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Biomedical subjects

Billy R Hammond

Publications and source records attributed to Billy R Hammond.

15 recordsLinked to original sources
PubMed2006

Scotopic sensitivity in schizophrenia.

Among participants with schizophrenia there is evidence for early-stage visual processing deficits, which may arise in the rod pathways. Input to the earliest level of this pathway, however, has not been tested in this population. It has been widely hypothesized that schizophrenia participants have magnocellular deficits that occur at the pre-cortical level. To address this hypothesis, we studied absolute scotopic (dark-adapted) sensitivity in fifteen schizophrenia and fifteen matched control participants. Scotopic thresholds were assessed using a 1.85-deg, 510-nm circular test stimulus located at 10 degrees eccentricity in the left visual field and presented in Maxwellian-view. Thresholds were obtained using a two-alternative forced-choice paradigm (an average of 200 trials per participant was obtained). Threshold estimates were derived using probit analysis. In this procedure the transformed binomial data (the inverse of the normal probability integral) is fit with a weighted linear regression. Noise was defined as the average deviation from this line. Lens optical density was also assessed by comparing absolute scotopic thresholds to the extinction spectrum of rhodopsin. Scotopic thresholds and lens density values of the two groups were evaluated using independent samples t-tests. The scotopic thresholds, and associated noise, did not differ between the schizophrenia and control participants. Lens density was also nearly identical between groups. These results suggest that magnocellular deficits in schizophrenia may not be due to problems at the level of the rods but are more likely to occur later in the visual pathway.

Adult↗
PubMed2006

Compensation for light loss resulting from filtering by macular pigment: relation to the S-cone pathway.

PURPOSE: Macular pigment (MP) filters short-wavelength light before it reaches the visual pigments. At peak absorbance (460 nm), transmission of light through MP can range from almost 100% transmission to as little as 3%. As a result of the uneven topographic distribution of MP, spatial nonuniformities in visual perception would result if the visual system did not compensate for filtering differences across the central retina. This study characterizes compensation for different densities of MP. METHODS: Sixteen young subjects (aged 24-40 years) with a wide range of MP density were studied. Increment thresholds were measured at 440 and 500 nm in the center of the fovea and at 6 degrees to 7 degrees eccentricity using conditions chosen to isolate the pi-1 mechanism. For six of the subjects, increment thresholds were also obtained for eccentricities of 1 degrees , 1.75 degrees , and 3 degrees . MP density was measured using heterochromatic flicker photometry at the same locations as the increment thresholds. RESULTS: Peak sensitivity of the short-wavelength pathway across the central retina was constant despite MP density differences as large as 1.0 log unit. CONCLUSIONS: These results suggest that the visual system increases gain of the S-cone pathway to offset light absorption by MP.

Adaptation, Ocular↗
PubMed2005

Assessment of the validity of in vivo methods of measuring human macular pigment optical density.

PURPOSE: As a result of the putative protective role of lutein and zeaxanthin at the site of the retina (macular pigment [MP]), interest in the noninvasive measurement of these retinal carotenoids is increasing. In principle, any method used to measure MP in vivo should provide spectral absorption curves that match extinction spectra of MP measured ex vivo. METHODS: In this article, we assess published spectral curves with respect to this criterion. RESULTS AND DISCUSSION: Variation in derived MP measures resulting from methodological differences between techniques is noted and discussed.

Adult↗
PubMed2005

Spectral absorbance and spatial distribution of macular pigment using heterochromatic flicker photometry.

PURPOSE: The validity of the heterochromatic flicker photometric (HFP) technique for measuring macular pigment (MP) was assessed by evaluating the spatial density and spectral absorption curves of normal subjects. METHODS: Standard Maxwellian view optics were used to measure MP spatial density and spectral absorption curves. Four subjects with normal color vision and one protanope were tested. All of the subjects were experienced psychophysical observers. RESULTS: Spatial density profiles for all subjects were nearly symmetric. The width of the spatial distribution, at half maximal density, averaged 0.70 degrees (standard deviation = 0.15). A first-order exponential decay with eccentricity described the distribution profile well (average r = 0.95). The shape of the spectral absorption curves matched an ex vivo template closely at wavelengths greater than approximately 430 nm. Stimulus size did not influence the derived spectral curves. CONCLUSION: Based on data from this study and others, the HFP method appears to be a valid method for measuring MP density in subjects without retinal disease.

Absorption↗
PubMed2005

CFF thresholds: relation to macular pigment optical density.

PURPOSE: Only two small studies have evaluated the relation between individual differences in macular pigment (MP) levels and visual health prior to retinal disease. In this study, we assess the relation between MP and a general measure of visual health, the critical flicker fusion (CFF) threshold. METHODS: A total of 134 subjects were assessed. CFF values were determined psychophysically using a 570 nm, circular 1-degree test field centrally fixated. MP optical density was measured using flicker photometry and a 1-degree circular 458 nm test field centrally fixated. Personal characteristics were determined by questionnaire. RESULTS: Similar to past studies, we found a significant decline in CFF values with age (r = -0.56). There was a significant positive relationship (r = 0.30, p < 0.001) between MP density and CFF values that was independent of age. CONCLUSIONS: These results are consistent with a protective effect of MP on visual health across the lifespan.

Adolescent↗
PubMed2005

Dietary lutein and zeaxanthin: possible effects on visual function.

Of the many carotenoids circulating in human sera, only lutein and zeaxanthin are accumulated throughout the tissues of the eye. Within the eye, they reach their highest concentration in the central retina, where they are clinically referred to as the macula lutea. Lutein and zeaxanthin, more commonly referred to as macular pigments, may serve a variety of roles in the specialized vision of higher primates. This paper reviews recent studies investigating the influence of macular pigments on human visual performance. Such studies have offered insight into why lutein and zeaxanthin are uniquely concentrated in ocular tissues.

Antioxidants↗
PubMed2004

Macular pigment density and aging, assessed in the normal elderly and those with cataracts and age-related macular degeneration.

PURPOSE: Increasing evidence has linked retinal lutein and zeaxanthin (termed macular pigment, MP) to the risk of age-related macular degeneration (AMD). Currently, however, studies differ regarding the question of whether MP declines with age or age has an effect in patient populations being assessed. This study assessed MP across the lifespan with an emphasis on assessing MP in a cross-section of elderly including those with lenticular or age-related macular degeneration, or both. DESIGN: Prospective, observational, cross-sectional study. METHODS SETTING: Institution. STUDY POPULATION: Cross-sectional study of normal, cataractous, and AMD subjects tested in Indianapolis, Indiana, including 390 subjects, 22 with cataracts and 59 with age-related macular degeneration. OBSERVATIONAL PROCEDURE: MP density was measured with a one-degree diameter test field at 460 nm using a psychophysical method based on heterochromatic flicker photometry. MAIN OUTCOME MEASURES: MP optical density. RESULTS: MP does not appear to change as a function of age (r = +.04) when examining subjects across the lifespan (from 18-88 years). There was a slight tendency (slope = -.0027, r = -.11) for MP to decline when only the elderly subjects were considered, but this trend was not significant (P < .12) for any of the groups considered (normal, cataractous, or AMD). CONCLUSIONS: MP does not change significantly with age, even when elderly subjects with cataracts and AMD are considered. Using heterochromic flicker photometry, elderly subjects display a full range of MP density that is similar to young subjects.

Adolescent↗
PubMed2004

Critical flicker fusion frequency: relation to resting systolic blood pressure.

OBJECTIVE: Past studies have shown that a number of factors, both acute and chronic, influence individual differences in critical flicker fusion frequency (CFF). In this study, we assessed whether CFF co-varied with resting systolic blood pressure (SBP). METHODS: SBP and CFF were assessed in one session in 221 subjects (mean age, 19.6 +/- 2.5 years), and SBP and CFF were assessed over 10 sessions in 12 subjects. CFF values were determined psychophysically using a 570 nm circular 1 degrees test field centrally fixated. Blood pressure was determined with an automated sphygmomanometer. RESULTS: There was a significant (p < 0.0002) positive relation between CFF and SBP for the larger sample (n = 221). Significant within-subject relations were found for 6 of the 12 subjects tested repeatedly. CONCLUSIONS: The combination of results strongly suggests that CFF and resting SBP are positively related.

Adolescent↗
PubMed2002

Macular pigment: influences on visual acuity and visibility.

There is increasing evidence that the macular pigment (MP) carotenoids lutein (L) and zeaxanthin (Z) protect the retina and lens from age-related loss. As a result, the use of L and Z supplements has increased dramatically in recent years. An increasing number of reports have suggested that L and Z supplementation (and increased MP density) are related to improved visual performance in normal subjects and patients with retinal and lenticular disease. These improvements in vision could be due either to changes in the underlying biology and/or optical changes. The optical mechanisms, i.e., preferential absorption of short-wave light, underlying these putative improvements in vision, however, have not been properly evaluated. Two major hypotheses are discussed. The acuity hypothesis posits that MP could improve visual function by reducing the effects of chromatic aberration. The visibility hypothesis is based on the idea that MP may improve vision through the atmosphere by preferentially absorbing blue haze (short-wave dominant air light that produces a veiling luminance when viewing objects at a distance).

Humans↗
PubMed2002

In vivo assessment of retinal carotenoids: macular pigment detection techniques and their impact on monitoring pigment status.

Of the many carotenoids found within human tissue, only the carotenoids within the human retina can be assessed noninvasively at present. Such assessment should eventually provide a more complete understanding of the functional role of retinal lutein (L) and zeaxanthin (Z) (termed macular pigment, MP) in human vision. The emerging data allow for some initial observations. For example, there appears to be wide variation (>factor of 10) in the concentration of MP. Although MP levels have been recorded from nondetectable to 1.20 OD (optical density), the "average" levels, relative to what is possible, appear low. This may be due in part to the low average dietary intake of L and Z in the typical U.S. diet. Nonetheless, individual differences in MP may also be influenced by nondietary factors such as genetics, demographics and lifestyle characteristics. Some evidence indicates that the MP carotenoids may protect the retina and lens, and could improve vision through some optical mechanisms. Consequently, efforts to determine typical MP levels and the factors that influence individual differences in MP density should be continued.

Carotenoids↗
PubMed2002

The age-related eye disease study (AREDS).

Some of the earliest and best known manifestations of malnutrition involve visual disorders, such as blindness that results from vitamin A deficiency. Interest is now being focused on the role of nutrition in the prevention and treatment of chronic age-related disorders. The Age-related Eye Disease Study (AREDS) was conducted in order to determine whether high-dose supplementation could influence the natural progres sion of eye disease in older Americans. AREDS found that prospective administration of purified vitamin C, vitamin E, beta-carotene, zinc, and copper had some benefits for age-related maculopathy, but had virtually no impact on the progression of cataracts.

Adult↗
PubMed2002

Macular pigment density is reduced in obese subjects.

PURPOSE: Because of the potential protective function of lutein (L) and zeaxanthin (Z) within the retina and lens, a better understanding of factors influencing tissue deposition is needed. The largest fractions of L and Z are stored in adipose tissue. Thus, higher body fat content and body mass index (BMI) may be expected to influence the quantities of L and Z in the retina (measured as macular pigment optical density, MPOD). METHODS: Six hundred eighty subjects were tested. Information on MPOD, body mass index (BMI), body fat percentage (n = 400, using bioelectric impedance), dietary intake (n = 280, using a food frequency questionnaire), and serum carotenoid content (n = 280, using reversed phase high-performance liquid chromatography) was obtained. RESULTS: There was an inverse relationship between MPOD and BMI (n = 680, r = -0.12, P < 0.0008) and between MPOD and body fat percentage (n = 400, r = -0.12, P < 0.01). These relationships were largely driven by data from the subjects with higher BMI (more than 29, 21% less MP) and higher body fat percentage (more than 27%, 16% less MP). Dietary carotenoid intake and serum carotenoid levels were also lower in subjects with higher BMI (n = 280). CONCLUSIONS: Obese subjects tend to have lower retinal L and Z. This reduction may be due to decreased dietary intake of L and Z and/or competition between retina and adipose tissue for uptake of L and Z.

Adipose Tissue↗
PubMedDate not supplied

Resonance Raman spectroscopic measurement of carotenoids in the skin and retina.

Carotenoid pigments within the skin and retina are measured using resonance Raman spectroscopy (RS). These RS instruments are unique in that they have been designed to obtain vibrational spectra in normal and diseased subjects using noninvasive procedures. Raman spectra have traditionally been used as a means of identifying a given chemical within some substrate. The new generation of RS instruments, however, has been designed to quantify the amount of carotenoids within the retina and skin. These amounts are typically reported in nonstandardized units called Raman counts (RC). These RCs are dependent on many factors intrinsic to their measurement, such as the specific optics used for stimulation and acquisition. The question of whether RCs can be used to derive valid quantitative measures of the carotenoid pigments in vivo is discussed.

Algorithms↗