Binocular depth reversals despite familiarity cues.

Stereoscopic depth can be reversed by interchanging the left- and right-eye views (pseudoscopy) when abstract stereograms are used, but not when stereograms contain natural objects or scenes. This resistance to reversal of depth has traditionally been attributed to familiarity with the shape of objects and the presence of monocular depth cues. However, when texture disparity is neutralized by making the texture perspective of surfaces identical for both eyes, even a highly familiar object, like a monocularly recognizable human face, appears as concave (nose pointing inwards) when viewed pseudoscopically.

A neural correlate of working memory in the monkey primary visual cortex.

The brain frequently needs to store information for short periods. In vision, this means that the perceptual correlate of a stimulus has to be maintained temporally once the stimulus has been removed from the visual scene. However, it is not known how the visual system transfers sensory information into a memory component. Here, we identify a neural correlate of working memory in the monkey primary visual cortex (V1). We propose that this component may link sensory activity with memory activity.

Receptive field organization of the S-potential.

The receptive fields of S-potentials have been studied in carp retinas. The relationship between the stimulus intensity and area of stimulation was examined for each component of three different types of S-potential. It appears that for each component there is full summation over a large portion of the retina, a type of organization different from that found in the ganglion cell.

The representation of erroneously perceived stimuli in the primary visual cortex.

In order to attain a correct interpretation of an ambiguous visual stimulus, the brain may have to elaborate on the sensory evidence. Are the neurons that carry the sensory evidence also involved in generating an interpretation? To address this question, we studied the activity of neurons in the primary visual cortex of macaque monkeys involved in a task in which they have to trace a curve mentally, without moving their eyes. On a percentage of trials, the monkeys made errors and traced the wrong curve. Here, we show that these errors are predicted by activity in area V1. Thus, neurons in the primary visual cortex do not only represent sensory events, but also the way in which they are interpreted by the monkey.

Two distinct modes of sensory processing observed in monkey primary visual cortex (V1).

Even salient sensory stimuli are sometimes not detected. What goes wrong in the brain in that case? Here we show that a late (> 100-ms) component of the neural activity in the primary visual cortex of the monkey is selectively suppressed when stimuli are not seen. As there is evidence that this activity depends on feedback from extrastriate areas, these findings suggest a specific role for recurrent processing when stimuli are reaching a perceptual level. Further results show that this perceptual level is situated between purely sensory and decision or motor stages of processing.

Feedforward, horizontal, and feedback processing in the visual cortex.

The cortical visual system consists of many richly interconnected areas. Each area is characterized by more or less specific receptive field tuning properties. However, these tuning properties reflect only a subset of the interactions that occur within and between areas. Neuronal responses may be modulated by perceptual context or attention. These modulations reflect lateral interactions within areas and feedback from higher to lower areas. Recent work is beginning to unravel how horizontal and feedback connections each contribute to modulatory effects and what the role of these modulations is in vision. Whereas receptive field tuning properties reflect feedforward processing, modulations evoked by horizontal and feedback connections may reflect the integration of information that underlies perception.

The prenatal development of the human orbit.

During the 1970s, as part of his work for a doctor's thesis in which he described the development of the human orbit in great detail, the first author established the largest anatomical collection of embryonic and fetal orbits ever. Unfortunately, he died before the thesis could be finished. The thousands of sections have now been scanned at high resolution and made publicly available on the Internet at www.visible-orbit.org; 3-D reconstruction software is being developed. The Discussion and part of the 'Methods' section of this thesis are published in translation in this article. The conclusions of the first author at the time read as follows: (1) initially, the developing orbit is vaguely indicated by condensations in the mesenchymal connective tissue area; (2) in this connective tissue area, chondral, osseous and muscular structures develop and grow until, in the fully developed stage, the orbital content is surrounded by bony surfaces with a thin layer of connective tissue as periosteum, and by a muscle fragment; (3) the embryonic and early fetal phase, during which one can only speak of a 'regio orbitalis,' is followed by a period in which we can speak of a primordial orbit; (4) the phase of the primordial orbit extends until after birth; (5) the surface area of all orbital walls increases more or less linearly; (6) the 'musculus orbitalis Mülleri' occupies a special place in the orbital wall; (7) the so-called 'regio craniolateralis' is the primordium, which, in the fully developed stage, is occupied by the thick intersection of the frontolateral and the horizontal part of the frontal bone; (8) in the frontal plane, the shape of the primordial orbit, as well as that of the fully developed orbit, is more or less round; (9) the prenatal development of an eye socket is a complex event, characterized by changes in composition, shape and size of the orbital wall; and (10) the orbit can only be denoted by the term "eye socket" when it is fully developed. At the end of the thesis, he also presented the following postulates: (1) in the prenatal orbit, the development of the so-called 'periorbita' is at the forefront; (2) the mutual rotation of the orbital axes and the frontalization of the eyes from approx. 180 degrees in the early prenatal stages to approx. 50 degrees in adulthood do not seem to be caused by mechanical influences of the surrounding tissue; (3) the pterygopalatine fossa and the 'cavum cerebri' are not part of the orbit at any developmental stage; (4) in the prenatal skull, the inferior nasal concha, which forms part of the maxilla in the fully developed skull, is part of the 'capsula nasalis'; and (5) in order to achieve normal development of the eye socket in microphthalmus and anophthalmus, the normal orbital content should be restored.

The dynamic characteristics of color-coded S-potentials.

A linear analysis approach has been applied to determine the dynamic characteristics of the color-coded S-potentials. Using a sinusoidally modulated light stimulus it could be shown that the monophasic S-potential as well as each of the different components of biphasic and triphasic S-potentials behaves linearly. However, for high modulation depths and high average intensities nonlinear effects, such as saturation, become obvious. The transfer characteristics of the monophasic potentials and each component of the biphasic and triphasic potentials are indistinguishable. Their latencies, however, differ. These findings suggest that the three different types of S-potentials not only originate from functionally comparable cells but also that the dynamic characteristics of the cells presynaptic to the S-potential sources are identical.

Dynamic characteristics of retinal ganglion cell responses in goldfish.

A cross-correlation technique has been applied to quantify the dependence of the dynamic characteristics of retinal ganglion cell responses in goldfish on intensity, wavelength, spatial configuration, and spot size. Both theoretical and experimental evidence justify the use of the cross-correlation procedure which allows the completion of rather extensive measurements in a relatively short time. The findings indicate the following. (a) The shape of the amplitude characteristics depends on the energy per unit of time (power) falling within the center of a receptive field rather than on the intensity of the stimulus spot. For spot diameters of up to 1 mm, identical amplitude characteristics can be obtained by interchanging area and intensity. Therefore the receptor processes do not contribute to the change in the amplitude characteristics as a function of the power of the stimulus light. (b) For high frequencies the amplitude characteristics obtained as a function of power join together in a common envelope if plotted on an absolute sensitivity scale. For spontaneous ganglion cells this envelope holds over a range of three log units and the shape is identical for central and peripheral processes. (c) The amplitude characteristics of the central and peripheral processes converging to a ganglion cell are identical, irrespective of the sign (on or off) and the spectral coding of the response. Therefore we have no evidence for interneurons in the goldfish retina unique to the periphery of the receptive field.

Color opponency in cone-driven horizontal cells in carp retina. Aspecific pathways between cones and horizontal cells.

The spectral and dynamic properties of cone-driven horizontal cells in carp retina were evaluated with silent substitution stimuli and/or saturating background illumination. The aim of this study was to describe the wiring underlying the spectral sensitivity of these cells. We will present electrophysiological data that indicate that all cone-driven horizontal cell types receive input from all spectral cone types, and we will present evidence that all cone-driven horizontal cell types feedback to all spectral cone types. These two findings are the basis for a model for the spectral and dynamic behavior of all cone-driven horizontal cells in carp retina. The model can account for the spectral as well as the dynamic behavior of the horizontal cells. It will be shown that the strength of the feedforward and feedback pathways between a horizontal cell and a particular spectral cone type are roughly proportional. This model is in sharp contrast to the Stell model, where the spectral behavior of the three horizontal cell types is explained by a cascade of feedforward and feedback pathways between cones and horizontal cells. The Stell model accounts for the spectral but not for the dynamic behavior of the horizontal cells.

Lateral feedback from monophasic horizontal cells to cones in carp retina. II. A quantitative model.

About half of the monophasic horizontal cells in carp retina receive input from both red- and green-sensitive cones. Since the horizontal cells feed back to cones, the color and feedback pathways result in wavelength- and intensity-dependent changes of the dynamics and of the receptive field amplitude profile of the horizontal cell responses. In this paper we present a quantitative model that describes adequately the color and spatial coding and the dynamics of the responses from monophasic horizontal cells in carp. Lateral feedback plays a distinct role in this model.

The nature of surround-induced depolarizing responses in goldfish cones.

Cones in the vertebrate retina project to horizontal and bipolar cells and the horizontal cells feedback negatively to cones. This organization forms the basis for the center/surround organization of the bipolar cells, a fundamental step in the visual signal processing. Although the surround responses of bipolar cells have been recorded on many occasions, surprisingly, the underlying surround-induced responses in cones are not easily detected. In this paper, the nature of the surround-induced responses in cones is studied. Horizontal cells feed back to cones by shifting the activation function of the calcium current in cones to more negative potentials. This shift increases the calcium influx, which increases the neurotransmitter release of the cone. In this paper, we will show that under certain conditions, in addition to this increase of neurotransmitter release, a calcium-dependent chloride current will be activated, which polarizes the cone membrane potential. The question is, whether the modulation of the calcium current or the polarization of the cone membrane potential is the major determinant for feedback-mediated responses in second-order neurons. Depolarizing light responses of biphasic horizontal cells are generated by feedback from monophasic horizontal cells to cones. It was found that niflumic acid blocks the feedback-induced depolarizing responses in cones, while the shift of the calcium current activation function and the depolarizing biphasic horizontal cell responses remain intact. This shows that horizontal cells can feed back to cones, without inducing major changes in the cone membrane potential. This makes the feedback synapse from horizontal cells to cones a unique synapse. Polarization of the presynaptic (horizontal) cell leads to calcium influx in the postsynaptic cell (cone), but due to the combined activity of the calcium current and the calcium-dependent chloride current, the membrane potential of the postsynaptic cell will be hardly modulated, whereas the output of the postsynaptic cell will be strongly modulated. Since no polarization of the postsynaptic cell is needed for these feedback-mediated responses, this mechanism of synaptic transmission can modulate the neurotransmitter release in single synaptic terminals without affecting the membrane potential of the entire cell.

Lateral feedback from monophasic horizontal cells to cones in carp retina. I. Experiments.

The spatial and color coding of the monophasic horizontal cells were studied in light- and dark-adapted retinae. Slit displacement experiments revealed differences in integration area for the different cone inputs of the monophasic horizontal cells. The integration area measured with a 670-nm stimulus was larger than that measured with a 570-nm stimulus. Experiments in which the diameter of the test spot was varied, however, revealed at high stimulus intensities a larger summation area for 520-nm stimuli than for 670-nm stimuli. The reverse was found for low stimulus intensities. To investigate whether these differences were due to interaction between the various cone inputs to the monophasic horizontal cell, adaptation experiments were performed. It was found that the various cone inputs were not independent. Finally, some mechanisms for the spatial and color coding will be discussed.

Intrinsic cone adaptation modulates feedback efficiency from horizontal cells to cones.

Processing of visual stimuli by the retina changes strongly during light/dark adaptation. These changes are due to both local photoreceptor-based processes and to changes in the retinal network. The feedback pathway from horizontal cells to cones is known to be one of the pathways that is modulated strongly during adaptation. Although this phenomenon is well described, the mechanism for this change is poorly characterized. The aim of this paper is to describe the mechanism for the increase in efficiency of the feedback synapse from horizontal cells to cones. We show that a train of flashes can increase the feedback response from the horizontal cells, as measured in the cones, up to threefold. This process has a time constant of approximately 3 s and can be attributed to processes intrinsic to the cones. It does not require dopamine, is not the result of changes in the kinetics of the cone light response and is not due to changes in horizontal cells themselves. During a flash train, cones adapt to the mean light intensity, resulting in a slight (4 mV) depolarization of the cones. The time constant of this depolarization is approximately 3 s. We will show that at this depolarized membrane potential, a light-induced change of the cone membrane potential induces a larger change in the calcium current than in the unadapted condition. Furthermore, we will show that negative feedback from horizontal cells to cones can modulate the calcium current more efficiently at this depolarized cone membrane potential. The change in horizontal cell response properties during the train of flashes can be fully attributed to these changes in the synaptic efficiency. Since feedback has major consequences for the dynamic, spatial, and spectral processing, the described mechanism might be very important to optimize the retina for ambient light conditions.

Modulations of primary visual cortex activity representing attentive and conscious scene perception.

In the visual cortex, information is transferred from one area to the next by means of feedforward connections. These connections shape the receptive field properties of neurons in subsequent visual areas. Horizontal and feedback connections modulate this neuronal activity, resulting in the phenomenon of contextual modulation. In area V1, where receptive field properties reflect only low level processing, contextual modulation can be observed that represents fully evaluated perceptual saliency of the features within the receptive field. Here, we discuss to what extent these modulations are related to high level visual processes like perceptual organization, attention and visual awareness. Contextual modulation appears to reflects a process very distinct from receptive field based processing. This process seems to integrate information from distant areas in visual cortex to neurophysiologically 'highlight' those neurons that represent image elements or features of objects that stand out perceptually. Moreover, similar modulations are observed in relation to whether objects are attended to or not. Finally, these modulations are only present when subjects are aware of the visual input.

Ethambutol changes the color coding of carp retinal ganglion cells reversibly.

The influence of ethambutol on retinal function was studied by recording ganglion cell responses in isolated carp retinas superfused with a Ringer solution containing different concentrations of ethambutol (0 mg/liter, 10 mg/liter, 20 mg/liter, 30 mg/liter). The results indicate that ethambutol reversibly affects color opponency, without changing the sensitivity of the underlying receptor processes. The amacrine and bipolar cells are the most likely candidates to be affected by ethambutol.

Origin of notches in CSF: optical or neural?

Grating contrast sensitivity was measured across a range of 1 to 32 cycles per degree (c/deg) in normal observers with a computer-automated method of ascending limits. Monocular contrast sensitivity functions (CSF) were obtained for vertical, oblique and horizontal orientations, with or without full refractive correction. Small amounts of astigmatic error resulted in loss of sensitivity at selective spatial frequencies. Coincident with these CSF "notches" was the presence of monocular diplopia induced, in this study, by the condition of astigmatic error. Experimental manipulation of the selective spatial frequency losses was possible by the introduction of slight cylindrical defocus and by changes in grating orientation. Determination of the angular displacement and orientation of the monocular double images allowed prediction of the spatial frequencies which would show reduced sensitivity due to partial cancellation of contrast. The close fit between the predicted and measured sensitivity loss supports the suggestion that refractive error can affect narrowly-tuned notches. These results indicate that before the presence of a notch in the CSF can be attributed to neural abnormality, an optical cause must be eliminated.

Pattern evoked potentials in awake rhesus monkeys.

The pattern evoked potential (EP) in man to checkerboard stimulation has been shown to consist of various components originating in different regions of the visual cortex. Surface recordings, however, cannot unambiguously localize the sources of these components; for precision, depth recordings seem to be indicated. Considering the close correspondence in cortical architecture to man, rhesus monkeys (Macaca mulatta) seem to be a suitable animal for such experiments. However, Padmos et al. demonstrated that in monkeys anesthetized by pentobarbital (Nembutal), no pattern EP as found in man could be recorded. The present experiments were carried out to investigate whether the lack of contrast-specific EPs in monkey can be attributed to the effects of anesthesia. Four rhesus monkeys were trained to fixate at a television screen on which checkerboard and bar patterns of various sizes could be presented. The results from this study demonstrate that in monkey as in man, pattern EPs can be obtained that can be distinguished from those evoked by luminance variations. Therefore the awake rhesus monkey seems to be a suitable experimental model in the search for the origin of the pattern EP.

Comparison of acuity tests and pattern evoked potential criteria: two mechanisms underly acuity maturation in man.

A comparative study of acuity tests and pattern evoked potential (EP) criteria was performed on a total of 307 subjects, 214 of them at an age between 2 months post-term and 12 years. All were examined ophthalmologically prior to testing. It was shown that both psychophysical and EP estimated acuity improve in the same way until puberty. From birth to about 6 months a rapid improvement is found. This fast phase can probably be attributed to retinal morphological maturation. During this period a fair estimate of acuity can be obtained by determining the checksize that yields the largest EP; a conclusion of practical importance for screening. The subsequent slow improvement phase, which ends around puberty, is reflected in the development of the waveform of the pattern onset EP. Since it correlates with the growth of a spatial contrast specific component of extrastriate origin in the EP, the slow improvement phase most likely reflects maturation of central processes.

Visual stimulation reduces EEG activity in man.

Data are presented which show that background electric activity of the human brain is reduced by visual stimulation. Occipital EEG amplitude decreases 5-15% for all frequencies analyzed (0.2-40 Hz) upon pattern stimulation. The reduction is stimulus-specific, i.e. is the strongest for stimuli that activate a large number of visual cortical neurons.