The role of correlated firing in representing information has been a subject of much discussion. Several studies in retina, visual cortex, somatosensory cortex, and motor cortex, have suggested that it plays only a minor role, carrying < 10% of the total information carried by the neurons (Gawne and Richmond, 1993; Nirenberg et al., 2001; Oram et al., 2001; Petersen et al., 2001; Rolls et al., 2003). A limiting factor of these studies, however, is that they were carried out using pairs of neurons; how the results extend to large populations wasn’t clear. Recently, new methods for modeling network firing patterns have been developed (Pillow et al., 2008; Nirenberg and Pandarinath, 2010), opening the door to answering this question for more complete populations. One study, Pillow et al. (2008), showed that including correlations increased information by a modest amount, ~20%; however, this work used only a single retina (primate) and a white noise stimulus. Here we performed the analysis using several retinas (mouse) and both white noise and natural scene stimuli. The results showed that correlations added little information when white noise stimuli were used (~13%), similar to Pillow et al.’s findings, and essentially no information when natural scene stimuli were used. Further, the results showed that ignoring correlations did not change the quality of the information carried by the population (as measured by comparing the full pattern of decoding errors). These results suggest generalization: the pairwise analysis in several species show that correlations account for very little of the total information. Now, the analysis with large populations in two species show a similar result, that correlations still account for only a small fraction of the total information, and, most significantly, the amount is not statistically significant when natural stimuli are used, making rapid advances in the study of population coding possible.
correlations; noise correlation; retinal ganglion cell; parallel processing; information; vision; population coding; decoding; spike trains
The processing of texture patterns has been characterized by a model that postulates a first-stage linear filter to highlight a component texture, a pointwise rectification stage to convert contrast for the highlighted texture into mean response strength, followed by a second-stage linear filter to detect the texture-defined pattern. We estimated the spatial-frequency bandwidth of the second-stage filter mediating orientation-discrimination of orientation-modulated second-order gratings by measuring threshold elevation in the presence of filtered noise added to the modulation signal. This experiment yielded no evidence for frequency tuning. A second experiment, in which subjects had to detect similar second-order gratings while judging their modulation frequency, produced bandwidth estimates of 1–1.5 octaves, similar to estimated bandwidths of first-order channels. We propose that an additional dominant-response-selection nonlinearity can account for these apparently contradictory results.
second-order vision; texture
The extent of perceived blur produced by a moving retinal image is less when the image motion occurs during pursuit eye movements compared to fixation. This study examined the effect of this reduced perception of motion blur during pursuit on spatial-interval acuity. Observers judged during pursuit at 4 or 8 deg/s whether the horizontal separation between two stationary lines was larger or smaller than a standard. Three different line separations were tested for each pursuit velocity. Each observer performed these judgments also during fixation, for spatial-interval stimuli that moved with the same mean and standard deviation of speeds as the distribution of eye velocities during pursuit. Spatial-interval acuity was better during pursuit than fixation for small or intermediate line separations. The results indicate that a reduction of perceived motion blur during pursuit eye movements can lead to improved visual performance.
pursuit eye movement; retinal image motion; motion blur; motion smear; motion deblurring; spatial-interval acuity; separation discrimination
In this report, we describe a male subject who presents with a complex phenotype of myopia associated with cone dysfunction and a protan vision deficiency. Retinal imaging demonstrates extensive cone disruption, including the presence of non-waveguiding cones, an overall thinning of the retina, and an irregular mottled appearance of the hyper reflective band associated with the inner segment ellipsoid portion of the photoreceptor. Mutation screening revealed a novel p.Glu41Lys missense mutation in a hybrid L/M opsin gene. Spectral analysis shows that the mutant opsin fails to form a pigment in vitro and fails to be trafficked to the cell membrane in transfected Neuro2a cells. Extensive sequence and quantitative PCR analysis identifies this mutant gene as the only gene present in the affected subject’s L/M opsin gene array, yet the presence of protanopia indicates that the mutant opsin must retain some activity in vivo. To account for this apparent contradiction, we propose that a limited amount of functional pigment is formed within the normal cellular environment of the intact photoreceptor, and that this requires the presence of chaperone proteins that promote stability and normal folding of the mutant protein.
Colour vision; visual pigments; dichromacy; opsin mutation; cone dysfunction; retinal imaging
How we find what we are looking for in complex visual scenes is a seemingly simple ability that has taken half a century to unravel. The first study to use the term visual search showed that as the number of objects in a complex scene increases, observers’ reaction times increase proportionally (Green and Anderson, 1956). This observation suggests that our ability to process the objects in the scenes is limited in capacity. However, if it is known that the target will have a certain feature attribute, for example, that it will be red, then only an increase in the number of red items increases reaction time. This observation suggests that we can control which visual inputs receive the benefit of our limited capacity to recognize the objects, such as those defined by the color red, as the items we seek. The nature of the mechanisms that underlie these basic phenomena in the literature on visual search have been more difficult to definitively determine. In this paper, I discuss how electrophysiological methods have provided us with the necessary tools to understand the nature of the mechanisms that give rise to the effects observed in the first visual search paper. I begin by describing how recordings of event-related potentials from humans and nonhuman primates have shown us how attention is deployed to possible target items in complex visual scenes. Then, I will discuss how event-related potential experiments have allowed us to directly measure the memory representations that are used to guide these deployments of attention to items with target-defining features.
visual attention; visual working memory; long-term memory; visual search; electrophysiology; event-related potentials
We explored the perception of image focus in patients with cataracts, and how this perception changed following cataract removal and implantation of an intraocular lens. Thirty-three patients with immature senile cataract and with normal retinal function were tested before surgery and 2 days after surgery, with 18 of the patients retested again at 2 months following surgery. The subjective focus of natural images was quantified in each session by varying the slope of the image amplitude spectra. At each time, short-term adaptation to the spectral slope was also determined by repeating the measurements after exposure to images with blurred or sharpened spectra. Despite pronounced acuity deficits, before surgery images appeared “best-focused” when they were only slightly blurred, consistent with a strong compensation for the acuity losses. Post-operatively, the image slopes that were judged “in focus” before surgery appeared too sharp. This bias remained strong at 2 months, and was independent of the rapid blur after-effects induced by viewing filtered images. The focus settings tended to renormalize more rapidly in patients with higher post-operative acuity, while acuity differences were unrelated to the magnitude of the short-term blur aftereffects. Our results suggest that subjective judgments of image focus are largely compensated as cataracts develop, but potentially through a very long-term form of adaptation that results in persistent biases after the cataract is removed.
Cataract; Blur adaptation; Spatial vision
Performance for many visual tasks improves with training. The magnitude of improvement following training depends on the training task, number of trials per training session and the total amount of training. Does the magnitude of improvement also depend on the frequency of training sessions? In this study, we compared the learning effect for three groups of normally sighted observers who repeatedly practiced the task of identifying crowded letters in the periphery for six sessions (1000 trials per session), according to three different training schedules — one group received one session of training everyday, the second group received a training session once a week and the third group once every two weeks. Following six sessions of training, all observers improved in their performance of identifying crowded letters in the periphery. Most importantly, the magnitudes of improvement were similar across the three training groups. The improvement was accompanied by a reduction in the spatial extent of crowding, an increase in the size of visual span and a reduction in letter-size threshold. The magnitudes of these accompanied improvements were also similar across the three training groups. Our finding that the effectiveness of visual perceptual learning is similar for daily, weekly and biweekly training has significant implication for adopting perceptual learning as an option to improve visual functions for clinical patients.
perceptual learning; crowding; letter identification; peripheral vision
Autism is characterized by disruption in multiple dimensions of perception, emotion, language and cognition. Many hypotheses for the underlying neurophysiological basis have been proposed. Among these is the excitation/inhibition (E/I) imbalance hypothesis, which states that levels of cortical excitation and inhibition are disrupted in autism. We tested this theory in the visual system, because vision is one of the better understood systems in neuroscience, and because the E/I imbalance theory has been proposed to explain hypersensitivity to sensory stimuli in autism. We conducted two experiments on binocular rivalry, a well-studied psychophysical phenomenon that depends critically on excitation and inhibition levels in cortex. Using a computational model, we made specific predictions about how imbalances in excitation and inhibition levels would affect perception of two aspects of binocular rivalry: mixed perception (Experiment 1) and traveling waves (Experiment 2). We found no significant differences in either of these phenomena between high-functioning adults with autism and controls, and no evidence for a relationship between these measurements and the severity of autism. These results do not conclusively rule out an excitation/inhibition imbalance in visual system of those with autism, but they suggest that such an imbalance, if it exists, is likely to be small in magnitude.
binocular rivalry; psychophysics; computational model; autism
Brightness induction refers to a class of visual illusions where the perceived intensity of a region of space is influenced by the luminance of surrounding regions. These illusions are significant because they provide insight into the neural organization and processing strategies employed by the visual system. The nature of these processing strategies, however, has long been debated. Here we investigate the spatial characteristics of grating induction as a function of the distance from the inducing field edge to evaluate the viability of various competing models. In particular multiscale spatial filtering models and homogeneous filling-in models make very different predictions in regard to the magnitude of induction as a function of this distance. Filling-in explanations predict that the brightness/lightness of the filled-in region will be homogeneous, whereas multiscale filtering predicts a fall-off in induction magnitude with distance from the inducing field edge. Induction magnitude was measured using a narrow probe version of the quadrature-phase motion-cancellation paradigm (Blakeslee & McCourt, 2011) and a point-by-point brightness matching paradigm (McCourt, 1994; Blakeslee & McCourt, 1997; 1999). Both techniques reveal a decrease in the magnitude of induction with increasing distance from the inducing edge. A homogeneous filling-in mechanism cannot explain the induced structure in the test fields of these stimuli. The results argue strongly against filling-in mechanisms as well as against any mechanism that posits that induction is homogeneous. The structure of the induction is, however, well accounted for by the multiscale filtering (ODOG) model of Blakeslee and McCourt (1999). These results support models of brightness/lightness, such as filtering models, which preserve these gradients of induction.
Brightness induction; grating induction; cancelation; point-by-point brightness matching; filling-in; multiscale filtering; quadrature motion
Covert visual search has been studied extensively in humans, and has been used as a tool for understanding visual attention and cueing effects. In contrast, much less is known about covert search performance in monkeys, despite the fact that much of our understanding of the neural mechanisms of attention is based on these animals. In this study, we characterize the covert visual search performance of monkeys by training them to discriminate the orientation of a briefly-presented, peripheral Landolt-C target embedded within an array of distractor stimuli while maintaining fixation. We found that target discrimination performance declined steeply as the number of distractors increased when the target and distractors were of the same color, but not when the target was an odd color (color pop-out). Performance was also strongly affected by peripheral spatial precues presented before target onset, with better performance seen when the precue coincided with the target location (valid precue) than when it did not (invalid precue). Moreover, the effectiveness of valid precues was greatest when the delay between precue and target was short (~80 to 100 ms), and gradually declined with longer delays, consistent with a transient component to the cueing effect. Discrimination performance was also significantly affected by prior knowledge of the target location in the absence of explicit visual precues. These results demonstrate that covert visual search performance in macaques is very similar to that of humans, indicating that the macaque provides an appropriate model for understanding the neural mechanisms of covert search.
attention; transient; cueing; rhesus monkey; set-size effects
One of the most fundamental functions of the visual system is to code the positions of objects. Most studies, especially those using fMRI, widely assume that the location of the peak retinotopic activity generated in the visual cortex by an object is the position assigned to that object—this is a simplified version of the local sign hypothesis. Here, we employed a novel technique to compare the pattern of responses to moving and stationary objects and found that the local sign hypothesis is false. By spatially correlating populations of voxel responses to different moving and stationary stimuli in different positions, we recovered the modulation transfer function for moving patterns. The results show that the pattern of responses to a moving object is best correlated with the response to a static object that is located behind the moving one. The pattern of responses across the visual cortex was able to distinguish object positions separated by about 0.25 deg visual angle, equivalent to approximately 0.25 mm cortical distance. We also found that the position assigned to a pattern is not simply dictated by the peak activity—the shape of the luminance envelope and the resulting shape of the population response, including the shape and skew in the response at the edges of the pattern, influences where the visual cortex assigns the object’s position. Therefore, visually coded position is not conveyed by the peak but by the overall profile of activity. © 2006 Elsevier Ltd. All rights reserved.
Vision; Perception; Motion; Motion perception; fMRI; Retinotopy; Topography; Local sign; Localization; Labeled line; Visual cortex; Striate cortex
The sensory outer segments (OS) of vertebrate retinal photoreceptors, which detect photons of light, resemble the distal segments of C. elegans sensory cilia, which detect chemical ligands that influence the chemotactic movements of the animal. Based on fluorescence microscopy assays performed in sensory cilia of living, transgenic “wild type” and mutant C. elegans, combined with in vitro motility assays using purified motors, we have proposed that two types of kinesin-2 motor, heterotrimeric kinesin-II and homodimeric OSM-3, cooperate to build amphid and phasmid sensory cilia on chemosensory neurons. Specifically, we propose that these motors function together in a redundant manner to build the axoneme core (aka middle segments (MS)), whereas OSM-3 alone serves to build the distal segments (DS). Furthermore, our data suggest that these motors accomplish this by driving two sequential steps of anterograde transport of cargoes consisting of IFT-particles, retrograde dynein motors, and ciliary tubulin subunits, from the transition zone to the tips of the axonemal microtubules (MTs). Homologs of kinesin-II (KIF3) and OSM-3 (KIF17) are also proposed to contribute to the assembly of vertebrate photoreceptors, although how they do so is currently unclear. Here I review our work on kinesin-2 motors, intraflagellar transport (IFT) and cilium biogenesis in C. elegans sensory cilia, and comment on its possible relevance to current research on vertebrate photoreceptor cilia assembly and function.
Senior-Løken syndrome (SLS) is an autosomal recessive disease characterized by development of a retinitis pigmentosa (RP)- or Leber congenital amaurosis (LCA)-like retinal dystrophy and a medullary cystic kidney disease, nephronophthisis. Mutations in several genes (called nephrocystins) have been shown to cause SLS. The proteins encoded by these genes are localized in the connecting cilium of photoreceptor cells and in the primary cilium of kidney cells. Nephrocystins are thought to have a role in regulating transport of proteins bound to the outer segment/primary cilium; however, the precise molecular mechanisms are largely undetermined. This review will survey the biochemistry, cell biology and existing animal models for each of the nephrocystins as it relates to photoreceptor biology and pathogenesis of retinal degeneration.
Senior-Løken syndrome; nephronophthisis; nephrocystins; retinal degeneration; ciliopathy; primary cilium
This review will focus on the conserved molecular mechanisms for the specific targeting of rhodopsin and rhodopsin-like sensory receptors to the primary cilia. We will discuss the molecular assemblies that control the movement of rhodopsin from the central sorting station of the cell, the trans-Golgi network (TGN), into membrane-enclosed rhodopsin transport carriers (RTCs), and their delivery to the primary cilia and the cilia-derived sensory organelles, the rod outer segments (ROS). Recent studies reveal that these processes are initiated by the synergistic interaction of rhodopsin with the active form of the G-protein Arf4 and the Arf GTPase activating protein (GAP) ASAP1. During rhodopsin progression, ASAP1 serves as an activation platform that brings together the proteins necessary for transport to the cilia, including the Rab11a-Rabin8-Rab8 complex involved in ciliogenesis. These specialized molecular assemblies, through successive action of discrete modules, cooperatively determine how rhodopsin and other rhodopsin-like signaling receptors gain access to primary cilia.
Arfs; Arf GAPs; Cilium; Rabs; Rhodopsin; Trafficking
Expressed ubiquitously, PrBP/δ functions as chaperone/co-factor in the transport of a subset of prenylated proteins. PrBP/δ features an immunoglobulin-like β-sandwich fold for lipid binding, and interacts with diverse partners. PrBP/δ binds both C-terminal C15 and C20 prenyl side chains of phototransduction polypeptides and small GTP-binding (G) proteins of the Ras superfamily. PrBP/δ also interacts with the small GTPases, ARL2 and ARL3, which act as release factors (GDFs) for prenylated cargo. Targeted deletion of the mouse Pde6d gene encoding PrBP/δ resulted in impeded trafficking to the outer segments of GRK1 and cone PDE6 which are predicted to be farnesylated and geranylgeranylated, respectively. Rod and cone transducin trafficking was largely unaffected. These trafficking defects produce progressive cone-rod dystrophy in the Pde6d−/− mouse.
cGMP phosphodiesterase 6 (PDE6); Prenyl binding protein PrBP/δ; prenylation; PrBP/δ crystal structure; small GTPases; rod photoreceptors; G-protein coupled receptor kinase 1 (GRK1)
Whirlin is a gene responsible for Usher syndrome type II (USH2) and congenital deafness. In photoreceptors, it organizes a protein complex through binding to proteins encoded by other USH2 genes, usherin (USH2A) and G-protein-coupled receptor 98 (GPR98). Recently, Cav1.3α1 (α1D) has been discovered to interact with whirlin in vitro and these two proteins are localized to the same subcellular compartments in photoreceptors. Accordingly, it is proposed that Cav1.3α1 is in the USH2 protein complex and that the USH2 protein complex is involved in regulating Ca2+ in photoreceptors. To test this hypothesis, we investigated the interdependence of Cav1.3α1 and whirlin expression in photoreceptors. We found that lack of Cav1.3α1 did not change the whirlin distribution or expression level in photoreceptors. In the retina, several Cav1.3α1 splice variants were found at the RNA level. Among them, the whirlin-interacting Cav1.3α1 long variant had no change in its protein expression level in the absence of whirlin. The localization of Cav1.3α1 in photoreceptors, published previously, cannot be confirmed. Therefore, the mutual independence of whirlin and Cav1.3α1 expressions in photoreceptors suggests that Cav1.3α1 may not be a key member of the USH2 protein complex at the periciliary membrane complex.
Usher syndrome; L-type voltage-gated Ca2+ channel; periciliary membrane complex; retinitis pigmentosa; whirlin
Inositol phosphatases are important regulators of cell signaling and membrane trafficking. Mutations in inositol polyphosphate 5-phosphatase, INPP5E, have been identified in Joubert syndrome, a rare congenital disorder characterized by midbrain malformation, retinitis pigmentosa, renal cysts, and polydactyly. Previous studies have implicated primary cilia abnormalities in Joubert Syndrome, yet the role of INPP5E in cilia formation is not well understood. In this study, we examined the function of INPP5E in cilia development in zebrafish. Using specific antisense morpholino oligonucleotides to knockdown Inpp5e expression, we observed phenotypes of microphthalmia, pronephros cysts, pericardial effusion, and left-right body axis asymmetry. The Inpp5e morphant zebrafish exhibited shortened and decreased cilia formation in the Kupffer’s vesicle and pronephric ducts as compared to controls. Epinephrine-stimulated melanosome trafficking was delayed in the Inpp5e zebrafish morphants. Expression of human INPP5E expression rescued the phenotypic defects in the Inpp5e morphants. Taken together, we showed that INPP5E is critical for the cilia development in zebrafish.
Inositol phosphatase; primary cilia; Kupffer’s vesicle; INPP5E; zebrafish
Most cells in the human body elaborate cilia which serve a wide variety of functions, including cell and tissue differentiation during development, sensing physical and chemical properties of the extracellular milieu and mechanical force generation. Common among cilia is the transduction of external stimuli into signals that regulate the activities of the cilia and the cells that possess them. These functions require the transport and localization of specialized proteins to the cilium, a process that many recent studies have shown to be vital for normal cell function and, ultimately, the health of the organism. Here we discuss several mechanisms proposed for the transport and localization of soluble and peripheral membrane proteins to, or their exclusion from the ciliary compartment with a focus on how the structure of the cytoplasm and the size and shape of proteins influence these processes. Additionally, we examine the impact of cell and protein structure on our ability to accurately measure the relative concentrations of fluorescently tagged proteins amongst various cellular domains, which is integral to our understanding of the molecular mechanisms underlying protein localization and transport.
Cilia; photoreceptor; diffusion; signal transduction
The mechanism by which myristoylated proteins are targeted to specific subcellular membrane compartments is poorly understood. Two novel acyl-binding proteins, UNC119A and UNC119B, have been shown recently to function as chaperones/co-factors in the transport of myristoylated G protein α-subunits and src-type tyrosine kinases. UNC119 polypeptides feature an immunoglobulin-like β-sandwich fold that forms a hydrophobic pocket capable of binding lauroyl (C12) and myristoyl (C14) side chains. UNC119A in rod photoreceptors facilitates the transfer of transducin α subunits (Tα) from inner segment to outer segment membranes by forming an intermediate diffusible UNC119-Tα complex. Similar complexes are formed in other sensory neurons, as the G proteins ODR-3 and GPA-13 in C. elegans unc-119 mutants traffic inappropriately. UNC119B knockdown in IMCD3 cells prevents trafficking of myristoylated nephrocystin-3 (NPHP3), a protein associated with nephronophthisis, to cilia. Further, UNC119A was shown to transpot myristoylated src-type tyrosine kinases to cell membranes and to affect T-cell receptor (TCR) and interleukin-5 receptor (IL-5R) activities. These interactions establish UNC119 polypeptides as novel lipid-binding chaperones with specificity for a diverse subset of myristoylated proteins.
Uncoordinated 119 (UNC 119); acyl binding proteins; UNC119 crystal structure; C. elegans olfactory neurons; rod photoreceptors; G protein α-subunits; src-type tyrosine kinases; nephrocystin-3; T-cell receptor
This review focuses on recent advances in the understanding of kinesin-2 family motors in vertebrate photoreceptor development. Zebrafish photoreceptors develop by the 3rd day of embryogenesis, making it possible to study mutant phenotypes without the use of conditional alleles. Recent work using a zebrafish kif3b mutant allele validates the concept that the heterotrimeric kinesin II motor is generally required for ciliogenesis. In zebrafish photoreceptors, however, loss of kif3b function delays but does not block cilium formation. This is thought to occur because both kif3b or kif3c can dimerize with kif3a and function redundantly. The second ciliary kinesin thought to function in photoreceptor cells is kif17. Prior work has shown that either morpholino knockdown of this gene or the overexpression of its dominant negative form can reduce or delay photoreceptor cilium development without any evident impact on ciliogenesis in general. This has led to the idea that kif17 may play an important role only in some specialized cilium types, such the one in photoreceptor cells. In a recently identified kif17 mutant, however, photoreceptor outer segments are formed by 5 dpf and an obvious delay of outer segment formation is seen only at the earliest stage analyzed (3 dpf). This work suggests that kif17 plays a significant role mainly at an early stage of photoreceptor development. Taken together, these studies lead to an intriguing concept that as they differentiate photoreceptors alter their kinesin repertoire.
Multiple proteins are targeted to photoreceptor outer segments (OS) where they function in phototransduction. Intraflagellar transport (IFT), a highly conserved bidirectional transport pathway occurring along the microtubules of the ciliary axoneme has been implicated in OS trafficking. The canonical anterograde motor for IFT is the heterotrimeric kinesin II or KIF3 complex. Previous work from our laboratory has demonstrated a role for an additional kinesin 2 family motor, the homodimeric KIF17. To gain a better understanding of KIF17 function in photoreceptor OS we utilized transgenic zebrafish expressing zfKIF17-GFP to assess the localization and dynamics of zfKIF17. Our data indicate that both endogenous KIF17 and KIF17-GFP are associated with the axoneme of zebrafish cones at both early (5 dpf) and late (21 dfp) stages of development. Strikingly, KIF17-GFP accumulates at the OS distal tip in a phenomenon referred to as “tipping”. Tipping occurs in the large majority of photoreceptors and also occurs when mammalian KIF17-mCherry is expressed in ciliated epithelial cells in culture. In some cases KIF17-GFP is shed with the OS tip as part of the disc shedding process. We have also found that KIF17-GFP moves within the OS at rates consistent with those observed for IFT and other kinesins.
Photoreceptors; Cilia; Kinesin; Intraflagellar transport; Zebrafish
For being a polarized neuron and having a sensory cilium, photoreceptors attract remarkable attention. This is due their highly polarized structure and active visual signal transduction cascades and for the enrichment of complex networks of proteins in the cilium. Structural and functional maintenance of the photoreceptor sensory cilium, also called outer segment, ensures that light signal is received and relayed appropriately to the brain. Any perturbations in the protein content of the outer segment result in photoreceptor dysfunction, degeneration and eventually, blindness. This review focuses on the importance of photoreceptor sensory cilium to carry out signal transduction cascade for vision.
Despite several Findings of perceptual asynchronies between object features, it remains unclear whether independent neuronal populations necessarily code these perceptually unbound properties. To examine this, we investigated the binding between an object’s spatial frequency and its rotational motion using contingent motion aftereffects (MAE). Subjects adapted to an oscillating grating whose direction of rotation was paired with a high or low spatial frequency pattern. In separate adaptation conditions, we varied the moment when the spatial frequency change occurred relative to the direction reversal. After adapting to one stimulus, subjects made judgments of either the perceived MAE (rotational movement) or the position shift (instantaneous phase rotation) that accompanied the MAE. To null the spatial frequency-contingent MAE, motion reversals had to physically lag changes in spatial frequency during adaptation. To null the position shift that accompanied the MAE, however, no temporal lag between the attributes was required. This demonstrates that perceived motion and position can be perceptually misbound. Indeed, in certain conditions, subjects perceived the test pattern to drift in one direction while its position appeared shifted in the opposite direction. The dissociation between perceived motion and position of the same test pattern, following identical adaptation, demonstrates that distinguishable neural populations code for these object properties.
Vision; Perception; Motion; Color-motion asynchrony; Binding; Spatial frequency; Texture; Contingent aftereffect; Motion aftereffect; MAE; Localization; Differential latency
Although second-order motion may be detected by early and automatic mechanisms, some models suggest that perceiving second-order motion requires higher-order processes, such as feature or attentive tracking. These types of attentionally mediated mechanisms could explain the motion aftereffect (MAE) perceived in dynamic displays after adapting to second-order motion. Here we tested whether there is a second-order MAE in the absence of attention or awareness. If awareness of motion, mediated by high-level or top-down mechanisms, is necessary for the second-order MAE, then there should be no measurable MAE if the ability to detect directionality is impaired during adaptation. To eliminate the subject’s ability to detect directionality of the adapting stimulus, a second-order drifting Gabor was embedded in a dense array of additional crowding Gabors. We found that a significant MAE was perceived even after adaptation to second-order motion in crowded displays that prevented awareness. The results demonstrate that second-order motion can be passively coded in the absence of awareness and without top-down attentional control.
Vision; Attention; Perception; Motion; Motion perception; Second-order motion; Crowding; Awareness; Consciousness; First-order motion; MAE; Contrast-defined
Pursuit and saccades almost always select the same target. Is this the results of a common selection process or does smooth pursuit obligatorily follow the stimulus targeted by saccades? To address this question, we used microstimulation of the primate superior colliculus (SC) to redirect the eyes from a selected pursuit target to a distracter moving in the opposite direction. During each trial, monkeys pursued a horizontally moving array of colored target stimuli. In half of the trials, this target array was accompanied by a distracter array moving horizontally in the opposite direction, offset by the vertical amplitude of the stimulation-evoked saccade. We stimulated the SC during maintained pursuit on half of the trials, and measured pursuit eye velocity during the 50-ms interval immediately following the stimulation-evoked saccade to the distracter array. Saccades evoked by SC stimulation did not alter pursuit target selection. Pursuit velocity on average changed by less than 10% of that expected if the monkey had completely switched targets. Moreover, the same changes in velocity occurred when there was no distracter, indicating that even these small changes in pursuit velocity were a direct effect of the evoked saccade, not partial selection of the distracter. These results show that motor execution of saccades is not sufficient to select a pursuit target, and support the idea that the coordination of pursuit and saccades is accomplished by a shared target selection process.
target selection; attention; saccade; smooth pursuit; superior colliculus; eye movement