The accessory optic system in the newt, Triturus cristatus: unitary response properties from the basal optic neuropil

Response properties of optic flow neurons in the accessory optic system of hummingbirds versus zebra finches and pigeons

Optokinetic responses function to maintain retinal image stabilization by minimizing optic flow that occurs during self-motion. The hovering ability of hummingbirds is an extreme example of this behavior. Optokinetic responses are mediated by direction-selective neurons with large receptive fields in the accessory optic system (AOS) and pretectum. Recent studies in hummingbirds showed that, compared with other bird species, 1) the pretectal nucleus lentiformis mesencephali (LM) is hypertrophied, 2) LM has a unique distribution of direction preferences, and 3) LM neurons are more tightly tuned to stimulus velocity. In this study, we sought to determine if there are concomitant changes in the nucleus of the basal optic root (nBOR) of the AOS. We recorded the visual response properties of nBOR neurons to large-field-drifting random dot patterns and sine-wave gratings in Anna's hummingbirds and zebra finches and compared these with archival data from pigeons. We found no differences with respect to the distribution of direction preferences: Neurons responsive to upward, downward, and nasal-to-temporal motion were equally represented in all three species, and neurons responsive to temporal-to-nasal motion were rare or absent (<5%). Compared with zebra finches and pigeons, however, hummingbird nBOR neurons were more tightly tuned to stimulus velocity of random dot stimuli. Moreover, in response to drifting gratings, hummingbird nBOR neurons are more tightly tuned in the spatiotemporal domain. These results, in combination with specialization in LM, support a hypothesis that hummingbirds have evolved to be "optic flow specialists" to cope with the optomotor demands of sustained hovering flight.NEW & NOTEWORTHY Hummingbirds have specialized response properties to optic flow in the pretectal nucleus lentiformis mesencephali (LM). The LM works with the nucleus of the basal optic root (nBOR) of the accessory optic system (AOS) to process global visual motion, but whether the neural response specializations observed in the LM extend to the nBOR is unknown. Hummingbird nBOR neurons are more tightly tuned to visual stimulus velocity, and in the spatiotemporal domain, compared with two nonhovering species.

Membrane properties and monosynaptic retinal excitation of neurons in the turtle accessory optic system

Using an eye-attached isolated brain stem preparation of a turtle, Pseudemys scripta elegans, in conjunction with whole cell patch techniques, we recorded intracellular activity of accessory optic system neurons in the basal optic nucleus (BON). This technique offered long-lasting stable recordings of individual synaptic events. In the reduced preparation (most of the dorsal structures were removed), large spontaneous excitatory synaptic inputs [excitatory postsynaptic potentials (EPSPs)] were frequently recorded. Spontaneous inhibitory postsynaptic potentials were rarely observed except in few cases. Most EPSPs disappeared after injection of lidocaine into the retina. A few EPSPs of small size remained, suggesting that these EPSPs either were from intracranial sources or may have been miniature spontaneous synaptic potentials from retinal ganglion cell axon terminals. Population EPSPs were synchronously evoked by electrical stimulation of the contralateral optic nerve. Their constant onset latency and their ability to follow short-interval paired stimulation indicated that much of the population EPSP's response was monosynaptic. Visually evoked BON spikes and EPSP inputs to BON showed direction sensitivity when a moving pattern was projected onto the entire contralateral retina. With the use of smaller moving patterns, the receptive field of an individual BON cell was identified. A small spot of light, projected within the receptive field, guided the placement of a bipolar stimulation electrode to activate retinal ganglion cells that provided input to that BON cell. EPSPs evoked by this retinal microstimulation showed features of unitary EPSPs. Those EPSPs had distinct low current thresholds. Recruitment of other inputs was only evident when the stimulation level was increased substantially above threshold. The average size of evoked unitary EPSPs was 7.8 mV, confirming the large size of synaptic inputs of this system relative to nonsynaptic noise. EPSP shape was plotted (rise time vs. amplitude), with the use of either evoked unitary EPSPs or spontaneous EPSPs. Unlike samples of spontaneous EPSPs, data from many unitary EPSPs formed distinct clusters in these scatterplots, indicating that these EPSPs had a unique shape among the whole population of EPSPs. In most BON cells studied, hyperpolarization-activated channels caused a slow depolarization sag that reached a plateau within 0.5-1 s. This property suggests that BON cells may be more complicated than a simple site for convergence of direction-sensitive retinal ganglion cells to form a central retinal slip signal for control of oculomotor reflexes.

Morphology of retinal ganglion cells in lungless salamanders (fam. Plethodontidae): an HRP and Golgi study

The family Plethodontidae consists of nearly two-thirds of all living urodeles; most of them possess highly developed visual abilities. We investigated the morphology of retinal ganglion cells (RGCs) in four representative species by means of the horseradish peroxidase method in flatmounts and in transverse sections and with the Golgi method in transverse sections. In flatmount preparations, four classes of RGCs were found, differing in dendritic arborization, dendritic field size, and stratification pattern of dendrites in the inner plexiform layer (IPL). Class-1 cells had small dendritic fields (29-44 microns 2) and arborized throughout the entire depth of the IPL. Class-2 cells had medium to large dendritic fields (75-206 microns 2) and mostly arborized in two or three laminae or in a diffuse fashion in the IPL. Class-3 cells had medium to large dendritic fields (72-200 microns 2) but sparse dendritic arborization. They only arborized in the proximal lamina of the IPL. Class-4 cells had large dendritic fields (273-626 microns 2) and branched in the most sclerad stratum of the IPL. No large differences in intraspecific soma size of the different RGC classes were detected (although interspecific soma size varied to a considerable degree) and no "giant" cells typically found in other vertebrate retinas were present. The results suggest that, with respect to the pattern of arborization and stratification of dendrites, lungless salamanders possess morphological classes of RGC similar to those found in frogs, but the morphology of RGCs in lungless salamanders seems to be simplified in comparison to frog RGCs. This simplification might be a consequence of paedomorphosis.

Directional selectivities of visual afferents to the pretectal neuropil in the fire salamander

Single-unit recordings from visual afferents in the pretectal neuropil of Salamandra salamandra were performed to characterize the properties of direction specific units which are probably involved in the optokinetic reflex. It was shown that afferents from the contralateral eye were more sharply tuned than those from the ipsilateral eye. However, the majority of both were selective for temporonasal movements.

Vision and the skin camouflage reactions of Ambystoma larvae: the effects of eye transplants and brain lesions

Salamander larvae typically adapt their dermal melanophores to achieve camouflage, and it has been known for some time that removal of the eyes abolishes the response. Here we survey the contribution of the optic system to the bright and dark camouflage reactions and report that: the stimulus depends on an interaction between the direct and reflected light; an eye mounted atop the head and oriented vertically tended not to support camouflage, even though the animal responded to visual cues and learned a vision-dependent task; deviating the transplanted eye off the vertical axis enhanced the recovery of camouflage reactions; amputating or reorienting the telencephalon, epithalamus, pretectum or tectum did not abolish either camouflage reaction whereas lesions of the ventral optic pathway blocked brightening; transection near the midbrain-hindbrain junction--well posterior to known optic terminals--retarded the dark reaction; when the latter lesion was combined with disconnection of the telencephalon and epithalamus, contrary to predictions from the lesions executed separately, the animals lost the bright reaction; the hypophysis is necessary for darkening, but the organ supported this reaction even though detached, displaced or reoriented; and the pineal body was not essential for the grosser aspects of camouflage in Ambystoma larvae but may play an adjunctive role in fine tuning.

Projections of medial terminal accessory optic nucleus, ventral tegmental nuclei, and substantia nigra of rabbit and rat as studied by retrograde axonal transport of horseradish peroxidase

Projections of the medial terminal nucleus (MTN) of the accessory optic system, the ventral tegmental area of Tsai, and the substantia nigra of the rabbit and the rat have been studied by the method of retrograde axonal transport of horseradish peroxidase. The data show that MTN projections are remarkably similar in the rabbit and rat. The MTN projects heavily to the ipsilateral nucleus of the optic tract and dorsal terminal nucleus of the accessory optic system and to a portion of the contralateral ventral tegmental area of Tsai that we have termed the visual tegmental relay zone (VTRZ). Further, the MTN sends projections to the ipsilateral mesencephalic (deep mesencephalic nucleus, pars medialis) and pontine (nucleus reticularis pontis oralis) reticular formations; the contralateral dorsolateral division of the basal pontine complex; the superior and lateral vestibular nuclei (contralateral in rat; bilateral in rabbit); and the ipsi- and contralateral interstitial nucleus of Cajal, nucleus of Darkschewitsch, and supraoculomotor-periaqueductal gray. The findings also indicate that the MTN has a small bilateral, but mainly ipsilateral, projection to the dorsal cap, its ventrolateral outgrowth, and the B division of the inferior olivary complex. This study further reveals that ventral tegmental nuclei (n. parabrachialis pigmentosus and n. paranigralis) and subdivisions of the substantia nigra (pars compacta and pars reticulata) project to many brain stem targets of the MTN. Thus, the VTRZ projections are similar to those of the MTN in both distribution and density except that the VTRZ projection to the inferior olive is substantially stronger. The nucleus parabrachialis pigmentosus sends a small contralateral projection to the VTRZ and a moderate-sized bilateral projection to the supraoculomotor-periaqueductal gray. The nucleus paranigralis sends a moderate number of axons to the ipsilateral deep mesencephalic nucleus, pars medialis, and the nucleus reticularis pontis oralis and provides a strong bilateral projection to the supraoculomotor-periaqueductal gray. The pars compacta of the substantia nigra provides a sparse input to the ipsilateral deep mesencephalic nucleus, pars medialis, and nucleus reticularis pontis oralis, and to the contralateral VTRZ and sends a moderate number of axons, bilaterally, to the supraoculomotor-periaqueductal gray. The pars reticulata of the substantia nigra sends an ipsiateral projection of moderate size to the intermediate and deep layers of the superior colliculus, sparse ipsilateral projections to the deep mesencephalic nucleus, pars medialis, and nucleus reticularis pontis oralis, and a sparse bilateral projection to

Electrophysiology and anatomy of direction-specific pretectal units in Salamandra salamandra

Pretectal cells of the European fire salamander were recorded extracellularly during binocular and monocular horizontal optokinetic stimulations. The locations of the individual units within the pretectal nucleus were verified with Alcian blue injections. The particular anatomical properties of single cells were demonstrated after single-unit recordings by means of horseradish peroxidase preparations. Direction selective pretectal cells were found to be predominantly (2/3) sensitive to temporo-nasal movements in the visual field of the contralateral eye. They usually possessed large receptive fields centered on the visual equator with restricted diameters in the dorso-ventral axis. Their resting discharge was low, and in some cells no spontaneous discharges were observed. The cells preferred low stimulus velocities, most of them being optimally stimulated with velocities of 1 to 10 deg/s. A group of units was exclusively sensitive to accelerated movements. A subclass of them was transiently responsive when the stimulus stopped. In the anterior and most dorsal part of the pretectal nucleus, binocularly influenced units were found. These cells responded best with binocular optokinetic stimulations and less vigorously or with less pronounced direction selectivity if only the contralateral eye was stimulated. With ipsilateral stimulations alone no response could be elicited. This response type could be explained by inhibitory inputs from the ipsilateral eye via direct ipsilateral projections or crossing pretectal fibers. The responses of these cells are well correlated to behavioral results showing that OKN performance in salamanders, as in some other vertebrates, is different with binocular as compared to monocular stimulations. The direction-sensitive pretectal cells usually possess extensive dendritic arborizations within the ipsilateral pretectal neuropil. Most of the cell bodies were scattered in the white substance or in the superficial layers of the periventricular gray. In the cases where the efferent fiber of a particular cell could be clearly recognized, the axon projected to the basal optic neuropil of the accessory optic system, the contralateral pretectum or, in two cases, to the medulla oblongata into a region which might be homologous to the inferior olive of higher vertebrates.

Single unit activity in the nucleus of the basal optic root (nBOR) during optokinetic, vestibular and visuo-vestibular stimulations in the alert pigeon (Columbia livia)

Extracellular recordings were performed in the nucleus of the basal optic root (nBOR) of alert pigeons during optokinetic nystagmus (OKN), vestibulo-ocular reflex (VOR) and combined visuo-vestibular stimulation. Cell identification was assessed either by histological control or by electrophysiological testing (antidromic response to vestibulo-cerebellar or oculomotor complex stimulation). 1) OKN was induced in 8 directions by a binocular stimulation. During the fast phase of OKN, optokinetic after nystagmus (OKAN) or reversed OKAN, most cells showed an inhibition which varied in magnitude independent of the direction of stimulation. A few cells however showed a phasic discharge for some OKN directions. 2) During the slow phase of OKN induced by a binocular stimulation, cells displayed either a tonic activation or a more or less strong inhibition according to the direction of the OKN. Cells were classified in 4 groups, according to their degree of directional specificity. The best OKN direction (slow phase) for maximal cell activation was upwards and naso-upwards, and next to best, naso-temporal and downwards. Maximal cell inhibition occurred during downward, and for some cells during upward, directions. 3) During OKN induced by stimulating the eye contralateral to the recorded nBOR, cell responses resembled those obtained during binocular stimulation, but, during ipsilaterally induced OKN, the cells lost their directional specificity. As a result of binocular integration, neuronal activation seems to originate from contralateral input whereas cell inhibition would mainly come from ipsilateral input. 4) During sinusoidal optokinetic stimulation induced in the temporo downward-naso upward axis, cells showed a more or less marked modulation (according to their directional selectivity) that was closely in phase with the stimulation velocity, and therefore probably with retinal slip. 5) nBOR cells appeared generally unaffected during both the slow phase and the fast phase of the VOR. However, some cells showed a slight but irregular modulation which might imply a weak vestibular input. During visuo-vestibular stimulation, the response resembled that obtained with sinusoidal optokinetic stimulation but the fast phase inhibition was often strengthened in the downwards direction (fast phase).