Biomedical subjects

Brad Fortune

Publications and source records attributed to Brad Fortune.

At least 19 recordsLinked to original sources

PubMed2006

Microspheres method for ocular blood flow measurement in rats: size and dose optimization.

This study modified the microspheres method by optimizing the dose and size of microspheres (MS) to enable accurate ocular blood flow measurement in rats. Fluorescent MS, either 6, 8, 10 or 15 microm diameter, were administered into the left ventricle of anesthetized adult Brown Norway rat in a dose of either 10(6), 5x10(6), or 10(7). The total number of MS entrapped in retina, choroid and optic nerve (Ntissue) was quantified and compared between size and dose groups. The MS distribution in the retina and their reentry into systemic circulation were evaluated for different sized MS. The results showed that at the 5x10(6) dose, the Ntissue of 8 microm MS was significantly more than either 6 or 10 microm MS in the retina (P<0.02) and optic nerve (P<0.03). The 10 microm MS produced the highest Ntissue for the choroid, as compared with either 8 or 6 microm MS (P<0.03). At the 10(6) dose, no difference of N(tissue) was found between 8, 10, and 15 microm MS in the retina. The 10 microm MS yielded the highest Ntissue in the choroid as compared to 8 and 15 microm MS (P<0.003). The Ntissue for 8 microm MS was higher than both 10 and 15 microm (P<0.01) MS in the optic nerve. No MS (>or=8 microm) reentered the systemic circulation. The 15 microm MS tended to lodge in pre-capillary arterioles and caused significant blood pressure increase during the injection. The blood flow measured with the optimal size MS (mean+/-SE) were 19+/-3.4 and 170+/-35 microl/min in the retina and choroid, respectively; and 0.18+/-0.03 microl/min per mm optic nerve. It is concluded that the 8 microm MS are the optimal size for both retinal and optic nerve blood flow estimation; the 10 microm for the choroid. The optimal dose for the retina was approximately 2.5x10(6), 0.5x10(6) for the choroid, and 5x10(6) approximately 10(7) for the optic nerve. The 15 microm MS are inappropriate for ocular blood flow measurements in rats.

Animals↗

PubMed2006

Endothelin B receptor in human glaucoma and experimentally induced optic nerve damage.

OBJECTIVE: To assess endothelin B receptor (ETbR) expression in human glaucomatous optic nerves and the spatial relationship between ETbR and astrocytes. METHODS: Twenty-six eyes from 16 glaucoma patients and 10 normal control subjects were immunohistochemically labeled with antibodies to ETbR. The immunoreactivity was quantified and compared between normal and glaucomatous eyes with an image analysis system. Tissues were also double-labeled for ETbR and astrocytes. In addition, the optic nerve of a monkey with regional degeneration induced by laser coagulation was examined with the same techniques. RESULTS: The frequency of positive ETbR immunoreactivity was higher in human glaucomatous optic nerves as compared with age-matched controls (9/16 vs 1/10, P = .02). The ETbR immunoreactivity colocalized with astrocytic processes and was quantitatively higher in the glaucomatous eyes (P = .02). In the monkey, the regions of degeneration showed increased ETbR associated with reactive astrocytes and was highest at the borders between normal areas and degeneration. CONCLUSION: Increased ETbR immunoreactivity in diseased optic nerves and its association with astrocytes suggest that the glia-endothelin system may be involved in the pathologic mechanisms of neuronal degeneration. Clinical Relevance The study supports the clinical observation of endothelin involvement in glaucoma and provides direct evidence that the endothelin system is associated with glaucomatous pathologic abnormalities.

Aged↗

PubMed2006

Repeatability of normal multifocal VEP: implications for detecting progression.

PURPOSE: To assess the repeatability of the multifocal visual evoked potential (mfVEP) and to compare it with the repeatability of standard automated perimetry (SAP) in the same group of 50 normal controls retested after 1 year. Our second aim was to assess the repeatability of false alarm rates determined previously for the mfVEP using various cluster criteria. METHODS: Fifty individuals with normal vision participated in this study (33 females and 17 males). The age range was 26.7 to 77.9 years and the group average age (+/- SD) was 51.4 (+/- 12.1) years. Pattern-reversal mfVEPs were obtained using a dartboard stimulus pattern in VERIS and two 8-minute runs per eye were averaged. The average number of days between the first and second mfVEP tests was 378 (+/- 58). SAP visual fields were obtained within 17.4 (+/- 20.3) days of the mfVEP using the SITA-standard threshold algorithm. Repeatability of mfVEPs and SAP total deviation values were evaluated by calculating point-wise limits of agreement (LOA). Specificity (1-false alarm rate) was evaluated for a range of cluster criteria, whereby the number and probability level of the points defining a cluster were varied. RESULTS: Point-wise LOA for the mfVEP signal-to-noise ratio (SNR) ranged from 2.0 to 4.3 dB, with an average of 2.9 dB across all 60 locations. For SAP, LOA ranged from 2.4 to 8.9 dB, with an average of 4.0 dB (excluding the points immediately above and below the blind spot). Clusters of abnormal points were not likely to repeat on either mfVEP or SAP. When an mfVEP abnormality was defined as the repeat presence (confirmation) of a 3-point (P < 0.05) cluster anywhere within a single hemifield, only 1 (of 200) monocular hemifield was deemed abnormal. Although the LOA of the mfVEP were similar throughout the field, the limited dynamic range of SNR at superior field locations will limit the ability to follow progression in "depth" at those locations. CONCLUSIONS: Repeatability of the mfVEP was slightly better than SAP visual fields in this group of controls with a 1-year retest interval. This suggests that progression in early stages should be more easily detectable by mfVEP. However, in certain field locations (eg, superior periphery), the relatively more narrow dynamic range of the SNR of the mfVEP may limit detection of progression to just 1 event. Confirmation of a 3-point cluster abnormality is highly suggestive of a true defect on the mfVEP.

Adult↗

PubMed2005

The gradient of retinal functional changes during acute intraocular pressure elevation.

PURPOSE: To characterize retinal function during a period of acutely elevated intraocular pressure (IOP) across a wide range of IOPs, including those typically observed in animals with experimental glaucoma. METHODS: Unilateral elevation of IOP was achieved manometrically in adult Brown Norway rats (nine experimental groups; n=4-7 in each; 10-100 mmHg and sham control). Full-field ERGs were recorded simultaneously from treated and control eyes, beginning 75 minutes after IOP elevation. Scotopic ERG stimuli were brief white flashes (-6.1 to 2.7 log cd-s/m2). Photopic ERGs were recorded (1.2-2.7 log cd-s/m2) after 15 minutes of light adaptation (150 cd/m2). Relative amplitude (treated/control, %) of ERG components versus IOP was described with a cumulative normal function. RESULTS: Resting IOP was 12.1 +/- 2.8 mmHg and mean femoral artery pressure was 97.6 +/- 10.7 mmHg. ERG components showed a graded effect dependent on IOP. Systematic delays in the timing of the scotopic threshold response (STR) and photopic b-wave were observed between IOPs of 30 and 40 mmHg. Analysis of amplitudes revealed that the negative STR component (nSTR) and the photopic OPs were the most sensitive to acute IOP elevation. These components were first significantly affected at 50 mmHg, whereas all parameters of middle and outer retinal function (scotopic P2 and P3) remained normal. The nSTR and photopic OPs declined by 50% at IOP <61 mmHg. The scotopic P2, OPs, and positive STR (pSTR) had intermediate sensitivity, such that they were reduced by 50% at IOPs between 61 and 66 mmHg. Scotopic P2 amplitude, but not sensitivity, was significantly reduced by 60 mmHg. At 60 and 70 mm Hg, the decline in P2 amplitude was not attributable to changes in photoreceptor response (P3) amplitude or sensitivity. The least sensitive component was the scotopic a-wave (RmP3) showing a 50% reduction at an IOP of 71 mmHg. CONCLUSIONS: During acute IOP elevation, functional changes progress from the proximal to the distal retina. Alterations in ganglion-cell-related ERG potentials occurred at IOPs (30-50 mmHg) commonly observed in rat experimental glaucoma models. Nonspecific functional changes were observed at acute IOP above 50 mmHg, suggesting that IOP should be maintained below this level in experimental glaucoma models if selective ganglion cell injury is to be sought. Repeated IOP spikes above this level may cause permanent, nonspecific damage, perhaps via ischemic mechanisms. Thus, IOP should be monitored frequently in these models.

Acute Disease↗

PubMed2005

Idiopathic bilateral optic atrophy in the rhesus macaque.

PURPOSE: To document the existence of idiopathic bilateral optic atrophy (BOA) in rhesus macaque monkeys and to characterize the structural and functional consequences of this condition. METHODS: In vivo assessment of retinal and optic nerve structure included fundus biomicroscopy and stereophotography. Functional analyses included transient pattern-reversal electroretinography (PERG) and full-field flash ERG, with both white flashes while dark adapted and red flashes on a blue background used to assess the photopic negative response (PhNR). Also measured were visual evoked cortical potentials (VEPs) and multifocal (mf)ERGs, with both a standard fast and slowed (7F) stimulation sequence. Post mortem histologic evaluation was performed on a subset of five animals with BOA and compared with data from 22 healthy normal animals. Blood tests, including vitamin E, B(12), folate, lead, and complete blood cell count with differential were obtained on the four animals that remained alive. RESULTS: Animals with BOA showed temporal pallor of the optic nerve head and thinning of the retinal nerve fiber layer (RNFL) between the temporal vascular arcades (i.e., of the papillomacular bundle). Severity of optic atrophy and RNFL loss varied between animals from mild to severe, but was similar in the two eyes of each animal. Functional changes included greater loss of the PERG N95, compared with the P50 component and substantial reduction of mfERG high-frequency components. The mfERG low-frequency components were slightly larger than normal. None of the full-field flash ERG amplitudes (a-wave, b-wave, oscillatory potentials, or PhNR) was significantly different from normal. There were no consistent abnormalities found in the results of any blood test. Histologic findings included axonal loss and gliosis limited to the temporal optic nerve, reduction of nuclei within the retinal ganglion cell layer, and thinning of the temporal retinal RNFL. CONCLUSIONS: The existence of BOA in nonhuman primates warrants caution on the part of investigators who use these animals in experimental models of ophthalmic disease.

Animals↗

PubMed2004

Chronic ischemia induces regional axonal damage in experimental primate optic neuropathy.

OBJECTIVES: To evaluate the effects of chronic optic nerve ischemia in a nonhuman primate model and to evaluate the regional variability of axonal loss. METHODS: Unilateral ischemic optic neuropathy was induced by administration of endothelin-1 to the retrobulbar space via osmotic pumps in 12 primates for 6 to 12 months. The transversely cut sections were stained and divided into 16 regions. Average axonal density in each region was quantified and compared with the untreated contralateral control eyes. RESULTS: Mean axonal density was 208 310/mm(2) and 220 661/mm(2) in treated and control eyes, respectively (P = .03, 1-tailed paired t test), for the entire group. Two-way analysis of variance showed a significant effect of endothelin-1 on overall axonal density for the experimental group (P<.001). Among the nerves with significant axonal loss, the mean axonal loss was 11.6% (4%-21%). Regional mapping of the damage showed the axonal loss varied in the damaged nerves; the damaged regions often clustered within specific quadrants. CONCLUSION: Chronic ischemia induced by local administration of endothelin-1 causes significant loss of optic nerve axons with varying regional susceptibility. Clinical Relevance Localized damage occurs in other types of optic neuropathy, such as glaucoma, and may result from regional differences in anatomy, metabolism, or vasculature of the primate optic nerve.

Animals↗

PubMed2004

Normative ranges and specificity of the multifocal VEP.

PURPOSE: To describe a normative database for the multifocal VEP (mfVEP) and to evaluate specificity for a range of cluster criteria. METHODS: One hundred persons (62 females and 38 males) with normal visual fields and ranging in age from 21.6 to 92.4 years participated in this study. Self-reported race in 80 of these 100 persons was 'White or Caucasian,' eight were 'Black or African-American,' eight were 'Asian,' and four were 'Hispanic or Latino.' Pattern-reversal mfVEPs were obtained using a dartboard stimulus pattern in VERIS and two 8-min runs per eye were averaged. A bootstrap technique was used to estimate the normal range of mfVEP response signal-to-noise ratio (SNR) and inter-ocular amplitude ratio at each location. Specificity (1 - false alarm rate) was evaluated for a range of cluster criteria, whereby the number and probability level of the points defining a cluster were varied. RESULTS: There was no overall effect of age on SNR (r2 = 0.16, p = 0.22) nor was the interaction between age and location significant (F = 0.83, p = 0.82, ANOVA). The location with the largest age effect had an r2 of only 0.13. There was a small but significant effect of sex (t = 2.1, p = 0.04) such that SNR was slightly (11%) larger in females than males, but there was no significant interaction between sex and age (t = 0.82, p = 0.41). There was a slight trend toward higher SNR in the Asian group and lower SNR in the African-American group, but the overall effect of race was not significant (F = 1.99, p = 0.12). Specificity depended on the number and probability level of the points defining a cluster. Specificity did not vary by age group in a simple monotonic manner. False positive rates were slightly higher in females than males, and slightly higher in the African-American group as compared with the Asian group. CONCLUSIONS: Excellent specificity can be achieved for the mfVEP by using particular cluster criteria for monocular and inter-ocular tests. The effects of age, sex, and race were all very small and only the effect of sex was statistically significant. This normative database can be used for analyses of mfVEP results from individual patients with little risk that demographic factors such as age and sex will confound diagnostic accuracy.

Adult↗

PubMed2004

Determining abnormal interocular latencies of multifocal visual evoked potentials.

PURPOSE: To describe methods for measuring interocular latency differences of multifocal visual evoked potentials (mfVEP) and for determining regions with abnormal interocular latencies in patients. METHODS: The mfVEPs from 100 individuals with normal visual fields and normal fundus examinations were analyzed. Individuals ranged in age from 21.6 to 92.4 years. The stimulus was a 60 sector, pattern-reversing dartboard display. Each sector had 16 checks, 8 white (200 cd/m2) and 8 black (< 1 cd/m2). Interocular latency was measured as the temporal shift producing the best cross-correlation value between the corresponding responses of each eye. The 'corrected interocular latency' was defined as the difference between this shift and the mean interocular latency (shift) for a particular sector and recording channel. RESULTS: The variability of the corrected interocular latency decreased as the signal-to-noise ratio (SNR) of the mfVEP responses increased. For example, the 95% confidence intervals decreased from over 16 ms to under 4 ms as SNR increased. Grouping and summing the responses also lead to an increase in SNR and a decrease in the confidence interval. The results of various cluster criteria were also derived. A cluster criterion (e.g. two or more contiguous points within a hemisphere exceeding a given confidence interval), can serve to increase the specificity for detection of eyes or individuals with abnormal interocular latencies. For example, while 21% of the eyes had 3 or more points exceeding the 5% confidence interval, only 1.8% of the eyes had a cluster of 3 or more of these points. Finally, interocular latency was only weakly correlated with age (r = 0.26). CONCLUSION: In testing for abnormalities in interocular latencies, the confidence interval should be based upon the SNR of the response. Grouping and summing responses to increase SNR or employing a cluster test may also prove useful.

Acute Disease↗

PubMed2004

Determining abnormal latencies of multifocal visual evoked potentials: a monocular analysis.

PURPOSE: To describe a methodology for measuring abnormal timing of monocular multifocal visual evoked potentials (mfVEP). METHODS: The mfVEPs from 100 individuals with normal visual fields and normal fundus exams were analyzed. The stimulus was a 60 sector, pattern-reversing dartboard display. For each of the 60 locations of the dartboard and each channel and each eye, a template was derived based upon the average of the responses from the 100 normal individuals. In deriving this template, care was taken to exclude those responses reversed in polarity as compared to the average response. The best array of responses for each individual was compared to these templates. The relative latency of each response was measured as the temporal shift producing the best cross-correlation value. RESULTS: The 95% confidence interval (CI) decreased as the signal-to-noise ratio (SNR) of the mfVEP responses increased. For example, the 95% CI decreased from over 17 ms to under 9 ms as the SNR increased. Grouping and summing the responses also lead to an increase in SNR and a decrease in CI. Because the number of points exceeding the CI is not randomly distributed among normal individuals, a cluster criterion (e.g. two or more contiguous points within a hemisphere exceeding a given confidence interval) can be helpful. For example, while 18% of the eyes had 5 or more points exceeding the 5% confidence interval, only 6.5% of the eyes had a cluster of 5 of these points. The correlation between relative latency and age was relatively low (r = 0.46). CONCLUSION: For detecting abnormalities in the timing of monocular, mfVEP responses, a template method provides a reasonable approach. In devising a particular test for abnormal timing, the CI should be based upon the SNR of the response. In addition, grouping and summing responses to increase SNR or employing a cluster test may also prove useful.

Acute Disease↗

PubMed2004

Inter-ocular and inter-session reliability of the electroretinogram photopic negative response (PhNR) in non-human primates.

PURPOSE: To assess the inter-ocular and inter-session reliability for a range of parameters derived from the photopic electroretinogram (ERG) in a group of normal non-human primates. METHODS: Inter-ocular differences for photopic ERGs were assessed in a group of normal anesthetized adult rhesus monkeys (Macaca mulatta, n=29); inter-session reliability was assessed for 23 eyes of 23 animals tested 3 months later. Signals were acquired using Burian-Allen contact lens electrodes, whereby the contralateral cornea served as a reference. Photopic ERGs were elicited using red Ganzfeld flashes (-0.5-0.67 log photopic cd.sm(-2)) on a rod suppressing blue-background (30 scotopic cdm(-2)). Measurement reliability was established for a-wave, b-wave, photopic negative response (PhNR) and oscillatory potential (OP) amplitudes, as well as for their implicit times, by calculation of the 95% limits-of-agreement (LOA) and the coefficient-of-variation (COV) for each parameter. RESULTS: OP and a-wave amplitudes increased with intensity up to 0.67 log photopic cd.sm(-2), following a typical saturating function, whereas b-wave and PhNR amplitudes both declined above 0.42 log photopic cd.sm(-2). Inter-session variability was greater than inter-ocular variability. The inter-session COVs for PhNR amplitude (10-20%) were similar to the other photopic ERG components (a-wave: 12-17%, b-wave: 12-17%, OPs: 13-19%). Inter-session LOAs were also similar across components, but on average, were smallest for responses to moderate intensities (0.0-0.42 log photopic cd.sm(-2)). CONCLUSION: In non-human primates, the 95% LOA for inter-session measurements of the photopic ERG a-wave, b-wave, OPs and PhNR are all similar. Inner-retinal damage may best be measured using the PhNR amplitude for moderately bright stimulus intensities. B-wave and PhNR amplitudes for brighter flashes are smaller and more variable. The ratio of PhNR:b-wave amplitudes manifests smaller variability and may therefore be useful for detection of selective PhNR loss.

Analysis of Variance↗

PubMed2004

Selective ganglion cell functional loss in rats with experimental glaucoma.

PURPOSE: To characterize retinal functional consequences of elevated intraocular pressure (IOP) in a rat model of experimental glaucoma. METHODS: Unilateral elevation of IOP was produced by hypertonic saline injection into an episcleral vein in 20 adult male Brown-Norway rats. IOP was measured in both eyes of awake animals four to five times per week. After 5 weeks, animals were dark adapted overnight (>12 hours) and full-field electroretinograms (ERGs) were obtained simultaneously from both eyes. Scotopic ERG stimuli were brief white flashes (-6.64-2.72 log cd-s/m(2)). Photopic responses were also obtained (0.97-2.72 log cd-s/m(2)) after 15 minutes of light adaptation (150 cd/m(2)). Eyes were processed the following day for histologic evaluation by light microscopy, including masked determination of optic nerve injury grade (ONIG; 1, normal; 5, severe, diffuse damage). RESULTS: Among experimental eyes, the group average IOP (+/-SD) was 34.5 +/- 4.1 mm Hg, whereas the average for control eyes was 28.1 +/- 0.5 mm Hg (t = 7.1, P < 0.0001). The average ONIG for experimental and control eye groups, respectively, was 3.4 +/- 1.7 and 1.0 +/- 0.02 (t = 6.3, P < 0.0001). The ONIG increased with mean IOP in experimental eyes (r(2) = 0.78, P < 0.0001) and was unrelated to mean IOP in control eyes (r(2) = 0.09, P = 0.18). In experimental eyes with relatively mild IOP elevation (mean IOP < 31 mm Hg) and no structural (histologic) damage to the optic nerve evident by light microscopy (ONIG = 1.1 +/- 0.2, n = 5), there was a selective reduction of the positive scotopic threshold response (pSTR; P < 0.001), whereas other ERG components remained unaltered. In four of the five eyes, pSTR amplitude was reduced by more than 50%, whereas all five had normal scotopic a-wave, b-wave, and OP amplitudes. Eyes with mean IOP of more than 35 mm Hg had reduced a-wave, b-wave, and oscillatory potential (OP) amplitudes. CONCLUSIONS: As demonstrated by prior studies, selective loss of the pSTR is indicative of selective retinal ganglion cell (RGC) injury. In this rat model of experimental glaucoma, selective RGC functional injury occurred before the onset of structural damage, as assessed by light microscopy of optic nerve tissue. The highest IOP levels resulted in nonselective functional loss. Thus, in rodent models of experimental glaucoma, lower levels of chronically elevated IOP may be more relevant to human primary chronic glaucoma.

Animals↗

PubMed2003

Ganglion cell contributions to the rat full-field electroretinogram.

The purpose of this study was to determine what contributions are made to the rat full-field electroretinogram (ERG) by ganglion cells (GCs). To that end, the ERG was assessed longitudinally following optic nerve transection (ONTx). Additional studies were conducted using intravitreal injections of pharmacologically active substances. The ERG was recorded simultaneously from both eyes of anaesthetized adult Brown-Norway rats (ketamine: xylazine: acepromazine, 55: 5: 1 mg kg(-1)) using custom silver chloride electrodes. Stimuli were brief, white xenon discharges delivered via a Ganzfeld under dark-adapted and light-adapted conditions (150 cd m(-2)). ERGs were obtained 1, 2, 3, 4 and 9 weeks after ONTx (n = 8) or sham (n = 8) operations. ONTx reduced both positive and negative components of the scotopic threshold response (pSTR and nSTR). Scotopic ERG responses to brighter flashes, including a-waves, b-waves and oscillatory potentials (OPs) were unaffected by ONTx. ONTx reduced the photopic b-wave and OPs. TTX (6 microM) reduced the pSTR and nSTR, but not the scotopic a-wave, b-wave or OPs. TTX had dramatic effects on the photopic ERG, surpassing the effects of ONTx. TTX application 9 weeks post-ONTx had little additional effect on the STR. Inhibition of inner retinal responses using GABA (10 mM) or NMDA (0.8 mM) reduced the nSTR substantially. Similar results were obtained with antagonists of AMPA/KA ionotropic glutamate receptors 6-cyano-7-nitroquinoxaline-2,3(1H,4H)-dione (CNQX, 0.2 mM) or cis-2,3-piperidinedicarboxylic acid (PDA, 5 mm); however, both also reduced the scotopic b-wave by approximately 40 %. By contrast, the NMDA receptor antagonist D(-)-2-amino-7-phosphonoheptanoic acid (D-AP7, 0.2 mM) had no effect alone, but the combination of D-AP7 and CNQX completely abolished the STR. The results of this study indicate that: (1) both pSTR and nSTR components in the rat depend directly upon intact GC responses, and that amacrine cell contributions to these components are relatively small; (2) scotopic ERG response components to brighter flashes receive little influence from GCs; (3) the rat photopic ERG also reflects GC signals and may serve as an additional useful test of GC function; (4) TTX had dramatic effects on the rat photopic ERG that were not attributable to GC currents, but rather to voltage-gated sodium currents in amacrine or interplexiform cells; (5) a small residual negative STR persisted after ONTx that was likely to be generated by graded responses of third-order retinal cells, most likely amacrine cells.

Animals↗

PubMed2003

Appearance of the frequency doubling stimulus in normal subjects and patients with glaucoma.

PURPOSE: To determine whether the spatial structure of the frequency doubling technology (FDT) perimetry stimulus is visible at detection-contrast threshold in normal observers and those with glaucoma and to assess its perceived spatial frequency at threshold and suprathreshold contrast. METHODS: Three subject groups were assessed: 10 young normal observers (aged <40 years), 10 older normal observers (aged >50 years), and 10 subjects with glaucoma. Detection thresholds for centrally and eccentrically presented 10 degrees squares, 0.25-cyc/deg, 25-Hz counterphase flicker sine-wave gratings were obtained by using a yes-no staircase procedure. Eccentric locations were in areas of loss of FDT sensitivity (< or =21 degrees ) in subjects with glaucoma, or at 7 degrees or 21 degrees inferonasally in normal observers. Resolution-contrast thresholds were determined by a two-alternative, forced-choice staircase procedure in which subjects selected the orientation of the grating stimulus tilted at +/-45 degrees. Perceived spatial frequency was determined by having subjects alter the spatial frequency of a temporally interleaved stationary sine-wave grating to match the FDT stimulus. RESULTS: No significant difference was found between detection- and resolution-contrast thresholds, implying that spatial structure was visible at detection threshold. In general, subjects perceived the spatial structure to have a spatial frequency closer to doubled than to veridical, although the young normal subjects reported a lower apparent spatial frequency than older individuals. CONCLUSIONS: When instructed as for clinical testing, subjects respond to the presence of the structure of the grating, and perceive the FDT stimulus to have a spatial frequency greater than its true spatial frequency. These findings were consistent across both normal observers and those with glaucoma, at both central and eccentric test locations.

Adult↗

PubMed2003

Varicosities of intraretinal ganglion cell axons in human and nonhuman primates.

PURPOSE: To describe varicosities of intraretinal ganglion cell axons in the nerve fiber layer of human and nonhuman primate retinas. METHODS: Intraretinal ganglion cell axons of seven human donors (1-85 years old) and two nonhuman primates (Macaca mulatta, 15 and 17 years old) were immunohistochemically stained with an antibody of neurofilament on flatmounted retinas and examined with light microscopy. In addition, the axons within the retinal nerve fiber layer were examined with transmission electron microscopy in one human and one nonhuman retina. The variations of diameters of single axons were measured on transverse- and parallel-cut sections, and the frequency distributions of the diameters were statistically evaluated. RESULTS: Varicosities of the intraretinal ganglion cell axons were found throughout the retinas in both nonhuman primate and human eyes of all ages examined. The varicosities were rich in mitochondria and had desmosome- and hemidesmosome-like junctions with other axons and retinal glial cells. Measured on parallel-cut axons, the mean diameter (+/-SD) of varicosities was 2.7 +/- 0.9 micro m, whereas the mean diameter of intervaricosity regions was 0.7 +/- 0.3 micro m. The diameter distribution for transverse-cut axons was also bimodal, but the two peaks were much closer because the peak of the larger-diameter group decreased. CONCLUSIONS: The results demonstrated that intraretinal ganglion cell axons are predominantly varicose fibers in both human and nonhuman primates. Size variations exist within a single axon's diameter and thereby affect the patterns of diameter distribution seen in transverse-cut preparations. The mitochondria-rich varicosities and the presence of intercellular junctions suggest that the varicosities may be functional sites that serve local high-energy demands of unmyelinated fibers and signal transmission.

Adolescent↗

PubMed2003

Conventional pattern-reversal VEPs are not equivalent to summed multifocal VEPs.

PURPOSE: To compare conventional pattern-reversal visual evoked potentials (cVEPs) with multifocal VEPs (mfVEPs). METHODS: mfVEPs and cVEPs were recorded during the same session in 12 normal subjects with an active electrode at Oz referenced to the inion (Oz-In) or to a midfrontal position, Fz (Oz-Fz). The mfVEP stimulus, a 60-sector dartboard, had a mean luminance of 100 cd/m(2) and a diameter of 42.2 degrees. The cVEP checkerboard stimulus subtended 21 degrees, had a mean luminance of 75 cd/m(2) and a contrast of 90%. Transient responses (2.5 Hz) were recorded for check sizes ranging from 12 to 50 minutes of arc (minarc). White cardboard masks were used to isolate upper and lower hemifields, within various field windows, for comparison with corresponding parts of the mfVEP. In a second experiment, VEPs were obtained using slowed m-sequences (8 and 16 video frames per m-step), as well as square-wave periodic reversals (2.4 Hz), for both the scaled dartboard display and an unscaled checkerboard display (check size of 50 minarc). RESULTS: The mfVEPs to fast m-sequence stimulation showed a strong polarity reversal between waveforms from the upper versus the lower hemifield. The cVEPs had larger amplitudes (approximately 3x) and longer implicit times (approximately 15-20 ms) and did not show the polarity reversal. Amplitude asymmetry between upper and lower hemifields was larger for cVEPs than for mfVEPs. As the stimulation rate was slowed, response amplitudes and implicit times of the major features increased, the upper versus lower polarity reversal was generally lost, and asymmetry of hemifield amplitudes grew. The same pattern of results was observed for scaled and unscaled spatial displays and for Oz-Fz and Oz-In signal derivations. CONCLUSIONS: Full-field cVEPs cannot be simply related to the sum of mfVEPs when each are recorded under their typical conditions. The stimulation rate has the largest influence on the differences between the two response types. The findings suggest that contributions from extrastriate sources are greater with the cVEP paradigm or the slowed mfVEP sequence than with the standard mfVEP paradigm.

Adult↗

PubMed2003

Local ganglion cell contributions to the macaque electroretinogram revealed by experimental nerve fiber layer bundle defect.

PURPOSE: To assess the structural and functional consequences of local ganglion cell (GC) loss in an experimental model of a retinal nerve fiber layer (NFL) bundle defect. To evaluate and compare three commonly used multifocal electroretinogram (mfERG) stimuli, as well as the standard transient pattern-reversal ERG (pERG) and the photopic full-field ERG, for detection of local GC damage. METHODS: Intraretinal axotomy was achieved by multiple treatments with a diode laser adapted to a slit lamp biomicroscope. Retinal laser burns were applied along an arc, subtending approximately 60 degrees, about one disc diameter superotemporal to the optic nerve. Functional measures were acquired before laser application and at numerous time points thereafter. These included mfERGs for three different stimuli: a standard fast m-sequence flicker, a global-field flash paradigm (MOFO), and a slowed m-sequence (with seven dark frames inserted to each m-step [7F]). pERGs were measured for a 24 degrees x 32 degrees checkerboard stimulus (0.56 cyc/deg, 90% contrast, 75 cd/m(2), 5 reversals/s). Photopic full-field ERGs were measured for red flashes (0.42 log photopic cd-s/m(2)) on a blue rod-saturating background (30 scotopic cd/m(2)). Retinal photography, fluorescein angiography and postmortem histologic evaluation of the optic nerve, NFL, and retinal tissues were performed. RESULTS: After six laser sessions, the NFL bundle defect appeared to be complete and contiguous and was visible both proximal to and distal to the site of the photoablation by clinical examination of the fundus and stereoscopic photographs. Histologic evaluation demonstrated localized loss of GC axons, confirmed at the level of the retrobulbar optic nerve. Retinal cross sections in the temporal retina (distal to the axotomy) showed loss of GC soma and NFL degeneration, whereas all other layers appeared intact. mfERGs showed loss of high-frequency components (HFCs) for responses located within the arcuate region corresponding to the NFL defect. Local GC damage was most easily detected using the slowed 7F m-sequence stimulus. This stimulus elicited relatively large HFCs that were significantly reduced from local responses after axotomy and that were tetrodotoxin (TTX)-sensitive in a control experiment. Low-frequency component loss with the 7F stimulus did not reach statistical significance. The photopic full-field ERG was not significantly affected. pERG amplitudes declined significantly from baseline but remained within normal limits. CONCLUSIONS: Focal loss of GC function in the macaque retina is most easily detected using the slowed-sequence mfERG. Local 7F HFCs depend on intact GC function.

Animals↗

PubMed2002

Factors affecting the use of multifocal electroretinography to monitor function in a primate model of glaucoma.

While elevated intraocular pressure (IOP) undoubtedly plays a crucial role in many glaucoma patients, vascular dysregulation and chronic regional ischemia are also thought to contribute to the pathophysiology of glaucoma. In an effort to critically evaluate hypotheses involving vascular abnormalities in glaucoma, Cioffi, Van Buskirk and co-workers have developed a model of optic neuropathy based on chronic regional ischemia. The multifocal electroretinogram (MERG) has previously been used to assess function in non-human primates with experimental glaucoma induced by high-IOP. In this study, the MERG was used to monitor function in macaque monkeys with experimental glaucoma induced by chronic anterior optic nerve ischemia. Initial recordings from experimental eyes, which were later documented histologically to have moderate axon loss, revealed little difference from recordings of control eyes. This suggested that many of the signal components in the macaque MERG, which are known (from other studies) to be eliminated by intravitreal injections of NMDA/TTX or by high-IOP experimental glaucoma, may also be affected by the choice of anesthetic agents and MERG recording parameters. Subsequent experiments were performed to specifically evaluate the effects of bipolar versus monopolar signal derivation, anesthetic agents, MERG stimulus design and spatial scale. The results demonstrate that successful measurement of inner retinal and optic nerve head MERG components, especially those which have been shown by other investigators to originate with ganglion cell spiking activity, will depend critically upon the choice of anesthetic agents and recording parameters. One of the most important parameters seems to be use of a monopolar signal derivation, with the contralateral cornea serving as the reference position.

Anesthesia, General↗

PubMed2002

Decline of photopic multifocal electroretinogram responses with age is due primarily to preretinal optical factors.

Age-related changes in photopic retinal function were evaluated topographically with the multifocal electroretinogram (mfERG). Thirty-two subjects between the ages of 16 and 69 participated. There was a strong dependence on age for all mfERG response measures that was strongest for the group of central retinal responses (i.e., within 5 deg eccentricity) and approximately equal for responses between 5 and 20 deg. After adjustment for crystalline lens optical density and pupil diameter, significant effects of age were limited to central first-order (i.e., within 5 deg) and second-order response kernels. Simulation studies support an optical basis for the observed age-related changes. It is concluded that mfERG changes between the ages of 20 and 70 are due predominantly to preretinal optical factors.

Adolescent↗