Cerebellar tDCS Dissociates the Timing of Perceptual Decisions from Perceptual Change in Speech

Neuroimaging studies suggest that the cerebellum might play a role in both speech perception and speech perceptual learning. However, it remains unclear what this role is: does the cerebellum directly contribute to the perceptual decision? Or does it contribute to the timing of perceptual decisions? To test this, we applied transcranial direct current stimulation (tDCS) to the right cerebellum during a speech perception task. Participants experienced a series of speech perceptual tests designed to measure and then manipulate their perception of a phonetic contrast. One group received cerebellar tDCS during speech perceptual learning and a different group received "sham" tDCS during the same task. Both groups showed similar learning-related changes in speech perception that transferred to a different phonetic contrast. For both trained and untrained speech perceptual decisions, cerebellar tDCS significantly increased the time it took participants to indicate their decisions with a keyboard press. The results suggest that cerebellar tDCS disrupted the timing of perceptual decisions, while leaving the eventual decision unaltered. In support of this conclusion, we use the drift diffusion model to decompose the data into processes that determine the outcome of perceptual decision-making and those that do not. The modeling suggests that cerebellar tDCS disrupted processes unrelated to decision-making. Taken together, the empirical data and modeling demonstrate that right cerebellar tDCS dissociates the timing of perceptual decisions from perceptual change. The results provide initial evidence in healthy humans that the cerebellum critically contributes to speech timing in the perceptual domain

State estimation for early feedback responses in reaching: Intramodal or multimodal?

Contains fulltext : 180764.pdf (publisher's version ) (Open Access)Humans are highly skilled in controlling their reaching movements, making fast and task-dependent movement corrections to unforeseen perturbations. To guide these corrections, the neural control system requires a continuous, instantaneous estimate of the current state of the arm and body in the world. According to Optimal Feedback Control theory, this estimate is multimodal and constructed based on the integration of forward motor predictions and sensory feedback, such as proprioceptive, visual and vestibular information, modulated by context, and shaped by past experience. But how can a multimodal estimate drive fast movement corrections, given that the involved sensory modalities have different processing delays, different coordinate representations, and different noise levels? We develop the hypothesis that the earliest online movement corrections are based on multiple single modality state estimates rather than one combined multimodal estimate. We review studies that have investigated online multimodal integration for reach control and offer suggestions for experiments to test for the existence of intramodal state estimates. If proven true, the framework of Optimal Feedback Control needs to be extended with a stage of intramodal state estimation, serving to drive short-latency movement corrections.8 p

Characteristics of online movement corrections

Smeets, J.B.J. [Promotor]Brenner, E.M. [Copromotor

Evidence for optimal integration of visual feature representations across saccades

Contains fulltext : 151415.pdf (publisher's version ) (Open Access)We explore the visual world through saccadic eye movements, but saccades also present a challenge to visual processing by shifting externally stable objects from one retinal location to another. The brain could solve this problem in two ways: by overwriting preceding input and starting afresh with each fixation or by maintaining a representation of presaccadic visual features in working memory and updating it with new information from the remapped location. Crucially, when multiple objects are present in a scene the planning of eye movements profoundly affects the precision of their working memory representations, transferring limited memory resources from fixation toward the saccade target. Here we show that when humans make saccades, it results in an update of not just the precision of representations but also their contents. When multiple item colors are shifted imperceptibly during a saccade the perceived colors are found to fall between presaccadic and postsaccadic values, with the weight given to each input varying continuously with item location, and fixed relative to saccade parameters. Increasing sensory uncertainty, by adding color noise, biases updating toward the more reliable input, which is consistent with an optimal integration of presaccadic working memory with a postsaccadic updating signal. We recover this update signal and show it to be tightly focused on the vicinity of the saccade target. These results reveal how the nervous system accumulates detailed visual information from multiple views of the same object or scene. SIGNIFICANCE STATEMENT This study examines the consequences of saccadic eye movements for the internal representation of visual objects. A saccade shifts the image of a stable visual object from one part of the retina to another. We show that visual representations are built up over these different views of the same object, by combining information obtained before and after each saccade. The weights given to presaccadic and postsaccadic information are determined by the relative reliability of each input. This provides evidence that the visual system combines inputs over time in a statistically optimal way.8 p

Topographical thresholds for ephemeral gully initiation in intensively cultivated areas of the Mediterranean

Ephemeral gullying is an important erosion process in Mediterranean areas suffering from land degradation because of increased drought and human impact. An ephemeral gully is formed when a hydraulic threshold for incision into a resistant soil surface is exceeded. The flow intensity at the soil surface of any landscape position is controlled by topographical parameters, such as local slope (S) and drainage-basin area (A). The objective of this study was to investigate to what extent one could use field measurements of S and A to predict the location of ephemeral gully initiation in intensively cultivated Mediterranean environments. Two relatively homogeneous study areas in Southeast Spain and Northeast Portugal with widespread ephemeral gullying were selected. S and A were measured by standard methods at the initiation point of 50 ephemeral gullies in each study area. The relationship between S and A was statistically investigated. On a double logarithmic scale, the S and A data showed considerable scatter. Orthogonal regression resulted in the threshold relationship S=aA−b with b=0.133 for Spain, and b=0.226 for Portugal. The analysis also showed that S and A were weakly correlated for the Spanish dataset, and more strongly correlated for the Portuguese dataset. The correlation was considerably strengthened by including additional information on planform curvature and on tillage direction relative to the gully direction. Greater soil stoniness and the presence of a less permeable horizon generally increased the threshold level in both study areas. The threshold relationships resulting from this study were compared with literature data. The indices AcS and ln(Ac/S) were applied to our study sites but neither relationship individually described the general trend between the data of S and A. Therefore, an alternative index, AcbS, with 0.133≤b≤0.226, according to the empirical trend between drainage basin area and local slope, was proposed to describe the topographical threshold conditions for ephemeral gully initiation in similar Mediterranean areas

Analysis of methods to determine the latency of online movement adjustments

When studying online movement adjustments, one of the interesting parameters is their latency. We set out to compare three different methods of determining the latency: the threshold, confidence interval, and extrapolation methods. We simulated sets of movements with different movement times and amplitudes of movement adjustments, all with the same known latency. We applied the three different methods in order to determine when the position, velocity, and acceleration of the adjusted movements started to deviate from the values for unperturbed movements. We did so both for averaged data and for the data of individual trials. We evaluated the methods on the basis of their accuracy and precision, and according to whether the latency was influenced by the intensity of the movement adjustment. The extrapolation method applied to average acceleration data gave the most reliable estimates of latency, according to these criteria. © 2013 Psychonomic Society, Inc

Online manual movement adjustments in response to target position changes and apparent target motion

This study set out to determine whether the fastest online hand movement corrections are only responses to changing judgments of the targets' position or whether they are also influenced by the apparent target motion. Introducing a gap between when a target disappears and when it reappears at a new position in a double-step paradigm disrupts the apparent motion, so we examined the influence of such a gap on the intensity of the response. We found that responses to target perturbations with disrupted apparent motion were less vigorous. The response latency was 10 ms shorter when there was a gap, which might be related to the gap effect that has previously been described for initiating eye and hand movements. © 2014 Human Kinetics, Inc

Compétences spécifiques. S'approprier la règle du non contact

We explore the visual world through saccadic eye movements, but saccades also present a challenge to visual processing, by shifting externally-stable objects from one retinal location to another. The brain could solve this problem in two ways: by overwriting preceding input and starting afresh with each new fixation, or by storing a representation of pre-saccadic visual features in memory and updating it with new information from the spatiotopically-matched location. When multiple objects are present in a scene, the planning of eye movements profoundly alters the precision of their working memory representations, transferring memory resources from fixation toward the saccadic target. Here we show that enacting a saccade updates not only the precision of representations but also their contents. When multiple item colours are shifted imperceptibly during a saccade, the perceived colours are found to fall between pre- and post-saccadic values, with the weight given to each input varying continuously with item location, and fixed relative to saccade parameters. Increasing sensory uncertainty, by adding colour noise, biases updating towards the more reliable input, consistent with an optimal integration of pre-saccadic working memory with a post-saccadic update signal. We recover this update signal and show it to be tightly-focused on the vicinity of the saccade target. These results reveal how the nervous system accumulates detailed visual information from multiple views of the same object or scene. Meeting abstract presented at VSS 201

Movement adjustments have short latencies because there is no need to detect anything

Contains fulltext : 156475.pdf (publisher's version ) (Closed access)We can adjust an on-going movement to a change in the target’s position with a latency of about 100 ms, about half of the time that is needed to start a new movement in response to the same change in target position (reaction time). In this opinion paper, we discuss factors that could explain the difference in latency between initiating and adjusting a movement in response to target displacements. We consider the latency to be the sum of the durations of various stages in information processing. Many of these stages are identical for adjusting and initiating a movement, but for movement initiation it is essential to detect that something has changed in order to respond, whereas adjustments to movements can be based on updated position information without detecting that the position has changed. This explanation for the shorter latency for movement adjustments also explains why we can respond to changes that we do not detect.12 p