Emily D. Grossman

Brain Areas Involved in Perception of Biological Motion

These experiments use functional magnetic resonance imaging (fMRI) to reveal neural activity uniquely associated with perception of biological motion. We isolated brain areas activated during the viewing of point-light figures, then compared those areas to regions known to be involved in coherent-motion perception and kinetic-boundary perception. Coherent motion activated a region matching previous reports of human MT/MST complex located on the temporo-parieto-occipital junction. Kinetic boundaries activated a region posterior and adjacent to human MT previously identified as the kinetic-occipital (KO) region or the lateral-occipital (LO) complex. The pattern of activation during viewing of biological motion was located within a small region on the ventral bank of the occipital extent of the superior-temporal sulcus (STS). This region is located lateral and anterior to human MT/MST, and anterior to KO. Among our observers, we localized this region more frequently in the right hemisphere than in the left. This was true regardless of whether the point-light figures were presented in the right or left hemifield. A small region in the medial cerebellum was also active when observers viewed biological-motion sequences. Consistent with earlier neuroimaging and single-unit studies, this pattern of results points to the existence of neural mechanisms specialized for analysis of the kinematics defining biological motion.

Brain Areas Active during Visual Perception of Biological Motion

Theories of vision posit that form and motion are represented by neural mechanisms segregated into functionally and anatomically distinct pathways. Using point-light animations of biological motion, we examine the extent to which form and motion pathways are mutually involved in perceiving figures depicted by the spatio-temporal integration of local motion components. Previous work discloses that viewing biological motion selectively activates a region on the posterior superior temporal sulcus (STSp). Here we report that the occipital and fusiform face areas (OFA and FFA) also contain neural signals capable of differentiating biological from nonbiological motion. EBA and LOC, although involved in perception of human form, do not contain neural signals selective for biological motion. Our results suggest that a network of distributed neural areas in the form and motion pathways underlie the perception of biological motion.

Brain activity evoked by inverted and imagined biological motion.

Previous imaging research has identified an area on the human posterior superior temporal sulcus (STS) activated upon viewing biological motion. The current experiments explore the relationship between neural activity within this region and perceptual experience. Biological motion perception is orientation dependent: inverting point-light animations make them more difficult to see. We measured activity levels within this region as observers viewed inverted point-light animations. We also measured neural activity while observers imagined biological motion and compared it to that measured while observers viewed the animations. In both experiments we found that the BOLD response was modulated with perceptual experience. Viewing inverted biological motion activated posterior STS more than scrambled motion, but less than upright biological motion. Mental imagery of biological motion was also sufficient to activate this region in most of our observers, but the level of activity was weaker than during actual viewing of the motion animations.

Repetitive TMS over posterior STS disrupts perception of biological motion

Biological motion perception, the recognition of human action depicted in sparse dot displays, is supported by a network of brain areas including the human posterior superior temporal sulcus (pSTS). We have used repetitive transcranial magnetic stimulation (rTMS) to temporarily disrupt cortical activity within the pSTS and subsequently measured sensitivity to biological motion. Sensitivity was measured for canonical (upright) point-light animations and for animations inverted 180 deg, a manipulation that renders biological motion more difficult to recognize. Observers were markedly less sensitive to upright biological motion following pSTS stimulation. In contrast, performance remained normal for inverted biological motion following pSTS stimulation, and normal for upright and inverted biological motion following stimulation over visual motion sensitive area MT+/V5. In connection with previous brain imaging results, our findings demonstrate that normal functioning of the posterior STS is required for intact perception of biological motion.

Concepts Are More than Percepts: The Case of Action Verbs

Several regions of the posterior-lateral-temporal cortex (PLTC) are reliably recruited when participants read or listen to action verbs, relative to other word and nonword types. This PLTC activation is generally interpreted as reflecting the retrieval of visual-motion features of actions. This interpretation supports the broader theory, that concepts are comprised of sensory–motor features. We investigated an alternative interpretation of the same activations: PLTC activity for action verbs reflects the retrieval of modality-independent representations of event concepts, or the grammatical types associated with them, i.e., verbs. During a functional magnetic resonance imaging scan, participants made semantic-relatedness judgments on word pairs varying in amount of visual-motion information. Replicating previous results, several PLTC regions showed higher responses to words that describe actions versus objects. However, we found that these PLTC regions did not overlap with visual-motion regions. Moreover, their response was higher for verbs than nouns, regardless of visual-motion features. For example, the response of the PLTC is equally high to action verbs (e.g., to run) and mental verbs (e.g., to think), and equally low to animal nouns (e.g., the cat) and inanimate natural kind nouns (e.g., the rock). Thus, PLTC activity for action verbs might reflect the retrieval of event concepts, or the grammatical information associated with verbs. We conclude that concepts are abstracted away from sensory–motor experience and organized according to conceptual properties.

Perception of coherent motion, biological motion and form-from-motion under dim-light conditions

Three experiments investigated several aspects of motion perception at high and low luminance levels. Detection of weak coherent motion in random dot cinematograms was unaffected by light level over a range of dot speeds. The ability to judge form from motion was, however, impaired at low light levels, as was the ability to discriminate normal from phase-scrambled biological motion sequences. The difficulty distinguishing differential motions may be explained by increased spatial pooling at low light levels.

fMR-adaptation reveals invariant coding of biological motion on the human STS

Neuroimaging studies of biological motion perception have found a network of coordinated brain areas, the hub of which appears to be the human posterior superior temporal sulcus (STSp). Understanding the functional role of the STSp requires characterizing the response tuning of neuronal populations underlying the BOLD response. Thus far our understanding of these response properties comes from single-unit studies of the monkey anterior STS, which has individual neurons tuned to body actions, with a small population invariant to changes in viewpoint, position and size of the action being viewed. To measure for homologous functional properties on the human STS, we used fMR-adaptation to investigate action, position and size invariance. Observers viewed pairs of point-light animations depicting human actions that were either identical, differed in the action depicted, locally scrambled, or differed in the viewing perspective, the position or the size. While extrastriate hMT+ had neural signals indicative of viewpoint specificity, the human STS adapted for all of these changes, as compared to viewing two different actions. Similar findings were observed in more posterior brain areas also implicated in action recognition. Our findings are evidence for viewpoint invariance in the human STS and related brain areas, with the implication that actions are abstracted into object-centered representations during visual analysis.

Necessary but not sufficient: Motion perception is required for perceiving biological motion

Researchers have argued that biological motion perception from point-light animations is resolved from stationary form information. To determine whether motion is required for biological motion perception, we measured discrimination thresholds at isoluminance. Whereas simple direction discriminations falter at isoluminance, biological motion perception fails entirely. However, when performance is measured as a function of contrast, it is apparent that biological motion is contrast-dependent, while direction discriminations are contrast invariant. Our results are evidence that biological motion perception requires intact motion perception, but is also mediated by a secondary mechanism that may be the integration of form and motion, or the computation of higher-order motion cues.

Differential activation patterns of occipital and prefrontal cortices during motion processing: Evidence from normal and schizophrenic brains

Visual motion perception is normally mediated by neural processing in the posterior cortex. Focal damage to the middle temporal area (MT), a posterior extrastriate region, induces motion perception impairment. It is unclear, however, how more broadly distributed cortical dysfunction affects this visual behavior and its neural substrates. Schizophrenia manifests itself in a variety of behavioral and perceptual abnormalities that have proved difficult to understand through a dysfunction of any single brain system. One of these perceptual abnormalities involves impaired motion perception. Motion processing provides an opportunity to clarify the roles of multiple cortical networks in both healthy and schizophrenic brains. Using fMRI, we measured cortical activation while participants performed two visual motion tasks (direction discrimination and speed discrimination) and one nonmotion task (contrast discrimination). Normal controls showed robust cortical activation (BOLD signal changes) in MT during the direction and speed discrimination tasks, documenting primary processing of sensory input in this posterior region. In patients with schizophrenia, cortical activation was significantly reduced in MT and significantly increased in the inferior convexity of the prefrontal cortex, an area that is normally involved in higher level cognitive processing. This shift in cortical responses from posterior to prefrontal regions suggests that motion perception in schizophrenia is associated with both deficient sensory processing and compensatory cognitive processing. Furthermore, this result provides evidence that in the context of broadly distributed cortical dysfunction, the usual functional specificity of the cortex becomes modified, even across the domains of sensory and cognitive processing.

Diagnostic spatial frequencies and human efficiency for discriminating actions

Humans extract visual information from the world through spatial frequency (SF) channels that are sensitive to different scales of light-dark fluctuations across visual space. Using two methods, we measured human SF tuning for discriminating videos of human actions (walking, running, skipping and jumping). The first, more traditional, approach measured signal-to-noise ratio (s/n) thresholds for videos filtered by one of six Gaussian band-pass filters ranging from 4 to 128 cycles/image. The second approach used SF “bubbles”, Willenbockel et al. (Journal of Experimental Psychology. Human Perception and Performance, 36(1), 122–135, 2010), which randomly filters the entire SF domain on each trial and uses reverse correlation to estimate SF tuning. Results from both methods were consistent and revealed a diagnostic SF band centered between 12-16 cycles/image (about 1-1.25 cycles/body width). Efficiency on this task was estimated by comparing s/n thresholds for humans to an ideal observer, and was estimated to be quite low (>.04%) for both experiments.