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PubMed1999

Afterimages: a tool for defining the neural correlate of visual consciousness.

Our visual system not only mediates information about the visual environment but is capable of generating pictures of nonexistent worlds: afterimages, illusions, phosphenes, etc. We are "aware" of these pictures just as we are aware of the images of natural, physical objects. This raises the question: is the neural correlate of consciousness (NCC) of such images the same as that of images of physical objects? Images of natural objects have some properties in common with afterimages (e.g., stability of verticality) but there are also obvious differences (e.g., images maintain size constancy, whereas afterimages follow Emmert's Law: when seen while screens at different distances are observed, an afterimage looks larger, the greater the distance of the screen). The differences can be explained by differences in the retinal extent of images and afterimages, which favors the view that both have the same NCC. It seems reasonable to assume that before neural activity can produce awareness, all the computations necessary for a veridical representation of, e.g., an object, must be completed within the neural substrate and that information characteristic of a particular object must be available within the NCC. Given these assumptions, it can be shown that no retinotopic (in a strict sense) cortical areas can serve as the NCC, although some type of topographic representation is necessary. It seems also to be unlikely that neurons classified as cardinal cells alone can serve as NCC.

Afterimage↗
PubMed1989

Suppression of OKN and VOR by afterimages and imaginary objects.

Optokinetic nystagmus (OKN) is suppressed if attention is directed to a centrally placed afterimage superimposed on a moving display. Imagining a stationary object has little or no effect. An afterimage does not provide the retinal slip and misfoveation error signals provided by a stationary object and we have shown that an effective error signal does not arise from occlusion or masking of the display by the afterimage. Although a lack of relative motion between afterimage and moving display could indicate when OKN gain is one, there is no unique relative motion signal associated with a gain of zero. Subjects could partially inhibit the vestibulo-ocular reflex (VOR) in the dark when they imagined a head-fixed object. They could suppress the response more effectively by attending to an afterimage, but the suppression was still only partial. When OKN and VOR were evoked simultaneously, pursuit movements of the eyes could not be suppressed until the vestibular inputs had subsided. We conclude that signals associated with OKN, are fully available to the mechanism that assesses the headcentric motion of objects but that signals associated with VOR are only partially available to that mechanism.

Adult↗
PubMed1987

Duration of visual afterimages on modulated backgrounds: postreceptoral processes.

A foveal afterimage produced by a small photoflash increases in duration when the luminance of a 5.8 degrees diameter background on which it is seen is temporally modulated. At a modulation frequency of 1 Hz and a depth of modulation of 52%, the duration of the afterimage is prolonged by 335% compared to the duration obtained on a steady background. This increase has been attributed to the functional border resulting from the difference in excitability between bleached and unbleached photoreceptors. Afterimage duration is also prolonged, although only by 20%, when the luminance of the background is kept constant, while the luminance of an annular surround is modulated. This finding suggests a weak effect of neural lateral interaction (via area contrast). If the background luminance is modulated only in the contralateral eye (dichoptic presentation), afterimage duration increases by as much as 54% compared to the unmodulated state. This result indicates that afterimages, in part, are sustained by processes mediated by the visual cortex.

Adult↗
PubMed1987

Electrophysiological correlates of positive and negative afterimages.

We report here measurements of visual evoked potential (VEP) activity after induction of afterimages in human observers. Drifting gratings normally produce no measurable phase synchronized VEP. However, after an afterimage of a high contrast grating of the same orientation and spatial frequency of that of the drifting grating has been flashed on the retina. VEPs in synchrony with the drift frequency are produced. For up to about two minutes after the flash, the VEPs were all clustered in one phase, then declined for a minute or so, to reappear 180 degrees out of phase from the first VEPs. The first group of VEPs coincides with the percept of a positive afterimage, and the second with that of a negative afterimage. Possible explanations for the existence of positive and negative afterimages and the associated VEPs are considered.

Afterimage↗
PubMed2004

Unveiling the foveal blue scotoma through an afterimage.

The absence of short-wave-sensitive (S-) cones in the human foveola normally goes unnoticed, but the resulting foveal S-cone, or blue, scotoma can be visualized as the negative afterimage of a short-wavelength adapting field on a larger white background. The afterimage has an annular shape with a lighter inner region that corresponds to Maxwell's spot, and a small bright spot in the center corresponding to the foveal blue scotoma. We have shown that the visibility of the center spot in the afterimage approximately follows the spectral sensitivity curve of the S-cones. We further demonstrate that the central bright spot subtends a retinal area that is coincident with the tritanopic region of the foveola. The macular pigment distribution measured for the same observers also peaks in the central fovea, but has a relatively high density over a broader retinal region than the bright spot in the negative afterimage, and more closely corresponds to the lighter annular region of the afterimage. The results support the hypothesis of an active post-receptoral process for filling-in of chromatic scotomas.

Adaptation, Ocular↗
PubMed2002

Afterimages in fly motion vision.

Afterimage-like effects modulate the responses of fly wide-field motion-sensitive cells following adaptation to stationary or slowly moving patterns. The origin of these afterimages is unclear. They have been interpreted as either the result of adaptation in the early visual system or as a direct consequence of the correlation scheme of motion detection. Using a combination of intracellular recording and computer modelling, we find that afterimage-like effects cannot be satisfactorily explained by a simple version of the correlation model previously proposed by Egelhaaf and Borst (J. Opt. Soc. Am. A 6 (1) (1989) 116). We propose a modified variant of the correlation model featuring a short delay filter and temporal high-pass filtering prior to motion correlation. Our model gives superior predictions of afterimage-like effects induced by a range of stimuli. Our model also predicts changes in cells' image step responses following exposure to motion, suggesting that previous experimental evidence for the "shortening delay" theory of motion adaptation (Biol. Cybern. 54 (1986) 223; Visual Neurosci. 14 (4) (1997) 741) should be re-interpreted in terms of afterimage effects.

Adaptation, Physiological↗
PubMed1998

Auditory afterimage: tonotopic representation in the auditory cortex.

The auditory afterimage is a sensation which occurs for several seconds after the exciting acoustic signal has been switched off, and which roughly corresponds to the inverse of the spectrum of the exciting signal. In contrast to the well-known visual afterimage, the physiological mechanism generating the auditory afterimage has been questionable so far. Neuromagnetic source imaging revealed that the source of cortical neural activity which coincides with the sensation of the afterimage is located in the auditory cortex and exhibits a tonotopic organization similar to that of the sustained response which occurs during continuous presentation of an acoustic stimulus. It is concluded that the neural processes leading to the generation of the two phenomena -sustained response and auditory afterimage - are similar.

Acoustic Stimulation↗
PubMed2005

Using afterimages for orientation and color to explore mechanisms of visual filling-in.

Simulations of Grossberg's FACADE model of visual perception have previously been used to explain afterimage percepts produced by viewing a sequence of orthogonally oriented gratings. Additional simulations of the model are now used to predict new afterimage percepts. One simulation emphasizes that the afterimage percepts are the result of orientation afterresponses and color afterresponses that interact at a filling-in stage. We report experimental data that agree with FACADE's prediction. A second simulation emphasizes the properties of the model's filling-in stage and predicts a situation where the afterimage percept should not appear. We report experimental data indicating that this model prediction is incorrect. We argue that the model is unable to account for this result unless the filling-in stage mechanisms are different from a diffusive-type process. We propose an alternative mechanism, and simulations demonstrate the system's ability to account for the afterimage data.

Afterimage↗
PubMed2001

Effects of voluntary attention on structured afterimages.

The effect of voluntary attention on afterimage fragmentation was explored in two experiments. The afterimage, in the form of a 30 degrees-tilted star of David, was generated after prolonged steady fixation in the first experiment, and with a brief and intense flash in the second experiment. Subjects were instructed to select various target shapes in the afterimage for attention and, at the same time, observe what was visible or invisible. Verbal reports and manual responses to afterimage changes were analyzed. Attended shapes were found to disappear from awareness Faster than unattended ones (experiment 1), and complementary shapes were found to predominate visual awareness when one of the pair was selected for attention (experiment 2). Voluntary attention was also found to affect closure (filling-in of enclosed regions) and smoothing of line figures in afterimages.

Attention↗
PubMed1988

Positive colored afterimages from the figure-ground configurations of colored lights: effects of chromaticity, luminance and a spatial parameter of the adapting stimuli.

Colored afterimages were obtained after the eye was exposed to the adapting field consisting of the figure-ground configuration of colored lights with the same hue and different saturation. When a colored patch was surrounded by a more saturated field, the color of the afterimage was similar to that of the previous patch (positive afterimage). By measuring the threshold chromaticity difference between the patch and the surround for the production of the positive afterimage, the effects of the chromaticity, luminance and a spatial parameter of the adapting field were determined. The obtained threshold chromaticity differences increased with an increase in the saturation of the adapting field, and with a decrease in the surround luminance. When the adapting field consisted of an equiluminous saturation-modulated rectangular grating, the threshold chromaticity difference curves showed similar properties as those of the contrast thresholds for equiluminous chromaticity-modulated gratings. The results suggest the existence of a set of mechanisms at higher-order level in the visual system, tuning the difference in hue and/or saturation across contours.

Adaptation, Ocular↗
PubMed2003

Pupil responses associated with coloured afterimages are mediated by the magno-cellular pathway.

Sustained fixation of a bright coloured stimulus will, on extinction of the stimulus and continued steady fixation, induce an afterimage whose colour is complementary to that of the initial stimulus; an effect thought to be caused by fatigue of cones and/or of cone-opponent processes to different colours. However, to date, very little is known about the specific pathway that causes the coloured afterimage. Using isoluminant coloured stimuli recent studies have shown that pupil constriction is induced by onset and offset of the stimulus, the latter being attributed specifically to the subsequent emergence of the coloured afterimage. The aim of the study was to investigate how the offset pupillary constriction is generated in terms of input signals from discrete functional elements of the magno- and/or parvo-cellular pathways, which are known principally to convey, respectively, luminance and colour signals. Changes in pupil size were monitored continuously by digital analysis of an infra-red image of the pupil while observers viewed isoluminant green pulsed, ramped or luminance masked stimuli presented on a computer monitor. It was found that the amplitude of the offset pupillary constriction decreases when a pulsed stimulus is replaced by a temporally ramped stimulus and is eliminated by a luminance mask. These findings indicate for the first time that pupillary constriction associated with a coloured afterimage is mediated by the magno-cellular pathway.

Afterimage↗
PubMed1999

Afterimages, grating induction and illusory phantoms.

Under some conditions (dark or light inspection areas) illusory gratings often appear to be in-phase with the inducing gratings and under others (gray inspection area) illusory gratings often appear to be out-of-phase with the inducing gratings. McCourt reported that point-by-point brightness matches reveal only out-of-phase illusory gratings, no matter what the luminance of the inspection area (McCourt, M. E. (1994). Vision Research, 34, 1609-1617). Since the technique used might have led to afterimages which mimic out-of-phase illusory gratings, the present series of experiments was undertaken to determine how such afterimages might bias illusory grating judgments. Afterimages were induced during fixation with brief flashes of inducing gratings within the inspection area (Experiment 1), or by vertical shifts in the entire stimulus which exposed the retina to real gratings prior to judgments within the inspection area (Experiment 2). Experiment 2 was replicated with drifting inducing gratings (Experiment 3). The subjects were asked to indicate whether illusory gratings appeared in- or out-of-phase. The results of all three experiments reveal that out-of-phase illusory gratings predominate, and that afterimages can only bias judgments with stationary displays. It is suggested that grating induction is perceived when subjects attend to local contrast differences, while phantom visibility is facilitated when attention is captured by the more global aspects of the stimulus.

Afterimage↗
PubMed1977

Spatial-frequency adaptation and afterimages.

The contribution of afterimages to spatial-frequence adaptation was studied by comparing a number of different fixation paradigms designed to maximize of minimize afterimages. While it is clear that adaptation is not an afterimage artifact, nevertheless afterimages are produced at low spatial frequencies and can considerably distort the results of adaptation experiments unless steps are taken to eliminate them.

Adaptation, Ocular↗
PubMed1979

Sex differences in the perception of autokinetic movement of an afterimage.

The effects of stimulus color and gender upon the perception of an afterimage were examined. The Ss were 32 male and 32 female college undergraduate volunteers. Eight Ss of each gender viewed a pinpoint flash of light through one of four filters: blue-green (Wratten 44A), red (Wratten 92), yellow (Wratten 9), or a neutral (Wratten 96) filter. Each S was given three trials, and on each trial, the duration of the afterimage was recorded, along with changes of direction, and changes in perceived color. Males reported significantly (p less than .05) more autokinetic movement of the afterimage. The color of the stimulus affected afterimage duration differentially for the two genders (p less than .05), and there was also a significant interaction (p less than .02) of gender with filter color for the total number of color changes reported. Thus, it is likely that both outflow monitoring and error signal variables in the autokinetic effect may be affected by gender, and the two sexes may also have different retinal and/or central processing of visual information.

Afterimage↗
PubMed1978

Complementary afterimages and the unequal adapting effects of steady and flickering light.

Prolonged exposure to flickering chromatic light of moderate intensity produce a much weaker complementary afterimage than that produced by exposure to stead light of the same average intesity. This difference in adapting effectiveness was investigated by determining theretinal illuminance of the study adapting field required to produce an afterimage equal to that produced by the flickering adapting field. In a variety of conditions, the greater effectiveness of the steady adapting field was confirmed; in several instances a steady field having an average retinal illuminance of only 0.05 times that of the flickering field gave rise to an equally saturated afterimage. The results are taken as evidence that complementary afterimages produced by extended moderate intensity exposure are primarily a consequence of neural adaption.

Adaptation, Ocular↗
PubMed1975

Temporal and spatial aftereffects of a border on an afterimage and a possible laterality difference.

Five subjects were asked to report the brightness and duration of afterimages formed in a region where a border had previously been exposed. The temporal and spatial aftereffects of the border on the formation of the afterimage varied with the duration of the border and these aftereffects were within the area predicted by a photochemical bleaching hypothesis. In addition to these major findings, the experiment yielded some surprising secondary observations. For example, increasing the degree of bleaching or light adaptation made the afterimage brighter if it was on the right side but decreased the brightness if on the left. The difference between afterimages on the right and left sides was discussed in terms of spatial effects of borders and laterality differences.

Adaptation, Ocular↗
PubMed1996

Subitizing and counting depend on different attentional mechanisms: evidence from visual enumeration in afterimages.

Two experiments showed that, when selective eye movements were disabled by the presentation of stimuli in the form of afterimages, increased inspection time and facilitative stimulus configurations failed to increase the subitizing limit of 4 objects. Afterimages of two to eight dots induced by a photographic flashgun were shown to 3 adult subjects. For more than 4 objects, enumeration errors occurred at a rate of 20%-30%. Enumeration was effectively perfect for 2-4 linearly configured dots, with occasional errors surprisingly occurring in that range when dots appeared in groups of up to 3 items. No errors occurred in nonafterimage control conditions. Enumeration errors were attributed to failures of individuating dots to be counted due to the deactivation of selective eye movements in afterimages. A third experiment supported this interpretation by disabling eye movements with briefly presented stimuli and producing results much like those of the afterimage conditions.

Adult↗
PubMed1996

Influence of foveal afterimage on non-visually induced pursuit eye movements.

Smooth pursuit eye movements (SPEMs) were measured during trials with an acoustic target, a somatosensory target (finger tip), and an imaginary target. Under these non-visual conditions the induced SPEMs were investigated both with and without a foveal afterimage. Ten normal subjects were examined. Considerable inter-individual differences were observed in performance of the non-visual SPEMs. However, the presence of an afterimage did not influence SPEMs during acoustic or somatosensory pursuit. It is concluded that the capacity for "visualization" may not involve the ability for non-visual SPEMs. As in imaginary target condition the subjects could not elicit SPEM without the afterimage, clearly the memory of target motion alone cannot generate SPEMs. The afterimage may help the subject to convert the memory of target motion into the perception of virtual target motion.

Acoustic Stimulation↗