Cortical and subcortical influences on the nucleus of the optic tract of the opossum

The commissural transfer of the horizontal optokinetic signal in the rat: a c-Fos study

We applied the Fos method in rats subjected to horizontal optokinetic stimulation (OKS) to study whether optokinetic information is transferred through the commissural pretectal fibres from one optic tract nucleus (NOT) to the opposite. In binocular as well as in monocular nasalward OKS, the highest Fos immunoreactivity was found in the NOT contralateral to the nasalward stimulation, as expression of the activation either of direction-selective cells and of commissural neurons. Even the opposite NOT showed many Fos-positive cells activated by the opposite nucleus throughout the commissural pretectal pathway. They might be the GABA positive cells, which are thought to allow the activation in one nucleus to be transformed into inhibition of the opposite side. In monocular temporalward OKS, the inhibition on direction-selective cells and the consequent silencing of commissural neurons caused the faint immunoreactivity in the NOT contralateral to eye stimulated. In the opposite nucleus the few Fos-positive cells emerged as a consequence of the lack of the normal tonic commissurally mediated inhibition.

Contrast and Temporal Frequency-Related Adaptation in the Pretectal Nucleus of the Optic Tract

In mammals, many cells in the retino-geniculate-cortical pathway adapt during stimulation with high contrast gratings. In the visual cortex, adaptation to high contrast images reduces sensitivity at low contrasts while only moderately affecting sensitivity at high contrasts, thus generating rightward shifts in the contrast response functions (contrast gain control). Similarly, motion adaptation at particular temporal frequencies (TFs) alters the temporal tuning properties of cortical cells. For the first time in any species, this paper investigates the influence of motion adaptation on both the contrast and TF responses of neurons in the retino-pretectal pathway by recording from direction-selective neurons in the nucleus of the optic tract (NOT) of the marsupial wallaby, Macropus eugenii. This species is of interest because its NOT receives almost all input directly from the retina, with virtually none from the visual cortex (unlike cats and primates). All NOT cells show changes in their contrast response functions after adaptation, many revealing contrast gain control. Contrast adaptation is direction-dependent, preferred directions producing the largest changes. The lack of cortical input suggests that contrast adaptation is generated independently from the cortex in the NOT or retina. Motion adaptation also produces direction-selective effects on the TF tuning of NOT neurons by shifting the location of the optimum TF. Cells that show strong adaptation to contrast also tend to show large changes in TF tuning, suggesting similar intracellular mechanisms. The data are discussed in terms of the generality of contrast adaptation across mammalian species and across unconnected brain regions within the same species.

Telencephalic projections to the nucleus of the basal optic root and pretectal nucleus lentiformis mesencephali in pigeons

In birds, the nucleus of the basal optic root (nBOR) of the accessory optic system (AOS) and the pretectal nucleus lentiformis mesencephali (LM) are involved in the analysis of optic flow and the generation of the optokinetic response. In several species, it has been shown that the AOS and pretectum receive input from visual areas of the telencephalon. Previous studies in pigeons using anterograde tracers have shown that both nBOR and LM receive input from the visual Wulst, the putative homolog of mammalian primary visual cortex. In the present study, we used retrograde and anterograde tracing techniques to further characterize these projections in pigeons. After injections of the retrograde tracer cholera toxin subunit B (CTB) into either LM or nBOR, retrograde labeling in the telencephalon was restricted to the hyperpallium apicale (HA) of the Wulst. From the LM injections, retrograde labeling appeared as a discrete band of cells restricted to the lateral edge of HA. From the nBOR injections, the retrograde labeling was more distributed in HA, generally dorsal and dorso-medial to the LM-projecting neurons. In the anterograde experiments, biotinylated dextran amine (BDA) was injected into HA and individual axons were reconstructed to terminal fields in the LM and nBOR. Those fibers projecting to the nBOR also innervated the adjacent ventral tegmental area. However, tracing of BDA-labeled axons revealed no evidence that individual neurons project to both LM and nBOR. In summary, our results suggest that the nBOR and LM receive input from different areas of the Wulst. We discuss how these projections may transmit visual and/or somatosensory information to the nBOR and LM.

Telencephalic Input to the Pretectum of Pigeons: An Electrophysiological and Pharmacological Inactivation Study

The pretectal nucleus lentiformis mesencephali (LM) and the nucleus of the basal optic root (nBOR) of the avian accessory optic system (AOS) are retinal-recipient visual nuclei involved in the analysis of optic flow that results from self-motion, and in the generation of the optokinetic response. Neurons in these nuclei show direction selectivity in response to large-field motion and are tuned in the spatiotemporal domain. In addition to retinal afferentation, both the nBOR and LM receive afferents from the Wulst, which is thought to be the avian homolog of the primary visual cortex. We examined the effects of Wulst electrical stimulation on the activity of LM neurons and recorded the directional and spatiotemporal tuning of LM neurons in pigeons before, during, and after the Wulst was temporarily inactivated by lidocaine injection. In response to Wulst electrical stimulation, LM neurons showed either short-latency excitation followed by longer-latency inhibition (W+ cells), or only a longer-latency inhibition (W– cells). The average response latencies for W+ and W– cells were 13.5 and 28.3 ms, respectively. The effects of Wulst stimulation did not correlate with either the directional or spatiotemporal tuning of the LM neurons. Injection of lidocaine into the nBOR reduced the longer-latency oscillations of W+ and W– cells. When the Wulst was temporarily inactivated by lidocaine neither the directional nor spatiotemporal response properties of LM neurons were affected. The possible functions of the projection from the Wulst to the LM are discussed.

Video-oculography in the gerbil

Normative vestibulo-ocular and optokinetic reflexes (VOR and OKR) and pupil diameter were measured in young adult gerbils using infrared video-oculography with 60 Hz sampling during head-fixed binocular recordings. The pupillary light-sink technique was preferred over a single-beam retinal reflection method because its measurements were less affected by pupil size. Eye movements were generally conjugate with occasional independent saccadic movements, and independent drifting movements in the dark. The horizontal optokinetic response to sinusoidal motion of a randomly spaced white dot pattern was maximal at low velocities (5 degrees/s), stronger temporonasally, and dropped off quickly at approximately 20 degrees/s. Constant velocity gain was near unity through 60-80 degrees/s with a sharp drop-off. Monocular viewing revealed almost no nasotemporal optokinetic response. Pupil diameter was found to vary as a saddle function with optokinetic gain from cycle to cycle, but also have a circadian rhythm (smaller at dusk) that related inversely to mean horizontal VOR gain. Gerbils with eyes open sometimes had no optokinetic response during long stimulus periods, which then resumed after a brief vestibular stimulus. The horizontal angular VOR gain was relatively flat across 0.1-1.0 Hz and 30-120 d/s sines (phase near zero), with a mean gain of approximately 0.78 in the dark, and 1.0 with the fixed pattern surround (n=15, for both raw calibrated and normalized data). Most animals also revealed a strong slow phase eye velocity asymmetry (dominant during ipsilateral rotation) in the half-cycle gain of their horizontal angular VOR response in the dark. A constant velocity horizontal optokinetic bias velocity did not change the gain or symmetry of the sinusoidal VOR response, but shifted the VOR response velocity in an additive (linear) fashion. Both cross-coupling (pitch or roll while rotating) and pseudo-OVAR (off-axis counter-rotation) stimuli generated horizontal nystagmus. The findings suggest that the gerbil, like other lateral-eyed rodents, relies on otolith cues to interpret angular motion.

Fundamental mechanisms of visual motion detection: models, cells and functions

Taking a comparative approach, data from a range of visual species are discussed in the context of ideas about mechanisms of motion detection. The cellular basis of motion detection in the vertebrate retina, sub-cortical structures and visual cortex is reviewed alongside that of the insect optic lobes. Special care is taken to relate concepts from theoretical models to the neural circuitry in biological systems. Motion detection involves spatiotemporal pre-filters, temporal delay filters and non-linear interactions. A number of different types of non-linear mechanism such as facilitation, inhibition and division have been proposed to underlie direction selectivity. The resulting direction-selective mechanisms can be combined to produce speed-tuned motion detectors. Motion detection is a dynamic process with adaptation as a fundamental property. The behavior of adaptive mechanisms in motion detection is discussed, focusing on the informational basis of motion adaptation, its phenomenology in human vision, and its cellular basis. The question of whether motion adaptation serves a function or is simply the result of neural fatigue is critically addressed.