Direction biasing by brief apparent motion stimuli

Serial Dependence in Perception

Much evidence has shown that perception is biased towards previously presented similar stimuli, an effect recently termed serial dependence. Serial dependence affects nearly every aspect of perception, often causing gross perceptual distortions, especially for weak and ambiguous stimuli. Despite unwanted side-effects, empirical evidence and Bayesian modeling show that serial dependence acts to improve efficiency and is generally beneficial to the system. Consistent with models of predictive coding, the Bayesian priors of serial dependence are generated at high levels of cortical analysis, incorporating much perceptual experience, but feed back to lower sensory areas. These feedback loops may drive oscillations in the alpha range, linked strongly with serial dependence. The discovery of top-down predictive perceptual processes is not new, but the new, more quantitative approach characterizing serial dependence promises to lead to a deeper understanding of predictive perceptual processes and their underlying neural mechanisms.

Visual priming of two-step motion sequences

Perception of an ambiguous apparent motion is influenced by the immediately preceding motion. In positive priming, when an observer is primed with a slow-pace (1-3 Hz) sequence of motion frames depicting unidirectional drift (e.g., Right-Right-Right-Right), subsequent sequences of ambiguous frames are often perceived to continue moving in the primed direction (illusory Right-Right …). Furthermore, priming an observer with a slow-pace sequence of rebounding apparent motion frames that alternate between opponently coded motion directions (e.g., Right-Left-Right-Left) leads to an illusory continuation of the two-step rebounding sequence in subsequent random frames. Here, we show that even more arbitrary two-step motion sequences can be primed; in particular, two-step motion sequences that alternate between non-opponently coded directions (e.g., Up-Right-Up-Right; staircase motion) can be primed to be illusorily perceived in subsequent random frames. We found that staircase sequences, but not drifting or rebounding sequences, were primed more effectively with four priming frames compared with two priming frames, suggesting the importance of repeating the sequence element for priming arbitrary two-step motion sequences. Moreover, we compared the effectiveness of motion primes to that of symbolic primes (arrows) and found that motion primes were significantly more effective at producing prime-consistent responses. Although it has been proposed that excitatory and rivalry-like mechanisms account for drifting and rebounding motion priming, current motion processing models cannot account for our observed priming of staircase motion. We argue that higher order processes involving the recruitment and interaction of both attention and visual working memory are required to account for the type of two-step motion priming reported here.

Common and independent processing of visual motion perception and oculomotor response

Visual motion signals are used not only to drive motion perception but also to elicit oculomotor responses. A fundamental question is whether perceptual and oculomotor processing of motion signals shares a common mechanism. This study aimed to address this question using visual motion priming, in which the perceived direction of a directionally ambiguous stimulus is biased in the same (positive priming) or opposite (negative priming) direction as that of a priming stimulus. The priming effect depends on the duration of the priming stimulus. It is assumed that positive and negative priming are mediated by high- and low-level motion systems, respectively. Participants were asked to judge the perceived direction of a π-phase-shifted test grating after a smoothly drifting priming grating during varied durations. Their eye movements were measured while the test grating was presented. The perception and eye movements were discrepant under positive priming and correlated under negative priming on a trial-by-trial basis when an interstimulus interval was inserted between the priming and test stimuli, indicating that the eye movements were evoked by the test stimulus per se. These findings suggest that perceptual and oculomotor responses are induced by a common mechanism at a low level of motion processing but by independent mechanisms at a high level of motion processing.

Effect of spatial attention on spatiotopic visual motion perception

We almost never experience visual instability, despite retinal image instability induced by eye movements. How the stability of visual perception is maintained through spatiotopic representation remains a matter of debate. The discrepancies observed in the findings of existing neuroscience studies regarding spatiotopic representation partly originate from differences in regard to how attention is deployed to stimuli. In this study, we psychophysically examined whether spatial attention is needed to perceive spatiotopic visual motion. For this purpose, we used visual motion priming, which is a phenomenon in which a preceding priming stimulus modulates the perceived moving direction of an ambiguous test stimulus, such as a drifting grating that phase shifts by 180°. To examine the priming effect in different coordinates, participants performed a saccade soon after the offset of a primer. The participants were tasked with judging the direction of a subsequently presented test stimulus. To control the effect of spatial attention, the participants were asked to conduct a concurrent dot contrast-change detection task after the saccade. Positive priming was prominent in spatiotopic conditions, whereas negative priming was dominant in retinotopic conditions. At least a 600-ms interval between the priming and test stimuli was needed to observe positive priming in spatiotopic coordinates. When spatial attention was directed away from the location of the test stimulus, spatiotopic positive motion priming completely disappeared; meanwhile, the spatiotopic positive motion priming at shorter interstimulus intervals was enhanced when spatial attention was directed to the location of the test stimulus. These results provide evidence that an attentional resource is requisite for developing spatiotopic representation more quickly.

Dissociating Higher and Lower Order Visual Motion Systems by Priming Illusory Apparent Motion

Motion processing is thought of as a hierarchical system composed of higher and lower order components. Past research has shown that these components can be dissociated using motion priming paradigms in which the lower order system produces negative priming while the higher order system produces positive priming. By manipulating various stimulus parameters, researchers have probed these two systems using bistable test stimuli that permit only two motion interpretations. Here we employ maximally ambiguous test stimuli composed of randomly refreshing pixels in a task that allows observers to report more than just two types of motion percepts. We show that even with such stimuli, motion priming can constrain the unstructured random pixel patterns into coherent percepts of positive or negative apparent motion. Moreover, we find that the higher order system is uniquely susceptible to cognitive influences, as evidenced by a significant suppression of positive priming in the presence of alternative response options.

Adaptation to Complex Pictures: Exposure to Emotional Valence Induces Assimilative Aftereffects

Aftereffects have been documented for a variety of perceptual categories spanning from body gender to facial emotion, thus becoming an important tool in the study of high-level vision and its neural bases. We examined whether the perceived valence of a complex scene is subject to aftereffects, by observing the participants’ evaluation of the valence of a test picture preceded by a different picture. For this study, we employed an adaptation paradigm with positive and negative images used as adapters, and positive, negative, and neutral images used as tests. Our results show that adaptation to complex emotional pictures induces assimilative aftereffects: participants judged neutral tests more positively following positive adapters and more negatively following negative adapters. This depended on the prolonged adaptation phase (10 s), as the results of a second experiment, in which adapters lasted for 500 ms, did not show aftereffects. In addition, the results show that assimilative aftereffects of negative and positive adapters also manifested themselves on non-neutral (negative and positive) targets, providing evidence that the global emotional content of complex pictures is suitable to induce assimilative aftereffects.

Individual differences in visual motion perception and neurotransmitter concentrations in the human brain

Recent studies have shown that interindividual variability can be a rich source of information regarding the mechanism of human visual perception. In this study, we examined the mechanisms underlying interindividual variability in the perception of visual motion, one of the fundamental components of visual scene analysis, by measuring neurotransmitter concentrations using magnetic resonance spectroscopy. First, by psychophysically examining two types of motion phenomena-motion assimilation and contrast-we found that, following the presentation of the same stimulus, some participants perceived motion assimilation, while others perceived motion contrast. Furthermore, we found that the concentration of the excitatory neurotransmitter glutamate-glutamine (Glx) in the dorsolateral prefrontal cortex (Brodmann area 46) was positively correlated with the participant's tendency to motion assimilation over motion contrast; however, this effect was not observed in the visual areas. The concentration of the inhibitory neurotransmitter γ-aminobutyric acid had only a weak effect compared with that of Glx. We conclude that excitatory process in the suprasensory area is important for an individual's tendency to determine antagonistically perceived visual motion phenomena.This article is part of the themed issue 'Auditory and visual scene analysis'.

Different coding strategies for the perception of stable and changeable facial attributes

Perceptual systems face competing requirements: improving signal-to-noise ratios of noisy images, by integration; and maximising sensitivity to change, by differentiation. Both processes occur in human vision, under different circumstances: they have been termed priming, or serial dependencies, leading to positive sequential effects; and adaptation or habituation, which leads to negative sequential effects. We reasoned that for stable attributes, such as the identity and gender of faces, the system should integrate: while for changeable attributes like facial expression, it should also engage contrast mechanisms to maximise sensitivity to change. Subjects viewed a sequence of images varying simultaneously in gender and expression, and scored each as male or female, and happy or sad. We found strong and consistent positive serial dependencies for gender, and negative dependency for expression, showing that both processes can operate at the same time, on the same stimuli, depending on the attribute being judged. The results point to highly sophisticated mechanisms for optimizing use of past information, either by integration or differentiation, depending on the permanence of that attribute.

Perceived causality can alter the perceived trajectory of apparent motion

When objects collide, human observers perceive not only motion but also causal relations, such as which object caused the other to move. In the present experiments, we investigated whether such causal interpretations can actually influence the perceived path of apparent motion. Displays contained two alternately flashing motion targets positioned at either end of a semicircular occluder. Two additional "context objects" moved in such a way that the motion targets appeared to collide with and launch them. The collision was manipulated so that it was consistent with apparent motion either along the straight path between the targets or along a curved path passing behind the occluder. Subjects almost exclusively perceived motion consistent with the implied launch, which suggests that causally coherent interpretations can influence basic perceptual processes.

[Reversal of visual motion priming under mesopic vision]

It is known that a directionally ambiguous test stimulus is perceived to move in the same direction as a brief preceding priming stimulus when both stimuli are presented at the same retinal location (visual motion priming). To examine the spatial properties of visual motion priming under different retinal illuminance, we manipulated the distance between the priming and test stimuli. Participants judged the perceived direction of 180 deg phase-shifted, thus directionally ambiguous, sine-wave gratings (test stimulus) displayed immediately after the offset of a smoothly drifting priming stimulus. The distance between priming and test stimuli was varied from 0 to 10 deg in visual angle. Since the spatial summation area broadens under low retinal illuminance, we predicted that visual motion priming would be more conspicuous under mesopic vision than under photopic vision. Contrary to this prediction, as the retinal illuminance decreased and the distance between the primer and the test stimulus increased, the test stimulus was perceived to move in the direction opposite to the priming stimulus. We speculate that different motion integration systems are functioning depending on the retinal illuminance.

The effect of retinal illuminance on visual motion priming

The perceived direction of a directionally ambiguous stimulus is influenced by the moving direction of a preceding priming stimulus. Previous studies have shown that a brief priming stimulus induces positive motion priming, in which a subsequent directionally ambiguous stimulus is perceived to move in the same direction as the primer, while a longer priming stimulus induces negative priming, in which the following ambiguous stimulus is perceived to move in the opposite direction of the primer. The purpose of this study was to elucidate the underlying mechanism of motion priming by examining how retinal illuminance and velocity of the primer influences the perception of priming. Subjects judged the perceived direction of 180-deg phase-shifted (thus directionally ambiguous) sine-wave gratings displayed immediately after the offset of a primer stimulus. We found that perception of motion priming was greatly modulated by the retinal illuminance and velocity of the primer. Under low retinal illuminance, positive priming nearly disappeared even when the effective luminance contrast was equated between different conditions. Positive priming was prominent when the velocity of the primer was low, while only negative priming was observed when the velocity was high. These results suggest that the positive motion priming is induced by a higher-order mechanism that tracks prominent features of the visual stimulus, while a directionally selective motion mechanism induces negative motion priming.

Priming by motion too rapid to be consciously seen

When a rapidly rotating ring of dots was briefly flashed, observers saw only a solid ring with no discriminable rotation. However, when this stimulus served as a prime that was followed by a target that consisted of a clearly rotating ring of dots, response times (RTs) to report the target's rotation were shorter when the prime and target directions were congruent than when they were incongruent. In accord with shape priming data, this priming effect increased monotonically with the prime-target stimulus-onset asynchrony (SOA). The prime also biased the perceived direction of an ambiguous apparent motion target, but only at an intermediate SOA. At the same SOA, we also found that target presentations enabled above-chance discrimination of prime's rotation direction. These outcomes demonstrate the processing of motion direction information that is not phenomenally represented. They suggest a common mechanism may mediate the priming of RTs by shape and motion, whereas a different mechanism mediates perceptual measures of motion priming.

Automatic detection of motion direction changes in the human brain

The possibility that the visual system is able to register unattended changes is still debated in the literature. However, it is difficult to understand how a sensory system becomes aware of unexpected salient changes in the environment if attention is required for detecting them. The ability to automatically detect unusual changes in the sensory environment is an adaptive function which has been confirmed in other sensory modalities (i.e. audition). This deviance detector mechanism has proven to be based on a preattentive nonrefractory memory-comparison process. To investigate whether such automatic change detection mechanism exists in the human visual system, we recorded event-related potentials to sudden changes in a biologically important feature, motion direction. Unattended sinusoidal gratings varying in motion direction in the peripheral field were presented while subjects performed a central task with two levels of difficulty. We found a larger negative displacement in the electrophysiological response elicited by less frequent stimuli (deviant) at posterior scalp locations. Within the latency range of the visual evoked component N2, this differential response was elicited independently of the direction of motion and processing load. Moreover, the results showed that the negativity elicited by deviants was not related to a differential refractory state between the electrophysiological responses to frequent and infrequent directions of motion, and that it was restricted to scalp locations related to motion processing areas. The present results suggest that a change-detection mechanism sensitive to unattended changes in motion direction may exist in the human visual system.

Mental extrapolation of target position is strongest with weak motion signals and motor responses

Some accounts hold that the position of moving objects is extrapolated either in visual perception or visual short-term memory ("representational momentum"). However, some studies did not find forward displacement of the final position when smooth motion was used, whereas reliable displacement was observed with implied motion. To resolve this conflict, the frequency of position changes was varied to sample motion types between the extreme cases of implied and smooth motion. A continuous function relating frequency of target change and displacement was found: Displacement increased when the frequency of position changes was reduced. Further, the response mode was varied. Probe judgments produced less forward displacement than motor judgments such as mouse or natural pointing movements. Also, localization judgments were susceptible to motion context, but not to variations of probe shape or expectancy about trajectory length. It is suggested that forward displacement results from the extrapolation of the next step in the observed motion sequence.

Neural correlates of perceptual priming of visual motion

In two experiments, the temporal dynamics of neural activity underlying perceptual priming of visual motion was examined using event-related potentials (ERPs) during directional judgments of the apparent motion of two-dimensional sine-wave gratings. Compared to perceptually ambiguous motion, unambiguous left- or rightward motion was associated with enhanced ERP activity about 300 ms after the onset of apparent motion. In the second experiment, ERPs were recorded to two successive motion jumps in which an unambiguous motion jump served as a prime for a subsequent target motion that was ambiguous. The prime-target time interval was varied between 200, 400, and 1000 ms. In a control (motion reversal) condition, the two motion jumps were both unambiguous but in opposite directions. Compared to the motion reversal condition, motion priming was associated with an enhancement of ERP amplitudes at 100 ms and 350 ms following target stimulus onset. ERP enhancement was greatest at a short prime-target interval of 200 ms, which was also associated behaviorally with the strongest priming. The ERP enhancement and behavioral priming were both eliminated at the long 1000 ms prime-target interval. Functional magnetic resonance imaging (fMRI) data from a subset of subjects supported the view that motion priming involves modulation of neural responses both in early visual cortex and in later stages of visual processing.

Direction-specific changes of sensitivity after brief apparent motion stimuli

Direction-specific losses in sensitivity were found for a test grating which was superimposed on a stationary contrast pedestal and which moved either in the same or opposite direction as a prior biasing stimulus. Three types of biasing stimuli were employed: a grating swept through 270 degrees in 45 degrees steps, a single 90 degrees step of a grating, and a single 90 degrees step of a grating which contained a blank IFI and whose perceived direction was reversed. For the biasing sweep and the single 90 degrees step, the response of directionally selective mechanisms (directional motion energy) is greatest for the direction which corresponds to the actual physical displacement of the stimulus. For the biasing step with an IFI, the response is maximum for the opposite direction. For all three types of biasing stimuli, directional sensitivity for a test stimulus was reduced most when it moved in the biasing direction, i.e. the direction which produced the strongest signal in directionally selective mechanisms. Unlike the effects of the same types of biasing stimuli on the perceived direction of a suprathreshold 180 degrees step of a grating [Pinkus, A., & Pantle, A. (1997). Probing motion signals with a priming paradigm. Vision Research, 37, 541-52; Pantle, A., Gallogly, D.P., & Piehler, O.C. (2000). Direction biasing by brief apparent motion stimuli. Vision Research, 40, 1979-91], all the direction-specific losses of sensitivity can be explained by changes in the response characteristics of directionally selective mechanisms.