Recovery of fMRI activation in motion area MT following storage of the motion aftereffect

We used functional magnetic resonance imaging (fMRI) during storage of the motion aftereffect (MAE) to examine the relationship between motion perception and neural activity in the human cortical motion complex MT+ (including area MT and adjacent motion-selective cortex). MT+ responds not only to physical motion but also to illusory motion, as in the MAE when subjects who have adapted to continuous motion report that a subsequent stationary test stimulus appears to move in the opposite direction. In the phenomenon of storage, the total decay time of the MAE is extended by inserting a dark period between adaptation and test phases. That is, when the static test pattern is presented after a storage period equal in duration to the normal MAE, the illusory motion reappears for almost as long as the original effect despite the delay. We examined fMRI activation in MT+ during and after storage. Seven subjects viewed continuous motion, followed either by an undelayed stationary test (immediate MAE) or by a completely dark storage interval preceding the test (stored MAE). Like the perceptual effect, activity in MT+ dropped during the storage interval then rebounded to reach a level much higher than after the same delay without storage. Although MT+ activity was slightly enhanced during the storage period following adaptation to continuous motion (compared with a control sequence in which the adaptation grating oscillated and no MAE was perceived), this enhancement was much less than that observed during the perceptual phenomenon. These results indicate that following adaptation, activity in MT+ is pronounced only with the presentation of an appropriate visual stimulus, during which the MAE is perceived.     

Reduction of a pattern-induced motion aftereffect by binocular rivalry suggests the involvement of extrastriate mechanisms

Previous research suggests that plaid-induced motion aftereffects (MAEs) involve extrastriate mechanisms (Wenderoth et al., 1988). There is evidence also that binocular rivalry occurs beyond V1 and that it disrupts the processing of MAEs which are believed to be based upon extrastriate mechanisms (e.g. the spiral MAE) but not MAEs, such as linear MAE induced by a drifting grating, which are thought to arise in striate cortex (Wiesenfelder & Blake, 1990). The logical inference is that binocular rivalry during drifting plaid-induced adaptation should reduce the MAEs which result. We report experiments which confirm this prediction.     

Different mechanisms underlie three inhibitory phenomena in cat area 17

Recently, it has been proposed that all suppressive phenomena observed in the primary visual cortex (V1) are mediated by a single mechanism, involving inhibition by pools of neurons, which, between them, represent a wide range of stimulus specificities. The strength of such inhibition would depend on the stimulus that produces it (particularly its contrast) rather than on the firing rate of the inhibited cell. We tested this hypothesis by measuring contrast-response functions (CRFs) of neurons in cat V1 for stimulation of the classical receptive field of the dominant eye with an optimal grating alone, and in the presence of inhibition caused by (1) a superimposed orthogonal grating (cross-orientation inhibition); (2) a surrounding iso-oriented grating (surround inhibition); and (3) an orthogonal grating in the other eye (interocular suppression). We fitted hyperbolic ratio functions and found that the effect of cross-orientation inhibition was best described as a rightward shift of the CRF ('contrast-gain control'), while surround inhibition and interocular suppression were primarily characterised as downward shifts of the CRF ('response-gain control'). However, the latter also showed a component of contrast-gain control. The two modes of suppression were differently distributed between the layers of cortex. Response-gain control prevailed in layer 4, whereas cells in layers 2/3, 5 and 6 mainly showed contrast-gain control. As in human observers, surround gratings caused suppression when the central grating was of high contrast, but in over a third of the cells tested, enhanced responses for low-contrast central stimuli, hence actually decreasing threshold contrast.     

Directions of motion after-effects induced by gratings and plaids

In three experiments the direction of motion after-effect (MAE) is measured following adaptation to two gratings moving in different directions presented in alternation (component-induced MAEs: CMAEs), and to moving plaid patterns composed of superimposed pairs of these gratings (plaid-induced MAEs; PMAEs). These MAEs are compared to: (i) the vector sum direction of the component gratings; (ii) the IOC-predicted direction of the plaids; and (iii) the perceived direction of the plaids as reported by observers. Contrary to previous findings (Burke D, Wenderoth P. Vis Res 1993;33:351-9), directions of PMAEs are shown to approximate the vector sum direction of the components, whereas directions of CMAEs are shown to approximate the mean (unweighted) direction of the components. This difference is attributed to the activity, and adaptation, of an additional population of neurones whose stimulus), or a counterphase moving plaid (a combined Fourier and non-Fourier stimulus), rules out the possibility that the discrepancy between PMAE direction and actual plaid direction is due to the use of test stimuli that do not adequately reflect adaptation by the Fourier and non-Fourier components of the adapting plaids (HR, Ferrera VP, Yo C. Vis Neurosci 1992;9:79-97). Various explanations of this paradoxical result are discussed, including: (i) that MAEs produced by Fourier components out-weigh (and possibly even mask) MAEs produced by non-Fourier plaid components; (ii) PMAEs are influenced by adaptation of a population of component-selective neurones that do not contribute to plaid perception; and, (iii) PMAEs are influenced by component-specific adaptation effects that are weighted according to relative component sensitivity, rather than relative component speed (Pantle A. Vis Res 14;1974:1229-36). We review psychophysical and neurophysiological evidence consistent with these explanations.     

Contextual modulation of the motion aftereffect.

The authors examined center-surround effects for motion perception in human observers. The magnitude of the motion aftereffect (MAE) elicited by a drifting grating was measured with a nulling task and with a threshold elevation procedure. A surround grating of the same spatial frequency, temporal frequency, and orientation significantly reduced the magnitude of the MAE elicited by adaptation to the center grating. This effect was bandpass tuned for spatial frequency, orientation, and temporal frequency. Plaid surrounds but not contrast-modulated surrounds that moved in the same direction also reduced the MAE. These results provide psychophysical evidence for center-surround interactions analogous to those previously observed in electrophysiological studies of motion processing in primates. Collectively, these results suggest that motion processing, similar to texture processing, is organized for the purpose of highlighting regions of directional discontinuity in retinal images. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Conformational analysis of amphotericin B molecule

We examined the effects of adaptation and test contrasts on the duration of two types of motion aftereffect (MAE) that presumably reveal different levels of motion processing: MAE with a static test stimulus (static MAE), and that with a counterphasing test stimulus (flicker MAE). MAE duration increased with increasing adaptation contrast. When the test contrast was low, it increased rapidly, and saturated at a low adaptation contrast. When the test contrast was high, however, it gradually increased over a wide range of adaptation contrasts. These complex effects of stimulus contrasts could be well described by a dependency on adaptation contrast normalized by test contrast on a logarithmic axis. Little difference was found between the results for two types of MAE. The interaction between adaptation and test contrasts leads us to reject the idea that the shape of adaptation contrast dependency of MAE duration reflects that of the sensitivity function of motion detecting mechanisms. The results also suggest a functional similarity between the processes underlying static and flicker MAEs with regard to their responses to contrasts.     

Second-order motion perception in peripheral vision: limits of early filtering

Spatial and temporal analysis of contrast-modulated sine-wave gratings reveals that the second-order motion stimulus contains two sidebands, with equal energy but moving in opposite directions, flanking a stationary carrier. Any early linear spatial filtering process in the visual system that attenuates one sideband more than the other will be detrimental to the balance between the two sidebands, so that the perceived direction of the carrier might be opposite to that of the envelope motion. We tested this hypothesis by using contrast-modulated gratings presented centrally or at 20 deg in the horizontal nasal field with a two-alternative forced-choice staircase paradigm. We found that when the envelope frequency was close to that of the carrier, a second-order stimulus whose envelope motion direction was correctly identified in the fovea appeared to drift in the opposite direction in the periphery. Further increasing the envelope spatial frequency resulted in a reversed motion percept in both central and peripheral viewing conditions. For subjects to identify correctly the direction of motion of the envelope, the spatial frequency ratio of the carrier to the envelope had to be more than 2 in the fovea and more than 6 in the periphery. These phenomena in second-order motion perception can be explained by a linear model of motion detection with an early spatial filtering process. Further experiments and computer simulation show that undersampling of the carrier has little effect on second-order motion perception in the periphery, as long as the carrier is detectable.     

Temporal resolution of dichoptic and second-order motion mechanisms

We addressed the question of whether low-level motion analysers can integrate signals binocularly. We compared the temporal sensitivity in motion discrimination tasks using monocular and dichoptic first-order motion and monocular and dichoptic second-order motion. Three human observers were required to discriminate the direction of motion of either sinusoidal gratings (1 c/deg), used as a stimulus for first-order motion analysers, or the envelopes of contrast-modulated stationary sinusoidal gratings (carrier frequency 5 c/deg, carrier contrast 0.1, modulation frequency 1 c/deg), used as a stimulus for second-order motion analysers. Contrast sensitivity was measured as a function of temporal frequency. The moving grating or envelope was generated by summing two non-moving sinusoidally flickering gratings or envelopes in spatiotemporal quadrature. These were either combined monocularly or presented dichoptically. Sensitivity to the moving envelope was highest at a temporal frequency between 0.5 and 2 Hz, depending on the observer, and declined rapidly at high temporal frequencies. None of the observers was able to discriminate the direction of motion of envelopes moving faster than 4 Hz. Dichoptic and monocular presentation produced very similar results. Sensitivity to a monocularly presented moving grating was fairly uniform between 1 and 8 Hz, and declined slightly at 16 Hz. In one of three observers sensitivity to the dichoptically presented grating was very close to that of the monocularly presented grating at all temporal frequencies tested (from 1 to 16 Hz). All observers could discriminate the direction of motion of the dichoptically presented grating at 8 Hz, but two of the three were unable to discriminate its direction of motion at 16 Hz. These results indicate that second-order motion analysers have very poor temporal resolution and that dichoptic motion analysers have very good resolution. We suggest that this implies that there are low-level motion analysers that are capable of integrating information binocularly.     

Disparity tuning of the stereoscopic (cyclopean) motion aftereffect

Across five experiments this study investigated the disparity tuning of the stereoscopic motion aftereffect (adaptation from moving retinal disparity). Adapting and test stimuli were moving and stationary stereoscopic grating patterns, respectively, created from dynamic random-dot stereograms. Observers adapted to moving stereoscopic grating patterns presented with a given disparity and viewed stationary test patterns presented with the same or differing disparity to examine whether the motion aftereffect is disparity contingent. Across experiments aftereffect duration was greatest when adapting motion and test pattern both were presented with zero disparity and in the plane of fixation. Aftereffect declined as disparity of adapting motion and/or test pattern increased away from fixation, even under conditions in which depth position of adapt and test was equal. This argues against a relative depth separation explanation of the decline, and instead suggests that the amount of adaptable substrate decreases away from fixation.     

Spatial interactions in perceived speed

Previous research has shown that the perception of motion within a local region is influenced by other motions within neighboring areas (eg induced motion). Here, a study is reported of the perceived speed of dots moving within a circular target region, which was surrounded by other motions within a larger surrounding area. The perceived speed of the central dots was found to be fastest when the surround was stationary; it became slower as the speed of motion in the surround was increased. This decrease in the perceived target speed with increases in surround velocity occurred regardless of whether the direction in which the surround moved was the same as or opposite to the motion of the target region. This result cannot be explained by using simple models of perceived speed that depend only upon such factors as the magnitude of relative motion between center and surround. The spatial area over which these motion interactions occur was also investigated.     

Infant color vision: moving tritan stimuli do not elicit directionally appropriate eye movements in 2- and 4-month-olds

The purpose of the present study was to investigate the capacity of infants to code the direction of motion of moving tritan-modulated gratings. Infant and adult subjects were tested with 0.2 c/d sinusoidal gratings moving at a speed of 20 deg/sec. Three conditions were tested: luminance-modulated gratings, tritan-modulated gratings, and luminance- vs tritan-modulated gratings superimposed and moving in opposite directions in a chromatic motion nulling paradigm. Two-month-old infants were tested in all three conditions, while 4-month-olds were tested in only the first two conditions. For infant subjects, an adult observer reported the direction of the slow phase of the infant's eye movements; adult subjects judged the perceived direction of motion of the stimuli. Luminance-modulated gratings produced directionally appropriate eye movements (DEM) in all age groups. Tritan gratings presented alone did not produce DEM in either 2- or 4-month-olds, but did so in adults. Mean equivalent luminance contrasts were near zero in 2-month-olds, and small but reliably above zero in adults. In sum, the present study provides no evidence that infants can code the direction of motion of moving tritan gratings.     

Motion capture of luminance stimuli by equiluminous color gratings and by attentive tracking

Two experiments demonstrated motion capture of luminance-defined dots by gratings with no net luminance-based motion. In a series of two-frame experimental trials, we superimposed bright dots and a color grating rotating in opposite directions. Capture was observed at equiluminance and was facilitated by the presence of color in gratings over a range of luminance contrasts. In a second experiment, observers noted that when a counterphase grating was tracked in either direction with attention, the superimposed dots were captured in that direction. These results suggest motion capture is supported not only by luminance-based motion, but also by color- and attention-based motion. Indeed, we suggest that the most parsimonious explanation is that all capture is mediated by attention.     

What is the denominator for contrast normalisation?

The perceived speed of 1 c/deg sinusoidal gratings of contrast 0.02 was measured in the presence of high contrast (0.50) 1 c/deg sinusoidal gratings (called modifiers). The modifiers drifted or were counterphase modulated at various temporal frequencies. The presence of a modifier with temporal frequencies (0 and 3 Hz) lower than the low contrast moving grating decreased its perceived speed while the presence of modifiers with higher temporal frequencies (8, 12 and 16 Hz) increased its perceived speed. A modifier of the same temporal frequency (6 Hz) as the standard grating had no effect upon the perceived speed of the low contrast gratings. Moving modifiers are more effective than counterphase flickering modifiers in biasing the perceived speed of low contrast gratings if they move in the same direction as the test grating and less effective if they move in the opposite direction. Finally, a modifier presented in an annulus surrounding the test grating is more effective than a modifier presented in a circular patch above or below the test grating in raising the perceived speed of low contrast gratings. This suggests that perceived speed depends on the ratio of low and high temporal frequency signals averaged over a significant area of the visual field.     

Second-order motions contribute to vection

First- and second-order motions differ in their ability to induce motion aftereffects (MAEs) and the kinetic depth effect (KDE). To test whether second-order stimuli support computations relating to motion-in-depth we examined the vection illusion (illusory self motion induced by image flow) using a vection stimulus (V, expanding concentric rings) that depicted a linear path through a circular tunnel. The set of vection stimuli contained differing amounts of first- and second-order motion energy (ME). Subjects reported the duration of the perceived MAEs and the duration of their vection percept. In Experiment 1 both MAEs and vection durations were longest when the first-order (Fourier) components of V were present in the stimulus. In Experiment 2, V was multiplicatively combined with static noise carriers having different check sizes. The amount of first-order ME associated with V increases with check size. MAEs were found to increase with check size but vection durations were unaffected. In general MAEs depend on the amount of first-order ME present in the signal. Vection, on the other hand, appears to depend on a representation of image flow that combines first- and second-order ME.     

A note on formal learning theory

Previous investigations have challenged the generality of the claim that perceived motion in an effective stimulus for smooth pursuit eye movements. The experiments extend the scope of these investigations. Three experiments test the hypothesis that perceived motion can serve as the stimulus for pursuit when the eye movement does not generate constraining retinal error information. Observers viewed retinally stabilized displays that elicited the perception that a stationary target was moving or that a moving target was moving faster than it was actually moving. The results failed to confirm the hypothesis. Relevant literature is reviewed. We conclude that perceived movement can act as a stimulus for pursuit only when the "perceptual target" has no retinal counterpart.     

Reversed visual motion and self-sustaining eye oscillations

A random-dot field undergoing counterphase flicker paradoxically appears to move in the same direction as head and eye movements, i.e. opposite to the optic-flow field. The effect is robust and occurs over a wide range of flicker rates and pixel sizes. The phenomenon can be explained by reversed phi motion caused by apparent pixel movement between successive retinal images. The reversed motion provides a positive feedback control of the display, whereas under normal conditions retinal signals provide a negative feedback. This altered polarity invokes self-sustaining eye movements akin to involuntary optokinetic nystagmus.     

Position displacement, not velocity, is the cue to motion detection of second-order stimuli

Motion detection can be achieved either with mechanisms sensitive to a target's velocity, or sensitive to change in a target's position. Using a procedure to dissociate these two provided by Nakayama and Tyler (Vis Res 1981;21:427-433), we explored detection of first-order (luminance-based) and various second-order (texture-based and stereo-based) motion. In the first experiment, observers viewed annular gratings oscillating in rotational motion at various rates. For each oscillation temporal frequency, we determined the minimum displacement of the pattern for which observers could reliably see motion. For first-order motion, these motion detection thresholds decreased with increasing temporal frequency, and thus were determined by a minimum velocity. In contrast, motion detection thresholds for second-order motion remained roughly constant across temporal frequency, and thus were determined by a minimum displacement. In Experiment 2, luminance-based gratings of different contrasts were tested to show that the velocity-dependence was not an artifact of pattern visibility. In the remaining experiments, results similar to Experiment 1 were obtained with a central presentation of a linear grating, instead of an annular grating (Experiment 3), and with a motion discrimination (phase discrimination) rather than motion detection task (Experiment 4). We conclude that, within the ranges tested here, second-order motion is more readily detected with a mechanism which tracks the change of position of features over time.     

Perception of induced visual motion: Effects of relative position, shape, and size of the surround.

In the 1st of 3 experiments with 64 undergraduates, the induced motion perceived in a stationary central point of light was primarily determined by the movement of the outermost of 2 oppositely moving surrounds, regardless of surround shape. Exp II found that moving square surrounds were more effective than moving circular surrounds in generating induced motion. In Exp III, perceived motion of the stationary light was directly related to the size of the moving square surround. These results, which indicate that induced motion is a function of the relative position, shape, and size of the moving surround(s), may be due to changes in the observer's egocentric orientation and perception of straight ahead. (French abstract) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Cellular basis for the negative dromotropic effect of adenosine on rabbit single atrioventricular nodal cells

Detection and resolution of square patches of sinusoidal gratings were measured in central and peripheral vision (30 degrees horizontal temporal visual field) for high-contrast gratings as a function of the number of cycles in the stimulus. We determined performance in a forced-choice paradigm for a fixed number of stimulus cycles by arranging for stimulus diameter to vary inversely with spatial frequency. For both psychophysical tasks and for both target locations, the psychometric function relating performance to log spatial frequency shifted to higher frequencies without changing slope significantly as the number of cycles in the stimulus was increased. Thus the entire effect could be captured by an analysis of spatial acuity, which increased with increasing number of grating cycles over the range 0.5-6 cycles but remained constant over the range 6-14 cycles. In the central field, resolution acuity and detection acuity were equal regardless of the number of cycles in the stimulus. In the peripheral field, detection acuity exceeded resolution acuity and perceptual aliasing occurred for stimuli in the range 1-14 cycles. From this result we conclude that resolution acuity is sampling limited in the periphery, provided that the stimulus contains at least one full cycle of the grating. Essential features of the results could be accounted for by Fourier analysis of the stimulus.     

Perceptual depth synthesis in the visual system as revealed by selective adaptation.

Selective adaptation was used to determine the degree of interactions between channels processing relative depth from stereopsis, motion parallax, and texture. Monocular adaptations with motion parallax or binocular stationary adaptation caused test surfaces, viewed either stationary binocularly or monocularly with motion parallax, to appear to slant in the opposite direction compared with the slant initially adapted to. Monocular adaptations on frontoparallel surfaces covered with a pattern of texture gradients caused a subsequently viewed test surface, viewed either monocularly with motion parallax or stationary binocularly, to appear to slant in the opposite direction as the slant indicated by the texture in the adaptation condition. No aftereffect emerged in the monocular stationary test condition. A mechanism of independent channels for relative depth perception is dismissed in favor of a view of an asymmetrical interactive processing of different information sources. The results suggest asymmetrical inhibitory interactions among habituating slant detector units receiving inputs from static disparity, dynamic disparity, and texture gradients. (PsycINFO Database Record (c) 2010 APA, all rights reserved)