Lucia M. Vaina

Functional neuroanatomy of biological motion perception in humans

We used whole brain functional MRI to investigate the neural network specifically engaged in the recognition of “biological motion” defined by point-lights attached to the major joints and head of a human walker. To examine the specificity of brain regions responsive to biological motion, brain activations obtained during a “walker vs. non-walker” discrimination task were compared with those elicited by two other tasks: (i) non-rigid motion (NRM), involving the discrimination of overall motion direction in the same “point-lights” display, and (ii) face-gender discrimination, involving the discrimination of gender in briefly presented photographs of men and women. Brain activity specific to “biological motion” recognition arose in the lateral cerebellum and in a region in the lateral occipital cortex presumably corresponding to the area KO previously shown to be particularly sensitive to kinetic contours. Additional areas significantly activated during the biological motion recognition task involved both, dorsal and ventral extrastriate cortical regions. In the ventral regions both face-gender discrimination and biological motion recognition elicited activation in the lingual and fusiform gyri and in the Brodmann areas 22 and 38 in superior temporal sulcus (STS). Along the dorsal pathway, both biological motion recognition and non-rigid direction discrimination gave rise to strong responses in several known motion sensitive areas. These included Brodmann areas 19/37, the inferior (Brodmann Area 39), and superior parietal lobule (Brodmann Area 7). Thus, we conjecture that, whereas face (and form) stimuli activate primarily the ventral system and motion stimuli primarily the dorsal system, recognition of biological motion stimuli may activate both systems as well as their confluence in STS. This hypothesis is consistent with our findings in stroke patients, with unilateral brain lesions involving at least one of these areas, who, although correctly reporting the direction of the point-light walker, fail on the biological motion task.

Representation and Recognition of the Movements of Shapes

The problems posed by the representation and recognition of the movements of 3-D shapes are analysed. A representation is proposed for the movements of shapes that lie within the scope of the Marr & Nishihara (1978) 3-D model representation of static shapes. The basic problem is how to segment a stream of movement into pieces, each of which can be described separately. The representation proposed here is based upon segmenting a movement at moments when a component axis, e. g. an arm, starts to move relative to its local coordinate frame (here the torso). For example, walking is divided into a segment of the stationary states between each swing of the arms and legs, and the actual motions between the stationary points (relative to the torso, not the ground). This representation is called the state─motion─state (SMS) moving shape representation, and several examples of its application are given.

Mapping the Signal-To-Noise-Ratios of Cortical Sources in Magnetoencephalography and Electroencephalography

Although magnetoencephalography (MEG) and electroencephalography (EEG) have been available for decades, their relative merits are still debated. We examined regional differences in signal‐to‐noise‐ratios (SNRs) of cortical sources in MEG and EEG. Data from four subjects were used to simulate focal and extended sources located on the cortical surface reconstructed from high‐resolution magnetic resonance images. The SNR maps for MEG and EEG were found to be complementary. The SNR of deep sources was larger in EEG than in MEG, whereas the opposite was typically the case for superficial sources. Overall, the SNR maps were more uniform for EEG than for MEG. When using a noise model based on uniformly distributed random sources on the cortex, the SNR in MEG was found to be underestimated, compared with the maps obtained with noise estimated from actual recorded MEG and EEG data. With extended sources, the total area of cortex in which the SNR was higher in EEG than in MEG was larger than with focal sources. Clinically, SNR maps in a patient explained differential sensitivity of MEG and EEG in detecting epileptic activity. Our results emphasize the benefits of recording MEG and EEG simultaneously. Hum Brain Mapp 2009.

Selective impairment of visual motion interpretation following lesions of the right occipito-parietal area in humans

A group of eighteen patients selected on the basis of the anatomical locus of the lesion, normal visual acuity and the ability to discriminate visual motion, were assessed on the perception of Julesz Random-Dot stereograms and on three tasks of visual motion interpretation: Speed Discrimination, 3D-Structure-from-Motion and 2D-Form-from-motion. The results on these experimental tasks demonstrate a double dissociation of deficits on the visual analysis of motion and stereopsis in the patients with lesions to the posterior right hemisphere. The right occipital-parietal (ROP) group failed on the Stereopsis task and showed a dramatic impairment on the Speed Comparison and on the Structure-from-Motion experiments. They performed in the normal range, however, on the 2D-Form-from-Motion task. The right occipital-temporal (ROT) group, on the other hand, were severely impaired on the identification of two dimensional forms from motion or stereopsis. In both cases, however, they were able to obtain a coarse segregation of the figure from the background. The ROT group did not present significant deficits on the Speed Discrimination and the Structure-from-Motion tasks. The results are discussed in the light of recent physiological and psychophysical findings, and it is hypothesized that, in the human brain, visual deficits ofmotion interpretation and ofstereopsis are associated with right occipital-parietal lesions.

Impairment of the perception of second order motion but not first order motion in a patient with unilateral focal brain damage

Unlike first order motion, which is based on spatiotemporal variations in luminance, second-order motion relies on spatiotemporal variation of attributes derived from luminance, such as contrast. Here we show that a patient with a small unilateral cortical lesion adjacent to human cortical area MT (V5) has an apparently permanent disorder in perceiving several forms of second-order but not first-order motion in his contralateral visual field. This result indicates that separate pathways for motion perception exist, either as divergent pathways from area MT or even from prim ary visual cortex, or as separate pathways from subcortical areas to extrastriate visual areas.

Optic flow and beyond

Optic flow provides all the information necessary to guide a walking human or a mobile robot to its target. Over the past 50 years, a body of research on optic flow spanning the disciplines of neurophysiology, psychophysics, experimental psychology, brain imaging and computational modelling has accumulated. Today, when we survey the field, we find independent lines of research have now converged and many arguments have been resolved; simultaneously the underpinning assumptions of flow theory are being questioned and alternative accounts of the visual guidance of locomotion proposed. At this critical juncture, this volume offers a timely review of what has been learnt and pointers to where the field is going.

Large receptive fields for optic flow detection in humans

We used a psychophysical summation technique to study the properties of detectors tuned to radial, circular and translational motion, and to determine the spatial extent of their receptive fields. Signal-to-noise motion thresholds were measured for patterns curtailed spatially in various ways. Sensitivity for radial, circular and translational motion increased with stimulus area at a rate predicted by an ideal integrator. When sectors of noise were added to the stimulus, sensitivity decreased at a rate consistent with an ideal integrator. Summation was tested for large annular stimuli, and shown to hold up to 70 degrees in some cases, suggesting very large receptive fields for this type of motion (consistent with the physiology of neurones in the dorsal region of the medial superior temporal area (MSTd)). This is a far greater area than observed for summation of contrast sensitivity to gratings (Anderson SJ and Burr DC, Vis Res 1987;29:621-635, and to this type of stimuli (Morrone MC, Burr DC and Vaina LM, Nature 1995;376:507-509, consistent with the suggestion that the two techniques examine different levels of motion analysis.

The selective impairment of the perception of first-order motion by unilateral cortical brain damage

First-order (Fourier) motion consists of stable spatiotemporal luminance variations. Second-order (non-Fourier) motion consists instead of spatiotemporal modulation of contrast, flicker, or spatial frequency. In spite of extensive psychophysical and computational analysis of the nature and relationship of these two types of motion, it remains unclear whether they are detected by the same mechanism or whether separate mechanisms are involved. Here we report the selective impairment of first-order motion, on a range of local and global motion tasks, in the contralateral visual hemifield of a patient with unilateral brain damage centered on putative visual areas V2 and V3 in the medial part of the occipital lobe. His perception of second-order motion was unimpaired. As his disorder is the obverse of that reported after damage in the vicinity of human visual area MT (V5), the results support models of motion processing in which first- and second-order motion are, at least in part, computed separately at the extrastriate cortical level.

Visual areas involved in the perception of human movement from dynamic form analysis

The perception of biological motion combines the analysis of form and motion. However, patient observations by Vaina et al. and psychophysical experiments by Beintema and Lappe showed that humans could perceive human movements (a walker) without local image motion information. Here, we examine the specificity of brain regions responsive to a biological motion stimulus without local image motion, using functional magnetic resonance imaging. We used the stimulus from Beintema and Lappe and compared the brain activity with a point-light display that does contain local motion information and was often used in previous studies. Recent imaging studies have identified areas sensitive to biological motion in both the motion-processing and the form-processing pathways of the visual system. We find a similar neuronal network engaged in biological motion perception, but more strongly manifested in form-processing than in motion-processing areas, namely, fusiform-/occipital face area and extrastriate body area.

The perception and discrimination of speed in complex motion

Random dot kinematograms were used to simulate radial, rotational and spiral optic flow. The stimuli were designed so that, while dot speed increased linearly with distance from the centre of the display, the density of dots remained uniform throughout their presentation. In two experiments, subjects were required to perform a temporal 2AFC speed discrimination task. Experiment 1 measured the perceived speed of a range of optic flow patterns against a rotational comparison stimulus. Radial motions were found to appear faster than rotations by approximately 10%, with a smaller but significant effect for spirals. Experiment 2 measured discrimination thresholds for pairs of similar optic flow stimuli identical in all respects except mean speed. No consistent differences were observed between the speed discrimination thresholds of radial, rotational and spiral motions and a control stimulus with the same speed profile in which motion followed fixed random trajectories. The perceived speed results are interpreted in terms of a model satisfying constraints on motion-in-depth and object rigidity, while speed discrimination appears to be based upon the pooled responses of elementary motion detectors.