Jonathon Toft-Nielsen
University of Miami
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Featured researches published by Jonathon Toft-Nielsen.
Investigative Ophthalmology & Visual Science | 2014
Tsung Han Chou; Jorge Bohorquez; Jonathon Toft-Nielsen; Özcan Özdamar; Vittorio Porciatti
PURPOSE We recorded pattern electroretinograms (PERGs) simultaneously from each eye in mice using binocular stimulation and a common noncorneal electrode. METHODS The PERG was derived simultaneously from each eye in 71 ketamine/xylazine anesthetized mice (C57BL/6J, 4 months old) from subcutaneous needles (active, snout; reference, back of the head; ground, root of the tail) in response to contrast-reversal of gratings (0.05 cycles/deg, >95% contrast) generated on two custom-made light-emitting diode (LED) tablets alternating at slight different frequencies (OD, 0.984 Hz; OS, 0.992 Hz). Independent PERG signals from each eye were retrieved using one channel continuous acquisition and phase-locking average (OD, 369 epochs of 492 ms; OS, 372 epochs of 496 ms). The PERG was the average of three consecutive repetitions. RESULTS Binocular snout PERGs had high amplitude (mean, 25.3 μV, SD 6.6) and no measurable interocular cross-talk. Responses were reliable (test-retest variability within-session, 14%, SD 7; between sessions, 25%, SD 9; interocular asymmetry within-session, 9%, SD 7; between sessions, 13%, SD 5). Retinal ganglion cells (RGCs) were the main source of the binocular snout PERG, as optic nerve crush in three mice abolished the signal. CONCLUSIONS The PERG, a sensitive measure of RGC function, is used increasingly in mouse models of glaucoma and optic nerve disease. Compared to current methods, the binocular snout PERG represents a substantial improvement in terms of simplicity and speed. It also overcomes limitations of corneal electrodes that interfere with invasive procedures of the eye and facilitates experiments based on comparison between the responses of the two eyes.
international conference of the ieee engineering in medicine and biology society | 2011
Jonathon Toft-Nielsen; Jorge Bohorquez; Özcan Özdamar
Pattern Reversal (PR) stimulation is a frequently used tool in the evaluation of the visual pathway. The PR stimulus consists of a field of black and white segments (usually checks or bars) of constant luminance, which change phase (black to white and white to black) at a given reversal rate. The Pattern Electroretinogram (PERG) is a biological potential that is evoked from the retina upon viewing PR display. Likewise, the Pattern Visual Evoked Potential (PVEP) is a biological potential recorded from the occipital cortex when viewing a PR display. Typically, PR stimuli are presented on a Cathode Ray Tube (CRT) or Liquid Crystal Display (LCD) monitor. This paper presents three modalities to generate pattern reversal stimuli. The three methods are as follows: a display consisting of array of Light Emitting Diodes (LEDs), a display comprised of two miniature projectors, and a display utilizing a modified LCD display in conjunction with a variable polarizer. The proposed stimulators allow for the recording of PERG and PVEP waveforms at much higher rates than are capable with conventional stimulators. Additionally, all three of the alternative PR displays will be able to take advantage of advanced analysis techniques, such as the recently developed Continuous Loop Averaging Deconvolution (CLAD) algorithm.
Investigative Ophthalmology & Visual Science | 2014
Özcan Özdamar; Jonathon Toft-Nielsen; Jorge Bohorquez; Vittorio Porciatti
PURPOSE We determined if the overlap of transient (tr) pattern electroretinograms (PERG(tr)) can explain the generation of the steady-state (SS) pattern electroretinogram (PERG(SS)), and investigated the relationship between the two types of responses. METHODS Slightly jittered pattern reversals were used to generate quasi SS (QSS) PERG(SS) responses from eight normal subjects, recorded using lower eyelid skin electrodes, at rates between 6.9 and 26.5 reversals per second (rps). Jittered quasi PERG(SS) were deconvolved using the frequency domain continuous loop averaging deconvolution method. Additionally, conventional PERG(tr) at 2.2 rps and PERG(SS) at each of the QSS stimulation rates were obtained from all subjects. Two synthetic PERG(SS) responses were constructed at each stimulation rate, one using the PERG(tr) obtained at that rate, and the other using the conventional 2.2 rps PERG(tr). Synthetic responses then were compared to the recorded PERG(SS) using amplitude, latency, and spectral measurements. RESULTS Findings indicate that the PERG(SS) obtained at SS rates can be predicted using the superposition of deconvolved tr PERGs at each particular rate. Although conventional PERG(tr) can explain PERG(SS) obtained at rates below 15.4 rps (≥ 97% correlation), for higher reversal rates only deconvolved responses obtained at that rate can produce the recorded SS responses (96% vs. 65% correlation at 26.5 rps). CONCLUSIONS The study shows that PERG(SS) results from the overlapping of tr PERG(tr) waveforms generated at that reversal rate. The first two peaks (N(SS) and P(SS)) of the PERG(SS) reflect N35 and P50 waves of the tr PERG(tr). The N95 amplitude is reduced at conventional (16 rps) SS rates, but contributes to the overall PERG(SS) amplitude.
25th Southern Biomedical Engineering Conference 2009 | 2009
Jonathon Toft-Nielsen; Jorge Bohorquez; Özcan Özdamar
The Pattern Electroretinogram (PERG) is a biological potential that is evoked from the retina upon viewing a temporally modulated stimulus pattern of constant total luminance. Typically, PERG recordings are limited by the method of conveying the stimulus. This paper presents an alternative method to generate PERG stimuli utilizing Light Emitting Diodes (LEDs) and optical diffusing materials. The proposed stimulator will allow for the recording of PERG waveforms at much higher rates than are capable with conventional stimulators. Also, the proposed stimulator will be able to take advantage of advanced analysis techniques, such as the recently developed Continuous Loop Averaging Deconvolution (CLAD) algorithm.
Translational Vision Science & Technology | 2017
Pedro Monsalve; Giacinto Triolo; Jonathon Toft-Nielsen; Jorge Bohorquez; Amanda D. Henderson; Rafael E. Delgado; Edward Miskiel; Özcan Özdamar; William J. Feuer; Vittorio Porciatti
Purpose To compare a new method for steady-state pattern electroretinogram (PERGx) with a validated method (PERGLA) in normal controls and in patients with optic neuropathy. Methods PERGx and PERGLA were recorded in a mixed population (n = 33, 66 eyes) of younger controls (C1; n = 10, age 38 ± 8.3 years), older controls (C2; n = 11, 57.9 ± 8.09 years), patients with early manifest glaucoma (G; n = 7, 65.7 ±11.6 years), and patients with nonarteritic ischemic optic neuropathy (N; n = 5, mean age 59.4 ± 8.6 years). The PERGx stimulus was a black-white horizontal grating generated on a 14 × 14 cm LED display (1.6 cycles/deg, 15.63 reversals/s, 98% contrast, 800 cd/m2 mean luminance, 25° field). PERGx signal and noise were averaged over 1024 epochs (∼2 minutes) and Fourier analyzed to retrieve amplitude and phase. Partial averages (16 successive samples of 64 epochs each) were also analyzed to quantify progressive changes over recording time (adaptation). Results PERGLA and PERGx amplitudes and latencies were correlated (Amplitude R2 = 0.59, Latency R2 = 0.39, both P < 0.0001) and were similarly altered in disease. Compared to PERGLA, however, PERGx had shorter (16 ms) latency, higher (1.39×) amplitude, lower (0.37×) noise, and higher (4.2×) signal-to-noise ratio. PERGx displayed marked amplitude adaptation in C1 and C2 groups and no significant adaptation in G and N groups. Conclusions The PERGx high signal-to-noise ratio may allow meaningful recording in advanced stages of optic nerve disorders. In addition, it quantifies response adaptation, which may be selectively altered in glaucoma and optic neuropathy. Translational Relevance A new PERG method with increased dynamic range allows recording of retinal ganglion cell function in advanced stages of optic nerve disorders. It also quantifies the response decline during the test, an autoregulatory adaptation to metabolic challenge that decreases with age and presence of disease.
2013 29th Southern Biomedical Engineering Conference | 2013
Jorge Bohorquez; Sebastian Lozano; Alexander Kao; Jonathon Toft-Nielsen; Özcan Özdamar
Visual Evoked Potentials (VEPs) are brain signals generated in response to visual stimuli which are used in the diagnosis of neuro-ophthalmic disorders and the development of brain computer interfaces (BCI). When a pattern-reversal stimulus is delivered at low rates, the acquired transient VEP response (VEPtr) represents the activation of neural structures in the visual pathway. For stimulation rates above six reversals per second (rps), response overlapping occurs and steady state responses are formed (VEPss). This study investigates VEP adaptation by deconvolving responses to temporally jittered stimuli at high rates. VEPs were obtained from normal subjects stimulated with a LED based pattern reversal stimulator delivering three steady-state (1.63, 8.14 and 13.02 rps) and two low-jitter (8.14 and 13.02 mean rps) sequences. Using the Continuous Loop Averaging Deconvolution (CLAD) method VEPtrs were extracted from the jittered responses. To verify the quality of the deconvolution process, the high-rate isochronic VEPss were compared with a synthetic response estimated with the VEPtr. Consistent VEPs (P100-N135) were obtained from all subjects at all rates. At high rates, latencies were slightly shorter while amplitudes were reduced differentially for P100 (80%) and N135 (20%). Models constructed using VEPtr, reliably predicted 8rps and 13rps VEPss confirming high SNR obtained in BCI applications.
Archive | 2018
Tsung Han Chou; Jonathon Toft-Nielsen; Vittorio Porciatti
We describe a new method for obtaining user-friendly, robust pattern electroretinograms (PERG) simultaneously from each eye using asynchronous binocular stimulation and one-channel acquisition of signals recorded from a subcutaneous needle in the snout.
Clinical Neurophysiology | 2014
Jonathon Toft-Nielsen; Jorge Bohorquez; Özcan Özdamar
Investigative Ophthalmology & Visual Science | 2010
Jonathon Toft-Nielsen; Jorge Bohorquez; Erdem Yavuz; Özcan Özdamar
120th ASEE Annual Conference and Exposition | 2013
Jorge Bohorquez; Jonathon Toft-Nielsen