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Biomedical subjects

G T Timberlake

Publications and source records attributed to G T Timberlake.

At least 19 recordsLinked to original sources
PubMed1994

Light distribution of ocular endophotocoagulator probes and its surgical implications.

Ophthalmic endophotocoagulator probe cone angle affects the spot size, working distance, laser output power requirement, tissue exposure time, and uniformity of tissue irradiance, which all affect ease and safety of clinical use. The cone angle and irradiance distribution of several ophthalmic endophotocoagulator delivery systems were studied by directing the laser energy emitted by them on a CCD video sensor at several angles of incidence. The irradiances followed a Gaussian distribution. The measured irradiances were compared to a mathematical model of tissue irradiance that adjusted for the cone angle, probe-tissue distance, and angle of incidence. Using this model, laser irradiances produced by endophotocoagulator probes can be estimated under a wide variety of conditions. At highly oblique angles of incidence, wide-angle probes produce unexpectedly higher and uneven tissue irradiance. When numerous characteristics are considered, probes with a cone angle in the range of 10 degrees have many advantages over narrower or wider angle probes.

Fiber Optic Technology↗
PubMed1992

Short-term, low-contrast visual acuity reduction associated with in vivo contact lens drying.

Hydrogel contact lens surfaces partially desiccate during extended interblink intervals, producing a microscopically rough and irregular surface that scatters light. Such light scattering could reduce retinal image contrast, elevating thresholds for target perception. To test this idea, we measured low (7%)-contrast visual acuity of subjects who wore: (1) hydrogel contact lenses, (2) rigid gas permeable (RGP) lenses, and (3) no contact lenses when the subjects blinked normally and when they suppressed blinking. Acuity thresholds were determined using computer-generated acuity stimuli and a staircase psychophysical procedure. Cessation of blinking resulted in small reductions in low-contrast acuity for subjects wearing RGP lenses (mean loss: 0.1 line) or no lenses (mean loss: 0.3 lines). Subjects wearing hydrogel lenses, however, generally exhibited substantial reduction of acuity (mean loss: 4.1 lines) when blinking was suppressed.

Adult↗
PubMed1992

Use of scrolled text in a scanning laser ophthalmoscope to assess reading performance at different retinal locations.

A new technique is described for assessing reading performance using a scanning laser ophthalmoscope. Letters of different sizes and contrasts were projected onto specific retinal locations of normal and low vision observers. Successive letters were scrolled in a horizontal direction at different speeds through a 'window'. Throughout the experiments the subjects' fundus and the retinal location of the stimuli could be visualized. With this scanning laser ophthalmoscope text-scrolling computer program the subject does not search for adjacent letters, and because the eye is held relatively stationary the tedious eye movement analysis incurred in other studies is reduced. Five retinal areas were investigated in two normal observers. The percentage of letters correctly identified decreased with eccentricity, increased velocity of the text and reduced text contrast. The reading performance of two patients, one with age-related macular degeneration and the other with juvenile macular disease, was investigated. Decrements in performance were related to morphology of the lesions.

Adult↗
PubMed1989

Recording pattern reversal visual evoked response with the scanning laser ophthalmoscope.

We recorded visual evoked responses (VERs) to alternating, checkerboard pattern stimuli using the scanning laser ophthalmoscope (SLO). Retinal position and focus of checkerboard stimuli were monitored on the SLO video monitor throughout testing. Checkerboard size, check size, and retinal positions were varied. Consistent with other, well-established pattern reversal techniques, the SLO method produced: 1) reliable VERs with amplitudes of 2 to 10 microvolts, 2) maximum amplitudes at an intermediate check size for a fixed overall pattern size, and 3) variations in VER amplitude depending on stimulus retinal position relative to the fovea. Hence, the SLO-VER technique would be useful for clinical VER measurements when precise retinal stimulus position and focus are desired.

Computer Systems↗
PubMed1987

What is psychophysically perfect stabilization? Do perfectly stabilized images always disappear?: reply to comment.

We acknowledge the earlier work cited by Ditchburn in his comment [R. W. Ditchburn, J. Opt. Soc. Am. A 4, 405-406 (1987).] and point out a crucial difference between that work and our novel approach [L. E. Arend and G. T. Timberlake, J. Opt. Soc. Am. A 3, 235 (1986)]. Our procedure provides measurements uncontaminated by residual errors of stabilization. The visual system's extreme sensitivity to small temporal changes and the indirectness of the evidence cited by Ditchburn leave the meaning of stabilized-image reappearance unclear.

Humans↗
PubMed1987

Reading with a macular scotoma. II. Retinal locus for scanning text.

To elucidate how patients with macular scotomas use residual functional retina for inspecting visual detail and reading, we tested three patients with dense macular scotomas using a scanning laser ophthalmoscope that allows an examiner to view and record stimuli on the retina while the patient views them. Using standard psychophysical techniques, we determined the retinal position of scotomas, the retinal areas used for fixating and inspecting acuity targets, and the retinal area used for reading simple, three-letter, nonsense syllables. We found that each patient used a single, idiosyncratic retinal area, immediately adjacent to the scotoma, for fixating, inspecting acuity targets, and scanning simple, nonsense-syllable text. This preferred retinal locus (PRL) was at different retinal eccentricities (relative to the foveola) for each patient and was not always as close as possible to the foveola. There appears to be no simple rule by which patients "select" a particular PRL. Plots of text placement on the retina revealed considerable differences in patients' abilities to execute an orderly text scan. Two patients read text more rapidly with a novel retinal area than with their PRL, suggesting that the PRL may not be optimal for text reading.

Eye Movements↗
PubMed1986

What is psychophysically perfect image stabilization? Do perfectly stabilized images always disappear?

High-contrast luminance gratings stabilized on the retina with a Purkinje image eyetracker do not disappear completely. This could be due to small errors of stabilization, or the visual system could include mechanisms capable of responding to temporally constant images. We examined the visual system's sensitivity to small movements of gratings. We (1) replicated previous measurements of contrast sensitivity for gratings with controlled retinal-drift velocities, (2) developed a method for calculating sensitivity to small oscillations of gratings using thresholds for flickering stabilized gratings, and (3) examined the calculations empirically. We calculated that movements of only 8 sec of arc peak to peak produce detectable temporal changes. Since existent stabilization systems cannot eliminate movements this small, residual stabilized-grating detectability does not require detectors sensitive to temporally constant images.

Humans↗
PubMed1986

Reading with a macular scotoma. I. Retinal location of scotoma and fixation area.

To investigate how patients with macular scotomas use residual functional retinal areas to inspect visual detail, a scanning laser ophthalmoscope (SLO) was used to map the retinal locations of scotomas and areas used to fixate. Three patients with dense macular scotomas of at least 20 months duration and with no explicit low vision training were tested. SLO stimuli were produced by computer modulation of the scanned laser beam, and could be placed on known retinal loci by direct observation of the retina on a television monitor. Videotaped SLO images were analyzed to produce retinal maps that are corrected for shifts of stimulus position due to fixational eye movement, thus showing the true retinal locations of scotomas and fixation loci. Major findings were as follows: 1) each patient used a single, idiosyncratic retinal area, immediately adjacent to the scotoma to fixate, and did not attempt to use the nonfunctional foveola, 2) fixation stability with the eccentric fixation locus was as good as, or better than, that of ocularly normal subjects trying to fixate at comparable eccentricities, 3) fixation stability was not systematically related to clinical visual acuity, and 4) there is good agreement as to the shape and overall size of SLO and standard clinical tangent screen scotoma maps for these three patients.

Adult↗
PubMed1982

Scanning laser ophthalmoscopy. Clinical applications.

The scanning laser ophthalmoscope (SLO) provides a high-quality television image of the retina using less than 1/1000 of the light required for conventional indirect ophthalmoscopy. The SLO employs a new ophthalmoscopic principle in which a dim laser beam scans across the fundus, and light is collected only from one retinal point at a time. Since the instrument is highly light efficient, illumination levels are comfortable for the patient, and fluorescein angiography can be performed with one tenth of the usual fluorescein dose. Since a continuous, large depth of field view is displayed on the SLO screen and stored on video tape, repeated dynamic inspection of the vitreous, retina and vitreoretinal interface is afforded. In addition, any graphical material that can be displayed on a microcomputer monitor (such as text of video games) can also be impressed on the retinal pattern formed by the sweeping laser beam. The graphical material is thus observed directly by the patient and on the patient's retina by the clinician. Since the exact retinal locus of each point in the graphical material is viewed directly, it is possible to perform perimetry directly on the retina, to measure acuity at arbitrary retinal loci, to study how patients with macular disease use residual functional retina for reading, and to perform distortometry with a retinal (Amsler-type) grid.

Fluorescein Angiography↗
PubMed1982

Retinal localization of scotomata by scanning laser ophthalmoscopy.

The scanning laser ophthalmoscope (SLO) system has been modified to permit direct retinal perimetry. A movable pinhole in a retinal conjugate plane furnishes a mapping stimulus whose retinal locus is directly observable on a video image of the fundus. Scotoma maps in patients with macular disease and physiologic scotomata associated with normal optic discs are presented. Clinical applications of the method are discussed.

Humans↗
PubMed1981

Oculomotor changes in cats reared without experiencing continuous retinal image motion.

Eye movements recorded with the magnetic-field search coil technique from six cats raised in an 8 Hz stroboscopically illuminated environment were found to be normal with the exception of fixation and optokinetic nystagmus. Fixation eye movements were characterized by a 4-5 Hz conjugate pendular nystagmus superimposed on a wandering drift. Optokinetic nystagmus was very asymmetrical on monocular tests. Depriving the oculomotor system of experience differentially effects fixation and smooth pursuit.

Animals↗
PubMed1981

Automated variable contrast acuity testing.

The results of clinical trials with a simple system for automating visual acuity measurements are presented. An inexpensive microcomputer: (1) presents acuity targets on a television screen, (2) uses patients' responses entered through a remote response box to control a staircase testing procedure, and (3) prepares a printed record of a patient's mean visual acuity and its standard deviation. The automated method: (1) offers a stable measure of visual performance, (2) provides an immediate analysis of the statistical significance of changes measured in visual acuity from one office visit to the next, and (3) gives results that are in general agreement with conventional chart testing methods when high contrast targets are used on the television screen. In addition, when medium and low contrast targets are used, the automated method provides a variable contrast acuity profile (VCAP), which describes visual performance at contrast levels that are vital to a patient's performance in daily life but not examined in conventional chart testing. VCAPs are measured clinically, and it is shown that the extent that a patient's acuity declines with decreasing contrast cannot be predicted from the patient's high contrast acuity. Automated variable contrast acuity testing is found to provide a simple, reproducible, clinically practicable method of documenting and quantitating the extent of a patient's visual difficulties at medium and low contrasts.

Adolescent↗
PubMed1980

Automated clinical visual acuity testing.

We developed a method for testing visual acuity using a general-purpose microcomputer which displays visual acuity targets on a television monitor, controls a staircase psychophysical testing procedure, and provides a printed record of mean visual acuity and standard deviation. This automated procedure can be used to increase the precision of clinical visual acuity testing and to determine if changes in a patient's visual acuity are statistically significant.

Adolescent↗
PubMed1967

Voluntary control of microsaccades during maintained monocular fixation.

A contact-lens technique was used to record eye movements made by two subjects instructed either to "fixate" stationary white-light targets or to "hold" their eyes in position in the presence of the same targets. A marked reduction in saccade rate, frequently reaching zero throughout 9.8-second trials, was observed under the "hold" instruction.

Contact Lenses↗
PubMedDate not supplied

Corneal haze after excimer laser refractive surgery: objective measurements and functional implications.

Postoperative changes in corneal transparency is a major concern after PRK. Data were obtained from 69 human eyes treated with excimer laser photorefractive keratectomy in order to evaluate a relationship between objective measurements of corneal transparency and visual performance. A CCD-camera device was employed and by using polarizing filters the system could discriminate between reflected and scattered light. We observed two groups of postoperative behaviour in corneal transparency. Each group showed an increase in both scattered and reflected light signals with a maximum at around two months postoperatively. In the majority group (70%) this was followed by a subsequent decline of the scattered light signal, whereas the combined signal generated by reflected and scattered light showed a biphasic curve with a second peak at around 4 to 5 months postoperatively. In the minority patient group (30%) the timebase variations in pattern of both signals were indistinguishable throughout the period of observation. In all patients we observed a good correlation between the signal generated by scattered light alone and the reduction in the 5% contrast visual acuity performance, whereas correlation was poor when the combined signal of reflected and scattered light was considered. These disturbances in low contrast visual performance were only significant during the first three to four months postoperatively and thereafter most patients returned to their preoperative value. Eighteen percent of our patients discontinued topical steroids postoperatively. No differences in either corneal transparency or final refraction was observed. We consider that PRK is an effective form of refractive surgery and the marginal loss of corneal transparency should only be a problem during the first three or four months postoperatively.

Adult↗
PubMedDate not supplied

Corneal light scattering after excimer laser photorefractive keratectomy: the objective measurements of haze.

BACKGROUND: After photorefractive keratectomy using excimer lasers (193 nm), most corneas show a marginal loss of transparency (haze) and the assessment of its magnitude in clinical studies has been subjective. To address this problem, we have developed a new device for the objective measurement of haze by measuring corneal light scattering. METHODS: A CCD-camera was fixed at 40 degrees to a slit-lamp microscope and connected via frame-grabber to a computer. By incorporating polarizing filters, the system could discriminate between reflected and scattered light. The intensity of light coming from the cornea was measured in gray scale levels using in-house image analysis software. The system was calibrated against three different sizes of microspheres (0.25, 0.50, and 5.00 microns) which corresponded to the size of cellular and extracellular elements known to occur at sites of corneal surgery. Data were obtained from three treated human eyes with measurements before treatment and at five different postoperative intervals with a maximum follow up of 4 months. RESULTS: All three sizes of microspheres caused disturbances in gray scale levels (36 to 255 units) in the same range of those observed in corneal measurements. Disturbances in corneal light scattering were noted from 1 week postoperatively and persisted throughout the period of observation. We observed an increase in reflected and scattered light until the 2nd postoperative month followed by a subsequent decline. CONCLUSIONS: It appears that this device is very useful to defect and measure objectively disturbances in corneal transparency after excimer laser photorefractive keratectomy.

Adult↗