as reported behaviorally. Nineteen young, right-handed male subjects participated. The main factor influencing the latency and amplitude of N100 was that of contralateral versus ipsilateral stimulation. Shorter N100 latency and larger amplitude were recorded over the hemisphere contralateral to the visual field stimulated. In contrast, the factors influencing the P300 parameters were the visual field stimulated and the hemisphere over which the ERP was recorded. P300 amplitude was significantly larger and P300 latency significantly shorter over the LH than over the RH. Significantly shorter P300 latency and larger peak amplitude were found for RVF than for LVF stimulation.     

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.     

Cortical and spinal evoked potentials in parkinsonism. Part I. Visual potentials evoked with a checkerboard pattern

In 21 patients with parkinsonism and 20 healthy controls visual potentials evoked with checker pattern used as an alternating stimulus were studied. The left and right eye were examined separately recording visual cortical responses in the central occipital area. The analysed elements included the latency of the first highest positive wave P100 and the amplitude of the P100/N120 complex. Prolongation of the mean latency of P100 was found in patients with parkinsonism, but it was not significant statistically. No differences were found of the P100/N120 amplitude in the group with parkinsonism in relation to healthy controls. In the patients with parkinsonian syndrome (mainly of atherosclerotic origin) the mean latency of the visual potentials was longer and the amplitude was lower than in cases of Parkinson's disease. Attention is called to the high variability of the visual evoked potentials related to the clinical state and origin of the disease (parkinsonian syndrome and Parkinson's disease).     

Time course and hemispheric lateralization effects of complex pitch processing: evoked magnetic fields in response to rippled noise stimuli

To delineate the time course and processing stages of pitch encoding at the level of the supratemporal plane, the present study recorded evoked magnetic fields in response to rippled noise (RN) stimuli. RN largely masks simple tonotopic representations and addresses pitch processing within the temporal domain (periodicity encoding). Four dichotic stimulus types (111 or 133 Hz RN at one ear, white noise to the other one) were applied in randomized order during either visual distraction or selective auditory attention. Strictly periodic signals, noise-like events, and mixtures of both signals served as control conditions. (1) Attention-dependent ear x hemisphere interactions were observed within the time domain of the M50 field, indicating early streaming of auditory information. (2) M100 responses to strictly periodic stimuli were found lateralized to the right hemisphere. Furthermore, the higher-pitched stimuli yielded enhanced activation as compared to the lower-pitch signals (pitch scaling), conceivably reflecting sensory memory operations. (3) Besides right-hemisphere pitch scaling, the relatively late M100 component in association with the RN condition (latency = 136 ms) showed significantly stronger field strengths over the left hemisphere. Control experiments revealed this lateralization effect to be related to noise rather than pitch processing. Furthermore, subtle noise variations interacted with signal periodicity. Obviously, thus, complex task demands such as RN encoding give rise to functional segregation of auditory processing across the two hemispheres (left hemisphere: noise, right hemisphere: periodicity representation). The observed noise/periodicity interactions, furthermore, might reflect pitch-synchronous spectral evaluation at the level of the left supratemporal plane, triggered by right-hemisphere representation of signal periodicity.     

Iconic recognition of musical symbols in the lateral visual fields.

Subjects with formal musical training were tested tachistoscopically in order to obtain thresholds for recognizing single notes a staff, presented successively to the lateral visual fields. They were also tested on backward visual making of the same stimuli to determine the interstimllus interval needed to escape the masking effect (critical ISI). Response requirements in threshold and ISI conditions were nonverbal (depressing an organ key), or verbal (indicating the stimulus by name). Recognition threshold for notes were lower in the right visual field than in the left, and verbal responses were easier than manual responses in the threshold condition. However, critical ISIs were lower in the left field than in the right, and ISIs from the left visual field were lower with manual responses than with verbal responses. These findings support the idea that (a) threshold and ISI represent separate levels of perceptual processing, and (b) in trained subjects, the ability to recognize musical symbols may be governed by the left hemisphere, while the information-processing stage of iconic storage may occur in the right hemisphere.     

Differences in semantic category priming in the left and right cerebral hemispheres under automatic and controlled processing conditions.

The contribution of each cerebral hemisphere to the generation of semantic category meanings at automatic and strategic levels of processing was investigated in a priming experiment where prime and target words were independently projected to the left or right visual fields (LVF or RVF). Non-associated category exemplars were employed as related pairs in a lexical decision task and presented in two experimental conditions. The first condition was designed to elicit automatic processing, so related pairs comprised 20% of the positive set, stimulus pairs were temporally separated by a stimulus onset asynchrony (SOA) of 250 ms, and there was no allusion to the presence of related pairs in the instructions to subjects. The second condition, designed to invoke controlled processing, incorporated a relatedness proportion of 50%, stimulus pairs separated by an SOA of 750 ms, and instructions which informed subjects of the presence and use of category exemplar pairs in the stimulus set. In the first condition, a prime directed to either visual field facilitated responses to categorically related targets subsequently projected to the RVF, while in the second condition a prime directed to either visual field facilitated responses to related targets projected to the LVF. The facilitation effects obtained in both conditions appeared to reflect automatic processes, while strategic processes were invoked in the left, but not the right hemisphere in the second condition. The results suggest that both hemispheres have automatic access to semantic category meanings, although the timecourse of activation of semantic category meanings is slower in the right hemisphere than in the left.     

Event-related potentials in children with specific visual cognitive disability

Event-related potentials (ERPs) elicited by novel and well memorized non-verbal visual patterns were recorded over the right and left hemispheres from ten children with Specific Visual Cognitive Disability (SVCD) and ten matched Control subjects (C). ERPs of SVCD children were generally longer in latency (particularly N2) and decreased in amplitude (particularly P3) relative to the ERPs of Cs. No hemisphere differences were observed in the SVCD group, while the latency of N2 in the C group tended to be slightly shorter and the amplitude of P3 was significantly larger over the right than over the left hemisphere. The results are discussed in terms of deficits in visual learning and visual pattern analysis. The different hemispheric pattern in SVCD children as compared to Cs is suggested to be a symptom secondary to basic visual cognitive deficits.     

Hemispheric differences in odour recognition

Two experiments were designed in order to study hemispheric differences in odour recognition in normal subject. In the first experiment subjects first smelled an odour and then a visual stimulus (a picture or a word related or unrelated to the odour) was flashed either to the left or to the right hemisphere for 150 milliseconds. Subjects had to press a key if the two stimuli (olfactory and visual) matched and another key if they did not. Reaction time analysis showed that the responses were faster when the second stimulus (either picture or word) was presented to the right hemisphere. In a second experiment the first stimulus was auditory rather than olfactory. Subjects were instructed to listen to names of odorants and then to respond to pictures or written names as in the first experiment. In this case a left hemisphere advantage emerged, presumably related to the priming of the left hemisphere by the auditorially presented verbal stimulus.     

An investigation of hemispheric asymmetry in size and semantic discriminations and related visual ERPs

The present study examined possible hemispheric differences in discriminations of different sizes of geometric shapes (rectangles) and different meanings of words, and determined whether left and right hemisphere derived visual event related potentials (ERPs) were related to performance. Eighteen right-handed subjects (10 male and 8 female) participated in two separate sessions conducted on different days. The visual ERPs were recorded from over left parietal (P3) and right parietal (P4) scalp locations. Subjects were required to make discriminations of three words (PARE, PAIR, PEAR) and three sizes of rectangles (small, medium, large). Each word and rectangle was singly presented for 40 msec at 1 degree 24 minutes of arc to the left and right of central fixation (LVF and RVF respectively). The major findings were as follows: 1) there were no performance and ERP differences between hemispheres in the verbal task; 2) the left hemisphere excelled in the spatial task; however, ERPs derived from the two hemispheres were similar; 3) subjects experienced greater difficulty in their discriminations of geometric size, as compared to words, regardless of field of presentation. Discrimination of size seemed to have influenced P3 (P300) latency, i.e., it was longer (both hemispheres) when subjects made size discriminations as compared to words. It was proposed that the more difficult discriminations involved in size discrimination required a greater time for stimulus evaluation and that this was reflected in the delayed P3 response.     

Half-field sinusoidally modulated light stimulation at subject's alpha frequency.

Interhemispheric responses of alpha activity were investigated by use of half-field sinusoidally modulated light (SML) stimulation at the subject's alpha frequency. The left and right visual half-fields as well as full-field were separately stimulated by the SML. The power spectrum of SML responses, averaged with Wiener filtering, was obtained at the left and the right occipital area (O1 and O2) in 11 normal subjects. Power in each hemisphere, phase difference and coherence between O1 and O2 were estimated at an alpha frequency. A laterality index of power was defined as (P(l)-P(r))/(P(l) + P(r)) where P(l) is a power value at O1, and P(r) at O2. In most of the subjects, laterality index decreased in the order: left half-field, full-field and right half-field stimulation. This relationship revealed greater SML responses on the ipsilateral occipital area. There were significant differences in laterality index among 3 visual field conditions, thus showing that full-field responses ranged in lateralization between the left and right half-field responses in most of the subjects. This hemispheric distribution suggests that half-field SML stimulation affected alpha activity selectively in each hemisphere, mostly in the ipsilateral hemisphere.     

Ethanol and menstrual cycle interactions in the visual evoked response

Right and left hemisphere visual evoked responses from central locations were collected from 10 normal right-handed women in the follicular and luteal phases of their menstrual cycle. Subjects were tested sober and under 3 doses of ethanol. Amplitude and latency characteristics of the N120 and P180 components for each hemisphere were determined. Asymmetry between hemispheres for the various measurements was then calculated. The follicular phase was associated with a significant P180 latency asymmetry under baseline or placebo conditions. Virtually no asymmetry for P180 latency was present in the luteal phase. Ethanol eliminated the follicular P180 latency asymmetry and reduced the amplitude of both N120 and P180 in a dose-related fashion. The latency of the N120 component was prolonged by ethanol in a similar fashion. Interactions between menstrual cycle and ethanol occurred for both amplitude and latency of P180, but only for the response from the left hemisphere. These occurred at low or moderate doses of ethanol and illustrate the need to consider both the biphasic effects of low doses of ethanol and the possibility of lateralization of these effects to one cerebral hemisphere.     

Late auditory evoked potentials asymmetry revisited.

OBJECTIVE: To investigate brain asymmetries of the auditory evoked potential (AEP) N100, T-complex, and P200 in response to monaural stimulation. METHODS: Electroencephalographic (EEG) recordings from 68 channels were used to record auditory cortex responses to monaural stimulation from normal hearing participants (N=16). White-noise stimuli and 1000Hz tones were repeatedly presented to either the left or right ear. Source localization of the AEP N100 response was carried out with two symmetric regional sources placed into left and right auditory cortex. Regional source waveform amplitude and latency asymmetries were analyzed for tangential and radial activity explaining the N100, T-complex and P200 AEP components. RESULTS: Regional source waveform analysis showed that early tangential activity in the N100 latency range exhibited larger contralateral amplitudes and shorter latencies for both tone and noise monaural stimuli. Lateralized activity was significantly greater when tones or noise was presented to the left compared to the right ear (p<.001). The ear difference in the degree of lateralization arose due to hemispheric asymmetry. Significantly more tangential activity in the N100 latency range was recorded in the right compared to the left hemisphere in response to stimulation by either tones or noise (p<.001). Neither the radial activity modelling the T-complex, nor activity modelling the P200, showed robust ear or hemisphere differences. CONCLUSIONS: Regional source waveform analysis revealed that the extent of auditory evoked potential asymmetries depends on the ear and hemisphere examined. These findings have implications for future studies utilizing AEP asymmetries to examine normal auditory function or experience-related changes in the auditory cortex. SIGNIFICANCE: The right compared to the left auditory cortex may be more involved in processing monaurally presented tone and noise stimuli.     

Primary visual response (M100) delays in adolescents with FASD as measured with MEG

Fetal alcohol spectrum disorders (FASD) are debilitating, with effects of prenatal alcohol exposure persisting into adolescence and adulthood. Complete characterization of FASD is crucial for the development of diagnostic tools and intervention techniques to decrease the high cost to individual families and society of this disorder. In this experiment, we investigated visual system deficits in adolescents (12–21 years) diagnosed with an FASD by measuring the latency of patients' primary visual M100 responses using MEG. We hypothesized that patients with FASD would demonstrate delayed primary visual responses compared to controls. M100 latencies were assessed both for FASD patients and age‐matched healthy controls for stimuli presented at the fovea (central stimulus) and at the periphery (peripheral stimuli; left or right of the central stimulus) in a saccade task requiring participants to direct their attention and gaze to these stimuli. Source modeling was performed on visual responses to the central and peripheral stimuli and the latency of the first prominent peak (M100) in the occipital source timecourse was identified. The peak latency of the M100 responses were delayed in FASD patients for both stimulus types (central and peripheral), but the difference in latency of primary visual responses to central vs. peripheral stimuli was significant only in FASD patients, indicating that, while FASD patients' visual systems are impaired in general, this impairment is more pronounced in the periphery. These results suggest that basic sensory deficits in this population may contribute to sensorimotor integration deficits described previously in this disorder. Hum Brain Mapp 34:2852–2862, 2013. © 2012 Wiley Periodicals, Inc.     

Sorting of hemifield presented temporal and spatial stimuli

An experiment was performed, using a sorting task (choice reaction-time), to study the processing of stimuli, which differed along a temporal or a spatial dimension, presented to the right or to the left visual hemifield. The results indicate more accurate responses and shorter reaction times to a temporal stimulus when it appears in the right-visual-field than when it appears in the left-visual-field. Conversely, more accurate responses and shorter reaction times are found to a spatial stimulus when it appears in the left-visual-field than when it appears in the right-visual-field. In addition to this major interaction, three more interactions are found, all of which involve response direction and one or two other stimulus variables. The results are consistent with the hypothesized hemispheric functional specificity, i.e., that the initial processing of the temporal dimension of visual stimuli is performed better by the left hemisphere than by the right hemisphere and that the converse is true for the initial processing of the spatial dimension of visual stimuli.     

Lateralisation of escape responses in the stripe-faced dunnart, Sminthopsis macroura (Dasyuridae: Marsupialia)

Although lateralisation has been observed in many vertebrate species, marsupials have been neglected in the study of lateralisation. We investigated the behavioural responses of the stripe-faced dunnart (Sminthopsis macroura) to a mechanical model of a snake approaching into the monocular (left and right) or the binocular visual field. The snake model was presented to 30 adult subjects. The behavioural responses and the latency to react to the stimulus were scored. Reactivity was calculated by pooling scores for retreat, startle, ears back, and orientation. Retreat tended to be the most common of these responses. Approach of the snake into the dunnarts' left monocular visual field elicited a significantly higher reactivity compared to approach into the right or binocular visual field. Half of the animals did not respond in the 60 seconds allocated when the stimulus approached on their right side, whereas only seven did not respond when the stimulus approached on the left, and ten when the stimulus was presented binocularly. These results are consistent with the right hemisphere's known specialisation for controlling fear and escape responses. Our results suggest that marsupials are lateralised in a way similar to other vertebrates.     

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.     

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.     

Neuroelectrical correlates of categorical perception for place of articulation in normal and lead-treated rhesus monkeys

Categorical perception of place of articulation contrasts was evaluated in rhesus monkeys. The monkeys had been chronically exposed to subclinical levels of lead, either from conception to birth, or for approximately 6 months beginning at birth, or were never exposed to lead. Auditory evoked responses were recorded from scalp electrodes placed over left and right hemispheres during stimulus presentation. The brain responses recorded from the right hemisphere of the normal control group of monkeys discriminated between the categories of [dae] and [gae]. Categorical discriminations were also noted for monkeys exposed to lead either prenatally or postnatally. These discriminations, in contrast, were found over only the left hemisphere. In additions, postnatal exposure resulted in categorical discrimination associated with slower latency components, suggesting a less mature pattern than that obtained for prenatally exposed monkeys. Finally, the brain responses of the animals in the normal control and postnatal exposure conditions evidenced reliable within-category, as well as between-category, discriminations. These results suggest that the neurocortical mechanisms associated with categorical perception of place information may differ between human and nonhuman primates and that early exposure to lead alters these processes.     

Lateralisation of escape responses in the stripe-faced dunnart,Sminthopsis macroura (Dasyuridae: Marsupialia)

Although lateralisation has been observed in many vertebrate species, marsupials have been neglected in the study of lateralisation. We investigated the behavioural responses of the stripe-faced dunnart (Sminthopsis macroura) to a mechanical model of a snake approaching into the monocular (left and right) or the binocular visual field. The snake model was presented to 30 adult subjects. The behavioural responses and the latency to react to the stimulus were scored. Reactivity was calculated by pooling scores for retreat, startle, ears back, and orientation. Retreat tended to be the most common of these responses. Approach of the snake into the dunnarts' left monocular visual field elicited a significantly higher reactivity compared to approach into the right or binocular visual field. Half of the animals did not respond in the 60 seconds allocated when the stimulus approached on their right side, whereas only seven did not respond when the stimulus approached on the left, and ten when the stimulus was presented binocularly. These results are consistent with the right hemisphere's known specialisation for controlling fear and escape responses. Our results suggest that marsupials are lateralised in a way similar to other vertebrates.