1,721,012 research outputs found
Visual sensitivity and bias oscillate phase-locked to saccadic eye movements
Full text linkOscillations in perceptual performance have been observed before and after a voluntary action, like hand, finger, and eye movements. In particular, discrimination accuracy of suprathreshold contrast stimuli oscillates in the delta range (2-3 Hz) phase-locked to saccadic eye movements. Importantly, saccadic suppression is embedded in phase with these long-lasting perceptual oscillations. It is debated whether these rhythmic modulations affect only appearance of high-contrast stimuli or whether absolute detection threshold is also modulated rhythmically. Here we measured location discrimination of a brief Gabor patch presented randomly between 1 s before and after a voluntary saccade and demonstrated that absolute contrast thresholds oscillated at a similar frequency to suprathreshold contrast discrimination. Importantly, saccadic suppression is also embedded in phase with absolute threshold oscillations. Interestingly, response bias was also found to oscillate at the same frequency in both tasks. However, the frequency was in the alpha range for bias, while it was in the delta range for sensitivity. These results demonstrate the presence of perisaccadic delta oscillations in visual sensitivity phase-locked to saccadic onset, and independent from response bias alpha oscillations. Overall, the present findings reinforce the suggestion of a leading role of oscillations in the temporal binding between eye-movement and visual processing timing
Neuroplasticity in adult human visual cortex
Full text linkBetween 1-5:100 people worldwide have never experienced normotypic vision due to a condition called amblyopia, and about 1:4000 suffer from inherited retinal dystrophies that progressively lead to blindness. While a wide range of technologies and therapies are being developed to restore vision, a fundamental question still remains unanswered: would the adult visual brain retain a sufficient plastic potential to learn how to ‘see’ after a prolonged period of abnormal visual experience? In this review we summarize studies showing that the visual brain of sighted adults retains a type of developmental plasticity, called homeostatic plasticity, and this property has been recently exploited successfully for adult amblyopia recovery. Next, we discuss how the brain circuits reorganize when blindness occurs and when visual stimulation is partially restored by means of a ‘bionic eye’ in late blind adults with Retinitis Pigmentosa. The primary visual cortex in these patients slowly became activated by the artificial visual stimulation, indicating that sight restoration therapies can rely on a considerable degree of spared plasticity in adulthood
Active vision gates ocular dominance plasticity in human adults
Full text linkPrimary visual cortex (V1) retains a form of plasticity in adult humans: a brief period of monocular deprivation induces an enhanced response to the deprived eye, which can stabilize into a consolidated plastic change1,2 despite unaltered thalamic input3. This form of homeostatic plasticity in adults is thought to act through neuronal competition between the representations of the two eyes, which are still separate in primary visual cortex4,5. During monocular occlusion, neurons of the deprived eye are thought to increase response gain given the absence of visual input, leading to the post-deprivation enhancement. If the decrease of reliability of the monocular response is crucial to establish homeostatic plasticity, this could be induced in several different ways. There is increasing evidence that V1 processing is affected by voluntary action, allowing it to take into account the visual effects of self-motion6, important for efficient active vision7. Here we asked whether ocular dominance homeostatic plasticity could be elicited without degrading the quality of monocular visual images but simply by altering their role in visuomotor control by introducing a visual delay in one eye while participants actively performed a visuomotor task; this causes a discrepancy between what the subject sees and what he/she expects to see. Our results show that homeostatic plasticity is gated by the consistency between the monocular visual inputs and a person's actions, suggesting that action not only shapes visual processing but may also be essential for plasticity in adults
The visual white matter connecting human area prostriata and the thalamus is retinotopically organized
Full text linkThe human visual system is capable of processing visual information from fovea to the far peripheral visual field. Recent fMRI studies have shown a full and detailed retinotopic map in area prostriata, located ventro-dorsally and anterior to the calcarine sulcus along the parieto-occipital sulcus with strong preference for peripheral and wide-field stimulation. Here, we report the anatomical pattern of white matter connections between area prostriata and the thalamus encompassing the lateral geniculate nucleus (LGN). To this end, we developed and utilized an automated pipeline comprising a series of Apps that run openly on the cloud computing platform brainlife.io to analyse 139 subjects of the Human Connectome Project (HCP). We observe a continuous and extended bundle of white matter fibers from which two subcomponents can be extracted: one passing ventrally parallel to the optic radiations (OR) and another passing dorsally circumventing the lateral ventricle. Interestingly, the loop travelling dorsally connects the thalamus with the central visual field representation of prostriata located anteriorly, while the other loop travelling more ventrally connects the LGN with the more peripheral visual field representation located posteriorly. We then analyse an additional cohort of 10 HCP subjects using a manual plane extraction method outside brainlife.io to study the relationship between the two extracted white matter subcomponents and eccentricity, myelin and cortical thickness gradients within prostriata. Our results are consistent with a retinotopic segregation recently demonstrated in the OR, connecting the LGN and V1 in humans and reveal for the first time a retinotopic segregation regarding the trajectory of a fiber bundle between the thalamus and an associative visual area
Perceptual Oscillations in Gender Classification of Faces, Contingent on Stimulus History
Full text linkPerception is a proactive ‘‘predictive’’ process, in which the brain takes advantage of past experience to make informed guesses about the world to test against sensory data. Here we demonstrate that in the judgment of the gender of faces, beta rhythms play an important role in communicating perceptual experience. Observers classified in forced choice as male or female, a sequence of face stimuli, which were physically constructed to be male or female or androgynous (equal morph). Classification of the androgynous stimuli oscillated rhythmically between male and female, following a complex waveform comprising 13.5 and 17 Hz. Parsing the trials based on the preceding stimulus showed that responses to androgynous stimuli preceded by male stimuli oscillated reliably at 17 Hz, whereas those preceded by female stimuli oscillated at 13.5 Hz. These results suggest that perceptual priors for face perception from recent perceptual memory are communicated through frequency-coded beta rhythms
Two systems for spatial location during saccades
No full textWe investigated whether there may exist separate spatial representations for perception and action, taking advantage of a strong perceptual distortion that occurs with saccadic eye movements. Subjects were required to make 20í horizontal saccades, and report the position of a bar briefly displayed on a touch screen just prior to the saccade, either verbally or by pointing to a touch screen (with hand obscured from view). The screen was viewed through a liquid crystal shutter that could close 75 ms after bar display. With verbal reports, subjects reported large errors in localization, showing the characteristic compression of perceived positions towards the saccadic target (Ross et al, Nature, 1997). This occurred both with clear viewing and when the shutter closed after bar presentation. However, under the same conditions, subjects pointed accurately to the target position (with shutter or eyes closed at time of pointing). Interestingly, when subjects pointed to a screen in clear view (but with their hands concealed), the pointing showed the same compression as verbal reports. We conclude that there are separate representations of visual space, one plastic and subject to distortion, the other not. The plastic representation, used for conscious visual perception, seems to dominate when both are available
Perception during double-step saccades
Full text linkHow the visual system achieves perceptual stability across saccadic eye movements is a long-standing question in neuroscience. It has been proposed that an efference copy informs vision about upcoming saccades, and this might lead to shifting spatial coordinates and suppressing image motion. Here we ask whether these two aspects of visual stability are interdependent or may be dissociated under special conditions. We study a memory-guided double-step saccade task, where two saccades are executed in quick succession. Previous studies have led to the hypothesis that in this paradigm the two saccades are planned in parallel, with a single efference copy signal generated at the start of the double-step sequence, i.e. before the first saccade. In line with this hypothesis, we find that visual stability is impaired during the second saccade, which is consistent with (accurate) efference copy information being unavailable during the second saccade. However, we find that saccadic suppression is normal during the second saccade. Thus, the second saccade of a double-step sequence instantiates a dissociation between visual stability and saccadic suppression: stability is impaired even though suppression is strong
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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