We examined contrast, direction of motion, and concentration dependencies of the effects of GABAergic and cholinergic antagonists, and anticholinesterases on responses to movement of On—Off directionally selective (DS) ganglion cells of the rabbit's retina. The drugs tested were curare and hexamethonium bromide (cholinergic antagonists), physostigmine (anticholinesterase), and picrotoxin (GABAergic antagonist). They all reduced the cells' directional selectivity, while maintaining their preferred-null axis. However, cholinergic antagonists did not block directional selectivity completely even at saturating concentrations. The failure to eliminate directional selectivity was probably not due to an incomplete blockade of cholinergic receptors. In a extension of a Masland and Ames (1976) experiment, saturating concentrations of antagonists blocked the effects of exogenous acetylcholine or nicotine applied during synaptic blockade. Consequently, a noncholinergic pathway may be sufficient to account for at least some directional selectivity. This putative pathway interacts with the cholinergic pathway before spike generation, since physostigmine eliminated directional selectivity at contrasts lower than those saturating responses. This elimination apparently resulted from cholinergic-induced saturation, since reduction of contrast restored directional selectivity. Under picrotoxin, directional selectivity was lost in 33% of the cells regardless of contrast. However, 47% maintained their preferred direction despite saturating concentrations of picrotoxin, and 20% reversed the preferred and null directions. Therefore, models based solely on a GABAergic implementation of Barlow and Levick's asymmetric-inhibition model or solely on a cholinergic implementation of asymmetric-excitation models are not complete models of directional selectivity in the rabbit. We propose an alternate model for this retinal property.

1. Spontaneous and visually evoked action potentials were recorded from single retinal ganglion cells in superfused retina-eyecups prepared from cats between postnatal day 5 (P5) and adulthood. The development of functional responses was studied quantitatively with contrast-modulated spots and sinusoidal grating stimuli. Some functionally characterized cells were impaled and injected intracellularly with horseradish peroxidase to permit direct structure-function comparisons during the period of synaptogenesis and the expression of transient morphological features. 2. Around the time of eye opening at postnatal day 7-postnatal day 10, the spontaneous discharge of ganglion cells was characterized by bursts of action potentials separated by periods of no activity lasting up to tens of seconds. This "burst-type" pattern gradually changed to a more regular spontaneous discharge during the first 2-3 postnatal wk. The burst-type discharge was completely blocked by 10 mM Mg2+, but was largely unaffected by bath application the excitatory amino acid receptor antagonist 6-cyano-7-nitroquinoxaline-2,3- dione at concentrations that eliminated both spontaneous and light-driven activity of cells that had developed a regular spontaneous discharge. DL-2-amino-7-phosphonoheptanoic acid, a competitive antagonist of the N-methyl-D-aspartate (NMDA) receptor, also had little effect on the burst-type discharge but did block the depolarizing effects of exogenous NMDA. These results demonstrate that the burst-type spontaneous discharge is a calcium-dependent process but probably is not mediated by synapses utilizing receptors for excitatory amino acids. 3. On P5, a minority of ganglion cells responded to a light stimulus delivered to the retina at the electrode tip. The percentage of responsive cells increased over time so that all cells responded by P10. During this transition period, the light responses were often weak, fatigued with repeated stimulation, and displayed poor temporal resolution. There was a rapid increase in the briskness of the response to a standard light stimulus during the second through fourth postnatal weeks. However, cells that responded sluggishly to visual stimulation were observed at all ages studied. 4. Both ON- and OFF-type receptive field (RF) centers were observed in approximately equal numbers as soon as the light response developed, and the proportion of the two types was independent of age. Three percent (10/342) of visually responsive ganglion cells had ON-OFF RFs, and the incidence of this type also appeared to be invariant with age.(ABSTRACT TRUNCATED AT 400 WORDS)

Synchronous spiking has been postulated to be a meta-signal in visual cortex and other CNS loci that tags neuronal spike responses to a single entity. In retina, however, synchronized spikes have been postulated to arise via mechanisms that would largely preclude their carrying such a code. One such mechanism is gap junction coupling, in which synchronous spikes would be a by-product of lateral signal sharing. Synchronous spikes have also been postulated to arise from common-source inputs to retinal ganglion cells having overlapping receptive fields, and thus code for stimulus location in the overlap area. On-Off directionally selective ganglion cells of the rabbit retina exhibit a highly precise tiling pattern in which gap junction coupling occurs between some neighboring, same-preferred-direction cells. Depending on how correlated spikes arise, and for what purpose, one could postulate that synchronized spikes in this system (1) always arise in some subset of same-direction cells because of gap junctions, but never in non-same-preferred-directional cells; (2) never arise in same-directional cells because their receptive fields do not overlap, but arise only in different-directional cells whose receptive fields overlap, as a code for location in the overlap region; or (3) arise in a stimulus-dependent manner for both same- and different-preferred-direction cells for a function similar to that postulated for neurons in visual cortex. Simultaneous, extracellular recordings were obtained from neighboring On-Off directionally selective (DS) ganglion cells having the same and different preferred directions in an isolated rabbit retinal preparation. Stimulation by large flashing spots elicited responses from DS ganglion-cell pairs that typically showed little synchronous firing. Movement of extended bars, however, often produced synchronous spikes in cells having similar or orthogonal preferred directions. Surprisingly, correlated firing could occur for the opposite contrast polarity edges of moving stimuli when the leading edge of a sweeping bar excited the receptive field of one cell as its trailing edge stimulated another. Pharmacological manipulations showed that the spike synchronization is enhanced by excitatory cholinergic amacrine-cell inputs, and reduced by inhibitory GABAergic inputs, in a motion-specific manner. One possible interpretation is that this synchronous firing could be a signal to higher centers that the outputs of the two DS ganglion cells should be "bound" together as responding to a contour of a common object.

We recorded the responses to visual stimulation of single neurons in the A-layers of the dorsal lateral geniculate nucleus (LGNd) of 4- to 5-wk-old kittens and adult cats. Visual stimuli were generated on a cathode-ray tube (CRT) display and consisted of circular spots and annuli whose contrast was twice the threshold for each neuron and was modulated about a background luminance of 28 cd/m2 at 0.5 Hz. Neural responses were collected as interspike intervals and displayed as instantaneous firing rates for individual trials. From the responses to a series of sizes of spot stimuli, area-response functions were constructed and used to derive a quantitative measure of the strength of the receptive field (RF) surround inhibition of each neuron, the spatial density minimum ([SDmin[). To separate neural from optical factors that affect measurements of surround inhibition, published values for the posterior nodal distances of the kitten and adult eye were used to scale stimuli in terms of the retinal area subtended. Of 153 kitten and 95 adult LGNd neurons studied, the responses to a complete series of spot stimuli of different sizes (areas) were obtained for 52 kitten neurons [44 with linear spatial summation (L) and 8 with nonlinear spatial summation (NL)] and 45 adult (24 X-and 21 Y-) neurons. In addition, intracellular recordings were made from 30 of the kitten neurons that were filled iontophoretically with horseradish peroxidase (HRP) and were evaluated structurally. In the adult, neurons were classified as X-or Y-cells on the basis of a battery of physiological properties, including linearity of spatial summation, latency to electrical stimulation of the optic chiasm, and ability to respond reliably to rapidly moving stimuli. Kitten neuronal responses allowed them to be clearly identified as exhibiting linear or nonlinear spatial summation, but application of additional criteria produced ambiguous results for classification into X-or Y-categories. Kitten L or NL neurons showed differences typical of adult X-and Y-cells on some [e.g., RF center size (P less than 0.01)] but not other [e.g., latency to stimulation of optic chiasm (P greater than 0.40)] properties. In addition, by direct comparison of morphological features with these physiological responses, some kitten cells with adult X-cell physiological properties on these tests were found to have typical adult Y-cell somadendritic structure.(ABSTRACT TRUNCATED AT 400 WORDS)

Purpose Alabama Medicaid reimburses “objective” vision screening (VS), i.e, by acuity or similar quantitative method, and well child checks (WCCs) separately. We analyzed the frequency each service was obtained. Methods Claims for WCC and VS provided between 10/1/2002 and 9/30/2003 for children ages 3-18 years, and summary data for all enrolled children, were obtained from Alabama Medicaid. We used univariate analysis followed by logistic regression to explore the potential influence of factors (patient age, provider type, and provider’s volume of WCCs) on the receipt of VS at preschool ages. Results Children receiving WCCs were 55% African-American, 40% white, and 5% other. Percentages of children with WCC claims were highest at 4 years (57%) and thereafter declined to 30% at 6-14 years and to less than 10% at 18 years. Nearly all VS (>98% at each age) occurred the same day as the WCC. Pediatricians provided 68% of all WCCs. Multivariate analysis, after adjusting for nesting of preschool patients within provider, showed the odds of VS were increased by patient age (5 years vs. 3 years OR=3.57, p<0.0001), non-physician provider type (non-physician vs. pediatrician OR=1.80, p=0.0004) and high WCC volume (at or above vs. below the median number (N=8) of WCC per provider per year (OR=7.11, p<0.0001)). Because VS rates were high when attendance to WCC visits was low, few enrolled children received vision screening at any age (6% at age 3, 13% at age 4, and a maximum of 20% at age 5). Conclusions National efforts to reduce preventable vision loss from amblyopia are hampered because children are not available for screening and because providers miss many opportunities to screen vision at preschool age. Efforts to improve vision screening should target pediatrician-led practices, since these serve greater numbers of children.

The image on the retina is almost never static. Eye, head, and body movements, and externally generated motion create rapid and continual changes in the retinal image ("active vision"). Virtually all vision in animals such as primates, which make saccades as often as 3-4 times/s, is based on information that must be derived from the first few hundred milliseconds after sudden, global changes in the retinal image. These changes may be accompanied by large changes in area mean luminance, as well as higher order image contrast statistics. This study investigated how retinal ganglion cell responses, whose response properties have been typically studied and defined in a stable stimulus regime, are affected by sudden changes in mean luminance that are characteristic of active vision. Specifically, the steady-state responses of retinal ganglion cells to static or moving square-wave grating stimuli were recorded in an isolated, superfused rabbit eyecup preparation and compared to responses after saccade-like changes in luminance. The manner of coding after luminance changes was different for different ganglion cell classes; both suppression and enhancement of responses to patterns following luminance changes were found. Brisk-transient Off cells unambiguously signaled the darkening of the overall image, but were also modulated by the subsequently appearing grating stimulus. Several types of On-center cell behavior were observed, ranging from strong suppression of the subsequent response by luminance changes, to strong enhancement. Overall, most ganglion cells distinguished static patterns after a luminance change via differences in their spike discharges nearly as well as before, although there were clear asymmetries between the On and Off pathways. Changes in mean luminance in some ganglion cells, such as On-Off directionally selective ganglion cells, could create large phase shifts in the response to patterned, moving stimuli, although these stimuli were still detected immediately after luminance changes. The results of this study show that the image dynamics of active vision may be a fundamental challenge for the visual system because of strong effects on retinal ganglion cell function. However, rapid extraction of unambiguous information after luminance changes appears to be encoded in differences in the spike discharges in different retinal ganglion cell classes. Asymmetries among ganglion cell classes in sensitivity to luminance changes may provide a basis by which some provide the "context" for interpreting the firing of others.

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The sensitivity to spatial contrast patterns of single retinal ganglion cell axons and neurons in the A-layers of the dorsal lateral geniculate nucleus (LGNd) was measured in 4 1/2- and 6 1/2-week-old kittens and adult cats. Drifting sinusoidal grating stimuli were presented at 6-12 spatial frequencies to obtain spatial contrast sensitivity functions (SCSFs). The SCSFs were normalized for the postnatal growth of the kitten eye and were interpreted using a difference of Gaussians model of the receptive field (RF). The average optimal spatial frequency, spatial frequency bandwidth, and the proportion of cells that were selective for spatial frequency did not differ significantly between the kittens and adults for ganglion cells belonging to the cluster 1 (X-) or cluster 2 (Y-) functional types. The spatial resolution of kitten ganglion cells was also adultlike, except for that of Y-ganglion cells with peripheral RFs, which was significantly higher than in the adult. The spatial resolution of X-LGNd neurons with peripheral RFs was significantly poorer at 4 1/2 weeks than in the older animals. The proportion of X-LGNd neurons that were selective for spatial frequency increased between 4 1/2 and 6 1/2 weeks postnatally, but the spatial frequency bandwidth of selective cells did not change. The increased proportion of spatially selective LGNd neurons is probably due to the maturation of intrageniculate inhibitory circuits. Developmental changes in spatial resolution were interpreted as resulting from an increase (Y-retinal ganglion cells) or decrease (X-LGNd neurons) in RF center size. A model of retinogeniculate development is presented that attributes postnatal expansion of Y-retinal ganglion cell RF centers to increased functional convergence from more distal retinal neurons and reduction in LGNdX-cell RF center size to decreased convergence from X-retinal ganglion cells.