Dopamine is a major neurotransmitter and neuromodulator in vertebrate retina. Although its pharmacological and physiological actions are well understood, the biochemical mechanisms of its signal transduction are less clear. Acting via D1 receptors, dopamine was shown to increase cyclic AMP levels in intact retina and to activate adenylate cyclase in retinal homogenates. The action via activation of D2 receptors is controversial: it was reported to decrease cyclic AMP levels in intact retina but inhibition of cyclase could not be demonstrated in retinal homogenates; also it was reported to activate rod outer segment cyclic GMP phosphodiesterase in vitro but did not decrease cyclic GMP levels in aspartate-treated retinas. We made an attempt to fractionate bovine retinal membranes and to investigate the effects of dopamine, via Dl and D2 receptors, on the synthesis and hydrolysis of cyclic AMP and cyclic GMP. Activation of cyclic AMP synthesis was noted in all fractions, but no effects were evident on cyclic nucleotide hydrolysis or cyclic GMP synthesis in any fraction. Also, D2 agonist did not inhibit cyclic AMP synthesis. These observations suggest that D2 receptors may not be directly coupled to cyclic nucleotide metabolizing enzymes in bovine retina.

Adult female praying mantises, Sphodromantis lineola (Burr.), were presented with computer-generated black rectangular stimuli that moved horizontally or vertically at 82 deg/s against a homogeneous white background. Both stimulus configuration (orientation in relation to direction) and the retinal location of the stimulus image affected the rate at which mantises responded appetitively (approached or struck at the stimulus). Mantises responded most to square stimuli (12.5 deg × 12.5 deg) when they moved horizontally or vertically through, or horizontally at 24.5 deg below the center of their visual field. Mantises also responded most to vertically (vs. horizontally) oriented rectangular stimuli (12.5 deg × 47 deg) that moved through their visual-field center, irrespective of whether the stimuli moved downward or horizontally. Upward moving stimuli elicited intermediate amounts of behavior with no configuration preference. Mantises did not demonstrate a configuration preference when rectangular stimuli moved ≥24.5 deg outside of the visual-field center. Furthermore, mantises responded very little and demonstrated no configuration preferences to stimuli that moved less than ≈83 deg through their visual field even if the stimuli moved through the visual-field center.

Maturation of temporal resolution was investigated in a visual evoked potential study in 77 infants from birth to 9 months of age. Luminance evoked potential measures in response to homogeneous sinusoidal flickering light (1–64 Hz) were recorded under behavioral state-defined conditions. Behavioral state was determined by direct observation and by polygraphic recording of the electroencephalogram (EEG), eye movements (EOG), muscle activity (EMG), heart rate (ECG), and respiration. Temporal-frequency functions of the amplitude of the fundamental response across the temporal-frequency range were recorded during sleep and wakefulness. The highest temporal-frequency response recorded during wakefulness was accepted as a measure for inclusion in a growth function of temporal-frequency responsiveness. The resulting temporal resolution frequency vs. age function showed three separate maturational phases. Maturational phases were defined as (1) an initial slow phase from 1–32 days postnatal during which maturation of temporal vision is unremarkable; (2) an intermediate rapid phase of improvement from age 26 to 170 days; and (3) an overlapping but final slow phase from 151 to at least 270 days during which adult-like flicker resolution is approximated. This study suggests that the multiple maturational phases of the infant's responses to flickering light are due to maturational differences, which correspond with maturation of structural factors of brain function. Finally, across the age span tested, high-frequency responsivity was influenced significantly by the degree of infant arousal.

Previous studies indicate that cortical areas 17 and 18 play a prominent role in generating the direction selectivities of neurons in the superior colliculus of the cat. This hypothesis was tested by quantifying the activities of neurons in the superficial collicular layers in intact cats and cats which incurred ablation of areas 17 and 18 and part of area 19. In addition, since behavioral and anatomical studies suggest a functional adjustment in the superior colliculus following removal of inputs from areas 17, 18, and 19 in the neonatal cat, we included a group of neonatally lesioned cats. Computation of an index of directionality indicated that the majority of neurons in intact cats preferred movement in one direction, thus confirming reports of others. Following ablation of areas 17 and 18 and part of area 19 in both groups of lesioned cats, only modest changes in the population indices were detected when poorly responsive neurons were eliminated from the analyses. Based upon levels of visually evoked neuronal activity, our data suggest a physiological compensation by neurons in stratum griseum superficiale following removal of areas 17, 18, and 19 inputs. In the intact and neonatally operated groups, activity in stratum griseum superficiale is high, whereas in the adult lesioned group activity is low. In stratum opticum, neuronal activity was similar in all three groups of cats. These results show that neurons in stratum griseum superficiale undergo a physiological compensation following removal of immature areas 17 and 18.

Experiments on the lateral geniculate nucleus (LGN) of the cat based on 14C 2-deoxyglucose (2-DG) autoradiography and intraocular injections of 2-amino-4-phosphonobutyric acid (APB) provided evidence for gradients of metabolic activity in the ON and OFF pathways in layer A, but only very weakly, if at all, in layer Al. Alert and freely moving cats were exposed to square-wave gratings over a 45-min period after injection of the 2-DG. When one eye had been treated previously with APB, contralateral layer A showed a clear gradient of 2-DG label indicating that the remaining OFF pathway was most active ventrally in the layer and, by implication, that the ON pathway is normally most active dorsally. No gradient was apparent in layer Al ipsilateral to the APB eye. Control experiments based on binocular injections of tetrodotoxin (TTX) demonstrated that no gradients were present in the baseline activity within the layers. Finally, monocular injections of TTX provided evidence for gradients of nondominant eye activity in layers A and Al that were maximal near the interlaminar zone between layers A and A1 and declined in mirror-symmetric fashion toward the dorsal border of A and the ventral border of A1.

Combined with earlier anatomical studies showing depth-dependent patterns of geniculo-cortical projection, these results indicate that in the cat, as in several other species, the visual input to striate cortex is partly organized around ON and OFF pathways. In addition, the results suggest that a systematic variation of binocular interaction, perhaps related to ocular dominance, exists through the depths of the geniculate layers. Understanding how the ON and OFF pathways, and binocular interactions, are organized in the thalamus may provide insight into the functional merging of these systems in the cortex.

In the turtle retina, dopamine has been observed in a small population of amacrine cells. Whereas the effect of dopamine has been intensively studied, knowledge about the release of this transmitter and the neuronal control of its release are still poorly understood. We therefore decided to study the release of endogenous dopamine. Isolated retinas were superfused with Ringer’s solutions and stimulated with increased potassium, light, or drugs which interfere with neurotransmitter systems. Dopamine was analyzed by using aluminum-oxide extraction and high-pressure liquid chromatography (HPLC) with electrochemical detection. Increased potassium (25 mM) caused a five-fold increase in the basal release. When calcium was replaced by cobalt, no increase was induced by 25 mM potassium. Flickering light increased the basal release of endogenous dopamine by a factor of three. The effect of flickering light was greater in the presence of additional steady background illumination. Kainate (10 μM), an agonist for excitatory amino acids, doubled the basal dopamine release. Bicuculline (10 μM), a γ-amino butyric acid (GABA) antagonist, increased the release to about six times the basal level. Naloxone (10 μM), an opiate antagonist, increased the release to eight times the basal level. These findings suggest that dopamine is released from amacrine cells in the turtle retina in a calcium-dependent manner, which is most likely a vesicular release. Dopamine release is induced by flickering light vs. darkness and vs. steady background illumination. A moderate background illumination alone does not significantly increase basal dopamine release. Drug treatment during the release experiments suggests that dopaminergic neurons receive an excitatory input either directly or indirectly from glutamatergic bipolar and/or amacrine cells and inhibitory inputs either directly or indirectly from GABAergic amacrine cells and from enkephalinergic amacrine cells or efferent fibers.

Newborn rhesus monkeys were raised under monocular (lid suture, aphakia, aphakia corrected optically with contact lenses, and occlusion with opaque occluder lenses) and under binocular visual deprivation conditions (aphakia combined with occlusion or optical undercorrection of the fellow eye). Routine immunohistochemical methods with an antibody to the calcium-binding protein parvalbumin (PV) were used to examine the distribution of PV+ neurons and PV+ processes in the LGN of these monkeys. Under all rearing conditions, we found no obvious difference in PV density in neurons in any lamina, although in all monocularly deprived and in two of the three binocularly deprived monkeys neurons connected to the deprived eye were of reduced size. Furthermore, PV-immunoreactive processes in the neuropil of deprived laminae were as numerous and of the same morphologies as those in nondeprived laminae or as in normal controls. Thus, in the LGN of rhesus monkeys, the calcium-binding protein parvalbumin is resistant to monocular as well as binocular visual deprivation during the postnatal maturation process of the visual system.

Winter flounder (Pseudopleuronectes americanus) are hatched as bilaterally symmetric larvae which live near the ocean surface. At metamorphosis, they become laterally compressed, one eye migrates to the opposite side of the head, and they live the remainder of their lives lying on their blind side on the ocean floor. The present study characterizes and quantifies retinal cell distribution throughout the larval period and contrasts it with the adult retina. Based on light- and electron-microscopic analyses, retinas of larval flounder contain only a single cone-like photoreceptor type, arranged in a hexagonal array. In contrast, after metamorphosis, the adult retina has three types of photoreceptors: rods, single cones, and double cones. Rod photoreceptors are numerous in the ventral retina and decrease in density dorsad. The cone photoreceptor density, in contrast to rods, is higher in the dorsal retina decreasing ventrad. Adult cone photoreceptors are arranged in a square mosaic with four double cones surrounding one single cone. The differences in larval and adult retinal morphology reflect the distinctly different habitat each occupies.

The habitat occupied by larval winter flounder (Pseudopleuronectes americanus) differs considerably in light regime from that of the adult. To understand how the visual system has adapted to such changes, photoreceptor spectral absorbance was measured microspectrophotometrically in premetamorphic and postmetamorphic specimens of winter flounder. Before metamorphosis, larval flounder retinas contain only one kind of photoreceptor which is morphologically cone-like with peak absorbance at 519 nm. After metamorphosis, the adult retina has three types of photoreceptors: single cones, double cones, and rods. The visual pigment in single cones has a peak absorbance at λmax = 457 nm, the double cones at λmax = 531 and 547 nm, and the rod photoreceptors at λmax = 506 nm. Double cones were morphologically identical, but the two members contained either different (531/547 nm) or identical pigments (531/531 nm). The latter type were found only in the dorsal retina. The measured spectral half-bandwidths (HBW) were typical of visual pigments with chromophores derived from vitamin A1 with the possible exception of the long-wavelength absorbing pigment in double cones which appeared slightly broader. Because the premetamorphic pigment absorbance has a different λmax than those of the postmetamorphic pigments, different opsin genes must be expressed before and after metamorphosis.

Two basic types of cholinergic receptor have been identified in nervous systems: nicotinic and muscarinic. In the mammalian visual system, the balance of evidence suggests that nicotinic activity is associated primarily with transmission and processing of information while muscarinic activity reflects modulatory influences arising in the brainstem and basal forebrain. We have measured contrast sensitivity functions using a two-alternative forced-choice procedure in young human volunteers with and without administration of nicotine (1.5 mg by buccal absorption) or the muscarinic antagonist scopolamine (1.2 mg orally). Scopolamine elevates contrast-detection thresholds uniformly at all spatial frequencies, consistent with blocking of a nonspecific arousal system. Nicotine, in contrast, improves sensitivity at low spatial frequencies (below about 4 cycle/deg); at higher spatial frequencies sensitivity is, if anything, impaired. Using counterphase gratings, we find that scopolamine elevates thresholds uniformly at all temporal frequencies. Nicotine lowers thresholds at high but not low temporal frequencies. The results obtained with nicotine suggest that contrast sensitivity reflects the activity of two mechanisms, or sets of spatiotemporal filters, that are pharmacologically distinct, the contrast sensitivity function reflecting the envelope of their sensitivities.

In in-vitro preparations of both macaque and human retina, intracellular injections of Neurobiotin and horseradish peroxidase were used to characterize the morphology, depth of stratification, and mosaic organization of a type of bistratified ganglion cell. This cell type, here called the small bistratified cell, has been shown to project to the parvocellular layers of the dorsal lateral geniculate nucleus (Rodieck, 1991) and is therefore likely to show color-opponent response properties.

In both human and macaque, the two dendritic tiers of the bistratified cell are narrowly stratified close to the inner and outer borders of the inner plexiform layer. The inner tier is larger in diameter and more densely branched than the outer tier and gives rise to distinct spine-like branchlets bearing large, often lobulated heads. By contrast the smaller, outer tier is sparsely branched and relatively spine-free.

In human retina, the small bistratified cells range in dendritic field diameter from ∼50 µm in central retina to ∼400 µm in the far periphery. The human small bistratified cells are about 20% larger in dendritic-field diameter than their counterparts in the macaque. However, when the difference in retinal magnification between human and macaque is taken into account, the small bistratified cells are similar in size in both species. In macaque, the small bistratified cell has a dendritic-field size that is ~10% larger than that of the magnocellular-projecting parasol ganglion cell. Human small bistratified ganglion cells tend to have smaller dendritic-field diameters than parasol cells. This is because parasol ganglion cells are larger in human than in macaque retina (Dacey & Petersen, 1992).

In macaque retina, intracellular injections of Neurobiotin revealed heterotypic tracer coupling to a distinct mosaic of amacrine cells and probable homotypic coupling to an array of neighboring ganglion cells around the perimeter of the injected cell's dendritic tree. The amacrine cell mosaic had a density of 1700 cells/mm2 in peripheral retina. Individual amacrines had small, densely branched and bistratified dendritic fields. From the homotypic coupling, it was possible to estimate for the small bistratified cell a coverage factor of ~1.8, and a density of ~1% of the total ganglion cells in central retina, increasing to ~6–10% in the retinal periphery.

The estimated density, dendritic-field size, and depth of stratification all suggest that the small bistratified ganglion cell type is the morphological counterpart of the common short-wavelength sensitive or ‘blue-ON’ physiological type.

The performance of pigeons on a color-reversal learning task was assessed after thalamic lesions disrupting the thalamofugal and tectofugal visual pathways. Successful performance of a simultaneous color discrimination was accomplished after surgery, and a series of reversals of the original discrimination followed during which the positive and negative consequences associated with the stimuli were interchanged. Shimizu and Hodos (1989) had reported that lesions of two laminae in the visual wulst (IHA and HD), both targets of the avian thalamofugal pathway, resulted in increased errors in a color-reversal learning task in pigeons. This finding suggested that the thalamofugal pathway might play a role in visual discrimination involving stimulus context changes. In the present study, lesions of the OPT complex (the thalamic source of afferents to IHA and HD) were found to have no effect on color-reversal learning performance. Instead, we found that damage to nucleus rotundus (the thalamic component of the tectofugal pathway) resulted in deficits that were far in excess of those that had been obtained after IHA and HD lesions. We suggest that the color-reversal learning deficits after Wulst lesions are not due to the Wulst's connections with the thalamofugal pathway, but rather to its connections with the tectofugal pathway.

The compartmental organization of visual cortical neurons was examined across species of primates by directly comparing the pattern of immunoreactivity for the 28-kD vitamin D-dependent calcium-binding protein (calbindin) in area 17 of squirrel monkeys, macaques, and neurologically normal adult humans. Area 17 of macaques and squirrel monkeys was similar in that somata and processes intensely immunoreactive for calbindin were present in the same layers (II-III, IVB, and V) and in both species formed a well-stained matrix that surrounded the CO-rich puffs in layer III. These intensely calbindin-immunoreactive neurons were identified as subpopulations of GABA-immunoreactive neurons. Among the most obvious differences in the two monkey species was the distribution of calbindin-positive elements outside of layer III: a dense immunostained matrix surrounded the puffs in layers II, IVB, V, and VI of squirrel monkeys but the immunostained neurons adopted no regular pattern outside layer III in macaques. In addition, although somata lightly immunoreactive for calbindin were present in both species, they were much more abundant in squirrel monkeys than macaques. The pattern of calbindin immunostaining in human area 17 resembled that of macaques in forming an intense matrix that surrounded puffs only in layer III, yet also resembled that of squirrel monkeys by including large numbers of lightly immunoreactive somata. These lightly immunostained somata included a very dense population forming a prominent band in layer IVA of human visual cortex. We conclude that for layer III of primary visual cortex, a similar pattern of neuronal chemistry exists across species of primates which is related to this layer's compartmental organization. Yet for other layers, the expression of calbindin immunoreactivity varies from one species to the next, perhaps reflecting variations in other neuronal properties.

Immunocytochemical techniques have been used to examine the distribution of substance-P (SP)-labeled neurons in the superior colliculus of rats from birth to adulthood. At birth, there are almost no SP-immunopositive neurons in the tectum. A small number of SP neurons appear over the next several days. However, the vast majority of SP neurons appear between P9 and P10, and by P12 have attained adult-like numbers and distribution. Neurons are confined to the superficial layers of the colliculus, specifically the upper two-thirds of the stratum griseum superficiale (SGS). There is no indication of a differential developmental sequence along rostrocaudal or mediolateral axes. Neuronal types can be distinguished as early as P6 and include horizontal, vertical, and multipolar cells.

Substance-P-immunoreactive axons and boutons are also present in the superior colliculus at birth, and are for the most part confined to the deep layers. Boutons are generally of the en-passant type. The density of labeled axons and boutons increases progressively, and by P10 there is an almost adult-like lamination and patchiness. In the adult, labeled axons and boutons are most dense in the stratum opticum and stratum griseum intermedium. Bridges of dorsoventrally oriented labeled axons span the relatively label-poor stratum album intermedium. SP label in the stratum griseum profundum is dense and patchy, and there is also dense label in the stratum album profundum bordering the periaqueductal grey.

The role of substance-P-labeled neurons in the superior colliculus is still a matter of speculation. The findings of this study indicate that SP neurons may play a role in intrinsic collicular circuitry.

It has been argued that in order for the visual system to detect edges accurately under a range of conditions, the visual system needs to adapt to the local contrast level to preserve sensitivity (Blakemore & Campbell, 1969). Cells in the primary visual cortex of cats adapt to stimuli with low to moderate contrast. Curiously, macaque monkey neurons in primary visual cortex (V1) do not show evidence for similar adaptation. To address the question of whether this differential sensitivity in contrast adaptation might be due to phylogenetic variation between cats and primates or to specializations for visual niche (e.g. nocturnal vs. diurnal), contrast adaptation to temporally and spatially optimized gratings was examined in 30 V1 cells of three nocturnal primate bush babies (Galago crassicaudatus). A second objective was to examine the relationship between the degree of contrast adaptation and cell classification or cell location relative to cortical layers or compartments [i.e. cytochrome-oxidase (CO) blobs and interblobs]. All cells were classified (simple vs. complex) and anatomically localized relative to cortical layers and cytochrome-oxidase (CO) blob and interblob compartments. Two independent measures of contrast adaptation were used. In the first test, contrast was sequentially increased from 3–56% and then decreased. The contrast required to maintain a half-maximum response amplitude in the 30 cells tested increased an average of 0.24 (±0.12) log units during the sequential decrements in contrast. For the second test, four sets of five interleaved contrasts within ±1 octave of a central adapting contrast (10%, 14%, 20%, and 28%, respectively) were presented. The cells produced a mean adaptation index of 0.57 (±0.47) which is very similar to that exhibited by cat cortical neurons (0.54 ± 0.41). Interestingly, cells in interblobs showed a trend toward greater adaptation than did blob cells. Moreover, cells in the supragranular layers exhibited greater adaptation than cells in the infragranular layers. No significant differences in adaptation were found to correlate with other cell classification indices. Taken together, our results suggest that contrast adaptation may be more important for maintaining sensitivity in nocturnal species (primates or cats) than in diurnal species (macaque monkeys), and that in the nocturnal bush baby, cells in cortical layers and compartments may be differentially specialized for contrast adaptation.

Spatial summation curves were determined in the circumscribed visual-field defects of five patients with blindsight. Under light-adapted conditions that favor the color-opponent system, increment thresholds for white and red targets presented on a white background were measured as a function of stimulus size which ranged from 9–110 min arc. In both normal and defective hemifields, summation was linear for the red stimuli. In contrast, the curves measured with the white stimuli showed some nonlinearity in the normal hemifield, and a pronounced eccentricity-dependent notch in the field defect. The results indicate that the neurons mediating sensitivity differ in their summation properties for wavelength and intensity information.

One striking transformation in response properties that occurs in the geniculo-cortical pathway is the appearance of a high degree of orientation selectivity in the cortex. This property may be conceived as arising purely from the excitatory inputs to the cell, as being structured largely by the inhibition a cortical cell receives or could be due to a combination of the two. We have studied the contributions of excitatory and inhibitory inputs to cortical cells' orientation selectivity by analyzing the postsynaptic potentials evoked in cat striate neurones by flashing stimuli of different orientations. We made these recordings using the in vivo whole-cell technique (Xing Pei et al., 1991), which provides more stable and reliable results than classical intracellular recording methods. Our results show that the cat striate cortex exhibits a variety of mechanisms to achieve orientation selectivity. Orientation selectivity of a particular cell can be created by excitatory, by inhibitory, or by a combination of both mechanisms.

Dynamic random-dot stimuli have been widely used to explore central mechanisms of motion processing. We have measured the responses of neurons in area MT of the alert monkey while we varied the strength and direction of the motion signal in such displays. The strength of motion is controlled by the proportion of spatiotemporally correlated dots, which we term the correlation of the stimulus. For many MT cells, responses varied approximately linearly with stimulus correlation. When they occurred, nonlinearities were equally likely to be either positively or negatively accelerated. We also explored the relationship between response magnitude and response variance for these cells and found, in general agreement with other investigators, that this relationship conforms to a power law with an exponent slightly greater than 1. The variance of the cells' discharge is little influenced by the trial-to-trial fluctuations inherent in our stochastic display, and is therefore likely to be of neural origin. Linear responses to these stochastic motion stimuli are predicted by simple, low-level motion models incorporating sensors having relatively broad spatial and temporal frequency tuning.