Multisensory-inspired modeling and neural correlates for two key binocular interactions.

Most binocular vision models assume that the two eyes sum incompletely. However, some facilitatory cortical neurons fire for only one eye, but amplify their firing rates if both eyes are stimulated. These 'binocular gate' neurons closely resemble subthreshold multisensory neurons. Binocular amplification for binocular gate neurons follows a power law, with a compressive exponent. Unexpectedly, this rule also applies to facilitatory true binocular neurons; although driven by either eye, binocular neurons are well modeled as gated amplifiers of their strongest monocular response, if both eyes are stimulated. Psychophysical data follows the same power law as the neural data, with a similar exponent; binocular contrast sensitivity can be modeled as a gated amplification of the more sensitive eye. These results resemble gated amplification phenomena in multisensory integration, and other non-driving modulatory interactions that affect sensory processing. Models of incomplete summation seem unnecessary for V1 facilitatory neurons or contrast sensitivity. However, binocular combination of clearly visible monocular stimuli follows Schrödinger's nonlinear magnitude-weighted average. We find that putatively suppressive binocular neurons closely follow Schrödinger's equation. Similar suppressive multisensory neurons are well documented but seldom studied. Facilitatory binocular neurons and mildly suppressive binocular neurons are likely neural correlates of binocular sensitivity and binocular appearance respectively.

Exploring the Interaction Between Head-Supported Mass, Posture, and Visual Stress on Neck Muscle Activation.

OBJECTIVE: Assess neck muscle activity for varying interactions between helmet, posture, and visual stress in a simulated "helo-hunch" posture. BACKGROUND: Military aviators frequently report neck pain (NP). Risk factors for NP include head-supported mass, awkward postures, and mental workload. Interactions between these factors could induce constant low-level muscle activation during helicopter flight and better explain instances of NP. METHOD: Interactions between physical loading (helmet doffed/donned), posture (symmetric/asymmetric), and visual stress (low/high contrast) were studied through neck muscle electromyography (EMG), head kinematics, subjective discomfort, perceived workload, and task performance. Subjects (n = 16) performed eight 30-min test conditions (varied physical loading, posture, and visual stress) while performing a simple task in a simulated "helo-hunch" seating environment. RESULTS: Conditions with a helmet donned had fewer EMG median frequency cycles (which infer motor unit rotation for rest/recovery, where more cycles are better) in the left cervical extensor and left sternocleidomastoid. Asymmetric posture (to the right) resulted in higher normalized EMG activity in the right cervical extensor and left sternocleidomastoid and resulted in less lateral bending compared with neutral across all conditions. Conditions with high visual stress also resulted in fewer EMG cycles in the right cervical extensor. CONCLUSION: A complex interaction exists between the physical load of the helmet, postural stress from awkward postures, and visual stress within a simulated "helo-hunch" seating environment. APPLICATION: These results provide insight into how visual factors influence biomechanical loading. Such insights may assist future studies in designing short-term administrative controls and long-term engineering controls.

Color Vision Testing, Standards, and Visual Performance of the U.S. Military.

INTRODUCTION: Color vision deficiency (CVD) is a disqualifying condition for military special duty occupations. Color vision testing and standards vary slightly among the U.S. military branches. Paper-based pseudoisochromatic plates (PIPs) remain a screening tool. Computer-based color vision tests (CVTs), i.e., the Cone Contrast Test (CCT), the Colour Assessment and Diagnosis (CAD) test, and the Waggoner Computerized Color Vision Test (WCCVT), are now replacing the Farnsworth Lantern Test (FALANT) and its variants to serve as a primary or secondary test in the U.S. Armed Forces. To maintain consistency in recruitment, performance, and safety, the study objectives were to examine military color vision testing, passing criteria, and color discrimination performance. METHODS: Study participants were 191 (17% female) students, faculty, and staff of the U.S. Air Force Academy and the Naval Aerospace Medical Institute. All subjects performed six CVTs, and 141 participants completed two additional military relevant color discrimination tasks. Friedman non-parametric test and Wilcoxon signed-rank post hoc test with Bonferroni adjusted P values were used to compare CVTs and standards. Analysis of variance and Bonferroni adjusted post hoc test were used to describe effects on color discrimination performance. RESULTS: The Heidelberg Multicolor-Moreland and Rayleigh (HMC-MR) anomaloscope diagnosed 58 CVD (30.4%). There were no statistically significant differences in identifying red-green CVD by the HMC-MR, CCT, CAD, WCCVT, and PIP tests (P = .18), or classifying deutan, protan, and normal color vision (CVN) by the HMC-MR and the CVT (P = .25). Classification of tritan CVD was significantly different depending on which CVT was used (P < .001). Second, overall passing rates were 79.1% on the CAD (≤6 standard normal unit (SNU)), 78.5% on the combined PIP/FALANT, 78.0% on the CCT (≥55%), and 75.4% on the WCCVT (mild) military standards. The CVTs and the PIP/FALANT standards were not significantly different in number of personnel selected, but CAD and CCT passed significantly more individuals than WCCVT (P = .011 and P = .004, respectively). The previous U.S. Air Force standard (CCT score ≥75%) passed significantly fewer individuals relative the U.S. Navy pre-2017 PIP/FALANT or the current CVT standards (P ≤ .001). Furthermore, for those who failed the PIP (<12/14), the FALANT (9/9 or ≥16/18) agreed with the CVTs on passing the same CVN (n = 5); however, it also passed moderate-to-severe CVD who did not pass WCCVT (n = 6), CCT (n = 3), and CAD (n = 1). Lastly, moderate/severe CVD were significantly slower and less accurate than the "mild" CVD or CVN in the two color discrimination tasks (P < .001). In comparison to CVN in the in-cockpit display color discrimination task, mild CVD (CCT ≥55% and <75%) were significantly slower by 1,424 ± 290 milliseconds in reaction time (P < .001) while maintaining accuracy. CONCLUSIONS: CVTs are superior to paper-based PIP in diagnosing, classifying, and grading CVD. Relative to the PIP/FALANT standard in personnel selection, the current U.S. military CVT passing criteria offer comparable passing rates but are more accurate in selecting mild CVD. Nevertheless, military commanders should also consider specific operational requirements in selecting mild CVD for duty as reduced job performance may occur in a complex color critical environment.

Coincidence Anticipation Timing Responses with Head Tracking and Eye Tracking.

BACKGROUND: Head tracking movements are common in interceptive tasks. The benefits of these movements are unclear. The purpose of this study was to compare coincidence anticipation timing (CAT) responses for a simulated approaching object when the eyes were used in tracking the object and when the head was used in tracking the object.METHODS: A total of 29 subjects participated. A Bassin Anticipation Timer consisting of a track of sequentially illuminated lights was used to simulate an approaching object at velocities of 223 cm · s-1 to 894 cm · s-1. Each velocity was used 10 times under 2 conditions. In one condition, subjects were told to turn the eyes with the stimulus. In the other condition, subjects viewed the stimulus through apertures and were told to turn the head with the stimulus. Subjects pushed a button to coincide with illumination of the final light on the track.RESULTS: Signed CAT errors, unsigned CAT errors, and variable CAT errors were compared between the head movement (HM) and eye movement (EM) conditions. No significant differences were noted for the signed errors (mean signed error at 894 cm · s-1; 10.3 ± 75.4 ms (HM), -16.1 ± 51.0 ms (EM). However, the unsigned and variable errors were significantly larger at some stimulus velocities in the head movement condition [mean unsigned error at 894 cm · s-1: 82.6.0 ± 45.9 ms (HM), 59.0 ± 22.4 ms (EM); mean variable error at 894 cm · s-1; 78.0 ± 37.8 ms (HM), 49.2 ± 17.1ms (EM)].DISCUSSION: Head movement did not result in improved CAT performance compared to eye movements. Further work will be required to determine whether these results are generalizable to situations where head tracking is required but apertures are not worn.Ross E, Kinney M, Fogt N. Coincidence anticipation timing responses with head tracking and eye tracking. Aerosp Med Hum Perform. 2022; 93(2):79-88.

A simple vector-like law for perceptual information combination is also followed by a class of cortical multisensory bimodal neurons.

An interdisciplinary approach to sensory information combination shows a correspondence between perceptual and neural measures of nonlinear multisensory integration. In psychophysics, sensory information combinations are often characterized by the Minkowski formula, but the neural substrates of many psychophysical multisensory interactions are unknown. We show that audiovisual interactions - for both psychophysical detection threshold data and cortical bimodal neurons - obey similar vector-like Minkowski models, suggesting that cortical bimodal neurons could underlie multisensory perceptual sensitivity. An alternative Bayesian model is not a good predictor of cortical bimodal response. In contrast to cortex, audiovisual data from superior colliculus resembles the 'City-Block' combination rule used in perceptual similarity metrics. Previous work found a simple power law amplification rule is followed for perceptual appearance measures and by cortical subthreshold multisensory neurons. The two most studied neural cell classes in cortical multisensory interactions may provide neural substrates for two important perceptual modes: appearance-based and performance-based perception.

Deficits in Visual Processing During Hypoxia as Evidenced by Visual Mismatch Negativity.

INTRODUCTION: Hypoxia is an ever-present threat in tactical aviation and gained recent attention due to its putative role in physiological episodes. Previous work has demonstrated that hypoxia negatively impacts a variety of sensory, cognitive, and motor systems. In particular, the visual system is one of the earliest systems affected by hypoxia. While the majority of previous studies have relied on self-report and behavioral testing, the use of event-related potentials as a novel tool to monitor responses to low oxygen in humans has recently been investigated. Specifically, ERP components that are evoked passively in response to unattended changes in background sensory stimulation have been explored.METHOD: Subjects (N 28) completed a continuous visuomotor tracking task while EEG was recorded. During the tracking task, a series of standard color checkerboard patterns were presented in the periphery while occasionally a deviant color checkerboard was presented. The visual mismatch negativity (MMN) component was assessed in response to the deviant compared to the standard stimuli. Subjects completed two sessions in counterbalanced order that only differed by the oxygen concentration breathed (10.6% vs. 20.4%).RESULTS: Results demonstrated a significant reduction in the amplitude of the visual MMN under hypoxic compared to normoxic conditions, showing a 50% reduction in amplitude during hypoxia. Our results suggest that during low-oxygen exposure the ability to detect environmental changes and process sensory information is impaired.DISCUSSION: The visual MMN may represent an early and reliable predictor of sensory and cognitive deficits during hypoxia exposure, which may be of great use to the aviation community.Blacker KJ, Seech TR, Funke ME, Kinney MJ. Deficits in visual processing during hypoxia as evidenced by visual mismatch negativity. Aerosp Med Hum Perform. 2021; 92(5):326332.

Effects of Intermediate-Tinted Lenses on Pistol Marksmanship and Visual Performance.

INTRODUCTION: The standard issue clear or sun Military Combat Eye Protection (MCEP) is often inadequate in visually challenging training or combat environment. Intermediate-tinted lenses may offer a viable option for warfighters operating in dynamic visual conditions such as moving rapidly from bright sunny areas to darker building interiors in combat. Because the use of intermediate-tinted lenses has been delayed as a result of a potential negative impact on vision performance such as color perception, this investigation evaluated several commercially available intermediate-tinted lenses for operation performance during a U.S. military field training. Test lenses complied with all the requirements of the current MCEP except for the visible light transmittance (VLT) values that ranged between 32 and 62%. MATERIALS AND METHODS: Study subjects consisted of 22 service members who attended a military Close Combat and Marksmanship training course in May 2019. Visual and pistol marksmanship performance of three intermediate-tinted lenses (Eye Safety Systems [ESS] Copper [32% VLT], Oakley Prizm TR45 [44% VLT], and ESS Bronze [62% VLT]) was compared to that of a standard issue MCEP, ESS Clear (90% VLT). Quick contrast sensitivity function test was used to assess quality vision (AULCSF, area under a log contrast sensitivity function) and visual acuity (CSF Acuity). Color vision was assessed by Cone Contrast Test. Pistol marksmanship (Bill Drill) was used for performance testing. A pre-survey inquired about MCEP use, and a MCEP survey during the pistol marksmanship testing inquired about lens performance and ranking. RESULTS: AULCSF and CSF Acuity were significantly affected by the lenses (general linear model, repeated measures, P < .05). Bonferroni post hoc test showed a significant reduction of binocular AULCSF from ESS Clear to Oakley Prizm TR45 (P = .003) and ESS Copper (P < .001) and a significant reduction in binocular CSF Acuity from ESS Clear to ESS Copper (P = .001). Color vision and pistol marksmanship performance were not significantly affected by wearing different lenses (P > .05). Subjectively, there were no statistically significant differences among study lenses in perceived "clarity of vision," "ability to clearly identify the target," or "overall performance" (Friedman test and Wilcoxon signed-rank post hoc test with Bonferroni adjustment, P > .017). Participants ranked Prizm TR45 (44% VLT) and ESS Bronze (62% VLT) lenses significantly more favorably than EES Clear (P = .001 and P = .009). CONCLUSIONS: Quality of vision and visual acuity decreased with darker lenses; however, the study lenses had insignificant impacts on pistol marksmanship and subjective acceptance. Our surveys indicated that Intermediate-tinted lenses were operationally acceptable and preferred over a standard issue MCEP. While more evaluations for color vision deficient subjects are needed, overall results suggest that commercially available intermediate-tinted lenses may be a viable option to enhance protection and performance in a visually dynamic combat environment.

Temporal Effects of 2% Pilocarpine Ophthalmic Solution on Human Pupil Size and Accommodation.

INTRODUCTION: First responders and those who work with organophosphate (OP) compounds can experience ocular symptoms similar to those caused by exposure to low levels of nerve agents. This study was designed to examine the efficacy of a safe, clinically available, simulant that reproduces ocular symptoms associated with low-level OP exposure. Among these ocular symptoms are a constriction of the pupils (miosis), decreased visual acuity, and changes in accommodation. MATERIALS AND METHODS: Volunteers aged 18-40 were assigned to groups receiving either a two-drop or three-drop dose of FDA approved 2% pilocarpine ophthalmic solution. Baseline visual performance measurements were taken before eye drop instillation and a timer was started following the first drop of pilocarpine. Once eye drops were administered, visual performance including distant and near vision, pupil size, and accommodation were measured every 5 minutes for 2 hours. RESULTS: Both groups experienced significant miosis in excess of 90 minutes. Visual acuity was significantly reduced because of accommodative changes. The three-drop group experienced longer lasting combined effects when compared to the two-drop group. CONCLUSIONS: 2% pilocarpine ophthalmic solution can safely simulate major ocular symptoms of OP exposure for behavioral research studies for at least 60 minutes.

Evaluation of Aircrew Low-Intensity Threat Laser Eye Protection.

Prototype low-intensity threat laser eye protection (LIT-LEP) spectacles were evaluated for US Coast Guard (USCG) cockpits and night vision goggle compatibility. The impetus for interest in aviation LIT-LEP is driven in part by the fact that easily accessible 0.5-2.0 W high-power laser pointers exceed safety standards for direct on-axis viewing. A repeated-measures experimental design was used to assess LIT-LEP performance relative to a no-LEP control for the following tasks: Near- and far contrast acuity, night vision goggle far-contrast acuity, emissive and non-emissive light source color-vision screening, and USCG multifunctional display color symbol discrimination reaction time and accuracy. Near- and far-contrast acuity results demonstrated good LIT-LEP performance for typical in- and out-of-cockpit lighting conditions. Night vision goggle performance suffered marginally at only one contrast level (85%; 20/30 acuity line). Color vision test results showed good color balance in that S-, M-, and L-cone performance did not demonstrate a clinical diagnostic color defect for emissive or non-emissive light sources when wearing LIT-LEP. Color symbol discrimination reaction-time-task results based on inverse efficiency scores revealed that some non-primary flight display colors exhibited a combination of slower speed and decreased accuracy. The findings will contribute to an acquisition decision as well as guide future LEP designs.