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PubMed2001

Perceived motion in orientational afterimages: direction and speed.

Two sets of experiments demonstrate new properties of motion in orientational after-effects. In a previous report, we showed that when observers adapted to a static bar grating whose elements varied in size or intensity from one side to the other, offset of the grating resulted in a motion after-effect, with the perceived motion in the direction of the largest or most intense bar. In the first new experiment, we show that similar results can be produced by varying the duration of the bar elements, with the direction of the motion after-effect toward the bar with the longest duration. In the second new experiment we demonstrate that the perceived speed of the motion after-effect is influenced by the spatial extent of the after-effect, with larger extents corresponding to faster speeds. The experimental findings are discussed in the context of a neural network theory of visual perception. In this theory, a moving oriented contour leaves a trail of activity among cortical cells tuned to orthogonal orientations. We hypothesize that the grating stimuli produce after-effects that mimic the pattern of oriented responses produced by a true moving contour, and the visual system interprets this pattern as a cue for motion. We also show how the model connects the properties of these motion after-effects to properties of visual persistence.

Figural Aftereffect↗
PubMed1995

The role of exocentric reference frames in the perception of visual direction.

One classic piece of evidence for an efference copy signal of eye position is that a small, positive afterimage viewed in darkness is perceived to move with the eye. When a small stationary reference point is visible the afterimage appears to move relative to the reference point. However, this is true only when the afterimage is localized to a small area. We have observed that when an extended afterimage of a complex scene is generated by a brief, bright flash it does not appear to move, even with large changes in eye position. When subjects were instructed to maintain their direction of gaze, we observed small saccades (typically < 1 deg) and slow drift movements often totalling more than 10 deg over a 30 sec period. When the instructions were to simply inspect the extended afterimage, subjects made larger saccades (up to 5 deg) which were not accompanied by afterimage movement. The smaller movements observed under the first instructions are greater than those observed in the dark or with small afterimages. When a visible reference is present with these large afterimages, the afterimage appears stationary, while the reference point appears to move. Eye position was monitored following the generation of such afterimages. In general, the perceived motion of the stationary reference point was in a direction opposite to the motion of the eye. Similar drift movements of smaller magnitude were observed with localized afterimages, but the motion was attributed to the afterimage.(ABSTRACT TRUNCATED AT 250 WORDS)

Afterimage↗
PubMed2006

An empirical test of formal equivalence between Emmert's law and the size-distance invariance hypothesis.

Emmert's law and the size-distance invariance hypothesis have been said to be formally equivalent, provided that Emmert's law means that the perceived size of an afterimage is proportional to the perceived distance of the projected surface of the afterimage. However, there have been very few studies that have attempted to verify this formal equivalence empirically. We measured both the perceived size and distance of afterimages and real objects with the same proximal size. Nineteen participants projected afterimages of 1 deg in visual angle on the wall located at distances of 1 to 23 meters from the participants. They also observed real objects, disc-shaped and made from a sheet of Styrofoam board, with the same proximal size as that of the afterimages, which were located at the same physical distances as those of the wall on which the afterimages were projected. Each participant reproduced the apparent sizes of the afterimages and real objects using the reproduction method and estimated the apparent distances using the magnitude estimation method. When the mean apparent sizes of the afterimages and real objects, represented as a function of apparent distance, were fitted to a linear function, the slopes for the afterimages and real objects did not differ significantly. These results are interpreted as evidence for the formal equivalence of Emmert's law and the size-distance invariance hypothesis.

Adolescent↗
PubMed2006

Visual-proprioceptive mismatch and the Taylor illusion.

When a participant moves a hand-held target in complete darkness after an afterimage of that target has been obtained, an illusory increase (with movements away from the participant) or decrease (with movements towards the participant) in the apparent size of the afterimage is reported (the Taylor illusion, reported first in Taylor, J Exp Psychol 29: 1941). Unlike typical Emmert's Law demonstrations, the Taylor illusion shows that a motor-related signal can be used to specify distance for the computation of real size. A study by Carey and Allan (Exp Brain Res 110: 1996) found that the Taylor illusion did not occur in a condition where an afterimage of one hand was obtained while the other hand performed a movement away from the participant from directly behind the first. It was proposed that, for the illusion to manifest itself, proprioceptive and visual information must be in strict "register" when the afterimage is obtained. To evaluate this hypothesis, 14 participants performed "towards" and "away" movements after obtaining afterimages of hand-held cards. Participants wore either plain lenses or prism lenses during the trials, the latter of which displaced visual stimuli 10 degrees to the left. No significant difference was found between the two lens conditions in terms of the effect on the perception of the Taylor illusion. It was concluded that the illusory size distortions may depend on register of visual and proprioceptive position in terms of depth, rather than in the picture plane. Several suggestions for future studies of the Taylor illusion are proposed, and limitations of size judgements of afterimages are outlined.

Afterimage↗
PubMed1988

The cyclopean eye and its implications: vergence state and visual direction.

The cyclopean illusion (Hering, 1861) is an anomalous lateral shift in the apparent direction to a monocularly seen target, which arises when a change in vergence is made by the opposite (nonobserving) eye. Surprisingly, this directional illusion does not arise, when the observed target is an afterimage or an intermittently illuminated (4-8 Hz) object. Instead, during convergence, a monocularly imprinted afterimage seems to move toward the observer, and a stroboscopically illuminated target seems to remain fully stationary. The apparent displacement of an afterimage in depth is particularly puzzling. Since binocular interactions in the persistence of monocularly induced afterimages can be demonstrated, it is conceivable that long-persistent afterimages arise, in part, from binocularly driven neurons in the visual cortex, and that a monocularly induced afterimage can thereby become the perceptual equivalent of a binocularly fused target.

Afterimage↗
PubMed1984

Role of local adaptation in the fading of stabilized images.

We provide evidence that the fading of stabilized images and the formation of negative afterimages result from the same local adaptive process. We measure thresholds for stabilized, static, sine-wave gratings and for stabilized flickering sine-wave gratings. We then measure the contrasts of the negative afterimages formed by the threshold-contrast stabilized, static stimuli. (The threshold-contrast flickering gratings produce no visible afterimages.) We find that the difference between the thresholds for stabilized, static gratings and the thresholds for slowly flickering gratings is equal to the contrasts of the afterimages produced by the stabilized, static gratings. We conclude that the fading of these stabilized gratings can be accounted for completely by local adaptation (the process underlying the formation of negative afterimages.

Adaptation, Ocular↗
PubMed2004

Verification of Emmert's law in actual and virtual environments.

We examined Emmert's law by measuring the perceived size of an afterimage and the perceived distance of the surface on which the afterimage was projected in actual and virtual environments. The actual environment consisted of a corridor with ample cues as to distance and depth. The virtual environment was made from the CAVE of a virtual reality system. The afterimage, disc-shaped and one degree in diameter, was produced by flashing with an electric photoflash. The observers were asked to estimate the perceived distance to surfaces located at various physical distances (1 to 24 m) by the magnitude estimation method and to estimate the perceived size of the afterimage projected on the surfaces by a matching method. The results show that the perceived size of the afterimage was directly proportional to the perceived distance in both environments; thus, Emmert's law holds in virtual as well as actual environments. We suggest that Emmert's law is a specific case of a functional principle of distance scaling by the visual system.

Afterimage↗
PubMed1996

A motor signal and "visual" size perception.

Recent models of the visual system in primates suggest that the mechanisms underlying visual perception and visuomotor control are implemented in separate functional streams in the cerebral cortex. However, a little-studied perceptual illusion demonstrates that a motor-related signal representing arm position can contribute to the visual perception of size. The illusion consists of an illusory size change in an afterimage of the hand when the hand is moved towards or away from the subject. The motor signal necessary for the illusion could be specified by feedforward and/or feedback sources (i.e. efference copy and/or proprioception/kinesthesis). We investigated the nature of this signal by measuring the illusion's magnitude when subjects moved their own arm (active condition, feedforward and feedback information available), and when arm movement was under the control of the experimenter (passive condition, feedback information available). Active and passive movements produced equivalent illusory size changes in the afterimages. However, the illusion was not obtained when an after-image of subject's hand was obtained prior to movement of the other hand from a very similar location in space. This evidence shows that proprioceptive/kinesthetic feedback was sufficient to drive the illusion and suggests that a specific three-dimensional registration of proprioceptive input and the initial afterimage is necessary for the illusion to occur.

Afterimage↗
PubMed1984

Stability of phase recognition in complex spatial waveforms.

Observers viewed 200 msec presentations of gratings containing first (0.5 c/deg) and third (1.5 c/deg) harmonic components. The phase of the third harmonic and the absolute position of the grating on the screen varied randomly from trial to trial. Classification of the phase relation (0, 90, 180 or 270 deg was 99% perfect. When a 2 sec period of inspection of the grating or its fundamental preceded the test presentation, strong shifts in perceived waveform were observed that depended on the test grating's position relative to the inspection grating, and resembled the effects seen during continuous viewing. No phase-specific effects were obtained. The pattern of results was exactly that predicted by negative afterimage. Phase recognition at low contrast, and at a high frequency, was also good. Triangular-wave gratings were misperceived (the "square-wave illusion") only when real or simulated afterimages were present. We conclude that recognition of phase relations in a complex waveform is stable when the predictable variation due to afterimages is eliminated.

Afterimage↗
PubMedDate not supplied

Vestibular function in the space environment.

Adaptation to the weightless state and readaptation after space flight to the 1-G environment on the ground are accompanied by various transitory symptoms of vestibular instability, kinetosis, and illusory sensations. Aside from the problem of how to treat and if possible prevent such symptoms, they offer a clue to a better understanding of normal vestibular functions. Weightlessness is a powerful new "tool" of vestibular research. Graybiel reported as early as 1952 that human subjects observed the illusion that a real target and the visual afterimage seemed to raise in the visual field during centrifugation when the subjects were looking toward the axis of rotation (oculogravic illusion). In aircraft parabolic-flight weightlessness, human subjects observed that fixed real targets appeared to have moved downward while visual afterimages appeared to have moved upward (oculoagravic illusion). It can be shown by electronystagmography as well as by a method employing double afterimages that part of this illusion is caused by eye movements that are triggered by the changing input from the otolith system. Another part of the illusion is based on a change of the subjective horizontal and must be caused by convergence of vestibular and visual impulses "behind" the eyes. This part was measured independently of the first one by using a new method. Eye movements could be prevented during these experiments by optical fixation with the right eye on a target at the end of a 24-in. long tube which was rigidly attached parallel to the longitudinal axis of an aircraft. At the same time the subject tried to line up a shorter tube, which was pivoting around his left eye, with the subjective horizon.

Acceleration↗
PubMed2005

Temporal image fusion in human vision.

We studied temporal integration by presenting sequences of orthogonal high-contrast sinusoidal gratings. In rapid alternation (38 Hz) gratings appear fused whereas at lower rates they appear segregated. To test whether afterimages are the source of the fused and segregated appearance, we probed the decay characteristics of a vertical grating's afterimage by flashing an otherwise identical horizontal grating at varying times after the grating's offset. The results show a rapid decay function for a negative afterimage with a 50-ms time constant, much shorter than expected from other paradigms. From dichoptic presentations we conclude that the site of the interaction is before that of binocular integration.

Afterimage↗
PubMed1986

The equivalent background of bleaching.

Stiles and Crawford proposed that a retinal region bleached by preexposure to intense light behaves as if it were illuminated by some steady veiling or background luminance. We test this notion by comparing the afterimage of a bleaching light with a steady (and retinally stabilized) light of adjustable intensity, in the manner of Barlow and Sparrock. With their matching procedure, and also with a new procedure, we find as they did that during the rod phase of recovery the afterimage does look like a stabilized field of an intensity which, presented as a background, brings visual sensitivity to the same level. It is as if the two conditions produce equal signals at some stage of the visual pathway. Liked Barlow and Sparrock we observe a rod-cone break in the afterimage matches. However, we argue that the appearance of the rod-cone break presents a paradox and we show a way to resolve it.

Afterimage↗
PubMed1990

Emmert's law in the Ames room.

The Ames distorted room illusion, in which the perceived sizes of objects placed within the room differ from their objective sizes, has been used to support arguments for indirect perception. A study is reported in which Emmert's law of the apparent size of after-images was examined in relation to the Ames room's illusory alteration of apparent and actual distances. Size judgments of afterimages projected into the Ames room were compared with control conditions in which both actual and apparent afterimage projection distances were reproduced. Results indicate that Emmert's law may not provide a simple geometrical relationship between proximal image size and actual viewing distance, and that the processes involved in making afterimage size judgments are similar to those processes involved in making size judgments of 'real world' objects.

Adult↗
PubMed1980

Motion and vision. III. Stabilized pattern adaptation.

It has been suggested that local variations of retinal sensitivity may be responsible for elevating the threshold in pattern-adaptation experiments of the Blakemore-Campbell type. Subjects are unable to scan high-contrast gratings uniformly enough to eliminate this possibility. To control this effect, we performed grating-adaptation experiments under stabilized-image conditions, while both adapting and test targets were moved at retinal velocities determined by the experimenter. By means of an afterimage technique, we also measured the strength of the retinal sensitivity mask that forms under these conditions. Varying the spatial frequency and velocity of the adapting stimulus, we inferred the spatial and temporal properties of the principal mechanism that contributes to the afterimage. We found that the Blakemore-Campbell effect persists at adapting velocities that are fast enough to rule out local variations of retinal sensitivity. More surprisingly, even the clearly visible afterimages that occur at a retinal velocity of 0.1 deg/s seem to have no effect on pattern adaptation. (Sensitivity masking can raise the adapted threshold, but only at adapting velocities slower than normal eye movements). By manipulating the image velocity, we were able to shift the spatial frequencies of some threshold-elevation curves, but these shifts were not great enough to suggest that velocity tuning plays important role in pattern adaptation.

Adaptation, Ocular↗
PubMed1990

Imagery vividness, hypnotic susceptibility, and the perception of fragmented stimuli.

Two experiments were conducted to determine the role of hypnotic susceptibility level (high or low) and imaging ability (vivid or poor) in the performance of gestalt closure tasks. In Experiment 1, subjects were required to identify fragmented stimuli in the Closure Speed Test and in the Street Test. In Experiment 2, subjects reported on fragmented stimuli that were projected to the right eye and subsequently produced an afterimage. Individuals were asked to identify the composite if possible and to report on the duration of the afterimage. In both experiments, hypnotic susceptibility level and imaging ability affected reports of gestalt closure. The greatest number of correct closures was reported by those who were both high in hypnotic susceptibility and vivid in imaging ability. In addition, in the second experiment, this group also reported the longest enduring afterimage. These results are discussed in terms of the processes required to perform in a gestalt closure task.

Adult↗