Grip Constancy but Not Perceptual Size Constancy Survives Lesions of Early Visual Cortex

The contribution of semantic distance knowledge to size constancy in perception and grasping when visual cues are limited

To achieve a stable perception of object size in spite of variations in viewing distance, our visual system needs to combine retinal image information and distance cues. Previous research has shown that, not only retinal cues, but also extraretinal sensory signals can provide reliable information about depth and that different neural networks (perception versus action) can exhibit preferences in the use of these different sources of information during size-distance computations. Semantic knowledge of distance, a purely cognitive signal, can also provide distance information. Do the perception and action systems show differences in their ability to use this information in calculating object size and distance? To address this question, we presented 'glow-in-the-dark' objects of different physical sizes at different real distances in a completely dark room. Participants viewed the objects monocularly through a 1-mm pinhole. They either estimated the size and distance of the objects or attempted to grasp them. Semantic knowledge was manipulated by providing an auditory cue about the actual distance of the object: "20 cm", "30 cm", and "40 cm". We found that semantic knowledge of distance contributed to some extent to size constancy operations during perceptual estimation and grasping, but size constancy was never fully restored. Importantly, the contribution of knowledge about distance to size constancy was equivalent between perception and action. Overall, our study reveals similarities and differences between the perception and action systems in the use of semantic distance knowledge and suggests that this cognitive signal is useful but not a reliable depth cue for size constancy under restricted viewing conditions.

Temporal features of size constancy for perception and action in a real-world setting: A combined EEG-kinematics study

A stable representation of object size, in spite of continuous variations in retinal input due to changes in viewing distance, is critical for perceiving and acting in a real 3D world. In fact, our perceptual and visuo-motor systems exhibit size and grip constancies in order to compensate for the natural shrinkage of the retinal image with increased distance. The neural basis of this size-distance scaling remains largely unknown, although multiple lines of evidence suggest that size-constancy operations might take place remarkably early, already at the level of the primary visual cortex. In this study, we examined for the first time the temporal dynamics of size constancy during perception and action by using a combined measurement of event-related potentials (ERPs) and kinematics. Participants were asked to maintain their gaze steadily on a fixation point and perform either a manual estimation or a grasping task towards disks of different sizes placed at different distances. Importantly, the physical size of the target was scaled with distance to yield a constant retinal angle. Meanwhile, we recorded EEG data from 64 scalp electrodes and hand movements with a motion capture system. We focused on the first positive-going visual evoked component peaking at approximately 90 ms after stimulus onset. We found earlier latencies and greater amplitudes in response to bigger than smaller disks of matched retinal size, regardless of the task. In line with the ERP results, manual estimates and peak grip apertures were larger for the bigger targets. We also found task-related differences at later stages of processing from a cluster of central electrodes, whereby the mean amplitude of the P2 component was greater for manual estimation than grasping. Taken together, these findings provide novel evidence that size constancy for real objects at real distances occurs at the earliest cortical stages and that early visual processing does not change as a function of task demands.

Behavioral examination of the role of the primary visual cortex in the perceived size representation

Previous research has shown that neural activity in the primary visual cortex (V1) and V1 surface area may be linked with subjective experience of size illusions. Here, we behaviorally measured the hallway illusion with experimental manipulations as a proxy of V1's influence on size perception. We first tested whether the hallway illusion can persist without further recurrent processing by using backward masking. Next, we examined relations among the hallway illusion magnitude and other perceptual measures that have been suggested to be correlated with V1 surface area. In Experiment 1, the magnitude of the hallway illusion was not affected by the stimulus duration and visual masking when the hallway context was previewed (i.e., complex depth information is already processed). It suggests that V1 activity could support the size illusion to some extent even when recurrent processing between V1 and higher areas is disturbed. In Experiment 2, the hallway illusion magnitude was correlated with the Vernier acuity threshold, but not with physical size discriminability. Our results provide converging evidence with the previous findings in that neural activity in V1 may contribute to size illusions and that V1 surface area is not the sole factor that mediates size perception and visual precision.

Binocular Viewing Facilitates Size Constancy for Grasping and Manual Estimation

A prerequisite for efficient prehension is the ability to estimate an object’s distance and size. While most studies demonstrate that binocular viewing is associated with a more efficient grasp programming and execution compared to monocular viewing, the factors contributing to this advantage are not fully understood. Here, we examined how binocular vision facilitates grasp scaling using two tasks: prehension and manual size estimation. Participants (n = 30) were asked to either reach and grasp an object or to provide an estimate of an object’s size using their thumb and index finger. The objects were cylinders with a diameter of 0.5, 1.0, or 1.5 cm placed at three distances along the midline (40, 42, or 44 cm). Results from a linear regression analysis relating grip aperture to object size revealed that grip scaling during monocular viewing was reduced similarly for both grasping and estimation tasks. Additional analysis revealed that participants adopted a larger safety margin for grasping during monocular compared to binocular viewing, suggesting that monocular depth cues do not provide sufficient information about an object’s properties, which consequently leads to a less efficient grasp execution.

Grasping of Real-World Objects Is Not Biased by Ensemble Perception

The visual system is known to extract summary representations of visually similar objects which bias the perception of individual objects toward the ensemble average. Although vision plays a large role in guiding action, less is known about whether ensemble representation is informative for action. Motor behavior is tuned to the veridical dimensions of objects and generally considered resistant to perceptual biases. However, when the relevant grasp dimension is not available or is unconstrained, ensemble perception may be informative to behavior by providing gist information about surrounding objects. In the present study, we examined if summary representations of a surrounding ensemble display influenced grip aperture and orientation when participants reached-to-grasp a central circular target which had an explicit size but importantly no explicit orientation that the visuomotor system could selectively attend to. Maximum grip aperture and grip orientation were not biased by ensemble statistics during grasping, although participants were able to perceive and provide manual estimations of the average size and orientation of the ensemble display. Support vector machine classification of ensemble statistics achieved above-chance classification accuracy when trained on kinematic and electromyography data of the perceptual but not grasping conditions, supporting our univariate findings. These results suggest that even along unconstrained grasping dimensions, visually-guided behaviors toward real-world objects are not biased by ensemble processing.

The Role of Haptic Expectations in Reaching to Grasp: From Pantomime to Natural Grasps and Back Again

When we reach to pick up an object, our actions are effortlessly informed by the object's spatial information, the position of our limbs, stored knowledge of the object's material properties, and what we want to do with the object. A substantial body of evidence suggests that grasps are under the control of "automatic, unconscious" sensorimotor modules housed in the "dorsal stream" of the posterior parietal cortex. Visual online feedback has a strong effect on the hand's in-flight grasp aperture. Previous work of ours exploited this effect to show that grasps are refractory to cued expectations for visual feedback. Nonetheless, when we reach out to pretend to grasp an object (pantomime grasp), our actions are performed with greater cognitive effort and they engage structures outside of the dorsal stream, including the ventral stream. Here we ask whether our previous finding would extend to cued expectations for haptic feedback. Our method involved a mirror apparatus that allowed participants to see a "virtual" target cylinder as a reflection in the mirror at the start of all trials. On "haptic feedback" trials, participants reached behind the mirror to grasp a size-matched cylinder, spatially coincident with the virtual one. On "no-haptic feedback" trials, participants reached behind the mirror and grasped into "thin air" because no cylinder was present. To manipulate haptic expectation, we organized the haptic conditions into blocked, alternating, and randomized schedules with and without verbal cues about the availability of haptic feedback. Replicating earlier work, we found the strongest haptic effects with the blocked schedules and the weakest effects in the randomized uncued schedule. Crucially, the haptic effects in the cued randomized schedule was intermediate. An analysis of the influence of the upcoming and immediately preceding haptic feedback condition in the cued and uncued random schedules showed that cuing the upcoming haptic condition shifted the haptic influence on grip aperture from the immediately preceding trial to the upcoming trial. These findings indicate that, unlike cues to the availability of visual feedback, participants take advantage of cues to the availability of haptic feedback, flexibly engaging pantomime, and natural modes of grasping to optimize the movement.

Vision: A Product of a Society of Independent Experts

A lesion of primary visual cortex, V1, can result in the perceived size of objects varying with the size of their retinal image. A new study shows that the pregrasp span of the hand of an individual with such a lesion remains tuned to the object's true size, providing evidence for separate representations mediating perceptual appearance and motor interactions.