Dopaminergic modulation of visual sensitivity in man

A large body of experimental evidence supports the hypothesis that dopamine is a functional neuromodulator at many levels of the visual system. Intrinsic dopaminergic neurons were characterized in most mammalian retina, including man. These neurons give rise to a dendritic plexus covering the retina. Thus, dopamine seems to be involved in the organization of the ganglion cell and the bipolar cell receptive fields and modulates physiological activity of photoreceptors, both processes which underlie sensitivity and spatial selectivity of visual processing in the early stage of the visual system. Moreover, few data are now available concerning the functional significance of dopaminergic modulation of visual sensitivity in man. Parkinson's disease is a specific disorder of central dopaminergic systems. Abnormalities in the pattern-evoked potentials and electroretinogram have been found in parkinsonian patients. Contrast sensitivity, a useful tool for measuring visual spatio-temporal sensitivity in man, has also been shown to be modified due to this affection. Dynamic contrast sensitivity is primarily decreased in these patients, distinguishing them from the normal aging process. Because these modifications in shape of the contrast sensitivity function are reversed by L-Dopa, and that neuroleptic administration could reproduce them in schizophrenia patients, it was suggested that dopamine might tune the contrast sensitivity function in man. We have recently shown that subcutaneous apomorphine induces changes in contrast sensitivity in healthy volunteers, which preferentially affect motion sensitivity. These dopaminergic sensitive modifications in the shape of the contrast sensitivity function might reflect a change in the range of sensitivity of the visual system, both in dynamic and spatial properties. This could be explained by a modification in the spatial and dynamic properties of the ganglion cell responses in the retina. Moreover, we suggest both from our results and from the review of the literature that human psychophysical data confirm the hypothesis that dopamine may be involved in light retinal adaptation, as light-induced and dopamine-induced modifications in the shape in the contrast sensitivity function are quite similar.