SUMMARY1. Intracellular responses were recorded from on-centre and off-centre ganglion cells in isolated eyecups of the mudpuppy, Necturus maculosus.2. Current-voltage relations were measured in darkness, during illumination of the receptive field centre, and after chemically mediated synaptic inputs were blocked by 4 mM-cobalt chloride.3. In on-centre cells the membrane potential in darkness was -56 + 6 mV (mean+ S.D.). Addition of Co2+ resulted in an average depolarization of 10 mV and an average decrease in conductance of 2-1 nS. These results suggest that in darkness on-centre cells are tonically inhibited by synaptic input which increases conductance and has a reversal potential more negative than the dark membrane potential.In off-centre cells the membrane potential in darkness was -46 + 5 mV. Addition of Co2+ caused an average hyperpolarization of 6 mV and an average decrease in conductance of 1-5 nS. These results suggest that in darkness off-centre cells receive a tonic excitatory input which increases conductance and has a reversal potential more positive than the dark membrane potential.4. In on-centre cells light causes a sustained depolarization. This response involves an increase in a tonic excitatory input which increases conductance and has a reversal potential more positive than the dark membrane potential.5. In off-centre cells, light causes a sustained hyperpolarization. This response involves an increase in a sustained inhibitory input which increases conductance and has a reversal potential more negative than the dark membrane potential.6. The depolarizing off-response of off-centre cells is associated with an increase in an excitatory input which increases conductance and has a reversal potential more positive than the dark membrane potential. This response may be due to a temporary increase in the excitatory input which is tonically active in darkness or may reflect an additional excitatory input.7. It is suggested that in both on-and off-centre ganglion cells the balance of sustained excitatory and inhibitory synaptic inputs determines the resting potential * Present address:

SUMMARY1. Transient and sustained inhibitory synaptic inputs to on-centre, off-centre, and on-offganglion cells in the mudpuppy retina were studied using intracellular recording in the superfused eye-cup preparation.2. When chemical transmission was blocked with 4 mM-Co2+, application of either glycine or y-aminobutyric acid (GABA) caused a hyperpolarization and conductance increase in all ganglion cells. For both amino acids, the responses were dose dependent in the range 0-05-10 mm, with a half-maximal response at about 0-7 mM. Glycine and GABA sensitivities were very similar in all three types of ganglion cells.3. The response to applied glycine was selectively antagonized by 10-5 M-strychnine and the response to applied GABA was selectively antagonized by 10-5 M-picrotoxin.4. In all ganglion cells, 10-5 M-strychnine eliminated the transient inhibitory events which occur at the onset and termination of a light stimulus. The block of transient inhibition was associated with a relative depolarization of membrane potential and decrease in conductance at these times. Strychnine had no effect on membrane potential or conductance in darkness or during sustained inhibitory responses to light.5. Picrotoxin (10-5 M) did not block transient inhibitory events in any ganglion cells, but did affect other components of their responses.6. The results suggest that in all three classes of ganglion cells transient inhibition, but not sustained inhibition, may be mediated by glycine or a closely related substance.

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1. Spatial processing of visual signals in the fly's movement-detecting pathway was studied by recording the responses of directionally-selective movement-detecting (DSMD) neurons in the lobula plate. The summarized results pertain to a type of neuron which preferentially responds to horizontal movement directed toward the animal's midline. Three kinds of visual stimuli were used: moving gratings, reversing-contrast gratings and reversing-contrast bars. 2. Contrast-sensitivity functions were measured for reversing-contrast gratings. With horizontally-oriented gratings, sensitivity is maximum at the lowspatial-frequency end and falls off toward high frequencies. With vertically-oriented gratings, sensitivity is maximum at an intermediate spatial frequency (Fig. 7). These results are consistent with a neural organization in which the DSMD neuron receives its input through an array of small-field (" sampling") units, each unit having a receptive field comprising an excitatory centre and horizontally-extending inhibitory flanks (Fig. 17). 3. Threshold contrast functions were measured for reversing-contrast bars (Figs. 11 and 12). The results for horizontally-oriented bars differ from those for vertically-oriented bars in a way that is consistent with the hypothesized neural organization. 4. Response to horizontally-moving, verticallyoriented gratings of various spatial frequencies were measured (Figs. 13 and 14) and the results used to infer the azimuthal angle A~ between the visual axes of sampling units participating in directionally-selectire movement detection (Fig. 18). At a mean luminance of 10 cd/m e, the inferred value of A'~ is approximately equal to the angle between the visual axes of