Liquid-Crystal Display (LCD) of achromatic, mean-modulated flicker in clinical assessment and experimental studies of visual systems.

Achromatic, mean-modulated flicker-wherein luminance increments and decrements of equal magnitude are applied, over time, to a test field-is commonly used in both clinical assessment of vision and experimental studies of visual systems. However, presenting flicker on computer-controlled displays is problematic; displays typically introduce luminance artifacts at high flicker frequency or contrast, potentially interfering with the validity of findings. Here, we present a battery of tests used to weigh the relative merits of two displays for presenting achromatic, mean-modulated flicker. These tests revealed marked differences between a new high-performance liquid-crystal display (LCD; EIZO ColorEdge CG247X) and a new consumer-grade LCD (Dell U2415b), despite displays' vendor-supplied specifications being almost identical. We measured displayed luminance using a spot meter and a linearized photodiode. We derived several measures, including spatial uniformity, the effect of viewing angle, response times, Fourier amplitude spectra, and cycle-averaged luminance. We presented paired luminance pulses to quantify the displays' nonlinear dynamics. The CG247X showed relatively good spatial uniformity (e.g., at moderate luminance, standard deviation 2.8% versus U2415b's 5.3%). Fourier transformation of nominally static test patches revealed spectra free of artifacts, with the exception of a frame response. The CG247X's rise and fall times depended on both the luminance from which, and to which, it responded, as is to be generally expected from LCDs. Despite this nonlinear behaviour, we were able to define a contrast and frequency range wherein the CG247X appeared largely artifact-free; the relationship between nominal luminance and displayed luminance was accurately modelled using a causal, linear time-invariant system. This range included contrasts up to 80%, and flicker frequencies up to 30 Hz. This battery of tests should prove useful to others conducting clinical assessment of vision and experimental studies of visual systems

Biological–machine systems integration : engineering the neural interface

The state of the art of biological-machine systems integration (BMSI) with an emphasis on neural interfacing is reported. The goal of BMSI from a medical viewpoint is to effectively replace or facilitate activation of, or recording from, neural elements in a part of the nervous system. BMSI will be firstly examined from a generic level whereby the current technologies for noninvasive and invasive neural recording and neural stimulation will be detailed. A case study in the area of visual neuroprosthesis will be presented to elucidate the current and future issues facing BMSI. This will include a discussion on biocompatibility, design of stimulating electrodes, the implanted microelectronic neurostimulator, and parallelization of stimulus encoding and delivery

Optical imaging of electrically evoked visual signals in cats. I, Responses to corneal and intravitreal electrical stimulation

Microelectronic retinal prostheses have been shown to restore the perception of light to the blind through electrical stimulation. Conventional recording techniques such as recording electrode arrays on the visual cortex can give a basic understanding of the events that occur during such stimulation events, but their finite size and number limits the spatial resolution achievable with them. Optical imaging of intrinsic signals (OIS imaging) allows for greater resolution (approximately 50 mum) of the activity in the cortex. This can be used to facilitate a greater understanding of the complex neurophysiological events that allow prosthetic vision. This paper shows responses to visual and electrical stimulation of the retina, and demonstrates that OIS imaging may be an effective technique in further refining stimulation techniques and implant designs for retinal prostheses

Optical imaging of electrically evoked visual signals in cats. II, ICA 'harmonic filtering' noise reduction

Optical imaging of intrinsic signals (OIS) is a tool for visualizing differential areas in the primary visual cortex devoted to visual functions such as ocular dominance and spatial orientation preferences. The OIS methodology was employed to verify visual cortical response to a retinal vision prosthesis whereby electrical stimulation is applied to the neural retina in order to elicit visual percepts. However, OIS recording is quite susceptible to cardiac and respiratory artifact, and inherent noise related to the measurement process. This complicates the identification of evoked signals using standard ensemble averaging based image processing. We therefore developed an independent component analysis (ICA) “harmonic filtering” technique to improve the signal-to-noise ratio. This technique is capable of reducing noise components, highlighting response signals to visual and electrical stimuli. Particularly, we demonstrated extraction of an ocular dominance map due to corneal stimulation and localized cortical activation due to intravitreal stimulation

Advances in Retinal Neuroprosthetics

Submitted as a book chapter to the IEEE/Wiley edited book on Neural Engineering/Neuro