Left Visual Hemifield Research Articles

Study of target and non-target interplay in spatial attention task

Selective visual attention is the ability to selectively pay attention to the targets while inhibiting the distractors. This paper aims to study the targets and non-targets interplay in spatial attention task while subject attends to the target object present in one visual hemifield and ignores the distractor present in another visual hemifield. This paper performs the averaged evoked response potential (ERP) analysis and time-frequency analysis. ERP analysis agrees to the left hemisphere superiority over late potentials for the targets present in right visual hemifield. Time-frequency analysis performed suggests two parameters i.e. event-related spectral perturbation (ERSP) and inter-trial coherence (ITC). These parameters show the same properties for the target present in either of the visual hemifields but show the difference while comparing the activity corresponding to the targets and non-targets. In this way, this study helps to visualise the difference between targets present in the left and right visual hemifields and, also the targets and non-targets present in the left and right visual hemifields. These results could be utilised to monitor subjects’ performance in brain–computer interface (BCI) and neurorehabilitation.

Sex differences and functional hemispheric asymmetries during number comparison

BackgroundGlobal-local stimuli are hierarchical structures consisting of a larger global structure which is composed of smaller local stimuli. Numbers are also constructed hierarchically, with multi-digit numbers being made up from single digits. During two-digit number comparison, compatible items (larger number contains larger unit digit, e.g., 53 vs. 68) are processed faster and more accurately than incompatible items (smaller number contains larger unit digit, e.g., 58 vs. 63). This so-called unit-decade-compatibility effect has challenged the holistic model of number processing and suggests that the processing of number magnitudes occurs at least in part, decomposed, i.e., separately for each digit. Thus, the compatibility effect is indicative of how decomposed numbers are processed, thereby sharing similarities with traditional global-local processing of hierarchical stimuli.The goal of this study was to investigate whether factors that have been shown to reliably influence global-local processing also affect the compatibility effect during number comparison. Those include visual hemifield, sex, and menstrual cycle phase in women.MethodOne hundred sixty participants, 77 naturally cycling women and 83 men, completed a two-digit number comparison task twice, with test-sessions time-locked to the early follicular or mid-luteal cycle phase in women. Number comparison stimuli were presented to the right or left hemifield, respectively.ResultsWe observed a stronger compatibility effect in the right visual hemifield compared to left visual hemifield and in women compared to men, but no evidence for an influence of menstrual cycle phase in women could be found.ConclusionHemispheric asymmetries in holistic versus decomposed number processing could be demonstrated for the first time, suggesting a similar hemispheric modulation for number magnitude processing as for global-local processing.

Right but not left hemispheric discrimination of faces in infancy.

The ontogeny of the functional asymmetries of the human brain is poorly understood. Are they a consequence of differential development based on competition mechanisms, or are they constitutive of the human brain architecture from the start? Using structural magnetic resonance imaging and a face-discrimination electroencephalography paradigm with lateralized presentation of faces, we studied face perception in infants over the first postnatal semester. We showed that the corpus callosum is sufficiently mature to transfer visual information across hemispheres, but the inter-hemispheric transfer time of early visual responses is modulated by callosal fibre myelination. We also revealed that only the right hemisphere shows evidence of face discrimination when presented in the left visual hemifield. This capability improved throughout the first semester with no evidence of discrimination in the left hemisphere. Face-processing lateralization is thus a characteristic of the infant's extra-striate visual cortex, highlighting the differential left-right organization of the human brain already established in infanthood.

Attentional asymmetry between visual hemifields is related to habitual direction of reading and its implications for debate on cause and effects of dyslexia

A major controversy regarding dyslexia is whether any of the many visual and phonological deficits found to be correlated with reading difficulty cause the impairment or result from the reduced amount of reading done by dyslexics. We studied this question by comparing a visual capacity in the left and right visual hemifields in people habitually reading scripts written right-to-left or left-to-right. Selective visual attention is necessary for efficient visual search and also for the sequential recognition of letters in words. Because such attentional allocation during reading depends on the direction in which one is reading, asymmetries in search efficiency may reflect biases arising from the habitual direction of reading. We studied this by examining search performance in three cohorts: (a) left-to-right readers who read English fluently; (b) right-to-left readers fluent in reading Farsi but not any left-to-right script; and (c) bilingual readers fluent in English and in Farsi, Arabic, or Hebrew. Left-to-right readers showed better search performance in the right hemifield and right-to-left readers in the left hemifield, but bilingual readers showed no such asymmetries. Thus, reading experience biases search performance in the direction of reading, which has implications for the cause and effect relationships between reading and cognitive functions.

Simultaneous multi-slice inverse imaging of the human brain

Ultrafast functional magnetic resonance imaging (fMRI) can measure blood oxygen level dependent (BOLD) signals with high sensitivity and specificity. Here we propose a novel method: simultaneous multi-slice inverse imaging (SMS-InI) — a combination of simultaneous multi-slice excitation, simultaneous echo refocusing (SER), blipped controlled aliasing in parallel imaging echo-planar imaging (EPI), and regularized image reconstruction. Using a 32-channel head coil array on a 3 T scanner, SMS-InI achieves nominal isotropic 5-mm spatial resolution and 10 Hz sampling rate at the whole-brain level. Compared with traditional inverse imaging, we found that SMS-InI has higher spatial resolution with lower signal leakage and higher time-domain signal-to-noise ratio with the optimized regularization parameter in the reconstruction. SMS-InI achieved higher effective resolution and higher detection power in detecting visual cortex activity than InI. SMS-InI also detected subcortical fMRI signals with the similar sensitivity and localization accuracy like EPI. The spatiotemporal resolution of SMS-InI was used to reveal that presenting visual stimuli with 0.2 s latency between left and right visual hemifield led to 0.2 s relative hemodynamic response latency between the left and right visual cortices. Together, these results indicate that SMS-InI is a useful tool in measuring cortical and subcortical hemodynamic responses with high spatiotemporal resolution.

Plasticity of the human visual brain after an early cortical lesion

In adults, partial damage to V1 or optic radiations abolishes perception in the corresponding part of the visual field, causing a scotoma. However, it is widely accepted that the developing cortex has superior capacities to reorganize following an early lesion to endorse adaptive plasticity. Here we report a single patient case (G.S.) with near normal central field vision despite a massive unilateral lesion to the optic radiations acquired early in life. The patient underwent surgical removal of a right hemisphere parieto-temporal-occipital atypical choroid plexus papilloma of the right lateral ventricle at four months of age, which presumably altered the visual pathways during in utero development. Both the tumor and surgery severely compromised the optic radiations. Residual vision of G.S. was tested psychophysically when the patient was 7 years old. We found a close-to-normal visual acuity and contrast sensitivity within the central 25° and a great impairment in form and contrast vision in the far periphery (40–50°) of the left visual hemifield. BOLD response to full field luminance flicker was recorded from the primary visual cortex (V1) and in a region in the residual temporal-occipital region, presumably corresponding to the middle temporal complex (MT+), of the lesioned (right) hemisphere. A population receptive field analysis of the BOLD responses to contrast modulated stimuli revealed a retinotopic organization just for the MT+ region but not for the calcarine regions. Interestingly, consistent islands of ipsilateral activity were found in MT+ and in the parieto-occipital sulcus (POS) of the intact hemisphere. Probabilistic tractography revealed that optic radiations between LGN and V1 were very sparse in the lesioned hemisphere consistently with the post-surgery cerebral resection, while normal in the intact hemisphere. On the other hand, strong structural connections between MT+ and LGN were found in the lesioned hemisphere, while the equivalent tract in the spared hemisphere showed minimal structural connectivity. These results suggest that during development of the pathological brain, abnormal thalamic projections can lead to functional cortical changes, which may mediate functional recovery of vision.

Differential hemispheric and visual stream contributions to ensemble coding of crowd emotion.

In crowds, where scrutinizing individual facial expressions is inefficient, humans can make snap judgments about the prevailing mood by reading "crowd emotion". We investigated how the brain accomplishes this feat in a set of behavioral and fMRI studies. Participants were asked to either avoid or approach one of two crowds of faces presented in the left and right visual hemifields. Perception of crowd emotion was improved when crowd stimuli contained goal-congruent cues and was highly lateralized to the right hemisphere. The dorsal visual stream was preferentially activated in crowd emotion processing, with activity in the intraparietal sulcus and superior frontal gyrus predicting perceptual accuracy for crowd emotion perception, whereas activity in the fusiform cortex in the ventral stream predicted better perception of individual facial expressions. Our findings thus reveal significant behavioral differences and differential involvement of the hemispheres and the major visual streams in reading crowd versus individual face expressions.

Face processing is enhanced in the left and upper visual hemi-fields

ABSTRACTWe tested whether two known hemi-field asymmetries would affect visual search with face stimuli. Holistic processing of spatial configurations is better in the left hemi-field, reflecting a right hemisphere specialization, and object recognition is better in the upper visual field, reflecting stronger projections into the ventral stream. Faces tap into holistic processing and object recognition at the same time, which predicts better performance in the left and upper hemi-field, respectively. In the first experiment, participants had to detect a face with a gaze direction different from the remaining faces. Participants were faster to respond when targets were presented in the left and upper hemi-field. The same pattern of results was observed when only the eye region was presented. In the second experiment, we turned the faces upside-down, which eliminated the typical spatial configuration of faces. The left hemi-field advantage disappeared, showing that it is related to holistic processing of faces, whereas the upper hemi-field advantage related to object recognition persisted. Finally, we made the search task easier by asking observers to search for a face with open among closed eyes or vice versa. The easy search task eliminated the need for complex object recognition and, accordingly, the advantage of the upper visual field disappeared. Similarly, the left hemi-field advantage was attenuated. In sum, our findings show that both horizontal and vertical asymmetries affect the search for faces and can be selectively suppressed by changing characteristics of the stimuli.

The Location and Effects of Visual Hemisphere-Specific Stimulation on Reading Fluency in Children With the Characteristics of Dyslexia

Fluency is used as an indicator of reading proficiency. Many students with reading disabilities are unable to benefit from typical interventions. This study is designed to replicate Lorusso, Facoetti, Paganoni, Pezzani, and Molteni’s (2006) work using FlashWord, a computer program that tachistoscopically presents words in the right or left visual hemi-field in English and locates through fMRI imaging the processing areas involved in fluency development. Our participants were 15 students who were ages 8 to 19 years and had reading disabilities randomly assigned to Intervention (n = 9) and Delayed Intervention (n = 6) groups. Functional imaging studies focused on analyzing activations in the left hemisphere (LH) superior temporal gyrus, the inferior frontal gyrus, and the LH inferior occipito-temporal/fusiform area (visual-word form area [VWFA]). Analysis of intervention data showed that 6 of the 9 Intervention group participants (67%) achieved levels of automatic processing and increased their reading rate by an average of 20 words per minute after participating in the FlashWord intervention. Analyses of fMRI group activation maps and mean activation levels in regions of interest document processing changes in VWFA activations that could be related to the increase in reading speed and confirm these locations as essential to developing fluency.

Stronger interference from distractors in the right hemifield during visual search

ABSTRACTThe orientation-bias hypothesis states that there is a bias to attend to the right visual hemifield (RVF) when there is spatial competition between stimuli in the left and right hemifield [Pollmann, S. (1996). A pop-out induced extinction-like phenomenon in neurologically intact subjects. Neuropsychologia, 34(5), 413–425. doi:10.1016/0028-3932(95)00125-5]. In support of this hypothesis, stronger interference was reported for RVF distractors with contralateral targets. In contrast, previous studies using rapid serial visual presentation (RSVP) found stronger interference from distractors in the left visual hemifield (LVF). We used the additional singleton paradigm to test whether this discrepancy was due to the different distractor features that were employed (colour vs. orientation). Interference from the colour distractor with contralateral targets was larger in the RVF than in the LVF. However, the asymmetrical interference disappeared when observers had to search for an inconspicuous colour target instead of the inconspicuous shape target. We suggest that the LVF orienting-bias is limited to situations where search is driven by bottom-up saliency (singleton search) instead of top-down search goals (feature search). In contrast, analysis of the literature suggests the opposite for the LVF bias in RSVP tasks. Thus, the attentional asymmetry may depend on whether the task involves temporal or spatial competition, and whether search is based on bottom-up or top-down signals.

Relative Spatial Frequency Processing Drives Hemispheric Asymmetry in Conscious Awareness.

Visual stimuli with different spatial frequencies (SFs) are processed asymmetrically in the two cerebral hemispheres. Specifically, low SFs are processed relatively more efficiently in the right hemisphere than the left hemisphere, whereas high SFs show the opposite pattern. In this study, we ask whether these differences between the two hemispheres reflect a low-level division that is based on absolute SF values or a flexible comparison of the SFs in the visual environment at any given time. In a recent study, we showed that conscious awareness of SF information (i.e., visual perceptual selection from multiple SFs simultaneously present in the environment) differs between the two hemispheres. Building upon that result, here we employed binocular rivalry to test whether this hemispheric asymmetry is due to absolute or relative SF processing. In each trial, participants viewed a pair of rivalrous orthogonal gratings of different SFs, presented either to the left or right of central fixation, and continuously reported which grating they perceived. We found that the hemispheric asymmetry in perception is significantly influenced by relative processing of the SFs of the simultaneously presented stimuli. For example, when a medium SF grating and a higher SF grating were presented as a rivalry pair, subjects were more likely to report that they initially perceived the medium SF grating when the rivalry pair was presented in the left visual hemifield (right hemisphere), compared to the right hemifield. However, this same medium SF grating, when it was paired in rivalry with a lower SF grating, was more likely to be perceptually selected when it was in the right visual hemifield (left hemisphere). Thus, the visual system’s classification of a given SF as “low” or “high” (and therefore, which hemisphere preferentially processes that SF) depends on the other SFs that are present, demonstrating that relative SF processing contributes to hemispheric differences in visual perceptual selection.

Neural signatures of adaptive post-error adjustments in visual search

Errors in speeded choice tasks can lead to post-error adjustments both on the behavioral and on the neural level. There is an ongoing debate whether such adjustments result from adaptive processes that serve to optimize performance or whether they reflect interference from error monitoring or attentional orientation. The present study aimed at identifying adaptive adjustments in a two-stage visual search task, in which participants had to select and subsequently identify a target stimulus presented to the left or right visual hemifield. Target selection and identification can be measured by two distinct event-related potentials, the N2pc and the SPCN. Using a decoder analysis based on multivariate pattern analysis, we were able to isolate the processing stages related to error sources and post-error adjustments. Whereas errors were linked to deviations in the N2pc and the SPCN, only for the N2pc we identified a post-error adjustment, which exhibits key features of source-specific adaptivity. While errors were associated with an increased N2pc, post-error adjustments consisted in an N2pc decrease. We interpret this as an adaptive adjustment of target selection to prevent errors due to disproportionate processing of the task-irrelevant target location. Our study thus provides evidence for adaptive post-error adjustments in visual search.

Visual Search for Letters in the Right vs. Left Visual Hemifields: The Role of Perceptual Load and Set

The aim of the study was to better understand interconnection of visual attention, word processing and visual field asymmetries. Three experiments investigated visual search for a prespecified letter in displays containing 6-letter words or non-words placed 7 degrees left and right to the fixation, with a variable target letter position within word and non-word strings. In Experiment 1, two letter strings of the same type (either words or non-words) were presented to both sides of the visual field. In Experiment 2, there was only one letter string presented right or left to the fixation. In Experiment 3, two letter strings presented to both sides of the visual field were of different type (one word and one non-word). RT and accuracy data were collected. The results of Experiment 1 provide evidence for serial search for a letter within a word in the left visual field (LVF) and within a non-word in the right visual field (RVF) and parallel search for a letter within a non-word in the LVF and within a word in the RVF. The results of Experiment 3 were similar except for serial search for words in RVF. In Experiment 2, where perceptual load was twice lower, the search within both types of letter strings in both hemifields was serial. These results demonstrate the influence of the perceptual load and readiness to process a certain type of letter string, on the observers' choice of search strategy.

Hemifield columns co-opt ocular dominance column structure in human achiasma

In the absence of an optic chiasm, visual input to the right eye is represented in primary visual cortex (V1) in the right hemisphere, while visual input to the left eye activates V1 in the left hemisphere. Retinotopic mapping In V1 reveals that in each hemisphere left and right visual hemifield representations are overlaid (Hoffmann et al., 2012). To explain how overlapping hemifield representations in V1 do not impair vision, we tested the hypothesis that visual projections from nasal and temporal retina create interdigitated left and right visual hemifield representations in V1, similar to the ocular dominance columns observed in neurotypical subjects (Victor et al., 2000). We used high-resolution fMRI at 7T to measure the spatial distribution of responses to left- and right-hemifield stimulation in one achiasmic subject. T2-weighted 2D Spin Echo images were acquired at 0.8mm isotropic resolution. The left eye was occluded. To the right eye, a presentation of flickering checkerboards alternated between the left and right visual fields in a blocked stimulus design. The participant performed a demanding orientation-discrimination task at fixation. A general linear model was used to estimate the preference of voxels in V1 to left- and right-hemifield stimulation. The spatial distribution of voxels with significant preference for each hemifield showed interdigitated clusters which densely packed V1 in the right hemisphere. The spatial distribution of hemifield-preference voxels in the achiasmic subject was stable between two days of testing and comparable in scale to that of human ocular dominance columns. These results are the first in vivo evidence showing that visual hemifield representations interdigitate in achiasmic V1 following a similar developmental course to that of ocular dominance columns in V1 with intact optic chiasm.

Use of backward masking test for the study of visual information processing in healthy subjects and schizophrenic patients

The study of 14 healthy subjects and 15 schizophrenic patients was conducted under visual backward masking conditions. Sensory thresholds were identified using the method of constant stimuli. A special modification of the backward masking technique with lateralized presentation of test and masking stimuli was used to study the lateral characteristics of visual attention. It was found that the thresholds of letter stimulus identification were significantly higher in patients with schizophrenia than in healthy subjects. Only in patients the asymmetry of visual perception was revealed with the higher recognition thresholds in the left visual hemifield. The overall data analysis suggests an association between increased recognition thresholds in schizophrenic patients and changes in the interruption mechanism functioning at the neocortex level.

198 Dutasteride for men with androgenic alopecia unresponsive to finasteride

198 Dutasteride for men with androgenic alopecia unresponsive to finasteride IG Motofei, DL Rowland, SR Georgescu, M Tampa, VD Constantin, S Paunica, BC Baleanu and I Sinescu 1 Surgery and Urology, “Carol Davila” University of Medicine and Pharmacy, Bucharest, Romania, 2 Psychology, Valparaiso University, Valparaiso, IN, 3 Dermatology, “Carol Davila” University of Medicine and Pharmacy, Bucharest, Romania, 4 Urology, Hermann-Josef KH, Kreisfreie Stadt Aachen Area, Germany and 5 Urology, “Carol Davila” University of Medicine and Pharmacy, Bucharest, Romania About 30% of men with androgenic alopecia have no significant improvement at six months after Finasteride administration. For this subgroup, dutasteride could represent an alternative therapy being more efficient and with reasonable adverse effects, according to some authors. Starting from this perspective, we intended to verify the dutasteride efficiency and side effects in androgenic alopecia, on unresponsive men to finasteride. From ninety-two men treated with Finasteride (1 mg/ day) for androgenic alopecia, twenty three subjects finished the treatment with no result on the target hair area, as confirmed by global photograph assessment and phototrichogram. From those 23 unresponsive men, 16 started a new treatment with dutasteride 0.5 mg/ day, being evaluated after six months again by dermatologists and urologists. In response to dutasteride, 9 subjects (56.25%) registered a significant improvement (hair density/ thickness) after six months, according to photograph and phototrichogram assessments. The other 7 subjects were appreciated as unresponsives to dutasteride, no aggravation being reported. Dutasteride adverse effects were reasonable, consisting in transient erectile dysfunction (in two responsive patients) and erectile dysfunction plus slight depressive episode (in 1 unresponsive patient). In contrast to Finasteride, dutasteride inhibits both type I and II of 5-a reductase enzyme, being able to ameliorate androgenic alopecia in some subjects who registered no improvement after finasteride administration. Dutasteride could be thus viewed as a secondary therapeutic option for androgenic alopecia, and a possible first therapeutic choice at subjects with predisposed (anticipated) finasteride side effects. 199 Hand preference and sexual orientation as useful elements to predict finasteride side effects in male androgenic alopecia DL Rowland, IG Motofei, SR Georgescu, M Tampa and I Sinescu 1 Psychology, Valparaiso University, Valparaiso, IN, 2 Surgery and Urology, “Carol Davila” University of Medicine and Pharmacy, Bucharest, Romania, 3 Dermatology, “Carol Davila” University of Medicine and Pharmacy, Bucharest, Romania and 4 Urology, “Carol Davila” University of Medicine and Pharmacy, Bucharest, Romania This study further investigated the distribution of finasteride adverse effects (according to hand preference and sexual orientation) on men treated for male androgenic alopecia, based on the existing interrelation among sexual hormones, pheromones, cognitive and sexual laterality of information processing. Finasteride 1 mg daily, human axillary pheromones, questionnaires regarding hand preference (Edinburgh Handedness Inventory) and related to sexual orientation/ behavior were used, to delineate within the brain hormonal and pheromonal neuromodulation of sexuality on 68 men with androgenic alopecia. Participants’ responses delineated four distinct groups. Group Awith 12 men, involving especially the left hemibrain: sensitive to female axillary pheromones, left nostril and right visual hemifield, with left hand preference and predominantly heterosexuals (9 men). Group B (16 men) related to the right hemibrain: sensitive to male axillary pheromones, right nostril, and left visual hemifield, with right hand preference, six being homosexuals and ten heterosexuals. Group C with 11 men related to the left hemibrain: sensitive to androgens, with right hand preference and heterosexuals. Group D (29 men) related to right hemibrain: sensitive to estrogens, with left hand preference and mixed sexual orientation (17 heterosexuals and 12 homosexuals). Finasteride adverse effects were encountered especially in groups C (on 8 men) and group D (on 7 men), specifically on subjects who were either right handed heterosexuals or left handed homosexuals. In psycho-physiological terms, androgens seem to modulate only the left hemibrain, and more precisely, the left dorsal thalamic route.

EP 54. Alpha activity in Fronto-Parietal Areas is modulated by Dopamine in Switching Visuospatial Attention in Parkinson’s Disease

Alpha-band activity (8–12 Hz) is a prominent electrophysiological signal in the human brain. Alpha power is known to be modulated by visuospatial attention in healthy humans( Thut et al., 2006 , Klimesch, 2012 ). Patients with Parkinson’s disease (PD) show a decrease in attentional control and deficits in switching of attention ( Aarsland et al., 2010 ). Here, we studied the role of alpha power in switching of visuospatial attention in PD. In PD, motor symptoms improve with dopamine replacement medication. We additionally studied whether dopaminergic medication alters the network activity in the alpha band during visuospatial attention. 20 subjects with diagnosed PD (15 male, 55 ± 10 y) performed the task in which they had to attend either to the left or right visual hemifield, indicated by a central cue. After a delay (1200 ms) a visual stimulus was presented in each hemifield. After another randomly chosen time, one of the two stimuli changed its color. A button press was required when the color change appeared on the cued side. We measured reaction times, and brain activity with magnetoencephalography. To analyze frequency bands of brain activity, we applied fast Fourier transform on the neural data. Subjects performed the task after 12 h withdrawal from dopamine replacement medication (“OFF”) and after receiving L-DOPA (“ON”). Before and after receiving L-DOPA, we performed the Unified Parkinson’s Disease Rating Scale (UPDRS) to assess efficiency of L-DOPA. We compared switching with not-switching trials, with “switching” meaning two successive trials required attention to opposite hemifields, “not-switching” meaning two successive trials required attention to the same hemifield. Comparison was done between and within medication factor (ON/OFF).UPDRS scores showed significant improvement of motor symptoms after receiving L-DOPA. During OFF, subjects responded faster in not-switching than in switching, whereas during ON there was no difference in reaction times. During OFF, we found a decrease of alpha power for switching relative to not-switching in right parietal (5–15 Hz) and right frontal areas (10–17 Hz) during the delay period. During ON, we found a decrease of alpha power for switching relative to not-switching in right occipital regions (2.5–15 Hz) during the cue period. Finally, we found a positive correlation between alpha power (8–12 Hz) and reaction times during OFF for switching, but not for not-switching. Faster reaction times for not-switching during OFF might reflect a strong focus on the previously attended side. Switching attention during OFF is therefore more difficult, which is reflected in less relative alpha power for switching, and thus more activity, in the fronto-parietal network. L-DOPA, therefore, increases flexibility for switching by lessening suppressing alpha-band activity in the fronto-parietal network. This could be achieved by better visual processing of the cue, as reflected in less alpha power during the cue period in visual areas.

Glioblastoma multiforme: a devastating diagnosis

It declared itself abruptly with bright flashing lights in the left visual hemifield. Our 71-year-old left-handed dad (figure 1), a general practitioner and master handyman, had been painting the ceiling,...

Behavioral Differences in the Upper and Lower Visual Hemifields in Shape and Motion Perception

Perceptual accuracy is known to be influenced by stimuli location within the visual field. In particular, it seems to be enhanced in the lower visual hemifield (VH) for motion and space processing, and in the upper VH for object and face processing. The origins of such asymmetries are attributed to attentional biases across the visual field, and in the functional organization of the visual system. In this article, we tested content-dependent perceptual asymmetries in different regions of the visual field. Twenty-five healthy volunteers participated in this study. They performed three visual tests involving perception of shapes, orientation and motion, in the four quadrants of the visual field. The results of the visual tests showed that perceptual accuracy was better in the lower than in the upper visual field for motion perception, and better in the upper than in the lower visual field for shape perception. Orientation perception did not show any vertical bias. No difference was found when comparing right and left VHs. The functional organization of the visual system seems to indicate that the dorsal and the ventral visual streams, responsible for motion and shape perception, respectively, show a bias for the lower and upper VHs, respectively. Such a bias depends on the content of the visual information.

The left hemisphere learns what is right: Hemispatial reward learning depends on reinforcement learning processes in the contralateral hemisphere

Orienting biases refer to consistent, trait-like direction of attention or locomotion toward one side of space. Recent studies suggest that such hemispatial biases may determine how well people memorize information presented in the left or right hemifield. Moreover, lesion studies indicate that learning rewarded stimuli in one hemispace depends on the integrity of the contralateral striatum. However, the exact neural and computational mechanisms underlying the influence of individual orienting biases on reward learning remain unclear. Because reward-based behavioural adaptation depends on the dopaminergic system and prediction error (PE) encoding in the ventral striatum, we hypothesized that hemispheric asymmetries in dopamine (DA) function may determine individual spatial biases in reward learning. To test this prediction, we acquired fMRI in 33 healthy human participants while they performed a lateralized reward task. Learning differences between hemispaces were assessed by presenting stimuli, assigned to different reward probabilities, to the left or right of central fixation, i.e. presented in the left or right visual hemifield. Hemispheric differences in DA function were estimated through differential fMRI responses to positive vs. negative feedback in the left vs. right ventral striatum, and a computational approach was used to identify the neural correlates of PEs. Our results show that spatial biases favoring reward learning in the right (vs. left) hemifield were associated with increased reward responses in the left hemisphere and relatively better neural encoding of PEs for stimuli presented in the right (vs. left) hemifield. These findings demonstrate that trait-like spatial biases implicate hemisphere-specific learning mechanisms, with individual differences between hemispheres contributing to reinforcing spatial biases.