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At least 271 records · Page 15Linked to original sources
PubMed1999

Toward a theory of visual consciousness.

The visual brain consists of several parallel, functionally specialized processing systems, each having several stages (nodes) which terminate their tasks at different times; consequently, simultaneously presented attributes are perceived at the same time if processed at the same node and at different times if processed by different nodes. Clinical evidence shows that these processing systems can act fairly autonomously. Damage restricted to one system compromises specifically the perception of the attribute that that system is specialized for; damage to a given node of a processing system that leaves earlier nodes intact results in a degraded perceptual capacity for the relevant attribute, which is directly related to the physiological capacities of the cells left intact by the damage. By contrast, a system that is spared when all others are damaged can function more or less normally. Moreover, internally created visual percepts-illusions, afterimages, imagery, and hallucinations-activate specifically the nodes specialized for the attribute perceived. Finally, anatomical evidence shows that there is no final integrator station in the brain, one which receives input from all visual areas; instead, each node has multiple outputs and no node is recipient only. Taken together, the above evidence leads us to propose that each node of a processing-perceptual system creates its own microconsciousness. We propose that, if any binding occurs to give us our integrated image of the visual world, it must be a binding between microconsciousnesses generated at different nodes. Since any two microconsciousnesses generated at any two nodes can be bound together, perceptual integration is not hierarchical, but parallel and postconscious. By contrast, the neural machinery conferring properties on those cells whose activity has a conscious correlate is hierarchical, and we refer to it as generative binding, to distinguish it from the binding that might occur between the microconsciousnesses.

Agnosia↗
PubMed1993

Oculovestibular interactions under microgravity.

On a space mission in March 1992 a set of experiments were performed aimed at clarifying the interaction between visual, proprioceptive and vestibular inputs to the equilibrium system. Using the VESTA goggle facility from the European Space Agency we investigated the effect of pure neck receptor stimulation on eye position as measured by the flash afterimage method and on perception of a head-fixed luminous line in space. Space vestibular adaptation processes were measured by rotating pattern perception during prescribed head movements. It was found that static ocular counterrotation does not occur under microgravity conditions. This result suggests that the neck receptors apparently do not contribute to a measurable extent. The subjective orientation of a vertical line was perceived correctly inflight. Obviously neck receptors on the perception level can fully substitute for the ineffective equilibrium organs of the inner ear within less than 4 days. The rotating pattern perception during different head motion patterns is not influenced by the absence of a gravity reference.

Adaptation, Physiological↗
PubMed1981

Linking quantal absorption rate to the visual response.

A photochemical system is proposed that offers a possible link between the rate of quantal absorption and the visual response. In the proposed system the time courses of the concentrations of certain photoproducts turn out to be equal to the quantal absorption rate passed through various linear filters. The proposed system is consistent with the cone pigment kinetics proposed by Rushton (1958). The system produces afterimages and can provide Weber's law behavior.

Humans↗
PubMed1997

Looking at the task in hand: vergence eye movements and perceived size.

A retinal afterimage of the hand changes size when the same unseen hand is moved backwards and forwards in darkness. We demonstrate that arm movements per se are not sufficient to cause a size change and that vergence eye movements are a necessary and sufficient condition for the presence of the illusory size change. We review previous literature to illustrate that changing limb position in the dark alters vergence angle and we explain the illusion via this mechanism. A discussion is provided on why altering limb position causes a change in vergence and we speculate on the underlying mechanisms.

Convergence, Ocular↗
PubMed2000

Electrophysiological correlates of human intrasaccadic processing.

Visual discrimination performance is thought to be suppressed during saccades in order to contribute to space constancy. However, under certain experimental conditions, visual inhibition may not take place, suggesting a more complex underlying mechanism. We tested the discrimination ability of 20 healthy subjects during visually guided horizontal saccades and recorded simultaneously the evoked brain activity from 30 channels over occipital, parietal, and temporal areas. During the execution of saccadic eye movements, visual stimuli were presented for 30 ms. In order to prevent retinal afterimages, stimuli were followed by a visual mask. In a control condition, the same stimuli were presented with stationary eyes. Electro-oculogram (EOG) and electroencephalogram (EEG) signals were recorded continuously together with information about the stimuli and the subject's response. Evoked potentials were computed offline, and component latency, field strength (global field power), and topography were compared between conditions. During saccades, subjects showed only slightly reduced discrimination performance which remained very high above the chance level; thus, there was no evidence for strong saccadic suppression with the supra-threshold stimuli employed. However, the cortical activation patterns exhibited large alteration when a physically identical stimulus was presented during the eye movement: around 130 ms latency, field strength was significantly smaller than when stationary targets were processed, and scalp topography was also different. These effects on evoked field distributions may be attributed to neural interactions of an efference copy signal (linked to the oculomotor command) with the afferent excitation following the visual stimulus.

Adult↗
PubMed2006

Associative visual learning, color discrimination, and chromatic adaptation in the harnessed honeybee Apis mellifera L.

We studied associative visual learning in harnessed honeybees trained with monochromatic lights associated with a reward of sucrose solution delivered to the antennae and proboscis, to elicit the proboscis extension reflex (PER). We demonstrated five properties of visual learning under these conditions. First, antennae deprivation significantly increased visual acquisition, suggesting that sensory input from the antennae interferes with visual learning. Second, covering the compound eyes with silver paste significantly decreased visual acquisition, while covering the ocelli did not. Third, there was no significant difference in the visual acquisition between nurse bees, guard bees, and foragers. Fourth, bees conditioned with a 540-nm light stimulus exhibited light-induced PER with a 618-nm, but not with a 439-nm light stimulus. Finally, bees conditioned with a 540-nm light stimulus exhibited PER immediately after the 439-nm light was turned off, suggesting that the bees reacted to an afterimage induced by prior adaptation to the 439-nm light that might be similar to the 540-nm light.

Acclimatization↗
PubMed1984

Prolonged complementary chromatopsia in users of video display terminals.

Induced prolonged complementary chromatopsia in four subjects (two men, 26 and 34 years old, and two women, 34 and 45 years old) was documented by abnormal responses to pseudoisochromatic color plates 30 minutes after cessation of monochrome video display terminal use. This phenomenon was unrelated to preexisting color vision anomalies, known ocular disease, or drug use, and defines a specific phenomenon probably related to afterimages. All four patients missed AOH-R-R plate 3 after using the video display terminal. One patient also had a delayed response to plate 6. One patient patched one eye before using the video display terminal and noted that the complementary chromatopsia occurred only in the unpatched eye.

Adult↗
PubMed1992

A visual nonlinearity fed by single cones.

An intensive nonlinearity in the visual system can produce distortion products, or difference frequency gratings, when observers view two high contrast, high spatial frequency interference fringes of slightly different frequency or orientation added together at the retina. These distortion products are visible even when the two fringes imaged on the retina are above the resolution limit. Our experiments take advantage of this nonlinearity to measure the spatial filtering in the visual system following the formation of the retinal image, but preceding the site of the nonlinearity. The point spread function corresponding to this spatial filter is so small that it can be entirely explained by light integration within the apertures of foveal and parafoveal cones. The small size of this point spread function implies that (1) laser interferometry avoids contrast losses inherent in the eye's optics at spatial frequencies as high as 130 c/deg, (2) retinal scatter causes negligible image degradation in the fovea and parafoveal retina, (3) eye movements have little or no effect on contrast sensitivity to the distortion product and (4) that there is no neural spatial summation in the visual system prior to the site of the nonlinearity. Distortion products could also be observed when a bright interference fringe was briefly flashed on the fovea and a test interference fringe was viewed through the resulting afterimage. Measurements of the point spread function at stages in the visual system that precede the generation of this distortion product were similar to those obtained with simultaneous presentation of the two fringes, implying that the aftereffect of light adaptation is extremely local, no larger than the dimensions of single cones.

Adaptation, Ocular↗
PubMed1996

No binocular rivalry in the LGN of alert macaque monkeys.

Orthogonal drifting gratings were presented binocularly to alert macaque monkeys in an attempt to find neural correlates of binocular rivalry. Gratings were centered over lateral geniculate nucleus (LGN) receptive fields and the corresponding points for the opposite eye. The only task of the monkey was to fixate. We found no difference between the responses of LGN neurons under rivalrous and nonrivalrous conditions, as determined by examining the ratios of their respective power spectra. There was, however, a curious "temporal afterimage" effect in which cell responses continued to be modulated at the drift frequency of the grating for several seconds after the grating disappeared.

Animals↗
PubMed1994

Ocular torsion during microgravity on a space mission in 1992.

On a space mission in 1992 we investigated the effect of pure neck receptor stimulation on eye roll position in space. To do this, we used the flash afterimage method. We found that eye rotations in static tilted head positions are absent in weightlessness. This suggests that in microgravity the neck position receptors do not contribute to a measurable extent to static OCR.

Adult↗
PubMed2001

Effects of temporal gaps between successive fixation targets on discrimination performance and evoked brain activity.

Planning and executing of action in real-world conditions require continuous sensory input from many modalities. At the same time, sensory functions depend on reafferent and efference-copy information flow as imposed by motor actions. We studied how a specific oculomotor task influences afferent visual processing. Twenty healthy adults performed visually guided saccades. Between the offset of a fixation light and the onset of a new visual target a temporal gap of a duration of about 200 ms was introduced. This time structure is known from previous studies to elicit saccades at express latencies. In a control condition, 'no gap' was used. During eye movements one of four visual patterns with different orientations was presented, triggered by the horizontal electro-oculogram. We analyzed discrimination performance and the simultaneously recorded multichannel EEG activity. In the gap condition, shorter saccadic latencies were accompanied by significant more correct perceptual judgments. However, brain activity, as quantified by global field power, evoked component latency and topographical descriptors (centers of gravity or centroids) were not affected by the gap. This contrasts the notion that parieto-occipital areas are the most important sites of sensorimotor integration. Furthermore, the presence of a visual masking stimulus did not degrade discrimination performance, demonstrating that local retinal afterimages were not used for perceptual decisions. We conclude that intra-saccadic visual processing is influenced by pre-saccadic events. Under the short-time constraints prevalent in the saccadic task, fixation target cues are not only used for motor planning but also influence the visibility of visual patterns presented during the eye movement.

Adult↗
PubMed2003

Interaction of retinal image and eye velocity in motion perception.

When we move our eyes, why does the world look stable even as its image flows across our retinas, and why do afterimages, which are stationary on the retinas, appear to move? Current theories say this is because we perceive motion by summation: if an object slips across the retina at r degrees/s while the eye turns at e degrees/s, the object's perceived velocity in space should be r + e. We show that activity in MT+, the visual-motion complex in human cortex, does reflect a mix of r and e rather than r alone. But we show also that, for optimal perception, r and e should not summate; rather, the signals coding e interact multiplicatively with the spatial gradient of illumination.

Confidence Intervals↗
PubMed2000

Nervous control of blood flow microkinetics in the infrared organs of pit vipers.

The pit organ of pit vipers contains a membrane which serves as an infrared retina, processing infrared information by the degree to which the temperature of trigeminal nerve receptors (terminal nerve masses) is raised. The receptors are arranged in a monolayer array within the pit membrane and irrigated by a capillary network which both supplies energy to the terminal nerve masses and serves as a heat exchange mechanism. This mechanism maintains the receptors at a stable temperature level to increase or decrease their sensitivity and to reduce to a minimum the afterimage effect of a moving stimulus. We used a Doppler laser blood flow meter to measure the local changes in blood flow in response to a point heat source (a small soldering iron) and to direct stimuli (red and infrared lasers). Resection of any one of the trigeminal A-delta fiber trunks innervating the pit membrane abolished blood flow response in the area innervated, but resection of the main trunk between the primary neurons and the medulla left the response intact. In addition to the A-delta fibers the pit membrane contains autonomic and sensory C-fiber innervation, but preganglionic resection of parasympathetic neurons, and chemical blocking of postganglionic fibers with atropine and capsaicin had no influence on the blood flow changes. Therefore, on the basis of the rapid response time and the similarity of the blood flow curves to electrophysiological recordings from the receptors, we surmised that all blood flow changes were due to a vasomotor reaction, modulated by the terminal nerve masses directly, resulting in a change in local heat capacity that cools the stimulated receptors back to a basal temperature.

Animals↗
PubMed1989

Visual misalignment in arc and chevron figures.

Five experiments investigated an apparent misalignment effect in 90 degrees arc figures. Preliminary observations showed that the effect occurs also in chevron figures, in an afterimage of the arc figure, and haptically in arc- and chevron-shaped objects. The experiments showed that the effect is greater with 3 radial lines than with 2, absent without them, and present in a figure consisting of only 3 radial lines. The effect with arc figures was consistently greater than that with chevron figures, a difference found not to be due to an apex marking the midpoint of the latter, and it was of intermediate size in figures with 1 arc boundary and 1 chevron boundary. The misalignment was also greater in narrow, elongated figures. The issues singled out for discussion are the effect of context on the misalignment effect with 3 radial lines, a possible explanation in terms of perceptual compromise, the difference in the effect between arcs and chevrons, and the relationship between this illusion and the Morinaga illusion.

Adult↗
PubMed1984

On the relationship between stimulus intensity and duration of visible persistence.

Stimulus intensity and duration of visible persistence have been found to vary directly in some studies but inversely in others. Hawkins and Shulman (1979) have proposed that this inconsistency can be resolved by separating the studies that employed a decrement-threshold measure of persistence (Type I measure) from those that measured the total duration of persistence (Type II measure). They suggested that Type I measures yield an inverse relationship, whereas Type II measures yield a direct relationship between intensity and persistence. Hawkins and Shulman's model is incomplete in ways that are easily remedied. However, the model is totally contradicted by the experimental evidence. A new resolution of the inconsistent results is proposed in terms of retinal afterimages.

Attention↗
PubMed1998

Medieval theories of mental representation.

Throughout most of the Middle ages, it was generally held that stored mental representations of perceived objects or events preserved the forms or species of such objects. This belief was consistent with a metaphor used by Plato. It was also consistent with the medieval belief that a number of cognitive processes took place in the ventricles of the brain and with the phenomenology of afterimages and imagination itself. In the 14th century, William of Ockham challenged this belief by claiming that mental representations are not stored but instead constructed in the basis of past learned experiences.

Brain↗
PubMed1976

The magic number 4 +/- 0: a new look at visual numerosity judgements.

Visual numerosity judgements were made for tachistoscopically presented linear arrays of dots or lines. The interelement interval (which could be specified in spatial frequency terms) was constant for a given array but varied across conditions. A clear limit in the accuracy of numerosity judgements was found to be set at 4 for regularly spaced elements with spatial frequencies below approximately 10 cycles/deg (element and interelement interval of 0-05 deg). This limit in terms of accuracy is accompanied by a fast and almost constant response time for arrays of 4 or less, compared to response times for arrays of more than 4 elements. The limit in accuracy falls to 2 elements rather than 4 for spacing narrower than 0-05 deg alghough with such spacing the elements are still easily resolved. The limit of 4 is found if the stimulus is a bright afterimage, lasting for approximately 60 s. This result suggests that the limit is independent of the time allowed for a single fixation and is a perceptual limit rather than a limit in some memory buffer. 'Numerosity' units are propsoed to account for the results.

Humans↗
PubMed1980

The graph-paper effect: subjective stereoscopic patterns induced by moving gratings.

Smooth tracking across an oblique grid pattern produced hallucinations of vertical and/or horizontal striations which moved with the eyes. The effect was produced by single or multiple gratings, monocularly or binocularly, but in the latter case it appeared to lie steresoscopically in the plane of fixation. Gratings containing thin lines or sawtooth edges of moderate contrast were particularly effective stimuli but sine or square waves were not. The subjective stripes had an apparent edge polarity which was opposite to that of the inducing edges or, with thin inducing lines, was determined by the lines' polarity and movement direction. Conventional explanations (eg strobe, afterimage, or moiré effects) can be ruled out. Neither the present effect nor the "pincushion-grid illusion" are due to the presence of spurious Fourier components in the stimulus pattern. An extension of a previous model, involving disinhibitory interaction between movement and pattern channels, accounts for many aspects of this elaborate phenomenon. The detailed dependence of polarity on spatial waveform and movement direction implies: (i) the spatial second harmonic is a necessary component, (ii) the generating is approximately linear and has 90 degree phase preference, and (iii) there is a movement-induced phase lag of about 45 degrees in the response to the second harmonic.

Depth Perception↗