According to the traditional inferential theory of perception, percepts of object motion or stationarity stem from an evaluation of afferent retinal signals (which encode image motion) with the help of extraretinal signals (which encode eye movements). According to direct perception theory, on the other hand, the percepts derive from retinally conveyed information only. Neither view is compatible with a perceptual phenomenon that occurs during visually induced sensations of ego motion (vection). A modified version of inferential theory yields a model in which the concept of extraretinal signals is replaced by that of reference signals, which do not encode how the eyes move in their orbits but how they move in space. Hence reference signals are produced not only during eye movements but also during ego motion (i.e., in response to vestibular stimulation and to retinal image flow, which may induce vection). The present theory describes the interface between self-motion and object-motion percepts. An experimental paradigm that allows quantitative measurement of the magnitude and gain of reference signals and the size of the just noticeable difference (JND) between retinal and reference signals reveals that the distinction between direct and inferential theories largely depends on: (1) a mistaken belief that perceptual veridicality is evidence that extraretinal information is not involved, and (2) a failure to distinguish between (the perception of) absolute object motion in space and relative motion of objects with respect to each other. The model corrects these errors, and provides a new, unified framework for interpreting many phenomena in the field of motion perception.

There is evidence that neurons in medial superior temporal area (MST) respond to rotation in depth of textured planes. MST neurons project to the ventral intraparietal area (VIP) and the question arises whether VIP neurons are responsive to rotation in depth as well. In the present study on awake monkeys, we have simulated movement of a flat board, covered with dots, by a computer. The two-dimensional images corresponded to the projection of structured planes rotating around a fronto-parallel axis. In the literature this stimulus is called fanning. Fanning effectively induced responses in VIP neurons. Most often the responses were nearly as strong as for translation, expansion/contraction, or rotation, indicating that there was no special sensitivity for rotation in depth. For neurons, sensitive to expansion, the response to fanning could often be explained by the positioning of the expanding part of the fanning stimulus over the area which was most responsive to expansion. For neurons which were direction selective to translation, the optimal direction of fanning was usually the same as the preferred direction for translation. It is concluded that VIP neurons may be sensitive to movement of structured planes but they are not specialized for the detection of such movement.

Although the neural location of the plaid motion coherence process is not precisely known, the middle temporal (MT) cortical area has been proposed as a likely candidate. This claim rests largely on the neurophysiological findings showing that in response to plaid stimuli, a subgroup of cells in area MT responds to the pattern direction, whereas cells in area V1 respond only to the directions of the component gratings. In Experiment 1, we report that the coherent motion of a plaid pattern can be completely abolished following adaptation to a grating which moves in the plaid direction and has the same spatial period as the plaid features (the so-called “blobs”). Interestingly, we find this phenomenon is monocular: monocular adaptation destroys plaid coherence in the exposed eye but leaves it unaffected in the other eye. Experiment 2 demonstrates that adaptation to a purely binocular (dichoptic) grating does not affect perceived plaid coherence. These data suggest several conclusions: (1) that the mechanism determining plaid coherence responds to the motion of plaid features, (2) that the coherence mechanism is monocular, and thus (3), that it is probably located at a relatively low level in the visual system and peripherally to the binocular mechanisms commonly presumed to underlie two-dimensional (2-D) motion perception. Experiment 3 examines the spatial tuning of the monocular coherence mechanism and our results suggest it is broadly tuned with a preference for lower spatial frequencies. In Experiment 4, we examine whether perceived plaid direction is determined by the motion of the grating components or the features. Our data strongly support a feature-based model.

A series of psychophysical tests examining early and later aspects of image-motion processing were conducted in a patient with bilateral lesions involving the posterior visual pathways, affecting the lateral parietal-temporal-occipital cortex and the underlying white matter (as shown by magnetic resonance imaging studies and confirmed by neuro-ophthalmological and neuropsychological examinations). Visual acuity, form discrimination, color, and contrast-sensitivity discrimination were normal whereas spatial localization, line bisection, depth, and binocular stereopsis were severely impaired. Performance on early motion tasks was very poor. These include seeing coherent motion in random noise (Newsome & Paré, 1988), speed discrimination, and seeing two-dimensional form from relative speed of motion. However, on higher-order motion tasks the patient was able to identify actions from the evolving pattern of dots placed at the joints of a human actor (Johansson, 1973) as well as discriminating three-dimensional structure of a cylinder from motion in a dynamic random-dot field. The pattern of these results is at odds with the hypothesis that precise metrical comparison of early motion measurements is necessary for higher-order “structure from motion” tasks.

Although gradient schemes for detecting the motion of images and measuring their velocities are commonly used in computer vision, and although there is increasing evidence to support the existence of such schemes in biological vision, little attention has been directed to suggesting how such computations might be realized by neural hardware. This paper proposes two simple models, consisting of physiologically realistic networks of neurons, that approximate the gradient scheme. Computer simulations demonstrate that the models measure the speed of an object or pattern independently of its structural properties.

We showed high-contrast, second-order motion stimuli to subjects whilst recording their horizontal eye movements. These stimuli were very poor at evoking optokinetic nystagmus. Smooth-pursuit eye movements and fixation were reduced by a masking band ±2.5 deg above and below an imaginary fixation point. First-order stimuli evoked vigorous optokinetic nystagmus (OKN) under identical conditions and also when matched for apparent contrast. These findings are discussed in terms of the site of detection of second-order motion.

Direction-selective neurons in cat striate cortex were tested with bar-shaped stimuli, sequentially flashed at spatially displaced positions chosen to elicit maximal direction selectivity. Temporally overlapping flash exposures of prolonged duration (400–1000 ms) were employed at a series of onset asynchronies to explore the nature of temporal tuning of the direction-selective mechanism. In most neurons studied, direction selectivity was found to be supported by a surprisingly broad range of stimulus onset asynchronies, which was greater for longer exposure durations. These findings imply the existence of a sustained input to the direction-selective mechanism, in spite of the relatively transient nature of most cortical neurons' step responses. A model is described to illustrate how different front-end temporal filters can affect the dependence of two-flash direction selectivity on stimulus onset asynchrony. The versions of the model which successfully predict the form of the observed responses are those which combine inputs from sustained and transient filters.

Measurements were made of the optimal velocity for drifting bar-shaped stimuli to excite striate cortex neurons of the cat. These data were compared to the optimal spatial and temporal frequencies of the same neurons, as determined with drifting sine-wave grating stimuli. A systematic relationship was revealed, whereby those neurons preferring higher velocities of bar motion also preferred lower spatial and higher temporal frequencies of gratings. The optimal bar velocity for a given neuron could be quantitatively predicted from the ratio of that neuron's optimal temporal frequency to its optimal spatial frequency.

We evaluated the subcortical pathways’ contribution to human adults’ horizontal OKN by using a method similar to that used previously with cats (Harris & Smith, 1990; Smith & Harris, 1991). Five normal adults viewed plaids composed of two drifting sinusoidal gratings arranged such that their individual directions of drift were 60 deg or more from the direction of coherent motion of the overall pattern. Physiological evidence indicates that under monocular viewing, nasalward coherent motion gives advantage to any crossed subcortical contribution while temporalward coherent motion minimizes it. We recorded horizontal eye movement by infrared reflection and asked subjects to report the perceived direction of motion.

During both binocular and monocular viewing, the direction of the slow phase of OKN fell closer to the direction of coherent movement than to that of the oriented components. Monocular viewing produced no nasal-temporal asymmetries in the influence of coherent motion on the direction of OKN. This suggests that in humans the influence of coherent motion is mediated primarily by cortical mechanisms and, unlike in cats, with little or no involvement of subcortical mechanisms in the generation of horizontal OKN.

The posterior cingulate cortex (PCC) is involved in higher order sensory and sensory-motor integration while the planum temporale/parietal operculum (PT/PO) junction takes part in auditory motion and vestibular processing. Both regions are activated during different types of visual stimulation. Here, we describe the response characteristics of the PCC and PT/PO to basic types of visual motion stimuli of different complexity (complex and simple coherent as well as incoherent motion). Functional magnetic resonance imaging (fMRI) was performed in 10 healthy subjects at 3 Tesla, whereby different moving dot stimuli (vertical, horizontal, rotational, radial, and random) were contrasted against a static dot pattern. All motion stimuli activated a distributed cortical network, including previously described motion-sensitive striate and extrastriate visual areas. Bilateral activations in the dorsal region of the PCC (dPCC) were evoked using coherent motion stimuli, irrespective of motion direction (vertical, horizontal, rotational, radial) with increasing activity and with higher complexity of the stimulus. In contrast, the PT/PO responded equally well to all of the different coherent motion types. Incoherent (random) motion yielded significantly less activation both in the dPCC and in the PT/PO area. These results suggest that the dPCC and the PT/PO take part in the processing of basic types of visual motion. However, in dPCC a possible effect of attentional modulation resulting in the higher activity evoked by the complex stimuli should also be considered. Further studies are warranted to incorporate these regions into the current model of the cortical motion processing network.

The present study examines whether illusory movement (IM) of a horizontal line, induced by a moving background (MB), influences line-bisection performance in normal subjects. The first experiment attempted to identify the speeds of MB that induce IM. We found that when speed is increased from 1.53° to 13.3°/sec, IM increases, but that with further speed increases, IM decreases. Leftward MB induces rightward IM, and vice versa. In the second experiment, we had subjects bisect lines at MB speeds that had been shown to induce IM in the first experiment. We found that leftward MB induced a rightward bias, and vice versa. We also found that there was a relationship between the magnitude of IM and the degree of bias. In the third experiment, by making the target line larger than the MB, we made the conditions where IM was presumably absent. Unlike the results of bisection performed with IM, subjects showed a bias in the direction of the MB. Overall, these experiments demonstrated that the perception of motion induces subjects to attend in the direction of movement. (JINS, 2005, 11, 881–888.)

Intense scrutiny has been focused on whether chromatic stimuli contribute to motion perception. The present study considers a related but different question: how does motion affect chromatic detection? Detection thresholds were measured for a disk that underwent a brief (13.3 ms) chromatic change in the L/(L+M) chromatic direction. The disk's presentation sequence and speed (0–16 deg/s) were manipulated. In the coherent presentation sequence, the disk moved smoothly along a circular path centered on the fixation point. In the random presentation sequence, the disk appeared randomly at positions along the circular path. In both types of sequences, the disk underwent a brief chromatic change midway through the temporal presentation sequence. Threshold was elevated in the coherent condition compared to the random condition, and threshold decreased with an increase in speed. The threshold elevation observed in the coherent presentation sequence can be accounted for by temporal integration. The decrease in threshold with an increase in speed can be accounted for by spatial integration. The results, therefore, can be explained by spatiotemporal integration, without invoking a neural mechanism specialized for motion.

One type of retinal ganglion cells prefers object motion in a particular direction. Neuronal mechanisms for the computation of motion direction are still unknown. We quantitatively mapped excitatory and inhibitory regions of receptive fields for directionally selective retinal ganglion cells in the Japanese quail, and found that the inhibitory regions are displaced about 1–3 deg toward the side where the null sweep starts, relative to the excitatory regions. Directional selectivity thus results from delayed transient suppression exerted by the nonconcentrically arranged inhibitory regions, and not by local directional inhibition as hypothesized by Barlow and Levick (1965).