Papers by Bart Krekelberg
medRxiv (Cold Spring Harbor Laboratory), Dec 30, 2020
doi: medRxiv preprint NOTE: This preprint reports new research that has not been certified by pee... more doi: medRxiv preprint NOTE: This preprint reports new research that has not been certified by peer review and should not be used to guide clinical practice.
Journal of Vision, Sep 6, 2019
Schizophrenia Bulletin, Apr 1, 2019
Background: Schizophrenia patients (SZ) poorly integrate multiple spatially segregated oriented e... more Background: Schizophrenia patients (SZ) poorly integrate multiple spatially segregated oriented elements into cohesive contours and shapes, and this impairment becomes more pronounced as the stimulus is globally scaled down in size (d=1.7; Keane et al., 2016, J Abnorm Psychol). But what stimulus properties drive this scaling effect and how specific is it to the schizophrenia phenotype? Addressing this issue will yield important clues for building behavioral tasks that flag current or impending psychosis; it will also clarify the visual mechanisms disturbed. Methods: We compared SZ patients (N=5), bipolar disorder patients (BDs; N=9), and well-matched healthy controls (N=7) on a psychophysical task in which subjects sought to identify the screen quadrant location of an integrated circular target. Each target was rendered visible by eight cocircular edge elements distributed along its circumference. Task difficulty was staircase controlled and depended on the number of noise elements copresented with the target. Noise and target elements were drawn as Gabors, which are sinusoidal luminance gratings multiplied by a circularly symmetric Gaussian filter. There were 16 different stimulus conditions corresponding to the crossings of four different stimulus parameters that would change with spatial scaling: (1) Gabor spatial frequency (6 or 12 cycles/ deg), (2) Gabor width (Gaussian envelope SD of 2.4 or 4.8 arcmin), (3) target eccentricity (2.3 or 4.7 degrees from central fixation), and (4) target radius (.74 or 1.5 deg). Results: As expected, targets were easier to detect when closer to fixation and having a smaller radius (resulting in smaller inter-element distance) (ps<.05), and when individual elements had a lower spatial frequency and larger width (ps<.05). None of these main effects depended on subject group (p>.08). Five of the six two-way-interactions were significant (exception: eccentricity by Gabor width) but none depended on subject group (p>.28). Subject group interacted with spatial frequency, eccentricity, and Gabor width (four-way interaction, p<.01); it also interacted with eccentricity, radius, and spatial frequency (four-way interaction, p<.05). Comparing controls and BDs, there was no group differences or interactions (ps>.05). Comparing controls to SZs, there was the same pair of four-way interactions just described (ps<.01). Comparing SZs to BDs, there was a four-way interaction as before with spatial frequency, eccentricity, and Gabor width (p=.04). To state this last result in another way, among bipolar patients and controls, increasing the Gabor width improved contour integration performance more for low than for high spatial frequency stimuli, and this effect was more apparent for high eccentricity displays (p<.001). Among SZs, increasing eccentricity did not accentuate the two-way interaction and appeared to have the opposite effect (p>.2). Thus, SZ may involve more uncertainty at locations closer to fixation such that broadening the window of the Gabor (to reveal more of the waveform) offers more benefit to those subjects at those same locations. Discussion: These results, while still very preliminary, suggest that contour integration deficits worsen in SZ when the spatial scaling factors jointly vary, viz., when the eccentricity is reduced, the Gabor width is reduced, the spatial frequency is increased, and the target radius is minimized. In a word, it appears that potentially all aspects of spatial scaling may contribute to contour integration deficits in schizophrenia perhaps because of inadequate sampling within parafoveal relative to peripheral retinotopic locations; these deficits appear to be fairly specific to schizophrenia.
Journal of Vision, Sep 1, 2018
Journal of Vision, Jul 25, 2013
Perception of visual features is thought to arise from noisy population codes in the brain. For m... more Perception of visual features is thought to arise from noisy population codes in the brain. For most tasks, not all members of a population are equally suited to the task at hand. For example, for fine discrimination, the most informative neurons are those that have the steepest part of their tuning curve on the decision-boundary. Choice probability analysis suggests that primates assign higher weights to these maximally informative neurons to optimize decision-making. Under more natural conditions, however, such optimization could be difficult because tuning curves (and their derivatives) depend critically on the stimulus set used for their definition. Accordingly, sub-optimal inference is inevitable for real-world perceptual decision-making. We examined effects of optimal and sub-optimal inference on discrimination using a combination of computational modeling and psychophysics in humans and monkeys. We simulated a population of direction-selective neurons and mapped tuning curves for translational motion in random dot patterns. Even though model parameters were fixed, empirical tuning widths could be adjusted (and therefore, which neurons were the most informative) by varying the width of the distribution from which dot-directions were drawn ("external noise"). Discrimination thresholds were measured at different levels of external noise and with two different states of optimization. When optimized for narrow-distribution ("clean") stimuli, thresholds increased monotonically with external noise. When optimized for broad-distribution ("noisy") stimuli, however, thresholds for moderate levels of external noise were actually lower than those for low levels of external noise-even though the latter objectively carried more information about direction. We confirmed this unexpected result psychophysically in humans and monkeys using comparable methods (the two optimization-states were encouraged by presenting different proportions of high and low external-noise trials across blocks). Our results suggest that the visual system can optimize decisionmaking for expected noisy stimuli at the expense of reduced sensitivity to unexpected high-fidelity signals.
Current Biology, Jun 1, 2011
A car driving through the fog appears tomovemore slowly than one driving on a clear and sunny day... more A car driving through the fog appears tomovemore slowly than one driving on a clear and sunny day. In the laboratory, this observation has been confirmed as a pronounced reduction of perceived speed caused by a reduction in contrast. We measured the influence of contrast on cells in the middle temporal area (MT) of the macaque, which has been hypothesized to underlie the perception of speed. The influence of contrast on the responsiveness and speed tuning of these cellswas pervasive andhighly regular.As expected,most cells responded less at lowcontrast.More importantly, thepreferred speedofmost cells shifted to lower speeds at lower contrasts.Moreover, approximately one-third of cells surprisingly respondedmore strongly to slow low-contrast stimuli than to slow high-contrast stimuli. Currentmodels of speedperception suggest that eachMTcell votes for its preferred speed,with a vote determinedby its firing rate.We tested anumber of these labeled-linemodels by entering theneural respon...
Number: 949 Submitted By: Kohitij Kar Last Modified: December 1 2010 Retinal And Cortical Effects... more Number: 949 Submitted By: Kohitij Kar Last Modified: December 1 2010 Retinal And Cortical Effects Of Transcranial Electric Stimulation Kohitij Kar, Bart Krekelberg Center for Molecular and Behavioral Neuroscience, Rutgers University Transcranial electric current stimulation (tES) of the human brain is assumed to affect cortical excitability by invoking subthreshold modulation of neuronal activity. This assumption has been used to interpret clinical studies where tES improved stroke recovery, alleviated chronic pain, or treated depression. Our goal is to understand how electrical stimulation leads to changes in brain and behavior. A recent study has shown that application of transcranial alternating current stimulation (tACS) over the visual cortex induces the perception of flashes of light (phosphenes). The claim that these phosphenes were generated cortically has led to considerable debate. We applied tACS over visual cortex with a reference electrode on the vertex; and measured cu...
Synopsis This review discusses theoretical, behavioral, and physiological studies of motion mecha... more Synopsis This review discusses theoretical, behavioral, and physiological studies of motion mechanisms. The three main schemes for motion detection (space-time correlation, orientation, and gradients) are contrasted using experimental data from insects, rabbits, cats, monkeys, and humans. These schemes provide a basic understanding of the organization of many neural motion detection systems. However, few neural systems are pure implementations of any of these three detection schemes. It is suggested that using a mixture of motion detection mechanisms may be advantageous to a neural system faced with the difficult, but important task of detecting motion under widely varying conditions.
in the incidence and distribution of neurons whose chemosensory mechanisms are temperature-sensit... more in the incidence and distribution of neurons whose chemosensory mechanisms are temperature-sensitive. However, the absence of signi®cant bitterness during warming and the reports of bitterness during cooling on circumvallate papillae raise the possibility that thermal sensitivity in some gustatory neurons may arise from cellular processes that are unrelated to chemosensory transduction. We note that thermal taste was nearly discovered 35 years ago by von Be Âke Âsy. In a well-known but controversial paper, von Be Âke Âsy 15 reported that taste and thermal stimuli (heated or cooled water) presented to opposite sides of the tongue merged into a single sensation when warm water was paired with sucrose or quinine, or when cold water was paired with citric acid or NaCl. This observation led him to propose the`Duplexity Theory of Taste' 15 , in which he posited that``warm and cold stimuli act similarly to the four primary taste stimuli¼'' Our results now suggest that von Be Âke Âsy's subjects may have reported a single sensation in the middle of the tongue when bilateral thermal and chemical stimuli evoked the same taste quality. M Methods Thermal taste screening procedure
PLOS ONE, 2021
Multiple Sclerosis (MS) is an autoimmune demyelinating disease that damages the insulation of ner... more Multiple Sclerosis (MS) is an autoimmune demyelinating disease that damages the insulation of nerve cell fibers in the brain and spinal cord. In the visual system, this demyelination results in a robust delay of visually evoked potentials (VEPs), even in the absence of overt clinical symptoms such as blurred vision. VEPs, therefore, offer an avenue for early diagnosis, monitoring disease progression, and, potentially, insight into the differential impairment of specific pathways. A primary hypothesis has been that visual stimuli driving the magno-, parvo-, and konio-cellular pathways should lead to differential effects because these pathways differ considerably in terms of myelination. Experimental tests of this hypothesis, however, have led to conflicting results. Some groups reported larger latency effects for chromatic stimuli, while others found equivalent effects across stimulus types. We reasoned that this lack of pathway specificity could, at least in part, be attributed to t...
Frontiers in Systems Neuroscience, 2019
Adaptation is a multi-faceted phenomenon that is of interest in terms of both its function and it... more Adaptation is a multi-faceted phenomenon that is of interest in terms of both its function and its potential to reveal underlying neural processing. Many behavioral studies have shown that after exposure to an oriented adapter the perceived orientation of a subsequent test is repulsed away from the orientation of the adapter. This is the well-known Tilt Aftereffect (TAE). Recently, we showed that the dynamics of recurrently connected networks may contribute substantially to the neural changes induced by adaptation, especially on short time scales. Here we extended the network model and made the novel behavioral prediction that the TAE should be attractive, not repulsive, on a time scale of a few 100 ms. Our experiments, using a novel adaptation protocol that specifically targeted adaptation on a short time scale, confirmed this prediction. These results support our hypothesis that recurrent network dynamics may contribute to short-term adaptation. More broadly, they show that understanding the neural processing of visual inputs that change on the time scale of a typical fixation requires a detailed analysis of not only the intrinsic properties of neurons, but also the slow and complex dynamics that emerge from their recurrent connectivity. We argue that this is but one example of how even simple recurrent networks can underlie surprisingly complex information processing, and are involved in rudimentary forms of memory, spatio-temporal integration, and signal amplification.
Schizophrenia Bulletin, 2019
A quarter of patients with schizophrenia endorse visual hallucinations while two-thirds report su... more A quarter of patients with schizophrenia endorse visual hallucinations while two-thirds report subtle perceptual anomalies of visual experience. Yet, little is known about the origin of these visual disturbances and their significance to the disorder. Does schizophrenia represent a unique class of perceptual anomalies or do common mechanisms affect other disorders? Understanding the nature of this defining feature of schizophrenia could help elucidate etiology and address whether visual perceptual phenomena should be considered diagnostic. This symposium will present new findings from four independent laboratories regarding the nature and diagnostic specificity of visual disturbances in schizophrenia. Brian Keane will present results demonstrating visual shape completion deficits in first-episode and chronic schizophrenia patients (d>.8; p<10-6). The deficits are maximally present by the first psychotic break; linked to cognitive disorganization and poor premorbid functioning; reduced in bipolar disorder; and not explicable by medication use, poor orientation discrimination, or impoverished attention/motivation. At the same time, patients exhibit normal sensitivity to the illusory contour boundaries that compose the discriminated shapes, indicating that dysfunctional visual shape completion stems from a lessened ability to notice and use illusory contours at a conceptual stage of analysis. Scott Sponheim will present findings of early visual cortical abnormalities in schizophrenia. Analysis of MEG and functional MRI data reveal that schizophrenia is associated with basic perceptual abnormalities during initial sensory registration of a stimulus, while aberrant use of perceptual context occurs later across visual cortex. Data from biological relatives of people with schizophrenia and bipolar affective disorder patients indicate that early visual cortical anomalies do not mark unexpressed genetic liability for schizophrenia and are generally not shared with bipolar affective disorder. Jason Johannesen will present on the perception of animacy in visual motion. Individuals with schizophrenia have difficulty deriving causal inferences from social scenes, whether portrayed by human actors or simple geometric shapes. In the latter case, when reliant only on object motion, performance is reliable using different stimuli, related to affect recognition and Theory of Mind, and distinguishes schizophrenia from non-psychotic samples with higher accuracy than IQ or visual attention to still images. Ivy Tso will examine the high-level function of gaze perception. Using psychophysical methods, two cognitive processes underlying gaze perception (visual perceptual sensitivity, self-referential bias) are isolated, and patients with schizophrenia and bipolar disorder show abnormalities in both. Dynamic causal modeling of fMRI data demonstrates three characteristics of neural dynamics during gaze perception in schizophrenia: reduced visual cortical response to sensory input; weakened feedforward connectivity from the visual cortex; and aberrant increase of top-down suppression of visual cortex when analyzing gaze direction (vs. gender) of face stimuli. These findings suggest impaired visual processing and potentially compensatory reliance on higher-level functions (e.g., beliefs) contribute to altered social cognition in psychosis. Sohee Park will serve as discussant and speak to how the diverse set of visual disturbances in schizophrenia informs our understanding of the disorder.
Nature, 2000
in the incidence and distribution of neurons whose chemosensory mechanisms are temperature-sensit... more in the incidence and distribution of neurons whose chemosensory mechanisms are temperature-sensitive. However, the absence of signi®cant bitterness during warming and the reports of bitterness during cooling on circumvallate papillae raise the possibility that thermal sensitivity in some gustatory neurons may arise from cellular processes that are unrelated to chemosensory transduction. We note that thermal taste was nearly discovered 35 years ago by von Be Âke Âsy. In a well-known but controversial paper, von Be Âke Âsy 15 reported that taste and thermal stimuli (heated or cooled water) presented to opposite sides of the tongue merged into a single sensation when warm water was paired with sucrose or quinine, or when cold water was paired with citric acid or NaCl. This observation led him to propose the`Duplexity Theory of Taste' 15 , in which he posited that``warm and cold stimuli act similarly to the four primary taste stimuli¼'' Our results now suggest that von Be Âke Âsy's subjects may have reported a single sensation in the middle of the tongue when bilateral thermal and chemical stimuli evoked the same taste quality. M Methods Thermal taste screening procedure
Frontiers in systems neuroscience, 2017
To discriminate visual features such as corners and contours, the brain must be sensitive to spat... more To discriminate visual features such as corners and contours, the brain must be sensitive to spatial correlations between multiple points in an image. Consistent with this, macaque V2 neurons respond selectively to patterns with well-defined multipoint correlations. Here, we show that a standard feedforward model (a cascade of linear-non-linear filters) does not capture this multipoint selectivity. As an alternative, we developed an artificial neural network model with two hierarchical stages of processing and locally recurrent connectivity. This model faithfully reproduced neurons' selectivity for multipoint correlations. By probing the model, we gained novel insights into early form processing. First, the diverse selectivity for multipoint correlations and complex response dynamics of the hidden units in the model were surprisingly similar to those observed in V1 and V2. This suggests that both transient and sustained response dynamics may be a vital part of form computations....
Cortex; a journal devoted to the study of the nervous system and behavior, Jan 19, 2016
We investigated how neural activity in the middle temporal area of the macaque monkey changes aft... more We investigated how neural activity in the middle temporal area of the macaque monkey changes after 3 sec of exposure to a visual stimulus and used this to gain insight into the assumptions underlying the fMRI adaptation method (fMRIa). We studied both changes in tuning curves following weak and strong motion stimuli (adaptation) and the differences between a first and second exposure to the same stimulus (repetition suppression). Typically, tuning curves had smaller amplitudes and narrower tuning widths after strong adaptation; this was true for single neurons, multi-unit activity (MUA), the evoked local field potential (LFP), as well as gamma band activity. Repetition typically led to reduced responses. This reduction was correlated with direction selectivity and not explained by neural fatigue. Our data, however, warn against a simplistic view of the consequences of adaptation. First, a considerable fraction of neurons and sites showed response enhancements after adaptation, espe...
Frontiers in systems neuroscience, 2016
Eye movements are essential to primate vision but introduce potentially disruptive displacements ... more Eye movements are essential to primate vision but introduce potentially disruptive displacements of the retinal image. To maintain stable vision, the brain is thought to rely on neurons that carry both visual signals and information about the current direction of gaze in their firing rates. We have shown previously that these neurons provide an accurate representation of eye position during fixation, but whether they are updated fast enough during saccadic eye movements to support real-time vision remains controversial. Here we show that not only do these neurons carry a fast and accurate eye-position signal, but also that they support in parallel a range of time-lagged variants, including predictive and post dictive signals. We recorded extracellular activity in four areas of the macaque dorsal visual cortex during a saccade task, including the lateral and ventral intraparietal areas (LIP, VIP), and the middle temporal (MT) and medial superior temporal (MST) areas. As reported prev...
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Papers by Bart Krekelberg