AVRC: Clinical and Physiological Optics research group

Learning from the pupil - studies of basic mechanisms and clinical applications

John Barbur and Alister Harlow.

Summary:

Studies of pupil response mechanisms and clinical applications have been pursued with great interest over several years. New instrumentation for pupillometry and eye-movement recording and a range of psychophysical tests based on advances in digital imaging and visual display technology have been implemented on the P_SCAN 100 system. This instrument makes possible the measurement of extremely small pupil responses that are normally elicited in response to changes in neural activity in extrastriate visual areas of the cortex. Studies in normal subjects and patients with damaged visual pathways have resulted in the discovery of a number of pupil response components. Such measurements yield useful information about the processing of spatially structured patterns, motion or chromatic signals. The P_SCAN instrumentation is currently being used in a number of collaborative projects both in UK and abroad.

Isolation of pupil response components

This study involves the measurement of pupil responses to stimulus specific attributes in normal subjects and in patients with damage to either the primary visual cortex or to midbrain nuclei. We have two principal objectives:

to establish the extent to which the geniculostriate pathway is involved in the pupil light reflex response,
to discover the pathways involved in the mediation of pupil colour and pupil grating responses.

Collaborators in this work are Barbara Wilhelm (The Eye Hospital, University of Tübingen, Germany) and Randy Kardon (Department of Ophthalmology, University of Iowa, USA).

Pupil responses in studies of blindsight

It is commonly assumed that the primary function of the pupil of the eye is to respond to changes in the amount of light that enters the eye, with its major neural complex and the origin of its final efferent pathway residing in the midbrain. But recent studies have demonstrated that the pupil constricts in a systematic manner to stimulus attributes such as spatial structure, colour, and movement, even when there is no change in mean light flux level or, indeed, even when there is a net reduction in light flux in a visual stimulus. Such stimulus properties are associated with electrophysiological activity in extrastriate cortical neurones, and thus there might be a down-stream modulation of midbrain centres by the cortex. Recent psychophysical and pupillometric findings in subjects with long-standing lesions in the primary visual cortex show that some capacity for processing gratings and chromatic signals.Evidence of processing of these signals remains, especially for large, "red" stimuli, even when the subjects are unaware of the stimulus. There are many questions that remain unanswered in relation to the retinal projections and the areas of the brain involved in the processing of visual signals in the absence of conscious visual perception. We are in the process of exploring such questions using techniques based on psychophysics, pupillometry and fMRI.

Collaborators in this work are Larry Weiskrantz and Alan Cowey (University of Oxford, UK).

Comparison of reaction time and pupil response amplitudes and latencies to light flux increments, grating stimuli and chromatic modulation

Response latencies associated with the processing of light flux increments, stimulus structure, colour and coherent motion have been studied using manual reaction times and pupil responses. Our aim was to investigate latency differences between subcortical and central components of the pupil response and the correlation with manual reaction times measured for the same stimuli.

Collaborators in this work are Alison Finlay and Janet Wolf (City University) and Peter Lennie (University of Rochester, NY).

The use of stimulus-specific pupil responses to study the processing of spatially structured and chromatic stimuli in rhesus monkeys

Visual stimuli that isolate pupil color and pupil grating responses in human vision have been used to investigate the properties of stimulus-specific pupil responses in the rhesus monkey. Pupil responses to light flux increments, isoluminant chromatic stimuli, and gratings of equal and lower space-averaged luminance were measured and compared. The results demonstrate clearly the existence of pupil color and pupil grating responses similar to those observed in human vision but of significantly shorter latencies.

Collaborators in this work are Paul Gamlin and Hongyu Zhang (Vision Science Research Center, University of Alabama at Birmingham, USA).

Isolation of pupil light reflex response components - clinical applications

New techniques have been developed and applied to isolate different components in the Pupil Light Reflex response. The aim of this project was to understand and account for some of the observed variations in the pattern of abnormal pupil response. We want to explain why lesions to the primary visual cortex (V1) either eliminate or reduce the PLR response and why patients with optic nerve lesions that show clear, steady state, relative afferent pupil defects fail to show any constriction abnormality in response to brief flashes of light (i.e., the dynamic PLR response).

Collaborators in this work are Randy Kardon (University of Iowa, USA).

Relative loss and recovery of chromatic and achromatic sensitivity in patients with acute optic neuritis

The aim of this study was to establish the extent to which pupil responses to either light flux or chromatic modulation are affected in optic neuritis. The test employs a new technique based on automatic extraction of pupil response aptitude and latency in response to sinusoidal modulation of either light flux increments or chromatic modulation. The use of pure chromatic stimuli makes it possible to establish whether the chromatic or the achromatic mechanisms are affected most in acute optic neuritis. We also wish to establish how the expected recovery of visual sensitivity to chromatic and achromatic stimuli parallels the recovery of pupil response characteristics.

Collaborators in this work are Sancho Moro (City University), Byron Lam and Mu Liu (Bascom Palmer Eye Center, University of Miami, USA) and Gordon Plant (National Hospital, London, UK)

References:

Barbur, J. L., Harlow, A. J., Moro, S., and Levy, I. S. (2000). Perimetric study of relative afferent pupil defects . 35, 26-29. In Trends in Optics and Photonics Series - Vision Science and its Applications. Ed. Lakshminarayanan, V. Optical Society of America, Washington DC.

Barbur, J. L., J. A. Harlow, R. Schmid, and B. Wilhelm (2000). Patterns of abnormal pupil response in patients with damaged primary visual cortex. Investigative Ophthalmology & Visual Science 41: S563.

Liu, M., B. L. Lam, J. L. Barbur, and S. Moro (2000). Pupil responses to colour and light flux changes in normal subjects and in patients with acute optic neuritis. Investigative Ophthalmology & Visual Science 41: S35.

Barbur, J. L. and Moro, S. (2000). Component pupil perimetry in subjects with acute optic neuritis. Ophthalmic Research 32, 146-146.

Barbur, J. L., L. Weiskrantz, and J. A. Harlow (1999). The unseen color aftereffect of an unseen stimulus: insight from blindsight into mechanisms of color afterimages. Proc. Natl. Acad. Sci. U. S. A. 96: 11637-11641.

Weiskrantz, L., A. Cowey, and J. L. Barbur (1999). Differential pupillary constriction and awareness in the absence of striate cortex. Brain 122: 1533-1538.

Wolf, J. E., A. L. Finlay, K. Bisseseur, A. J. Harlow, and J. L. Barbur (1999). Pupil latencies to sinusoidally modulated stimulus attributes. Investigative Ophthalmology & Visual Science 40: S45.

Barbur, J. L. and Kardon, R. H. (1998) Isolation of pupil light-reflex response components - clinical applications. Ophthalmic Research 30, 124-124.

Barbur, J. L., J. Wolf, and P. Lennie (1998). Visual processing levels revealed by response latencies to changes in different visual attributes. Proc. R. Soc. Lond. B. Biol. Sci. 265: 2321-2325.

Barbur, J. L., P. D. R. Gamlin, A. J. Harlow, G. Wood, and J. E. Wolf (1998). Comparison of pupil responses in man and monkey. Perception 27: 5-5.

Gamlin, P. D., H. Zhang, A. Harlow, and J. L. Barbur (1998). Pupil responses to stimulus color, structure and light flux increments in the rhesus monkey. Vision Res. 38: 3353-3358.

Harlow, A. J., J. L. Barbur, G. Wood, and J. E. Wolf (1998). Pupil responses to real & illusory light flux changes. Investigative Ophthalmology & Visual Science 39: 395-395.

Barbur, J. L., V. A. Cole, A. Sahraie, A. Simmons, L. Weiskrantz, and S. C. R. Williams (1997). A study of pupil responses in a subject with damaged primary visual cortex. Investigative Ophthalmology & Visual Science 38: 71-71.

Freedman, D., J. L. Barbur, and P. Lennie (1997). Pupil response latencies & reaction times to chromatic and achromatic stimuli. Investigative Ophthalmology & Visual Science 38: 1012-1012.

Sahraie, A. and J. L. Barbur (1997). Pupil response triggered by the onset of coherent motion. Graefes. Arch. Clin. Exp. Ophthalmol. 235: 494-500.

Barbur, J. L., V. A. Cole, and A. J. Harlow (1996). Investigation of pupil light reflex response components: spatial summation and contrast gain. Investigative Opthalmology & Visual Science 37: 160-160.

Barbur, J. L., Cole, V. A., Harlow, A. J., and Levy, I. S. (1996). Isolation of pupil light reflex response components: selective loss of function in a subject with optic nerve drusen. In Vision Science and its Application (Technical Digest Series) Vol. , 50-53. 1996. Optical Society of America, Washington, DC.

Barbur, J. L., V. A. Cole, A. J. Harlow, and A. Sahraie (1995). Pupil colour and light reflex responses in the periphery of the visual field. Investigative Ophthalmology & Visual Science 36: 660-660.

Barbur, J. L. (1995). A study of pupil response components in human vision., p. 3-18. In Basic and Clinical Perspectives in Vision Research J. G. Robbins, M. B. A. Djamgoz, and A. Taylor (eds.). Plenum Publishing Company, New York.

Edgar, D. F., J. L. Barbur, and E. G. Woodward (1995). Pupil size measurements in relation to light scatter in the eye. Investigative Ophthalmology & Visual Science 36: 938-938.

Barbur, J. L., R. F. Hess, and H. D. Pinney (1994). Pupillary function in human amblyopia. Ophthalmic Physiol. Opt. 14: 139-149.

Barbur, J. L., A. J. Harlow, and A. Sahraie (1992). Pupillary responses to stimulus structure, colour and movement. Ophthalmic & Physiological Optics 12: 137-141.

Barbur, J. L., J. Birch, A. J. Harlow, and G. Plant (1992). The pupil colour response (evidence for involvement of central mechanisms). Perception 21: 74-74.

Barbur, J. L. (1991). Pupillary responses to grating stimuli. Journal of Psychophysiology 5: 259-263.

Barbur, J. L. (1991). Pupillary responses to stimulus structure and colour: possible mechanisms. In Non-invasive Assessment of the Visual System (Technical Digest Series) 1, 68-71. Optical Society of America, Washington DC.

Barbur, J. L. and P. M. Forsyth (1986). Can the pupil response be used as a measure of the visual input associated with the geniculo-striate pathway? Clinical Vision Science 1: 107-111.

Supported by:

Wellcome Trust, MRC, The Royal Society and City University.

John Barbur

Applied Vision Research Centre | Clinical and Physiological Optics

avrc@city.ac.uk - last updated on 16 March 2001.