Erez Freud

Accurate visuomotor control below the perceptual threshold of size discrimination.

Background Human resolution for object size is typically determined by psychophysical methods that are based on conscious perception. In contrast, grasping of the same objects might be less conscious. It is suggested that grasping is mediated by mechanisms other than those mediating conscious perception. In this study, we compared the visual resolution for object size of the visuomotor and the perceptual system. Methodology/Principal Findings In Experiment 1, participants discriminated the size of pairs of objects once through perceptual judgments and once by grasping movements toward the objects. Notably, the actual size differences were set below the Just Noticeable Difference (JND). We found that grasping trajectories reflected the actual size differences between the objects regardless of the JND. This pattern was observed even in trials in which the perceptual judgments were erroneous. The results of an additional control experiment showed that these findings were not confounded by task demands. Participants were not aware, therefore, that their size discrimination via grasp was veridical. Conclusions/Significance We conclude that human resolution is not fully tapped by perceptually determined thresholds. Grasping likely exhibits greater resolving power than people usually realize.

Representation of possible and impossible objects in the human visual cortex: evidence from fMRI adaptation.

Impossible objects are defined as 2D drawings that represent objects that could not exist in real 3D space. Previous studies have proposed that such objects are represented differently in long-term memory due to their invalid spatial structure. Nevertheless, it is not clear whether these discrepancies reflect early perceptual processes or late, post-recognition, processes. Here, we address this issue by using fMRI adaptation. Interestingly, equivalent adaptation levels were obtained for repeated compared to non-repeated possible and impossible objects in object selective regions within the human high-level visual cortex. Yet, a significant correlation between fMRI adaptation and behavioral facilitation was found for possible but not for impossible objects, thus exhibiting some dissociation in the representation of the two object categories. Overall, our findings suggest that the initial perception of possible and impossible objects is mediated by shared neural mechanisms whereas the observed differences between the two types of objects may reflect higher post-recognition processes.

Visual Control of Action Directed toward Two-Dimensional Objects Relies on Holistic Processing of Object Shape

Visual perception relies on holistic processing of object shape. In contrast to perception, previous studies demonstrated that vision-for-action operates in a fundamentally different manner based on an analytical representation of objects. This notion was mainly supported by the absence of Garner interference for visually guided actions, compared to robust interference effects for perceptual estimations of the same objects. This study examines the nature of the representations that subserve visually guided actions toward two-dimensional (2D) stimuli. Based on recent results suggesting that actions directed toward 2D objects are mediated by different underlying processes compared to normal actions, we predicted that visually guided actions toward 2D stimuli would rely on perceptually driven holistic representations of object shape. To test this idea, we asked participants to grasp 2D rectangular objects presented on a computer monitor along their width while the values of the irrelevant dimension of length were either kept constant (baseline condition) or varied between trials (filtering condition). Worse performance in the filtering blocks is labeled Garner interference, which indicates holistic processing of object shape. Unlike in previous studies that used real objects, the results showed that grasping toward 2D objects produced a significant Garner interference effect, with more variable within-subject performance in the filtering compared to the baseline blocks. This finding suggests that visually guided actions directed toward 2D targets are mediated by different computations compared to visually guided actions directed toward real objects.

General holistic impairment in congenital prosopagnosia: Evidence from Garner's speeded-classification task

Congenital prosopagnosia (CP), a lifelong impairment in face processing in the absence of brain damage, is often ascribed to impairment in holistic processing. It is still debated whether such difficulties are restricted to faces or whether they can also be observed for nonfacial stimuli. Here, we investigate this issue by examining CP individuals and their controls on two variations of the Garner speeded classification task tailored to assess holistic processing of nonfacial stimuli. In Experiment 1, participants were asked to judge the width of visually presented rectangles while ignoring their irrelevant height, or to judge changes in width while height remained constant. Critically, while controls exhibited the expected Garner interference, no such interference was observed for the CPs, indicating impaired holistic processing of integral, nonfacial shape dimensions. Experiment 2, utilized the same Garner paradigm, but here participants were asked to judge integral dimensions that are unrelated to shape (colour). Importantly, both CPs and controls exhibited the same level of Garner interference, indicating intact integral processing of colour dimensions. This dissociation between the performance on the two Garner tasks indicates that CPs do not exhibit a general local processing bias or impaired integration of any perceptual dimensions, but rather a deficit that is restricted to tasks requiring holistic integral perception of shape dimensions. Taken together, these results provide evidence for the existence of a general impairment in holistic shape perception in CP, which may be related to the mechanisms underlying this disorder.

Holistic processing of impossible objects: Evidence from Garner’s speeded-classification task

Holistic processing, the decoding of the global structure of a stimulus while the local parts are not explicitly represented, is a basic characteristic of object perception. The current study was aimed to test whether such a representation could be created even for objects that violate fundamental principles of spatial organization, namely impossible objects. Previous studies argued that these objects cannot be represented holistically in long-term memory because they lack coherent 3D structure. Here, we utilized Garners speeded classification task to test whether the perception of possible and impossible objects is mediated by similar holistic processing mechanisms. To this end, participants were asked to make speeded classifications of one object dimension while an irrelevant dimension was kept constant (baseline condition) or when this dimension varied (filtering condition). It is well accepted that ignoring the irrelevant dimension is impossible when holistic perception is mandatory, thus the extent of Garner interference in performance between the baseline and filtering conditions serves as an index of holistic processing. Critically, in Experiment 1, similar levels of Garner interference were found for possible and impossible objects implying holistic perception of both object types. Experiment 2 extended these results and demonstrated that even when depth information was explicitly processed, participants were still unable to process one dimension (width/depth) while ignoring the irrelevant dimension (depth/width, respectively). The results of Experiment 3 replicated the basic pattern found in Experiments 1 and 2 using a novel set of object exemplars. In Experiment 4, we used possible and impossible versions of the Penrose triangles in which information about impossibility is embedded in the internal elements of the objects which participant were explicitly asked to judge. As in Experiments 1-3, similar Garner interference was found for possible and impossible objects. Taken together, these findings emphasize the centrality of holistic processing style in object perception and suggest that it applies even for atypical stimuli such as impossible objects.

The highs and lows of object impossibility: effects of spatial frequency on holistic processing of impossible objects.

Holistic processing, the decoding of a stimulus as a unified whole, is a basic characteristic of object perception. Recent research using Garner’s speeded classification task has shown that this processing style is utilized even for impossible objects that contain an inherent spatial ambiguity. In particular, similar Garner interference effects were found for possible and impossible objects, indicating similar holistic processing styles for the two object categories. In the present study, we further investigated the perceptual mechanisms that mediate such holistic representation of impossible objects. We relied on the notion that, whereas information embedded in the high-spatial-frequency (HSF) content supports fine-detailed processing of object features, the information conveyed by low spatial frequencies (LSF) is more crucial for the emergence of a holistic shape representation. To test the effects of image frequency on the holistic processing of impossible objects, participants performed the Garner speeded classification task on images of possible and impossible cubes filtered for their LSF and HSF information. For images containing only LSF, similar interference effects were observed for possible and impossible objects, indicating that the two object categories were processed in a holistic manner. In contrast, for the HSF images, Garner interference was obtained only for possible, but not for impossible objects. Importantly, we provided evidence to show that this effect could not be attributed to a lack of sensitivity to object possibility in the LSF images. Particularly, even for full-spectrum images, Garner interference was still observed for both possible and impossible objects. Additionally, performance in an object classification task revealed high sensitivity to object possibility, even for LSF images. Taken together, these findings suggest that the visual system can tolerate the spatial ambiguity typical to impossible objects by relying on information embedded in LSF, whereas HSF information may underlie the visual system’s susceptibility to distortions in objects’ spatial layouts.

Impossible expectations: fMRI adaptation in the lateral occipital complex (LOC) is modulated by the statistical regularities of 3D structural information

fMRI adaptation (fMRIa), the attenuation of fMRI signal which follows repeated presentation of a stimulus, is a well-documented phenomenon. Yet, the underlying neural mechanisms supporting this effect are not fully understood. Recently, short-term perceptual expectations, induced by specific experimental settings, were shown to play an important modulating role in fMRIa. Here we examined the role of long-term expectations, based on 3D structural statistical regularities, in the modulation of fMRIa. To this end, human participants underwent fMRI scanning while performing a same-different task on pairs of possible (regular, expected) objects and spatially impossible (irregular, unexpected) objects. We hypothesized that given the spatial irregularity of impossible objects in relation to real-world visual experience, the visual system would always generate a prediction which is biased to the possible version of the objects. Consistently, fMRIa effects in the lateral occipital cortex (LOC) were found for possible, but not for impossible objects. Additionally, in alternating trials the order of stimulus presentation modulated LOC activity. That is, reduced activation was observed in trials in which the impossible version of the object served as the prime object (i.e. first object) and was followed by the possible version compared to the reverse order. These results were also supported by the behavioral advantage observed for trials that were primed by possible objects. Together, these findings strongly emphasize the importance of perceptual expectations in object representation and provide novel evidence for the role of real-world statistical regularities in eliciting fMRIa.

Evidence for similar early but not late representation of possible and impossible objects

The perceptual processes that mediate the ability to efficiently represent object 3D structure are still not fully understood. The current study was aimed to shed light on these processes by utilizing spatially possible and impossible objects that could not be created in real 3D space. Despite being perceived as exceptionally unusual, impossible objects still possess fundamental Gestalt attributes and valid local depth cues that may support their initial successful representation. Based on this notion and on recent findings from our lab, we hypothesized that the initial representation of impossible objects would involve common mechanisms to those mediating typical object perception while the perceived differences between possible and impossible objects would emerge later along the processing hierarchy. In Experiment 1, participants preformed same/different classifications of two markers superimposed on a display containing two objects (possible or impossible). Faster reaction times were observed for displays in which the markers were superimposed on the same object (“object-based benefit”). Importantly, this benefit was similar for possible and impossible objects, suggesting that the representations of the two object categories rely on similar perceptual organization processes. Yet, responses for impossible objects were slower compared to possible objects. Experiment 2 was designed to examine the origin of this effect. Participants classified the location of two markers while exposure duration was manipulated. A similar pattern of performance was found for possible and impossible objects for the short exposure duration, with differences in accuracy between these two types of objects emerging only for longer exposure durations. Overall, these findings provide evidence that the representation of object structure relies on a multi-level process and that object impossibility selectively impairs the rendering of fine-detailed description of object structure.

Functional dissociation between action and perception of object shape in developmental visual object agnosia.

According to the two visual systems model, the cortical visual system is segregated into a ventral pathway mediating object recognition, and a dorsal pathway mediating visuomotor control. In the present study we examined whether the visual control of action could develop normally even when visual perceptual abilities are compromised from early childhood onward. Using his fingers, LG, an individual with a rare developmental visual object agnosia, manually estimated (perceptual condition) the width of blocks that varied in width and length (but not in overall size), or simply picked them up across their width (grasping condition). LGs perceptual sensitivity to target width was profoundly impaired in the manual estimation task compared to matched controls. In contrast, the sensitivity to object shape during grasping, as measured by maximum grip aperture (MGA), the time to reach the MGA, the reaction time and the total movement time were all normal in LG. Further analysis, however, revealed that LGs sensitivity to object shape during grasping emerged at a later time stage during the movement compared to controls. Taken together, these results demonstrate a dissociation between action and perception of object shape, and also point to a distinction between different stages of the grasping movement, namely planning versus online control. Moreover, the present study implies that visuomotor abilities can develop normally even when perceptual abilities developed in a profoundly impaired fashion.

Simon in action: the effect of spatial congruency on grasping trajectories.

The Simon effect, one of the well-known stimulus–response compatibility effects, is usually explained as an expression of a conflict that occurs at the response selection stage. Here, we extended previous findings to provide evidence for post-response selection expression of the Simon effect. Following a presentation of a visual stimulus, participants grasped one of two objects that differed slightly in size. The results showed that visual stimulus congruency modulated grasping trajectories. Particularly, movements were more lateralized in congruent trials. This lateralization decreased as reaction time (RT) increased and therefore this effect could not be fully dissociated from the response selection stage. However, size sensitivity, as measured by the time taken to reach the maximum grip aperture between the fingers, was decreased for incongruent trials, unrelated to RT. This finding provides novel evidence for an independent expression of the Simon effect in post-response selection stages. Overall, our findings extend previous studies and demonstrate that the spatial conflict evoked by the Simon task encompasses several components and independently affects response selection stages as well as other components of motor execution.