Intensity coding in the frog retina. Quantitative relations between impulse and graded activity

Zebrafish inner retina: local signals for spatial position, luminance, and color contrast

The retina of the zebrafish (Danio rerio) provides an unusually favorable preparation for genetic and developmental studies of the retina. Although the retina has been studied extensively for two decades, the neuronal response of the inner retina is largely unknown. This report describes a prominent local field potential of the inner retina, the Proximal Negative Response (PNR). It is best evoked by small (100 μm) precisely positioned spots of light and is exceedingly sensitive to negative luminance contrast. The polarity, waveform, and other properties of the PNR suggest that it arises primarily from ON-OFF neurons of the proximal retina. The dominant response to negative contrast and its enhancement by light adaptation is believed due to a dominant presynaptic input from OFF bipolar cells. Color contrast was investigated by analyzing responses to a green bar moving on green versus red backgrounds. Over an intermediate range of irradiance, the response to green on red was larger than the response to green on green, thereby providing evidence for the encoding of color contrast. The present findings complement the classic principle of color contrast for human vision known as Kirschmann's third law and bring to mind the view of Walls that color contrast may have been the driving force for the evolution of color vision in lower vertebrates. In sum, the PNR of zebrafish provides clear evidence for the encoding of color and luminance contrast in the inner retina. It exhibits the defining properties common to many other vertebrates, reinforcing the view that the zebrafish may further serve as a model for retinal function and that the PNR may provide a new approach for studies of development, genetics, and retinal degeneration in zebrafish.

Basal Ganglia preferentially encode context dependent choice in a two-armed bandit task

Decision is a self-generated phenomenon, which is hard to track with standard time averaging methods, such as peri-event time histograms (PETHs), used in behaving animals. Reasons include variability in duration of events within a task and uneven reaction time of animals. We have developed a temporal normalization method where PETHs were juxtaposed all along task events and compared between neurons. We applied this method to neurons recorded in striatum and GPi of behaving monkeys involved in a choice task. We observed a significantly higher homogeneity of neuron activity profile distributions in GPi than in striatum. Focusing on the period of the task during which the decision was taken, we showed that approximately one quarter of all recorded neurons exhibited tuning functions. These so-called coding neurons had average firing rates that varied as a function of the value of both presented cues, a combination here referred to as context, and/or value of the chosen cue. The tuning functions were used to build a simple maximum likelihood estimation model, which revealed that (i) GPi neurons are more efficient at encoding both choice and context than striatal neurons and (ii) context prediction rates were higher than those for choice. Furthermore, the mutual information between choice or context values and decision period average firing rate was higher in GPi than in striatum. Considered together, these results suggest a convergence process of the global information flow between striatum and GPi, preferentially involving context encoding, which could be used by the network to perform decision-making.

Inhibition in the Limulus lateral eye in situ

Inhibition in the Limulus lateral eye in situ is qualitatively similar to that in the excised eye. In both preparations ommatidia mutually inhibit one another, and the magnitude of the inhibitory effects are linear functions of the response rate of individual ommatidia. The strength of inhibition exerted between single ommatidia is also about the same for both preparations; however, stronger effects can converge on a single ommatidium in situ. At high levels of illumination of the retina in situ the inhibitory effects are often strong enough to produce sustained oscillations in the discharge of optic nerve fibers. The weaker inhibitory influences at low levels of illumination do not produce oscillations but decrease the variance of the optic nerve discharge. Thresholds for the inhibitory effects appear to be determined by both presynaptic and postsynaptic cellular processes. Our results are consistent with the idea that a single ommatidium can be inhibited by more of its neighbors in an eye in situ than in an excised eye. Leaving intact the blood supply to the eye appears to preserve the functional integrity of the retinal pathways which mediate inhibition.

GABA-mediated control of transient signals in the inner retina

The effects of chemicals on visual activity was investigated by recording the proximal negative response (PNR) in the perfused eyecup of frogs. Various amino acids reversibly decrease or abolish the PNR over an equivalent concentration range but differ in this depressive action in the presence of convulsant alkaloids. Depression produced by GABA was antagonized by picrotoxin or bicuculline; while glycine-produced depression was selectively antagonized by strychnine. In addition to selective antagonism of amino acids, the convulsants produced differential enhancement of sustained and transient phases of the PNR. In this respect, the effects of picrotoxin and bicuculline are similar to those of semicarbazide and chloride-free solutions which may also inactivate GABA pathways. The effects of these chemicals on the time course and input-output relation of the PNR suggest that processes involving GABA may regulate the transformation from sustained to transient signals which occurs in the inner retina.

A comparison of the proximal negative response and ganglion cell responses to sinusoidal flicker

In the mudpuppy retina, sinusoidal light stimulation of the proximal negative response (PNR) demonstrates two main components whose phases are essentially the same as those of the spike discharges of ON/OFF ganglion cells. Oscillations superimposed upon these components are synchronized with the spike discharges of ON/OFF cells, but not of ON or OFF cells. Intracellular recordings from ON/OFF ganglion cells reveal slow potentials with nearly identical waveforms and phases to those of the PNR. These results, together with previously published comparisons between the flesh-evoked PNR and amacrine cell responses, support the suggestion that the PNR arises from activity in the on/off system (amacrine and/or ganglion cells).

The proximal negative response and visual adaptation in the skate retina

The proximal negative response (PNR), a complex extracellular potential derived mainly from amacrine cell activity, was studied in the all-rod retina of the skate. Tetrodotoxin (10(-6) mg/ml) did not affect either the waveform or the latency of the response, indicating that the PNR reflects the graded, nonregenerative components of the amacrine cell potential. As regards its adaptive properties, the PNR exhibited both the extreme sensitivity to weak background light and the slow time course of light and dark adaptation that are characteristic of other responses from the proximal retina. Thus, the PNR, like the b-wave and ganglion cell discharge, appears to reflect adaptive processes located within the neural network of the inner retina.

Receptive field organization of ganglion cells in the frog retina: contributions from cones, green rods and red rods

1. The impulse discharge of ganglion cells was recorded with extracellular micro-electrodes in the excised and opened eye of the common frog, Rana temporaria. 2. When a single unit was isolated, the cell type was first determined according to the Maturana, Lettvin, McCulloch & Pitts (1960) classification with the aid of varying moving and stationary stimuli. 3. Class 4 cells respond only to a decrease of light when cones are stimulated but respond to an increase of light when green rods are stimulated. A distinct class of deviating class 4 cells was found that give a brief high frequency burst at 'off' from their small excitatory receptive fields (ERF); unlike typical class 4 cells they possess a purely inhibitory surrounding field (IRF).4. The contributions from the cones and the green and red rods were isolated by measuring the thresholds of the discharges with on-off stimuli of varying wave-lengths against strong yellow backgrounds, or against a very weak background or no background at all. The spatial distribution of the contributions to the ERF was determined by mapping threshold profiles, and additional information about ERF and IRF was obtained from area-threshold curves. 5. The cone-mediated ERFs were found to be 0-06-0-50 mm wide (1-5-12 degrees of visual field), which agrees well with the sizes of the dendritic trees of the ganglion cells. The green rod-mediated ERFs can be 0-5-1-5 mm wide and have less distinct boundaries than the cone-mediated. The green rod-mediated ERF of an individual ganglion cell is always larger than the cone-mediated ERF of the same cell. The red rod-mediated ERFs seem to be somewhat larger than the cone-mediated but smaller than the green rod-mediated. 6. The green rods contribute only to the on thresholds of class 1, 2 and 4 cells, but both to on and off in typical class 3 cells, while the cones contribute to on and off in classes 1-3 and only to off in class 4.7. When the red rods begin to contribute during dark adaptation they seem to enter the cone but not the green rod channels. 8. All three receptor types contribute to the IRF surrounding the ERF of classes 1, 2, 3 and deviating class 4 cells. Normal class 4 cells have no IRF. 9. The organization of the receptive fields is discussed in relation to the anatomy and electrophysiology of the cell types transmitting the signals from the receptors to the ganglion cells.

Sensitization and centre-surround antagonism in Necturus retina

1. The impulse discharge of ganglion cells and the proximal negative response were recorded with extracellular micro-electrodes in Necturus retina and spatial influences of background illumination studied.2. When a steady background of constant illuminance was extended in diameter from 0.25 to 1 mm, the response to a small concentrically placed test flash was enhanced (sensitization).3. This sensitization was tonic, showed marked spatial summation, was also found by surrounding a central background with an annulus, and when quantified by threshold measurements, closely resembled the sensitization effect found psychophysically in human vision.4. Sensitization was prominent in intracellular recordings from bipolar cells and usually apparent in horizontal cells. In bipolar cells, a background change which produced sensitization evoked a large but predominantly transient potential whose polarity was opposite to that evoked by a central flash.5. The tonicity and large spatial extent of the sensitization effect strongly suggest that it is mediated by horizontal cells. They might do this by either acting upon receptors, bipolar cells, or both.