From Receptive to Perceptive Fields: Size-Dependent Asymmetries in Both Negative Afterimages and Subcortical On and Off Post-Stimulus Responses.

Negative afterimages are perceptual phenomena that occur after physical stimuli disappear from sight. Their origin is linked to transient post-stimulus responses of visual neurons. The receptive fields (RFs) of these subcortical ON- and OFF-center neurons exhibit antagonistic interactions between central and surrounding visual space, resulting in selectivity for stimulus polarity and size. These two features are closely intertwined, yet their relationship to negative afterimage perception remains unknown. Here we tested whether size differentially affects the perception of bright and dark negative afterimages in humans of both sexes, and how this correlates with neural mechanisms in subcortical ON and OFF cells. Psychophysically, we found a size-dependent asymmetry whereby dark disks produce stronger and longer-lasting negative afterimages than bright disks of equal contrast at sizes >0.8°. Neurophysiological recordings from retinal and relay cells in female cat dorsal lateral geniculate nucleus showed that subcortical ON cells exhibited stronger sustained post-stimulus responses to dark disks, than OFF cells to bright disks, at sizes >1°. These sizes agree with the emergence of center-surround antagonism, revealing stronger suppression to opposite-polarity stimuli for OFF versus ON cells, particularly in dorsal lateral geniculate nucleus. Using a network-based retino-geniculate model, we confirmed stronger antagonism and temporal transience for OFF-cell post-stimulus rebound responses. A V1 population model demonstrated that both strength and duration asymmetries can be propagated to downstream cortical areas. Our results demonstrate how size-dependent antagonism impacts both the neuronal post-stimulus response and the resulting afterimage percepts, thereby supporting the idea of perceptual RFs reflecting the underlying neuronal RF organization of single cells.SIGNIFICANCE STATEMENT Visual illusions occur when sensory inputs and perceptual outcomes do not match, and provide a valuable tool to understand transformations from neural to perceptual responses. A classic example are negative afterimages that remain visible after a stimulus is removed from view. Such perceptions are linked to responses in early visual neurons, yet the details remain poorly understood. Combining human psychophysics, neurophysiological recordings in cats and retino-thalamo-cortical computational modeling, our study reveals how stimulus size and the receptive-field structure of subcortical ON and OFF cells contributes to the parallel asymmetries between neural and perceptual responses to bright versus dark afterimages. Thus, this work provides a deeper link from the underlying neural mechanisms to the resultant perceptual outcomes.

Dyskinetopsic Palinopsia: Palinopsia Accompanied by Moving Afterimages.

Palinopsia refers to the abnormal persistence, or recurrence, of visual images after a visual stimulus has subsided. We describe here a case of palinopsia accompanied by a visual motion perception disorder as manifested by moving afterimages. A 71-year-old man presented to us after having experienced acute-onset, vivid, visual hallucinations for 1 week. A detailed history revealed that he was hallucinating multiple living and nonliving objects. He also complained of a persistence of afterimages, particularly in the left visual field. He reported that, on a few occasions, while sitting by the window in his room, he had seen a moving car on the road; immediately after the car had disappeared from his sight, he had then seen the same car moving backward at almost the same speed-as if the driver had applied the reverse gear. A neuropsychological assessment did not reveal any deficits in attention, language, or episodic memory. Visual field testing by confrontational perimetry suggested left hemianopia. An MRI of the brain revealed an arteriovenous malformation in the medial part of the right occipital lobe, affecting both the lingual gyrus and the inferior occipital gyrus. Palinopsia has generally been described in reference to static afterimages. In our case, not only was the afterimage that was perceived by the patient in motion, but the direction of the movement was also opposite to that of the actual object. We propose the term dyskinetopsic palinopsia, or simply motion-related palinopsia, for this particular condition.

Adaptation aftereffects reveal how categorization training changes the encoding of face identity.

Previous research suggests that learning to categorize faces along a novel dimension changes the perceptual representation of such dimension, increasing its discriminability, its invariance, and the information used to identify faces varying along the dimension. A common interpretation of these results is that categorization training promotes the creation of novel dimensions, rather than simply the enhancement of already existing representations. Here, we trained a group of participants to categorize faces that varied along two morphing dimensions, one of them relevant to the categorization task and the other irrelevant to the task. An untrained group did not receive such categorization training. In three experiments, we used face adaptation aftereffects to explore how categorization training changes the encoding of face identities at the extremes of the category-relevant dimension and whether such training produces encoding of the category-relevant dimension as a preferred direction in face space. The pattern of results suggests that categorization training enhances the already existing norm-based coding of face identity, rather than creating novel category-relevant representations. We formalized this conclusion in a model that explains the most important results in our experiments and serves as a working hypothesis for future work in this area.

An Optical Illusion Pinpoints an Essential Circuit Node for Global Motion Processing.

Direction-selective (DS) neurons compute the direction of motion in a visual scene. Brain-wide imaging in larval zebrafish has revealed hundreds of DS neurons scattered throughout the brain. However, the exact population that causally drives motion-dependent behaviors-e.g., compensatory eye and body movements-remains largely unknown. To identify the behaviorally relevant population of DS neurons, here we employ the motion aftereffect (MAE), which causes the well-known "waterfall illusion." Together with region-specific optogenetic manipulations and cellular-resolution functional imaging, we found that MAE-responsive neurons represent merely a fraction of the entire population of DS cells in larval zebrafish. They are spatially clustered in a nucleus in the ventral lateral pretectal area and are necessary and sufficient to steer the entire cycle of optokinetic eye movements. Thus, our illusion-based behavioral paradigm, combined with optical imaging and optogenetics, identified key circuit elements of global motion processing in the vertebrate brain.

The contribution of stereopsis in Emmert's law.

Size constancy is the ability to perceive objects as remaining constant in size regardless of their distance from the observer. Emmert's law demonstrates that viewing distance determines the perceived size of afterimages according to the amount of depth cues that are available. Using an afterimage paradigm, we examined to what extent removing stereopsis and other depth cues affects size-distance scaling. Thirty participants 'projected' afterimages onto a surface presented at different distances under binocular, monocular, and eyes-closed conditions. The perceived size of the afterimages closely followed the size-distance scaling predictions made by Emmert's law under binocular testing conditions, when all depth cues were available. In contrast, monocular testing decreased adherence to Emmert's law, while the eyes-closed condition resulted in a greater breakdown of size-distance scaling. Because we used an afterimage paradigm, this study provides the first demonstration of how perceived size is modulated by the availability of depth cues under conditions with a constant retinal image stimulus.

Personality and opponent processes: Shyness, sociability, and visual afterimages to emotion.

The opponent process theory of emotion posits that emotional states evoke opposite emotion states as they wane, resulting in sequential approach and withdrawal motivations. However, whether opponent processes are associated with individual differences in personality remains an empirical question. Using visual afterimage responses to emotional faces as an index of opponent processes, we found that young adults (N = 101; Mage = 19.41 years, SD = 2.06 years) characterized by relatively high shyness and high sociability (i.e., conflicted shyness) were more likely to perceive a negative face emotion afterimage after adapting to happy faces and a positive face emotion afterimage after adapting to angry faces, compared with young adults classified by other combinations of high and low shyness and sociability. We speculate that conflicted shyness may result from strong opponent processes to both positive and negative emotions to real or anticipated social situations in some individuals, resulting in conflicting social motivations. (PsycInfo Database Record (c) 2020 APA, all rights reserved).

Reduced Fading of Visual Afterimages after Transcranial Magnetic Stimulation over Early Visual Cortex.

In the complete absence of small transients in visual inputs (e.g., by experimentally stabilizing an image on the retina or in everyday life during intent staring), information perceived by the eyes will fade from the perceptual experience. Although the mechanisms of visual fading remain poorly understood, one possibility is that higher level brain regions actively suppress the stable visual signals via targeted feedback onto early visual cortex (EVC). Here, we used positive afterimages and multisensory conflict to induce gestalt-like fading of participants' own hands. In two separate experiments, participants rated the perceived quality of their hands both before and after transcranial magnetic stimulation (TMS) was applied over EVC. In a first experiment, triple-pulse TMS was able to make a faded hand appear less faded after the pulses were applied, compared with placebo pulses. A second experiment demonstrated that this was because triple-pulse TMS slowed down fading of the removed hand that otherwise occurs naturally over time. Interestingly, TMS similarly affected the left and right hands, despite being applied only over the right EVC. Together, our results suggest that TMS over EVC attenuates the effects of visual fading in positive afterimages, and it might do so by crossing transcollosal connections or via multimodal integration sites in which both hands are represented.

Concentrative (Sahaj Samadhi) meditation training and visual awareness: An fMRI study on color afterimages.

All of us consciously experience the world around us through our sensory modalities. Empirical studies on the relationship between attention and awareness have shown that attention does influence perceptual experience or appearance in addition to better performance in perceptual tasks. The practice of meditation also changes perceptual experience in addition to better perceptual performance. For example, a study with Sahaj Samadhi meditators utilizing negative color afterimages had shown that concentrative meditation influences visual experience. However the brain regions that are modified by meditation practice leading to such changes in visual experience or awareness are still not known. Here using negative color afterimages in a functional MRI study, we investigated the brain mechanisms underlying the changes in visual awareness as a function of attentional enhancement achieved through long-term concentrative meditation practice. We found increased activity in right lateralized inferior occipital and inferior frontal cortex, which suggests the importance of attentional control in modulating visual awareness. The results of this study indicate that the link between attention and conscious experience is possibly changed by meditation practices.

Retinal spatiotemporal dynamics on emergence of visual persistence and afterimages.

Visual persistence (stimulus perception that prolongs for a few milliseconds after the physical disappearance of the stimulus) and afterimages (an illusory percept that lingers after the physical disappearance of the stimulus at the retinotopic location of the preceding stimulus) are classic perceptual phenomena reflecting temporal characteristics of the visual system. These phenomena are modulated by some common stimulus aspects: A longer stimulus generates shorter persistence and a longer afterimage and a lower spatial-frequency stimulus generates shorter persistence and a stronger afterimage. The current study proposes that these spatiotemporal characteristics of visual persistence and afterimages can be explained by a generic retinal processing architecture. Wilson (1997) developed a neural network model of retinal circuitry and demonstrated that afterimages emerge due to a retinal light-adaptive gain control mechanism. In this study, we provide an overview of the retinal physiology to assess the feasibility of his retinal model, and simulate psychophysical experiments on persistence and afterimages in the same model to provide systematic explanations to the stimulus duration and spatial frequency effects. Our results suggest that these characteristics emerge from the spatiotemporal characteristics of each cell (response gain and time course, receptive-field structure) that comprises a part of the feedforward-feedback laminar network in the retina. The retinal circuitry performs short- and long-term adaptive operations as the signal transmission is recurrently regulated by various feedback mechanisms and consequently engenders complicated spatiotemporal dynamics in the ganglion cell responses that match the patterns of the perceptual phenomena. (PsycINFO Database Record (c) 2019 APA, all rights reserved).

Visual processing in migraineurs depends on the migraine cycle.

We investigated changes of after-image duration in migraineurs and healthy controls (HCs) and throughout the migraine cycle to depict changes in the excitatory/inhibitory equilibrium within the visual cortex. Forty-seven episodic (EMs) and 39 chronic migraineurs (CMs; interictal) were compared to 34 HCs for visual after-image duration. Additionally, seven EMs were investigated every consecutive day over 20 to 32 days using the identical paradigm throughout the migraine cycle. Interictally, the after-image duration was shorter compared to HCs, but significantly longer in the ictal compared to interictal phase. These data suggest an altered excitatory/inhibitory equilibrium in migraineurs, which oscillates over the migraine cycle. ANN NEUROL 2019;85:280-283.

Adaptation to dynamic faces produces face identity aftereffects.

Face aftereffects are well established for static stimuli and have been used extensively as a tool for understanding the neural mechanisms underlying face recognition. It has also been argued that adaptive coding, as demonstrated by face aftereffects, plays a functional role in face recognition by calibrating our face norms to reflect current experience. If aftereffects tap high-level perceptual mechanisms that are critically involved in everyday face recognition then they should also occur for moving faces. Here we asked whether face identity aftereffects can be induced using dynamic adaptors. The face identity aftereffect occurs when adaptation to a particular identity (e.g., Dan) biases subsequent perception toward the opposite identity (e.g., antiDan). We adapted participants to video of real faces that displayed either rigid, non-rigid, or no motion and tested for aftereffects in static antifaces. Adapt and test stimuli differed in size, to minimize low-level adaptation. Aftereffects were found in all conditions, suggesting that face identity aftereffects tap high-level mechanisms important for face recognition. Aftereffects were not significantly reduced in the motion conditions relative to the static condition. Overall, our results support the view that face aftereffects reflect adaptation of high-level mechanisms important for real-world face recognition in which faces are moving.

Cortical suppression in human primary visual cortex predicts individual differences in illusory tilt perception.

Neural responses to visual stimuli are modulated by spatial and temporal context. For example, in primary visual cortex (V1), responses to an oriented target stimulus will be suppressed when embedded within an oriented surround stimulus. This suppression is orientation-specific, with the largest suppression observed when stimuli in the neuron's classical receptive field and surround are of similar orientation. In human psychological experiments, the tilt illusion and tilt aftereffect demonstrate an effect of context on perceived orientation of a target stimulus. Similar to the neurophysiological data, the strength of these effects is modulated by the orientation difference between the target stimulus and context. It has been hypothesized that the neural mechanism underlying both the tilt illusion and tilt aftereffect involves orientation-tuned inhibition in V1. However, to date there is no direct evidence linking human perception of these illusions with measurements of inhibition from human visual cortex. Here, we measured context-induced suppression of neural responses in human visual cortex using functional magnetic resonance imaging (fMRI). In the same participants, we also measured magnitudes of their tilt illusion and tilt aftereffect. Our data revealed a significant relationship between the magnitude of neural suppression in V1 and size of the tilt illusion and tilt aftereffect. That is, participants who showed stronger blood oxygenation level dependent (BOLD) suppression in V1 also perceived stronger shifts in illusory tilt. This agreement between perception and neural responses in human V1 suggests a shared inhibitory mechanism that mediates both spatial and temporal effects of context in human perception.

Disambiguating auditory information causes priming, but not aftereffects, in the perception of ambiguous faces.

In five experiments, we used a visual aftereffects paradigm to probe whether emotion- and gender-relevant information presented in the auditory domain would affect the formation of visual aftereffects or would instead create a priming effect. In experiment 1, participants fixated on surprise facial expressions while listening to a story that described the surprise as either happy or sad, and then were asked to classify the expression of a briefly presented neutral face. Subsequently, the identity of the model (experiment 2) and the timing of the auditory presentation (experiment 3) were manipulated. In experiment 4, this approach was extended to judgments of gender. Experiment 5 serves as a control experiment in which the story, but no visual stimuli, was presented during the adaptation phase. In each case results revealed evidence of priming, but no evidence that information in the auditory domain affected the formation of aftereffects.

Color Afterimages in Autistic Adults.

It has been suggested that attenuated adaptation to visual stimuli in autism is the result of atypical perceptual priors (e.g., Pellicano and Burr in Trends Cogn Sci 16(10):504-510, 2012. doi: 10.1016/j.tics.2012.08.009 ). This study investigated adaptation to color in autistic adults, measuring both strength of afterimage and the influence of top-down knowledge. We found no difference in color afterimage strength between autistic and typical adults. Effects of top-down knowledge on afterimage intensity shown by Lupyan (Acta Psychol 161:117-130, 2015. doi: 10.1016/j.actpsy.2015.08.006 ) were not replicated for either group. This study finds intact color adaptation in autistic adults. This is in contrast to findings of attenuated adaptation to faces and numerosity in autistic children. Future research should investigate the possibility of developmental differences in adaptation and further examine top-down effects on adaptation.

Re-examining the influence of attention and consciousness on visual afterimage duration.

The relationship between visual attention and conscious perception has been the subject of debate across a number of fields, including philosophy, psychology, and neuroscience. Whereas some researchers view attention and awareness as inextricably linked, others propose that the two are supported by distinct neural mechanisms that can be fully dissociated. In a pioneering study, van Boxtel, Tsuchiya, and Koch (2010b) reported evidence for a dissociation between attention and conscious perception using a perceptual adaptation task in which participants' perceptual awareness and visual attention were manipulated independently. They found that participants' awareness of an adapting stimulus increased afterimage duration, whereas attending to the adaptor decreased it. Given the important theoretical implications of these findings, we endeavored to replicate them using an identical paradigm while dealing with some potential shortcomings of the original study by adding more trials and a larger participant sample. Consistent with van Boxtel, Tsuchiya, and Koch, we found that afterimage duration was reliably increased when participants were aware of the adapting stimulus. In contrast to the original findings, however, attention to the adaptor also increased afterimage duration, suggesting that attention and awareness had the same-rather than opposing-effects on afterimage duration. We discuss possible reasons for this discrepancy. (PsycINFO Database Record

Different Signal Enhancement Pathways of Attention and Consciousness Underlie Perception in Humans.

It is not yet known whether attention and consciousness operate through similar or largely different mechanisms. Visual processing mechanisms are routinely characterized by measuring contrast response functions (CRFs). In this report, behavioral CRFs were obtained in humans (both males and females) by measuring afterimage durations over the entire range of inducer stimulus contrasts to reveal visual mechanisms behind attention and consciousness. Deviations relative to the standard CRF, i.e., gain functions, describe the strength of signal enhancement, which were assessed for both changes due to attentional task and conscious perception. It was found that attention displayed a response-gain function, whereas consciousness displayed a contrast-gain function. Through model comparisons, which only included contrast-gain modulations, both contrast-gain and response-gain effects can be explained with a two-level normalization model, in which consciousness affects only the first level and attention affects only the second level. These results demonstrate that attention and consciousness can effectively show different gain functions because they operate through different signal enhancement mechanisms.SIGNIFICANCE STATEMENT The relationship between attention and consciousness is still debated. Mapping contrast response functions (CRFs) has allowed (neuro)scientists to gain important insights into the mechanistic underpinnings of visual processing. Here, the influence of both attention and consciousness on these functions were measured and they displayed a strong dissociation. First, attention lowered CRFs, whereas consciousness raised them. Second, attention manifests itself as a response-gain function, whereas consciousness manifests itself as a contrast-gain function. Extensive model comparisons show that these results are best explained in a two-level normalization model in which consciousness affects only the first level, whereas attention affects only the second level. These findings show dissociations between both the computational mechanisms behind attention and consciousness and the perceptual consequences that they induce.

Cortical mechanisms for afterimage formation: evidence from interocular grouping.

Whether the retinal process alone or retinal and cortical processes jointly determine afterimage (AI) formation has long been debated. Based on the retinal rebound responses, recent work proposes that afterimage signals are exclusively generated in the retina, although later modified by cortical mechanisms. We tested this notion with the method of "indirect proof". Each eye was presented with a 2-by-2 checkerboard of horizontal and vertical grating patches. Each corresponding patch of the two checkerboards was perpendicular to each other, which produces binocular rivalry, and can generate percepts ranging from complete interocular grouping to either monocular pattern. The monocular percepts became more frequent with higher contrast. Due to adaptation, the visual system is less sensitive during the AIs than during the inductions with AI-similar contrast. If the retina is the only origin of AIs, comparable contrast appearance would require stronger retinal signals in the AIs than in the inductions, thus leading to more frequent monocular percepts in the AIs than in the inductions. Surprisingly, subjects saw the fully coherent stripes significantly more often in AIs. Our results thus contradict the retinal generation notion, and suggest that in addition to the retina, cortex is directly involved in the generation of AI signals.

Visual Discomfort From Flash Afterimages of Riloid Patterns.

Op-art-based stimuli have been shown to be uncomfortable, possibly due to a combination of fixational eye movements (microsaccades) and excessive cortical responses. Efforts have been made to measure illusory phenomena arising from these stimuli in the absence of microsaccades, but there has been no attempt thus far to decouple the effects of the cortical response from the effect of fixational eye movements. This study uses flash afterimages to stabilise the image on the retina and thus reduce the systematic effect of eye movements, in order to investigate the role of the brain in discomfort from op-art-based stimuli. There was a relationship between spatial frequency and the magnitude of the P300 response, showing a similar pattern to that of discomfort judgements, which suggests that there might be a role of discomfort and excessive neural responses independently from the effects of microsaccades.

Size Constancy is Preserved but Afterimages are Prolonged in Typical Individuals with Higher Degrees of Self-Reported Autistic Traits.

Deficits in perceptual constancies from early infancy have been proposed to contribute to autism and exacerbate its symptoms (Hellendoorn et al., Frontiers in Psychology 6:1-16, 2015). Here, we examined size constancy in adults from the general population (N = 106) with different levels of self-reported autistic traits using an approach based on negative afterimages. The afterimage strength, as indexed by duration and vividness, was also quantified. In opposition to the Hellendoorn and colleagues' model, we were unable to demonstrate any kind of relationship between abilities in size constancy and autistic traits. However, our results demonstrated that individuals with higher degrees of autistic traits experienced more persistent afterimages. We discuss possible retinal and post-retinal explanations for prolonged afterimages in people with higher levels of autistic traits.

Illusory Distance Modulates Perceived Size of Afterimage despite the Disappearance of Depth Cues.

It is known that the perceived size of an afterimage is modulated by the perceived distance between the observer and the depth plane on which the afterimage is projected (Emmert's law). Illusions like Ponzo demonstrate that illusory distance induced by depth cues can also affect the perceived size of an object. In this study, we report that the illusory distance not only modulates the perceived size of object's afterimage during the presence of the depth cues, but the modulation persists after the disappearance of the depth cues. We used an adapted version of the classic Ponzo illusion. Illusory depth perception was induced by linear perspective cues with two tilted lines converging at the upper boundary of the display. Two horizontal bars were placed between the two lines, resulting in a percept of the upper bar to be farther away than the lower bar. Observers were instructed to make judgment about the relative size of the afterimage of the lower and the upper bars after adaptation. When the perspective cues and the bars were static, the illusory effect of the Ponzo afterimage is consistent with that of the traditional size-distance illusion. When the perspective cues were flickering and the bars were static, only the afterimage of the latter was perceived, yet still a considerable amount of the illusory effect was perceived. The results could not be explained by memory of a prejudgment of the bar length during the adaptation phase. The findings suggest that cooccurrences of depth cues and object may link a depth marker for the object, so that the perceived size of the object or its afterimage is modulated by feedback of depth information from higher-level visual cortex even when there is no depth cues directly available on the retinal level.