Gregory D Horwitz

Wisconsin National Primate Research Center, Madison, Wisconsin, United States

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Publications (27)167.48 Total impact

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    ABSTRACT: Chromatic sensitivity cannot exceed limits set by noise in the cone photoreceptors. To determine how close neurophysiological and psychophysical chromatic sensitivity come to these limits, we developed a parameter-free model of stimulus encoding in the cone outer segments, and we compared the sensitivity of the model to the psychophysical sensitivity of monkeys performing a detection task and to the sensitivity of individual V1 neurons. Modeled cones had a temporal impulse response and a noise power spectrum that were derived from in vitro recordings of macaque cones, and V1 recordings were made during performance of the detection task. The sensitivity of the simulated cone mosaic, the V1 neurons, and the monkeys were tightly yoked for low-spatiotemporal-frequency isoluminant modulations, indicating high-fidelity signal transmission for this class of stimuli. Under the conditions of our experiments and the assumptions for our model, the signal-to-noise ratio for these stimuli dropped by a factor of ∼3 between the cones and perception. Populations of weakly correlated V1 neurons narrowly exceeded the monkeys' chromatic sensitivity but fell well short of the cones' chromatic sensitivity, suggesting that most of the behavior-limiting noise lies between the cone outer segments and the output of V1. The sensitivity gap between the cones and behavior for achromatic stimuli was larger than for chromatic stimuli, indicating greater postreceptoral noise. The cone mosaic model provides a means to compare visual sensitivity across disparate stimuli and to identify sources of noise that limit visual sensitivity.
    No preview · Article · Nov 2015 · Journal of Vision
  • Zachary Lindbloom-Brown · Leah J Tait · Gregory D Horwitz
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    ABSTRACT: Spectral sensitivity of humans and rhesus monkeys was compared using identical displays and similar procedures. Detection thresholds were measured for the following: 1) 15-Hz modulation of a blue and a green cathode-ray tube phosphor; 2) 15-Hz modulation of all three phosphors together; and 3) slow (<1 Hz) modulations of a blue and a green phosphor under scotopic conditions. Monkeys had lower blue-to-green threshold ratios than humans at all eccentricities tested (0.5 to 7°), consistent with a lower lens optical density in monkeys. In addition to apparently having a lower lens density than humans, monkeys were more sensitive to 15-Hz red-green isoluminant modulations than humans, an effect that cannot be explained by optical factors.
    No preview · Article · Dec 2014 · Journal of Neurophysiology
  • G D Horwitz
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    ABSTRACT: Animal models are a necessary component of systems neuroscience research. Determining which animal model to use for a given study involves a complicated calculus. Some experimental manipulations are easily made in some animal models but impossible in others. Some animal models are similar to humans with respect to particular scientific questions, and others are less so. In this review, I discuss work done in my laboratory to investigate the neural mechanisms of color vision in the rhesus macaque. The emphasis is on the strengths of the macaque model, but shortcomings are also discussed. Copyright © 2014. Published by Elsevier Ltd.
    No preview · Article · Oct 2014 · Neuroscience
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    Amy M Ni · Scott O Murray · Gregory D Horwitz
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    ABSTRACT: Stimuli that project the same retinal visual angle can appear to occupy very different proportions of the visual field if they are perceived to be at different distances [1-8]. Previous research shows that perceived angular size alters the spatial distribution of activity in early retinotopic visual cortex [7, 9-11]. For example, a sphere superimposed on the far end of a corridor scene appears to occupy a larger visual angle and activates a larger region of primary visual cortex (V1) compared with the same sphere superimposed on the near end of the corridor [7]. These previous results, however, were obtained from human subjects using psychophysics and fMRI, a fact that fundamentally limits our understanding of the underlying neuronal mechanisms. Here, we present an animal model that allows for a finer examination of size perception at the level of single neurons. We first show that macaque monkeys perceive a size-distance illusion similarly to humans. Then, using extracellular recordings, we test the specific hypothesis [12] that neurons in V1 shift the position of their receptive fields (RFs) in response to complex monocular depth cues. Consistent with this hypothesis, we found that when ring-shaped stimuli appeared at the back of the corridor, RFs of V1 neurons shifted toward the center of the rings. When the same stimuli appeared at the front of the corridor, RFs shifted outward. Thus, our results show for the first time that V1 RFs can shift, potentially serving as the neural basis for the perception of angular size.
    Preview · Article · Jul 2014 · Current Biology
  • Mijung Park · J Patrick Weller · Gregory D Horwitz · Jonathan W Pillow
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    ABSTRACT: A firing rate map, also known as a tuning curve, describes the nonlinear relationship between a neuron's spike rate and a low-dimensional stimulus (e.g., orientation, head direction, contrast, color). Here we investigate Bayesian active learning methods for estimating firing rate maps in closed-loop neurophysiology experiments. These methods can accelerate the characterization of such maps through the intelligent, adaptive selection of stimuli. Specifically, we explore the manner in which the prior and utility function used in Bayesian active learning affect stimulus selection and performance. Our approach relies on a flexible model that involves a nonlinearly transformed gaussian process (GP) prior over maps and conditionally Poisson spiking. We show that infomax learning, which selects stimuli to maximize the information gain about the firing rate map, exhibits strong dependence on the seemingly innocuous choice of nonlinear transformation function. We derive an alternate utility function that selects stimuli to minimize the average posterior variance of the firing rate map and analyze the surprising relationship between prior parameterization, stimulus selection, and active learning performance in GP-Poisson models. We apply these methods to color tuning measurements of neurons in macaque primary visual cortex.
    No preview · Article · May 2014 · Neural Computation
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    Charles A Hass · Gregory D Horwitz
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    ABSTRACT: To elucidate the cortical mechanisms of color vision, we recorded from individual V1 neurons in macaque monkeys performing a chromatic detection task. Roughly 30% of the neurons we encountered were unresponsive at the monkeys' psychophysical detection threshold. The other 70% were responsive at threshold, but on average, were slightly less sensitive than the monkey. For these neurons, the relationship between neurometric and psychometric threshold was consistent across the four isoluminant color directions tested. A corollary of this result is that neuronal thresholds were roughly four times lower for L-M stimuli than S-cone isolating stimuli. Nearly half of the neurons that responded to chromatic stimuli at the monkeys' detection threshold also responded to high contrast luminance modulations, suggesting a role for jointly color-luminance tuned neurons in chromatic detection. Analysis of neuronal contrast-response functions and signal-to-noise ratios yielded no evidence for a special set of "cardinal color directions" for which V1 neurons are particularly sensitive. We conclude that at detection threshold - as shown previously with high contrast stimuli - V1 neurons are tuned for a diverse set of color directions and do not segregate naturally into red-green and blue-yellow categories.
    Preview · Article · Feb 2013 · Journal of Neurophysiology
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    Mehrdad Jazayeri · Zachary Lindbloom-Brown · Gregory D Horwitz
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    ABSTRACT: Optogenetics has advanced our understanding of the neural basis of simple behaviors in rodents and small animals. In primates, however, for which more sophisticated behavioral assays exist, optogenetic manipulations of behavior have been unsuccessful. We found that monkeys reliably shifted their gaze toward the receptive field of optically driven channelrhodopsin-2-expressing neurons of the primary visual cortex. This result establishes optogenetics as a viable tool for the causal analysis of behavior in primate brain.
    Full-text · Article · Sep 2012 · Nature Neuroscience
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    Gregory D Horwitz · Charles A Hass
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    ABSTRACT: Understanding color vision requires knowing how signals from the three classes of cone photoreceptor are combined in the cortex. We recorded from individual neurons in the primary visual cortex (V1) of awake monkeys while an automated, closed-loop system identified stimuli that differed in cone contrast but evoked the same response. We found that isoresponse surfaces for half the neurons were planar, which is consistent with linear processing. The remaining isoresponse surfaces were nonplanar. Some were cup-shaped, indicating sensitivity to only a narrow region of color space. Others were ellipsoidal, indicating sensitivity to all color directions. The major and minor axes of these nonplanar surfaces were often aligned to a set of three color directions that were previously identified in perceptual experiments. These results suggest that many V1 neurons combine cone signals nonlinearly and provide a new framework in which to decipher color processing in V1.
    Preview · Article · May 2012 · Nature Neuroscience
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    Charles A Hass · Gregory D Horwitz
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    ABSTRACT: Microsaccades can elevate contrast detection thresholds of human observers and modulate the activity of neurons in monkey visual cortex. Whether microsaccades elevate contrast detection thresholds in monkey observers is not known and bears on the interpretation of neurophysiological experiments. To answer this question, we trained two monkeys to perform a 2AFC contrast detection task. Performance was worse on trials in which a microsaccade occurred during the stimulus presentation. The magnitude of the effect was modest (threshold changes of <0.2 log unit) and color specific: achromatic sensitivity was impaired, but red-green sensitivity was not. To explore the neural basis of this effect, we recorded the responses of individual V1 neurons to a white noise stimulus. Microsaccades produced a suppression of spiking activity followed by an excitatory rebound that was similar for L - M cone-opponent and L + M nonopponent V1 neurons. We conclude that microsaccades in the monkey increase luminance contrast detection thresholds and modulate the spiking activity of V1 neurons, but the luminance specificity of the behavioral suppression is likely implemented downstream of V1.
    Preview · Article · Mar 2011 · Journal of Vision
  • G. Horwitz · E. J. Chichilnisky · T. Albright

    No preview · Article · Dec 2010 · Journal of Vision
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    ABSTRACT: Color has become a premier model system for understanding how information is processed by neural circuits, and for investigating the relationships among genes, neural circuits, and perception. Both the physical stimulus for color and the perceptual output experienced as color are quite well characterized, but the neural mechanisms that underlie the transformation from stimulus to perception are incompletely understood. The past several years have seen important scientific and technical advances that are changing our understanding of these mechanisms. Here, and in the accompanying minisymposium, we review the latest findings and hypotheses regarding color computations in the retina, primary visual cortex, and higher-order visual areas, focusing on non-human primates, a model of human color vision.
    Full-text · Article · Nov 2010 · The Journal of Neuroscience : The Official Journal of the Society for Neuroscience
  • G. D Horwitz · E. J. Chichilnisky · T. D Albright

    No preview · Article · Oct 2010 · Journal of Vision
  • V. R. Posina · G. D. Horwitz · T. D. Albright

    No preview · Article · Jun 2010 · Journal of Vision
  • G.D. Horwitz · T.D. Albright
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    ABSTRACT: Color is an important part of our visual experience. The swirls of blues, yellows, and greens in Van Gogh's Starry Night affect a viewer in a way that a black-and-white rendition cannot. Color vision allows us to find a green bell pepper among red ones and predict how good it will taste. This article describes briefly the aspects of light that are most important for color vision, the processing of color information in the retina and thalamus, and color processing in the cerebral cortex.
    No preview · Article · Jan 2010
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    Gregory D Horwitz · E J Chichilnisky · Thomas D Albright
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    ABSTRACT: Rules by which V1 neurons combine signals originating in the cone photoreceptors are poorly understood. We measured cone inputs to V1 neurons in awake, fixating monkeys with white-noise analysis techniques that reveal properties of light responses not revealed by purely linear models used in previous studies. Simple cells were studied by spike-triggered averaging that is robust to static nonlinearities in spike generation. This analysis revealed, among heterogeneously tuned neurons, two relatively discrete categories: one with opponent L- and M-cone weights and another with nonopponent cone weights. Complex cells were studied by spike-triggered covariance, which identifies features in the stimulus sequence that trigger spikes in neurons with receptive fields containing multiple linear subunits that combine nonlinearly. All complex cells responded to nonopponent stimulus modulations. Although some complex cells responded to cone-opponent stimulus modulations too, none exhibited the pure opponent sensitivity observed in many simple cells. These results extend the findings on distinctions between simple and complex cell chromatic tuning observed in previous studies in anesthetized monkeys.
    Full-text · Article · May 2007 · Journal of Neurophysiology
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    ABSTRACT: Genetic strategies for perturbing activity of selected neurons hold great promise for understanding circuitry and behavior. Several such strategies exist, but there has been no direct demonstration of reversible inactivation of mammalian neurons in vivo. We previously reported quickly reversible inactivation of neurons in vitro using expression of the Drosophila allatostatin receptor (AlstR). Here, adeno-associated viral vectors are used to express AlstR in vivo in cortical and thalamic neurons of rats, ferrets, and monkeys. Application of the receptor's ligand, allatostatin (AL), leads to a dramatic reduction in neural activity, including responses of visual neurons to optimized visual stimuli. Additionally, AL eliminates activity in spinal cords of transgenic mice conditionally expressing AlstR. This reduction occurs selectively in AlstR-expressing neurons. Inactivation can be reversed within minutes upon washout of the ligand and is repeatable, demonstrating that the AlstR/AL system is effective for selective, quick, and reversible silencing of mammalian neurons in vivo.
    Full-text · Article · Aug 2006 · Neuron
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    Gregory D Horwitz · E J Chichilnisky · Thomas D Albright
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    ABSTRACT: We measured the color tuning of a population of S-cone-driven V1 neurons in awake, fixating monkeys. Analysis of randomly chosen color stimuli that were effective in evoking action potentials showed that these neurons received opposite sign input from the S cones and a combination of L and M cones. Surprisingly, these cells also responded to LM cone contrast irrespective of polarity, a nonlinear sensitivity that was masked by conventional linear analysis methods. Taken together, these observations can be summarized in a nonlinear model that combines nonopponent and opponent signals such that luminance contrast enhances color processing. These findings indicate that important aspects of the cortical representation of color cannot be described by classical linear analysis, and reveal a possible neural correlate of perceptual color-luminance interactions.
    Full-text · Article · May 2005 · Journal of Neurophysiology
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    Gregory D Horwitz · Thomas D Albright
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    ABSTRACT: The motion of a color-defined edge is often more difficult to perceive than the motion of a luminance-defined edge. Neurons subserving motion vision may therefore be particularly sensitive to luminance contrast. One class of neurons thought to play a critical role in motion perception is V1 neurons whose spatiotemporal receptive fields are oriented in space-time. We used the reverse correlation technique to study the relationship between color tuning and space-time receptive field orientation in V1 neurons of awake, fixating monkeys. Neurons with space-time oriented receptive fields were tuned almost exclusively for luminance, whereas neurons with nonoriented space-time receptive fields were tuned for luminance or for color. These results suggest that the special role of luminance contrast in motion perception is due in part to the establishment of space-time oriented receptive fields among luminance-tuned, but not color-tuned, V1 neurons.
    Preview · Article · Feb 2005 · Journal of Vision
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    Gregory D Horwitz · Edward M Callaway
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    ABSTRACT: Monkeys and humans work harder for immediate than for distant rewards. How are associations between reward immediacy and sensory stimuli established in the brain? A recent study suggests a crucial role for dopamine-mediated activity in the rhinal cortex.
    Preview · Article · Nov 2004 · Nature Neuroscience
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    Gregory D Horwitz · Aaron P Batista · William T Newsome
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    ABSTRACT: In a previous report, we described a heretofore undetected population of neurons in the intermediate and deep layers of the monkey superior colliculus (SC) that yielded directionally selective visual responses to stimuli presented within the central 4 degrees of the visual field. We observed these neurons in three monkeys that had been extensively trained to perform a visual direction discrimination task in this region of the visual field. The task required the monkeys to report the perceived direction of motion by making a saccadic eye movement to one of two targets aligned with the two possible directions of motion. We hypothesized that these neurons reflect a learned association between visual motion direction and saccade direction formed through extensive training on the direction discrimination task. We tested this hypothesis by searching for direction-selective visual responses in two monkeys that had been trained to perform a similar motion discrimination task in which the direction of stimulus motion was dissociated from the direction of the operant saccade. Strongly directional visual responses were absent in these monkeys, consistent with the notion that extensive training can induce highly specific visual responses in a subpopulation of SC neurons.
    Full-text · Article · Sep 2004 · Neuroscience Letters

Publication Stats

866 Citations
167.48 Total Impact Points


  • 2012-2014
    • Wisconsin National Primate Research Center
      Madison, Wisconsin, United States
  • 2010-2014
    • University of Washington Seattle
      • Department of Physiology and Biophysics
      Seattle, Washington, United States
  • 2004-2007
    • Salk Institute
      • • Systems Neurobiology Laboratory
      • • Vision Center Laboratory
      لا هویا, California, United States
    • Stanford University
      Palo Alto, California, United States
  • 1998-2004
    • Howard Hughes Medical Institute
      Ashburn, Virginia, United States