Enhanced reading abilities is modulated by faster visual spatial attention

Research has shown improved reading following visual magnocellular training in individuals with dyslexia. Many studies have demonstrated how the magnocellular pathway controls visual spatial attention. Therefore, we have investigated the relationship between magnocellular pathway and visual spatial attention deficits in dyslexia in order to better understand how magnocellular-based interventions may help children to learn to read. Magnocellular function, visual spatial attention, and reading abilities of thirty elementary school students with dyslexia, aged between 8 and 10, were measured. The experimental group received magnocellular-based visual motion training for 12 sessions, while the control group received neutral sessions. All tests were repeated at the end of the training and after 1 month. The magnocellular functioning, visual spatial attention, and reading abilities of the experimental group improved significantly compared to the controls. Additionally, improvement in reaction time of invalid conditions predicted improvements in saccadic eye movements. We conclude that visual magnocellular training improved saccadic eye movement control, visual spatial orientation, and reading ability.

Faster recognition of graspable targets defined by orientation in a visual search task

Peri-hand space is the area surrounding the hand. Objects within this space may be subject to increased visuospatial perception, increased attentional prioritization, and slower attentional disengagement compared to more distal objects. This may result from kinesthetic and visual feedback about the location of the hand that projects from the reach and grasp networks of the dorsal visual stream back to occipital visual areas, which in turn, refines cortical visual processing that can subsequently guide skilled motor actions. Thus, we hypothesized that visual stimuli that afford action, which are known to potentiate activity in the dorsal visual stream, would be associated with greater alterations in visual processing when presented near the hand. To test this, participants held their right hand near or far from a touchscreen that presented a visual array containing a single target object that differed from 11 distractor objects by orientation only. The target objects and their accompanying distractors either strongly afforded grasping or did not. Participants identified the target among the distractors by reaching out and touching it with their left index finger while eye-tracking was used to measure visual search times, target recognition times, and search accuracy. The results failed to support the theory of enhanced visual processing of graspable objects near the hand as participants were faster at recognizing graspable compared to non-graspable targets, regardless of the position of the right hand. The results are discussed in relation to the idea that, in addition to potentiating appropriate motor responses, object affordances may also potentiate early visual processes necessary for object recognition.

Eccentricity-dependent temporal contrast tuning in human visual cortex measured with fMRI

Cells in the peripheral retina tend to have higher contrast sensitivity and respond at higher flicker frequencies than those closer to the fovea. Although this predicts increased behavioural temporal contrast sensitivity in the peripheral visual field, this effect is rarely observed in psychophysical experiments. It is unknown how temporal contrast sensitivity is represented across eccentricity within cortical visual field maps and whether such sensitivities reflect the response properties of retinal cells or psychophysical sensitivities. Here, we used functional magnetic resonance imaging (fMRI) to measure contrast sensitivity profiles at four temporal frequencies in five retinotopically-defined visual areas. We also measured population receptive field (pRF) parameters (polar angle, eccentricity, and size) in the same areas. Overall contrast sensitivity, independent of pRF parameters, peaked at 10 Hz in all visual areas. In V1, V2, V3, and V3a, peripherally-tuned voxels had higher contrast sensitivity at a high temporal frequency (20 Hz), while hV4 more closely reflected behavioural sensitivity profiles. We conclude that our data reflect a cortical representation of the increased peripheral temporal contrast sensitivity that is already present in the retina and that this bias must be compensated later in the cortical visual pathway.

Bipolar cell gap junctions serve major signaling pathways in the human retina

Connexin36 (Cx36) constituent gap junctions (GJ) throughout the brain connect neurons into functional syncytia. In the retina they underlie the transmission, averaging and correlation of signals prior conveying visual information to the brain. This is the first study that describes retinal bipolar cell (BC) GJs in the human inner retina, whose function is enigmatic even in the examined animal models. Furthermore, a number of unique features (e.g. fovea, trichromacy, midget system) necessitate a reexamination of the animal model results in the human retina. Well-preserved postmortem human samples of this study are allowed to identify Cx36 expressing BCs neurochemically. Results reveal that both rod and cone pathway interneurons display strong Cx36 expression. Rod BC inputs to AII amacrine cells (AC) appear in juxtaposition to AII GJs, thus suggesting a strategic AII cell targeting by rod BCs. Cone BCs serving midget, parasol or koniocellular signaling pathways display a wealth of Cx36 expression to form homologously coupled arrays. In addition, they also establish heterologous GJ contacts to serve an exchange of information between parallel signaling streams. Interestingly, a prominent Cx36 expression was exhibited by midget system BCs that appear to maintain intimate contacts with bistratified BCs serving other pathways. These findings suggest that BC GJs in parallel signaling streams serve both an intra- and inter-pathway exchange of signals in the human retina.

Competition for attentional resources between low spatial frequency content of emotional images and a foreground task in early visual cortex

Low spatial frequency (LSF) image content has been proposed to play a superior functional role in emotional content extraction via the magnocellular pathway biasing attentional resources toward emotional content in visual cortex. We investigated whether emotionally unpleasant complex images that were presented either unfiltered or with LSF content only in the background while subjects performed a foreground task will withdraw more attentional resources from the task compared to unemotional, neutral images (distraction paradigm). We measured steady-state visual evoked potentials (SSVEPs) driven by flickering stimuli of a foreground task. Unfiltered unpleasant images resulted in a significant reduction of SSVEP amplitude compared to neutral images. No statistically significant differences were found with LSF background images. In a behavioral control experiment, we found no significant differences for complexity ratings between unfiltered and LSF pictures. Content identification was possible for unfiltered and LSF picture (correct responses > 74%). An additional EEG study examined typical emotion-related components for complex images presented either as unfiltered, LSF, or high spatial frequency (HSF, as an additional control) filtered, unpleasant, and neutral images. We found a significant main effect of emotional valence in the early posterior negativity. Late positive potential differences were only found for unfiltered and HSF images. Results suggest that, while LSF content is sufficient to allow for content and emotional cue extraction when images were presented alone, LSF content is not salient enough to serve as emotional distractor that withdraws attentional resources from a foreground task in early visual cortex.

Examining the role of red background in magnocellular contribution to face perception

This study examines the role of the magnocellular system in the early stages of face perception, in particular sex categorization. Utilizing the specific property of magnocellular suppression in red light, we investigated visually guided reaching to low and high spatial frequency hybrid faces against red and grey backgrounds. The arm movement curvature measure shows that reduced response of the magnocellular pathway interferes with the low spatial frequency component of face perception. This finding provides behavioral evidence for magnocellular contribution to non-emotional aspect of face perception.

A touchy subject: advancing the modulated visual pathways account of altered vision near the hand

A growing body of evidence demonstrates that human vision operates differently in the space near and on the hands; for example, early findings in this literature reported that rapid onsets are detected faster near the hands, and that objects are searched more thoroughly. These and many other effects were attributed to enhanced attention via the recruitment of bimodal visual-tactile neurons representing the hand and near-hand space. However, recent research supports an alternative account: stimuli near the hands are preferentially processed by the action-oriented magnocellular visual pathway at the expense of processing in the parvocellular pathway. This Modulated Visual Pathways (MVP) account of altered vision near the hands describes a hand position-dependent trade-off between the two main retinal-cortical visual pathways between the eye and brain. The MVP account explains past findings and makes new predictions regarding near-hand vision supported by new research.

Speed discrimination predicts word but not pseudo-word reading rate in adults and children

Visual processing in the magnocellular pathway is a reputed influence on word recognition and reading performance. However, the mechanisms behind this relationship are still unclear. To explore this concept, we measured reading rate, speed-discrimination, and contrast detection thresholds in adults and children with a wide range of reading abilities. We found that speed discrimination thresholds are higher in children than in adults and are correlated with age. Speed discrimination thresholds are also correlated with reading rates but only for real words, not pseudo-words. Conversely, we found no correlations between contrast detection thresholds and the reading rates. We also found no correlations between speed discrimination or contrast detection and WASI subtest scores. These findings indicate that familiarity is a factor in magnocellular operations that may influence reading rate. We suggest this effect supports the idea that the magnocellular pathway contributes to word reading through an analysis of letter position.

Developmental dissociation of visual dorsal stream parvo and magnocellular representations and the functional impact of negative retinotopic BOLD responses

Localized neurodevelopmental defects provide an opportunity to study structure-function correlations in the human nervous system. This unique multimodal case report of epileptogenic dysplasia in the visual cortex allowed exploring visual function across distinct pathways in retinotopic regions and the dorsal stream, in relation to fMRI retinotopic mapping and spike triggered BOLD responses. Pre-surgical EEG/video monitoring, MRI/DTI, EEG/fMRI, PET and SPECT were performed to characterize structure/function correlations in this patient with a very early lesion onset. In addition, we included psychophysical methods (assessing parvo/konio and magnocellular pathways) and retinotopic mapping. We could identify dorsal stream impairment (with extended contrast sensitivity deficits within the input magno system contrasting with more confined parvocellular deficits) with disrupted active visual field input representations in regions neighboring the lesion. Simultaneous EEG/fMRI identified perilesional and retinotopic bilaterally symmetric BOLD deactivation triggered by interictal spikes, which matched the contralateral spread of magnocellular dysfunction revealed in the psychophysical tests. Topographic changes in retinotopic organization further suggested long term functional effects of abnormal electrical discharges during brain development. We conclude that fMRI based visual field cortical mapping shows evidence for retinotopic dissociation between magno and parvocellular function well beyond striate cortex, identifiable in high level dorsal visual representations around visual area V3A which is consistent with the effects of epileptic spike triggered negative BOLD.