The pineal complex of anuran &hibians is a directly photosensory organ, encompassing both an extracranial portion, the frontal organ, and an intracranial portion, the pineal organ proper. The projection neurons of the frontal organ respond differentially according to the wavelengths of the light stimuli. The pineal organ, on the other hand, functions mainly as a luminosity meter. Most of its centrally projecting neurons respond to all increases in ambient illumination with decreases in spontaneous firing of action potentials, although some neural units in the pineal organ may respond according to wavelength. This difference in responses to light stimulation may be reflected in the neural organization of the two parts of the pineal complex. In the present study, we have analyzed the morphology of the projection neurons of the frontal and pineal organs of the frog, Rana esculenta, by backfilling of the neurons with horseradish peroxidase through their cut axons. In the pineal organ, several types of centrally projecting neurons were observed: peripherally situated unipolar and multipolar neurons, the dendrites of which extend into a superficial axon plexus that surrounds the pineal epithelium; smaller unipolar, bipolar, or multipolar neurons situated close to the central pineal tract; and radially oriented bipolar neurons, with short dendritic processes oriented towards the lumen of the pineal organ. This latter type was strongly reminiscent of photoreceptor cells. The centrally projecting neurons of the frontal organ were multipolar, and situated in the ventral part of the organ. One photoreceptor-like bipolar neuron was observed in one frontal organ. The neurons of the frontal organ did not form a superficial plexus of neurites. This difference may relate to the different ratio of chromaticity/luminosity units in the frontal and pineal organs.

The photosensory pineal complex of anurans comprises an extracranial part, the frontal organ, and an intracranial part, the pineal organ proper. Although the pineal organ functions mainly as a luminosity detector, the frontal organ monitor the relative proportions of short and intermediate/long wavelengths in the ambient illumination. The major pathway of information processing in the pineal and frontal organs is the photoreceptor to ganglion cell synapse. It is not known whether interneurons form part of the neural circuitry. In the present study, we demonstrate GABA-immunoreactive (GABA-IR) neurons in the pineal and frontal organs of the frog, Rana esculenta. No GABA-IR axons were observed in the pineal nerve between the frontal and pineal organs, or in the pineal tract that connects the pineal complex with the brain. The GABA-IR neurons differed in morphology from centrally projecting neurons visualized by retrograde labeling with horseradish peroxidase. Thus, we suggest that the GABA-IR neurons in the pineal and frontal organs represent local interneurons.

Axons of central origin, immunoreactive with a sensitive antiserum against the tetrapeptide Phe-Met-Phe-Arg-NH2 (FMRFamide), were observed in the intracranial portion of the photosensory pineal organ. The immunoreactive axons enter the caudal pole of the pineal organ via the posterior commissure. The largest density of axons was observed in the caudal part, while fewer axons were detected in the rostral portion. The uneven distribution of the FMRFamide-immunoreactive axons may be related to the distribution of different types of intrapineal neurons. FMRFamide-immunoreactive varicose axons were observed in the extracranial frontal organ. A central innervation of the pineal organ, previously known exclusively from amniotes, is probably not per se linked with the evolutionary transition of the pineal organ from a directly photosensory organ to a neuroendocrine organ. It could rather represent a centrifugal input to a sensory system which has been retained when the directly sensory functions have changed, during phylogency, to neuroendocrine functions.

An aftereffects paradigm was used to behaviorally measure contrast sensitivity of cats to gratings of three different test spatial frequencies after adaptation to gratings of various spatial frequencies, contrasts, and durations. Post-adaptation reductions in sensitivity occurred even after short periods of adaptation (<7 s) and could be as large as 1.0 log unit under some conditions. The magnitude of the adaptation effect varied monotonically with (1) adaptation grating contrast, (2) duration, and (3) the contrast sensitivity for the test grating. Average half-width (at half-height) of the spatial-frequency tuning curves constructed from the data was 1.4 octaves, and was not dependent upon the level of adaptation or the spatial frequency of the test grating. Post-adaptation psychometric functions of the cats showed reduced slopes and maxima suggesting that, unlike humans, in cats apparent contrast grows more slowly with increases in physical contrast after contrast adaptation. All of the characteristics observed are in excellent agreement with electrophysiologically measured properties of neurons in striate cortex of cats. In addition, there was a remarkable similarity of the cat tuning functions, both in shape and bandpass, to those measured in man with a similar paradigm suggesting that (1) the two visual systems are sufficiently similar to make the cat a useful spatial vision model and (2) there is a common functional plan to all mammalian visual systems despite significant anatomical differences between species.

Electrical reponses of luminosity horizontal cells (L cells) to monochromatic stimuli were analyzed by intracellular recordings in the retinas of the freshwater turtle (Pseudemys scripta elegans) and of the sea turtle (Chelonia mydas mydas). Light intensity, duration, and wavelength were varied to assess temporal effects. For a given intensity of monochromatic light, response amplitude increased with stimulus duration until maximum amplitude occurred at a specific duration. This suprathreshold metric of temporal integration is called here summation time, and it is wavelength-dependent.

L cells always had some level of red-sensitive cone input, although in some cells inputs from green- and blue-sensitive cones were also observed. For these latter cells, summation times were shorter for 640-nm than for 540-nm or 450-nm lights. These results were most evident in cells that received dominant inputs from blue- or green-sensitive cones.

Responses of some other L cells were almost completely dominated by inputs from red-sensitive cones. Summation times of these cells were not wavelength-dependent. But when these inputs also included green-sensitive cones, shorter summation times were obtained to 640-nm light than to 540-nm light, even though dominant inputs were still from red-sensitive cones. These results, obtained from both retinal and 3,4-dehydroretinal photopigment systems, are consistent with reported observations in Pseudemys scripta elegans that show linear responses of red-sensitive cones to have shorter integration times and times-to-peak than green-sensitive cones.

Responses from horizontal cells dominated by blue-sensitive cone inputs were the most sensitive of all; they also had the longest summation times. These results support the hypothesis that a gain in sensitivity occurs from the integration of absorbed photons over longer periods of time.

These intracellular responses are of particular importance because behavioral critical durations in turtle, as defined by Bloch&'s law, are similarly wavelength-dependent.

A postembedding immunogold procedure was used to estimate quantitatively, at the electron-microscopical level, the intensity of glutamate (GLU) immunoreactivity in different identifiable profiles of the lateral geniculate nucleus (LGN) and perigeniculate nucleus (PGN) of the cat. Synaptic terminals of retinal and cortical origins in the LGN, and of axon collaterals of geniculo-cortical relay cells in the PGN, were identified by previously determined ultrastructural features. Processes of interneurons or relay cells were identified by being immunoreactive or non-immunoreactive, respectively, in serial thin section reacted with a GABA antibody.

The results showed that synaptic terminals of geniculo-cortical relay cells in the PGN have significantly higher levels of GLU immunoreactivity than their parent somata or dendrites in the LGN; this suggests transmitter storage of this amino acid in these terminals. By contrast, synaptic terminals of interneurons did not show enrichment of GLU relative to their parent somata. This argues against the possibility that the relative enrichment of GLU in relay cells terminals is due to factors other than presynaptic storage. In addition, axon collateral terminals of relay cells in the PGN, as well as retinal and cortical terminals in the LGN, showed significantly higher GLU immunoreactivity than GABAergic terminals. These immunocytochemical results suggest that GLU is a neurotransmitter in the retino-geniculate, cortico-geniculate, and geniculo-cortical pathways in the cat.

The representation of the visual field in the architectonically defined transition zone between areas 17 and 18 of cat cerebral cortex was assessed by recording the activities and plotting the receptive fields of neurons at 2327 sites along 148 electrode penetrations made in 19 cats. The results show that the transition zone contains a significant representation of the ipsilateral visual hemifield although not all elevations in the visual field represented to the same extent. The shape of the field region represented resembles an hour glass, for the region represented is narrowest on the 0-deg horizontal meridian and increasingly wider at progressively more positive and negative elevations. When receptive-field centers are considered, the extent of the representation reaches to -2.5 deg on the 0-deg horizontal meridian and to 10 or more degrees towards the field periphery. When receptive-field areas are considered, the representation at the 0-deg horizontal meridian extends to -3.6 deg and to beyond 20 deg at other elevations. In contrast, the visual-field representations in flanking areas 17 and 18 are essentially limited to the contralateral hemifield. The presence of a distinct representation of part of the ipsilateral hemifield in the transition zone suggests that the zone may have connections distinctly different from those of the adjacent areas. The observations bear on the problems of understanding the visual pathways in hypopigmented cats and binocular disparity mechanisms about the midline.

The activity of neurons was recorded extracellurayly at the junction of visual cortical areas 17, 18, 20a, and 20b in the cat. The receptive fields of these neurons were striking for their size, which ranged from a diameter of more than 40 deg of visual angle to the complete visual of the contralateral eye. It is speculated that these large receptive fields may be generated by perturbations in the individual maps as the four areas merge together.

Desensitization of cGMP-activated channels was examined in excised, inside-out patches obtained from rod photoreceptors. Cyclic GMP was applied using a rapid-flow system in which concentration jumps are complete within 10–50 ms. In outer-segment patches containing many channels, the cGMP-dependent conductance reached a steady plateau that was maintained for tens of seconds in the presence of cGMP; thus, there was no indication of slow desensitization. However, rapid desensitization on the scale of milliseconds could not be ruled out because of limited speed of access of cGMP to the inner face of the patch membrane. To test for rapid desensitization, inner-segment patches containing only a single cGMP-activated channel were used. In these one-channel patches, there was no change in activity of the channel with time from its earliest onset after application of cGMP, indicating that rapid desensitization also did not occur.

In the rods of Bufo marinus and other species, bleaching of the rhodopsin in isolated cells leads to a loss of sensitivity and response amplitude and to a shortened response duration. These changes are permanent for cells bathed in Ringer&s solution. They are due to as yet unknown modulations in the transduction biochemistry. In this paper, we report that these changes can be partly or completely reversed by supplying biotin, pyruvate, and elevated glucose to the rod. The time course of this reversal and the substances which promote it imply that these are metabolically mediated effects. Based on the reported action of biotin and pyruvate on the one hand and on the changes of the response waveforms on the other hand, we believe that the phenomena we observe involve the later steps of the transduction cycle.

This commentary extends the review by Hiroyuki Uchiyama (1989) entitled “Centrifugal pathways to the retina: influence of the optic tectum” Visual Neuroscience 3, 183–206. A further scrutiny is made of the topography of the isthmo-retinal projection of the pigeon, which provides the most fully investigated example of a retinopetal projection. It is suggested that the topographical organization in this system is compatible with it's input being derived from the lower visual field (and upper retina) and it's output being directed to the horizon and lower retina. This is another form of attentional hypothesis for the centrifugal pathway, compatible with a role in ground feeding, and is testable by current tracing techniques in neurobiology.