Gregory D Horwitz

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

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Publications (22)139.04 Total impact

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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.
    Neural computation. 05/2014;
  • 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.
    Journal of Neurophysiology 02/2013; · 3.30 Impact Factor
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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.
    Nature Neuroscience 09/2012; 15(10):1368-70. · 15.25 Impact Factor
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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.
    Nature Neuroscience 05/2012; 15(6):913-9. · 15.25 Impact Factor
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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.
    Journal of Vision 01/2011; 11(3):1-17. · 2.48 Impact Factor
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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.
    Journal of Neuroscience 11/2010; 30(45):14955-63. · 6.91 Impact Factor
  • V. R. Posina, G. D. Horwitz, T. D. Albright
    Journal of Vision - J VISION. 01/2010; 6(6):121-121.
  • Journal of Vision - J VISION. 01/2010; 3(9):139-139.
  • Journal of Vision - J VISION. 01/2010; 7(15):3-3.
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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.
    Journal of Neurophysiology 05/2007; 97(4):3070-81. · 3.30 Impact Factor
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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.
    Neuron 08/2006; 51(2):157-70. · 15.77 Impact Factor
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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.
    Journal of Neurophysiology 05/2005; 93(4):2263-78. · 3.30 Impact Factor
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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.
    Journal of Vision 02/2005; 5(6):525-33. · 2.48 Impact Factor
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    Gregory D Horwitz, Edward M Callaway
    Nature Neuroscience 11/2004; 7(10):1023-4. · 15.25 Impact Factor
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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.
    Neuroscience Letters 09/2004; 366(3):315-9. · 2.03 Impact Factor
  • Gregory D Horwitz, Aaron P Batista, William T Newsome
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    ABSTRACT: We recorded from neurons in the intermediate and deep layers of the superior colliculus (SC) while monkeys performed a novel direction discrimination task. In contrast to the task we used previously, the new version required the monkey to dissociate perceptual judgments from preparation to execute specific operant saccades. The monkey discriminated between 2 opposed directions of motion in a random-dot motion stimulus and was required to maintain the decision in memory throughout a delay period before the target of the required operant saccade was revealed. We hypothesized that perceptual decisions made in this paradigm would be represented in an "abstract" or "categorical" form within the brain, probably in the frontal cortex, and that decision-related neural activity would be eliminated from spatially organized preoculomotor structures such as the SC. To our surprise, however, a small population of neurons in the intermediate and deep layers of the SC fired in a choice-specific manner early in the trial well before the monkey could plan the operant saccade. Furthermore, the representation of the decision during the delay period appeared to be spatial: the active region in the SC map corresponded to the region of space toward which the perceptually discriminated stimulus motion flowed. Electrical microstimulation experiments suggested that these decision-related SC signals were not merely related to covert saccade planning. We conclude that monkeys may employ, in part, a spatially referenced mnemonic strategy for representing perceptual decisions, even when an abstract, categorical representation might appear more likely a priori.
    Journal of Neurophysiology 06/2004; 91(5):2281-96. · 3.30 Impact Factor
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    Gregory D Horwitz, Thomas D Albright
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    ABSTRACT: We investigated the effect of peripheral visual stimulation on small-amplitude saccades that occur naturally during fixation. Two macaque monkeys were rewarded for fixating while a colorful stimulus flickered randomly in the periphery. Reverse correlation revealed a lawful relationship between the stimulus sequence and saccade occurrences: on average, a transient increase in stimulus intensity evoked saccades at a latency of approximately 70 ms. The spectral tuning of this increase was roughly, but not exactly, consistent with a pure luminance increase. We conclude that peripheral luminance increases can evoke fixational saccades.
    Journal of Neurophysiology 09/2003; 90(2):1333-9. · 3.30 Impact Factor
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    G D Horwitz, W T Newsome
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    ABSTRACT: We investigated the role of the superior colliculus (SC) in saccade target selection while macaque monkeys performed a direction-discrimination task. The monkeys selected one of two possible saccade targets based on the direction of motion in a stochastic random-dot display; the difficulty of the task was varied by adjusting the strength of the motion signal in the display. One of the two saccade targets was positioned within the movement field of the SC neuron under study while the other target was positioned well outside the movement field. Approximately 30% of the neurons in the intermediate and deep layers of the SC discharged target-specific preludes of activity that "predicted" target choices well before execution of the saccadic eye movement. Across the population of neurons, the strength of the motion signal in the display influenced the intensity of this "predictive" prelude activity: SC activity signaled the impending saccade more reliably when the motion signal was strong than when it was weak. The dependence of neural activity on motion strength could not be explained by small variations in the metrics of the saccadic eye movements. Predictive activity was particularly strong in a subpopulation of neurons with directional visual responses that we have described previously. For a subset of SC neurons, therefore, prelude activity reflects the difficulty of the direction discrimination in addition to the target of the impending saccade. These results are consistent with the notion that a restricted network of SC neurons plays a role in the process of saccade target selection.
    Journal of Neurophysiology 12/2001; 86(5):2543-58. · 3.30 Impact Factor
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    G D Horwitz, W T Newsome
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    ABSTRACT: We investigated the role of the superior colliculus (SC) in saccade target selection in rhesus monkeys who were trained to perform a direction-discrimination task. In this task, the monkey discriminated between opposed directions of visual motion and indicated its judgment by making a saccadic eye movement to one of two visual targets that were spatially aligned with the two possible directions of motion in the display. Thus the neural circuits that implement target selection in this task are likely to receive directionally selective visual inputs and be closely linked to the saccadic system. We therefore studied prelude neurons in the intermediate and deep layers of the SC that can discharge up to several seconds before an impending saccade, indicating a relatively high-level role in saccade planning. We used the direction-discrimination task to identify neurons whose prelude activity "predicted" the impending perceptual report several seconds before the animal actually executed the operant eye movement; these "choice predicting" cells comprised approximately 30% of the neurons we encountered in the intermediate and deep layers of the SC. Surprisingly, about half of these prelude cells yielded direction-selective responses to our motion stimulus during a passive fixation task. In general, these neurons responded to motion stimuli in many locations around the visual field including the center of gaze where the visual discriminanda were positioned during the direction-discrimination task. Preferred directions generally pointed toward the location of the movement field of the SC neuron in accordance with the sensorimotor demands of the discrimination task. Control experiments indicate that the directional responses do not simply reflect covertly planned saccades. Our results indicate that a small population of SC prelude neurons exhibits properties appropriate for linking stimulus cues to saccade target selection in the context of a visual discrimination task.
    Journal of Neurophysiology 12/2001; 86(5):2527-42. · 3.30 Impact Factor
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    G D Horwitz, W T Newsome
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    ABSTRACT: At any given instant, multiple potential targets for saccades are present in the visual world, implying that a "selection process" within the brain determines the target of the next eye movement. Some superior colliculus (SC) neurons begin discharging seconds before saccade initiation, suggesting involvement in target selection or, alternatively, in postselectional saccade preparation. SC neurons were recorded in monkeys who selected saccade targets on the basis of motion direction in a visual display. Some neurons carried a direction-selective visual signal, consistent with a role in target selection in this task, whereas other SC neurons appeared to be more involved in postselection specification of saccade parameters.
    Science 06/1999; 284(5417):1158-61. · 31.03 Impact Factor

Publication Stats

605 Citations
139.04 Total Impact Points

Institutions

  • 2012
    • Wisconsin National Primate Research Center
      Madison, Wisconsin, United States
  • 2007–2012
    • University of Washington Seattle
      • Department of Physiology and Biophysics
      Seattle, WA, United States
  • 1998–2012
    • Howard Hughes Medical Institute
      Ashburn, Virginia, United States
  • 2005–2006
    • Salk Institute
      • Vision Center Laboratory
      La Jolla, California, United States