Primate pupillary light reflex: receptive field characteristics of pretectal luminance neurons

Time-on-task effects on human pupillary and saccadic metrics after theta burst transcranial magnetic stimulation over the frontal eye field

Pupil size undergoes constant changes primarily influenced by ambient luminance. These changes are referred to as the pupillary light reflex (PLR), where the pupil transiently constricts in response to light. PLR kinematics provides valuable insights into autonomic nervous system function and have significant clinical applications. Recent research indicates that attention plays a role in modulating the PLR, and the circuit involving the frontal eye field (FEF) and superior colliculus is causally involved in controlling this pupillary modulation. However, there is limited research exploring the role of the human FEF in these pupillary responses, and its impact on PLR metrics remains unexplored. Additionally, although the protocol of continuous theta-burst stimulation (cTBS) is well-established, the period of disruption after cTBS is yet to be examined in pupillary responses. Our study aimed to investigate the effects of FEF cTBS on pupillary and saccadic metrics in relation to time spent performing a task (referred to as time-on-task). We presented a bright stimulus to induce the PLR in visual- and memory-delay saccade tasks following cTBS over the right FEF or vertex. FEF cTBS, compared to vertex cTBS, resulted in decreased baseline pupil size, peak constriction velocities, and amplitude. Furthermore, the time-on-task effects on baseline pupil size, peak amplitude, and peak time differed between the two stimulation conditions. In contrast, the time-on-task effects on saccadic metrics were less pronounced between the two conditions. In summary, our study provides the first evidence that FEF cTBS affects human PLR metrics and that these effects are modulated by time-on-task.

PyPlr: A versatile, integrated system of hardware and software for researching the human pupillary light reflex

We introduce PyPlr-a versatile, integrated system of hardware and software to support a broad spectrum of research applications concerning the human pupillary light reflex (PLR). PyPlr is a custom Python library for integrating a research-grade video-based eye-tracker system with a light source and streamlining stimulus design, optimisation and delivery, device synchronisation, and extraction, cleaning, and analysis of pupil data. We additionally describe how full-field, homogenous stimulation of the retina can be realised with a low-cost integrating sphere that serves as an alternative to a more complex Maxwellian view setup. Users can integrate their own light source, but we provide full native software support for a high-end, commercial research-grade 10-primary light engine that offers advanced control over the temporal and spectral properties of light stimuli as well as spectral calibration utilities. Here, we describe the hardware and software in detail and demonstrate its capabilities with two example applications: (1) pupillometer-style measurement and parametrisation of the PLR to flashes of white light, and (2) comparing the post-illumination pupil response (PIPR) to flashes of long and short-wavelength light. The system holds promise for researchers who would favour a flexible approach to studying the PLR and the ability to employ a wide range of temporally and spectrally varying stimuli, including simple narrowband stimuli.

The Pupillary Light Reflex as a Biomarker of Concussion

The size of our pupils changes continuously in response to variations in ambient light levels, a process known as the pupillary light reflex (PLR). The PLR is not a simple reflex as its function is modulated by cognitive brain function and any long-term changes in brain function secondary to injury should cause a change in the parameters of the PLR. We performed a retrospective clinical review of the PLR of our patients using the BrightLamp Reflex iPhone app. The PLR variables of latency, maximum pupil diameter (MaxPD), minimum pupil diameter (MinPD), maximum constriction velocity (MCV), and the 75% recovery time (75% PRT) were associated with significant differences between subjects who had suffered a concussion and those that had not. There were also significant differences in PLR metrics over the life span and between genders and those subjects with and without symptoms. The differences in PLR metrics are modulated not only by concussion history but also by gender and whether or not the person has symptoms associated with a head injury. A concussive injury to the brain is associated with changes in the PLR that persist over the life span, representing biomarkers that might be used in clinical diagnosis, treatment, and decision making.

Pupil responses to implied motion in figurative and abstract paintings

Motion can be perceived in static images, such as photos and figurative paintings, representing realistic subjects in motion, with or without directional information (e.g., motion blur or speed lines). Motion impression can be achieved even in non-realistic static images such as motion illusions and abstract paintings. It has been shown that visual motion processing affects the diameter of the pupil, responding differently to real, illusory, and implied motion in photographs (IM). It has been suggested that these different effects might be due to top-down modulations from different cortical areas underlying their processing. It is worthwhile to investigate pupillary response to figurative paintings, since they require an even higher level of interpretation than photos representing the same kind of subjects, given the complexity of cognitive processes involved in the aesthetic experience. Also, pupil responses to abstract paintings allows to study the effect of IM perception in representations devoid of real-life motion cues. We measured pupil responses to IM in figurative and abstract artworks depicting static and dynamic scenes, as rated by a large group of individuals not participating in the following experiment. Since the pupillary response is modulated by the subjective image interpretation, a motion rating test has been used to correct individual pupil data according to whether participants actually perceived the presence of motion in the paintings. Pupil responses to movies showing figurative and abstract subjects, and to motion illusions were also measured, to compare real and illusory motion with painted IM. Movies, both figurative and abstract, elicit the largest pupillary dilation of all static stimuli, whereas motion illusions cause the smallest pupil size, as previously shown. Interestingly, pupil responses to IM depend on the paintings’ style. Figurative paintings depicting moving subjects cause more dilation than those representing static figures, and pupil size increases with the strength of IM, as already found with realistic photos. The opposite effect is obtained with abstract artworks. Abstract paintings depicting motion produce less dilation than those depicting stillness. In any case, these results reflect the individual subjective perception of dynamism, as the very same paintings can induce opposite responses in observer which interpreted it as static or dynamic. Overall, our data show that pupil size depends on high-level interpretation of motion in paintings, even when they do not represent real-world scenes. Our findings further suggest that the pupil is modulated by multiple top-down cortical mechanisms, involving the processing of motion, attention, memory, imagination, and other cognitive functions necessary for enjoying a complete aesthetic experience.

Wearable light spectral sensor optimized for measuring daily α-opic light exposure

Light has many non-visual effects on human physiology, including alterations in sleep, mood, and alertness. These effects are mainly mediated by photoreceptors containing the photopigment melanopsin, which has a peak sensitivity to short wavelength ('blue') light. Commercially available light sensors are commonly wrist-worn and report photopic illuminance and are calibrated to perceive visual brightness and hence cannot be used to investigate the non-visual impacts of light. In this paper, we report the development of a wearable spectrophotometer designed to be worn as a pendant or affixed to clothing to capture spectral power density data close to eye level in the visible wavelength range 380-780 nm. From this, the relative impact of a given light stimulus can be determined for each photoreceptive input in the human eye by calculating effective illuminances. This device showed high accuracy for all effective illuminances while measuring a range of commonly encountered light sources by calibrating for directional response, dark noise, sensor saturation, non-linearity, stray-light and spectral response. Features of the device include IoT-integration, onboard data storage and processing, Bluetooth Low Energy (BLE) enabled data transfer, and cloud storage in one cohesive unit.

Effects of pupillary light and darkness reflex on the generation of pro- And anti-saccades

Saccades are often directed toward a stimulus that provides useful information for observers to navigate the visual world. The quality of visual signals of a stimulus is influenced by global luminance, and the pupil constricts or dilates after a luminance increase or decrease, respectively, to optimize visual signals for further information processing. Although luminance level changes regularly in the real environment, saccades are mostly studied in the luminance-unchanged setup. Whether pupillary responses triggered by global luminance changes modulate saccadic behavior are yet to be explored. Through varying background luminance level in an interleaved pro- and anti-saccade paradigm, we investigated the modulation of pupillary luminance responses on the generation of reflexive and voluntary saccades. Subjects were instructed to either automatically look at the peripheral stimulus (pro-saccade) or to suppress the automatic response and voluntarily look in the opposite direction from the stimulus (anti-saccade). Level of background luminance was increased (light), decreased (dark), or unchanged (control) during the instructed fixation period. Saccade reaction time distributions of correct pro- and anti-saccades in the light and dark conditions were differed significantly from those in the control condition. Moreover, the luminance condition modulated saccade kinematics, showing reduced performances in the light condition than in the control condition, particularly in pro-saccades. Modeling results further suggested that both pupil diameter and pupil size derivative significantly modulated saccade behavior, though effect sizes were small and mainly mediated by intersubject differences. Together, our results demonstrated the influence of pupillary luminance responses on the generation of pro- and anti-saccades.

Pupillary light reflex circuits in the macaque monkey: the preganglionic Edinger-Westphal nucleus

The motor outflow for the pupillary light reflex originates in the preganglionic motoneuron subdivision of the Edinger-Westphal nucleus (EWpg), which also mediates lens accommodation. Despite their importance for vision, the morphology, ultrastructure and luminance-related inputs of these motoneurons have not been fully described in primates. In macaque monkeys, we labeled EWpg motoneurons from ciliary ganglion and orbital injections. Both approaches indicated preganglionic motoneurons occupy an EWpg organized as a unitary, ipsilateral cell column. When tracers were placed in the pretectal complex, labeled terminals targeted the ipsilateral EWpg and reached contralateral EWpg by crossing both above and below the cerebral aqueduct. They also terminated in the lateral visceral column, a ventrolateral periaqueductal gray region containing neurons projecting to the contralateral pretectum. Combining olivary pretectal and ciliary ganglion injections to determine whether a direct pupillary light reflex projection is present revealed a labeled motoneuron subpopulation that displayed close associations with labeled pretectal terminal boutons. Ultrastructurally, this subpopulation received synaptic contacts from labeled pretectal terminals that contained numerous clear spherical vesicles, suggesting excitation, and scattered dense-core vesicles, suggesting peptidergic co-transmitters. A variety of axon terminal classes, some of which may serve the near response, synapsed on preganglionic motoneurons. Quantitative analysis indicated that pupillary motoneurons receive more inhibitory inputs than lens motoneurons. To summarize, the pupillary light reflex circuit utilizes a monosynaptic, excitatory, bilateral pretectal projection to a distinct subpopulation of EWpg motoneurons. Furthermore, the interconnections between the lateral visceral column and olivary pretectal nucleus may provide pretectal cells with bilateral retinal fields.

Pupillary light reflex circuits in the Macaque Monkey: the olivary pretectal nucleus

The olivary pretectal nucleus is the first central connection in the pupillary light reflex pathway, the circuit that adjusts the diameter of the pupil in response to ambient light levels. This study investigated aspects of the morphology and connectivity of the olivary pretectal nucleus in macaque monkeys by use of anterograde and retrograde tracers. Within the pretectum, the vast majority of neurons projecting to the preganglionic Edinger-Westphal nucleus were found within the olivary pretectal nucleus. Most of these neurons had somata located at the periphery of the nucleus and their heavily branched dendrites extended into the core of the nucleus. Retinal terminals were concentrated within the borders of the olivary pretectal nucleus. Ultrastructural examination of these terminals showed that they had clear spherical vesicles, occasional dense-core vesicles, and made asymmetric synaptic contacts. Retrogradely labeled cells projecting to the preganglionic Edinger-Westphal nucleus displayed relatively few somatic contacts. Double labeling indicated that these neurons receive direct retinal input. The concentration of retinal terminals within the nucleus and the extensive dendritic trees of the olivary projection cells provide a substrate for very large receptive fields. In some species, pretectal commissural connections are a substrate for balancing the direct and consensual pupillary responses to produce pupils of equal size. In the macaque, there was little evidence for such a commissural projection based on either anterograde or retrograde tracing. This may be due to the fact that each macaque retina provides nearly equal density projections to the ipsilateral and contralateral olivary pretectal nucleus.

Localization of Neuronal Gain Control in the Pupillary Response

Multifocal pupillographic objective perimetry (mfPOP) is being developed as an alternative to standard visual perimetry. In mfPOP, pupil responses to sparse multifocal luminance stimuli are extracted from the overall composite response. These individual test-region responses are subject to gain-control which is dependent on the temporal and spatial density of stimuli. This study aimed to localize this gain within the pupil pathway. Pupil constriction amplitudes of 8 subjects (41.5 ±12.7 y, 4 male) were measured using a series of 14 mfPOP stimulus variants. The temporal density of stimulus signal at the levels of retina, pretectal olivary nuclei (PON), and Edinger-Westphal nuclei (EWN) were controlled using a combination of manipulation of the mean interval between stimulus presentations (3 or 6 stimuli/s/hemiretina) and the restriction of stimuli to specific subsets of the 24 visual field test-regions per eye (left or right eye, left or right hemifield, or nasal or temporal hemifield). No significant difference was observed between mfPOP variants with differing signal density at the retina or PON but matched density at the other levels. In contrast, where signal density differed at the EWN but was the same at the retinal and PON levels e.g., between 3 stim/s homonymous hemifield and all test-region variants, significant reductions in constriction amplitudes were observed [t₍₃₀₎ = −2.07 to −2.50, all p < 0.05]. Similar, although more variable, relationships were seen using nasal, and temporal hemifield stimuli. Results suggest that the majority of gain-control in the subcortical pupillary pathway occurs at the level of the EWN.

A novel method of inducing endogenous pupil oscillations to detect patients with unilateral optic neuritis

Purpose

To use and test a new method of inducing endogenously generated pupillary oscillations (POs) in patients with unilateral optic neuritis (ON), to describe a signal analysis approach quantifying pupil activity and to evaluate the extent to which POs permit to discriminate patients from control participants.

Method

Pupil size was recorded with an eye-tracker and converted in real time to modulate the luminance of a stimulus (a 20° disk) presented in front of participants. With this biofeedback setting, an increasing pupil size transforms into a high luminance, entraining a pupil constriction that in turn decreases the stimulus luminance, and so on, resulting in endogenously generated POs. POs were recorded for 30 seconds in the affected eye, in the fellow eye and in binocular conditions with 22 patients having a history of unilateral ON within a period of 5 years, and with 22 control participants. Different signal analysis methods were used to quantify the power and frequency of POs.

Results

On average, pupil size oscillated at around 1 Hz. The amplitude of POs appears not to be a reliable marker of ON. In contrast, the frequency of POs was significantly lower, and was more variable over time, in the patients’ affected eye, as compared to their fellow eye and to the binocular condition. No such differences were found in control participants. Receiver operating characteristic analyses based on the frequency and the variability of POs to classify patients and control participants gave an area under the curve of 0.82, a sensitivity of 82% (95%CI: 60%-95%) and a specificity of 77% (95%CI: 55%-92%).

Conclusions

The new method used to induce POs allowed characterizing the visual afferent pathway defect in ON patients with encouraging accuracy. The method was fast, easy to use, only requiring that participants look ahead, and allows testing many stimulus parameters (e.g. color, stimulus location, size, etc).

Melanopsin photoreception contributes to human visual detection, temporal and colour processing

The visual consequences of melanopsin photoreception in humans are not well understood. Here we studied melanopsin photoreception using a technique of photoreceptor silent substitution with five calibrated spectral lights after minimising the effects of individual differences in optical pre-receptoral filtering and desensitising penumbral cones in the shadow of retinal blood vessels. We demonstrate that putative melanopsin-mediated image-forming vision corresponds to an opponent S-OFF L + M-ON response property, with an average temporal resolution up to approximately 5 Hz, and >10x higher thresholds than red-green colour vision. With a capacity for signalling colour and integrating slowly changing lights, melanopsin-expressing intrinsically photosensitive retinal ganglion cells maybe the fifth photoreceptor type for peripheral vision.

A control system analysis of the dynamic response of N-methyl-D-aspartate glutamate receptors to alcoholism and alcohol withdrawal

BACKGROUND

N-methyl-D-Aspartate (NMDA) and its receptors (NMDAR) play a critical role in glutamatergic neurotransmission. Ethanol molecules inhibit these receptors, and if the brain is exposed to ethanol chronically, NMDA-induced glutamatergic changes can result in physical dependence to ethanol in order to sustain normal brain function. In these cases, removal of ethanol from the system results in excitotoxic withdrawal. One compensatory mechanism the brain uses to regulate extracellular glutamate concentration is modulating the number of NMDARs at the synapse. Previous work has shown that the number of functional NMDARs at the synapse can be changed by three mechanisms: additional receptors can be synthesized and inserted, receptors can be recruited to the synapse from extrasynaptic regions, or the functionality of existing receptors can be modified.

METHODS

In this study, we consider the dynamic relocation control of NMDARs in response to chronic alcoholism and withdrawal. Specifically, we (1) propose and construct a mathematical model of the relocation control as a negative feedback system with an explicit set point, (2) investigate the effect of various ethanol consumption and withdrawal profiles on the NMDAR population, and (3) propose and calculate quantitative measures for the extent of withdrawal based on modeled NMDAR populations.

RESULTS

A relocation-only model with an explicit set point was developed. The model was shown to apply across a wide range of controller parameters. The results suggest that withdrawal severity does not depend upon the dynamics involved in the development of dependence, and that regulating the blood alcohol level throughout the progression of withdrawal can minimize excitotoxic withdrawal symptoms.

CONCLUSIONS

The negative feedback control system produced characteristic behaviors of NMDAR populations in response to simulations of alcohol dependence and abrupt withdrawal. The model can also predict the severity of excitotoxic withdrawal following various alcohol consumption and/or withdrawal patterns in order to generate testable hypotheses regarding ameliorating withdrawal.

Internal organization of medial rectus and inferior rectus muscle neurons in the C group of the oculomotor nucleus in monkey

Mammalian extraocular muscles contain singly innervated twitch muscle fibers (SIF) and multiply innervated nontwitch muscle fibers (MIF). In monkey, MIF motoneurons lie around the periphery of oculomotor nuclei and have premotor inputs different from those of the motoneurons inside the nuclei. The most prominent MIF motoneuron group is the C group, which innervates the medial rectus (MR) and inferior rectus (IR) muscle. To explore the organization of both cell groups within the C group, we performed small injections of choleratoxin subunit B into the myotendinous junction of MR or IR in monkeys. In three animals the IR and MR myotendinous junction of one eye was injected simultaneously with different tracers (choleratoxin subunit B and wheat germ agglutinin). This revealed that both muscles were supplied by two different, nonoverlapping populations in the C group. The IR neurons lie adjacent to the dorsomedial border of the oculomotor nucleus, whereas MR neurons are located farther medially. A striking feature was the differing pattern of dendrite distribution of both cell groups. Whereas the dendrites of IR neurons spread into the supraoculomotor area bilaterally, those of the MR neurons were restricted to the ipsilateral side and sent a focused bundle dorsally to the preganglionic neurons of the Edinger-Westphal nucleus, which are involved in the "near response." In conclusion, MR and IR are innervated by independent neuron populations from the C group. Their dendritic branching pattern within the supraoculomotor area indicates a participation in the near response providing vergence but also reflects their differing functional roles.

Autonomic control of the eye

The autonomic nervous system influences numerous ocular functions. It does this by way of parasympathetic innervation from postganglionic fibers that originate from neurons in the ciliary and pterygopalatine ganglia, and by way of sympathetic innervation from postganglionic fibers that originate from neurons in the superior cervical ganglion. Ciliary ganglion neurons project to the ciliary body and the sphincter pupillae muscle of the iris to control ocular accommodation and pupil constriction, respectively. Superior cervical ganglion neurons project to the dilator pupillae muscle of the iris to control pupil dilation. Ocular blood flow is controlled both via direct autonomic influences on the vasculature of the optic nerve, choroid, ciliary body, and iris, as well as via indirect influences on retinal blood flow. In mammals, this vasculature is innervated by vasodilatory fibers from the pterygopalatine ganglion, and by vasoconstrictive fibers from the superior cervical ganglion. Intraocular pressure is regulated primarily through the balance of aqueous humor formation and outflow. Autonomic regulation of ciliary body blood vessels and the ciliary epithelium is an important determinant of aqueous humor formation; autonomic regulation of the trabecular meshwork and episcleral blood vessels is an important determinant of aqueous humor outflow. These tissues are all innervated by fibers from the pterygopalatine and superior cervical ganglia. In addition to these classical autonomic pathways, trigeminal sensory fibers exert local, intrinsic influences on many of these regions of the eye, as well as on some neurons within the ciliary and pterygopalatine ganglia.

Central pupillary light reflex circuits in the cat: I. The olivary pretectal nucleus

The central pathways subserving the feline pupillary light reflex were examined by defining retinal input to the olivary pretectal nucleus (OPt), the midbrain projections of this nucleus, and the premotor neurons within it. Unilateral intravitreal wheat germ agglutinin-conjugated horseradish peroxidase (WGA-HRP) injections revealed differences in the pattern of retinal OPt termination on the two sides. Injections of WGA-HRP into OPt labeled terminals bilaterally in the anteromedian nucleus, and to a lesser extent in the supraoculomotor area, centrally projecting Edinger-Westphal nucleus, and nucleus of the posterior commissure. Labeled terminals, as well as retrogradely labeled multipolar cells, were present in the contralateral OPt, indicating a commissural pathway. Injections of WGA-HRP into the anteromedian nucleus labeled fusiform premotor neurons within the OPt, as well as multipolar cells in the nucleus of the posterior commissure. Connections between retinal terminals and the pretectal premotor neurons were characterized by combining vitreous chamber and anteromedian nucleus injections of WGA-HRP in the same animal. Fusiform-shaped, retrogradely labeled cells fell within the anterogradely labeled retinal terminal field in the OPt. Ultrastructural analysis revealed labeled retinal terminals containing clear spherical vesicles. They contacted labeled pretectal premotor neurons via asymmetric synaptic densities. These results provide an anatomical substrate for the pupillary light reflex in the cat. Pretectal premotor neurons receive direct retinal input via synapses suggestive of an excitatory drive, and project directly to nuclei containing preganglionic motoneurons. These projections are concentrated in the anteromedian nucleus, indicating its involvement in the pupillary light reflex.

Educating the blind brain: a panorama of neural bases of vision and of training programs in organic neurovisual deficits

Vision is a complex function, which is achieved by movements of the eyes to properly foveate targets at any location in 3D space and to continuously refresh neural information in the different visual pathways. The visual system involves five main routes originating in the retinas but varying in their destination within the brain: the occipital cortex, but also the superior colliculus (SC), the pretectum, the supra-chiasmatic nucleus, the nucleus of the optic tract and terminal dorsal, medial and lateral nuclei. Visual pathway architecture obeys systematization in sagittal and transversal planes so that visual information from left/right and upper/lower hemi-retinas, corresponding respectively to right/left and lower/upper visual fields, is processed ipsilaterally and ipsialtitudinally to hemi-retinas in left/right hemispheres and upper/lower fibers. Organic neurovisual deficits may occur at any level of this circuitry from the optic nerve to subcortical and cortical destinations, resulting in low or high-level visual deficits. In this didactic review article, we provide a panorama of the neural bases of eye movements and visual systems, and of related neurovisual deficits. Additionally, we briefly review the different schools of rehabilitation of organic neurovisual deficits, and show that whatever the emphasis is put on action or perception, benefits may be observed at both motor and perceptual levels. Given the extent of its neural bases in the brain, vision in its motor and perceptual aspects is also a useful tool to assess and modulate central nervous system (CNS) in general.

Pupil size reflects the focus of feature-based attention

We measured pupil size in adult human subjects while they selectively attended to one of two surfaces, bright and dark, defined by coherently moving dots. The two surfaces were presented at the same location; therefore, subjects could select the cued surface only on the basis of its features. With no luminance change in the stimulus, we find that pupil size was smaller when the bright surface was attended and larger when the dark surface was attended: an effect of feature-based (or surface-based) attention. With the same surfaces at nonoverlapping locations, we find a similar effect of spatial attention. The pupil size modulation cannot be accounted for by differences in eye position and by other variables known to affect pupil size such as task difficulty, accommodation, or the mere anticipation (imagery) of bright/dark stimuli. We conclude that pupil size reflects not just luminance or cognitive state, but the interaction between the two: it reflects which luminance level in the visual scene is relevant for the task at hand.

The Verriest Lecture: Visual properties of metameric blacks beyond cone vision

The generic framework of metamerism implies that the number of sensors is smaller than the dimension of the stimulus. The metameric black paradigm was introduced by Wyszecki [Farbe2, 39 (1953)] and developed by Cohen and Kappauf [Am. J. Psychol.95, 537 (1982)]. Within a multireceptor and multiprimary scheme, we investigate how far the choice of illumination can isolate a photoreceptor response. The spectral profiles of the fundamental metamers that correspond to a collection of (x,y) values over the chromaticity diagram are shown. When the luminance is set at a fixed value, the relative excitation of the melanopsin cells and of the rods elicited by the fundamental metamers varies over the chromaticity diagram. The range of excitation of the melanopsin cells and of the rods that could be achieved at a given chromaticity, by manipulating the metameric black content, is examined. When only the melanopsin excitation is manipulated, the range of melanopsin excitation that can be achieved is rather limited. On the chromaticity diagram, the largest range of variation of the rods and the melanopsin cells excitation is obtained for (x,y) chromaticity coordinates near (1/3,1/3). Extension of Cohen's procedure to rod and cone metamers is proposed. The higher the number of spectral bands, the wider the choice of metameric lights.

Photoresponse diversity among the five types of intrinsically photosensitive retinal ganglion cells

Intrinsically photosensitive retinal ganglion cells (ipRGCs) mediate non-image-forming visual responses, including pupillary constriction, circadian photoentrainment and suppression of pineal melatonin secretion. Five morphological types of ipRGCs, M1-M5, have been identified in mice. In order to understand their functions better, we studied the photoresponses of all five cell types, by whole-cell recording from fluorescently labelled ipRGCs visualized using multiphoton microscopy. All ipRGC types generated melanopsin-based ('intrinsic') as well as synaptically driven ('extrinsic') light responses. The intrinsic photoresponses of M1 cells were lower threshold, higher amplitude and faster than those of M2-M5. The peak amplitudes of extrinsic light responses differed among the ipRGC types; however, the responses of all cell types had comparable thresholds, kinetics and waveforms, and all cells received rod input. While all five types exhibited inhibitory amacrine-cell and excitatory bipolar-cell inputs from the 'on' channel, M1 and M3 received additional 'off'-channel inhibition, possibly through their 'off'-sublamina dendrites. The M2-M5 ipRGCs had centre-surround-organized receptive fields, implicating a capacity to detect spatial contrast. In contrast, the receptive fields of M1 cells lacked surround antagonism, which might be caused by the surround of the inhibitory input nullifying the surround of the excitatory input. All ipRGCs responded robustly to a wide range of motion speeds, and M1-M4 cells appeared tuned to different speeds, suggesting that they might analyse the speed of motion. Retrograde labelling revealed that M1-M4 cells project to the superior colliculus, suggesting that the contrast and motion information signalled by these cells could be used by this sensorimotor area to detect novel objects and motion in the visual field.

Chronic lithium feeding reduces upregulated brain arachidonic acid metabolism in HIV-1 transgenic rat

HIV-1 transgenic (Tg) rats, a model for human HIV-1 associated neurocognitive disorder (HAND), show upregulated markers of brain arachidonic acid (AA) metabolism with neuroinflammation after 7 months of age. Since lithium decreases AA metabolism in a rat lipopolysaccharide model of neuroinflammation, and may be useful in HAND, we hypothesized that lithium would dampen upregulated brain AA metabolism in HIV-1 Tg rats. Regional brain AA incorporation coefficients k* and rates J ( in ), markers of AA signaling and metabolism, were measured in 81 brain regions using quantitative autoradiography, after intravenous [1-(14) C]AA infusion in unanesthetized 10-month-old HIV-1 Tg and age-matched wildtype rats that had been fed a control or LiCl diet for 6 weeks. k* and J ( in ) for AA were significantly higher in HIV-1 Tg than wildtype rats fed the control diet. Lithium feeding reduced plasma unesterified AA concentration in both groups and J ( in ) in wildtype rats, and blocked increments in k* (19 of 54 regions) and J ( in ) (77 of 81 regions) in HIV-1 Tg rats. These in vivo neuroimaging data indicate that lithium treatment dampened upregulated brain AA metabolism in HIV-1 Tg rats. Lithium may improve cognitive dysfunction and be neuroprotective in HIV-1 patients with HAND through a comparable effect.

Defining the pupillary component of the perioculomotor preganglionic population within a unitary primate Edinger-Westphal nucleus

The primate Edinger-Westphal nucleus (EW) contains perioculomotor preganglionic (pIII(PG)) motoneurons that control the lens and pupil. Separate subdivisions have been described in EW and termed visceral columns, with the lateral visceral column (lvc) reportedly receiving pretectal inputs for the pupillary light reflex. However, choline acetyl transferase staining reveals a single paired column of cells dorsal to the oculomotor nucleus, suggesting the EW is not subdivided. We investigated this issue by transneuronal retrograde labelling of pIII(PG) neurons in three monkey species. In all three, pIII(PG) neurons were contained in a single column. We have also examined which part of the macaque pIII(PG) population receives pretectal input. Injections of biocytin into the pretectum anterogradely labelled terminals that lay in close association with pIII(PG) motoneurons retrogradely labelled by ciliary ganglion injections of WGA-HRP. These close associations were concentrated in the ventromedial portion of the middle third of EW, suggesting this pIII(PG) region mediates pupillary control. In other cases, pretectal WGA-HRP injections, in addition to labelling terminals in the EW, produced a circular field of labelled neurons, and terminals in the periaqueductal grey, dorsolateral to EW. This region may represent the previously designated lvc, but it does not contain pIII(PG) motoneurons.

GABAergic pathways in the rat subcortical visual system: a comparative study in vivo and in vitro

Inhibitory pathways project from the pretectal nuclear complex to the ipsilateral superior colliculus (SC) and dorsal lateral geniculate nucleus (dLGN). Both pathways arise from GABAergic neurones that are located in the dorsal pretectal nuclear complex. In the present experiments, we compared the anatomy and physiology of these two pathways with the objective of determining whether they have similar functions. First, we injected retrograde axonal tracers that fluoresce at different wavelengths in the dLGN and SC of single animals to determine if individual GABAergic neurones in the pretectum project to both structures. The results showed that the dLGN and SC receive input from different cell groups. Next, morphological reconstructions of cells labelled after in-vitro whole-cell patch-clamp recordings demonstrated that the pretectal-recipient cells in the dLGN are GABAergic interneurones, whereas those in the SC are projection cells. Finally, with in-vitro whole-cell patch-clamp recordings we showed that inhibitory currents generated by both pathways are mediated by GABA(A) receptors. Taken together, these results suggest that these inhibitory projections may function to facilitate the relay of information from the dLGN to the visual cortex by suppressing the activity of dLGN interneurones, and to reduce the level of activity leaving the SC by inhibiting the projection neurones. These hypotheses will be discussed in the context of the known functions of the pretectal complex.

Human and macaque pupil responses driven by melanopsin-containing retinal ganglion cells

Melanopsin, a novel photopigment, has recently been localized to a population of retinal ganglion cells that display inherent photosensitivity. During continuous light and following light offset, primates are known to exhibit sustained pupilloconstriction responses that resemble closely the photoresponses of intrinsically-photoreceptive ganglion cells. We report that, in the behaving macaque, following pharmacological blockade of conventional photoreceptor signals, significant pupillary responses persist during continuous light and following light offset. These pupil responses display the unique spectral tuning, slow kinetics, and irradiance coding of the sustained, melanopsin-derived ganglion cell photoresponses. We extended our observations to humans by using the sustained pupil response following light offset to document the contribution of these novel ganglion cells to human pupillary responses. Our results indicate that the intrinsic photoresponses of intrinsically-photoreceptive retinal ganglion cells play an important role in the pupillary light reflex and are primarily responsible for the sustained pupilloconstriction that occurs following light offset.

Shaping the pupil’s response to light in the hooded rat

The contribution of iris muscle steady state and dynamic response characteristics to the shaping of the pupil response to light in the hooded rat were studied using electrical stimulation of the parasympathetic fibers in the III nerve. The waveforms of pupillary contractions to single or brief trains of electrical impulses applied to the III nerve were virtually identical to those elicited with short duration light flashes. Individual contractions could be resolved at stimulation rates of 2 Hz and below, and the size of the contractions increased with the decrease in frequency. The pupil responded to long trains of stimuli above 2 Hz with smooth tonic contractions. Steady state contraction amplitude was linearly related to log stimulation frequency. The mean time constant of pupil constriction to stimulus trains was 1.41 s (SD +/- 0.71 s) and the shortest mean latency was 292 ms (SD +/- 30 ms). The fastest mean latency of pupil constriction to the brightest light flash used was 295 ms. In contrast, the time constant of pupillary dilation was 7 s (SD +/- 1.4 s) and the shortest latency was 485 ms (SD +/- 74 ms). Therefore, the sluggish dynamic properties of the iris musculature are responsible for the asymmetries in pupil contraction, dilation, and latencies as well as low flicker fusion frequency and constriction amplitude characteristics of pupil responses to light.

The pretectum: connections and oculomotor-related roles

Research over the past two decades in mammals, especially primates, has greatly improved our understanding of the afferent and efferent connections of two retinorecipient pretectal nuclei, the nucleus of the optic tract (NOT) and the pretectal olivary nucleus (PON). Functional studies of these two nuclei have further elucidated some of the roles that they play both in oculomotor control and in relaying oculomotor-related signals to visual relay nuclei. Therefore, following a brief overview of the anatomy and retinal projections to the entire mammalian pretectum, the connections and potential roles of the NOT and the PON are considered in detail. Data on the specific connections of the NOT are combined with data from single-unit recording, microstimulation, and lesion studies to show that this nucleus plays critical roles in optokinetic nystagmus, short-latency ocular following, smooth pursuit eye movements, and adaptation of the gain of the horizontal vestibulo-ocular reflex. Comparable data for the PON show that this nucleus plays critical roles in the pupillary light reflex, light-evoked blinks, rapid eye movement sleep triggering, and modulating subcortical nuclei involved in circadian rhythms.

Pupillary evaluation of retinal asymmetry: development and initial testing of a technique

Glaucomatous damage to upper and lower retina is often unequal. We have developed a rapid, objective, quantitative measure of asymmetry of retinal sensitivity, using infrared pupillometry and pairs of large stimuli that are symmetric about the horizontal meridian. Results for a group of 11 young subjects free of eye disease indicate that the distribution of asymmetry is close to a normal distribution centered near upper/lower symmetry. Some subjects showed modest amounts of asymmetry, which was relatively uniform within each eye, and between the two eyes, of the subject. This approach to determination of asymmetry within an eye is potentially applicable to testing patients with glaucoma. The narrowness of the distribution should make it possible to detect asymmetries caused by disease.

Non-image-forming ocular photoreception in vertebrates

It has been accepted for a hundred years or more that rods and cones are the only photoreceptive cells in the retina. The light signals generated in rods and cones, after processing by downstream retinal neurons (bipolar, horizontal, amacrine and ganglion cells), are transmitted to the brain via the axons of the ganglion cells for further analysis. In the past few years, however, convincing evidence has rapidly emerged indicating that a small subset of retinal ganglion cells in mammals is also intrinsically photosensitive. Melanopsin is the signaling photopigment in these cells. The main function of the inner-retina photoreceptors is to generate and transmit non-image-forming visual information, although some role in conventional vision (image detection) is also possible.

Chronic lithium administration to rats selectively modifies 5-HT2A/2C receptor-mediated brain signaling via arachidonic acid

The effects of chronic lithium administration on regional brain incorporation coefficients k* of arachidonic acid (AA), a marker of phospholipase A2 (PLA2) activation, were determined in unanesthetized rats administered i.p. saline or 1 mg/kg i.p. (+/-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane hydrochloride (DOI), a 5-HT2A/2C receptor agonist. After injecting [1-(14)C]AA intravenously, k* (brain radioactivity/integrated plasma radioactivity) was measured in each of 94 brain regions by quantitative autoradiography. Studies were performed in rats fed a LiCl or a control diet for 6 weeks. In the control diet rats, DOI significantly increased k* in widespread brain areas containing 5-HT2A/2C receptors. In the LiCl-fed rats, the significant positive k* response to DOI did not differ from that in control diet rats in most brain regions, except in auditory and visual areas, where the response was absent. LiCl did not change the head turning response to DOI seen in control rats. In summary, LiCl feeding blocked PLA2-mediated signal involving AA in response to DOI in visual and auditory regions, but not generally elsewhere. These selective effects may be related to lithium's therapeutic efficacy in patients with bipolar disorder, particularly its ability to ameliorate hallucinations in that disease.

Inhibition of superior colliculus neurons by a GABAergic input from the pretectal nuclear complex in the rat

The mammalian pretectal nuclear complex (PNC) is a visual and visuomotor control structure which is strongly connected to other subcortical visual structures. This indicates that the PNC also controls subcortical visual information flow during the execution of various oculomotor programs. A prominent, presumably GABAergic, projection from the PNC targets the superficial grey layer of the superior colliculus (SC), which itself is a central structure for visual information processing necessary for the generation of saccadic eye movements. In order to characterize the pretectotectal projection in vitro, we performed whole-cell patch-clamp recordings from SC and PNC neurons in slices obtained from 3-6-week-old pigmented rats. Focal glutamate injections into the PNC and electrical PNC stimulation were used to induce postsynaptic responses in SC neurons. Electrical stimulation of the SC allowed electrophysiological identification of PNC neurons that provide the inhibitory pretectotectal input. Only inhibitory postsynaptic currents could be elicited in SC neurons both by pharmacological and by electrical activation of the ipsilateral PNC. Concomitantly, a small number of PNC neurons could be antidromically activated from the ipsilateral SC. Most SC cells postsynaptic to the prectectal input showed the dendritic morphology of wide-field and narrow-field cells and are therefore regarded as projection neurons. All inhibitory currents evoked by PNC activation could be completely blocked by bath application of the selective GABA(A) receptor antagonist bicuculline. Together these results indicate that SC projection neurons receive a direct inhibitory input from the ipsilateral PNC and that this input is mediated by GABA(A) receptors.

Twitch and nontwitch motoneuron subgroups in the oculomotor nucleus of monkeys receive different afferent projections

Motoneurons in the primate oculomotor nucleus can be divided into two categories, those supplying twitch muscle fibers and those supplying nontwitch muscle fibers. Recent studies have shown that twitch motoneurons lie within the classical oculomotor nucleus (nIII), and nontwitch motoneurons lie around the borders. Nontwitch motoneurons of medial and inferior rectus are in the C group dorsomedial to nIII, whereas those of inferior oblique and superior rectus lie near the midline are in the S group. In this anatomical study, afferents to the twitch and nontwitch subgroups of nIII have been anterogradely labeled by injections of tritiated leucine into three areas and compared. 1) Abducens nucleus injections gave rise to silver grain deposits over all medial rectus subgroups, both twitch and nontwitch. 2) Laterally placed vestibular complex injections that included the central superior vestibular nucleus labeled projections only in twitch motoneuron subgroups. However, injections into the parvocellular medial vestibular nucleus (mvp), or Y group, resulted in labeled terminals over both twitch and nontwitch motoneurons. 3) Pretectal injections that included the nucleus of the optic tract (NOT), and the olivary pretectal nucleus (OLN), labeled terminals only over nontwitch motoneurons, in the contralateral C group and in the S group. Our study demonstrates that twitch and nontwitch motoneuron subgroups do not receive identical afferent inputs. They can be controlled either in parallel, or independently, suggesting that they have basically different functions. We propose that twitch motoneurons primarily drive eye movements and nontwitch motoneurons the tonic muscle activity, as in gaze holding and vergence, possibly involving a proprioceptive feedback system.

Spontaneous activity of rat pretectal nuclear complex neurons in vitro

Background

Neurons in the mammalian pretectum are involved in the control of various visual and oculomotor tasks. Because functionally independent pretectal cell populations show a wide variation of response types to visual stimulation in vivo, they may also differ in their intrinsic properties when recorded in vitro. We therefore performed whole-cell patch clamp recordings from neurons in the caudal third of the pretectal nuclear complex in frontal brain slices obtained from 3 to 6 week old hooded rats and tried to classify pretectal neurons electrophysiologically.

Results

Pretectal neurons showed various response types to intracellular depolarizations, including bursting and regular firing behavior. One population of pretectal nuclear complex neurons could be particularly distinguished from others because they displayed spontaneous activity in vitro. These cells had more positive resting potentials and higher input resistances than cells that were not spontaneously active. The maintained firing of spontaneously active pretectal cells was characterized by only small variances in interspike intervals and thus showed a regular temporal patterning. The firing rate was directly correlated to the membrane potential. Removing excitatory inputs by blockade of AMPA and/or NMDA receptors did not change the spontaneous activity. Simultaneous blockade of excitatory and inhibitory synaptic input by a substitution of extracellular calcium with cobalt neither changed the firing rate nor its temporal patterning. Each action potential was preceeded by a depolarizing inward current which was insensitive to calcium removal but which disappeared in the presence of tetrodotoxin.

Conclusions

Our results indicate that a specific subpopulation of pretectal neurons is capable of generating maintained activity in the absence of any external synaptic input. This maintained activity depends on a sodium conductance and is independent from calcium currents.