Foveal Threshold Research Articles

Critical flicker frequency and related symptoms in mild traumatic brain injury

Primary objective: To determine whether critical flicker frequency (CFF) thresholds are abnormal in individuals with mild traumatic brain injury (TBI) and, if so, if they are correlated with the degree of reported motion and light sensitivity.Methods and procedures: The foveal CFF threshold was assessed in individuals with mild TBI (n = 18) having varying degrees of reported light and motion sensitivity. Mean CFF values were obtained using the ascending and descending psychophysical method of limits with binocular viewing at 40 cm. A 7-item, rating-scale questionnaire was used to assess the degree of light and motion sensitivity. These parameters were also assessed in a large visually-normal, non-TBI cohort.Main outcomes and results: CFF in the mild TBI group was not significantly different across age groups from the visually-normal, non-TBI cohort. However, mean CFF among the mild TBI subjects was significantly higher for the ‘light sensitive’ and ‘motion sensitive’ sub-groups when compared to the ‘not light sensitive’ and ‘not motion sensitive’ sub-groups. The majority of TBI subjects manifested both light and motion sensitivity.Conclusion: CFF was found to be related to the reported degree of light and motion sensitivity in individuals with mild TBI. Neurological disinhibition as a result of brain injury may be causal of the subjective hypersensitivity to light and motion in the presence of normal CFF.

Development and validation of a computerized expert system for evaluation of automated visual fields from the Ischemic Optic Neuropathy Decompression Trial

BackgroundThe objective of this report is to describe the methods used to develop and validate a computerized system to analyze Humphrey visual fields obtained from patients with non-arteritic anterior ischemic optic neuropathy (NAION) and enrolled in the Ischemic Optic Neuropathy Decompression Trial (IONDT). The IONDT was a multicenter study that included randomized and non-randomized patients with newly diagnosed NAION in the study eye. At baseline, randomized eyes had visual acuity of 20/64 or worse and non-randomized eyes had visual acuity of better than 20/64 or were associated with patients refusing randomization. Visual fields were measured before treatment using the Humphrey Field Analyzer with the 24-2 program, foveal threshold, and size III stimulus.MethodsWe used visual fields from 189 non-IONDT eyes with NAION to develop the computerized classification system. Six neuro-ophthalmologists ("expert panel") described definitions for visual field patterns defects using 19 visual fields representing a range of pattern defect types. The expert panel then used 120 visual fields, classified using these definitions, to refine the rules, generating revised definitions for 13 visual field pattern defects and 3 levels of severity. These definitions were incorporated into a rule-based computerized classification system run on Excel® software. The computerized classification system was used to categorize visual field defects for an additional 95 NAION visual fields, and the expert panel was asked to independently classify the new fields and subsequently whether they agreed with the computer classification. To account for test variability over time, we derived an adjustment factor from the pooled short term fluctuation. We examined change in defects with and without adjustment in visual fields of study participants who demonstrated a visual acuity decrease within 30 days of NAION onset (progressive NAION).ResultsDespite an agreed upon set of rules, there was not good agreement among the expert panel when their independent visual classifications were compared. A majority did concur with the computer classification for 91 of 95 visual fields. Remaining classification discrepancies could not be resolved without modifying existing definitions.Without using the adjustment factor, visual fields of 63.6% (14/22) patients with progressive NAION and no central defect, and all (7/7) patients with a paracentral defect, worsened within 30 days of NAION onset. After applying the adjustment factor, the visual fields of the same patients with no initial central defect and 5/7 of the patients with a paracentral defect were seen to worsen.ConclusionThe IONDT developed a rule-based computerized system that consistently defines pattern and severity of visual fields of NAION patients for use in a research setting.

Influence of blue light spectrum filter on short-wavelength and standard automated perimetries

To evaluate the influence of a blue light spectrum filter (BLSF), similar in light spectrum transmittance to the intraocular lens Acrysof Natural, on standard automated perimetry (SAP) and short-wavelength automated perimetry (SWAP). Twenty young individuals (< 30 y.o.), without any systemic or ocular alterations (twenty eyes) underwent a random sequence of four Humphrey visual field tests: standard automated perimetry (SAP) and short-wavelength automated perimetry (SWAP) with and without a blue light spectrum filter. All patients had intraocular pressure lower than 21 mmHg, normal fundus biomicroscopy, and no crystalline lens opacity. Foveal threshold (FT), mean deviation (MD), and pattern standard deviation (PSD) indexes obtained from the visual field tests and the difference caused by eccentricity in short-wavelength automated perimetry examinations were analyzed using paired t test. Interindividual variability (standard deviation) was calculated using Pitman's test for correlated samples. Statistically significant reductions in the mean deviation (p < 0.001) and in the foveal threshold (p < 0.001) measured by short-wavelength automated perimetry with the use of the blue light spectrum filter in comparison to short-wavelength automated perimetry without the use of the blue light spectrum filter were observed, but not in standard automated perimetry exams. No other parameters showed statistically significant differences in the short-wavelength automated perimetry and standard automated perimetry tests. Interindividual standard deviation of the test points in the short-wavelength automated perimetry exams increased with eccentricity both with and without the use of the blue light spectrum filter, as sensitivity for inferior and superior hemifields (inferior hemifield minus superior hemifield), but no statistically significant difference in the variability when comparing the use or not of the blue light spectrum filter was noted. When comparing only the four most inferior points and the four most superior points, the inferior-superior difference increases in both situations - without and with the use of the blue light spectrum filter. The difference between without and with the use of the blue light spectrum filter was not statistically significant. Statistically significant reductions in mean deviation and foveal threshold in the short-wavelength automated perimetry with the use of the blue light spectrum filter were observed, but not in standard automated perimetry examinations. Additional studies are necessary to determine the influence of intraocular lenses with short-wavelength light filter after cataract extraction on short-wavelength automated perimetry.

Altered Visual Sensitivity in Axial High Myopia: A Local Postreceptoral Phenomenon?

The present study investigated retinal integrity in high myopia using spatial psychophysical tasks. Ten axial high myopes (-8.5 to -11.5 D) and 10 age-matched control subjects (+/-1.0 D) were recruited. All participants underwent clinical examination and ocular biometry and demonstrated no visible macular disease with visual acuities better than 6/12. Foveal summation thresholds were determined for white and S-cone-isolating spots of various diameters up to 5.4 degrees and spatial contrast sensitivity to luminance sine wave gratings (0.5-9.7 cyc/deg). Data were analyzed after correction for the magnification induced by eye size and correcting lens power. Spatial summation for both white and S-cone-isolating spots showed a generalized loss of sensitivity at all spot sizes in myopes relative to control subjects (P = 0.01). Critical areas at maximum summation were significantly larger in myopes, for S-cone isolating spots only, after image size correction (P = 0.048). Sensitivity at maximum summation correlated negatively with vitreous chamber depth for both targets (P = 0.005). Sensitivities for S-cone and luminance spots also correlated (P < 0.001), indicating widespread dysfunction. Myopes displayed contrast sensitivity losses at high spatial frequencies (P </= 0.006) with a normal peak contrast sensitivity. These data can be interpreted to indicate that highly myopic eyes have either (1) a reduction in the number of receptors and/or a reduction in their sensitivity or, (2) a reduction in the sensitivity of postreceptoral processes. The presence of normal contrast sensitivity at low spatial frequencies indicates dysfunction at a postreceptoral level in high myopes.

Neuroplasticity as a Double-edged Sword: Deaf Enhancements and Dyslexic Deficits in Motion Processing

We examined the hypothesis that aspects of processing that are most modifiable by experience (i.e., "plastic") display the most vulnerability in developmental disorders and the most compensatory enhancement after sensory deprivation. A large literature reports that motion processing and magnocellular visual function is selectively deficient in dyslexia. A smaller literature reports enhancements in such functions in deaf individuals. However, studies with dyslexic and deaf individuals have used different experimental paradigms to assess visual function, and no research has yet examined both sides of modifiability (i.e., enhancements and deficits) using the same experimental paradigm. In the present research, visual function was compared in dyslexic (n=15), deaf (n=17), and control adults by using automated peripheral kinetic and foveal static perimetry. In the kinetic perimetry task, the dyslexic group showed deficits ( p<.003), whereas the deaf group showed enhancements ( p<.001) for detecting moving light points in the periphery. In the foveal static perimetry task, neither the dyslexic ( p=.866) nor the deaf ( p=.632) group differed significantly from controls in foveal contrast sensitivity thresholds, and no group or individual approached ceiling performance on this task. Taken together, the present data bridge previous literatures and suggest that motion processing tasks are selectively modifiable, either to decrement or enhancement, whereas foveal contrast sensitivity does not differ in dyslexic or deaf groups.

MACULAR PHOTOSTRESS TESTING: SENSITIVITY AND RECOVERY WITH AN AUTOMATED PERIMETER

To describe a novel method for the evaluation of macular function with photostress testing and outline normative foveal threshold and recovery data for the adult population. In this prospective study, 50 adult subjects (25 men and 25 women aged 30-49 years) with no ocular disease underwent foveal threshold testing using the Humphrey Visual Field Perimeter Model 750. Baseline values and recovery time after photostress were measured. Baseline measurements were compared with threshold sensitivity after photostress at 1, 2, 4, 6, and 10 minutes. Mean foveal sensitivities were reduced from 38.53 db to 32.36 db after photostress, representing an average sensitivity reduction of 16%. The mean recovery time to baseline sensitivity was 6.58 minutes. There was no significant difference between sexes, races, or smokers and nonsmokers. This standardized protocol for testing the foveal threshold response to photostress is an inexpensive and noninvasive adjunct for following macular disease and also distinguishing optic neuropathy from macular disease.

Foveal blur discrimination of the human eye

Although the effect of retinal defocus on the foveal blur detection threshold has been well investigated, knowledge regarding the foveal blur discrimination threshold is limited. In the present study, both thresholds were assessed psychophysically using the ascending method of limits at the fovea with accommodation paralyzed. The unidirectional blur detection threshold was 0.87+/-0.18 D (+/-1 S.E.M.). The subsequent blur discrimination thresholds were relatively constant and significantly smaller than the blur detection threshold, with an average value of 0.48+/-0.006 D (+/-1 S.E.M.). We speculate that the difference in magnitude between these two thresholds may be attributed to the defocus-related change in the ocular modulation transfer function (MTF) and its interaction with contrast discrimination ability, as well as to the presence of a neuroperceptual blur buffering mechanism.

Effects of moderate smoking on the central visual field.

To investigate whether moderate cigarette smoking has any effects on the central visual field. This study included 30 healthy, moderate cigarette smokers (10-20 cigarettes per day for at least the past 5 years) and 22 healthy non-smokers. After two training test sessions, all individuals underwent computerized visual field examinations (Humphrey 30-2 Full Threshold Test) with both white-on-white (W-W) perimetry and blue-on-yellow (B-Y) perimetry. One eye of each subject with reliable visual field test results was evaluated. The foveal threshold, mean deviation (MD), pattern standard deviation (PSD), short-term fluctuation (SF), corrected pattern standard deviation (CPSD), glaucoma hemifield test (GHT) and number of significantly depressed points deviating at p < 5%, p < 2%, p < 1% and p < 0.5% on the pattern deviation probability map of the smokers were compared with those of the non-smokers. When the results of W-W perimetry were analysed, the smokers were found to have significantly lower foveal thresholds (p = 0.001) and mean retinal sensitivity (p = 0.02), and higher PSD (p = 0.002) and CPSD (p = 0.01) than the non-smokers. Short-term fluctuation was similar in both groups (p = 0.55). The number of significantly depressed points deviating at p < 5%, p < 2% and p < 1% on the pattern deviation probability map was similar for both groups (p > 0.05). The number of depressed points deviating at p < 0.5% on the pattern deviation probability map was higher for the smokers than for the non-smokers (p = 0.03). The results of B-Y perimetry showed the smokers to have a significantly lower foveal threshold than the non-smokers (p = 0.03). However, there were no significant differences in the global indices of the two groups (p > 0.05). The number of significantly depressed points deviating at p < 5%, p < 2%, p < 1% and p < 0.5% on the pattern deviation probability map was similar in both groups (p > 0.05). No significant difference in GHT was determined with either perimetry for the smokers compared with the non-smokers (p > 0.05). This study suggests that moderate cigarette smoking is associated with both diffuse and localized reductions in retinal sensitivity with W-W perimetry. Only reduction in the foveal threshold was observed with B-Y perimetry, with no hints of diffuse and localized reductions.

Foveal contrast thresholds exhibit spatial-frequency- and polarity-specific contour interactions

Traditionally, contour interaction has been investigated at the visual acuity limit using a Landolt C and flanking bars, performance being quantified in terms of a percent correct measure. More recently, it has been shown that the properties of the contour interaction are different when larger stimuli are used: Contour interaction is not polarity specific, and spatial frequency tuning for an unflanked C is broader. Here we quantify contour interaction for stimuli 5x larger than the resolution limit in terms of contrast thresholds. We show that polarity of bars has little effect on unfiltered stimuli but does show very different effects on the spatial-frequency-tuning curves for discrimination of the Landolt C. This explains the polarity dependence of crowding at the visual acuity limit and its independence for larger unfiltered targets. Thus the underlying filtering function is composed of more than one mechanism, affected differently depending on the relative polarity of the test and flank contours.

Full-threshold versus Swedish Interactive Threshold Algorithm (SITA) in normal individuals undergoing automated perimetry for the first time

Purpose To compare the full-threshold (FT) and Swedish Interactive Threshold Algorithm standard (SS) strategies in normal individuals undergoing automated perimetry for the first time. Design Randomized, comparative, observational case series. Participants Eighty perimetrically naive normal individuals. Methods All individuals underwent computerized visual field examinations (30-2, Humphrey 750, Humphrey-Zeiss, Dublin, CA) with both FT and SS strategies. One eye of each individual was tested. Test order between strategies was randomized. Main outcome measures The following variables were compared: test time, foveal threshold, fixation losses, false-positive and false-negative errors, mean deviation, pattern standard deviation, glaucoma hemifield test, and number of depressed points deviating at P < 5%, P < 2%, P < 1%, and P < 0.5% on the total and pattern deviation probability maps. Initially, we compared the results of all FT and SS tests, regardless of the order in which they were applied. Next, patients undergoing SS as the first examination were compared with those undergoing FT as the first test. Finally, the SS and FT results obtained in the second test were compared. Anderson’s criteria were applied to define abnormal examinations, allowing for the calculation of the specificity of the SS and FT strategies. Results When the results of all FT and SS tests were analyzed, the number of significantly depressed points deviating at P < 5%, P < 2%, and P < 1% on the pattern deviation probability maps was higher with the SS strategy ( P < 0.05); the specificity was 50% for SS and 72.5% for FT ( P < 0.01). When only first examinations were compared, the number of significantly depressed points deviating at P < 5%, P < 2%, and P < 1% on both the total and pattern deviation probability maps was higher with the SS strategy ( P < 0.05); the specificity was 38.1% and 63.2% for the SS and FT strategies, respectively ( P = 0.04). When only the second examinations were compared, there were no significant differences either between the number of depressed points or between the specificities of both strategies (73.7% for SS and 71.4% for FT) ( P = 0.98). Conclusions Normal individuals with no perimetric experience may present more significantly depressed points on the pattern deviation probability map when the SS strategy is used, reducing the test specificity in comparison with FT. These findings are probably due to a lower interindividual variability observed with SS. However, these differences disappeared in a second examination, suggesting that both strategies perform similarly in perimetrically experienced individuals.

The effect of a hockey visor and sports goggles on visual function

Background: Despite the potential for eye injury, most hockey players in the National Hockey League do not use a protective face shield. We examined the effect of a hockey visor and sports goggle on visual function to see whether visual impediment is a substantial factor in the resistance to use of protective eyewear.Methods: We measured Snellen visual acuity, Ishihara colour vision and contrast sensitivity and performed Humphrey central 30° and peripheral 30° to 60° threshold perimetry in eight volunteers (10 eyes) with and without a sports goggle and a hockey visor (half face shield). Subjects with normal acuity and normal confrontation fields who did not require presbyopic correction at near were enrolled. Five subjects (10 eyes) had further testing with threshold perimetry of the temporal crescent. The order in which the tests were performed (no mask first, visor first or goggle first) was alternated between subjects.Results: There was no difference in Snellen acuity or Ishihara colour vision, and no statistically significant difference in contrast sensitivity or foveal threshold with or without protective eyewear. The visor caused a statistically significant decrease in the central 30° mean deviation index compared to no mask (p = 0.00 1), but the goggle did not. Each of the peripheral quadrant threshold totals was significantly decreased with the visor compared with no mask (p ≤ 0.05). The 30° to 60° temporal field was better preserved with the goggle than with the visor, but the nasal peripheral field was worse with the goggle than with the visor. Both the visor and the goggle caused a statistically significant decrease of more than 4 decibels in the temporal crescent threshold (p = 0.000).Interpretation: Protective eyewear does not adversely affect Snellen acuity, contrast sensitivity, Ishihara colour vision or foveal threshold. The central 30° field of vision is not affected by the sports goggle and is minimally depressed by the hockey visor. The goggle better preserves the temporal midperipheral field, but, unlike the visor, the goggle caused a monocular nasal lens rim scotoma. Both devices caused a depression of more than 4 dB of the far temporal field beyond 60° (temporal crescent).

Estudo da circulação retrobulbar e do campo visual após dose única oral de citrato de sildenafil (Viagra®)

Objetivo: Estudar os efeitos na circulação retrobulbar e no campo visual de uma dose oral única de 100 mg de citrato de sildenafil (Viagra®). Métodos: Um estudo duplo-mascarado e controlado por placebo foi realizado em 10 voluntários do sexo masculino, com idade média de 27,7 + 5,68 anos. O olho direito de cada voluntário foi submetido aos exames de Doppler colorido de órbita e análise de campo visual por meio de perimetria computadorizada (Humphrey, programa 30-2, estratégia "Full Threshold") em 3 ocasiões: "baseline", 1 hora após placebo e 1 hora após 100 mg de sildenafil via oral. No campo visual, analisaram-se o limiar foveal e o "mean deviation" (MD) nas 3 ocasiões. No Doppler colorido, medimos a velocidade sistólica máxima (VSM), a velocidade diastólica final (VDF) e o índice de resistência (IR) da artéria central da retina (ACR) e da artéria oftálmica (AO) nas 3 ocasiões. Resultados: A administração do sildenafil não alterou significativamente o limiar foveal e o "mean deviation" em relação ao "baseline" e ao placebo. Houve um aumento significativo da velocidade sistólica máxima e velocidade diastólica final na artéria oftálmica após a administração do citrato de sildenafil (p<0,001). Não houve alteração significativa dos parâmetros hemodinâmicos na artéria central da retina e no índice de resistência da artéria oftálmica. Conclusões: A administração de 100 mg de citrato de sildenafil promoveu um aumento da velocidade de fluxo sanguíneo na artéria oftálmica de indivíduos normais e não promoveu alteração significativa no limiar foveal e "mean deviation" da campimetria computadorizada.

Achromatic impulses unmask L- and M-cone adaptive mechanisms.

The mechanisms underlying the adaptive response for achromatic impulses seen on achromatic fields were investigated. Foveal thresholds were measured for a static probe-impulse under two conditions of adaptation. Thresholds were obtained under gain-clamped conditions after observers had reached steady-state adaptation and with a probe-flash paradigm. It was found that thresholds isolated on steady-state fields cannot be modelled using a single mechanism. Likewise, the probe-flash condition failed to reflect the response of a single mechanism. Both threshold functions showed distinct breaks occurring at about the same field luminance (approximately 1.0 log cd/m2). Optimum data fits required the incorporation of two mechanisms implying the existence of independent processes mediating detection. Chromatic isolation confirmed that differential adaptation had been unmasked in the long- and medium-wavelength sensitive cone inputs to the achromatic channel.

Static perimetry and diabetic retinopathy: a long-term follow-up.

In order to evaluate if central static perimetry is useful to identify patients at risk of developing diabetic retinopathy, 60 (27 male, 33 female) adolescents and young adults (mean age, 15.9 years) with insulin-dependent diabetes mellitus were studied prospectively. No patient showed fluorescein angiographic signs of retinopathy initially. The patients were evaluated at the beginning of the study and after 8 years. At the beginning of the study, mean defect in the population was -2.34 dB as determined by perimetry; no patient showed significant impairment of foveal threshold (mean, 33.17 dB). After 8 years of follow-up, 7 patients had developed fluorangiographic signs of retinopathy. Life-table analysis showed that the overall probability of retinopathy development was significantly higher in subgroups of patients with mean sensitivity in areas 2 and 3 below the cutoff. These results suggest that central static perimetry is a useful tool in predicting the development of retinopathy in children with insulin-dependent diabetes mellitus who do not have fluorescein angiographic signs of retinopathy. This tool can help the physician to identify those patients at risk of developing fluorangiographic signs of retinopathy.

SITA visual field testing in children

Purpose: The Swedish Interactive Thresholding Algorithm (SITA) is a new testing strategy for the Humphrey perimeter. The standard SITA algorithm shortens test time in adults without increasing variability, but its usefulness for detecting field defects in children has not been investigated. Methods: We evaluated 92 standard SITA 24-2 visual fields of children, most of whom had various types of optic neuropathies (pediatric idiopathic intracranial hypertension, homonymous defects, bitemporal defects, papilledema from brain tumors), and compared them with 49 full threshold 24-2 fields obtained in similar patients. We evaluated outcome measures of foveal threshold, mean defect, pattern standard deviation, false-negative and false-positive rates, and test time. Five children (9 eyes) had both SITA and full threshold testing (FTT). Results: The SITA decreased test time by over 50% compared with FTT (12.6 ± 3.0 minutes vs 6.6 ± 1.6 minutes [P < .00001]). When patients with field defects were eliminated, the pattern standard deviation was lower with SITA than FTT (P < .002), indicating lower intratest variability of SITA in subjects with normal fields. No detectable difference was observed in the other outcome measures. Subjective analysis of gray-scale fields in patients who underwent testing with the use of both strategies showed marked similarities. Conclusions: SITA shortens test time significantly compared with FTT and does so without jeopardizing interpretability. SITA has less intratest variability than FTT and therefore should be better for detecting and following defects. Caution is advised when following a visual field defect unless the same strategy is used for each evaluation. Switching strategies in the absence of a stable field defect is not recommended. (J AAPOS 2001;5:114-7)

Effect of localized defocus on detection thresholds for different sized targets in the fovea and periphery.

Previous studies of optical blur in perimetry have measured the effect of foveal refractive error on peripheral perimetric detection thresholds. Since peripheral refractive error can be significantly different from that of the fovea we wished to remove the ambiguity of previous results by correcting the actual peripheral refractive error first before adding blur. We measured detection thresholds in the fovea and at 30 degrees in the horizontal temporal field in two trained observers. Peripheral refractive error was determined at each location and thresholds measured at the same locations for stimuli ranging in size from 0.2 to 6.4 degrees and refractive errors between +/-4.00 diopters. Foveal thresholds increased immediately with increasing refractive error, particularly for smaller stimulus sizes. At 30 degrees, thresholds for smaller stimuli were less affected by defocus initially and then increased more sharply. Larger stimuli were relatively unaffected by defocus such that when stimulus size reached 1.6 degrees there was little or no increase in threshold for refractive error between +/-4.00 diopters. Peripheral refractive error, largely forgotten by perimetrists, has a significant effect on performance, particularly for smaller stimuli. Differences in foveal vs peripheral viewing can be explained by differences in ganglion cell receptive field sizes.

Senescence of foveal and parafoveal cone sensitivities and their relations to macular pigment density.

Foveal and parafoveal increment thresholds were measured for 50 observers (12-88 years of age) under conditions that isolated retinal mechanisms dominated by short- (S-), middle- (M-), or long- (L-) wave-sensitive cones. Thresholds were obtained on the plateau of the threshold-versus-intensity function of each isolated mechanism and were referred to the retina by using individual measurements of ocular media and macular pigment density. Age-related increases in foveal thresholds, specified at the retina, were found for all three cone mechanisms. Parallel sensitivity losses for each cone mechanism were also observed at 4 degrees and 8 degrees in the temporal retina. A significant positive correlation was found between foveal macular pigment density and the S-cone, but not the M- and L-cone, log sensitivity difference (0 degrees-8 degrees) specified at the retina. This relation is expected from the hypothesis that the macular pigment protects the photoreceptors from senescent losses in sensitivity. However, because this result is independent of age, it is interpreted as being due to local gain changes resulting from differential filtering of incident light by the macular pigment between the fovea and the parafovea.

Characterisation of dark adaptation in human cone pathways: an application of the equivalent background hypothesis.

It is well accepted that in rod photoreceptors the photoproducts generated by a bleach cause desensitisation during dark adaptation. We examine whether this notion holds for cones. A model of cone dark adaptation is developed based on the equivalent background concept. The underlying theory of the model relies on a series of assumptions that link psychophysically determined detection thresholds to cone phototransduction. Correction of thresholds for the reduced quantum-catching ability of the cones (due to the depletion of photopigment caused by a bleaching light) is an important aspect of the model. Foveal detection thresholds were measured for a small test flash presented on a large steady background field or presented alone after adapting to the background field. Test and background fields were monochromatic, with wavelengths closely matched to promote detection by the luminance mechanism. The model provided a good description of the data collected under these conditions. Parameters of the model were similar for all wavelengths and each observer, as were the derived equivalent background relationships. Analysis of previously published data for Stiles' pi5 mechanism gave analogous results. The model is made up of two components. The early (fast) component is likely to be due to the direct action of the cone equivalent of inactivated Rh* on the G-protein cascade and/or the reverse reaction of the cone equivalent of inactivated Rh* to Rh*. The later (slow) component may be due to the direct action of cone opsin on the G-protein cascade.

Flicker adaptation can be explained by probability summation between ON- and OFF-mechanisms.

This study considered whether the dynamics of flicker adaptation can be explained in terms of probability summation over time between independent ON- and OFF-processors. Foveal thresholds were measured for flickering probes, and for static ON- and OFF-probes that have durations consistent with the flicker duty cycle. Thresholds were obtained using a probe-flash stimulus onset asynchrony paradigm. The outcome of the experiment suggests that flicker thresholds are lower than the component ON- and OFF-thresholds, and that the flicker response can be predicted by assuming probability summation between the ON- and OFF-mechanisms.

Foveal and peripheral thresholds for detection and resolution of vanishing optotype tumbling E’s

We measured detection and resolution acuity for vanishing optotype tumbling E stimuli in both the fovea and at 30° in the periphery to determine if peripheral resolution is sampling limited for this stimulus. In the fovea, where acuity is optically limited, detection and resolution were the same. At 30°, however, detection was markedly better than resolution indicating that peripheral resolution is sampling limited for this stimulus. Detection acuity was higher when contrast was 90% rather than 40%, but resolution did not change with contrast. The vanishing optotype is a legitimate perimetric stimulus to measure retinal ganglion cell density provided the task is resolution and not detection.