Psychophysical characterisation of early functional loss in glaucoma and ocular hypertension

Abstract

Aim: The psychophysical evaluation of selective cell loss in early glaucoma and ocular hypertension. Methods: Contrast sensitivity was measured for the detection of luminance modulated gratings at a range of spatial (0.5, 2, 8 c/deg) and temporal (0, 16 Hz) frequency combinations in three groups of age matched patients (primary open angle glaucoma, ocular hypertension, normal controls; n=16). Stimuli of 5°were presented foveally and at 15°along the nasal horizontal meridian under photopic conditions. Results: Fovea: Compared to the normal group, the thresholds for the glaucoma patients were significantly elevated at all spatial and temporal frequencies (p<0.0001), but this reduction was not significantly different at any particular spatial or temporal frequency (p>0.1). There was no difference in contrast sensitivity between the normals and OHTs (p>0.10). Periphery: The thresholds of the glaucoma patients were elevated compared to the normal controls (p<0.01). The loss of sensitivity was slightly greater at the higher spatial frequencies for both stationary and flickering patterns but this did not reach significance (p=0.09). The contrast sensitivity in normal and OHT groups was not significantly different (p>0.10). Conclusions: In early glaucoma, the reduction in contrast sensitivity to stimuli which isolate the magnocellular pathway (0.5 c/deg, 16 Hz) was not significantly different compared with the reduction in contrast sensitivity to stimuli that isolate the parvocellular pathway. These findings are not consistent with the hypothesis that the magnocellular pathway is selectively damaged in early glaucoma.H istological studies in the early stages of both human and experimental glaucoma have shown a selective reduction in the proportion of larger retinal ganglion cells. 1-3Since magnocellular cells tend to be larger at every eccentricity, it is reasonable to conclude that this would translate into a selective loss of magnocellular function. The important clinical implication is that the selective evaluation of magnocellular function should facilitate the earlier detection of ganglion cell loss in glaucoma, possibly before the development of field defects as detected by available clinical perimetric tests.Although a number of studies have selectively tested either magnocellular or parvocellular pathways in early glaucoma, 4-12 selective damage to the magnocellular pathway has not been a robust and consistent finding.13-15 When magnocellular (M) and parvocellular (P) functions are assessed using a battery of tests a selective deficit in magnocellular function does not appear to be a consistent feature of early glaucoma. 16The discrepancy between the anatomical data and psychophysical data questions the anatomical basis for selective cell death. A key point in extrapolating changes in cell size distributions to changes in the proportions of particular classes of cells has been the assumption that the normal relation between cell size and cell type is preserved in disease. Recent anatomical data in the experiment...

Clinical Psychophysical Assessment of the ON- and OFF-Systems of the Magnocellular and Parvocellular Visual Pathways

“…Many diseases of eye and retina could affect the function of MC and PC pathways. Alterations in the contrast sensitivity mediated by the PC pathway were evident in patients with ocular hypertensive glaucoma using sinusoidal gratings but no losses in the MC pathway were detected [19]. In another study, however, Anderson & OBrien [17] found a reduction in the PC and in the MC pathways measuring resolution acuity with high spatial frequency pattern reversal grating and with stationary sinusoidal gratings.…”

“…In another study, however, Anderson & OBrien [17] found a reduction in the PC and in the MC pathways measuring resolution acuity with high spatial frequency pattern reversal grating and with stationary sinusoidal gratings. In fact, clinical tests have contributed to show most of examples of separate functioning of the MC and PC visual pathways [8,17,19,25]. It has also been possible to successfully isolate the MC and PC contribution to contrast sensitivity in healthy subjects using the pedestal paradigm but the time to complete the protocol is long [9,18,21,23,30].…”

“…There are many retina and eye diseases that could affect the gross function of MC and PC pathways [17][18][19][20][21][22][23][24][25] or diseases that produce selective impairment in ON-or OFF-systems such as as melanomaassociated retinopathy [18], X-linked retinitis pigmentosa [26], congenital stationary night blindness [27,28] and Duchenne Muscular Dystrophy [29].…”

Spatio-temporal Contrast Sensitivity in the Cardinal Directions of the Colour Space. A Review

“…Cortical magnification is the size of the cortex area activated by a stimulus subtending 1º in the visual field. Although different formulae are possible, the change of cortical magnification with eccentricity is reasonably well fitted by the following equation: [62][63] (12) where M F is the cortical magnification at fovea, and E 2 is the eccentricity value for which the cortical magnification equals half of the value observed at fovea. Performance in certain spatial tasks deteriorates with eccentricity in accordance with cortical magnification, which would mean that that particular task is not limited by the sampling at the retina, but by that of the cortex.…”

“…[1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16] (Burr D, et al IOVS 2003;44: ARVO E-Abstract 3193). It has been shown, however, that the usefulness of these functions is rather limited in practice, for several reasons.…”