DOPAMINERGIC MODULATION OF VISUAL SENSITIVITY IN MAN

A large body of experimental evidence supports the hypothesis that dop amine is a functional neuromodulator at many levels of the visual syst em. Intrinsic dopaminergic neurons were characterized in most mammalia n retina, including man. These neurons give rise to a dendritic plexus covering the retina. Thus, dopamine seems to be involved in the organ ization of the ganglion cell and the bipolar cell receptive fields and modulates physiological activity of photoreceptors, both processes wh ich underlie sensitivity and spatial selectivity of visual processing in the early stage of the visual system. Moreover, few data are now av ailable concerning the functional significance of dopaminergic modulat ion of visual sensitivity in man. Parkinson's disease is a specific di sorder of central dopaminergic systems. Abnormalities in the pattern-e voked potentials and electroretinogram have been found in parkinsonian patients. Contrast sensitivity, a useful tool for measuring visual sp atio-temporal sensitivity in man, has also been shown to be modified d ue to this affection. Dynamic contrast sensitivity is primarily decrea sed in these patients, distinguishing them from the normal aging proce ss. Because these modifications in shape of the contrast sensitivity f unction are reversed by L-Dopa, and that neuroleptic administration co uld reproduce them in schizophrenian patients, it was suggested that d opamine might tune the contrast sensitivity function in man. We have r ecently shown that subcutaneous apomorphine induces changes in contras t sensitivity in healthy volunteers, which preferentially affect motio n sensitivity. These dopaminergic sensitive modifications in the shape of the contrast sensitivity function might reflect a change in the ra nge of sensitivity of the visual system, both in dynamic and spatial p roperties. This could be explained by a modification in the spatial an d dynamic properties of the ganglion cell responses in the retina. Mor eover, we suggest both from our results and from the review of the lit erature that human psychophysical data confirm the hypothesis that dop amine may be involved in light retinal adaptation, as light-induced an d dopamine-induced modifications in the shape in the contrast sensitiv ity function are quite similar.