1,721,020 research outputs found
The role of the retino-colliculo-extrastriate pathway in visual awareness and visual field recovery
No full textPatients with visual field defects resulting from post-chiasmatic lesions experience loss of visual function in up to one half of their visual field, with consequent impairments in their daily life activities. Therefore, effective strategies for compensating for the visual field loss are of great clinical relevance. After lesions to the primary visual pathway -which conveys visual information from the retina to the lateral geniculate nucleus, the optic radiations and, then, to the striate cortex-an alternative visual pathway, which projects from the superior colliculus to the extrastriate cortex, is usually spared in patients with visual field defects. In the present review, evidence for spared functioning of this alternative pathway in patients with visual field defects will be presented, both in terms of residual visual abilities, without awareness, for stimuli presented in the blind field, and the ability to integrate unseen visual signals presented in the blind field with concurrent auditory stimuli. Crucially, this review will discuss how the spared retino-colliculo-extrastriate pathway might be a useful tool for compensating for the loss of visual perception. Accordingly, evidence for the compensatory effects of systematic multisensory audio-visual stimulation in patients with visual field defects will be reviewed
Alpha-band sensory entrainment improves audiovisual temporal acuity
Full text linkVisual and auditory stimuli are transmitted from the environment to sensory cortices with different timing, requiring the brain to encode when sensory inputs must be segregated or integrated into a single percept. The probability that different audiovisual (AV) stimuli are integrated into a single percept even when presented asynchronously is reflected in the construct of temporal binding window (TBW). There is a strong interest in testing whether it is possible to broaden or shrink TBW by using different neuromodulatory approaches that can speed up or slow down ongoing alpha oscillations, which have been repeatedly hypothesized to be an important determinant of the TBWs size. Here, we employed a web-based sensory entrainment protocol combined with a simultaneity judgment task using simple flash-beep stimuli. The aim was to test whether AV temporal acuity could be modulated trial by trial by synchronizing ongoing neural oscillations in the prestimulus period to a rhythmic sensory stream presented in the upper (similar to 12 Hz) or lower (similar to 8.5 Hz) alpha range. As a control, we implemented a nonrhythmic condition where only the first and the last entrainers were employed. Results show that upper alpha entrainment shrinks AV TBW and improves AV temporal acuity when compared with lower alpha and control conditions. Our findings represent a proof of concept of the efficacy of sensory entrainment to improve AV temporal acuity in a trial-by-trial manner, and they strengthen the idea that alpha oscillations may reflect the temporal unit of AV temporal binding
Aberrant reward prediction error during Pavlovian appetitive learning in alexithymia
Full text linkExtensive literature shows that alexithymia, a subclinical trait defined by difficulties in identifying and describing feelings, is characterized by multifaceted impairments in processing emotional stimuli. Nevertheless, its underlying mechanisms remain elusive. Here, we hypothesize that alexithymia may be characterized by an alteration in learning the emotional value of encountered stimuli and test this by assessing differences between individuals with low (LA) and high (HA) levels of alexithymia in the computation of reward prediction errors (RPEs) during Pavlovian appetitive conditioning. As a marker of RPE, the amplitude of the feedback-related negativity (FRN) event-related potential was assessed while participants were presented with two conditioned stimuli (CS) associated with expected or unexpected feedback, indicating delivery of reward or no-reward. No-reward (vs reward) feedback elicited the FRN both in LA and HA. However, unexpected (vs expected) feedback enhanced the FRN in LA but not in HA, indicating impaired computation of RPE in HA. Thus, although HA show preserved sensitivity to rewards, they cannot use this response to update the value of CS that predict them. This impairment may hinder the construction of internal representations of emotional stimuli, leaving individuals with alexithymia unable to effectively recognize, respond and regulate their response to emotional stimuli
The effect of alexithymia on early visual processing of emotional body postures
Full text linkBody postures convey emotion and motion-related information useful in social interactions. Early visual encoding of body postures, reflected by the N190 component, is modulated both by motion (i.e., postures implying motion elicit greater N190 amplitudes than static postures) and by emotion-related content (i.e., fearful postures elicit the largest N190 amplitude). At a later stage, there is a fear-related increase in attention, reflected by an early posterior negativity (EPN) (Borhani et al., 2015). Here, we tested whether difficulties in emotional processing (i.e., alexithymia) affect early and late visual processing of body postures. Low alexithymic participants showed emotional modulation of the N190, with fearful postures specifically enhancing N190 amplitude. In contrast, high alexithymic participants showed no emotional modulation of the N190. Both groups showed preserved encoding of the motion content. At a later stage, a fear-related modulation of the EPN was found for both groups, suggesting that selective attention to salient stimuli is the same in both low and high alexithymia
Audiovisual integration in hemianopia: A neurocomputational account based on cortico-collicular interaction
No full textHemianopic patients retain some abilities to integrate audiovisual stimuli in the blind hemifield, showing both modulation of visual perception by auditory stimuli and modulation of auditory perception by visual stimuli. Indeed, conscious detection of a visual target in the blind hemifield can be improved by a spatially coincident auditory stimulus (auditory enhancement of visual detection), while a visual stimulus in the blind hemifield can improve localization of a spatially coincident auditory stimulus (visual enhancement of auditory localization). To gain more insight into the neural mechanisms underlying these two perceptual phenomena, we propose a neural network model including areas of neurons representing the retina, primary visual cortex (V1), extrastriate visual cortex, auditory cortex and the Superior Colliculus (SC). The visual and auditory modalities in the network interact via both direct cortical-cortical connections and subcortical-cortical connections involving the SC; the latter, in particular, integrates visual and auditory information and projects back to the cortices. Hemianopic patients were simulated by unilaterally lesioning V1, and preserving spared islands of V1 tissue within the lesion, to analyze the role of residual V1 neurons in mediating audiovisual integration. The network is able to reproduce the audiovisual phenomena in hemianopic patients, linking perceptions to neural activations, and disentangles the individual contribution of specific neural circuits and areas via sensitivity analyses. The study suggests i) a common key role of SC-cortical connections in mediating the two audiovisual phenomena; ii) a different role of visual cortices in the two phenomena: auditory enhancement of conscious visual detection being conditional on surviving V1 islands, while visual enhancement of auditory localization persisting even after complete V1 damage. The present study may contribute to advance understanding of the audiovisual dialogue between cortical and subcortical structures in healthy and unisensory deficit conditions
Machine learning methods detect arm movement impairments in a patient with parieto-occipital lesion using only early kinematic information
Full text linkPatients with lesions of the parieto-occipital cortex typically misreach visual targets that they correctly perceive (optic ataxia). Although optic ataxia was described more than 30 years ago, distinguishing this condition from physiological behavior using kinematic data is still far from being an achievement. Here, combining kinematic analysis with machine learning methods, we compared the reaching performance of a patient with bilateral occipitoparietal damage with that of 10 healthy controls. They performed visually guided reaches toward targets located at different depths and directions. Using the horizontal, sagittal, and vertical deviation of the trajectories, we extracted classification accuracy in discriminating the reaching performance of patient from that of controls. Specifically, accurate predictions of the patient's deviations were detected after the 20% of the movement execution in all the spatial positions tested. This classification based on initial trajectory decoding was possible for both directional and depth components of the movement, suggesting the possibility of applying this method to characterize pathological motor behavior in wider frameworks
Unseen fearful faces facilitate visual discrimination in the intact field
Full text linkImplicit visual processing of emotional stimuli has been widely investigated since the classical studies on affective blindsight, in which patients with primary visual cortex lesions showed discriminatory abilities for unseen emotional stimuli in the absence of awareness. In addition, more recent evidence from hemianopic patients showed response facilitation and enhanced early visual encoding of seen faces, only when fearful faces were presented concurrently in the blind field. However, it is still unclear whether unseen fearful faces specifically facilitate visual processing of facial stimuli, or whether the facilitatory effect constitutes an adaptive mechanism prioritizing the visual analysis of any stimulus. To test this question, we tested a group of hemianopic patients who perform at chance in forced-choice discrimination tasks of stimuli in the blind field. Patients performed a go/no-go task in which they were asked to discriminate simple visual stimuli (Gabor patches) presented in their intact field, while fearful, happy and neutral faces were concurrently presented in the blind field. The results showed a reduction in response times to the Gabor patches presented in the intact field, when fearful faces were concurrently presented in the blind field, but only in patients with left hemispheric lesions. No facilitatory effect was observed in patients with right hemispheric lesions. These results suggest that unseen fearful faces are implicitly processed and can facilitate the visual analysis of simple visual stimuli presented in the intact field. This effect might be subserved by activity in the spared colliculo-amygdala-extrastriate pathway that promotes efficient visual analysis of the environment and rapid execution of defensive responses. Such a facilitation is observed only in patients with left lesions, favouring the hypothesis that the right hemisphere mediates implicit visual processing of fear signals
Audiovisual Rehabilitation in Hemianopia: A Model-Based Theoretical Investigation
Full text linkHemianopic patients exhibit visual detection improvement in the blind field when audiovisual stimuli are given in spatiotemporally coincidence. Beyond this "online" multisensory improvement, there is evidence of long-lasting, "offline" effects induced by audiovisual training: patients show improved visual detection and orientation after they were trained to detect and saccade toward visual targets given in spatiotemporal proximity with auditory stimuli. These effects are ascribed to the Superior Colliculus (SC), which is spared in these patients and plays a pivotal role in audiovisual integration and oculomotor behavior. Recently, we developed a neural network model of audiovisual cortico-collicular loops, including interconnected areas representing the retina, striate and extrastriate visual cortices, auditory cortex, and SC. The network simulated unilateral V1 lesion with possible spared tissue and reproduced "online" effects. Here, we extend the previous network to shed light on circuits, plastic mechanisms, and synaptic reorganization that can mediate the training effects and functionally implement visual rehabilitation. The network is enriched by the oculomotor SC-brainstem route, and Hebbian mechanisms of synaptic plasticity, and is used to test different training paradigms (audiovisual/visual stimulation in eye-movements/fixed-eyes condition) on simulated patients. Results predict different training effects and associate them to synaptic changes in specific circuits. Thanks to the SC multisensory enhancement, the audiovisual training is able to effectively strengthen the retina-SC route, which in turn can foster reinforcement of the SC-brainstem route (this occurs only in eye-movements condition) and reinforcement of the SC-extrastriate route (this occurs in presence of survived V1 tissue, regardless of eye condition). The retina-SC-brainstem circuit may mediate compensatory effects: the model assumes that reinforcement of this circuit can translate visual stimuli into short-latency saccades, possibly moving the stimuli into visual detection regions. The retina-SC-extrastriate circuit is related to restitutive effects: visual stimuli can directly elicit visual detection with no need for eye movements. Model predictions and assumptions are critically discussed in view of existing behavioral and neurophysiological data, forecasting that other oculomotor compensatory mechanisms, beyond short-latency saccades, are likely involved, and stimulating future experimental and theoretical investigations
Crossmodal enhancement of visual orientation discrimination by looming sounds requires functional activation of primary visual areas: A case study
No full textApproaching or looming sounds are salient, potentially threatening stimuli with particular impact on visual processing. The early crossmodal effects by looming sounds (Romei, Murray, Cappe, & Thut, 2009) and their selective impact on visual orientation discrimination (. Leo, Romei, Freeman, Ladavas, & Driver, 2011) suggest that these multisensory interactions may take place already within low-level visual cortices. To investigate this hypothesis, we tested a patient (SDV) with bilateral occipital lesion and spared residual portions of V1/V2. Accordingly, SDV[U+05F3]s visual perimetry revealed blindness of the central visual field with some residual peripheral vision. In two experiments we tested for the influence of looming vs. receding and stationary sounds on SDV[U+05F3]s line orientation discrimination (. orientation discrimination experiment) and visual detection abilities (. detection experiment) in the preserved or blind portions of the visual field, corresponding to spared and lesioned areas of V1, respectively. In the visual orientation discrimination experiment we found that SDV visual orientation sensitivity significantly improved for visual targets paired with looming sounds but only for lines presented in the partially preserved visual field. In the visual detection experiment, where SDV was required to simply detect the same stimuli presented in the orientation discrimination experiment, a generalised sound-induced visual improvement both in the intact and in blind portion of the visual field was observed. These results provide direct evidence that early visual areas are critically involved in crossmodal modulation of visual orientation sensitivity by looming sounds. Thus, a lesion in V1 prevents the enhancement of visual orientation sensitivity. In contrast, the same lesion does not prevent the visual detection enhancement by a sound, probably due to alternative visual pathways (e.g. retino-colliculo-extrastriate) which are usually spared in these patients and able to mediate the crossmodal enhancement of basic visual abilities such as detection. © 2014 Elsevier Ltd
Multisensory stimulation in hemianopic patients boosts orienting responses to the hemianopic field and reduces attentional resources to the intact field
No full textPurpose: Lateralised lesions can disrupt inhibitory cross-callosal fibres which maintain interhemispheric equilibrium in attention networks, with a consequent attentional bias towards the ipsilesional field. Some evidence of this imbalance has also been found in hemianopic patients (Tant et al., 2002). The aim of the present study was to reduce this attentional bias in hemianopic patients by using multisensory stimulation capable of activating subcortical structures responsible for orienting attention, such as the superior colliculus. Methods: Eight hemianopic patients underwent a course of multisensory stimulation treatment for two weeks and their behavioural and electrophysiological performance was tested at three time intervals: baseline 1 (before treatment), control baseline 2 (two weeks after baseline 1 and immediately before treatment as a control for practice effects) and finally after treatment. Results: The results show improvements on various clinical measures, on orienting responses in the hemianopic field, and a reduction of electrophysiological activity (P3 amplitude) in response to stimuli presented in the intact visual field. Conclusions: These results suggest that the primary visual deficit in hemianopic patients might be accompanied by an ipsilesional attentional bias which might be reduced by multisensory stimulation
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