Function and morphology of vertebrate retina

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A major goal of our studies on vertebrate retina is to dissect neuronal circuitry by identifying the transfer function between neurons,i.e., to recover the neuronal filter that transforms the responses of the presynaptic neuron to those of the postsynaptic neuron. The most direct means of dissecting complex circuitry and of identifying such a filter, either in biology or in engineering, is to inject a test signal into one circuit element and to record the resulting and transformed signals in another element.Intracellular recordings were made simultaneously from two neighboring neurons in the eye-cup preparation of the channel catfish, Ictalurus punctatus. After identifying cell types by the responses evoked by light stimuli, an extrinsic current, modulated by pulse, sinusoid or Gaussian white-noise signal, was successively injected into one neuron and the resulting response was recorded from the other neuron. The tips of the two electrodes were about 0.2-0.4 mm part. In the case of Gaussian white-noise current, input current 20s in duration was cross-correlated against the response to compute first-and second-order Wiener kernels. Experiments were controlled by passing a current into nearby extracellular space to verify that the observed results were from the current injected into a neuron. For the study of synaptic organization of the retinal neurons, HRP-marked neurons were examined under an electron microscope.Our results indicate that retinal circuitry is a highly interconnected network. The pattern of interconnections may not be rigidly fixed in natural photic environment; rather the weights of connections may undergo modification as a function of spatially and temporally changing visual inputs. It appears, therefore, that signal interaction in the retina may involve new principles hitherto unknown in neurophysiology.