Afterimage duration depends on how deeply invisible stimuli were suppressed.

Quantifying visual responses to stimuli that are outside of awareness is a critical task for the study of visual consciousness. The current study psychophysically investigated whether afterimages reflect visual responses to stimuli that are not consciously visible throughout adaptation due to interocular suppression. A Gabor adaptor was presented to one eye of the observer, and a counterphase-flickering Gabor suppressor was presented to the other eye, thereby rendering the adaptor invisible during adaptation. To manipulate the depth of the suppression of the invisible adaptor, we varied the orientation difference between the adaptor and suppressor. We found that, even though the adaptor was not visible during adaptation, the afterimage duration varied depending on the orientation selectivity of interocular suppression. The duration was the shortest when the orientations of the adaptor and suppressor were identical and lengthened when the orientation differences increased. This finding could not be explained by confounding factors such as potential changes in contrast sensitivity that were caused by the suppressor. Our findings suggest that the magnitude of visual responses to stimuli suppressed below the threshold of awareness can be measured using the afterimage duration. Afterimages could be an effective tool for quantifying visual responses, irrespective of observers' conscious awareness of a presented stimulus.

Ptolemy's Optics, double-vision, and the technological afterimage.

In contrast to earlier theorists within the Greek optical tradition, who relied almost exclusively on geometrical diagrams to articulate and explain vision, Ptolemy employed several material instruments in his investigation of sight. These included rulers, glass cylinders, mirrors, and a bronze plaque designed to measure angles of incidence and reflection. These devices allowed Ptolemy to expand the operational definition of vision far beyond that of his predecessors, as he explicated several previously unexamined visual behaviors, including binocular vision, diplopia, and refraction. This article argues that these tools did more than make new phenomena visible; they also set the parameters for what these phenomena looked like-sometimes to such a degree that features of these instruments merged with the visual behaviors that they rendered visible. In some cases, this occurred as a type of "double-exposure," where the investigative tool became layered over top of the process of sight, such as when Ptolemy's "ruler" for investigating binocular vision became a template for imagining the mechanism of spatial perception employed by the eyes. In other cases, this merging occurred as a type of "technological afterimage," where the instrument provided an implicit model for phenomena it was not directly investigating. Ptolemy's bronze plaque stands as an example of this second type, insofar as it inspired his account of ocular geometry and facilitated novel assertions about the eye's operations, even though it did not directly inspect these features. In general, this article thus outlines how the technologies of investigation can structure patterns of thought and naturalize certain physical arguments, whether for the phenomena that they directly articulate or for those indirectly associated with their particular use cases.

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.

Negative afterimages facilitate the detection of real images.

Negative, or complementary afterimages are experienced following brief adaptation to chromatic or achromatic stimuli, and are believed to be formed in the post-receptoral layers of the retinae. Afterimages can be cancelled by the addition of real images, suggesting that afterimages and real images are processed by similar mechanisms. However given their retinal origin, afterimage signals represented at the cortical level might have different spatio-temporal properties from their real images counterparts. To test this we determined whether afterimages reduce the contrast threshold of added real images, i.e. produce the classic "dipper" function characteristic of contrast discrimination, a behavior believed to be cortically mediated. Stimuli were chromatic and achromatic disks on a grey background. Observers adapted for 1.0 s to two side-by-side disks of a particular color. Following stimulus offset, a test disk added to one side was ramped downwards for 1.5 s to approximately match the temporal characteristic of the afterimage, and the observer was required to indicate the side containing the test disk. The test hue/brightness was either the same as that of the afterimage or a different hue/brightness. The independent variable was the contrast of the adaptor. A dipper followed by masking was observed in most conditions in which the afterimage and test colors had the same hue or brightness. We conclude that afterimages are represented similarly to their real image counterparts at the cortical level.

Visual snow syndrome: A clinical and phenotypical description of 1,100 cases.

OBJECTIVE: To validate the current criteria of visual snow and to describe its common phenotype using a substantial clinical database. METHODS: We performed a web-based survey of patients with self-assessed visual snow (n = 1,104), with either the complete visual snow syndrome (n = 1,061) or visual snow without the syndrome (n = 43). We also describe a population of patients (n = 70) with possible hallucinogen persisting perception disorder who presented clinically with visual snow syndrome. RESULTS: The visual snow population had an average age of 29 years and had no sex prevalence. The disorder usually started in early life, and ≈40% of patients had symptoms for as long as they could remember. The most commonly experienced static was black and white. Floaters, afterimages, and photophobia were the most reported additional visual symptoms. A latent class analysis showed that visual snow does not present with specific clinical endophenotypes. Severity can be classified by the amount of visual symptoms experienced. Migraine and tinnitus had a very high prevalence and were independently associated with a more severe presentation of the syndrome. CONCLUSIONS: Clinical characteristics of visual snow did not differ from the previous cohort in the literature, supporting validity of the current criteria. Visual snow likely represents a clinical continuum, with different degrees of severity. On the severe end of the spectrum, it is more likely to present with its common comorbid conditions, migraine and tinnitus. Visual snow does not depend on the effect of psychotropic substances on the brain.

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).

Social contingency modulates the perceived distance between self and other.

Although social contingency, namely contingent reactions of other to one's own actions, critically affects attachment formation, can it also modulate the perceived distance between self and other? Previous studies have suggested a positive answer. However, these studies are criticized for not showing true top-down effects on perception because of pitfalls such as task demands. We show that social contingency reduced the perceived distance between self and other while avoiding pitfalls. According to Emmert's law, the perceived size of an afterimage increases with perceived distance. Thus, if social contingency modulates the perceived distance, the perceived size of afterimage should inevitably reflect it. The results showed that the size of the afterimages of a face that contingently responded to participants' actions was perceived as smaller than those of non-contingent and unresponsive faces. This effect was more salient with increasing viewing distances. Thus, prior knowledge of interaction with environment modulates online perceptual processing in size constancy, probably through its influence on perceived distance.

Illusory palinopsias induced by in vitro fertilization treatment: a case report.

BACKGROUND: In the current era, in vitro fertilization, a type of assisted reproductive technology, has been commonly used for infertility management and gestational surrogacy. The techniques that are routinely used in in vitro fertilization include ovarian hyperstimulation to generate multiple eggs, preparation of the ova and sperm, and culture and selection of resultant embryos before transfer into a uterus. These steps increase the chances of successful pregnancy following in vitro fertilization treatment many-fold, especially in young women. Complications reported with in vitro fertilization treatment include multiple gestations, ovarian hyperstimulation syndrome, and birth defects while ocular side effects reported include retinal detachment and progression of keratoconus. We report a case of visual illusory palinopsia following in vitro fertilization treatment in a patient with unexplained infertility. CASE PRESENTATION: A 31-year-old Asian woman was administered in vitro fertilization treatment for her unexplained infertility. She complained of visually disturbing flashes in her peripheral vision during her pregnancy. She described these flashes as occurring usually in the morning hours or while walking, coming in sets of three to four, occurring five-six times a day and lasting for less than 5-10 minutes. Her flashes were not accompanied by other ocular symptoms such as pain, redness, photophobia, or decrease in vision. Her ocular examination was normal. Neuroimaging with magnetic resonance imaging revealed no pathology. A diagnosis of visual illusory palinopsia secondary to in vitro fertilization treatment was made. CONCLUSION: Disturbing visual palinopsia and afterimages can occur following in vitro fertilization treatment for infertility due to increased estrogen levels. This rare ocular side effect caused by in vitro fertilization treatment is not reported in the literature to the best of our knowledge. Gynecologists and/or infertility experts should educate their patients regarding these possible ocular symptoms. Even ophthalmologists should be aware of this rare cause for visual palinopsia.

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.

The (limited) effect of emotional arousal in the regulation of accuracy in eyewitness memory / El (limitado) efecto de la activación emocional en la regulación de la exactitud de la memoria de testigos

Witnesses encoding a crime are likely to feel negative emotions with high arousal, e.g., anxiety or fear. Negative emotions improve memory for central information and impair memory for peripheral information. In this study we explored the effects of emotional arousal and type of information in the regulation of accuracy. The regulation of accuracy allows participants to maximize accuracy, for example, by deciding on the number of alternatives in their response (the plurality option). Participants were induced with high- and low-arousal negative emotions and then shown a slideshow of a crime. Afterwards, they answered questions about central and peripheral contents of the event. Questions followed the basic plurality option procedure. First, participants selected one alternative (single answer); second, they selected three alternatives (plural answer); and, finally, they decided on reporting either the single or the plural answer. Results showed successful manipulation of arousal, and that the regulation of accuracy led to a greater increase in accuracy for peripheral than for central information, but no differences depending on the level of arousal. We also identified two factors that increased accuracy in the plurality option: the ability to discard answers with low chances of being correct and the addition of answers with higher chances of being correct. Either one, or both, can increase witness accuracy

Es muy probable que los testigos de un crimen sientan emociones negativas con un alto grado de activación, por ejemplo, ansiedad o miedo. Las emociones negativas mejoran la memoria de información central y empeoran la memoria de información periférica. En esta investigación estudiamos el efecto de la activación emocional y el tipo de información en la regulación de la exactitud. La regulación de la exactitud permite a los participantes maximizar la exactitud, por ejemplo, decidiendo cuántas alternativas quieren incluir en su respuesta (la opción de pluralidad). Se indujeron en los participantes emociones negativas con un grado de activación alto y bajo, y después se presentó una serie de diapositivas sobre un crimen. Después los participantes respondieron preguntas sobre contenidos centrales y periféricos del crimen. Las preguntas siguieron el procedimiento básico de la opción de pluralidad. Primero los participantes seleccionaron una alternativa (respuesta única), segundo seleccionaron tres alternativas (respuesta plural), y finalmente decidieron si preferían escoger la respuesta única o la plural. Los resultados mostraron que la manipulación de la activación tuvo éxito, y que hubo un mayor aumento en la exactitud con información periférica que central gracias a la regulación de la exactitud, aunque no hubo diferencias en función del nivel de activación. También se identificaron dos factores que aumentan la exactitud en la opción de pluralidad: la capacidad de descartar respuestas con bajas probabilidades de ser correctas y la adición de respuestas con mayores probabilidades de ser correctas. Cada uno de esos factores, o los dos juntos, pueden aumentar la exactitud de los testigos de un crimen