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Visual Activity Evoked by Infrared in Humans After Dark Adaptation

2016-09-21 20:23:21 | BioPortfolio

Summary

This pilot study will evaluate the visual response to infrared (IR) in humans after dark adaptation. The investigators plan to determine which wavelength and intensity the human eye is most sensitive too, using a broad spectrum light source and wavelength-specific bandpass filters. The investigators will then evaluate the electrophysiologic response in healthy humans to IR, followed by studies in those with specific retinal diseases.

The long-term goal of this research is to better understand the role that IR plays in visual function, and whether this can be manipulated to allow for vision in certain retinal pathologies that result from loss of photoreceptor cells. The investigators central objective is to test the electrophysiologic response to IR in the dark-adapted retinal and visual pathways. The investigators central hypothesis is that IR evokes a visual response in humans after dark adaptation, and the characteristics of this response suggest transient receptor potential (TRP) channel involvement. The investigators rationale is that a better understanding of how IR impacts vision may allow for an alternative mechanism for vision in a number of diseases that cause blindness from the degradation or loss of function of photoreceptor cells. The investigators will test the investigators hypothesis with the following Aims:

Aim 1: To determine the optimal IR wavelength for visual perception in dark-adapted human participants. The investigators hypothesize that the healthy human eye will detect IR irradiation, with a maximum sensitivity at a specific wavelength. Using a broad-spectrum light source with wavelength-specific bandpass filters, the spectral range of visual perception to IR will be evaluated. The same will be done on colorblind participants.

Aim 2: To test the electrophysiologic response to IR in healthy humans after dark adaptation. The investigators hypothesize that IR will elicit an amplitude change on electroretinography (ERG) and visual evoked potential (VEP) responses after dark adaptation in healthy human participants. Participants will be tested with both test modalities to evaluate their response to IR.

Aim 3: To test the electrophysiologic response to IR after dark adaptation in humans with certain retinal diseases. Participants with retinitis pigmentosa, age related macular degeneration and congenital stationary night blindness, will be tested. Results will be compared to baselines and to those of healthy participants. The investigators hypothesize that there will be a response to IR on ERG and VEP, which will provide clues to the retinal cell layer location of the response to IR and the nature of potential TRP channel involvement.

Description

BACKGROUND: Visual impairment affects 285 million people worldwide. The prevalence of visual impairment in the US is expected to rise from 3.3 million in 2000 to 5.5 million in 2020. This will exacerbate the current economic burden of vision loss, which is already $38.2 billion per year in direct and indirect costs. The leading cause of blindness in high income countries is due to age-related macular degeneration (AMD), a disease that leads to gradual loss of the photoreceptor cell layer. An estimated 1.75 million people have AMD in the US and another 7.3 million are at risk. Importantly, despite the loss of photoreceptor cells in AMD, the other cellular layers in the retina remain largely intact.

The retina lines the back of the eye and is composed of structural layers. The outer nuclear layer contains photoreceptors called rods and cones. The inner nuclear layer includes bipolar, horizontal, and amacrine cells. Most anteriorly, the ganglion cell layer has axons that exit the eye as the optic nerve. Visual image formation begins when a photon of light enters the eye, passes through all retinal layers, and is absorbed by the photoreceptor cells. These cells transduce the photon of light into an electrochemical signal, which is communicated to bipolar cells, followed by the ganglion cells. Here, an action potential is generated and propagated via the optic nerve to the area of the brain where vision perception occurs. When the eye is dark adapted, the cells in this pathway are potentially more sensitive to other types of stimuli, such as IR. The investigators believe cation channels called TRP channels in ganglion cells are activated by IR in this dark adapted state, creating the visual response to IR. Heat is a known activator of certain subtypes these channels elsewhere in the body. TRP channels are also responsible for IR vision in pit vipers and vampire bats.

Palczewska et al. reported that visual perception to IR occurred through a process of direct two-photon isomerization of visual pigments. However, other evidence suggests IR perception can occur through single IR photon absorption. Studies that use IR to test the functionality of implanted visual prosthesis have noted a greater response to IR in the non-implanted eye when compared to the implanted eye on both VEP tests and ERG. On ERG, a specialized response specific to IR was found called the scotopic threshold response (STR). This response occurs under dark-adapted conditions and correlates with a response at the ganglion cell layer. Direct IR activation of TRP channels on ganglion cells could initiate a visual response. Based on these findings, the investigators hypothesize the human response to IR under dark adaptation occurs at the level of the ganglion cells through heat-activated TRP channels.

RESEARCH DESIGN AIM1: To determine the optimal IR wavelength for human visual perception while dark-adapted.

Introduction for Aim 1: The objective of this aim is to determine the optimal wavelength of IR to which the human eye is sensitive. To obtain this objective, the investigators will test the working hypothesis that the healthy human eye, and those with colorblindness, will detect a range of IR wavelengths, with a preference for a specific wavelength. The investigators will test the working hypothesis using a broad-spectrum light source with wavelength-specific bandpass filters in the IR range. The investigators rationale for this aim is that understanding the optimal IR wavelength of the human eye will aid in future investigations when testing the visual response to IR using diagnostic equipment. This is important because it could impact the way other ophthalmologic modalities use IR to diagnosis and treat visual pathologies.

Research Design for Aim 1: A total of healthy 25 participants (15 with normal vision and 10 with colorblindness) aged 18 and older will be recruited using the University of New Mexico (UNM) Clinical and Translational Science Center (CTSC) Clinical Research Volunteer Registry HRRC-06412. Informed consent, participant demographics, past medial and visual history, and a general eye exam will be obtained using the CTSC research coordinator. Each participant will be placed in a dark room for an hour to allow for optimal dark adaptation of the eye. The investigators will use a broad-spectrum light source with wavelength-specific bandpass filters of different IR wavelengths. A total of 12 filters will be used ranging from 850 nm to 1400nm. Intensity curves will be build for each wavelength, by slowing turning up the power until the participant indicates a visual response to the stimulus.

Data Analysis for Aim 1: Data will be analyzed by the investigators. Descriptive statistics will be used to evaluate demographics, general and visual health information, and reported optimal wavelengths. The investigators analysis will compare differences among responses for each wavelength. To the investigators knowledge there have been no studies evaluating which IR wavelength is optimal for human visual perception, thus the investigators assume a low effect size of 10%, which would produce an 82% chance of at least two out of 30 healthy participants giving a preferred response to a specific wavelength. The investigators will describe the estimated effect sizes in response to the findings.

Expected Outcomes for Aim 1: The investigators expect the human eye to perceive a range of IR wavelengths, but have a specific wavelength optimal in terms of brightness.

Potential Problems & Alternative Strategies for Aim 1: To prevent sampling bias, the investigators plan to obtain a representative sample from New Mexico; however, participants may be younger and more educated than the general population. Confounding bias of light pollution may occur, which would prevent dark adaptation and decrease the IR sensitivity. A photometer will assess the room for background photons.

AIM 2: To test the electrophysiologic response to IR in healthy humans after dark adaptation.

Introduction for Aim 2: The objective of this aim is to determine the site of IR transduction in the visual pathway. To obtain this objective, the investigators will test the working hypothesis that IR elicits an amplitude change in human subjects on ERG and VEP tests after dark adaptation. The investigators will test the working hypothesis with a clinical trial in which the electrophysiologic visual response to the baseline of visible light is compared to IR. The investigators rationale for this aim is that the proposed research will contribute to an important understanding of an alternative mechanism of vision. It is important to investigate this pathway to further understand general visual health and to demonstrate how IR directly elicits a visual response. Such a finding would be important because it would expand the visually responsive light spectrum to include IR.

Research Design for Aim 2: A total of 6 healthy participants aged 18 and older will be recruited using the same criteria as in Aim 1. The investigators will collect the same documentation and health information as in Aim 1. Participants will be tested in the UNM Eye Clinic using VEP and ERG at both baseline and under dark adapted IR conditions. Both tests are non-invasive and considered safe. The International Society for Clinical Electrophysiology of Vision (ISCEV) guidelines for clinical VEPs and full-field ERGs will be followed. These protocols will be extended to test the IR stimulus after dark adaptation. Total time for participants will be 5 hours and each will participate only once.

Data Analysis for Aim 2: Data will be analyzed by the investigators. Descriptive statistics will describe demographics and general and visual health information. As per ISCEV protocol, when experimenting outside of the normal laboratory ranges, the investigators will not assume a normal distribution. Reports will specify stimulus and recording parameters. The investigators primary analysis will test the underlying probability of a response to the stimulus using binomial distributions (HO = 0, HI > 0). Using exact tests, the investigators secondary analysis will compare differences among dark adaptation time intervals of 0, 15, 30, 45, 60 minutes for each stimulus type, and the tertiary analysis will compare differences between baseline and IR stimulus at each time interval. The investigators pilot data and animal models have demonstrated a consistent visual response to IR. However, to the investigators knowledge, there have been no electrophysiologic studies to an IR stimulus in humans. Thus, the investigators will assume a low effect size. However, due to the cost of the diagnostic test we are limited to 5 participants. The investigators will describe the estimated effect sizes in response to the finding.

Expected Outcomes for Aim 2: The investigators expect an IR response in ERG and VEP in dark-adapted humans.

Potential Problems & Alternative Strategies for Aim 2: Aim 2 shares the same potential problems as Aim 1, and the investigators will address these in an identical manner. In addition, calibration bias may occur with the ERG and VEP. The investigators will follow the ISCEV protocols for both of these tests. IR trials may require averaging additional stimulus repetitions to improve the signal-to-noise ratio of ERG and VEP signals. To avoid bias in interpretation of results, The investigators will use intra and inter-reliability comparisons.

AIM 3: To test the electrophysiologic response to IR in humans with retinal diseases or injuries after dark adaptation.

Introduction for Aim 3: The objective of this aim is to determine which retinal cell layer is responding to IR and the nature of TRP channel involvement. To obtain this objective, the investigators will test the working hypothesis that IR will not elicit an amplitude change in certain retinal diseases. The investigators will test the working hypothesis with a clinical trial by testing the visual response to IR in certain retinal diseases using ERG and VEP. The investigators rationale for this aim is that the proposed research will examine if certain retinal diseases are visually sensitive to IR. This is important to investigate because it could allow a different approach to visual research in certain retinal diseases. Such a finding would be important because it could provide the basis for a novel form of visual prosthesis.

Research Design for Aim 3: A total of twenty-five participants, or five per retinal disease, will be recruited using the CTSC Clinical Research Volunteer Registry HRRC-06412. Retinal diseases include retinitis pigmentosa, age related macular degeneration, congenital stationary night blindness, cataracts. Five participants with colorblindness will also be included. The same protocol will be followed as in aim 2 for demographic collection and ERG and VEP tests under baseline and dark-adapted conditions.

Data Analysis for Aim 3: In addition to using the same type of data analysis as in Aim 2, results will also be compared between retinal diseases and with healthy participants.

Expected Outcomes for Aim 3: The investigators expect an IR will not elicit a response on ERG and VEP in certain retinal diseases after dark-adaptation.

Potential Problems & Alternative Strategies for Aim 3: Same as in Aim 2.

Study Design

Allocation: Non-Randomized, Intervention Model: Parallel Assignment, Masking: Single Blind (Subject), Primary Purpose: Diagnostic

Conditions

Age Related Macular Degeneration

Intervention

Tungsten halogen light with narrow bandpass filters, ERG, VEP

Location

University of New Mexico
Albuquerque
New Mexico
United States
87131-0001

Status

Completed

Source

University of New Mexico

Results (where available)

View Results

Links

Published on BioPortfolio: 2016-09-21T20:23:21-0400

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