Craig S. Chapman

The effect of disrupting the human magnocellular pathway on global motion perception.

The purpose of this study was to demonstrate the effect of human magnocellular (M)-pathway disruption on global motion perception. Coherence thresholds for global motion direction discrimination in random dot patterns were determined at slow and moderate dot speeds: (1) after adaptation to full-field sinusoidal flicker or a steady gray field, and (2) on a red or a gray background. Adaptation to flicker and a red background increased motion coherence thresholds relative to the gray baseline conditions at both dot speeds. Physiological studies have shown that M cells in the retina and LGN are inhibited by red light and are a main contributor to flicker perception in monkeys. Therefore, our results suggest that interference with processing in the subcortical M pathway disrupts higher-level motion integration.

Missing in action: the effect of obstacle position and size on avoidance while reaching

When reaching to objects, our hand and arm rarely collide with non-target objects, even if our workspace is cluttered. The simplicity of these actions hides what must be a relatively sophisticated obstacle avoidance system. Recent studies on patients with optic ataxia and visual form agnosia have demonstrated that obstacle avoidance is an automatic process, likely governed by the dorsal stream (Schindler et al. 2004; Rice et al. 2006). The current study sought to quantify how normal participants react to changes in the size and position of non-target objects in and around their workspace. In the first experiment, 13 right-handed subjects performed reaches to a target strip in the presence of two non-target objects, which varied in depth and horizontal configuration. We found that objects with horizontal alignments that were asymmetric about midline created systematic deviations in reach trajectory away from midline, with participants seeming to maximize the distance away from the two objects. These deviations were significantly greater for objects nearer in depth and nearly disappeared when the objects were placed beyond the target strip. Accompanying this pattern of deviation were other significant obstructing effects whereby reaches were executed more slowly when objects were close in depth and close to the participants reaching (right) hand. In the second experiment, we varied the height of the two objects, as well as the depth. Object pairs were now both tall, both short, or one-short/one-tall. We replicated the significant depth effects of the first experiment, extending the finding to include sensitivity to the size of the objects. Here the obstructing effect caused by short objects was similar to tall objects when those objects were placed at the depth of the reach target, but less than the tall objects when placed at mid-reach. Taken together, these experiments suggest that humans possess a sophisticated obstacle avoidance system that is extremely sensitive and conservative in evaluating potential obstacles and adjusting the reach accordingly.

Activity patterns in the category-selective occipitotemporal cortex predict upcoming motor actions

Converging lines of evidence point to the occipitotemporal cortex (OTC) as a critical structure in visual perception. For instance, human functional magnetic resonance imaging (fMRI) has revealed a modular organisation of object‐selective, face‐selective, body‐selective and scene‐selective visual areas in the OTC, and disruptions to the processing within these regions, either in neuropsychological patients or through transcranial magnetic stimulation, can produce category‐specific deficits in visual recognition. Here we show, using fMRI and pattern classification methods, that the activity in the OTC also represents how stimuli will be interacted with by the body – a level of processing more traditionally associated with the preparatory activity in sensorimotor circuits of the brain. Combining functional mapping of different OTC areas with a real object‐directed delayed movement task, we found that the pre‐movement spatial activity patterns across the OTC could be used to predict both the action of an upcoming hand movement (grasping vs. reaching) and the effector (left hand vs. right hand) to be used. Interestingly, we were able to extract this wide range of predictive movement information even though nearly all OTC areas showed either baseline‐level or below baseline‐level activity prior to action onset. Our characterisation of different OTC areas according to the features of upcoming movements that they could predict also revealed a general gradient of effector‐to‐action‐dependent movement representations along the posterior–anterior OTC axis. These findings suggest that the ventral visual pathway, which is well known to be involved in object recognition and perceptual processing, plays a larger than previously expected role in preparing object‐directed hand actions.

“Real-time” obstacle avoidance in the absence of primary visual cortex

When we reach toward objects, we easily avoid potential obstacles located in the workspace. Previous studies suggest that obstacle avoidance relies on mechanisms in the dorsal visual stream in the posterior parietal cortex. One fundamental question that remains unanswered is where the visual inputs to these dorsal-stream mechanisms are coming from. Here, we provide compelling evidence that these mechanisms can operate in “real-time” without direct input from primary visual cortex (V1). In our first experiment, we used a reaching task to demonstrate that an individual with a dense left visual field hemianopia after damage to V1 remained strikingly sensitive to the position of unseen static obstacles placed in his blind field. Importantly, in a second experiment, we showed that his sensitivity to the same obstacles in his blind field was abolished when a short 2-s delay (without vision) was introduced before reach onset. These findings have far-reaching implications, not only for our understanding of the time constraints under which different visual pathways operate, but also in relation to how these seemingly “primitive” subcortical visual pathways can control complex everyday behavior without recourse to conscious vision.

One to Four, and Nothing More Nonconscious Parallel Individuation of Objects During Action Planning

Much of the current understanding about the capacity limits on the number of objects that can be simultaneously processed comes from studies of visual short-term memory, attention, and numerical cognition. Consistent reports suggest that, despite large variability in the perceptual tasks administered (e.g., object tracking, counting), a limit of three to four visual items can be independently processed in parallel. In the research reported here, we asked whether this limit also extends to the domain of action planning. Using a unique rapid visuomotor task and a novel analysis of reach trajectories, we demonstrated an upper limit to the number of targets that can be simultaneously encoded for action, a capacity limit that also turns out to be no more than three to four. Our findings suggest that conscious perceptual processing and nonconscious movement planning are constrained by a common underlying mechanism limited by the number of items that can be simultaneously represented.

To use or to move: goal-set modulates priming when grasping real tools

How we interact with objects depends on what we intend to do with them. In the current work, we show that priming and the kinematics of grasping depend on the goals of grasping, as well as the context in which tasks are presented. We asked participants to grasp familiar kitchen tools in order to either move them, grasp-to-move (GTM), or to demonstrate their common use, grasp-to-use (GTU). When tasks were blocked separately (Experiment 1), we found that priming was only evident for the GTU task. However, when tasks were presented in the same block of trials (Experiment 2), we observed priming for both tasks. Independent of priming, differences in kinematics and reaction times according to task were evident for both Experiments. Longer reaction times for the GTU task indicate more extensive planning, and differences in grasping reflect the characteristics of subsequent actions. Priming of real grasping is determined by task goals as well as task setting, both of which are likely to modulate how object features (affordances) are perceived and influence the planning of future actions.

Mental blocks: fMRI reveals top-down modulation of early visual cortex when obstacles interfere with grasp planning.

When grasping an object, the fingers, hand and arm rarely collide with other non-target objects in the workspace. Kinematic studies of neurological patients (Schindler et al., 2004) and healthy participants (Chapman and Goodale, 2010a) suggest that the location of potential obstacles and the degree of interference they pose are encoded by the dorsal visual stream during action planning. Here, we used a slow event-related paradigm in functional magnetic resonance imaging (fMRI) to examine the neural encoding of obstacles in normal participants. Fifteen right-handed participants grasped a square target object with a thumb-front or thumb-side wrist-posture with (1) no obstacle present, (2) an obstacle behind the target object (interfering with the thumb-front grasp), or (3) an obstacle beside the target object (interfering with the thumb-side grasp). Within a specified network of areas involved in planning, a group voxelwise analysis revealed that one area in the left posterior intraparietal sulcus (pIPS) and one in early visual cortex were modulated by the degree of obstacle interference, and that this modulation occurred prior to movement execution. Given previous reports of a functional link between IPS and early visual cortex, we suggest that the increasing activity in the IPS with obstacle interference provides the top-down signal to suppress the corresponding obstacle coding in early visual areas, where we observed that activity decreased with interference. This is the first concrete evidence that the planning of a grasping movement can modulate early visual cortex and provides a unifying framework for understanding the dual role played by the IPS in motor planning and attentional orienting.

Visual salience dominates early visuomotor competition in reaching behavior

In this study, we investigated whether visual salience influences the competition between potential targets during reach planning. Participants initiated rapid pointing movements toward multiple potential targets, with the final target being cued only after the reach was initiated. We manipulated visual salience by varying the luminance of potential targets. Across two separate experiments, we demonstrate that initial reach trajectories are directed toward more salient targets, even when there are twice as many targets (and therefore twice the likelihood of the final target appearing) on the opposite side of space. We also show that this salience bias is time-dependent, as evidenced by the return of spatially averaged reach trajectories when participants were given an additional 500-ms preview of the target display prior to the cue to move. This study shows both when and to what extent task-irrelevant luminance differences affect the planning of reaches to multiple potential targets.

Action plan co-optimization reveals the parallel encoding of competing reach movements.

Several influential cognitive theories propose that in situations affording more than one possible target of action, we prepare multiple competing movements before selecting one. Here we provide direct evidence for this provocative but largely untested idea and demonstrate why preparing multiple movements is computationally advantageous. Using a reaching task in which movements are initiated after one of two potential targets is cued, we show that the movement generated for the cued target borrows components of the movement that would have been required for the other, competing target. This interaction can only arise if multiple potential movements are fully specified in advance and we demonstrate that it reduces the time required to launch a given action plan. Our findings suggest that this co-optimization of motor plans is highly automatic and largely occurs outside conscious awareness.

Short-term motor plasticity revealed in a visuomotor decision-making task

Selecting and executing an action toward only one object in our complex environments presents the visuomotor system with a significant challenge. To overcome this problem, the motor system is thought to simultaneously encode multiple motor plans, which then compete for selection. The decision between motor plans is influenced both by incoming sensory information and previous experience-which itself is comprised of long-term (e.g. weeks, months) and recent (seconds, minutes, hours) information. In this study, we were interested in how the recent trial-to-trial visuomotor experience would be factored into upcoming movement decisions made between competing potential targets. To this aim, we used a unique rapid reaching task to investigate how reach trajectories would be spatially influenced by previous decisions. Our task required subjects to initiate speeded reaches toward multiple potential targets before one was cued in-flight. A novel statistical analysis of the reach trajectories revealed that in cases of target uncertainty, subjects initiated a spatially averaged trajectory toward the midpoint of potential target locations before correcting to the selected target location. Interestingly, when the same target location was consecutively cued, reaches were biased toward that location on the next trial and this effect accumulated across trials. Beyond providing supporting evidence that potential reach locations are encoded and compete in parallel, our results strongly suggest that this motor competition is biased by recent trial history.