Neil R A Parry

Central Manchester University Hospitals NHS Foundation Trust, Manchester, England, United Kingdom

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Publications (66)139.95 Total impact

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    Full-text · Article · Mar 2016 · Journal of the Optical Society of America A
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    ABSTRACT: The aim of the study was to determine orientation and spatial frequency sensitivity using reaction times (RTs) in an adaptation paradigm. Simple RTs were measured to the onset of a Gabor patch (SD = 1.2 deg, spatial frequency = 4 cycles deg-1). Observers adapted for 10 s to a 4 cycles deg-1 grating presented at a series of orientations (0, 2, 5, 10, 22.5, 45, 90°) or spatial frequencies (±0.5, 1, and 2 octaves). The contrast of the test grating was 4× each participant’s unadapted threshold. The effect of adaptation was evaluated by transforming RTs to effective contrast reduction using RT-based contrast response functions. RTs increased by between ~ 100 ms to 150 ms when the test and adapting gratings were of the same orientation or spatial frequency. The effect became less pronounced as the difference in orientation or spatial frequency increased. The average bandwidths for orientation and spatial frequency were 17.4° and 1.24 octaves, respectively. The method has some advantages over traditional approaches. It reveals a rapid time course of adaptation recovery with a half-life of about 13 s to 23 s. RTs form a rapid and easily implemented technique for assessing the underlying physiological mechanisms that control adaptation at suprathreshold levels of contrast.
    Full-text · Article · Nov 2015 · Perception
  • N. Mirza · H.J. Tahir · Y. Wang · N.R.A. Parry · I.J. Murray · T.M. Aslam

    No preview · Article · Oct 2015
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    ABSTRACT: Methylmalonic acidemia (MMA) and propionic acidemia (PA) are rare hereditary disorders of protein metabolism, manifesting early in life with ketoacidosis and encephalopathy and often resulting in chronic complications. Optic neuropathy (ON) has been increasingly recognised in both conditions, mostly through isolated case reports or small cases series. We here report the clinical features and visual outcomes of a case series of paediatric patients with a diagnosis of MMA or PA. Retrospective observational case series. A database of patients attending the Willink Biochemical Genetics unit in Manchester was interrogated. Fifty-three patients had a diagnosis of either isolated MMA or PA, of which 12 had been referred for ophthalmic review. Seven patients had clinical findings compatible with ON. Visual outcomes in these patients were poor, with slow clinical progression or stability over time in five cases with follow-up. Presentation was acute in a context of metabolic crisis in two of the cases. Four patients with ON had electrodiagnostics showing absent pattern evoked potentials, with one showing a preserved flash response. All four showed marked attenuation of the dark-adapted electroretinogram with better preservation of the light-adapted response. Our study suggests that ON is under-reported in patients with MMA and PA. Clinical presentation can be acute or insidious, and episodes of acute metabolic decompensation appear to trigger visual loss. Photoreceptor involvement may coexist. Active clinical surveillance of affected patients is important as comorbidities and cognitive impairment may delay diagnosis. Published by the BMJ Publishing Group Limited. For permission to use (where not already granted under a licence) please go to http://group.bmj.com/group/rights-licensing/permissions.
    No preview · Article · Jul 2015 · The British journal of ophthalmology
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    ABSTRACT: Abstract Purpose: The rate of recovery of rod vision following a bleach is emerging as highly clinically significant. It is systematically slowed in the older eye and in many clinical conditions, notably Age-Related Macular Degeneration (AMD). It is not known for certain whether this is a pan-retinal effect or if there are localised regions of impaired rod function. To address this issue a dual arc stimulus was developed that simultaneously samples sensitivity recovery in two retinal locations and we present data for three groups of observers, younger normals (n=13), older normals (n=13) and early AMD patients (AREDS grade 2 or 3, n=47). Methods: Arc-shaped white stimuli were presented on an otherwise black CRT screen at two locations in the inferior visual field. Recovery of sensitivity to the two stimuli was measured concurrently using method of adjustment by alternately presenting them following a localised bleach of at least 80%. Neutral density filters were used to extend the luminance range of the CRT. Data were fitted by non-linear regression to either a seven- or five- parameter model to characterise the dark adaptation curves. Results: The two stimuli produced similar cone recovery curves within each normal age group. Rod recovery slope, S2, was significantly different between the tested locations for the normal age groups (young 6°=-0.21±0.03, 11°=-0.24±0.03, p=0.02; old, 6°=-0.18±0.05, 11°=-0.20±0.04, p=0.01) but not for the AMD group (6°=-0.10±0.06, 11°=-0.11±0.05, p=0.11). S2 slopes were significantly shallower at both testing locations in the AMD group compared to the age-matched normals (6° p <0.001 and 11° p<0.001). Alpha and beta points were significantly delayed in the AMD group compared to the age-matched normals at both testing locations (α, 6° p=0.035, 11°=0.038 and β, 6° p<0.001, 11° p<0.001). ROC analysis showed testing the extra location enhanced the diagnostic capability of the test in detecting early AMD. Conclusions: The new technique increases the information yield without placing any additional demands on subjects. Previous findings regarding normal dark adaptation across the life span are confirmed. Additional novel data are revealed by the technique showing that the method has the potential to be of clinical benefit in detecting and monitoring early signs of disease of the outer retina, particularly AMD.
    No preview · Article · Jun 2015 · Investigative ophthalmology & visual science
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    ABSTRACT: We studied the spatial arrangement of Land M-cone driven electroretinograms (ERGs) reflecting the activity of magno-and parvocellular pathways. Land M-cone isolating sine wave stimuli were created with a four primary LED stimulator using triple silent substitution paradigms. Temporal frequencies were 8 and 12 Hz, to reflect cone opponent activity, and 30, 36 and 48 Hz to reflect luminance activity. The responses were measured for full-field stimuli and for different circular and annular stimuli. The ERG data confirm the presence of two different mechanisms at intermediate and high temporal frequencies. The responses measured at high temporal frequencies strongly depended upon spatial stimulus configuration. In the full-field conditions, the L-cone driven responses were substantially larger than the full-field M-cone driven responses and also than the L-cone driven responses with smaller stimuli. The M-cone driven responses at full-field and with 70° diameter stimuli displayed similar amplitudes. The Land M-cone driven responses measured at 8 and 12 Hz were of similar amplitude and approximately in counter-phase. The amplitudes were constant for most stimulus configurations. The results indicate that, when the ERG reflects lumi-nance activity, it is positively correlated with stimulus size. Beyond 35° retinal eccentricity, the retina mainly contains L-cones. Small stimuli are sufficient to obtain maximal ERGs at low temporal frequencies where the ERGs are also sensitive to cone-opponent processing.
    Full-text · Article · Mar 2015 · PLoS ONE
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    ABSTRACT: We studied the spatial arrangement of Land M-cone driven electroretinograms (ERGs) reflecting the activity of magno-and parvocellular pathways. Land M-cone isolating sine wave stimuli were created with a four primary LED stimulator using triple silent substitution paradigms. Temporal frequencies were 8 and 12 Hz, to reflect cone opponent activity, and 30, 36 and 48 Hz to reflect luminance activity. The responses were measured for full-field stimuli and for different circular and annular stimuli. The ERG data confirm the presence of two different mechanisms at intermediate and high temporal frequencies. The responses measured at high temporal frequencies strongly depended upon spatial stimulus configuration. In the full-field conditions, the L-cone driven responses were substantially larger than the full-field M-cone driven responses and also than the L-cone driven responses with smaller stimuli. The M-cone driven responses at full-field and with 70° diameter stimuli displayed similar amplitudes. The Land M-cone driven responses measured at 8 and 12 Hz were of similar amplitude and approximately in counter-phase. The amplitudes were constant for most stimulus configurations. The results indicate that, when the ERG reflects lumi-nance activity, it is positively correlated with stimulus size. Beyond 35° retinal eccentricity, the retina mainly contains L-cones. Small stimuli are sufficient to obtain maximal ERGs at low temporal frequencies where the ERGs are also sensitive to cone-opponent processing.
    Full-text · Article · Mar 2015 · PLoS ONE
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    ABSTRACT: Purpose: To define the phenotypic manifestation, confirm the genetic basis and delineate the pathogenic mechanisms underlying an oculo-auricular syndrome (OAS). Methods: Two individuals from a consanguineous family underwent comprehensive clinical phenotyping and electrodiagnostic testing (EDT). Genome-wide microarray analysis and Sanger sequencing of the candidate gene were used to identify the likely causal variant. Protein modelling, Western blotting, and dual luciferase assays were used to assess the pathogenic effect of the variant in vivo. Results: Complex developmental ocular abnormalities of congenital cataract, anterior segment dysgenesis, iris coloboma, early-onset retinal dystrophy and abnormal external ear cartilage presented in the affected family members. Genetic analyses identified a homozygous c.650A>C; p.(Gln217Pro) missense mutation within the highly conserved homeodomain of the H6 family homeobox 1 (HMX1) gene. Protein modelling suggested the variant would have a detrimental effect on protein folding and/or stability. In vitro analyses were able to confirm that the mutation has no effect on protein expression but adversely alters function. Conclusions: OAS is an autosomal recessive condition that has a profound effect on the development of the external ear, anterior segment and retina, leading to significant visual loss at an early age. This study has delineated the phenotype and confirmed HMX1 as the gene causative of OAS, enabling the description of only the second family with the condition. HMX1 is a key player in ocular development, possibly in both the pathway responsible for lens and retina development, and via the gene network integral to optic fissure closure. Copyright © 2015 by Association for Research in Vision and Ophthalmology.
    Full-text · Article · Jan 2015 · Investigative Ophthalmology & Visual Science
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    Humza J Tahir · Ian J Murray · Neil R A Parry · Tariq M Aslam
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    ABSTRACT: Technological advances have led to the development of powerful yet portable tablet computers whose touch-screen resolutions now permit the presentation of targets small enough to test the limits of normal visual acuity. Such devices have become ubiquitous in daily life and are moving into the clinical space. However, in order to produce clinically valid tests, it is important to identify the limits imposed by the screen characteristics, such as resolution, brightness uniformity, contrast linearity and the effect of viewing angle. Previously we have conducted such tests on the iPad 3. Here we extend our investigations to 2 other devices and outline a protocol for calibrating such screens, using standardised methods to measure the gamma function, warm up time, screen uniformity and the effects of viewing angle and screen reflections. We demonstrate that all three devices manifest typical gamma functions for voltage and luminance with warm up times of approximately 15 minutes. However, there were differences in homogeneity and reflectance among the displays. We suggest practical means to optimise quality of display for vision testing including screen calibration.
    Full-text · Article · Apr 2014 · PLoS ONE
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    ABSTRACT: L- and M-cone driven on- and off- ERG responses and their interactions were examined using full field stimuli with sawtooth temporal profiles. The effects of temporal frequency and contrast were studied. ERG recordings were obtained from 21 trichromatic, 1 protanopic, and 1 deuteranopic subjects. ERGs to L-cone increments and decrements resembled those to M-cone decrements and increments, respectively (i.e., of the opposite polarity). Temporal frequency and contrast had little effect on the implicit times. All response components varied linearly with contrast. When stimulated simultaneously, the responsivities of most components were larger for counterphase than for inphase modulation. The retinal processing leading to an ERG response is reversed for L- and M-cone driven responses.
    Full-text · Article · Apr 2014 · Journal of the Optical Society of America A
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    ABSTRACT: Electroretinograms (ERGs) elicited by transient, square-wave L- and M-cone isolating stimuli were recorded from human trichromatic (n=19) and dichromatic (n=4) observers. The stimuli were generated on a four primary LED stimulator and were equated in terms of cone modulation (cone contrast=0.11) and retinal illuminance (12,000 trolands). L- and M-cone isolated ERGs had waveforms similar to those observed for luminance responses. However, M-cone ERGs exhibited a phase reversal in their responses to onset and offset stimuli relative to the L-cone responses. This on-off response reversal was observed in trichromats but not dichromats. Simultaneous counterphase and inphase combinations of L- and M-cone isolating stimuli generated responses that reflected chromatic and luminance processing, respectively. We conclude that L- and M-cone specific ERGs provide a measure of how photoreceptors contribute to postreceptoral mechanisms.
    Full-text · Article · Apr 2014 · Journal of the Optical Society of America A
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    ABSTRACT: Cone isolating stimuli were used to assess the temporal frequency response characteristics of L- and M-cone electroretinograms (ERGs) in nine trichromatic and four dichromatic human observers. The stimuli comprised sinusoidal temporal modulations varying from 5 to 100 Hz. ERGs were recorded using corneal fiber electrodes and subjected to fast Fourier transform analysis. At low temporal frequencies (<10 Hz) the L- and M-cone ERGs had similar amplitude and exhibited minimal differences in apparent latency. At higher flicker rates (>20 Hz) L-cone ERGs had greater amplitudes and shorter apparent latencies than the M-cone responses. These differences between the L- and M-cone ERGs are consistent with their mediation by chromatic and luminance postreceptoral processing pathways at low and high temporal frequencies, respectively.
    Full-text · Article · Apr 2014 · Journal of the Optical Society of America A
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    Laura Patryas · Neil R A Parry · Dave Carden · Tariq Aslam · Ian J Murray
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    ABSTRACT: To investigate whether macular pigment optical density (MPOD) is related to dark adaptation in healthy subjects. Dark adaptation was measured after a minimum 30 % pigment bleach in 33 subjects (aged 15-68), using a white 1° stimulus presented 11° below fixation on a cathode ray tube monitor. The luminance range of the monitor was extended using neutral density filters. A heterochromatic flicker photometry based instrument (MPS 9000) was used to measure MPOD. The average MPOD for the whole group was 0.37 ±0.21 optical density units. Subjects with lighter irides had on average 40 % lower MPOD compared to those with darker irides (0.3 ± 0.20 vs 0.5 ± 0.19). Group mean MPOD was weakly associated with second (r = 0.32, p = 0.07) and third rod-mediated recovery rates (r = 0.31, p = 0.08) and with the rod threshold (r = -0.24, p = 0.18) 30 min after the onset of bleach. MPOD was unrelated to cone time constant (r = -0.02, p = 0.91), cone threshold (r = -0.01, p = 0.96), rod-cone break (r = 0.13, p = 0.45) or the rod-rod break (r = 0.11, p = 0.52). The second rod-mediated recovery rate (S2) for the lower 10th percentile of MPOD (n = 4) was 0.18 log cd.m(-2).min(-1) and 0.24 log cd.m(-2).min(-1) for the upper 10th percentile (n = 4). The two groups were significantly different (t = -2.67, p = 0.037). We report a statistically significant difference between subjects falling in the 10th percentile extremes of MPOD and rod-mediated but not cone-mediated sensitivity recovery. Further investigation into the relationship between MPOD and rod function is warranted, particularly extending the work to encompass those with low MPOD and poor night vision.
    Full-text · Article · Jan 2014 · Albrecht von Graæes Archiv für Ophthalmologie
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    Full-text · Article · Jan 2014 · Journal of Glaucoma
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    ABSTRACT: There are a multitude of applications using modern tablet computers for vision testing that are accessible to ophthalmology patients. While these may be of potential future benefit, they are often unsupported by scientific assessment. This report investigates the pertinent physical characteristics behind one of the most common highest specification tablet computers with regard to its capacity for vision testing. We demonstrate through plotting of a gamma curve that it is feasible to produce a precise programmable range of central luminance levels on the device, even with varying background luminance levels. It may not be possible to display very low levels of contrast, but carefully using the gamma curve information allows a reasonable range of contrast sensitivity to be tested. When the screen is first powered on, it may require up to 15 min for the luminance values to stabilize. Finally, luminance of objects varies towards the edge of the screen and when viewed at an angle. However, the resulting effective contrast of objects is less variable. Details of our assessments are important to developers, users and prescribers of tablet clinical vision tests. Without awareness of such findings, these tests may never reach satisfactory levels of clinical validity and reliability.
    Preview · Article · Apr 2013 · Journal of The Royal Society Interface
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    ABSTRACT: Purpose To characterize the rate of rod-mediated sensitivity decline with age using a PC-driven cathode ray tube (CRT) monitor. To provide data regarding the repeatability of the technique. Methods Dark adaptation was monitored for 30 min following a minimum 30 % pigment bleach, using a white 1° stimulus (modulated at 1 Hz), presented 11° below fixation on a CRT monitor. Thirty-three subjects with no ocular pathology and normal fundus photographs were divided into two groups: older (≥45, n = 16) and younger (<45, n = 17). Results Rod recovery was assessed using component S2 of dark adaptation. S2 was significantly slower in the older (0.19 ± 0.03 log cd.m−2.min−1) compared with the younger group (0.23 ± 0.03 log cd.m−2.min−1, t = −4.05, p < 0.0003), despite no difference in visual acuity and fundus appearance. Faster rates of S2 recovery were correlated with lower threshold at 30 min (T30) (r = −0.49). Correlation coefficients between first and second measurements for S2 and T30 were 0.49 (p < 0.009) and 0.84 (p < 0.0001) respectively. The coefficient of repeatability was 0.07 log cd.m−2.min−1 for S2 and 0.35 log cd.m−2 for T30. The coefficients of variation for S2 and T30 were 15 % and 10 % respectively. Conclusions Dark adaptation is slowed in normal ageing. CRT-based dark adaptometry is easily implemented and highly repeatable. The technique described in this article would be useful for documenting visual changes in future clinical trials assessing retinal health in the older eye with and without ocular pathology.
    Full-text · Article · Apr 2013 · Albrecht von Graæes Archiv für Ophthalmologie
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    Full-text · Dataset · Feb 2013
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    ABSTRACT: Purpose: We investigated the effect of daily supplementation with lutein (L) capsules on macular pigment optical density (MPOD) and visual acuity (VA) in patients with early age-related macular degeneration (AMD). Methods: A randomized, double-blind, placebo-controlled, two-center investigation of the effects of L supplementation in early AMD was conducted. The duration of the trial was 12 months. The centers were Manchester, United Kingdom and Maastricht, the Netherlands. L capsules (10 mg Ester) or a placebo (P) were taken daily. There were 72 patients (mean age 70.5 ± 8.7) assigned randomly to either L (n = 36) or P (n = 36) groups. MPOD using a flicker-based technique (MPS9000) and best corrected VA (LogMAR) were measured at the beginning and at 4-month intervals over the duration of the 12-month supplementation period. Blood serum samples were collected to monitor compliance. Results: At the end of the trial, an overall increase in the mean MPOD level was found for the L group from 0.38 ± 0.19 to 0.53 ± 0.22 optical density (OD) units. According to a mixed design ANOVA, this was statistically significant (P < 0.001). No change in MPOD was found for the P group. There was no significant change in VA in the L group (n = 36). The P group (n = 36) showed a statistically significant deterioration from 0.05 ± 0.13 to 0.09 ± 0.13 (P < 0.05). When comparing the change in VA over the supplementation period, there was a significant difference between the two groups (P < 0.05). To avoid ceiling effects, 2 subgroups of patients with VA worse than 0.06 at baseline were reanalyzed. In the L subgroup (n = 19) a mean improvement in VA from 0.23 ± 0.12 at baseline to 0.16 ± 0.10 at visit 4 was observed (P < 0.05). In the P subgroup (n = 14), there was a small deterioration from 0.18 ± 0.13 to 0.19 ± 0.12 (P = 0.70). The improvement in VA in the L subgroup was compared to the deterioration in VA in the P group and this effect reached statistical significance (P < 0.05). Conclusions: L supplementation increases MPOD levels in early stage AMD patients. According to the VA measurements, the progress of the disease might be slowed in some patients with augmented levels of MP. (ClinicalTrials.gov number NCT01042860.).
    Full-text · Article · Feb 2013 · Investigative ophthalmology & visual science
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    ABSTRACT: Background: The aim of the present study was to investigate the value of pattern visual-evoked potentials (pVEP) and pattern electroretinograms (pERG) in early glaucoma diagnosis. Materials and methods: Thirty-eight eyes of 38 patients were included. Patients were classified into normal control (NC) and glaucoma patient (GP) groups. Patients underwent a detailed clinical ophthalmic examination and an electrodiagnostic examination using steady-state pVEP and pERG. Differences between groups in the amplitudes of the second harmonic of the pVEP and pERG responses to 480' (A480) and 48' (A48) check sizes and the ratio of the above amplitudes (A48/A480) were examined. Results: Differences in the 48' and 480' pVEP between groups were not statistically significant. The pVEP A48/A480 ratio was significantly higher in NC than in GP. Differences in pERG between groups were statistically not significant for both 48' and 480' check sizes. In contrast, respective differences in pERG A48/A480 ratio were statistically significant. Conclusions: Steady-state pVEP and pERG A48/A480 ratio may be of value in glaucoma diagnosis.
    Full-text · Article · Jan 2013 · Journal of glaucoma
  • Neil R A Parry · Anthony G Robson
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    ABSTRACT: Large, nominally isoluminant chromatic gratings containing a short-wavelength component are prone to luminance contrast intrusions due to retinal inhomogeneity, especially as a result of the uneven distribution of macular pigment. Isoluminance is usually determined for a relatively small, central area, but a significantly larger stimulus cannot be isoluminant across the whole field, largely due to macular pigment absorption of short-wavelength light. This confounds attempts to maintain high selectivity, particularly in suprathreshold electrophysiological and brain-imaging studies that require large stimulus fields. Here we introduce the concept of a panisoluminant grating (PIG), which comprises a series of concentric annular regions, each adjusted to location-specific isoluminance for the observer. Gratings were modulated along subject-specific tritanopic confusion lines and the selectivity of responses to the PIG was tested according to both psychophysical and electrophysiological criteria. The psychophysically-determined temporal tuning function obtained using the PIG showed lower sensitivity and lower resolution than with a conventional tritan grating of equal diameter (18°). Chromatic onset visual evoked potentials (VEPs) to the PIG were dominated by a chromatic-specific negative wave and reduced achromatic response components that were prominent in VEPs to the conventional grating. These data demonstrate that a large tritan PIG is capable of eliciting selective responses of the S-cone-driven pathway at threshold and at suprathreshold levels. The PIG stimulus may prove beneficial in investigations that require large fields such as electrophysiological and brain imaging studies of chromatic processing.
    No preview · Article · Nov 2012 · Journal of Vision

Publication Stats

514 Citations
139.95 Total Impact Points

Institutions

  • 2013-2015
    • Central Manchester University Hospitals NHS Foundation Trust
      • Vision Science Centre
      Manchester, England, United Kingdom
  • 2010-2015
    • The University of Manchester
      Manchester, England, United Kingdom
  • 2003-2015
    • University of Bradford
      • Department of Optometry and Vision Science
      Bradford, England, United Kingdom
  • 2009-2010
    • Manchester Memorial Hospital
      Manchester, Connecticut, United States