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Dwight Burkhardt, Ph.D. Professor, Department of Psychology E-mail: burkh001@umn.edu

Research Interests:

The primary aim of research in our laboratory is to understand the relation between the electrical signals of nerve cells in the retina of the eye and visual information processing. What are the neural mechanisms which shape and control these signals? How are these signals related to visual perception in animals and humans? To search for answers to these questions, we study the electrical response of retinal neurons evoked by precisely-controlled light stimuli. Of late, we are using computer-controlled LCD panels to apply light patterns on the retina. We are primarily interested in the retinal mechanisms of contrast vision, color vision and adaptation.

Our recent work on contrast vision involves intracellular recording from retinal neurons in the tiger salamander retina, a widely-studied animal model for understanding the neurophysiology of the retina. Our goal is to understand how information for visual contrast (the"gray scale" of vision) is encoded and transformed across the synaptic levels of the retinal network. We have found that the ganglion cells (the output neurons of the retina) and amacrine cells are remarkably sensitive to very small steps of contrast and that much of this sensitivity arises from a great amplification of the contrast signal at the synapse between the cone photoreceptors and bipolar cells. We have also found that this amplification is modulated by light adaptation in so that the contrast gain increases as the ambient light level in the environment increases. This adaptation-dependent modulation is determined by at least two mechanisms: one occurs in the photoreceptors, the second is intrinsic to bipolar cells. We are currently studying linearity of signal generation and transmission in cones and bipolar cells with sinusoidal light modulation, signal averaging, and Fast Fourier Transforms of the response. Additional studies of the relation between natural images and the contrast encoding of retinal neurons and on the center-surround organization of the bipolar cell receptive field are in progress.

Selected Publications:

(For a comprehensive list of recent publications, refer to PubMed, a service provided by the National Library of Medicine.)

Burkhardt DA, Fahey PK, Sikora MA. Natural images and contrast encoding in bipolar cells in the retina of the land- and aquatic-phase tiger salamander. Vis Neurosci. 2006 Jan-Feb;23(1):35-47.

Burkhardt DA, Fahey PK, Sikora MA. Retinal bipolar cells: contrast encoding for sinusoidal modulation and steps of luminance contrast. Vis Neurosci.
2004 Nov-Dec;21(6):883-93.

Fahey, P.K. and Burkhardt, D.A. (2002). Center-surround organization in bipolar cells: symmetry for opposing contrasts. Visual Neuroscience.

Burkhardt, D.A. (2001). Contrast and light adaptation in the outer retina. Progress in Brain Research, 131:407-417.

Fahey, P.K. and Burkhardt, D.A. (2001). Effects of light adaptation on contrast processing in bipolar cells in the retina. Visual Neuroscience, 18:581-597.