Visual tasks that are perceptually diverse might be expected to elicit unique evoked-potential waveforms that exhibit differing topographic maps. To investigate this possibility, multichannel visual-evoked potentials (VEPs) were recorded in response to several dot spatial localization stimuli that are physically similar yet produce different percepts (vernier offsets, stereoscopic disparity, bisection, orientation, and relative displacement) to determine if the unique percepts arising from these stimuli reflect the activation of different cortical neural populations. The resulting evoked potentials were all similar in waveform, although the stereoscopic VEPs were relatively delayed. Topographic maps of the evoked-potential activity to each stimulus revealed a late major component with two independent foci: one 7 or more centimeters above the inion lateral to the midline, and the other at least 6 cm lateral to Oz. The scalp localization of both peaks was independent of both the position of the stimulus in the visual field and the particular stimulus cue presented. An asymmetric response to pattern appearance vs. disappearance indicated strong pattern specificity for each stimulus type except unreferenced motion. The timing of the VEP responses and relative insensitivity to retinal locus of stimulation suggest the involvement of higher cortical areas. The two map foci might be interpreted as activation of inferotemporal and parietal cortices whose roles are thought to be visual object interpretation and spatial attention and localization, respectively.

The turtle retina has been shown to have a variety of different morphological ganglion cell types as well as distinct physiological ganglion cell types. The major projection of the retina to the brain in nonmammalian vertebrates is to the optic tectum. In this study, we address the question of which retinal ganglion cell types project to the optic tectum in the turtle.

Fluorescent rhodamine-labeled microspheres were used to trace the retinal ganglion cell projection to the superficial layers of the optic tectum. The fluorescent ganglion cell somata, retrogradely marked by transport from the contralateral optic tectum, were impaled with micropipettes containing rhodamine-horseradish peroxidase solution and this dye was iontophoresed into the cells under visual control.

Most of the morphological ganglion cell types described in Golgi studies (Kolb, 1982; Kolb et al., 1988) were stained. Thus, the small cell types G1, G2, G3, G5, G6, and G7; the medium-sized types G10, G11, G12, G13, and G14; and the large-sized types G15, G16, G19, G20, and G21 project to the optic tectum in the turtle. We have added a new type, G2a, which proves to have some differences from the original G2 in branching pattern. We were unable to stain the small type G4, the medium-sized types G8 and G9, and the large cell types G17 and G18; this suggests that they might not project to the superficial layers of the dorsolateral optic tectum, at least, in the turtle.

Vertebrate photoreceptors are unusual neurons in that they are capable of continuous calcium-mediated release of neurotransmitter (Trifonov, 1968; Hagins et al., 1970). In this study, we have examined the development and characteristics of calcium currents in chick cone cells placed in culture on embryonic day 8. Cone cells were identified by their lectin-binding properties, rhodopsin-like immunoreactivity, and the presence of an oil droplet. Using the whole-cell patch-clamp method, we have seen calcium currents in these cells after three days in culture, slightly before the appearance of synapses (Gleason & Wilson, 1989). Because cone calcium currents are blocked by cadmium and nifedipine but are enhanced by Bay K 8644, they most closely resemble L-type current (Nowycky et al., 1985). An unexpected feature of these currents is that their gating ranges varied widely between cells so that some cells showed the foot of their activation range at —70 mV and others as positive as —25 mV. Calcium imaging of fura-2 loaded cells was used to confirm the time course of calcium current development and describe the distribution of cytosolic calcium. As expected, depolarization of young cells failed to increase cytosolic calcium but in older cells an increase of threefold to fourfold was usually observed. Both at rest and during depolarization, most cone cells showed regional differences in internal calcium concentration. In the most mature cones, depolarization strongly elevated cytosolic calcium at the terminal end of the cell while producing a lesser change around the oil droplet and the ellipsoid region, suggesting that calcium channels are localized to the terminal.

The lateral eyes of the horseshoe crab Limulus contain about 80 pmoles of retinal, 30 pmoles of retinol, and 4 pmoles of retinyl esters. More all-trans than 11-cis isomer was found in each category of retinoid. No consistent changes were observed in the amounts of retinal, retinol, or retinyl esters as a function of time of day. No 3,4-dehydro- nor hydroxyretinoids were found.

Aqueous extracts of the eye support the stereospecific formation of 11 -cis retinal from a11-trans retinal when irradiated with light. The reaction requires a protein that is apparently recognized by polyclonal antibodies raised against the retinal photoisomerase extracted from honeybee eyes. The isomerase is able to use as substrate either endogenous all-trans retinal in the extract or retinal supplied in vesicles of phospholipid. The spectral efficiency of this isomerization has λmax at 550 nm, but the spectrum appears too narrow compared with the absorbance spectrum of retinoid-binding proteins, probably because of inadequate correction for nonspecific isomerization at short wavelengths.

Previously, we have demonstrated that a subpopulation of visual cortical neurons exhibit oscillatory responses to their preferred stimuli at a frequency near 50 Hz (Gray & Singer, 1989). These responses can selectively synchronize over large distances of cortex in a stimulus-specific manner (Gray et al., 1989; Engel et al., 1990a). Here we report the results of a new analysis which reveals the fine temporal structure inherent in these interactions. We utilized pairs of recordings of the local field potential (LFP) activity from area 17 in the anesthetized cat which met two criteria. The LFP was correlated with the underlying unit activity at each site and the recording sites were at least 5 mm apart in cortex. A moving-window technique was applied to compute cross correlograms on 100-ms epochs of data repeated at intervals of 30 ms for a period of 3 s during each direction of stimulus movement. A statistical test was devised to determine the significance of detected correlations. In this way we were able to determine the magnitude, phase difference, frequency, and duration of correlated oscillations as a function of time. The results demonstrate that (1) the duration of synchrony is variable and lasts from 100–900 ms; (2) the phase differences between and the frequencies of synchronized responses are also variable within and between events and range from +3 to —3 ms and 40–60 Hz, respectively; and (3) multiple correlation events often occur within a single stimulus period. These results demonstrate a high degree of dynamic variability and a rapid onset and offset of synchrony among interacting populations of neurons which is consistent with the requirements of a mechanism for feature integration.

A protocol was developed to isolate enzymatically photoreceptors from the retina of the squid, Loligo pealei. The procedure routinely results in a high yield of intact cells. Examination of solitary photoreceptors under Nomarski optics revealed that the fine morphological features described in anatomical studies of retinal sections are retained. The distal segment is up to 250 μm long, 4–7 μm wide, covered in part by short microvilli; the inner segment and the cell body, with the initial portion of the axon, are also clearly discernible in solitary cells. Suction electrode measurements performed from the cell body confirmed that responsiveness to light survived cell isolation. Macroscopic membrane currents were measured using the whole-cell tight-seal technique, and the perforated-patch method. Step depolarizations of membrane voltage administered in the dark elicited a slowly activating, sustained outward current. Light stimulation evoked an inward current graded with stimulus intensity; the peak current could amply exceed 1000 pA. Intense photostimulation gave rise to a prolonged inward aftercurrent that lasted for tens of seconds. On-cell patch recording along the intermediate segment and most of the smooth areas of the distal segment showed a large incidence of silent patches, with the occasional presence of voltage-dependent channels. On the other hand, channel activity could be recorded more frequently from electrode placements near the apical tip of the cell, where the presence of microvilli could be confirmed visually. Some patches were unresponsive to voltage Stimulation applied in the dark but produced distinct bursts of channel openings after illumination. The feasibility of single-cell electrophysiology in isolated photoreceptors, together with the growing body of biochemical information on cephalopod preparations, makes squid an attractive model system to investigate the visual process in invertebrates using multiple experimental approaches.

Thresholds for detection of light by a dark-adapted test eye were measured while the other, non-test eye was either similarly dark adapted or while it was exposed to an intense red adapting field. An interocular effect that depends on the retinal location of the test was found: compared to the threshold during binocular dark adaptation, sensitivity decreased during contralateral light adaptation when the test was presented to the foveola and up to 4 deg above it; but sensitivity increased when the test was between 7 and 12 deg, showing a reversal at 5 deg.

Responses of 71 cells in areas 17 and 18 of the cat visual cortex were recorded extracellularly while stimulating with gratings drifting in each direction across the receptive field at a series of temporal frequencies. Direction selectivity was most prominent at temporal frequencies of 1–2 Hz. In about 20% of the total population, the response in the nonpreferred direction increased at temporal frequencies of around 4 Hz and direction selectivity was diminished or lost. In a few cells the preferred direction reversed.

One consequence of this behavior was a tendency for the preferred direction to have lower optimal temporal frequencies than the nonpreferred direction. Across the population, the preferred direction was tuned almost an octave lower. In spite of this, temporal resolution was similar in the two directions. It appeared that responses in the nonpreferred direction were suppressed at low frequencies, then recovered at higher frequencies.

This phenomenon might reflect the convergence in visual cortex of lagged and nonlagged inputs from the lateral geniculate nucleus. These afferents fire about a quarter-cycle apart (i.e. are in temporal quadrature) at low temporal frequencies, but their phase difference increases to a half-cycle by about 4 Hz. Such timing differences could underlie the prevalence of direction-selective cortical responses at 1 and 2 Hz and the loss of direction selectivity in many cells by 4 or 8 Hz.

Vasoactive intestinal polypeptide (VIP) immunoreactive (IR) neurons in the rabbit retina constitute a population of wide-field amacrine cells. To better define this cell population, we examined the coexpression of VIP with other putative retinal transmitters or their biosynthetic enzymes, including γ-aminobutyric acid (GABA), tyrosine hydroxylase (TH), and somatostatin (SRIF). Colchicine-treated retinas were immersion fixed in 4% paraformaldehyde. The retinas were cut either perpendicular or parallel to the vitreal surface and processed by double-label immunofluorescence techniques using antibodies directed to VIP, GABA, TH, and SRIF. The immunoreactive staining patterns obtained with these antibodies were the same as those described in previous studies. GABA-IR neurons were localized to the proximal inner nuclear layer (INL) and ganglion cell layer (GCL) and processes were distributed throughout the inner plexiform layer (IPL). TH- and SR1F-IR neurons were sparsely distributed to the proximal INL and GCL, respectively. TH-IR processes ramified in laminae 1, 3, and 5, and SRIF-1R processes in laminae 1 and 5 of the IPL. Colocalization experiments showed that all VIP-IR neurons contain GABA immunoreactivity. In contrast, colocalization of VIP and TH or SRIF immunoreactivities was never observed. These results demonstrate that VIP-IR wide-field amacrines of the rabbit retina make up a neurochemically and morphologically distinct subpopulation of the GABA-IR amacrine cell population. Furthermore, VIP-IR amacrine cells constitute a distinct group with respect to the TH- and SRIF-IR amacrine cells.

The Golgi-stained dendrites of a two-cone bipolar cell of a vervet monkey were serially sectioned for electron microscopy and shown to make basal synapses with two neighboring cones. The synapses were all on the cone pedicle membrane adjacent to the bipolar processes invaginating to form the triads. This is the characteristic position for flat midget bipolar cells. The numbers of triads in nine adjacent cone pedicles were not significantly different from those in the cones contacted by the bipolar. Based on this measure the two-cone bipolar does not contact a special population of cones.

The advantages and disadvantages of using vertical or horizontal sections for the determination by electron microscopy of the connectivity of the dendrites of Golgi-stained flat bipolar cells are discussed.