Department of Ophthalmology

Visual Physiology and Psychophysics


Dr. Joseph Harrison's research program complements his clinical role as Director of the Visual Function Testing Service for the department. The service uses:

electrophysiological measures --

  • electroretinogram (ERG)
  • electro-oculogram EOG)
  • visual evoked potential (VEP)
and psychophysical measures --
  • color vision
  • dark adaptation
  • contrast sensitivity to characterize visual dysfunction in patients with diseases of the visual system.

These non-invasive measures of visual function can frequently pinpoint dysfunction specific to a given physiological mechanism in a specific disease, even in diseases without other ophthalmic manifestations. In general, these non-invasive measures are used to characterize the effects of perturbation of the visual system from its normal optimal state.

Current Projects

Dr. Harrison is currently investigator on two projects examining the safety and efficacity of a medical treatment for cocaine dependence. He has a continuing interest in detecting the functional alteration associated with mutations (naturally occurring, knockout and transgenic) in mice and rats using ERG and VEP.

Significant Findings

Along with Dr. Izhak Nir (Department of Pathology, UTHSCSA) and Dr. Michael Iuvone (Emory University), Dr. Harrison found that the dopamine D4 receptor knockout mouse has a defective retinal adaptation mechanism and reduced retinal function under light adapted conditions. This may be a candidate mutation for disease causing visual complaints in some patients with unknown diseases affecting light and dark adaptation. Working with Dr. Wichard A. J. van Heuven, he found that in human immunodeficiency patients with cytomegalovirus, the EOG is more severely affected than the a-wave of the ERG reflecting the function of the outer retina. The data suggest that the retinal pigment epithelium may be an early site of patholophysiology in the development of cytomegalovirus retinitis and the data have implications with respect to the type of retinal detachments that occur in CMV and the appropriate repair. Working with Dr. Jeffrey Kiel, Dr. Harrison found that outer retinal function as measured by the ERG is not significantly affected by as great as a 50% reduction in choroidal blood flow for as long as five minutes.


Unique Technical and Clinical Research Capabilities/Instrumentation

Technical and Clinical Research Capabilities

  • Expertise in measurement of electromagnetic radiation by photometry and radiometry.
  • Many years’ experience in measuring contrast sensitivity and disability glare.


  • Instrumentation for measuring ERG, EOG and pattern and flash VEPs (at UT Medicine Eye Consultants office at University Hospital).
  • Numerous instruments for photometric and radiometric measurements.
  • Laboratory equipment (amplifiers, CRTs, digitizers, capacitive discharge flashes, analog and digital filters, etc.) enabling Dr. Harrison to carry out similar electrophysiological studies in experimental animals.
  • Instrumentation for measuring color vision contrast sensitivity.
  • Portable equipment that permits recording ERGs and flash VEPs in humans (such as infants in the operating room) and animals.

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For more information, please contact:
Joseph M. Harrison, Ph.D.
Director, Visual Function Testing Service
University of Texas Health Science Center at San Antonio
7703 Floyd Curl Drive, MC 6230
San Antonio, TX 78229-3900
Phone: (210) 567-8426
Fax: (210) 567-8413