Intracellular recordings were obtained from 73 cone-driven bipolar cells in the light-adapted retina of the tiger salamander (Ambystoma tigrinum). Responses to flashes of negative and positive contrast for centered spots and concentric annuli of optimum spatial dimensions were analyzed as a function of contrast magnitude. For both depolarizing and hyperpolarizing bipolar cells, it was found that remarkably similar responses were observed for the center and surround when comparisons were made between responses of the same response polarity and thus, responses to opposite contrast polarity. Thus, spatial information and contrast polarity appear to be rather strongly confounded in many bipolar cells. As a rule, the form of the contrast/response curves for center and surround approximated mirror images of each other. Contrast gain and C50 (the contrast required for half-maximal response) were quantitatively similar for center and surround when comparisons were made for responses of the same response polarity. The average contrast gain of the bipolar cell surround was 3–5 times higher than that measured for horizontal cells. Contrast/latency measurements and interactions between flashed spots and annuli showed that the surround response is delayed by 20–80 ms with respect to that of the receptive-field center. Cones showed no evidence for center-surround antagonism while for bipolar cells, the average strength of the surround ranged from about 50% to 155% of the center, depending on the test and response polarity. The results of experiments on the effects of APB (100 μM) on depolarizing bipolar cells suggest that the relative contribution of the feedback pathway (horizontal cell to cones) and the feedforward pathway (horizontal cell to bipolar cell) to the bipolar surround varies in a distributed manner across the bipolar cell population.

The spectral mechanisms of the ferret (Mustela putorious furo) were studied with electroretinogram (ERG) flicker photometry. Variations in adaptation state and flicker rate were used to define corneally based spectral sensitivities for the three classes of receptor present in the retina of this mustelid—rods (λmax = 505 nm), S cones (430 nm), and L cones (558 nm). The retinal distributions of the two classes of cone were determined using opsin antibody labeling. Ferret retinas contain a total of about 1.3 million cones with L cones outnumbering S cones in a ratio of approximately 14:1. ERGs were also recorded using 18.75-Hz flickering stimuli that were designed to isolate signals from individual cone classes. The contrast/response functions for signals originating from both S and L cones were linear over low-to-moderate levels of contrast, but with greatly different slopes for the two cone types. The L:S contrast gain ratio derived from a comparison of these slopes, as well as inferences drawn from another experiment in which responses to various combinations of L- and S-cone activation were analyzed, suggest that contributions of these two cone types to the flicker ERG have a relative weighting of about 4:1 to 5:1 (L/S).

To investigate the influence of voltage-sensitive conductances in shaping light-evoked responses of retinal bipolar cells, whole-cell recordings were made in the slice preparation of the tiger salamander, Ambystoma tigrinum. To study contrast encoding, the retina was stimulated with 0.5-s steps of negative and positive contrasts of variable magnitude. In the main, responses recorded under voltage- and current-clamp modes were remarkably similar. In general agreement with past results in the intact retina, the contrast/response curves were relatively steep for small contrasts, thus showing high contrast gain; the dynamic range was narrow, and responses tended to saturate at relatively small contrasts. For ON and OFF cells, linear regression analysis showed that the current response accounted for 83–93% of the variance of the voltage response. Analysis of specific parameters of the contrast/response curve showed that contrast gain was marginally higher for voltage than current in three of four cases, while no significant differences were found for half-maximal contrast (C50), dynamic range, or contrast dominance. In sum, the overall similarity between current and voltage responses indicates that voltage-sensitive conductances do not play a major role in determining the shape of the bipolar cell's contrast response in the light-adapted retina. The salient characteristics of the contrast response of bipolars apparently arise between the level of the cone voltage response and the postsynaptic current of bipolar cells, via the transformation between cone voltage and transmitter release and/or via the interaction between the neurotransmitter glutamate and its postsynaptic receptors on bipolar cells.

Light adaptation in rod photoreceptors is thought to involve down-regulation of the signaling activity of photoactivated rhodopsin (R*). However, electrophysiological evidence in support of this notion has come largely from studies of truncated, perfused rod outer segments and of rods genetically engineered to perturb known steps in R* deactivation. To test this hypothesis within intact native rods, we examined the effect of a fixed conditioning flash on rods prepared to contain 9-demethyl rhodopsin (9dR) in addition to residual rhodopsin. 9dR, an opsin-based photopigment containing 11-cis 9-demethylretinal as its chromophore, exhibits a blue-shifted excitation spectrum and sluggish deactivation kinetics, properties that distinguish the signaling activities of photoactivated 9dR (9dR*) from those of R*. Saturating photocurrent responses mediated preferentially by R* and 9dR* were obtained with test flash stimulation at 640 and 440 nm, respectively, under dark-adapted conditions (unconditioned response) and at a fixed time after a 640-nm conditioning flash of fixed high intensity. At each test wavelength, the decrease in photocurrent saturation period induced by the conditioning flash was analyzed to determine ψ, the sensitivity of the conditioned response relative that of the unconditioned response; ψ640 /ψ440, the ratio of relative sensitivities, was then obtained. Data obtained from 12 rods yielded ψ640 /ψ440 = 0.60 ± 0.13 (mean ± SD). As common pools of transducin and other downstream components mediate transduction initiated by both R* and 9dR*, the finding that ψ640 < ψ440 provides direct evidence for the down-regulation specifically of R*'s signaling activity during rod light adaptation.

Synaptotagmin I is the leading candidate for the calcium sensor that triggers exocytosis at conventional synapses. However, physiological characterization of the calcium sensor for phasic release at the ribbon-style synapses of the goldfish Mb1 bipolar cell demonstrates a lower than predicted affinity for calcium, suggesting that a modified or different sensor triggers exocytosis at this synapse. We examined synaptotagmin immunolabeling in goldfish retina using two different antibodies directed against synaptotagmin epitopes that specifically labeled the expected 65-kDa protein on western blots of goldfish and mouse retinal membranes. The first antiserum strongly labeled conventional synapses in the inner plexiform layer (IPL), but did not label the ribbon-style synapse-containing synaptic terminals of goldfish Mb1 bipolar cells or photoreceptors. The second antibody also specifically labeled the expected 65-kDa protein on western blots but did not label any synapses in the goldfish retina. A third synaptotagmin antibody that performed poorly on western blots selectively labeled goldfish photoreceptor terminals. These results suggest that synaptotagmin may exist in at least three distinct “forms” in goldfish retinal synapses. These forms, which are differentially localized to conventional synapses, bipolar cell, and photoreceptor terminals, may represent differences in isoform, posttranslational modifications, epitope availability, and protein-binding partners. Labeling with these antibodies in the salamander and mouse retina revealed species-specific differences, indicating that synaptotagmin epitopes can vary across species as well as among synapses.

We quantified and compared the effect of element spacing on contour integration between the achromatic (Ach), red–green (RG), and blue–yellow (BY) mechanisms. The task requires the linking of orientation across space to detect a contour in a stimulus composed of randomly oriented Gabor elements (1.5 cpd, σ = 0.17 deg), measured using a temporal 2AFC method. A contour of ten elements was pasted into a 10 × 10 cells array, and background elements were randomly positioned within the available cells. The effect of element spacing was investigated by varying the mean interelement distance between two and six times the period of the Gabor elements (λ = 0.66 deg) while the total number of elements was fixed. Contour detection was measured as a function of its curvature for jagged contours and for closed contours. At all curvatures, we found that performance for chromatic mechanisms declines more steeply with the increase in element separation than does performance for the achromatic mechanism. Averaged critical element separations were 4.6 ± 0.7, 3.6 ± 0.4, and 2.9 ± 0.2 deg for Ach, BY, and RG mechanisms, respectively. These results suggest that contour integration by the chromatic mechanisms relies more on short-range interactions in comparison to the achromatic mechanism. In a further experiment, we looked at the combined effect of element size and element separation in contour integration for the Ach mechanism.

The ability of the visual system to detect stimuli that vary along dimensions other than luminance or color— “second-order” stimuli—has been of considerable interest in recent years. An important unresolved issue is whether different types of second-order stimuli are detected by a single, all purpose, mechanism, or by mechanisms that are specific to stimulus type. Using a conventional psychophysical paradigm, we show that for a class of second-order stimuli—textures sinusoidally modulated in orientation (OM), spatial frequency (FM), and contrast (CM)—the human visual system employs mechanisms that are selective to stimulus type. Whereas the addition of a subthreshold mask to a test pattern of the same stimulus type was found to facilitate the detection of the test, no facilitation was observed when mask and test were of different types, suggesting mechanism independence for the different types of stimulus. This finding raises the important question of whether mechanism independence is compatible with the well-known filter-rectify-filter (FRF) model of second-order stimulus detection, since FRF mechanisms, in principle, do not discriminate between stimulus types. We show that for all mask/test combinations except those with CM masks, the FRF mechanism giving the largest response to the test modulation is largely unaffected by subthreshold levels of a different stimulus-type mask. For this reason, we cannot rule out the possibility that FRF mechanisms mediate the detection of our stimuli. For combinations involving CM masks, however, we propose that a process of contrast normalization renders the test stimulus insensitive to the mask stimulus.

Relative motion information, especially relative speed between different input patterns, is required for solving many complex tasks of the visual system, such as depth perception by motion parallax and motion-induced figure/ground segmentation. However, little is known about the neural substrate for processing relative speed information. To explore the neural mechanisms for relative speed, we recorded single-unit responses to relative motion in the primary visual cortex (area V1) of rhesus monkeys while presenting sets of random-dot arrays moving at different speeds. We found that most V1 neurons were sensitive to the existence of a discontinuity in speed, that is, they showed higher responses when relative motion was presented compared to homogenous field motion. Seventy percent of the neurons in our sample responded predominantly to relative rather than to absolute speed. Relative speed tuning curves were similar at different center–surround velocity combinations. These relative motion-sensitive neurons in macaque area V1 probably contribute to figure/ground segmentation and motion discontinuity detection.

A stimulus located outside the classic receptive field (CRF) of a striate cortical neuron can markedly influence its behavior. To study this phenomenon, we recorded from two cortical sites, recorded and peripheral, with separate electrodes in cats anesthetized with Propofol and nitrous oxide. The receptive fields of each site were discrete (2–7.3 deg between centers). A control orientation tuning (OT) curve was measured for a single recorded cell with a drifting grating. The OT curve was then remeasured while stimulating simultaneously the cell's CRF as well as the peripheral site with a stimulus optimized for that location. For 22/60 cells, the peripheral stimulus suppressed the peak response and/or shifted the center of mass of the OT curve. For 19 of these 22 cells, we then reversibly blocked stimulus-driven activity at the peripheral site by iontophoretic application of GABA (0.5 M). For 6/19 cells, the response returned to control levels, implying that for these cells the inhibitory influence arose from the blocked site. The responses of nine cells remained reduced during inactivation of the peripheral site, suggesting that influence was generated outside the region of local block in area 17. This is consistent with earlier findings suggesting that modulatory influences can originate from higher cortical areas. Three cells had mixed results, suggesting multiple origins of influence. The response of each cell returned to suppressed levels after dissipation of the GABA and returned to baseline values when the peripheral stimulus was removed. These findings support a cortical model in which a cell's response is modulated by an inhibitory network originating from beyond the receptive field that supplants convergence of excitatory lateral geniculate neurons.