A model for nonlinear stochastic behavior of the pupil

In binocular fusion, pairs of left and right stimuli yielding the same brightness perception constitute an equibrightness curve in a coordinate system whose ordinate and abscissa correspond to the left and right stimulus strengths. A neural network model is presented to elucidate the characteristics of the curve. According to the model, Fechner's paradox is due to the threshold characteristics of the neuron. If the shapes or movements are radically different between the left and right stimuli, the retinal rivalry is caused. That is, only the left stimulus is perceived at one moment and the right stimulus at another moment. The period of left or right eye dominance alternates randomly from time to time. The distribution of the period is approximate to the gamma distribution. In order to account for this fact, a neural network model is proposed, which consists of a pair of neurons receiving inputs with stochastic fluctuations. The computer simulation was carried out with satisfactory results. The model of retinal rivalry is integrated with that of brightness perception.     

Attention controls multisensory perception via two distinct mechanisms at different levels of the cortical hierarchy

To form a percept of the multisensory world, the brain needs to integrate signals from common sources weighted by their reliabilities and segregate those from independent sources. Previously, we have shown that anterior parietal cortices combine sensory signals into representations that take into account the signals’ causal structure (i.e., common versus independent sources) and their sensory reliabilities as predicted by Bayesian causal inference. The current study asks to what extent and how attentional mechanisms can actively control how sensory signals are combined for perceptual inference. In a pre- and postcueing paradigm, we presented observers with audiovisual signals at variable spatial disparities. Observers were precued to attend to auditory or visual modalities prior to stimulus presentation and postcued to report their perceived auditory or visual location. Combining psychophysics, functional magnetic resonance imaging (fMRI), and Bayesian modelling, we demonstrate that the brain moulds multisensory inference via two distinct mechanisms. Prestimulus attention to vision enhances the reliability and influence of visual inputs on spatial representations in visual and posterior parietal cortices. Poststimulus report determines how parietal cortices flexibly combine sensory estimates into spatial representations consistent with Bayesian causal inference. Our results show that distinct neural mechanisms control how signals are combined for perceptual inference at different levels of the cortical hierarchy.

A combination of psychophysics, computational modelling and fMRI reveals novel insights into how the brain controls the binding of information across the senses, such as the voice and lip movements of a speaker.     

A visual test based on a freeware software for quantifying and displaying night-vision disturbances: study in subjects after alcohol consumption

Background

In this work, we propose the Halo test, a simple visual test based on a freeware software for quantifying and displaying night-vision disturbances perceived by subjects under different experimental conditions, more precisely studying the influence of the alcohol consumption on visual function.

Methods

In the Halo test, viewed on a monitor, the subject's task consists of detecting luminous peripheral stimuli around a central high-luminance stimulus over a dark background. The test, performed by subjects before and after consuming alcoholic drinks, which deteriorate visual performance, evaluates the influence that alcohol consumption exerts on the visual-discrimination capacity under low illumination conditions. Measurements were made monocularly and binocularly. Pupil size was also measured in both conditions (pre/post). Additionally, we used a double-pass device to measure objectively the optical-quality of the eye and corroborate the results from the Halo test.

Results

We found a significant deterioration of the discrimination capacity after alcohol consumption, indicating that the higher the breath-alcohol content, the greater the deterioration of the visual-discrimination capacity. After alcohol intake, the graphical results showed a greater area of undetected peripheral stimuli around the central high-luminance stimulus. An enlargement of the pupil was also observed and the optical quality of the eye was deteriorated after alcohol consumption.

Conclusions

A greater influence of halos and other night-vision disturbances were reported with the Halo test after alcohol consumption. The Halo freeware software constitutes a positive contribution for evaluating nighttime visual performance in clinical applications, such as reported here, but also in patients after refractive surgery (where halos are present) or for monitoring (time course) some ocular pathologies under pharmacological treatment.

     

Panum's phenomenon and the confluence of signals from the two eyes in stereoscopy

The signals from the two eyes must be routed to allow either eye to have access to the processing mechanisms for position, shape, colour, etc.; at the same time, information as to the eye of origin must be retained for the purposes of stereoscopy. The study of this confluence of signals from the two eyes was approached psychophysically by studying induced position and depth changes of adjacent binocular and monocular stimuli in the human fovea. It was demonstrated that a monocular visual stimulus located near a binocular one acquires a depth signal, according to a scheme originally proposed by Panum. The effect is unspecific as regards feature shape and brightness, and falls off with a length constant of about 15 minutes of arc in the fovea. A monocular stimulus also affects the apparent depth of its binocular neighbour in a centre-surround manner; disparity pooling changes to disparity repulsion when features are separated by distances of about 3 minutes of arc in the fovea. The findings led to the development of a scheme of uniocular connectivity to a matrix of depth units. Excitation patterns here would depend on the state of the input lines, the intrinsic neuronal interaction properties, and contextural configuring influences from other parts of the nervous system. Experiments showing the spatial extent of pooling and repulsive interaction within the disparity domain help to characterize the stimulus processing in this neural ensemble.     

Unequal saccades generated by velocity interactions in the peripheral oculomotor system

Experiments with precision eye movement recordings show binocularly unequal saccades to be present under several stimulus conditions having as a common theme ongoing low velocities at the times of the saccades. Simulations using a model of eye muscles and eyeball dynamics reproduce these unequal saccades in quantitative agreement with the experimental findings. The model uses equal innervation for the saccades, and demonstrates a peripheral interaction between the muscle forces and the eye velocities to be the cause of the large inequality of the simulated binocular saccades. Thus, the simulations provide evidence that Hering's law continues to describe the innervation patterns to corresponding muscles producing these binocularly unequal saccades found in the experimental situation.     

Eye movements in Daphnia magna

Summary Three types of behavior of the compound eye of Daphnia magna are characterized: flick, a transient rotation elicited by a brief flash of light; fixation, a maintained eye orientation in response to a stationary light stimulus of long-duration; tracking, the smooth pursuit of a moving stimulus. The magnitudes of the flick and fixation responses vary with stimulus position and are generally proportional to stimulus intensity, although at high intensities there is an attenuation of both behaviors. When the stimulus is placed at a position 80° dorsal to the eye axis, there is no response; this area is called the null region. For stationary stimuli in other positions, the direction of the response is such as to bring the stimulus closer to the null region. During tracking, the relative positions of the eye and stimulus change; the eye velocity is approximately half that of the moving stimulus. The regions of the eye in which these behaviors may be induced are different, being largest for flick and smallest for tracking. It is proposed that flick and fixation responses are a means for rotating the eye so that the stimulus is within the area surrounding the null region which is used for tracking.     

Experience-Driven Axon Retraction in the Pharmacologically Inactivated Visual Cortex Does Not Require Synaptic Transmission

Background

Experience during early postnatal development plays an important role in the refinement of specific neural connections in the brain. In the mammalian visual system, altered visual experiences induce plastic adaptation of visual cortical responses and guide rearrangements of afferent axons from the lateral geniculate nucleus. Previous studies using visual deprivation demonstrated that the afferents serving an open eye significantly retract when cortical neurons are pharmacologically inhibited by applying a γ-aminobutyric acid type A receptor agonist, muscimol, whereas those serving a deprived eye are rescued from retraction, suggesting that presynaptic activity can lead to the retraction of geniculocortical axons in the absence of postsynaptic activity. Because muscimol application suppresses the spike activity of cortical neurons leaving transmitter release intact at geniculocortical synapses, local synaptic interaction may underlie the retraction of active axons in the inhibited cortex.

Method and Findings

New studies reported here determined whether experience-driven axon retraction can occur in the visual cortex inactivated by blocking synaptic inputs. We inactivated the primary visual cortex of kittens by suppressing synaptic transmission with cortical injections of botulinum neurotoxin type E, which cleaves a synaptic protein, SNAP-25, and blocks transmitter release, and examined the geniculocortical axon morphology in the animals with normal vision and those deprived of vision binocularly. We found that afferent axons in the animals with normal vision showed a significant retraction in the inactivated cortex, as similarly observed in the muscimol-treated cortex, whereas the axons in the binocularly deprived animals were preserved.

Conclusions

Therefore, the experience-driven axon retraction in the inactivated cortex can proceed in the absence of synaptic transmission. These results suggest that presynaptic mechanisms play an important role in the experience-driven refinement of geniculocortical axons.     

Experience-driven axon retraction without binocular imbalance in developing visual cortex

Refinement of the neural circuit during brain maturation is regulated by experience-driven neural activity. In the mammalian visual cortex, monocular visual deprivation (MD) in the early postnatal life causes a significant loss of cortical responses to a deprived eye and the retraction of input axons serving the deprived eye. A competitive interaction between inputs serving both eyes has been supposed to underlie the effects of MD because the loss of cortical response is much weaker when both eyes are deprived of vision. Also, the input axons do not retract after binocular deprivation. Here, we report that uncorrelated activity between presynaptic and postsynaptic neurons can solely lead to the retraction of geniculocortical axons in the absence of activity imbalance between two inputs. We analyzed the morphology of geniculocortical axons in a pharmacologically inhibited visual cortex of animals with normal vision and of binocularly deprived animals. In the normal vision animals, the axonal arbors in the inhibited cortex showed robust retraction. On the other hand, the arbors in binocularly deprived animals remained mostly intact. These results suggest that a homosynaptic associative mechanism, rather than a heterosynaptic competition between inputs, may play an important role in experience-driven axon retraction.     

The visual field and visually guided behavior in the zebra finch (Taeniopygia guttata)

Summary Measurements were made of the physical properties of the visual system of the zebra finch, a bird with laterally placed eyes. The use of the visual system in pecking and courtship behavior was examined. It was demonstrated that the optical axis and the fovea of the eye point in a direction about 62° from the sagittal axis of the head. The visual field of each eye covers about 170° in the horizontal plane. In the frontal region there is an overlap of about 30°–40° where the birds can see binocularly; caudally there is a gap in the visual field of 60°. The point of best binocular viewing is in the sagittal plane at 16.5° below the beak.Concerning movement detection, the upper threshold is 540°/s for the binocular (frontal) part of the visual field and about 1100°/s for the monocular (lateral) part. Most fixations before pecking occur monocularly. A preference for one eye during pecking was not detected. During the courtship song, a male bird directs its head towards the female. The results are discussed in comparison with findings in pigeons and chickens.     

Why do bees turn back and look?

The timing of learning of colour and shape of the food source, as well as of near-by landmarks, was examined exploiting a behaviour described recently, the Turn Back and Look behaviour (TBL): Bees departing from a novel food source after feeding turn around to view it at a short distance (Figs. 2, 3) before departing for the hive. They repeat this behaviour on several successive visits, termed the TBL phase (Fig. 5). To examine the function of the TBL, I trained individual bees in 4 different modes. In the first 3 they could view a food source or a landmark of a particular colour or shape during (i) arrival as well as departure, (ii) only arrival, and (iii) only departure; in the final mode (iv) the bees viewed one colour (or shape) on arrival, and another on departure. At the end of the TBL phase, the bees were tested by offering them a choice between the visual stimulus to which they were trained (modes i–iii) and a different (novel) one, or between the stimulus viewed on arrival and that viewed on departure (mode iv). The test results show that learning after feeding (while performing the TBL), i.e. backward conditioning, occurs regardless of whether the colour (Fig. 6, Fig. 10a) or shape (Fig. 7) of the food source, or the colour (Fig. 10b), shape (Fig. 11), and position (Fig. 12) of a near-by landmark is considered. Bees trained in mode (iv) preferred the stimulus learned on arrival over that learned on departure in almost all cases. However, a stimulus viewed exclusively on departure (mode iii) was often learned as well as when it was viewed exclusively on arrival (mode ii) (Figs. 10a, 11, 12), or both on arrival and departure (mode i) (Fig. 6). The finding that the timing of learning can be manipulated suggests that it is not based on hard wired predispositions to learn particular visual cues on arrival, and others on departure.     

Superposition and scattering of visual fields in a compound,double eye—II. Stimulation sequences for different distances in a stomatopod from a bright habitat

• 1.1. The inputs deriving from a point light moving in front of a column of ommatidia are plotted by a computer.
• 2.2. Overlap of visual fields causes one main, central maximum surrounded by smaller maxima intercalated by minima.
• 3.3. With increasing distance of the point from the eye the central maximum increases.
• 4.4. Ommatidia in each half of the eye are stimulated sequentially in two groups. Stimulation sequences are parallel between the two halves of the eye.
• 5.5. We postulate an integrative mechanism for processing the succession of simultaneous inputs and hypothesize that the animal derives information about speed and distance from that mechanism.

     

Misspecifications of Stimulus Presentation Durations in Experimental Psychology: A Systematic Review of the Psychophysics Literature

Background

In visual psychophysics, precise display timing, particularly for brief stimulus presentations, is often required. The aim of this study was to systematically review the commonly applied methods for the computation of stimulus durations in psychophysical experiments and to contrast them with the true luminance signals of stimuli on computer displays.

Methodology/Principal Findings

In a first step, we systematically scanned the citation index Web of Science for studies with experiments with stimulus presentations for brief durations. Articles which appeared between 2003 and 2009 in three different journals were taken into account if they contained experiments with stimuli presented for less than 50 milliseconds. The 79 articles that matched these criteria were reviewed for their method of calculating stimulus durations. For those 75 studies where the method was either given or could be inferred, stimulus durations were calculated by the sum of frames (SOF) method. In a second step, we describe the luminance signal properties of the two monitor technologies which were used in the reviewed studies, namely cathode ray tube (CRT) and liquid crystal display (LCD) monitors. We show that SOF is inappropriate for brief stimulus presentations on both of these technologies. In extreme cases, SOF specifications and true stimulus durations are even unrelated. Furthermore, the luminance signals of the two monitor technologies are so fundamentally different that the duration of briefly presented stimuli cannot be calculated by a single method for both technologies. Statistics over stimulus durations given in the reviewed studies are discussed with respect to different duration calculation methods.

Conclusions/Significance

The SOF method for duration specification which was clearly dominating in the reviewed studies leads to serious misspecifications particularly for brief stimulus presentations. We strongly discourage its use for brief stimulus presentations on CRT and LCD monitors.     

Optomotorische Reaktionen der Fliege Musca Domestica

Summary The torque exerted by the housefly Musca domestica during fixed flight was used as a measure of the optomotor reaction of the insect elicited by the rotation of cylindrical patterns with periodic distributions of surface brightness. Measurements were made of the dependence of the reaction on the wave length, speed of rotation, contrast, and mean brightness of the stimulus patterns. The effect on the reaction of modulation of the light illuminating the stimulus pattern was examined. Further experiments indicated that stimulation of only one of the two complex eyes is sufficient to elicit an optomotor reaction, and that there is overlap between the visual fields of neighboring photoreceptor units in the complex eye. Estimates of the rates of absorption of light quanta by individual ommatidia in the complex eye indicated that these rates are low enough that the Poisson statistics of the light quanta results in a significant level of noise in the light signals received by the photoreceptors, when the brightness of the stimulus pattern is low but still sufficient to elicit a measurable reaction. The contrast that is required of a rotating stimulus pattern in order to elicit a just-measurable reaction was found to depend upon the mean brightness of the pattern in a manner that is consistent with the hypothesis that the noise due to the statistics of the light quanta absorbed by the photoreceptors in the complex eye is a principle cause of the breakdown of the optomotor reaction at low values of the contrast and mean brightness of the stimulus pattern.

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Mit Unterstützung durch ein Fellowship des National Institute of Neurological Diseases and Blindness, US Public Health Service und ein Fellowship der National Science Foundation, USA.

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Herrn Freiberg danken wir für das Anfertigen der Abbildungen.     

Taxonomy of Gundlachia (Compositae: Astereae)

Gundlachia, a genus of shrubs occurring in the Caribbean islands, is treated as comprising two species, one of which has six varieties. Five new combinations are made: Gundlachia corymbosa var. apiculata (Britton & S. F. Blake) M. A. Lane, G. corymbosa var. compacta (Urb. & Ekman) M. A. Lane, G. corymbosa var. cubana (Britton & S. F. Blake) M. A. Lane, G. corymbosa var. foliosa (Britton & S. F. Blake) M. A. Lane, and G. corymbosa var. ocoana (Urb. & Ekman) M. A. Lane. Gundlachia is probably most closely related to Gymnosperma.     

An owl's eye: Schematic optics and visual performance inStrix aluco L.

Summary The schematic eye ofStrix aluco, a nocturnal owl species, is described. A comparative and ecological context is used to examine the relationships between optical parameters of the eye and its light gathering and resolving powers. It is concluded that the essentially nocturnal feature of the owl eye does not lie in either its light gathering power or the sensitivity of individual rod receptors. Differences in visual performance at low light levels between the owl and the diurnal pigeon appear to be attributable to differences in the retinal neural integration mechanisms of the two species. However, it is hypothesised that the neural mechanisms which mediate the extraction of spatial information from the retinal image throughout the nocturnal luminance range, can function in the owl eye only because of its absolutely large sized retinal image. Thus the primarily nocturnal feature of the owl eye is its absolutely large posterior nodal distance, retinal image brightness is maximised only as a secondary feature.Abbreviation PND posterior nodal distance     

Structure of visual pathways in the nervous system of freshwater pulmonate molluscs

Central nervous system of freshwater pulmonate molluscs Lymnaea stagnalis and Planorbarius corneus was stained using retrograde transport of neurobiotin in the optic tract fibers. In both species, perikarya and fibers of the stained neurons are found in all ganglia except the buccal ones. Afferent fibers of the optic nerve form dense sensory neuropil located in relatively small volume of cerebral ganglia. Typical neuronal groups sending their processes into the optic nerves of ipsilateral and contralateral body halves are described. Among them, neurons of visceral and parietal ganglia innervating both eyes concurrently as well as sending projections into peripheral nerves are revealed. These neurons, supposedly, have a function to integrate sensory signals, which may be a basis for regulation of light sensitivity of retina and functioning of peripheral organs. Bilateral links of the molluscan eye with the pedal ganglia cells and statocysts are found, which is, likely, a structural basis of certain known behavioral patterns related to stimulation of visual inputs in the studied gastropod molluscs.     

Optimum probabilistic processing in colour perception. II. Colour vision as template matching.

A statistical approach to account for psychophysical phenomena in human colour vision is presented. The central visual processor is viewed as an optimum recognizer of stochastic patterns supplied by the periphery. The processor makes an optimum estimate of the spectral parameters of the stimulus, given the wavelength filter characteristics of the periphery, the stochastic nature of the information and an internal template to which the external stimulus is matched. The estimate is constrained in ways inferred from empirical phenomena. Subjective brightness of monochromatic stimuli and related constant brightness manifolds in the colour space constitute the constraint for brightness estimation. Results analogous and in accord with those of earlier line element theories are obtained. The Bezold-Brücke hue shift constitutes the basic constraint for hue estimation. The hue estimate involves interrelation between the fields in the experiment. Similarities and differences both in basic conceptions and results introduced by the template matching notions are discussed.     

The plasticity of correspondence: After-effects,illusions and horopter shifts in depth perception

Retinal disparity is the cue for stereoscopic depth perception. Disparity detection begins with cortical single units driven binocularly from the two eyes. A previous paper (Nelson, 1975) has shown that inhibitory and facilitatory interactions are essential to insure successful disparity detection, particularly with repeating stimulus patterns, and that such a system will display all the appropriate properties of sensory fusion. This paper shows that most depth illusions occur as by-products of the same domain interactions. Such illusion effects fall into two classes: those caused by shifts in the distribution of activity along the appropriate sensory domain (here, the disparity domain) and those caused by changes in the average activity level within the domain. Profile shifts cause depth contrast illusions. The most important profile level change is an activity lowering due to disparity domain inhibition. This adversely affects fusional range (Panum's area). It is postulated that all domain interactions persist following cessation of stimulation. Persistent profile shifts cause depth after-effect illusions; persistent profile lowering is responsible for threshold elevation after-effects.Sensory fusion, the coding errors seen in illusions, the induced effect, and widespread failure to perceive depth from disparity input show that retinal correspondence is not stable in the normal individual. Yet horopter research has attempted to specify one set of retinal points as corresponding. Not surprisingly, horopter research shows systematic shifts in retinal correspondence linked to eye position. Small, simple, tonic modulations of the domain interactions responsible for so many other stereopsis system properties provide a satisfactory cortical mechanism for horopter changes.