Hemispheric differences in visual recognition of two Kanji characters

The purpose of the present study was to examine hemispheric differences in visual recognition of two kanji letters. Kanji phrases and nonsense kanji letters were presented unilaterally to the left or right visual hemifield in thirty normal right-handed men by a tachistoscope. The experiments were separated into three conditions; the mixing tasks (ten phrases and ten sets of nonsense letters are presented by mixture), the phase task (twenty phrases are presented), and the nonsense letters task (twenty sets of nonsense letters are presented). Under all conditions, significant right visual hemifield superiorities for the accuracy of recognition of phrase and nonsense letters were observed.     

Visual hemifield differences in recognition of kanji and hiragana and its relation to hemispheric cerebral asymmetries

Visual hemifield differences in recognition of kanji and hiragana were studied on forty male right handers. A letter of kanji or hiragana was presented unilaterally to the right or left visual hemifield on a CRT display for 123 msec. A hundred and twenty recognition trials were performed for each subject using 20 well-acquainted kanji, 20 unfamiliar kanji and 20 hiragana. Kanji was more accurately recognized in the left visual hemifield than in the right hemifield. This tendency was more prominent in unfamiliar kanji compared with well-acquainted kanji. There were no visual hemifield differences in recognition of hiragana. Learning effects were observed for the right hemifield on kanji and both hemifields on hiragana. The results were discussed in relation to cerebral asymmetries of function. Kanji might be processed in the right cerebral hemisphere as geometric forms. The results on hiragana may be explained by mental set. It is suggested that modes of processing may be different between kanji and hiragana.     

Hemispheric asymmetry of visual evoked potentials during human recognition of facial emotional expressions

Visual evoked potentials (VEP) in standard 16 EEG derivations were recorded in 26 young men and 20 women during recognition of facial emotional expressions and geometric figures. The stimuli were presented on a computer screen in the center of the visual field or randomly in the right or left vision hemifields. Peak VEP latency and mean amplitude in 50-ms epochs were measured; spatiotemporal VEP dynamics was analyzed in a series of topographic maps. The right hemisphere was shown to be more important in processing emotional faces. The character of the asymmetry was dynamic: at earlier stages of emotion processing the electrical activity was higher in the right inferior temporal region compared to the left symmetrical site. Later on the activity was higher in the right frontal and central areas. The dynamic mapping of "face-selective" component N180 of VEPs revealed the onset of activation over the right frontal areas that was followed by the fast activation of symmetrical left zones. Notably, this dynamics didn't correlate with the hemifield of stimuli exposition. The degree of asymmetry was lower during presentation of figures, especially in the inferior temporal and frontal regions. The prominent asymmetry of information processes in the inferior temporal and frontal areas was suggested to be specific for recognition of facial expression.     

Differences in latent inhibition between men and women under conditions of nonverbal visually-spaatial masking]

In order to study gender differences in development of the latent inhibition (LI) and the influence of laterality (hemisphere) factor, two groups of subjects were examined. In the preexposure phase, subjects of the control group were presented with a pair of geometric figures, which appeared either in the right or in the left visual hemifield (at random) with the exposure time of 80 ms. A subject had to compare the figures in a pair and to press a button in case of their identity. In the preexposure (experimental) group, the subject solved the same task, but half of them was presented with the same set of stimuli which appeared in the right hemifield, another half of subjects received these stimuli in the left hemifield. Simultaneously with the target stimuli, the opposite hemifield was stimulated with a nonattended symbol "[symbol: see text]". In the test phase, all the subjects (both of the control and experimental groups) had to respond to the symbol "[symbol: see text]" (previously nonattended in the experimental group), which appeared in part of the trials, by pressing the button. Pairs of stimuli which had been target ones in the preexposure phase, became masking stimuli in the test phase. Correct responses were reinforced by presentation of a special sound. The number of trials required for acquisition of the instrumental contingency served as an index of the LI. Analysis of variance showed that significant strong effect of the LI was observed only in men (comparison between the control and preexposure groups), while there was no significant difference between women of the control and preexposure groups. There was no laterality effect. Gender differences and hemispheric asymmetry in the LI organization are discussed.     

Bilateral field advantage in visual enumeration

A number of recent studies have demonstrated superior visual processing when the information is distributed across the left and right visual fields than if the information is presented in a single hemifield (the bilateral field advantage). This effect is thought to reflect independent attentional resources in the two hemifields and the capacity of the neural responses to the left and right hemifields to process visual information in parallel. Here, we examined whether a bilateral field advantage can also be observed in a high-level visual task that requires the information from both hemifields to be combined. To this end, we used a visual enumeration task--a task that requires the assimilation of separate visual items into a single quantity--where the to-be-enumerated items were either presented in one hemifield or distributed between the two visual fields. We found that enumerating large number (>4 items), but not small number (<4 items), exhibited the bilateral field advantage: enumeration was more accurate when the visual items were split between the left and right hemifields than when they were all presented within the same hemifield. Control experiments further showed that this effect could not be attributed to a horizontal alignment advantage of the items in the visual field, or to a retinal stimulation difference between the unilateral and bilateral displays. These results suggest that a bilateral field advantage can arise when the visual task involves inter-hemispheric integration. This is in line with previous research and theory indicating that, when the visual task is attentionally demanding, parallel processing by the neural responses to the left and right hemifields can expand the capacity of visual information processing.     

Visual Speech Perception in Foveal and Extrafoveal Vision: Further Implications for Divisions in Hemispheric Projections

When observing a talking face, it has often been argued that visual speech to the left and right of fixation may produce differences in performance due to divided projections to the two cerebral hemispheres. However, while it seems likely that such a division in hemispheric projections exists for areas away from fixation, the nature and existence of a functional division in visual speech perception at the foveal midline remains to be determined. We investigated this issue by presenting visual speech in matched hemiface displays to the left and right of a central fixation point, either exactly abutting the foveal midline or else located away from the midline in extrafoveal vision. The location of displays relative to the foveal midline was controlled precisely using an automated, gaze-contingent eye-tracking procedure. Visual speech perception showed a clear right hemifield advantage when presented in extrafoveal locations but no hemifield advantage (left or right) when presented abutting the foveal midline. Thus, while visual speech observed in extrafoveal vision appears to benefit from unilateral projections to left-hemisphere processes, no evidence was obtained to indicate that a functional division exists when visual speech is observed around the point of fixation. Implications of these findings for understanding visual speech perception and the nature of functional divisions in hemispheric projection are discussed.     

Movement in the normal visual hemifield induces a percept in the 'blind' hemifield of a human hemianope.

We have investigated visual responses to moving stimuli presented to the normal hemifield of a hemianope, GY, who exhibits residual visual function in his right, ''blind'' hemifield. Preliminary experiments established that his perception of moving stimuli localized in his ''blind'' hemifield is retained when a similar stimulus is presented simultaneously in the normal hemifield. In response to a grating stimulus moving horizontally towards fixation in the non-foveal region of the normal, left hemifield, he perceives in addition to a normal motion percept in the left hemifield, a sensation of movement localized in the right hemifield. Qualitatively, this latter is indistinguishable from responses elicited by direct stimulation localized within his ''blind'' hemifield by moving stimuli. We have investigated the characteristics of the mechanisms which induce the ''blind'' field component of GY''s responses to stimulation of the normal hemifield. We show that GY''s sensitivity for detection of movement localized within his ''blind'' hemifield is dependent on the direction of movement, the contrast and the velocity of a grating presented to the normal hemifield. No induced effects were recorded in response to colour or to non-moving, flickering stimuli. We examine the possible contribution of scattered light to our observations, and eliminate this factor by consideration of our experimental results. We discuss the neural mechanisms which may be involved in this response.     

Category learning induces position invariance of pattern recognition across the visual field

Human object recognition is considered to be largely invariant to translation across the visual field. However, the origin of this invariance to positional changes has remained elusive, since numerous studies found that the ability to discriminate between visual patterns develops in a largely location-specific manner, with only a limited transfer to novel visual field positions. In order to reconcile these contradicting observations, we traced the acquisition of categories of unfamiliar grey-level patterns within an interleaved learning and testing paradigm that involved either the same or different retinal locations. Our results show that position invariance is an emergent property of category learning. Pattern categories acquired over several hours at a fixed location in either the peripheral or central visual field gradually become accessible at new locations without any position-specific feedback. Furthermore, categories of novel patterns presented in the left hemifield are distinctly faster learnt and better generalized to other locations than those learnt in the right hemifield. Our results suggest that during learning initially position-specific representations of categories based on spatial pattern structure become encoded in a relational, position-invariant format. Such representational shifts may provide a generic mechanism to achieve perceptual invariance in object recognition.     

Assessment of the Hemispheric Lateralization of Grapheme-Color Synesthesia with Stroop-Type Tests

Grapheme-color synesthesia, the idiosyncratic, arbitrary association of colors to letters or numbers, develops in childhood once reading is mastered. Because language processing is strongly left-lateralized in most individuals, we hypothesized that grapheme-color synesthesia could be left-lateralized as well. We used synesthetic versions of the Stroop test with colored letters and numbers presented either in the right or the left visual field of thirty-four synesthetes. Interference by synesthetic colors was stronger for stimuli in the right hemifield (first experiment, color naming task). Synesthetes were also faster in the right hemifield when naming the synesthetic color of graphemes (second experiment). Overall, the lateralization effect was 7 ms (the 95% confidence interval was [1.5 12] ms), a delay compatible with an additional callosal transfer for stimuli presented in the left hemifield. Though weak, this effect suggests that the association of synesthetic colors to graphemes may be preferentially processed in the left hemisphere. We speculate that this left-lateralization could be a landmark of synesthetic grapheme-color associations, if not found for color associations learnt by non-synesthete adults.     

Interhemispheric differences in the spatial-frequency description of images and the consequences for visual recognition

Visual perception of images transformed by spatial-frequency filtering was investigated in tachistoscopic experiments. Evidences have been received that the left hemisphere describes preferentially low harmonics of an image, and right hemisphere--the high ones. A hypothesis is suggested that these differences are based on different sizes of the receptive fields of cortical modules. Some consequences of the proposed model are discussed--the consequences for the invariance of visual recognition (innate invariance mechanisms in the left hemisphere and learned invariance in the right hemisphere) and for the methods of visual image classification (discriminant or structural methods in the left and right hemisphere respectively). Some data confirming these predictions are presented.     

Effect of the loci of visual cues on ability of polyrhythmic bimanual movement

Abstract

The present study investigated whether the deviation of the performed movement cycle from the required cycle during polyrhythmic bimanual (BM) movement depends on the loci of the visual cues that guide the rhythm of finger movements. Twelve healthy right-handed males rhythmically abducted and adducted the index finger or index fingers with the rhythm of the visual cues. During UM movement, the visual cue guiding the rhythm of finger movement was provided in the left or right visual hemifield. During 2:3 polyrhythmic BM movement, two visual cues, one guiding the rhythm of the left finger movement and another guiding the rhythm of the right finger movement, were provided in a single visual hemifield, or each visual cue guiding each finger movement was provided in each visual hemifield. During polyrhythmic BM movement, the cycle duration of the slower side of the movement guided by the rhythm of the visual cues provided in one visual hemifield was shorter than the required cycle duration, and the magnitude of the shortage in this condition was greater than that guided by each visual cue provided in each visual hemifield. Slower side of the movement is more precisely performed by each visual cue guiding each finger movement in each visual hemifield rather than that guided by visual cues provided in one visual hemifield during polyrhythmic BM movement. This may be explained by bottle-neck model in which visual information overflows the processing capacity when two visual processes are simultaneously provided in a single visual cortex.     

Gravitation and the Hemispheric Specialization of the Brain in Bilateral Symmetry Determination

We studied the influence of weightlessness on bilateral symmetry detection during prolonged space flight. Supposing that weightlessness may affect visual information processing by the right and left hemispheres in different ways, we studied this phenomenon with regard for the part of the visual field where to a stimulus was presented (the sight fixation center or the left/right half of this field). We used two types of stimuli, i.e., closed figures (polygons) and distributed figures formed by dots. There was a distinct difference between the central and noncentral presentation of stimuli under terrestrial conditions. When a stimulus was presented noncentrally (on the left or right), a manifest dominance of the horizontal axis was observed. However, there was no substantial difference while stimulating the left and right parts of the visual field. This contradicts the hypothesis on hemispheric specialization of the brain in symmetry detection. When stimuli were presented eccentrically, weightlessness did not notably influence information processing. When they were presented centrally, the predominance of the vertical axis in closed figures tended to weaken under the impact of weightlessness. However, this predominance strengthened when multicomponent figures were presented in space. The different influences of weightlessness on perceiving symmetry of stimuli of different types shows that it may be detected at various levels with different degrees of using nonvisual sensory information.     

Dynamic spatial coding within the dorsal frontoparietal network during a visual search task

To what extent are the left and right visual hemifields spatially coded in the dorsal frontoparietal attention network? In many experiments with neglect patients, the left hemisphere shows a contralateral hemifield preference, whereas the right hemisphere represents both hemifields. This pattern of spatial coding is often used to explain the right-hemispheric dominance of lesions causing hemispatial neglect. However, pathophysiological mechanisms of hemispatial neglect are controversial because recent experiments on healthy subjects produced conflicting results regarding the spatial coding of visual hemifields. We used an fMRI paradigm that allowed us to distinguish two attentional subprocesses during a visual search task. Either within the left or right hemifield subjects first attended to stationary locations (spatial orienting) and then shifted their attentional focus to search for a target line. Dynamic changes in spatial coding of the left and right hemifields were observed within subregions of the dorsal front-parietal network: During stationary spatial orienting, we found the well-known spatial pattern described above, with a bilateral hemifield representation in the right hemisphere and a contralateral preference in the left hemisphere. However, during search, the right hemisphere had a contralateral preference and the left hemisphere equally represented both hemifields. This finding leads to novel perspectives regarding models of visuospatial attention and hemispatial neglect.     

Conditioned Sounds Enhance Visual Processing

This psychophysics study investigated whether prior auditory conditioning influences how a sound interacts with visual perception. In the conditioning phase, subjects were presented with three pure tones ( =  conditioned stimuli, CS) that were paired with positive, negative or neutral unconditioned stimuli. As unconditioned reinforcers we employed pictures (highly pleasant, unpleasant and neutral) or monetary outcomes (+50 euro cents, −50 cents, 0 cents). In the subsequent visual selective attention paradigm, subjects were presented with near-threshold Gabors displayed in their left or right hemifield. Critically, the Gabors were presented in synchrony with one of the conditioned sounds. Subjects discriminated whether the Gabors were presented in their left or right hemifields. Participants determined the location more accurately when the Gabors were presented in synchrony with positive relative to neutral sounds irrespective of reinforcer type. Thus, previously rewarded relative to neutral sounds increased the bottom-up salience of the visual Gabors. Our results are the first demonstration that prior auditory conditioning is a potent mechanism to modulate the effect of sounds on visual perception.     

Saccade latency during probability presentation of the visual targets in man

The latent periods of saccadic eye movements in response to peripheral visual stimuli were measured in 8 right-handed healthy subjects using Posner's paradigm "COST-BENEFIT". In 6 subjects, the saccade latency in response to visual target presented in expected location in valid condition was shorter than that in neutral condition ("benefit"). Increase in saccade latency in response to the visual target presented in unexpected location in valid condition versus neutral condition took place only in 4 subjects ("cost"). A decrease in left-directed saccade latency in response to expected target presented in the left hemifield and increase in saccade latency in response to unexpected left target in comparison with analogous right-directed saccades were observed in valid condition. This phenomenon can be explained by the dominance of the right hemisphere in the processes of spatial orientation and "disengage" of attention.     

Cortical Contributions to Saccadic Suppression

The stability of visual perception is partly maintained by saccadic suppression: the selective reduction of visual sensitivity that accompanies rapid eye movements. The neural mechanisms responsible for this reduced perisaccadic visibility remain unknown, but the Lateral Geniculate Nucleus (LGN) has been proposed as a likely site. Our data show, however, that the saccadic suppression of a target flashed in the right visual hemifield increased with an increase in background luminance in the left visual hemifield. Because each LGN only receives retinal input from a single hemifield, this hemifield interaction cannot be explained solely on the basis of neural mechanisms operating in the LGN. Instead, this suggests that saccadic suppression must involve processing in higher level cortical areas that have access to a considerable part of the ipsilateral hemifield.     

How birds use their eyes: Opposite left-right specialization for the lateral and frontal visual hemifield in the domestic chick

Recent evidence has demonstrated that, in animals with laterally placed eyes, functional cerebral asymmetry is revealed by preferential use of either the left or right eye in a range of behaviors (birds: [1, 2, 3]; fish: [4, 5]; reptiles: [6, 7]). These findings pose a theoretical problem. It seems that there would be disadvantages in having a substantial degree of asymmetry in the use of the two eyes; a deficit on one side would leave the organism vulnerable to attack on that side or unable to exploit resources appearing on one side. We here report a possible solution to the problem. We have found that domestic chicks show selective use of the lateral visual field of the left eye and of the right hemifield in the binocular, frontal visual field when they peck at strangers but not at cagemates. Thus, during social recognition, there seems to be opposite and complementary left-right specialization for the lateral and frontal visual fields of the two eyes. These findings can reconcile the computational advantages associated with asymmetry of the left and right sides of the brain with the ecological demands for an animal to perceive and respond equally well to the left and right sides of its midline.     

Mechanisms and asymmetries in visual perception of simultaneity and temporal order

Temporally overlapping, spatially separated visual stimuli were used for studying perception of simultaneity and temporal order. Pairs of flashes each of 100 ms duration were presented with stimulus onset asynchronies of 0, 30, 50, and 70 ms. Three spatial arrangements of flash presentation were tested: 1) both flashes were presented foveally; 2) one flash was presented foveally and the other at 9 deg in the left visual hemifield; 3) one flash was presented foveally and the other at 8 deg in the right visual hemifield. Onset asynchronies of 30 and 50 ms were not sufficient for correct identification of temporal order although the flashes were not perceived as simultaneous. Analysis of the response distributions suggests the existence of two-independent mechanisms for evaluating temporal interrelations: one for detecting simultaneity and the other for identifying temporal order. A better detection of simultaneity was found when synchroneous flashes were presented together with pairs of flashes separated by larger onset asynchronies. Reading habits may explain only part of the left-right asymmetries of the response distributions. The possible lateralization of the two suggested mechanisms within the cerebral hemispheres is discussed.     

Hemispheric Asymmetry in the Auditory Facilitation Effect in Dual-Stream Rapid Serial Visual Presentation Tasks

Even though auditory stimuli do not directly convey information related to visual stimuli, they often improve visual detection and identification performance. Auditory stimuli often alter visual perception depending on the reliability of the sensory input, with visual and auditory information reciprocally compensating for ambiguity in the other sensory domain. Perceptual processing is characterized by hemispheric asymmetry. While the left hemisphere is more involved in linguistic processing, the right hemisphere dominates spatial processing. In this context, we hypothesized that an auditory facilitation effect in the right visual field for the target identification task, and a similar effect would be observed in the left visual field for the target localization task. In the present study, we conducted target identification and localization tasks using a dual-stream rapid serial visual presentation. When two targets are embedded in a rapid serial visual presentation stream, the target detection or discrimination performance for the second target is generally lower than for the first target; this deficit is well known as attentional blink. Our results indicate that auditory stimuli improved target identification performance for the second target within the stream when visual stimuli were presented in the right, but not the left visual field. In contrast, auditory stimuli improved second target localization performance when visual stimuli were presented in the left visual field. An auditory facilitation effect was observed in perceptual processing, depending on the hemispheric specialization. Our results demonstrate a dissociation between the lateral visual hemifield in which a stimulus is projected and the kind of visual judgment that may benefit from the presentation of an auditory cue.