Abrupt darkening under high dynamic range (HDR) luminance invokes facilitation for high-contrast targets and grouping by luminance similarity.

When scanning across a scene, luminance can vary by up to 100,000-to-1 (high dynamic range, HDR), requiring multiple normalizing mechanisms spanning from the retina to the cortex to support visual acuity and recognition. Vision models based on standard dynamic range (SDR) luminance contrast ratios below 100-to-1 have limited ability to generalize to real-world scenes with HDR luminance. To characterize how orientation and luminance are linked in brain mechanisms for luminance normalization, we measured orientation discrimination of Gabor targets under HDR luminance dynamics. We report a novel phenomenon, that abrupt 10- to 100-fold darkening engages contextual facilitation, distorting the apparent orientation of a high-contrast central target. Surprisingly, facilitation was influenced by grouping by luminance similarity, as well as by the degree of luminance variability in the surround. These results challenge vision models based solely on activity normalization and raise new questions that will lead to models that perform better in real-world scenes.

A 100,000-to-1 high dynamic range (HDR) luminance display for investigating visual perception under real-world luminance dynamics.

BACKGROUND: Real-world illumination challenges both autonomous sensing and displays, because scene luminance can vary by up to 109-to-1, whereas vision models have limited ability to generalize beyond 100-to-1 luminance contrast. Brain mechanisms automatically normalize the visual input based on feature context, but they remain poorly understood because of the limitations of commercially available displays. NEW METHOD: Here, we describe procedures for setup, calibration, and precision check of an HDR display system, based on a JVC DLA-RS600U reference projector, with over 100,000-to-1 luminance dynamic range (636-0.006055 cd/m2), pseudo 11 bit grayscale precision, and 3 ms temporal precision in the MATLAB/Psychtoolbox software environment. The setup is synchronized with electroencephalography (EEG) and infrared eye-tracking measurements. RESULTS: We show display metrics including light scatter versus average display luminance (ADL), spatial uniformity, and spatial uniformity at high spatial frequency. We also show a luminance normalization phenomenon, contextual facilitation of a high contrast target, whose discovery required HDR display. COMPARISON WITH EXISTING METHODS: This system provides 100-fold greater dynamic range than standard 1000-to-1 contrast displays and increases the number of gray levels from 256 or 1024 (8 or 10 bits) to 2048 (pseudo 11 bits), enabling the study of mesopic-to-photopic vision, at the expense of spatial non-uniformities. CONCLUSIONS: This HDR research capability opens new questions of how visual perception is resilient to real-world luminance dynamics and will lead to improved visual modeling of dense urban and forest environments and of mixed indoor-outdoor environments such as cockpits and augmented reality. Our display metrics code can be found at https://github.com/USArmyResearchLab/ARL-Display-Metrics-and-Average-Display-Luminance.

Visual Evidence Accumulation Guides Decision-Making in Unrestrained Mice.

The ability to manipulate neural activity with precision is an asset in uncovering neural circuits for decision-making. Diverse tools for manipulating neurons are available for mice, but their feasibility remains unclear, especially when decisions require accumulating visual evidence. For example, whether mice' decisions reflect leaky accumulation is unknown, as are the relevant/irrelevant factors that influence decisions. Further, causal circuits for visual evidence accumulation are poorly understood. To address this, we measured decisions in mice judging the fluctuating rate of a flash sequence. An initial analysis (>500,000 trials, 29 male and female mice) demonstrated that information throughout the 1000 ms trial influenced choice, with early information most influential. This suggests that information persists in neural circuits for ∼1000 ms with minimal accumulation leak. Next, in a subset of animals, we probed strategy more extensively and found that although animals were influenced by stimulus rate, they were unable to entirely suppress the influence of stimulus brightness. Finally, we identified anteromedial (AM) visual area via retinotopic mapping and optogenetically inhibited it using JAWS. Light activation biased choices in both injected and uninjected animals, demonstrating that light alone influences behavior. By varying stimulus-response contingency while holding stimulated hemisphere constant, we surmounted this obstacle to demonstrate that AM suppression biases decisions. By leveraging a large dataset to quantitatively characterize decision-making behavior, we establish mice as suitable for neural circuit manipulation studies. Further, by demonstrating that mice accumulate visual evidence, we demonstrate that this strategy for reducing uncertainty in decision-making is used by animals with diverse visual systems.SIGNIFICANCE STATEMENT To connect behaviors to their underlying neural mechanism, a deep understanding of behavioral strategy is needed. This understanding is incomplete for mice. To surmount this, we measured the outcome of >500,000 decisions made by 29 mice trained to judge visual stimuli and performed behavioral/optogenetic manipulations in smaller subsets. Our analyses offer new insights into mice' decision-making strategies and compares them with those of other species. We then disrupted neural activity in a candidate neural structure and examined the effect on decisions. Our findings establish mice as suitable for visual accumulation of evidence decisions. Further, the results highlight similarities in decision-making strategies across very different species.

Listening for the right sounds.

In this issue of Neuron, Rodgers and DeWeese (2014) developed a new paradigm in which rats had to select or ignore an auditory stimulus, depending on its context. The authors recorded neurons in primary auditory and medial prefrontal cortex. Surprisingly, they found that stimulus context had the largest effect in the moments before the stimulus was presented.

Determination of electrode to nerve fiber distance and nerve conduction velocity through spectral analysis of the extracellular action potentials recorded from earthworm giant fibers.

Microneurography and the use of selective microelectrodes that can resolve single-unit nerve activity have become a tool to understand the coding within the nervous system and a clinical diagnostic tool to assess peripheral neural pathologies. Central to these techniques is the use of the differences in the shape of the extracellular action potential (AP) waveform to identify and discriminate units from one another. Theoretical modeling of the origins of these shape differences has shown that the position of the nerve fiber relative to the electrode and the conduction velocity of the unit contribute to these differences giving rise to the hypothesis that more information about the fiber and its relationship to the electrode could be extracted given further analysis of the AP waveform. This paper addresses this question by exploring the electrical coupling between the electrode and nerve fiber. Idealized models and the literature indicate that two parameters, the electrode-fiber distance and the unit conduction velocity, contribute to the amplitude of the extracellular AP detected by the electrode, which confounds the quantification of coupling using the spike amplitude alone. To resolve this, we develop a method that enables differential quantification of these two parameters using spectral analysis of the single-unit AP waveform and demonstrate that the two parameters could be effectively decoupled in an in vitro earthworm model. The method could open the way forward toward micro-scale in situ monitoring of the interaction of nerve fiber and neural interface.

Engineered T4 viral nanoparticles for cellular imaging and flow cytometry.

Viruses are of particular interest as scaffolds for biotechnology applications given their wide range of shapes and sizes and the possibility to modify them with a variety of functional moieties to produce useful virus-based nanoparticles (VNPs). In order to develop functional VNPs for cell imaging and flow cytometry applications, we used the head of the T4 bacteriophage as a scaffold for bioconjugation of fluorescent dyes. Bacteriophage T4 is a double-stranded DNA virus with an elongated icosahedron head and a contractile tail. The head is ∼100 nm in length and ∼90 nm in width. The large surface area of the T4 head is an important advantage for the development of functional materials since it can accommodate significantly larger numbers of functional groups, such as fluorescent dyes, in comparison with other VNPs. In this study, Cy3 and Alexa Fluor 546 were chemically incorporated into tail-less T4 heads (T4 nanoparticles) for the first time, and the fluorescent properties of the dye-conjugated nanoparticles were characterized. The T4 nanoparticles were labeled with up to 19 000 dyes, and in particular, the use of Cy3 led to fluorescent enhancements of up to 90% compared to free Cy3. We also demonstrate that the dye-conjugated T4 nanoparticles are structurally stable and that they can be used as molecular probes for cell imaging and flow cytometry applications.