Jeffrey M. Zacks

Coherent spontaneous activity accounts for trial-to-trial variability in human evoked brain responses.

Trial-to-trial variability in the blood oxygen level–dependent (BOLD) response of functional magnetic resonance imaging has been shown to be relevant to human perception and behavior, but the sources of this variability remain unknown. We demonstrate that coherent spontaneous fluctuations in human brain activity account for a significant fraction of the variability in measured event-related BOLD responses and that spontaneous and task-related activity are linearly superimposed in the human brain.

Perceiving, remembering, and communicating structure in events.

How do people perceive routine events, such as making a bed, as these events unfold in time? Research on knowledge structures suggests that people conceive of events as goal-directed partonomic hierarchies. Here, participants segmented videos of events into coarse and fine units on separate viewings; some described the activity of each unit as well. Both segmentation and descriptions support the hierarchical bias hypothesis in event perception: Observers spontaneously encoded the events in terms of partonomic hierarchies. Hierarchical organization was strengthened by simultaneous description and, to a weaker extent, by familiarity. Describing from memory rather than perception yielded fewer units but did not alter the qualitative nature of the descriptions. Although the descriptions were telegraphic and without communicative intent, their hierarchical structure was evident to naive readers. The data suggest that cognitive schemata mediate between perceptual and functional information about events and indicate that these knowledge structures may be organized around object/action units.

Human brain activity time-locked to perceptual event boundaries

Temporal structure has a major role in human understanding of everyday events. Observers are able to segment ongoing activity into temporal parts and sub-parts that are reliable, meaningful and correlated with ecologically relevant features of the action. Here we present evidence that a network of brain regions is tuned to perceptually salient event boundaries, both during intentional event segmentation and during naive passive viewing of events. Activity within this network may provide a basis for parsing the temporally evolving environment into meaningful units.

Segmentation in the perception and memory of events

People make sense of continuous streams of observed behavior in part by segmenting them into events. Event segmentation seems to be an ongoing component of everyday perception. Events are segmented simultaneously at multiple timescales, and are grouped hierarchically. Activity in brain regions including the posterior temporal and parietal cortex and lateral frontal cortex increases transiently at event boundaries. The parsing of ongoing activity into events is related to the updating of working memory, to the contents of long-term memory, and to the learning of new procedures. Event segmentation might arise as a side effect of an adaptive mechanism that integrates information over the recent past to improve predictions about the near future.

Imagined transformations of bodies : an fMRI investigation

A number of spatial reasoning problems can be solved by performing an imagined transformation of ones egocentric perspective. A series of experiments were carried out to characterize this process behaviorally and in terms of its brain basis, using functional magnetic resonance imaging (tMRI). In a task contrast designed to isolate egocentric perspective transformations, participants were slower to make left-right judgments about a human figure from the figures perspective than from their own. This transformation led to increased cortical activity around the left parietal-temporal-occipital junction, as well as in other areas including left frontal cortex. In a second task contrast comparing judgments about inverted figures to judgments about upright figures (always from the figures perspective), participants were slower to make left-right judgments about inverted figures than upright ones. This transformation led to activation in posterior areas near those active in the first experiment, but weaker in the left hemisphere and stronger in the right, and also to substantial left frontal activation. Together, the data support the specialization of areas near the parietal-temporal-occipital junction for egocentric perspective transformations. These results are also suggestive of a dissociation between egocentric perspective transformations and object-based spatial transformations such as mental rotation.

Bars and Lines: A Study of Graphic Communication

Interpretations of graphs seem to be rooted in principles of cognitive naturalness and information processing rather than arbitrary correspondences. These predict that people should more readily associate bars with discrete comparisons between data points because bars are discrete entities and facilitate point estimates. They should more readily associate lines with trends because lines connect discrete entities and directly represent slope. The predictions were supported in three experiments—two examining comprehension and one production. The correspondence does not seem to depend on explicit knowledge of rules. Instead, it may reflect the influence of the communicative situation as well as the perceptual properties of graphs.

Reading Stories Activates Neural Representations of Visual and Motor Experiences

To understand and remember stories, readers integrate their knowledge of the world with information in the text. Here we present functional neuroimaging evidence that neural systems track changes in the situation described by a story. Different brain regions track different aspects of a story, such as a characters physical location or current goals. Some of these regions mirror those involved when people perform, imagine, or observe similar real-world activities. These results support the view that readers understand a story by simulating the events in the story world and updating their simulation when features of that world change.

Transformations of Visuospatial Images

Transformations of visuospatial mental images are important for action, navigation, and reasoning. They depend on representations in multiple spatial reference frames, implemented in the posterior parietal cortex and other brain regions. The multiple systems framework proposes that different transformations can be distinguished in terms of which spatial reference frame is updated. In an object-based transformation, the reference frame of an object moves relative to those of the observer and the environment. In a perspective transformation, the observers egocentric reference frame moves relative to those of the environment and of salient objects. These two types of spatial reference frame updating rely on distinct neural processing resources in the parietal, occipital, and temporal cortex. They are characterized by different behavioral patterns and unique individual differences. Both object-based transformations and perspective transformations interact with posterior frontal cortical regions subserving the simulation of body movements. These interactions indicate that multiple systems coordinate to support everyday spatial problem solving.

Mental spatial transformations of objects and perspective

This study sought evidence for the independenceof two classes of mental spatialtransformation: object-based spatialtransformations and egocentric perspectivetransformations. Two tasks were designed toselectively elicit these two transformationsusing the same materials, participants, andtask parameters: one required same-differentjudgments about pairs of pictures, while theother required left-right judgments aboutsingle pictures. For pictures of human bodies,the two tasks showed strikingly differentpatterns of response time as a function ofstimulus orientation. Moreover, acrossindividuals, the two tasks had differentrelationships to psychometric tests of spatialability. The chronometric and individualdifference data converge withneuropsychological and neuroimaging data insuggesting that different mental spatialtransformations are performed by dissociableneural systems.

Imagined Viewer and Object Rotations Dissociated with Event-Related fMRI

Human spatial reasoning may depend in part on two dissociable types of mental image transformations: objectbased transformations, in which an object is imagined to move in space relative to the viewer and the environment, and perspective transformations, in which the viewer imagines the scene from a different vantage point. This study measured local brain activity with event-related fMRI while participants were instructed to either imagine an array of objects rotating (an object-based transformation) or imagine themselves rotating around the array (a perspective transformation). Object-based transformations led to selective increases in right parietal cortex and decreases in left parietal cortex, whereas perspective transformations led to selective increases in left temporal cortex. These results argue against the view that mental image transformations are performed by a unitary neural processing system, and they suggest that different overlapping systems are engaged for different image transformations.