Hemispheric processing of global form, local form, and texture.

Hemispheric processing of global form, local form, and texture of hierarchical patterns composed of many, relatively small elements and patterns composed of few, relatively large elements was examined in two experiments, employing a Stroop-type paradigm. In experiment 1 subjects were instructed to attend either to the global or the local level of the pattern and to identify the form at the designated level. In experiment 2 subjects were to identify the global form or the texture. A right visual field (left hemisphere) advantage was obtained for detection of local form, and a left visual field (right hemisphere) advantage was obtained for detection of global form. When many-element patterns were processed in terms of global form and texture, the results failed to show reliable hemispheric differences. The results suggest that the hemispheres differ in their sensitivity to the relatively more global versus the relatively more local aspects of visual patterns which require focused attention (as in global/local form detection). When the task involved distributed attention (as in texture detection) no lateralized effects were observed.     

Visual span of apprehension in patients with unilateral cerebral lesions

The visual span of apprehension for random letter and digit sequences, approximations to English sequences and non-symbolic line stimuli was measured in patients with unilateral cerebral lesions. The left hemisphere group was significantly impaired relative to the right hemisphere group and a control group on all three types of visual span task. The deficit was most marked in patients with left posterior lesions. The visual span deficits were not related to other language deficits. The findings are discussed in terms of a modality-specific defect of visual short-term memory.     

Effects of visual-field and matching instruction on event-related potentials and reaction time

Vertical letter pairs were presented randomly in the left and right visual hemifields in a physical identity match and name identity match condition. The reaction times showed a right visual field superiority for name matches, and a left visual field superiority for physical matches. Event-related potentials to letter pairs showed a sequence of three waves: a negative wave (N2, around 270 msec), a positive wave (P3, around 500 msec), and a broad positive slow wave (SW, around 600-700 msec), respectively. P3 and SW amplitudes were consistently larger at the left hemisphere than at the right hemisphere, regardless of the field of stimulation. At both hemispheres, N2 waves were always larger to stimuli presented in the visual field contralateral to a hemisphere than stimuli presented in the visual field ipsilateral to a hemisphere. The positive waves (P3, SW) showed the opposite pattern: smaller amplitudes to stimuli that were presented contralaterally than stimuli that were presented ipsilaterally to a given hemisphere. These results were attributed to a shift in sustained negativity on the directly stimulated hemisphere, relative to the indirectly stimulated hemisphere, reflecting either sensory at attentional processes in the posterior cerebral hemispheres.     

Parallel but temporally displaced visual half-field metacontrast functions

Using a classic letter-ring metacontrast paradigm, left and right visual field meta-contrast functions were separately determined. The parallel U-shaped recognition functions for both half-fields were found to interact differentially with stimulus onset asynchrony, the left visual field function being displaced by 13 ms toward longer test stimulus-masking stimulus separations. This result was consistent with the hypothesis of longer processing time requirements for verbal stimuli delivered to the right than to the left hemisphere. This indicates that the neural locus (loci) responsible for left visual field verbal processing delay is (are) capable of mediating metacontrast phenomena. It was tentatively concluded that a relative processing delay within the right hemisphere underlies the differing visual half-field metacontrast interaction with stimulus onset asynchrony.     

Visual field differences in the magnitude of the Oppel-Kundt illusion vary with processing time

Three experiments were performed to investigate hemispheric differences in susceptibility to the Oppel-Kundt illusion presented tachistoscopically to the two visual hemifields. Experiment 1 used a successive comparison mode, in which 16 undergraduate students indicated whether the second of two successive extents looked shorter or longer than the first. Experiment 2 (20 under-graduates) and Experiment 3 (1 commissurotomy patient) required judgments of two extents presented simultaneously. The first experiment found no significant visual field differences, although females were more susceptible than males. In the second experiment, the illusion magnitude was greater in the left visual field, and in the third experiment, it was greater in the right visual field. Post hoc analyses resolve the conflicting results and show that in all three experiments, the susceptibility of the right hemisphere declined more than that of the left hemisphere during illusion processing. An interpretation is offered in terms of two parallel processes, one that is fast, uses feature extraction, and is performed more effectively in the left hemisphere, and one that is slow, uses visuospatial analysis to compute distances between parts of the illusion figure, and is performed more effectively in the right hemisphere.     

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.     

Concurrent processing of spatial relations and objects in two cerebral hemispheres

This study examined hemispheric asymmetry for concurrent processing of object and spatial information. Participants viewed two successive stimuli, each of which consisted of two digits and two pictures that were randomly located and judged them as identical or different. A sample stimulus was presented in a central visual field, followed by a matching stimulus presented briefly in a left or right visual field. The matching stimuli were different from the sample stimuli with respect to the object (digit or picture) or spatial (locations or distances of items) aspect. No visual field asymmetry was found in the detection of object change. However, a left visual field advantage was found in the detection of spatial change. This result can be explained by the double filtering by frequency theory of Ivry and Robertson, who asserted that the left hemisphere has a bias for processing information contained in relatively high spatial frequencies whereas the right hemisphere has a bias for processing information contained in relatively low spatial frequencies. Based upon this evidence, the importance of interhemispheric integration for visual scene perception is discussed.     

Hemispheric specialization for emotional word processing is a function of SSRI responsiveness

Vulnerability to depression and non-response to Selective Serotonin Reuptake Inhibitors (SSRIs) are associated with specific neurophysiological characteristics including greater right hemisphere (RH) relative to left hemisphere (LH) activity. The present study investigated the relationship between hemispheric specialization and processing of emotional words using a divided visual field paradigm administered to never-depressed and previously-depressed individuals, who were subdivided into SSRI responders and non-responders. SSRI responders and never-depressed participants were similar in their left hemispheric lateralization for evaluating emotional words. In contrast, SSRI non-responders showed a relative shift towards RH processing of negative words, and a strong bias toward negative evaluation of words presented to the RH. The results are discussed within the context of a biological-cognitive model of vulnerability to depression.     

HEMISPHERIC DIFFERENCES IN AUDITORY PERCEPTION ARE SIMILAR TO THOSE FOUND IN VISUAL PERCEPTION

Abstract— In a pitch discrimination task, subjects were faster and more accurate in judging low-frequency sounds when these stimuli were presented to the left ear, compared with the right ear. In contrast, a right-ear advantage was found with high-frequency sounds. The effect was in terms of relative frequency and not absolute frequency, suggesting that the effect arisen from pastsensory mechanisms. A simitar laterality effect has been reported in visual perception with stimuli varying in spatial frequency. These multimodal laterality effects may reflect a general computational difference between the two cerebral hemispheres, with the left hemisphere biased for processing high-frequency information and the right hemisphere biased for processing low-frequency information.     

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.     

Hemispheric differences are found in the identification,but not the detection,of low versus high spatial frequencies

The processing of sine-wave gratings presented to the left and right visual fields was examined in four experiments. Subjects were required either to detect the presence of a grating (Experiments 1 and 2) or to identify the spatial frequency of a grating (Experiments 3 and 4). Orthogonally to this, the stimuli were presented either at threshold levels of contrast (Experiments 1 and 3) or at suprathreshold levels (Experiments 2 and 4). Visual field and spatial frequency interacted when the task required identification of spatial frequency, but not when it required only stimulus detection. Regardless of contrast level (threshold, suprathreshold), high-frequency gratings were identified more readily in the right visual field (left hemisphere), whereas low-frequency gratings showed no visual field difference (Experiment 3) or were identified more readily in the left visual field (right hemisphere) (Experiment 4). Thus, hemispheric asymmetries in the processing of spatial frequencies depend on the task. These results support Sergent’s (1982) spatial frequency hypothesis, but only when the computational demands of the task exceed those required for the simple detection of the stimuli.     

Hemispheric specialization for emotional word processing is a function of SSRI responsiveness

Vulnerability to depression and non-response to Selective Serotonin Reuptake Inhibitors (SSRIs) are associated with specific neurophysiological characteristics including greater right hemisphere (RH) relative to left hemisphere (LH) activity. The present study investigated the relationship between hemispheric specialization and processing of emotional words using a divided visual field paradigm administered to never-depressed and previously-depressed individuals, who were subdivided into SSRI responders and non-responders. SSRI responders and never-depressed participants were similar in their left hemispheric lateralization for evaluating emotional words. In contrast, SSRI non-responders showed a relative shift towards RH processing of negative words, and a strong bias toward negative evaluation of words presented to the RH. The results are discussed within the context of a biological–cognitive model of vulnerability to depression.     

Differential specialization of the cerebral hemispheres for the perception of sameness and difference

The Johns Hopkins University, Baltimore, Maryland 21218 Pairs of letters were presented 4 deg left or right of fixation, and Ss were asked to indicate as quickly as possible whether the letters in a pair were the same or different. Reaction times for “different” responses were faster when stimuli were in the left visual field than in the right, but reaction times for “same” responses were faster when stimuli were in the right visual field than in the left. These results may indicate that the right hemisphere is better specialized for difference detection, while the left hemisphere is better specialized for sameness detection.     

Hemispheric differences in the time-course of semantic priming processes: evidence from event-related potentials (ERPs)

To investigate hemispheric differences in the timing of word priming, the modulation of event-related potentials by semantic word relationships was examined in each cerebral hemisphere. Primes and targets, either categorically (silk-wool) or associatively (needle-sewing) related, were presented to the left or right visual field in a go/no-go lexical decision task. The results revealed significant reaction-time and physiological differences in both visual fields only for associatively related word pairs, but an electrophysiological difference also tended to reach significance for categorically related words when presented in the left visual field. ERP waveforms showed a different time-course of associative priming effects according to the field of presentation. In the right visual field/left hemisphere, both N400 and Late Positive Component (LPC/P600) were modulated by semantic relatedness, while only a late effect was present in the left visual field/ right hemisphere.     

Hemispheric differences in object identification

Although the right hemisphere is thought to be preferentially involved in visuospatial processing, the specialization of the two hemispheres with respect to object identification is unclear. The present study investigated the effects of hemifield presentation on object and word identification by presenting objects (Experiment 1) and words (Experiment 2) in a rapid visual stream of distracters. In Experiment 1, object images presented in the left visual field (i.e., to the right hemisphere) were identified with shorter display times. In addition, the left visual field advantage was greater for inverted objects. In Experiment 2, words presented in the right visual field (i.e., to the left hemisphere) under similar conditions were identified with shorter display times. These results support the idea that the right hemisphere is specialized with regard to object identification.     

Hemispheric specialization for categorical and coordinate spatial representations: A reappraisal

The purpose of the present study was to examine Kosslyn's (1987) claim that the left hemisphere (LH) is specialized for the computation of categorical spatial representations and that the right hemisphere (RH) is specialized for the computation of coordinate spatial representations. Categorical representations involve making judgements about the relative position of the components of a visual stimulus (e.g., whether one component is above/below another). Coordinate representations involve calibrating absolute distances between the components of a visual stimulus (e.g., whether one component is within 5 mm of another). Thirty-two male and 32 female undergraduates were administered two versions of a categorical or a coordinate task over three blocks of 36 trials. Within each block, items were presented to the right visual field-left hemisphere (RVF-LH), the left visual field-right hemisphere (LVF-RH), or a centralized position. Overall, results were more supportive of Kosslyn's assertions concerning the role played by the RH in the computation of spatial representations. Specifically, subjects displayed an LVF-RH advantage when performing both versions of the coordinate task. The LVF-RH advantage on the coordinate task, however, was confirmed to the first block of trials. Finally, it was found that males were more likely than females to display faster reaction times (RTs) on coordinate tasks, slower RTs on categorical tasks, and an LVF-RH advantage in computing coordinate tasks.     

Lateral asymmetry in visual detection under hypnosis

It has been suggested that the hypnotic state results in a greater relative activation or priming of the right cerebral hemisphere than of the left hemisphere. The experiment reported here employed hypnosis to produce such a priming effect in a visual-detection task. Subjects were required to detect the presence or absence of a gap in outline squares presented either to the left visual field or right visual field, with response time as the primary dependent measure. Those subjects who were hypnotized produced a 50-msec. response time difference favoring squares presented to the left visual field whereas control subjects and simulator-control subjects showed no lateral asymmetries. The result is classified as a material-nonspecific priming effect and discussed with regard to the nature of processing resources.     

Name and face matching in one or two visual fields: a test of models of hemispheric specialization

In two experiments a name and a face (each male or female) were simultaneously flashed to either the same or opposite visual fields (left or right), for matching congruent (same sex) or incongruent (opposite sex), to test the predictions of various models of hemispheric specialization. While overall best performance occurred with a face in the left visual field (LVF) and a name in the right visual field (RVF), and worst with the opposite configuration, the general pattern of results was incompatible with either a direct access model or an activational/attentional account. The results were, however, most compatible with the predictions of a semispecialized hemispheres account, whereby cerebral asymmetries are seen as relative rather than absolute, either hemisphere being capable of processing either kind of material (verbal or visuospatial), but to different levels of efficiency. However, despite the fact that the stimulus materials had previously been shown to produce stable and consistent lateral asymmetries in the predicted directions when presented in isolation, in the composite, integrative matching task the position of the name seemed to be the major determinant of the resultant asymmetries. It would seem therefore that when such stimuli are to be cross matched, either left hemisphere (language) processes somehow dominate right hemisphere (visuospatial) processing (though not in the way that would be predicted by a simple activational/attentional account) or the left hemisphere's greater capacity predominates.     

Hemispheric patterns in visual search

A visual search paradigm was employed to examine hemispheric serial and parallel processing. Stimulus arrays containing 4, 9, or 16 elements were tachistoscopically presented to the right visual field-left hemisphere (RVF-LH) or left visual field-right hemisphere (LVF-RH). Subjects judged whether all of the elements within an array were physically the same (all X's) or whether one (O) was different from the rest. Left hemisphere presentations were processed more quickly and accurately than LVF-RH presentations for all stimulus conditions. As the number of array elements increased, more errors and longer response times were obtained for different stimulus items whereas fewer errors and somewhat shorter response times were obtained for same stimulus items. These and previous results suggest that the left hemisphere obtains an advantage for visual search because of that hemisphere's superiority for fine-grained feature analysis rather than because of a fundamental hemispheric serial/parallel processing dichotomy.