Amacrine cells in Necturus retina: evidence for independent gamma-aminobutyric acid- and glycine-releasing neurons.

About one-half of on-off ganglion cells have inhibitory postsynaptic potentials (IPSP's) which are blocked by strychnine, while the remainder have IPSP's which are blocked by picrotoxin or bicuculline. These antagonists do not abolish light activity of the presynaptic inhibitory neuron, the amacrine cell. The existence of separate gamma-aminobutyric acid- and glycine-releasing amacrine cells is implied by these results.

Inhibitory influence of unstimulated rods in the human retina: evidence provided by examining cone flicker.

In the parafoveal retina of human observers, cone-mediated sensitivity to flicker decreases as rods become progressively more dark-adapted. This effect is greatest when a rod response to flicker is precluded. These results indicate that rods tonically inhibit cone pathways in the dark.

Rod-cone interaction in human scotopic vision.

Thresholds of a test flash were measured at various time intervals from onset of a conditioning flash under parafoveal scotopic conditions; rods or cones were selectively stimulated by utilizing either 420- or 680-nanometer light. Rod-cone interaction was indicated because conditioning flash presentation increased test threshold above control level for heterochromatic as well as for homochromatic stimulus pairs. The time course of these t.. reshold changes indicates that the rod system has a longer latency than the cone system.

Tonic interocular suppression, binocular summation, and the visual evoked potential.

PURPOSE: Psychophysical studies have shown that a dark-adapted eye exerts a tonic interocular suppression (TIS) upon spatial vision mediated by the contralateral eye. The present study was designed to demonstrate TIS by means of visual evoked potential (VEP) procedures. METHODS: Evoked cortical potentials were obtained in response to reversing checkerboard patterns with fundamental Fourier frequencies between 3 and 12 cycles per degree. Responses were obtained under monocular viewing conditions when the contralateral "adapting" eye was dark adapted, under monocular viewing conditions when the adapted state of the adapting eye was experimentally manipulated, or under binocular viewing conditions. Data were collected from three healthy young men, two native regarding purpose of experimentation. RESULTS: Regardless of spatial frequency, monocular responses evoked by stimulating a "test eye" were always smaller in amplitude when the contralateral adapting eye was dark adapted than when adapted to a dim, homogeneous field. The monocular evoked response obtained in the presence of an interocular adapting field was similar in amplitude to the binocular evoked response. During dark adaptation of the contralateral adapting eye, the amplitude of the monocular evoked response decreased: the time course of this decline follows that of psychophysically measured rod thresholds in the directly adapted eye. CONCLUSIONS: TIS is easily demonstrated by means of VEP as well as psychophysical procedures. The well-known increase in VEP amplitude resulting from binocular viewing may be attributable to the removal of TIS rather than to "physiologic, binocular summation."

Pathways and polarities of synaptic interactions in the inner retina of the mudpuppy: II. Insight revealed by an analysis of latency and threshold.

Intracellular recording experiments in the mudpuppy (Necturus maculosus) have demonstrated that depolarizing bipolars show a one-half to one log unit higher threshold to light stimuli and a longer latency when compared to hyperpolarizing bipolars. This threshold difference cannot be ascribed to differences in rod and cone connections. Experimentally it was possible to use these differences to evaluate postbipolar cell connections; such experiments support the idea that some neurons are connected to one or the other bipolar cell type, while on-off cells receive input from both.

Pathways and polarities of synaptic interactions in the inner retina of the mudpuppy: I. Synaptic blocking studies.

Intracellular recording techniques were applied to the perfused retina-eyecup of the mudpuppy (Necturus maculosus). The use of input resistance measurements and the application of cobalt as a synaptic blocking agent provided evidence about the mode of synaptic operation for depolarizing and hyperpolarizing bipolar cell input to cells of the inner retina. In addition, cobalt produced a more rapid block of hyperpolarizing bipolars compared to depolarizing bipolars; in some postbipolar neurons, a temporary simplification of neuronal response waveform was observed and such cells resembled the depolarizing bipolar. These experiments suggest that bipolar cell input is excitatory and that response characteristics of third-order neurons is partially dependent on the relative input from the two types of bipolar cells.

LEDs: convenient, inexpensive sources for visual experimentation.

This article presents the electrical and optical properties of LEDs for use as visual stimulators in neurophysiological and psychophysical research. one particular circuit is considered in detail which should enable an investigator with minimal technical expertise to build a high quality and versatile stimulator within a few hours and at a minimal cost. Improved circuits and anticipated developments in LED technology are discussed.

Frequency dependence in scotopic flicker sensitivity.

Sensitivity to rod-mediated (scotopic) flicker was parametrically studied in the parafoveal retina of human observers. Confirming prior studies, the present results show that sensitivity to scotopic flicker has many similarities to that at photopic levels. Specifically, our results show that the frequency response function for scotopic flicker is characterized by both low- and high-frequency cutoffs and that sensitivity to low frequencies is described by Weber's law. Overall, however, scotopic flicker sensitivity is characterized by higher increment thresholds and lower frequency tuning than photopic flicker. The influences of spatial factors and the prevailing level of illuminance on sensitivity is sufficiently different for relatively low (less than 3 Hz) and relatively high (greater than 5 Hz) temporal frequencies to suggest that they may be mediated by different channels. This possibility is also suggested by selective adaptation experiments. These show that adaptation to flicker frequencies of 3, 5, and 7 Hz have a similar influence on sensitivity to subsequent flicker which is different from the influence on 1 Hz flicker adaptation. Results are compared with prior evidence for channeling within both the scotopic and photopic visual systems.

Stimulation of rods can increase cone flicker ERGs in man.

Recent psychophysical studies in man and electrophysiological studies in lower vertebrates show that dark adapted, unstimulated rods inhibit cone mediated flicker. This investigation uses comparable psychophysical and ERG procedures in man to demonstrate rod-cone interaction of this type. With either procedure the rod cone interaction cannot be demonstrated with Ganzfeld stimulation. A single small, red, flickering test field, which is a common psychophysical stimulus for testing rod-cone interaction, elicits an immeasurably small cone ERG. But an array of many such targets, flickering synchronously, is an effective psychophysical stimulus and produces an ERG with larger cone than rod components. With such an array, it can be shown that a steady, rod-stimulating background selectively enhances cone ERG components.

Rod light and dark adaptation influence cone-mediated spatial acuity.

The influence of rod light and dark adaptation upon cone mediated spatial acuity was studied in the near parafoveal retina of normal human observers. The luminance just necessary to detect squarewave test gratings of variable frequency provided an index of spatial acuity. Such thresholds were determined in the presence of background fields which were varied in luminance, shape, and size, or throughout the time period of dark adaptation. Spectral controls determined the type of photoreceptors influenced by all stimuli. Cone mediated spatial acuity is improved by presenting background fields too dim to directly affect cones, and is increasingly suppressed during the rod recovery stage of dark adaptation. These effects are small with spatial frequencies less than 4 c/deg but increase with spatial frequency to greater than 1 log10 unit with the highest spatial frequency examined, 21 c/deg. These influences upon cone mediated spatial vision reflect the state of long-term adaptation of rods in a large annular area surrounding the locus to which the test grating is presented. Our results emphasize the differing influences of long-term dark adaptation and prevailing luminance level upon visual acuity. Ironically, spatial acuity is optimized under dim light conditions by selectively light adapting the receptors most sensitive to feeble stimuli, the rods.

The influence of cone adaptation upon rod mediated flicker.

The influence of annular fields on sensitivity to sinusoidal flicker was assessed in the dark adapted parafoveal retina. Test stimuli were 2 degrees 20' in diameter; annuli had a 2 degrees 20' inner and 7 degrees 30' outer diameter. Rod flicker was studied with a "green" stimulus too dim to influence cones. Selective cone flicker was obtained using red and green flicker in counterphase and yoked together in modulation depth and scotopic illuminance. Results showed the following. (1) Annular stimulation of rods slightly facilitated rod-mediated flicker sensitivity to frequencies less than 10 Hz. In contrast, annular stimulation of cones greatly facilitated rod-mediated sensitivity, particularly for flicker frequencies greater than 7 Hz. We designate this effect, cone-rod interaction. (2) Annular stimulation of cones has a negligible influence upon sensitivity to cone-mediated flicker frequencies less than 15 Hz. In contrast, annular stimulation of rods has a large influence upon sensitivity to cone-mediated flicker, an effect we designate rod-cone interaction. (3) Within limits, both rod-cone and cone-rod interaction increase as the annular illuminance increases and as flicker frequency increases; the limiting frequency and illuminance values, however, are different for the two forms of interaction. Results are compared with prior evidence that rod and cone signals summate to produce an absolute threshold or flicker sensation. We suggest that there are at least three mechanisms for interaction between rod- and cone-related signals.

The cellular basis for suppressive rod-cone interaction.

The response to spatially focal flicker is enhanced by dim, spatially diffuse, rod-stimulating backgrounds. This effect is called suppressive rod-cone interaction (SRCI) as it reflects a tonic, suppressive influence of dark-adapted rods upon cone pathways which is removed by selective rod-light adaptation. SRCI is observed in amphibian retina with intracellular recordings from most cone-driven cells including the cones themselves, and is most obvious using stimuli flickering at frequencies too rapid for rods to follow. SRCI is blocked by glutamate analogs which selectively block the photic response of horizontal cells (HCs). In the presence of these agents, flicker responses from bipolar cells and cones are enhanced to levels normally seen only with selective rod-light adaptation. In the HCs themselves, SRCI is similarly blocked by lead chloride which blocks rod-, but not cone-related activity. In amphibian and cat HCs and in human observers, SRCI is limited by a space constant of very similar value (between 100 and 150 microns). We suggest that SRCI in all three species is mediated by HCs: in amphibians, SRCI must at least partially reflect rod-modulation of HC feedback onto cones.

Suppressive rod-cone interaction in distal vertebrate retina: intracellular records from Xenopus and Necturus.

The influence of dim diffuse adapting fields upon the sensitivity to focal photic stimulation was studied by means of intracellular recording in retinal neurons of the south african clawed frog, Xenopus and the mudpuppy, Necturus. In cones and in most horizontal and bipolar cells lacking color opponency, dim diffuse backgrounds have little influence upon the response to diffuse flicker of low (less than 2 Hz) temporal frequencies; however, with small diameter test probes of higher temporal frequencies, presentation of dim backgrounds enhance the peak-to-peak amplitude of responses to sinusoidal flicker by as much as 800%. This background enhancement effect adheres to the spectral sensitivity of the green-absorbing rod photopigment, and appears to be largely independent of the influence of the adapting field upon cone photopigment or ambient membrane potential in the recorded neuron. This effect cannot be obtained with rod-driven flicker responses. We designate this background influence on flicker, suppressive rod-cone interaction (SRCI) and attribute it to a tonic suppressive (probably inhibitory) influence of rods upon cone pathways that is removed by rod light adaptation. SRCI is also observed in the response of most sustained ON and OFF ganglion cells. However, no corresponding effect occurs in rods, color-opponent second-order neurons, ON-OFF amacrine cells, or most ON-OFF ganglion cells. The spatial and temporal limitations of SRCI observed by means of intracellular recording in amphibians are very similar to those documented by means of psychophysical or electroretinogram (ERG) procedures in a wide variety of species including humans (2, 4, 11, 22, 23, 29). SRCI most probably reflects a process that is mediated by horizontal cells. The specifics of the underlying mechanism remain unclear.

Independent influences of rod adaptation on cone-mediated responses to light onset and offset in distal retinal neurons.

1. The influence of rod adaptation on cone-mediated intracellular responses of distal retinal neurons was examined in the larval tiger salamander. Rods were selectively stimulated by the use of 450-520 nm adapting stimuli too dim to appreciably influence cones. Cones were primarily stimulated with the use of deep-red stimuli (maximally sensitive to wavelengths greater than 650 nm). The qualitative properties of rod-cone interaction were assessed with the use of several different photic-stimulus paradigms. 2. Confirming results of prior studies, we showed that rod adaptation changed the time course of cone-mediated responses to the onset of square-wave light flashes in horizontal cells (HCs); rod adaptation had no similar influence in other distal retinal neurons. Rod adaptation also enhanced cone-mediated responses to rapid flicker in cones, hyperpolarizing (HPBCs) and depolarizing (DPBCs) bipolar cells, as well as HCs. 3. We also examined the influence of rod-stimulating background fields on cone-mediated responses to slow (approximately 1-Hz) flicker composed of sawteeth with a rapid onset (ramp offset) or with a rapid offset (ramp onset). Such stimulation maintained a constant state of long-term adaptation while providing cones with transient-ON or transient-OFF stimulation. 4. Rod adaptation speeds up the response of HCs to rapid onset and increases response amplitude. Rod adaptation had no reliable influence on response to rapid onset in cones or bipolar cells. 5. Rod adaptation enhanced the amplitude of responses of HCs to rapid offset without altering response time course. 6. Rod adaptation greatly enhanced the amplitude of DPBC responses to rapid offset having no reliable influence on the time course of the response. 7. Rod adaptation caused responses of HPBCs to rapid offset to become much more transient. Rod backgrounds had a similar but smaller and less reliable influence on the response of cones to rapid offset. 8. The foregoing results indicate that rod adaptation exerts a minimum of two separate influences on cone-mediated responses in distal amphibian retina. Changes at light onset must reflect the operation of a mechanism that alters response kinetics of the HC membrane. Changes at light offset reflect the operation of a separate mechanism or set of mechanisms that must act in part presynaptically to the HCs.

Influence of rod adaptation upon cone responses to light offset in humans: II. Results in an observer with exaggerated suppressive rod-cone interaction.

In normal observers, sensitivity of cones to rapid sinusoidal flicker decreases by about 0.7 log units as rods progressively dark adapt. However, Arden and Hogg (1985) described a night-vision disorder characterized by normal rod sensitivity but exaggerated suppressive rod-cone interaction (SRCI). We refer to this condition as the exaggerated SRCI syndrome (ESS). The present paper examines the influence of rod-adaptation upon cone-mediated responses to light onset and offset in an observer with ESS. Under all conditions of adaptation examined, sensitivity of cones to rapid-on waveforms is indistinguishable to that of a normal observer tested under identical circumstances; rod sensitivity is also normal. However, the sensitivity of cones to transient decreases in illumination is clearly subnormal under light-adapted conditions. This deficit in cone responsiveness to light offset becomes increasingly subnormal as rods dark adapt and, when completely dark adapted, the ESS observer is nearly blind to 1 Hz rapid-off sawtooth waveforms. These results strongly bolster previous results that suggest that suppressive rod-cone interaction is restricted to the response to transient decreases in illumination.