Structure and Function

THE STRUCTURE/FUNCTION CONUNDRUM

Tuesday, 12 March 2013 Written by Larry J Alexander OD FAAO Posted in Presentations

 

 

Until functional testing can match the sensitivity of functional testing… the question cannot be answered.

Until one can answer if functional testing is really testing what it is supposed to test… the question cannot be answered.

 

 

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..the magnocellular (fast conducting) stream, which carries information useful for motion analysis, and the parvocellular (slow conducting) stream, which carries information useful for analyses of shape and color…higher cortical areas that receive input either directly or indirectly from layer 4B are likely to be more strongly influenced by the parvocellular pathway than previously believed

Nature 380, 442 - 446 (04 April 1996)

The PARVOCELLULAR pathway accounts for approximately 80% of optic nerve fibers…loss of visual acuity and color vision are two visual functions thought to be mediated by the PC pathway…Optic neuritis produced large sensitivity losses mediated by the MC pathway and contrast gain losses in the inferred PC pathway. (Invest Ophthalmol Vis Sci. 2011;52:8900–8907)

The M ganglion cells in the retina, so called because of their larger cell body,3 project to layers one and 2 in the lateral geniculate nucleus (LGN). The P cells in the retina project to layers 3 through 6 in the LGN. The projections of the M and P cells appear to be distinct in the LGN4,5 and remain so from the LGN to the primary visual cortex (V1), with the M pathway terminating primarily in layer 4C of V1 and the P pathway terminating primarily in layers 4A and 4C of V1.6.

Because of the different physiologic responses of the M and P pathways, a common strategy to preferentially activate the pathways is to use a stimulus that is color-neutral, low in contrast, large in grid size, and reverses contrast at a fast rate for the M pathway. Conversely, the P pathway responds preferentially to stimuli that are different in color, high in contrast, small in grid size, and reverses contrast at a slow rate. These two pathways have different blood oxygen level dependencies.

Am J Neuroradiol 27:1628 –34 Sep 2006

Frequency doubling technology “perimetry” demonstrates high sensitivity and specificity for detection of early, moderate, and advanced glaucomatous visual field loss.

Am J Ophthalmol 2000;129:314–322.

FDT appears more sensitive to axonal injury reflected by the extent of optic disc pallor in altitudinal NAION than SAP and in some patients reveals visual dysfunction in the hemifield that appeared relatively uninvolved when evaluated using SAP.

Br J Ophthalmol 2004;88:1274-1279

FDT was developed based on the belief that the magnocellular (M) cells in the retina are preferentially stimulated. With few M cell fibers distributed throughout the retina, it was argued that FDT perimetry would be more sensitive in detecting early glaucomatous field loss. However, more recent evidence has suggested that the FDT effect is mediated by many neural cell types.

No difference was found suggesting that neurologic disease affects FDT similarly whether the insult is pre- or postchiasmal. However, it should be noted that in roughly 10% of cases (anterior and posterior) SAP and FDT did not correlate well. Our study was not designed to address this and the significance of this occasional discordance remains unknown.

Middle East Afr J Ophthalmol 2012;19:211-5

However, defects in patients with hemianopias may be missed with FDT technology because of the presence of scattered abnormal test locations and failure to detect test locations along the vertical meridian…This suggests FDT may not be isolating the magnocellular (M) cells with nonlinear responses to stimulus contrast (My cells) in patients with visual loss.

Comparison of Sensitivity and Specificity of CAP to FDT 

Type CAP Sens/Spec FDT Sens/Spec
Optic Neuropathy 86/81 78/76
Hemianopia 92/81 48/76
(Invest Ophthalmol Vis Sci. 2002;43:1277–1283)

The new Humphrey Matrix 24-2 testing strategy provides a visual field testing method for optic nerve and chiasmal disorders that has fair to good concordance with the Humphrey SITA Standard 24-2 program. Both tests have similar sensitivity and specificity.
Invest Ophthalmol Vis Sci. 2008;49:917–923

The FDT screening test can fail to demonstrate complete hemianopic and quadrantanopic field defects. Users should be aware of this deficiency when using FDT to screen for field defects.
Eye (2003) 17, 330–333.

The Humphrey Matrix was released in April 2005 with the aim of providing significant enhancements to the diagnostic capabilities. Additional tests having smaller targets and increased number of tested locations to improve the spatial resolution of VF defects have been created without greatly effecting the variability and sensitivity resolution.

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Larry J Alexander OD FAAO

Larry J Alexander OD FAAO

Dr. Alexander (1948-2016) was a 1971 graduate of Indiana University School of Optometry. He served in the US Navy then served as a Professor at the University of Alabama Birmingham School of Optometry. Larry contributed to a number of chapters in textbooks and has published three editions of Primary Care of the Posterior Segment, as well as contributed to the professional literature. He also lectured extensively in the area of ocular and systemic disease. His areas of special interest included dysfunctional tear syndrome, glaucoma and macular degeneration.  His lessons are the basis for this site and he will be dearly missed. 

Comments (1)

  • Agustin Gonzalez

    05 May 2013 at 16:17 | #

    VEP testing with high/ low contrast stimulus is a great way of assessing the magnocelullar and parvocellular pathway function.

    reply

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