David A. Westwood

An evolving view of duplex vision: separate but interacting cortical pathways for perception and action

In 1992, Goodale and Milner proposed a division of labour in the visual pathways of the primate cerebral cortex between a dorsal stream specialised for the visual control of action and a ventral stream dedicated to the perception of the visual world. In the years since this original proposal, support for the perception-action hypothesis has come from neuroimaging experiments, human neuropsychology, monkey neurophysiology, and human psychophysical experiments. Indeed, some of the strongest support for this hypothesis has come from behavioural experiments showing that visually guided actions are largely refractory to perceptual illusions. Although controversial, the findings from this literature both support the original hypothesis and suggest important modifications. The ongoing challenge for neurobiologists is to map these behavioural findings onto their corresponding neural substrates.

Perceptual illusion and the real-time control of action.

Participants were cued by an auditory tone to grasp a target object from within a size-contrast display. The peak grip aperture was unaffected by the perceptual size illusion when the target array was visible between the response cue and movement onset (vision trials). The grasp was sensitive to the illusion, however, when the target array was occluded from view when the response was cued (occlusion trials). This was true when the occlusion occurred 2.5 s before the response cue (delay), but also when the occlusion coincided with the response cue (no-delay). Unlike previous experiments, vision and occlusion trials were presented in random sequence. The results suggest that dedicated, real-time visuomotor mechanisms are engaged for the control of action only after the response is cued, and only if the target is visible. These visuomotor mechanisms compute the absolute metrics of the target object and therefore resist size-contrast illusions. In other situations (e.g. prior to the response cue, or if the target is no longer visible), a perceptual representation of the target object can be used for action planning. Unlike the real-time visuomotor mechanisms, perception-based movement planning makes use of relational metrics, and is therefore sensitive to size-contrast illusions.

Two distinct modes of control for object-directed action.

There are multiple routes from vision to action that play a role in the production of visually guided reaching and grasping. What remain to be resolved, however, are the conditions under which these various routes are recruited in the generation of actions and the nature of the information they convey. We argue in this chapter that the production of real-time actions to visible targets depends on pathways that are separate from those mediating memory-driven actions. Furthermore, the transition from real-time to memory-driven control occurs as soon as the intended target is no longer visible. Real-time movements depend on pathways from the early visual areas through to relatively encapsulated visuomotor mechanisms in the dorsal stream. These dedicated visuomotor mechanisms, together with motor centers in the premotor cortex and brainstem, compute the absolute metrics of the target object and its position in the egocentric coordinates of the effector used to perform the action. Such real-time programming is essential for the production of accurate and efficient movements in a world where the location and disposition of a goal object with respect to the observer can change quickly and often unpredictably. In contrast, we argue that memory-driven actions make use of a perceptual representation of the target object generated by the ventral stream. Unlike the real-time visuomotor mechanisms, perception-based movement planning makes use of relational metrics and scene-based coordinates. Such computations make it possible, however, to plan and execute actions upon objects long after they have vanished from view.

A double dissociation between sensitivity to changes in object identity and object orientation in the ventral and dorsal visual streams : A human fMRI study

We used an event-related fMR-adaptation paradigm to investigate changes in BOLD activity in the dorsal and ventral visual streams as a function of object identity and object orientation. Participants viewed successive paired images of real-world, graspable objects, separated by a visual mask. The second image of each pair was either: (i) the same as the first image, (ii) different only in identity, (iii) different only in orientation, or (iv) different in both identity and orientation. A region in the parieto-occipital cortex (dorsal stream) showed a selective increase in BOLD activity with changes in object orientation, but was insensitive to changes in object identity. In contrast, a region in the temporo-occipital cortex (ventral stream) showed a selective increase in activity with changes in identity, but was insensitive to changes in orientation. The differential sensitivity to orientation and identity is consistent with the idea that the dorsal stream plays a critical role in the visual control of object-directed actions while the ventral stream plays a critical role in object perception.

The influence of visual motion on fast reaching movements to a stationary object

One of the most important functions of vision is to direct actions to objects. However, every time that vision is used to guide an action, retinal motion signals are produced by the movement of the eye and head as the person looks at the object or by the motion of other objects in the scene. To reach for the object accurately, the visuomotor system must separate information about the position of the stationary target from background retinal motion signals—a long-standing problem that is poorly understood. Here we show that the visuomotor system does not distinguish between these two information sources: when observers made fast reaching movements to a briefly presented stationary target, their hand shifted in a direction consistent with the motion of a distant and unrelated stimulus, a result contrary to most other findings. This can be seen early in the hands trajectory (∼120 ms) and occurs continuously from programming of the movement through to its execution. The visuomotor system might make use of the motion signals arising from eye and head movements to update the positions of targets rapidly and redirect the hand to compensate for body movements.

Delayed grasping of a Müller-Lyer figure.

Abstract. Grasping movements are more sensitive to the Müller-Lyer (ML) illusion when the response is made after a brief period of visual occlusion. It is unclear whether this effect is due to (1) the elimination of on-line visual feedback, or (2) reliance on a stored perceptual representation of the target for movement planning. Here participants grasped objects from within two forms of a ML figure in four visual conditions (full vision, open-loop, brief delay, and 2-s delay) and estimated object size in the full-vision condition. Peak grasping aperture was influenced by the ML figure in the full-vision condition, although to a much smaller extent than was true for manual size estimation. The effect of the ML figure on peak grasping aperture was substantially increased in the open-loop and delay conditions, which did not differ from one another. These findings highlight the importance of on-line visual feedback for the resistance of grasping to the ML illusion and also call to attention the relevance of task factors such as target previewing, the visuomotor relevance of illusion-inducing elements, and participant strategies.

No Evidence for Accurate Visuomotor Memory: Systematic and Variable Error in Memory-Guided Reaching

Abstract The authors explored whether the motor system has access to highly accurate information about the aiming environment after visual occlusion. Participants (N = 14) reached to 1 of 3 midsagittal targets in 4 visual conditions (open-loop, brief-delay, 500-ms delay, and 2,000-ms delay). In all conditions, the aiming environment was first viewed for 2,000 ms. Movements were cued immediately after the initial viewing period in the open-loop and brief-delay conditions. Vision was not occluded until movement onset in the open-loop condition, whereas vision was occluded coincidentally with the movement cue in the brief-delay condition. In the 2 longer delay conditions, the movement was cued following a 500- or a 2,000-ms no-vision delay period. Participants overshot the target in the open-loop condition, but that tendency was significantly reduced in the 3 delay conditions. Moreover, end-point variability was greater in the 3 delay conditions than in the open-loop condition. A speed-accuracy tradeoff account could not explain the differences between open-loop and delayed reaching. Those findings suggest that the motor system does not have access to highly accurate information about the aiming environment for any appreciable period of time following visual occlusion, consistent with the view that the visuomotor system operates in real time.

Grasping two-dimensional images and three-dimensional objects in visual-form agnosia

Visually guided prehension is controlled by a specialized visuomotor system in the posterior parietal cortex. It is not clear how this system responds to visual stimuli that lack three-dimensional (3D) structure, such as two-dimensional (2D) images of objects. We asked a neurological patient with visual-form agnosia (patient D.F.) to grasp 3D objects and 2D images of the same objects and to estimate their sizes manually. D.F.’s grip aperture was scaled to the sizes of the 2D and 3D target stimuli, but her manual estimates were poorly correlated with object size. Control participants demonstrated appropriate size-scaling in both the grasping and manual size-estimation tasks, but tended to use a smaller peak aperture when reaching to grasp 2D images. We conclude that: (1) the dorsal stream grasping system does not discriminate in a fundamental way between 2D and 3D objects, and (2) neurologically normal participants might adopt a different visuomotor strategy for target objects that are recognized to be ungraspable. These findings are consistent with the view that the dorsal grasping system accesses a pragmatic, spatial representation of the target object, whereas the ventral system accesses a more comprehensive, volumetric description of the object.

Effect of body temperature on cold induced vasodilation

Cold-induced vasodilation (CIVD) is an acute increase in peripheral blood flow observed during cold exposures. It is hypothesized to protect against cold injuries, yet despite continuous research it remains an unexplained phenomenon. Contrary to the traditionally held view, we propose that CIVD is a thermoregulatory reflex mechanism contributing to heat loss. Ten adults (4 females; 23.8 ± 2.0 years) randomly underwent three 130-min exposures to −20°C incorporating a 10-min moderate exercise period at the 65th min, while wearing a liquid conditioning garment (LCG) and military arctic clothing. In the pre-warming condition, rectal temperature was increased by 0.5°C via the LCG before the cold exposure. In the warming condition, participants regulated the LCG throughout the cold exposure to subjective comfort. In the control condition, the LCG was worn but was not operated either before or during the cold exposure. Results demonstrated that the majority of CIVD occurred during the warming condition when the thermometrically-estimated mean body temperature (Tb) was at its highest. A thermoregulatory pattern was identified whereby CIVD occurred soon after Tb increased past a threshold (~36.65°C in warming and pre-warming; ~36.4°C in control). When CIVD occurred, Tb was reduced and CIVD ceased when Tb fell below the threshold. These findings were independent of extremity temperature since CIVD episodes occurred at a large range of finger temperatures (7.2–33.5°C). These observations were statistically confirmed by auto-regressive integrated moving average analysis (t = 9.602, P < 0.001). We conclude that CIVD is triggered by increased Tb supporting the hypothesis that CIVD is a thermoregulatory mechanism contributing to heat loss.

Manipulability and living/non-living category effects on object identification

Object naming studies have generally observed that both normal and brain damaged individuals are faster and more accurate at identifying non-living objects than living objects (). However, a potential confounding variable, manipulability, has been present in past studies that may mediate this effect. Previous studies that have observed a non-living advantage have often used manipulable and non-manipulable exemplars to represent the non-living and living groups, respectively. Under conditions which controlled for object manipulability and familiarity, results demonstrated advantages for the identification of non-manipulable and for living objects.