Natural scenes contain a variety of visual cues that facilitate boundary perception (e.g., luminance, contrast, and texture). Here we explore whether single neurons in early visual cortex can process both contrast and texture cues. We recorded neural responses in cat A18 to both illusory contours formed by abutting gratings (ICs, texture-defined) and contrast-modulated gratings (CMs, contrast-defined). We found that if a neuron responded to one of the two stimuli, it also responded to the other. These neurons signaled similar contour orientation, spatial frequency, and movement direction of the two stimuli. A given neuron also exhibited similar selectivity for spatial frequency of the fine, stationary grating components (carriers) of the stimuli. These results suggest that the cue-invariance of early cortical neurons extends to different kinds of texture or contrast cues, and might arise from a common nonlinear mechanism.

A functional role for retinal endocannabinoids has not been determined. We characterized retrograde suppression of membrane currents of goldfish cones in a retinal slice. Whole-cell recordings were obtained from cone inner segments under voltage clamp. IK(V) was elicited by a depolarizing pulse to +54 mV from a holding potential of −70 mV. A fifty-millisecond puff of saline with 70 mM KCl or Group I mGluR agonist DHPG was applied through a pipette directly at a mixed rod/cone (Mb) bipolar cell body. The amplitude of IK(V) decreased 25% compared to the pre-puff control. Retrograde suppression of IK(V) was blocked by CB1 receptor antagonist, SR141716A. The FAAH inhibitor URB597 had no effect on the suppression of IK(V), whereas nimesulide, a COX-2 inhibitor, prolonged the effects of the K+ puff 10-fold. Orlistat, a blocker of 2-AG synthesis, blocked the effect of the K+ puff. Group I mGluR activation of Gq/11 was demonstrated in that a puff with DHPG decreased IK(V) of cones by 32%, an effect blocked by SR141716A. The effect of DHPG was not blocked by the mGluR5 antagonist MPEP, indicating involvement of mGluR1. The suppressive effect of the K+ puff vanished in a Ca2+-free, 2 mM Co2+ saline. TMB-8 or ryanodine, blocked the effect of DHPG, but not that of the K+ puff, showing that calcium influx or release from intracellular stores could mediate retrograde release. We suggest that retrograde suppression of cone IK(V) is mediated by Ca2+-dependent release of 2-AG from Mb bipolar cell dendrites by separate mechanisms: (1) voltage-dependent, mimicked by the K+ puff, that may be activated by the depolarizing ON response to light; (2) voltage-independent, occurring under ambient illumination, mediated by tonic mGluR1 activation. The negative feedback of this latter mechanism could regulate tonic glutamate release from cones within narrow limits, regardless of ambient illumination.

The retina of salmonid fishes has two types of cone photoreceptors: single and double cones. At the nuclear level, these cones are distributed in a square mosaic such that the double cones form the sides of the square and the single cones occupy positions at the centre and at the corners of the square. Double cones consist of two members, one having visual pigment protein maximally sensitive to green light (RH2 opsin), the other maximally sensitive to red light (LWS opsin). Single cones can have opsins maximally sensitive to ultraviolet (UV) or blue light (SWS1 and SWS2 opsins, respectively). In Pacific salmonids, all single cones express UV opsin at hatching. Around the time of yolk sac absorption, single cones start switching opsin expression from UV to blue, in an event that proceeds from the ventral to the dorsal retina. This transformation is accompanied by a loss of single corner cones such that the large juvenile shows corner cones and UV opsin expression in the dorsal retina only. Previous research has shown that adult Pacific salmon have corner cones over large areas of retina suggesting that these cones may be regenerated and that they may express UV opsin. Here we used in-situ hybridization with cRNA probes and RT-PCR to show that: (1) all single cones in non-growth zone areas of the retina express blue opsin and (2) double cone opsin expression alternates around the square mosaic unit. Our results indicate that single cone driven UV sensitivity in adult salmon must emanate from stimulation of growth zone areas.

Humans discriminate approaching objects from receding ones shortly after birth, and optic flow associated with self-motion may activate distinctive brain networks, including the human MT+ complex. We sought evidence for evoked brain activity that distinguished radial motion from other optic flow patterns, such as translation or rotation by recording steady-state visual evoked potentials (ssVEPs), in both adults and 4–6 month-old infants to direction-reversing optic flow patterns. In adults, radial flow evoked distinctive brain responses in both the time and frequency domains. Differences between expansion/contraction and both translation and rotation were especially strong in lateral channels (PO7 and PO8), and there was an asymmetry between responses to expansion and contraction. In contrast, infants' evoked response waveforms to all flow types were equivalent, and showed no evidence of the expansion/contraction asymmetry. Infants' responses were largest and most reliable for the translation patterns in which all dots moved in the same direction. This pattern of response is consistent with an account in which motion processing systems detecting locally uniform motion develop earlier than do systems specializing in complex, globally non-uniform patterns of motion, and with evidence suggesting that motion processing undergoes prolonged postnatal development.

Given that the action potential output of retinal ganglion cells (RGCs) determines the nature of the visual information that is transmitted from the retina, an understanding of their intrinsic impulse firing characteristics is critical for an appreciation of the overall processing of visual information. Recordings from RGCs within an isolated whole-mount retina preparation showed that their normal impulse firing from the resting membrane potential (RMP) was linearly correlated in its frequency with the stimulus intensity. In addition to describing the relationship between the magnitude of the current injection and the resulting impulse frequency (F/I relationship), we have characterized the properties of individual action potentials when they are elicited from the RMP. In contrast, hyperpolarizing below the RMP revealed that RGCs displayed a time dependent anomalous rectification, manifested by the appearance of a depolarizing sag in their voltage response. When an adequate period of hyperpolarization was terminated, a fast phasic period of “rebound excitation” was observed, characterized by a brief phasic burst of impulse activity. When compared to equivalent action potential firing evoked by depolarizing from the RMP, rebound spiking was associated with a lower threshold and shorter latency for impulse activation as well as a prominent, phasic, burst-like doublet, or triplet of impulses. The rebound action potential had a more positive voltage overshoot and displayed a higher peak rate of rise in its upstroke than those correspondingly generated by depolarizing current pulses from the RMP. Blocking sodium spikes with TTX confirmed that the preceding hyperpolarization led to the recruitment and subsequent generation of a transient depolarizing voltage overshoot, which we have termed the net depolarizing overshoot (NDO). We propose that the NDO boosts the generation of sodium spikes by triggering rebound spikes on its upstroke and crest, thus accounting for the observed voltage dependent change in the firing pattern of RGCs.

Retinal Development, edited by E. Sernagor, S. Eglen, W. Harris, and R. Wong. 2006. New York: Cambridge University Press.

Fairly regularly some member of my lab, finding a new feature of retinal circuitry, wonders aloud if it might “somehow relate to development.” I always reply, “We don't study development.” This partly reflects my curmudgeonly insistence that we stay focused on explaining function in the adult. But even if the discovery did relate to development, the connection would be too difficult to find—because the large territory has lacked any useful guide. Next time, however, I will hand over my copy of Retinal Development and reply more kindly, “Check it out.”

Following up on a prior study of contour and brightness processing in visual masking (Breitmeyer et al., 2006), we investigated the effects of a binocular and dichoptic para-contrast masking on the visibility of the contour and brightness of a target presented to the other eye. Combined, the results support the contributions of several cortical processes to para-contrast: (1) two central sources of inhibition, one long-latency and prolonged and the other short-latency and brief; (2) binocular rivalry suppression; and (3) a facilitatory effect peaking at different SOAs for the contour and the brightness tasks, reflecting; (4) known properties of two separate cortical systems, one a fast contour-processing pathway and the other a slower brightness-processing pathway.

Acetylcholine (ACh) enhances the preferred direction responses of directionally selective ganglion cells (DS GCs; Ariel & Daw, 1982; Ariel & Adolph, 1985) through the activation of nicotinic acetylcholine receptors (nAChRs; Ariel & Daw, 1982; Massey et al., 1997; Kittila & Massey, 1997). DS GCs appear to express at least two types of nAChRs, those that are sensitive to the partially subtype-specific antagonist methyllycaconitine (MLA), and those that are MLA-insensitive (Reed et al., 2002). Our purpose was to confirm the expression of α7 nAChRs by DS GCs and to assess the contributions of other nAChR subtypes to DS GC responses. Using choline as a nAChR partially subtype-specific agonist, we found that the majority of DS GCs demonstrated responses to choline while under synaptic blockade. The blockade or reduction of choline-induced responses by bath application of nanomolar (nM) concentrations of MLA provided direct evidence that the choline responses were mediated by α7 nAChRs. Because choline is a partial agonist for α3β4 nAChRs (Alkondon et al., 1997), the residual choline responses are consistent with mediation by α3β4 nAChRs. Additionally, a subset of DS GCs responded to nicotine but not to choline, indicating the expression of a third nAChR subtype. The pharmacological results were supported by single cell reverse transcription polymerase chain reaction (RT-PCR) and immunohistochemistry experiments. The expression of α7 and specific non-α7 nAChR subtypes was correlated with the preferred direction. This indicates the possibility of differential responses to ACh depending on the direction of movement. This is the first description of differential expression of multiple nAChR subtypes by DS GCs.

The thalamic reticular nucleus (TRN) supplies an important inhibitory input to the dorsal thalamus. Previous studies in non-primate mammals have suggested that the visual sector of the TRN has a lateral division, which has connections with first-order (primary) sensory thalamic and cortical areas, and a medial division, which has connections with higher-order (association) thalamic and cortical areas. However, the question whether the primate TRN is segregated in the same manner is controversial. Here, we investigated the connections of the TRN in a New World primate, the marmoset (Callithrix jacchus). The topography of labeled cells and terminals was analyzed following iontophoretic injections of tracers into the primary visual cortex (V1) or the dorsal lateral geniculate nucleus (LGNd). The results show that rostroventral TRN, adjacent to the LGNd, is primarily connected with primary visual areas, while the most caudal parts of the TRN are associated with higher order visual thalamic areas. A small region of the TRN near the caudal pole of the LGNd (foveal representation) contains connections where first (lateral TRN) and higher order visual areas (medial TRN) overlap. Reciprocal connections between LGNd and TRN are topographically organized, so that a series of rostrocaudal injections within the LGNd labeled cells and terminals in the TRN in a pattern shaped like rostrocaudal overlapping “fish scales.” We propose that the dorsal areas of the TRN, adjacent to the top of the LGNd, represent the lower visual field (connected with medial LGNd), and the more ventral parts of the TRN contain a map representing the upper visual field (connected with lateral LGNd).

Action potentials in amacrine cells are important for lateral propagation of signals across the inner retina, but it is unclear how many subclasses of amacrine cells contain voltage-gated sodium channels or can fire action potentials. This study investigated the ability of amacrine cells with narrow (< 200 μm) and wide (> 200 μm) dendritic fields to fire action potentials in response to depolarizing current injections and light stimulation. The pattern of action potentials evoked by current injections revealed two distinct classes of amacrine cells; those that responded with a single action potential (single-spiking cells) and those that responded with repetitive action potentials (repetitive-spiking cells). Repetitive-spiking cells differed from single-spiking cells in several regards: Repetitive-spiking cells were more often wide field cells, while single-spiking cells were more often narrow field cells. Repetitive-spiking cells had larger action potential amplitudes, larger peak voltage-gated NaV currents lower action potential thresholds, and needed less current to induce action potentials. However, there was no difference in the input resistance, holding current or time constant of these two classes of cells. The intrinsic capacity to fire action potentials was mirrored in responses to light stimulation; single-spiking amacrine cells infrequently fired action potentials to light steps, while repetitive-spiking amacrine cells frequently fired numerous action potentials. These results indicate that there are two physiologically distinct classes of amacrine cells based on the intrinsic capacity to fire action potentials.

Color constancy is one of the most impressive features of color vision systems. Although the phenomenon has been studied for decades, its underlying neuronal mechanism remains unresolved. Literature indicates an early, possibly retinal mechanism and a late, possibly cortical mechanism. The early mechanism seems to involve chromatic spatial integration and performs the critical calculations for color constancy. The late mechanism seems to make the color manifest. We briefly review the current evidence for each mechanism. We discuss in more detail a model for the early mechanism that is based on direct measurements of goldfish outer retinal processing and induces color constancy and color contrast. In this study we extrapolate this model to primate retina, illustrating that it is highly likely that a similar mechanism is also present in primates. The logical consequence of our experimental work in goldfish and our model is that the wiring of the cone/horizontal cell system sets the reference point for color vision (i.e., it sets the white point for that animal).

To investigate the contributions of amacrine cells to red-green opponency, a linear computational model of the central macaque retina was developed based on a published cone mosaic. In the model, amacrine cells of ON and OFF types received input from all neighboring midget bipolar cells of the same polarity, but OFF amacrine cells had a bias toward bipolar cells whose center responses were mediated by middle wavelength sensitive cones. This bias might arise due to activity dependent plasticity because there are midget bipolar cells driven by short wavelength sensitive cones in the OFF pathway. The model midget ganglion cells received inputs from neighboring amacrine cells of both types. As in physiological experiments, the model ganglion cells showed spatially opponent responses to achromatic stimuli, but they responded to cone isolating stimuli as though center and surround were each driven by a single cone type. Without amacrine cell input, long and middle wavelength sensitive cones contributed to both the centers and surrounds of model ganglion cell receptive fields. According to the model, the summed amacrine cell input was red-green opponent even though inputs to individual amacrine cells were unselective. A key prediction is that GABA and glycine depolarize two of the four types of central midget ganglion cells; this may reflect lower levels of the potassium chloride co-transporter in their dendrites.

The horizontal cells are known to form a mono-layered mosaic in the adult retina, but are scattered at different retinal depths in early development. To help clarifying when and which spatial constraints appear in the relative positioning of these cells, we have performed a quantitative analysis of the three-dimensional (3D) organization of the horizontal cell mosaic at different developmental stages in the postnatal rat retina. We first analyzed the two-dimensional (2D) distribution of the horizontal cell projections onto a plane parallel to the upper retinal surface in retinal flat-mounts, and thus to the future mature horizontal cell mosaic. We found that this 2D distribution was non random since postnatal day 1 (P1), and had a subsequent stepwise improvement in regularity. This preceded the alignment of cells in a single monolayer, which was observed on P6. We then computed true horizontal cell spacing in 3D, finding non-random 3D positioning already on P1. Simulation studies showed that this order might simply derive from the 2D order observed in the projections of the cells in flat-mount, combined with their limited spread in retinal depth. Throughout the period analyzed, the relative positions of horizontal cells are in good agreement with a minimal spacing rule in which the exclusion zone corresponds to the average size of the inner core of the cell dendritic tree estimated from P1 samples. These data indicate the existence of different phases in the process of horizontal cell 3D spatial ordering, supporting the view that multiple mechanisms are involved in the development of the horizontal cell mosaic.

Goldfish (Carassius auratus) were trained to discriminate triangles and squares using a two choice procedure. In the first experiment, three goldfish were trained with food reward on a black outline triangle on a white background, while a black outline square was shown for comparison. In transfer tests, a Kanizsa triangle and a Kanizsa square were presented, perceived by humans as an illusory triangle- or square-shaped surface of slightly higher brightness than the background. The choice behavior in this situation indicates that goldfish are able to discriminate between both figures in almost the same way as in the training situation. In control experiments goldfish did not discriminate between shapes in which humans do not perceive the illusion. A series of generalization experiments was performed indicating the similarity between the tested shapes and the training triangle. From all these findings we conclude that goldfish are able to perceive an illusory triangle or square within the Kanizsa figures. In a second experiment, four goldfish were trained on a white outline triangle versus a white outline square, both on black background with white diagonal lines. In transfer tests in which the shapes were replaced by gaps within the white diagonal lines, goldfish were clearly able to discriminate between the two patterns based on the illusory contours. This was not the case in tranfer tests with phase shifted abutting lines.

A stimulus display was devised that enabled us to examine how effectively monkeys and humans can process shading and disparity cues for depth perception. The display allowed us to present these cues separately, in concert and in conflict with each other. An oddities discrimination task was used. Humans as well as monkeys were able to utilize both shading and disparity cues but shading cues were more effectively processed by humans. Humans and monkeys performed better and faster when the two cues were presented conjointly rather than singly. Performance was significantly degraded when the two cues were presented in conflict with each other suggesting that these cues are processed interactively at higher levels in the visual system. The fact that monkeys can effectively utilize depth information derived from shading and disparity indicates that they are a good animal model for the study of the neural mechanisms that underlie the processing of these two depth cues.

Genetically modified mice lacking the β2 laminin chain (β2null), the γ3 laminin chain (γ3 null), or both β2/γ3 chains (compound null) were produced. The development of tyrosine hydroxylase (TH) immunoreactive neurons in these mouse lines was studied between birth and postnatal day (P) 20. Compared to wild type mice, no alterations were seen in γ3 null mice. In β2 null mice, however, the large, type I TH neurons appeared later in development, were at a lower density and had reduced TH immunoreactivity, although TH process number and size were not altered. In the compound null mouse, the same changes were observed together with reduced TH process outgrowth. Surprisingly, in the smaller, type II TH neurons, TH immunoreactivity was increased in laminin-deficient compared to wild type mice. Other retinal defects we observed were a patchy disruption of the inner limiting retinal basement membrane and a disoriented growth of Müller glial cells. Starburst and AII type amacrine cells were not apparently altered in laminin-deficient relative to wild type mice. We postulate that laminin-dependent developmental signals are conveyed to TH amacrine neurons through intermediate cell types, perhaps the Müller glial cell and/or the retinal ganglion cell.

The preferred stimulus size of a V1 neuron decreases with increases in stimulus contrast. It has been supposed that stimulus contrast is the primary determinant of such spatial summation in V1 cells, though the extent to which it depends on other stimulus attributes such as orientation and spatial frequency remains untested. We investigated this by recording from single cells in V1 of anaesthetized cats and monkeys, measuring size-tuning curves for high-contrast drifting gratings of optimal spatial configuration, and comparing these curves with those obtained at lower contrast or at sub-optimal orientations or spatial frequencies. For drifting gratings of optimal spatial configuration, lower contrasts produced less surround suppression resulting in increases in preferred size. High contrast gratings of sub-optimal spatial configuration produced more surround suppression than optimal low-contrast gratings, and as much or more surround suppression than optimal high-contrast gratings. For sub-optimal spatial frequencies, preferred size was similar to that for the optimal high-contrast stimulus, whereas for sub-optimal orientations, preferred size was smaller than that for the optimal high-contrast stimulus. These results indicate that, while contrast is an important determinant of spatial summation in V1, it is not the only determinant. Simulation of these experiments on a cortical receptive field modeled as a Gabor revealed that the small preferred sizes observed for non-preferred stimuli could result simply from linear filtering by the classical receptive field. Further simulations show that surround suppression in retinal ganglion cells and LGN cells can be propagated to neurons in V1, though certain properties of the surround seen in cortex indicate that it is not solely inherited from earlier stages of processing.

We performed genome-wide chemical mutagenesis of C57BL/6J mice using N-ethyl-N-nitrosourea (ENU). Electroretinographic screening of the third generation offspring revealed two G3 individuals from one G1 family with a normal a-wave but lacking the b-wave that we named nob4. The mutation was transmitted with a recessive mode of inheritance and mapped to chromosome 11 in a region containing the Grm6 gene, which encodes a metabotropic glutamate receptor protein, mGluR6. Sequencing confirmed a single nucleotide substitution from T to C in the Grm6 gene. The mutation is predicted to result in substitution of Pro for Ser at position 185 within the extracellular, ligand-binding domain and oocytes expressing the homologous mutation in mGluR6 did not display robust glutamate-induced currents. Retinal mRNA levels for Grm6 were not significantly reduced, but no immunoreactivity for mGluR6 protein was found. Histological and fundus evaluations of nob4 showed normal retinal morphology. In contrast, the mutation has severe consequences for visual function. In nob4 mice, fewer retinal ganglion cells (RGCs) responded to the onset (ON) of a bright full field stimulus. When ON responses could be evoked, their onset was significantly delayed. Visual acuity and contrast sensitivity, measured with optomotor responses, were reduced under both photopic and scotopic conditions. This mutant will be useful because its phenotype is similar to that of human patients with congenital stationary night blindness and will provide a tool for understanding retinal circuitry and the role of ganglion cell encoding of visual information.

Regeneration of the teleost retina following surgical extirpation of 25% to 100% of the neural retina was investigated in goldfish (Carrasius auratus) and sunfish (Lepomis cyanellus). The retina will regenerate following removal of up to 95% of the neural retina, however complete extirpation prevented regeneration. Visual sensitivity was assessed by examining components of the electroretinogram (ERG) and the dorsal light reflex (DLR) during regeneration. B-wave amplitudes in the experimental eyes increased throughout the study and central connections were reestablished as indicated by the progressive improvement in the dorsal light reflex. The recovery of visual function was closely correlated with retinal regeneration. Visual recovery progressed more slowly than following complete cytotoxic destruction of the mature retina (Mensinger & Powers, 1999) because the surgery removed a large number of the pluripotent cell population and restricted the number and distribution of regenerating foci.

During visual fixation, small eye movements keep the retinal image continuously in motion. It is known that neurons in the visual system are sensitive to the spatiotemporal modulations of luminance resulting from this motion. In this study, we examined the influence of fixational eye movements on the statistics of neural activity in the macaque's retina during the brief intersaccadic periods of natural visual fixation. The responses of parvocellular (P) and magnocellular (M) ganglion cells in different regions of the visual field were modeled while their receptive fields scanned natural images following recorded traces of eye movements. Immediately after the onset of fixation, wide ensembles of coactive ganglion cells extended over several degrees of visual angle, both in the central and peripheral regions of the visual field. Following this initial pattern of activity, the covariance between the responses of pairs of P and M cells and the correlation between the responses of pairs of M cells dropped drastically during the course of fixation. Cell responses were completely uncorrelated by the end of a typical 300-ms fixation. This dynamic spatial decorrelation of retinal activity is a robust phenomenon independent of the specifics of the model. We show that it originates from the interaction of three factors: the statistics of natural scenes, the small amplitudes of fixational eye movements, and the temporal sensitivities of ganglion cells. These results support the hypothesis that fixational eye movements, by shaping the statistics of retinal activity, are an integral component of early visual representations.