Apparent Motion Stimuli Research Articles

Responses of extrastriate cortex to switching perception of ambiguous visual motion stimuli.

We recently found that in bistable apparent motion (AM) both the motion complex (hMT/V5+) and kinetic occipital area (KO) transiently activated whenever perception switched in the spinning wheel illusion. Here, we tested the specificity of this result with another bistable AM stimulus, the dynamic dot quartet, that does not involve kinetic contours. We observed significant activations in hMT/V5+, but not in KO. This indicates that neural activity in functionally specialized extrastriate visual areas during switching perception of ambiguous input depends on stimulus features in a finely tuned way suitable to encode perceptual content in the absence of sensory input changes.

Pursuit of the ineffable: perceptual and motor reversals during the tracking of apparent motion.

Pursuit can be guided by perceived rather than physical motion, but the temporal relationship between motion perception and pursuit is unknown. We used an apparent motion stimulus consisting of a horizontal row of evenly spaced Kanizsa illusory squares (1.44 deg2): the illusory contours appeared at the midpoints of the illusory squares presented in the previous frame, producing bi-directional apparent motion of the illusory contours (21.5 deg/s) that could be reversed at will. We measured eye movements in five subjects asked to (1) track the motion of the illusory squares, and (2) reverse the perceived direction while continuing to track the squares. We measured the timing of the voluntary perceptual reversals and compared this to the time course of the reversal in tracking direction. We found that subjects could smoothly track the apparent motion of illusory squares and also produce saccade-free reversals in pursuit velocity. The time course of these motor reversals closely followed the measurements of the perceptual reversal and, on average, the perceptual reversals preceded the pursuit reversals by 53 ms, a delay shorter than when the perceptual reversal was visually guided. Smooth pursuit and the perception of motion direction were in temporal register and highly correlated, suggesting that pursuit can provide a real-time readout for the state of motion perception.

Neural responses in motor cortex and area 7a to real and apparent motion

The neural activity in area 7a and the arm area of motor cortex was recorded while real or path-guided apparent motion stimuli were presented to behaving monkeys in the absence of a motor response. A smooth stimulus motion was produced in the real motion condition, whereas in the apparent motion condition five stimuli were flashed successively at the vertices of a regular pentagon. The stimuli moved along a low contrast circular path with one of five speeds (180-540 deg/s). We found strong neural responses to real and apparent motion in area 7a and motor cortex. In the motor cortex, a substantial population of neurons showed a selective response to real moving stimuli in the absence of a motor response. This activity was modulated in some cases by the stimulus speed, and some of the neurons showed a response during a particular part of the circular trajectory of the stimulus; the preferred stimulus angular locations were evenly distributed across this neuronal ensemble. It is likely that these neural signals are continuously available to the motor cortex in order to generate responses that demand immediate action. In area 7a, two overlapping populations of neurons were observed. The first comprised cells the activity of which was tuned to the angular location of a circularly moving stimulus in the real motion condition. These cells also responded to apparent motion at high stimulus speeds. A visual receptive field analysis showed that the angular tuning in most of the area 7a neurons did not depend on the spatial location of the stimulus in relation to their receptive field. The second population was selective to apparent moving stimuli and showed a periodic entrainment of activation with the period of the inter-stimulus interval of the flashing dots. Both the angular location and the inter-stimulus interval neural signals can be used to generate precise behavioral responses towards real or apparent moving stimuli.

Parietal activity and the perceived direction of ambiguous apparent motion.

We recorded from parietal neurons in monkeys (Macacca mulatta) trained to report the direction of an apparent motion stimulus consisting of regularly spaced columns of dots surrounded by an aperture. Displacing the dots by half their inter-column spacing produced vivid apparent motion that could be perceived in either the preferred or anti-preferred direction for each neuron. Many neurons in the lateral intraparietal area (LIP) responded more strongly on trials in which the animals reported perceiving the neurons' preferred direction, independent of the hand movement used to report their percept. This selectivity was less common in the medial superior temporal area (MST) and virtually absent in the middle temporal area (MT). Variations in activity of LIP and MST neurons just before motion onset were also predictive of the animals' subsequent perceived direction. These data suggest a hierarchy of representation in parietal cortex, whereby neuronal responses become more aligned with subjective perception in higher parietal areas.

Space-time maps and two-bar interactions of different classes of direction-selective cells in macaque V-1.

We used one-dimensional sparse noise stimuli to generate first-order spatiotemporal maps and second-order two-bar interaction maps for 65 simple and 124 complex direction-selective cells in alert macaque V1. Spatial and temporal phase differences between light and dark space-time maps clearly distinguished simple and complex cell populations. Complex cells usually showed similar direction preferences to light and dark bars, but many of the directional simple cells were much more direction selective to one sign of contrast than the reverse. We show that this is predicted by a simple energy model. Some of the direction-selective simple cells showed multiple space-time-slanted subregions, but others (previously described as S1 cells) had space-time maps that looked like just one subregion of an ordinary simple cell. Both simple and complex cells showed directional interactions (nonlinearities) to pairs of flashed bars (a 2-bar apparent-motion stimulus). The space-time slant of the simple cells correlated with the optimum dX/dT (velocity) of the paired-bar interactions. Some complex cells also showed a space-time slant; the direction of the slant usually correlated with the preferred direction of motion, but the degree of slant correlated with the inferred velocity tuning only when measured by a weighted-centroid calculation. Principal components analysis of the simple-cell space-time maps yielded one fast temporally biphasic component and a slower temporally monophasic component. We saw no consistent pattern for the spatial phase of the components, unlike previous reports; however, we show that principal components analysis may not distinguish between spatial offsets and phase offsets.

The dynamical foundations of motion pattern formation: stability, selective adaptation, and perceptual continuity.

A dynamical model is used to show that global motion pattern formation for several different apparent motion stimuli can be embodied in the stable distribution of activation over a population of concurrently activated, directionally selective motion detectors. The model, which is based on motion detectors being interactive, noisy, and self-stabilizing, accounts for such phenomena as bistability, spontaneous switching, hysteresis, and selective adaptation. Simulations show that dynamical solutions to the motion correspondence problem for a bistable stimulus (two qualitatively different patterns are formed) apply as well to the solution for a monostable stimulus (only one pattern is formed) and highlight the role of interactions among sequentially stimulated detectors in establishing the state dependence and, thereby, the temporal persistence of percepts.

Magnetoencephalographic study of speed-dependent responses in apparent motion.

There have been only few studies of visually-evoked cortical responses to apparent motion as a function of stimulus speed. Most earlier findings on evoked peak magnitudes and latencies, utilizing various types of smooth and apparent motion stimuli, have demonstrated that greater spatial separation/speed resulted in enhanced peak magnitudes, decreasing onset latencies in individual extrastriate neurons and in shorter motor reaction times in subjects. However, some reports using partial-coverage magnetoencephalography stated that increasing the stimulus displacement actually triggered a substantial reduction of the evoked main peak latency while the magnitude showed no clear change. To resolve the issue of the dependency of evoked responses on stimulus speed in apparent motion, we presented moving bar stimuli to 6 subjects at velocities within a 100 fold range and investigated the ensuing evoked visual cortical activity using a whole-cortex magnetoencephalograph. The magnitude and the latency of the first major evoked peak M1 was measured and compared for 6 discrete bar-stimuli displacements in all subjects. Our results showed clearly that the M1 peak response magnitudes increased in a nonlinear way with higher apparent speeds (larger displacements), in compliance with the logarithmic Fechner law. We observed also that the fluctuations of the mean evoked M1 peak latency (140+/-10.6 ms) did not reach significance over the tested range of stimulus velocities. These findings probably reflect global motion processing mechanisms which rely on nonlinear speed-dependent feedback connectivity between striate and extrastriate visual cortex areas.

Transient and steady-state VEPs—reappraisal

We report here the recent findings in transient and steady-state VEPs that were recorded in our laboratory. Firstly, we studied the neural generators of the pattern reversal transients VEPs (T-VEPs). The optic tract responses recorded during stereotactic pallidotomy showed a P50–N80 complex. Visual evoked magnetic fields (VEFs) showed that the equivalent current dipoles (ECDs) of the N75, P100 and N145 were located in the primary visual cortex (V1). Secondly, we have developed a technique using multimodality VEPs, which can evaluate the parvocellular (color and form) and magnocellular systems (motion). T-VEPs to isoluminant chromatic stimuli showed N120, which was absent or delayed in patients with acquired color deficits. T-VEPs to apparent motion (AM) showed P120. Visual evoked magnetic fields showed that the ECDs of N120 and P120 were located in V1. Abnormality rates to color and apparent motion stimuli were different in optic neuritis, suggesting that these potentials distinguish between the two parallel visual pathways. Thirdly, we studied the physiological properties of the second (2F) and fourth harmonic (4F) components of the steady-state VEPs (S-VEPs). It was found that the 2F and 4F were differently tuned to the spatial and temporal frequencies of achromatic and chromatic stimuli. The oblique effect was also clearly demonstrated in the S-VEPs. From the above findings, a combined use of T-VEPs and S-VEPs could allow us to explore the human visual function.

Neural Correlates of Spontaneous Direction Reversals in Ambiguous Apparent Visual Motion

Looking at bistable visual stimuli, the observer experiences striking transitions between two competing percepts while the physical stimulus remains the same. Using functional imaging techniques, it is therefore possible to isolate neural correlates of perceptual changes that are independent of the low-level aspects of the stimulus. Previous experiments have demonstrated distributed activations in human extrastriate visual cortex related to switches between competing percepts. Here we asked where extrastriate responses still occur with a bistable stimulus that minimizes the cognitive difference between the two percepts. We used the “spinning wheel illusion,” a bistable apparent motion stimulus of which both possible percepts correspond to the same object, share the same center, and are perceived as identically patterned stimuli moving at the same speed and changing only in direction. Using functional magnetic resonance imaging, we analyzed the spatial distribution of event-related activations occurring during spontaneous reversals of perceived direction of motion. In accordance with earlier neuroimaging findings for bistable percepts, we observed event-related activations in several frontal and parietal areas, including the superior parietal cortex bilaterally, the right inferior parietal cortex, and the premotor and inferior frontal cortex of both hemispheres. Furthermore, we found bilateral activations in the occipitotemporal junction (hMT+/V5) and in the lateral occipital sulcus (“KO”) posterior to hMT+/V5, but not in areas of the “ventral stream” of cortical visual processing. Our data suggest that, while a frontoparietal network subserves more general aspects in bistable visual perception, the activations in functionally specialized extrastriate visual cortex are highly category- or attribute-specific.

Counter-changing luminance: A non-Fourier, nonattentional basis for the perception of single-element apparent motion.

Motion perception was studied for generalized apparent motion stimuli composed of 2 simultaneously visible elements whose luminance alternated between 2 values (only 1 element is visible at a time for standard apparent motion). It was demonstrated that 1st-order motion energy is neither necessary nor sufficient for the perception of apparent motion. Instead, it was found that counter-changing luminance— simultaneous luminance changes at 2 element locations—is the informational basis for perceiving luminance-defined apparent motion: Motion starts where luminance changes toward the background luminance value and ends where luminance changes away from the background luminance. The results were not attributable to either 2nd-order motion mechanisms (for which rectification precedes the computation of motion energy) or attention-based, 3rd-order motion mechanisms. Single-element apparent motion has been a central perceptual phenomenon since the earliest days of experimental psychology (Exner, 1875; Wertheimer, 1912). In the standard version, a visual element, say a spot of light, is alternately presented against a dark background in one of two nearby spatial locations; it appears for a finite period of time in one location and almost instantly disappears and reappears for a finite period of time in its other location. Over a wide range of spatial and temporal conditions, a single element is perceived moving smoothly through the space between

Spatiotemporal interpolation and quality of apparent motion.

We investigate the conditions under which stimuli in apparent (sampled) motion are indistinguishable from those in smooth motion and compare this discrimination with the precision achieved by the visual system in interpolating apparent motion. In an initial experiment, observers were required to discriminate smooth from apparent motion, at variable step sizes, contrasts, velocities, and stimulus types (broadband line or bar stimuli and grating patches of different spatial frequency). Thresholds for discriminating smooth from sampled motion were approximately 40 arc min under optimal conditions, corresponding to the diameter of foveal photoreceptors. The tolerated step size between stations increased with velocity, more so for low- than for high-spatial-frequency stimuli. Tolerated step size decreased with presentation duration and with stimulus contrast. A separate experiment examined precision of interpolation. Vernier offsets were produced through temporal delays along the trajectory of an apparent motion, and thresholds for the discrimination of direction of offset were measured as a function of speed of motion and of distance between stations of apparent motion. Perfect interpolation was achieved for distances between stations of approximately 2 arc min. A model based on spatiotemporal filtering at an early stage of processing accounts well for the results of both types of experiments.

Motion processing in the auditory cortex of the rufous horseshoe bat: role of GABAergic inhibition.

This study examined the influence of inhibition on motion-direction-sensitive responses of neurons in the dorsal fields of auditory cortex of the rufous horseshoe bat. Responses to auditory apparent motion stimuli were recorded extracellularly from neurons while microiontophoretically applying gamma-aminobutyric acid (GABA) and the GABAA receptor antagonist bicuculline methiodide (BMI). Neurons could respond with a directional preference exhibiting stronger responses to one direction of motion or a shift of receptive field (RF) borders depending on direction of motion. BMI influenced the motion direction sensitivity of 53% of neurons. In 21% of neurons the motion-direction sensitivity was decreased by BMI by decreasing either directional preference or RF shift. In neurons with a directional preference, BMI increased the spike number for the preferred direction by a similar amount as for the nonpreferred direction. Thus, inhibition was not direction specific. BMI increased motion-direction sensitivity by either increasing directional preference or magnitude of RF shifts in 22% of neurons. Ten percent of neurons changed their response from a RF shift to a directional preference under BMI. In these neurons, the observed effects could often be better explained by adaptation of excitation rather than inhibition. The results suggest, that adaptation of excitation, as well as cortex specific GABAergic inhibition, contribute to motion-direction sensitivity in the auditory cortex of the rufous horseshoe bat.

Interactions between ON and OFF signals in directional motion detectors feeding the not of the wallaby.

An apparent motion stimulus is used to probe the interactions between signals representing brightness increments (ON stimuli) and decrements (OFF stimuli) in the directional motion detectors forming the input to the nucleus of the optic tract (NOT) of the wallaby, Macropus eugenii. Direction-selective NOT neurons increase their firing rates during image motion from temporal-to-nasal over the contralateral eye (preferred direction) and their spontaneous activities are inhibited by motion in the opposite, anti-preferred direction. An apparent motion stimulus, consisting of neighboring vertical bars, where the brightness can be manipulated independently, also produces directional responses. Preferred direction sequences of brightness changes of like polarities (ON-ON or OFF-OFF) produce increased firing rates while sequences of opposite polarities (ON-OFF or OFF-ON) in the same direction produce relatively small excitatory responses or inhibit the spontaneous rate. For apparent motion in the anti-preferred direction, these directional properties are reversed, showing that signals for brightness increments and decrements provide inputs to the same motion detectors. There is no evidence for segregation of motion detectors into those receiving only half-wave rectified inputs. Interactions between ON and OFF signals utilize the sign of the incoming signals. An array of Reichardt-type motion detectors receiving inputs represented as positive and negative values for ON and OFF stimuli, respectively, are used to simulate the NOT responses. The brightness signals enter band-pass temporal filters prior to motion detection. By altering the time constants of these prefilters, it was possible to accurately simulate the time courses of each cell's responses.

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° in 45° steps, a single 90° step of a grating, and a single 90° step of a grating which contained a blank IFI and whose perceived direction was reversed. For the biasing sweep and the single 90° 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° 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.

Perceptual learning of apparent motion mediated through ON- and OFF-pathways in human vision

We document the performance of 26 human observers practicing a motion discrimination task in foveal vision. Two blocks of 1960 apparent motion stimuli each were presented in succession. Stimuli in the two blocks were tailored to activate either the ON-pathway (ON-stimulus) or the OFF-pathway (OFF-stimulus). Initial performance of about half of the subjects was rather weak but improved with practice. Initial performance of the other subjects remained unaffected for the first block. Once performance had improved in one of the tasks it transferred to the other tasks. Improvement in performance to the ON-stimulus was found to extend over many more presentations than that to the OFF-stimulus.

Adaptive Eye Movements Induced by Cross-axis PursuitVestibular Interactions in Trained Monkeys

We showed previously that smooth pursuit training combined with whole-body rotation in the orthogonal plane induces adaptive cross-axis vestibulo-ocular reflex (VOR). To gain an insight into the possible pathways and the nature of error signals for cross-axis VOR adaptation, we examined further properties of adaptive responses. In the first series, we trained monkeys for vertical pursuit during sinusoidal yaw rotation at 0.5 Hz ( - 10°) by presenting a target spot either in phase with, or with phase shifts (lead or lag) of 90° to, the chair for 1 h. After training, sinusoidal or trapezoidal yaw rotation was tested in complete darkness without a target. Different training conditions resulted in different amounts of phase shift in cross-axis VOR. Trapezoidal yaw rotation (peak acceleration , 780°/s 2 ) revealed further differences in the direction, latency and time course of the adaptive responses depending on the conditions of the pursuit task. At least two (fast and slow) components with different latencies were induced in the cross-axis VOR by trapezoidal rotation after in-phase and phase-shift training. Adaptive responses were accurately simulated by the weighted sum of these two components. In the second series, we examined the effects of sequentially flashed (10 w s) targets in the horizontal plane during pitch rotation. The monkeys learned to track such targets by smooth pursuit, and cross-axis VOR was also induced after such apparent motion stimuli without retinal slip of the target image. These results indicate the importance of eye velocity for cross-axis VOR and suggest that this adaptation occurs most probably in the smooth pursuit pathways.

Evidence for multistability in the visual perception of pigeons

Perceptual multistability refers to cases where perception alternates between two or more interpretations of an unchanging sensory stimulus. In a first experiment we trained eight pigeons to discriminate horizontal and vertical apparent motion stimuli and then presented a multistable display. In five cases their behavior showed alternations similar to human experiments. In a second experiment we varied the aspect ratio of the display in order to support the hypothesis of a percept-driven nature of the switching behavior. The pecking rates and mean phase durations varied as predicted. This is the first evidence of visual multistability in animals confronted with classical ambiguous figures. The data suggest a stochastic mechanism.

Direction biasing by brief apparent motion stimuli

The perceived direction of a motion step (probe stimulus) can be influenced by an earlier motion step or a brief motion sweep containing a series of steps (biasing stimulus). Depending upon experimental conditions, the biasing of the direction of the probe step (a phase shift of 180°± Φ) by a biasing stimulus which precedes it by approximately 250 ms can either increase (positive filter biasing) or decrease (negative filter biasing) the tendency to see the probe move in the biasing direction as computed with a motion filter with a biphasic temporal impulse response. In a series of experiments it was found that biasing motions traversing 90° of phase angle in fewer than six steps in less than 100 ms produced positive filter biasing. Also, biasing of the probe direction could be dissociated from the consciously reported direction of the biasing stimulus, and it did not occur when the probe preceded rather than followed the biasing stimulus. A biasing sweep containing more than six steps traversing 90° or a sweep traversing 270° produced negative filter biasing. Perceptual fusion of the steps of the sweep was not a necessary condition for obtaining negative filter biasing. In general, the negative filter biasing effects were found to be the most pervasive for the conditions investigated, and they are suggestive of a direction-specific, adaptation-like (gain-control) process in first-order motion filters. The exception to the negative biasing rule was found only with biasing stimuli which were short in duration or distance spanned.

Perception of apparent motion is related to the neural activity in the human extrastriate cortex as measured by magnetoencephalography

To determine the neural correlate of apparent motion perception, we measured magnetic responses to visual stimuli in apparent motion and compared the results with subjective rating of the quality of perceived motion with varied stimulus timing. The latency of the magnetic response was about 150 ms, and its origin was estimated to be in the occipito-parieto-temporal junction. The strength of the first component in the response varied with the stimulus timing, the maximum value being at the interval 0. The change could not be explained by the simple summation of onset and offset responses and this value was related to the subjective rating of quality (smoothness) of motion measured of the stimulus. Results indicate there is a localized cortical region of neural activity which is closely related to the subjective assessment of quality of perceived motion.

Source localization of visually evoked magnetic fields to stimuli in apparent motion

When at least two spatially separated visual stimuli are delivered sequentially under appropriate temporal conditions, a continuous moving sensation, which is called apparent motion, can be perceived. To investigate the cortical activation associated with visual apparent motion, brain evoked magnetic fields in response to visual half field apparent motion stimuli were measured using a whole-head magnetometer. Peak magnetic fields were observed in the latency range of 170-190 msec after the stimulus onset. The neural sources in this epoch were estimated to lie in the border between occipital and temporal areas in the contralateral hemisphere, which corresponds to the extrastriate visual cortex, including the middle temporal area. The results suggest that the activities in the extrastriate cortex reflect apparent motion perception.