Grasping with the eyes: the role of elongation in visual recognition of manipulable objects

Reciprocal interactions among parietal and occipito-temporal representations support everyday object-directed actions

Everyday interactions with common manipulable objects require the integration of conceptual knowledge about objects and actions with real-time sensory information about the position, orientation and volumetric structure of the grasp target. The ability to successfully interact with everyday objects involves analysis of visual form and shape, surface texture, material properties, conceptual attributes such as identity, function and typical context, and visuomotor processing supporting hand transport, grasp form, and object manipulation. Functionally separable brain regions across the dorsal and ventral visual pathways support the processing of these different object properties and, in cohort, are necessary for functional object use. Object-directed grasps display end-state-comfort: they anticipate in form and force the shape and material properties of the grasp target, and how the object will be manipulated after it is grasped. End-state-comfort is the default for everyday interactions with manipulable objects and implies integration of information across the ventral and dorsal visual pathways. We propose a model of how visuomotor and action representations in parietal cortex interact with object representations in ventral and lateral occipito-temporal cortex. One pathway, from the supramarginal gyrus to the middle and inferior temporal gyrus, supports the integration of action-related information, including hand and limb position (supramarginal gyrus) with conceptual attributes and an appreciation of the action goal (middle temporal gyrus). A second pathway, from posterior IPS to the fusiform gyrus and collateral sulcus supports the integration of grasp parameters (IPS) with the surface texture and material properties (e.g., weight distribution) of the grasp target. Reciprocal interactions among these regions are part of a broader network of regions that support everyday functional object interactions.

Graspable foods and tools elicit similar responses in visual cortex

Extrastriatal visual cortex is known to exhibit distinct response profiles to complex stimuli of varying ecological importance (e.g., faces, scenes, and tools). The dominant interpretation of these effects is that they reflect activation of distinct "category-selective" brain regions specialized to represent these and other stimulus categories. We sought to explore an alternative perspective: that the response to these stimuli is determined less by whether they form distinct categories, and more by their relevance to different forms of natural behavior. In this regard, food is an interesting test case, since it is primarily distinguished from other objects by its edibility, not its appearance, and there is evidence of food-selectivity in human visual cortex. Food is also associated with a common behavior, eating, and food consumption typically also involves the manipulation of food, often with the hands. In this context, food items share many properties in common with tools: they are graspable objects that we manipulate in self-directed and stereotyped forms of action. Thus, food items may be preferentially represented in extrastriatal visual cortex in part because of these shared affordance properties, rather than because they reflect a wholly distinct kind of category. We conducted fMRI and behavioral experiments to test this hypothesis. We found that behaviorally graspable food items and tools were judged to be similar in their action-related properties, and that the location, magnitude, and patterns of neural responses for images of graspable food items were similar in profile to the responses for tool stimuli. Our findings suggest that food-selectivity may reflect the behavioral affordances of food items rather than a distinct form of category-selectivity.

Development of the mirror-image sensitivity for different object categories-Evidence from the mirror costs of object images in children and adults

Object recognition relies on a multitude of factors, including size, orientation, and so on. Mirrored orientation, particularly due to children's mirror confusion in reading, holds special significance among various object orientations. Brain imaging studies suggest that the visual ventral and dorsal streams exhibit distinct orientation sensitivity across diverse object categories. Yet, it remains unclear whether mirror orientation sensitivity also varies among these categories during development at the behavioral level. Here, we explored the mirror sensitivity of children and adults across five distinct categories, which encompass tools that activate both the visual ventral stream for function information and the dorsal stream for manipulation information, and animals and faces that mainly activate the ventral stream. Two types of symbols, letters and Chinese characters, were also included. Mirror sensitivity was assessed through mirror costs-that is, the additional reaction time or error rate in the mirrored versus the same orientation condition when judging the identity of object pairs. The mirror costs in reaction times and error rates consistently revealed that children exhibited null mirror costs for tools, and the mirror costs for tools in adults were minimal, if any, and were smaller than those for letters and characters. The mirror costs reflected in absolute reaction time and error rate were similar across adults and children, but when the overall difference in reaction times was considered, adults showed a larger mirror cost than children. Overall, our investigation unveils categorical distinctions and development in mirror sensitivity of object recognition across the ventral and dorsal streams.

Neural and behavioral signatures of the multidimensionality of manipulable object processing

Understanding how we recognize objects requires unravelling the variables that govern the way we think about objects and the neural organization of object representations. A tenable hypothesis is that the organization of object knowledge follows key object-related dimensions. Here, we explored, behaviorally and neurally, the multidimensionality of object processing. We focused on within-domain object information as a proxy for the decisions we typically engage in our daily lives - e.g., identifying a hammer in the context of other tools. We extracted object-related dimensions from subjective human judgments on a set of manipulable objects. We show that the extracted dimensions are cognitively interpretable and relevant - i.e., participants are able to consistently label them, and these dimensions can guide object categorization; and are important for the neural organization of knowledge - i.e., they predict neural signals elicited by manipulable objects. This shows that multidimensionality is a hallmark of the organization of manipulable object knowledge.

Semantic feature production norms for manipulable objects

Feature generation tasks and feature databases are important for understanding how knowledge is organized in semantic memory, as they reflect not only the kinds of information that individuals hold about objects but also how objects are conceptually represented. Traditionally, semantic norms focus on a variety of object categories and, as a result, have a small number of concepts per semantic category. Here, our main goal is to create a more fine-grained feature database exclusively for one category of objects-manipulable objects. This database contributes to the understanding of within-category, content-specific processing. To achieve this, we asked 130 participants to freely generate features for 80 manipulable objects and another group of 32 participants to generate action features for the same objects. We then compared our databases with other published semantic norms and found high similarity between them. In our databases, we calculated the similarity between objects in terms of visual, functional, encyclopaedic, and action feature types using Spearman correlation, Baker's gamma index, and cophenetic correlation. We discovered that objects were grouped in a distinctive and meaningful way according to feature type. Finally, we tested the validity of our databases by asking three groups of participants to perform a feature verification experiment while manipulating production frequency. Our results demonstrate that participants can recognize and associate the features of our databases with specific manipulable objects. Participants were faster to verify high-frequency features than low-frequency features. Overall, our data provide important insights into how we process manipulable objects and can be used to further inform cognitive and neural theories of object processing and identification.

Disentangling hand and tool processing: Distal effects of neuromodulation

Neural processing within a local brain region that responds to more than one object category (e.g., hands and tools) nonetheless have different functional connectivity patterns with other distal brain areas, which suggests that local processing can affect and/or be affected by processing in distal areas, in a category-specific way. Here we wanted to test whether administering either a hand- or tool-related training task in tandem with transcranial direct current stimulation (tDCS) to a region that responds both to hands and tools (posterior middle temporal gyrus; pMTG), modulated local and distal neural processing more for the trained than the untrained category in a subsequent fMRI task. After each combined tDCS/training session, participants viewed images of tools, hands, and animals, in an fMRI scanner. Using multivoxel pattern analysis, we found that tDCS stimulation to pMTG indeed improved the classification accuracy between tools vs. animals, but only when combined with a tool and not a hand training task. Surprisingly, tDCS stimulation to pMTG also improved classification accuracy between hands vs. animals when combined with a tool but not a hand training task. Our findings suggest that overlapping but functionally-specific networks may be engaged separately by using a category-specific training task together with tDCS - a strategy that can be applied more broadly to other cognitive domains using tDCS. By hypothesis, these effects on local processing are a direct result of within-domain connectivity constraints from domain-specific networks that are at play in the processing and organization of object representations.

Mugs and Plants: Object Semantic Knowledge Alters Perceptual Processing With Behavioral Ramifications

Neural processing of objects with action associations recruits dorsal visual regions more than the neural processing of objects without such associations. We hypothesized that because the dorsal and ventral visual pathways have differing proportions of magno- and parvocellular input, there should be behavioral differences in perceptual tasks between manipulable and nonmanipulable objects. This hypothesis was tested in college-age adults across five experiments (Ns = 26, 26, 30, 25, and 25) using a gap-detection task, suited to the spatial resolution of parvocellular processing, and an object-flicker-discrimination task, suited to the temporal resolution of magnocellular processing. Directly predicted from the cellular composition of each pathway, a strong nonmanipulable-object advantage was observed in gap detection, and a small manipulable-object advantage was observed in flicker discrimination. Additionally, these effects were modulated by reducing object recognition through inversion and by suppressing magnocellular processing using red light. These results establish perceptual differences between objects dependent on semantic knowledge.

Visual Attention and Cognitive Archaeology: An Eye-Tracking Study of Palaeolithic Stone Tools

The study of lithic technology can provide information on human cultural evolution. This article aims to analyse visual behaviour associated with the exploration of ancient stone artefacts and how this relates to perceptual mechanisms in humans. In Experiment 1, we used eye tracking to record patterns of eye fixations while participants viewed images of stone tools, including examples of worked pebbles and handaxes. The results showed that the focus of gaze was directed more towards the upper regions of worked pebbles and on the basal areas for handaxes. Knapped surfaces also attracted more fixation than natural cortex for both tool types. Fixation distribution was different to that predicted by models that calculate visual salience. Experiment 2 was an online study using a mouse-click attention tracking technique and included images of unworked pebbles and 'mixed' images combining the handaxe's outline with the pebble's unworked texture. The pattern of clicks corresponded to that revealed using eye tracking and there were differences between tools and other images. Overall, the findings suggest that visual exploration is directed towards functional aspects of tools. Studies of visual attention and exploration can supply useful information to inform understanding of human cognitive evolution and tool use.

Rapid category selectivity for animals versus man-made objects: An N2pc study

Visual recognition occurs rapidly at multiple categorization levels, including the superordinate level (e.g., animal), basic level (e.g., cat), or exemplar level (e.g., my cat). Visual search for animals is faster than for man-made objects, even when the images from those categories have comparable gist statistics (i.e., low- or mid-level visual information), which suggests that higher-level, conceptual influences may support this search advantage for animals. However, it remains unclear whether the search advantage can be explained in part by early visual search processes via the N2pc ERP component, which emerges earlier than behavioral responses, across different categorization levels. Participants searched for 1) an exact image (e.g., a specific squirrel image, Exemplar-level Search), 2) any images of an item (e.g., any squirrels, Basic-level Search), or 3) any items in a category (e.g., any animals, Superordinate-level Search). In addition to Target Present trials, Foil trials measured involuntary attentional selection of task-irrelevant images related to the targets (e.g., other squirrel images when searching for a specific squirrel image, or other animals when searching for squirrels). ERP results revealed 1) a larger N2pc amplitude during Foil trials in Exemplar-level Search for animals than man-made objects, and 2) faster onset latencies for animal search than man-made object search across all categorization levels. These results suggest that the search advantage for animals over man-made objects emerges early, and that attentional selection is more biased toward the basic-level (e.g., squirrel) for animals than for man-made objects during visual search.

Tool heads prime saccades

Tools are wielded by their handles, but a lot of information about their function comes from their heads (the action-ends). Here we investigated whether eye saccadic movements are primed by tool handles, or whether they are primed by tool heads. We measured human saccadic reaction times while subjects were performing an attentional task. We found that saccades were executed quicker when performed to the side congruent with the tool head, even though "toolness" was irrelevant for the task. Our results show that heads are automatically processed by the visual system to orient eye movements, indicating that eyes are attracted by functional parts of manipulable objects and by the characteristic information these parts convey.

Diffusion model-based understanding of subliminal affective priming in continuous flash suppression

Affective states influence our decisions even when processed unconsciously. Continuous flash suppression (CFS) is a new variant of binocular rivalry that can be used to render the prime subliminal. Nonetheless, how prior information from emotional faces suppressed by CFS influences subsequent decision-making remains unclear. Here, we employed a CFS priming task to examine the effect of the two main types of information conveyed by faces, i.e., facial identity and emotion, on the evaluation of target words as positive or negative. The hierarchical diffusion model was used to investigate the underlying mechanisms. A significant interaction effect on response time was observed following the angry face prime but not the happy or neutral face primes. The results of the diffusion model analyses revealed that the priming effects of facial identity were mapped onto the drift rate and erased the 'positive bias' (the processing advantage of positive over negative stimuli). Meanwhile, the positive emotional faces increased the nondecision time in response to negative target words. The model-based analysis implies that both facial identity and emotion are processed under CFS.

Continuous flash suppression: Known and unknowns

Studies utilizing continuous flash suppression (CFS) provide valuable information regarding conscious and nonconscious perception. There are, however, crucial unanswered questions regarding the mechanisms of suppression and the level of visual processing in the absence of consciousness with CFS. Research suggests that the answers to these questions depend on the experimental configuration and how we assess consciousness in these studies. The aim of this review is to evaluate the impact of different experimental configurations and the assessment of consciousness on the results of the previous CFS studies. We review studies that evaluated the influence of different experimental configuration on the depth of suppression with CFS and discuss how different assessments of consciousness may impact the results of CFS studies. Finally, we review behavioral and brain recording studies of CFS. In conclusion, previous studies provide evidence for survival of low-level visual information and complete impairment of high-level visual information under the influence of CFS. That is, studies suggest that nonconscious perception of lower-level visual information happens with CFS, but there is no evidence for nonconscious high-level recognition with CFS.

Overlapping but distinct: Distal connectivity dissociates hand and tool processing networks

The processes and organizational principles of information involved in object recognition have been a subject of intense debate. These research efforts led to the understanding that local computations and feedforward/feedback connections are essential to our representations and their organization. Recent data, however, has demonstrated that distal computations also play a role in how information is locally processed. Here we focus on how long-range connectivity and local functional organization of information are related, by exploring regions that show overlapping category-preferences for two categories and testing whether their connections are related with distal representations in a category-specific way. We used an approach that relates functional connectivity with distal areas to local voxel-wise category-preferences. Specifically, we focused on two areas that show an overlap in category-preferences for tools and hands-the inferior parietal lobule/anterior intraparietal sulcus (IPL/aIPS) and the posterior middle temporal gyrus/lateral occipital temporal cortex (pMTG/LOTC) - and how connectivity from these two areas relate to voxel-wise category-preferences in two ventral temporal regions dedicated to the processing of tools and hands separately-the left medial fusiform gyrus and the fusiform body area respectively-as well as across the brain. We show that the functional connections of the two overlap areas correlate with categorical preferences for each category independently. These results show that regions that process both tools and hands maintain object topography in a category-specific way. This potentially allows for a category-specific flow of information that is pertinent to computing object representations.

Within-category representational stability through the lens of manipulable objects

Our ability to recognize an object amongst many exemplars is one of our most important features, and one that putatively distinguishes humans from non-human animals and potentially from (current) computational and artificial intelligence models. We can recognize objects consistently regardless of when we see them suggesting that we have stable representations across time and different contexts. Importantly, little is known about how humans can replicate within-category object representations across time. Here, we investigate neural stability of within-category object representations by computing the similarity between representational geometries of activity patterns for 80 images of tools obtained on different functional magnetic resonance imaging (fMRI) scanning days. We show that within-category representational stability is observable in regions that span lateral and ventral temporal cortex, inferior and superior parietal cortex, and premotor cortex - regions typically associated with tool processing and visuospatial processing. We then focus on what kinds of representations best explain the representational geometries within these regions. We test the similarity of these geometries with those coming from the different layers of a convolutional neural network, and those coming from perceived and veridical visual similarity models. We find that regions supporting within-category representational stability show stronger relationship with higher-level visual/semantic features, suggesting that neural replicability is derived from perceived and higher-level visual information. Within category representational stability may thus originate from long-range cross talk between category-specific regions (and in this case strongly within ventral and lateral temporal cortex) over more abstract, rather than veridical/lower-level, visual (sensorial) representations, and perhaps in the service of object-centered representations.

It's not all about looks: The role of object shape in parietal representations of manual tools

The ability to build and expertly manipulate manual tools sets humans apart from other animals. Watching images of manual tools has been shown to elicit a distinct pattern of neural activity in a network of parietal areas, assumingly because tools entail a potential for action-a unique feature related to their functional use and not shared with other manipulable objects. However, a question has been raised whether this selectivity reflects a processing of low-level visual properties-such as elongated shape that is idiosyncratic to most tool-objects-rather than action-specific features. To address this question, we created and behaviourally validated a stimulus set that dissociates objects that are manipulable and nonmanipulable, as well as objects with different degrees of body extension property (tools and non-tools), while controlling for object shape and low-level image properties. We tested the encoding of action-related features by investigating neural representations in two parietal regions of interest (intraparietal sulcus and superior parietal lobule) using functional MRI. Univariate differences between tools and non-tools were not observed when controlling for visual properties, but strong evidence for the action account was nevertheless revealed when using a multivariate approach. Overall, this study provides further evidence that the representational content in the dorsal visual stream reflects encoding of action-specific properties.

Roles of Category, Shape, and Spatial Frequency in Shaping Animal and Tool Selectivity in the Occipitotemporal Cortex

Does the nature of representation in the category-selective regions in the occipitotemporal cortex reflect visual or conceptual properties? Previous research showed that natural variability in visual features across categories, quantified by image gist statistics, is highly correlated with the different neural responses observed in the occipitotemporal cortex. Using fMRI, we examined whether category selectivity for animals and tools would remain, when image gist statistics were comparable across categories. Critically, we investigated how category, shape, and spatial frequency may contribute to the category selectivity in the animal- and tool-selective regions. Female and male human observers viewed low- or high-passed images of round or elongated animals and tools that shared comparable gist statistics in the main experiment, and animal and tool images of naturally varied gist statistics in a separate localizer. Univariate analysis revealed robust category-selective responses for images with comparable gist statistics across categories. Successful classification for category (animals/tools), shape (round/elongated), and spatial frequency (low/high) was also observed, with highest classification accuracy for category. Representational similarity analyses further revealed that the activation patterns in the animal-selective regions were most correlated with a model that represents only animal information, whereas the activation patterns in the tool-selective regions were most correlated with a model that represents only tool information, suggesting that these regions selectively represent information of only animals or tools. Together, in addition to visual features, the distinction between animal and tool representations in the occipitotemporal cortex is likely shaped by higher-level conceptual influences such as categorization or interpretation of visual inputs.

SIGNIFICANCE STATEMENT Since different categories often vary systematically in both visual and conceptual features, it remains unclear what kinds of information determine category-selective responses in the occipitotemporal cortex. To minimize the influences of low- and mid-level visual features, here we used a diverse image set of animals and tools that shared comparable gist statistics. We manipulated category (animals/tools), shape (round/elongated), and spatial frequency (low/high), and found that the representational content of the animal- and tool-selective regions is primarily determined by their preferred categories only, regardless of shape or spatial frequency. Our results show that category-selective responses in the occipitotemporal cortex are influenced by higher-level processing such as categorization or interpretation of visual inputs, and highlight the specificity in these category-selective regions.

Transcranial Direct Current Stimulation Alters Functional Network Structure in Humans: A Graph Theoretical Analysis

Transcranial direct current stimulation (tDCS) is routinely used in basic and clinical research, but its efficacy has been challenged on a methodological, statistical and technical basis recently. The arguments against tDCS derive from an insufficient understanding of how this technique interacts with brain processes physiologically. Because of its potential as a central tool in neuroscience, it is important to clarify whether tDCS affects neuronal activity. Here, we investigate influences of offline tDCS on network architecture measured by functional magnetic resonance imaging. Applied to one network node only, offline tDCS affects the architecture of the entire functional network. Furthermore, offline tDCS exerts polarity-specific effects on the topology of the functional network attached. Our results confirm in a functioning brain and in a bias free and independent fashion that offline tDCS influences neuronal activity. Moreover, our results suggest that network-specific connectivity has an important role in improving our understanding of the effects of tDCS.

Size-invariant but location-specific object-viewpoint adaptation in the absence of awareness

Perceiving object viewpoint is important for appropriate action. Here we investigated whether viewpoint information could be represented in the absence of awareness, by measuring viewpoint adaptation aftereffect from visual objects rendered invisible through interocular suppression. Participants adapted to either a visible or an invisible line-drawing cube with unambiguous viewpoint, then viewed an ambiguous Necker cube and reported its perceived viewpoint. In both the visible and invisible adaptation conditions, participants more likely perceived the Necker cube in opposite viewpoint compared to the adapting cube. Interestingly, this viewpoint aftereffect was still observed when the adapting and testing cubes were different in size. However, when the testing Necker cube was in a different location, the viewpoint aftereffect was only observed following visible adapting cube, abolished when the adapting cube was invisible. Thus object viewpoint representation could be established without awareness, and such unconscious viewpoint representation is size-invariant but location-specific. Object viewpoint representation requires conscious awareness to be globally accessible.

Domain-Specific Diaschisis: Lesions to Parietal Action Areas Modulate Neural Responses to Tools in the Ventral Stream

Neural responses to small manipulable objects ("tools") in high-level visual areas in ventral temporal cortex (VTC) provide an opportunity to test how anatomically remote regions modulate ventral stream processing in a domain-specific manner. Prior patient studies indicate that grasp-relevant information can be computed about objects by dorsal stream structures independently of processing in VTC. Prior functional neuroimaging studies indicate privileged functional connectivity between regions of VTC exhibiting tool preferences and regions of parietal cortex supporting object-directed action. Here we test whether lesions to parietal cortex modulate tool preferences within ventral and lateral temporal cortex. We found that lesions to the left anterior intraparietal sulcus, a region that supports hand-shaping during object grasping and manipulation, modulate tool preferences in left VTC and in the left posterior middle temporal gyrus. Control analyses demonstrated that neural responses to "place" stimuli in left VTC were unaffected by lesions to parietal cortex, indicating domain-specific consequences for ventral stream neural responses in the setting of parietal lesions. These findings provide causal evidence that neural specificity for "tools" in ventral and lateral temporal lobe areas may arise, in part, from online inputs to VTC from parietal areas that receive inputs via the dorsal visual pathway.

What Role Does "Elongation" Play in "Tool-Specific" Activation and Connectivity in the Dorsal and Ventral Visual Streams?

Images of tools induce stronger activation than images of nontools in a left-lateralized network that includes ventral-stream areas implicated in tool identification and dorsal-stream areas implicated in tool manipulation. Importantly, however, graspable tools tend to be elongated rather than stubby, and so the tool-selective responses in some of these areas may, to some extent, reflect sensitivity to elongation rather than "toolness" per se. Using functional magnetic resonance imaging, we investigated the role of elongation in driving tool-specific activation in the 2 streams and their interconnections. We showed that in some "tool-selective" areas, the coding of toolness and elongation coexisted, but in others, elongation and toolness were coded independently. Psychophysiological interaction analysis revealed that toolness, but not elongation, had a strong modulation of the connectivity between the ventral and dorsal streams. Dynamic causal modeling revealed that viewing tools (either elongated or stubby) increased the connectivity from the ventral- to the dorsal-stream tool-selective areas, but only viewing elongated tools increased the reciprocal connectivity between these areas. Overall, these data disentangle how toolness and elongation affect the activation and connectivity of the tool network and help to resolve recent controversies regarding the relative contribution of "toolness" versus elongation in driving dorsal-stream "tool-selective" areas.

Battle of the Mondrians: Investigating the Role of Unpredictability in Continuous Flash Suppression

In continuous flash suppression (CFS), a dynamic sequence of Mondrian patterns presented to one eye suppresses a static target in the other eye for several seconds at a time. Its effectiveness has been linked to low-level properties such as spatial frequency and orientation, but the role of higher order influences remains unstudied. Here, using a tracking paradigm, we asked if the spatial and temporal predictability of the Mondrian sequence affects CFS dynamics. Predictable temporal sequences were regularly updated every 100 ms or modulated sinusoidally in pixel luminance at 2 Hz. Unpredictable temporal sequences were irregularly updated or had stochastic pixel luminance changes across time. To vary spatial predictability, sequences were either updated with different Mondrian patterns or had a fixed spatial pattern. We found a modest effect of spatial uncertainty when the target modulation was low (0.125 Hz) but not temporal uncertainty, which had no significant effects regardless of target modulation. Similar results were obtained when we pitted the standard Mondrian sequence against sequences with a fixed spatial pattern and temporally low-pass filtered sequences in a binocular rivalry paradigm. Thus, not only was the effect of information predictability was modest and spatial, but it was also dependent on the presence of higher temporal frequencies. Together, the results demonstrate the significance of low-level properties in affecting CFS dynamics and the possible involvement of pattern structure masking in CFS.

Visual and visuomotor processing of hands and tools as a case study of cross talk between the dorsal and ventral streams

A major principle of organization of the visual system is between a dorsal stream that processes visuomotor information and a ventral stream that supports object recognition. Most research has focused on dissociating processing across these two streams. Here we focus on how the two streams interact. We tested neurologically-intact and impaired participants in an object categorization task over two classes of objects that depend on processing within both streams-hands and tools. We measured how unconscious processing of images from one of these categories (e.g., tools) affects the recognition of images from the other category (i.e., hands). Our findings with neurologically-intact participants demonstrated that processing an image of a hand hampers the subsequent processing of an image of a tool, and vice versa. These results were not present in apraxic patients (N = 3). These findings suggest local and global inhibitory processes working in tandem to co-register information across the two streams.

Towards a unified perspective of object shape and motion processing in human dorsal cortex

Although object-related areas were discovered in human parietal cortex a decade ago, surprisingly little is known about the nature and purpose of these representations, and how they differ from those in the ventral processing stream. In this article, we review evidence for the unique contribution of object areas of dorsal cortex to three-dimensional (3-D) shape representation, the localization of objects in space, and in guiding reaching and grasping actions. We also highlight the role of dorsal cortex in form-motion interaction and spatiotemporal integration, possible functional relationships between 3-D shape and motion processing, and how these processes operate together in the service of supporting goal-directed actions with objects. Fundamental differences between the nature of object representations in the dorsal versus ventral processing streams are considered, with an emphasis on how and why dorsal cortex supports veridical (rather than invariant) representations of objects to guide goal-directed hand actions in dynamic visual environments.

More than Action: The Dorsal Pathway Contributes to the Perception of 3-D Structure

An evolving view in cognitive neuroscience is that the dorsal visual pathway not only plays a key role in visuomotor behavior but that it also contributes functionally to the recognition of objects. To characterize the nature of the object representations derived by the dorsal pathway, we assessed perceptual performance in the context of the continuous flash suppression paradigm, which suppresses object processing in the ventral pathway while sparing computation in the dorsal pathway. In a series of experiments, prime stimuli, which were rendered imperceptible by the continuous flash suppression, still contributed to perceptual decisions related to the subsequent perceptible target stimuli. However, the contribution of the prime to perception was contingent on the prime's structural coherence, in that a perceptual advantage was observed only for targets primed by objects with legitimate 3-D structure. Finally, we obtained additional evidence to demonstrate that the processing of the suppressed objects was contingent on the magnocellular, rather than the parvocellular, system, further linking the processing of the suppressed stimuli to the dorsal pathway. Together, these results provide novel evidence that the dorsal pathway does not only support visuomotor control but, rather, that it also derives the structural description of 3-D objects and contributes to shape perception.

Automatic Recruitment of the Motor System by Undetected Graspable Objects: A Motor-evoked Potential Study

Previous behavioral and neuroimaging studies have suggested that the motor properties associated with graspable objects may be automatically accessed when people passively view these objects. We directly tested this by measuring the excitability of the motor pathway when participants viewed pictures of graspable objects that were presented during the attentional blink (AB), when items frequently go undetected. Participants had to identify two briefly presented objects separated by either a short or long SOA. Motor-evoked potentials were measured from the right hand in response to a single TMS pulse delivered over the left primary motor cortex 250 msec after the onset of the second target. Behavioral results showed poorer identification of objects at short SOA compared with long SOA, consistent with an AB, which did not differ between graspable and nongraspable objects. However, motor-evoked potentials measured during the AB were significantly higher for graspable objects than for nongraspable objects, irrespective of whether the object was successfully identified or undetected. This provides direct evidence that the motor system is automatically activated during visual processing of objects that afford a motor action.

How do the two visual streams interact with each other?

The current consensus divides primate cortical visual processing into two broad networks or "streams" composed of highly interconnected areas (Milner and Goodale 2006, 2008; Goodale 2014). The ventral stream, passing from primary visual cortex (V1) through to inferior parts of the temporal lobe, is considered to mediate the transformation of the contents of the visual signal into the mental furniture that guides memory, recognition and conscious perception. In contrast the dorsal stream, passing from V1 through to various areas in the posterior parietal lobe, is generally considered to mediate the visual guidance of action, primarily in real time. The brain, however, does not work through mutually insulated subsystems, and indeed there are well-documented interconnections between the two streams. Evidence for contributions from ventral stream systems to the dorsal stream comes from human neuropsychological and neuroimaging research, and indicates a crucial role in mediating complex and flexible visuomotor skills. Complementary evidence points to a role for posterior dorsal-stream visual analysis in certain aspects of 3-D perceptual function in the ventral stream. A series of studies of a patient with visual form agnosia has been instrumental in shaping our knowledge of what each stream can achieve in isolation; but it has also helped us to tease apart the relative dependence of parietal visuomotor systems on direct bottom-up visual inputs versus inputs redirected via perceptual systems within the ventral stream.

'What' Is Happening in the Dorsal Visual Pathway

The cortical visual system is almost universally thought to be segregated into two anatomically and functionally distinct pathways: a ventral occipitotemporal pathway that subserves object perception, and a dorsal occipitoparietal pathway that subserves object localization and visually guided action. Accumulating evidence from both human and non-human primate studies, however, challenges this binary distinction and suggests that regions in the dorsal pathway contain object representations that are independent of those in ventral cortex and that play a functional role in object perception. We review here the evidence implicating dorsal object representations, and we propose an account of the anatomical organization, functional contributions, and origins of these representations in the service of perception.

Temporal Frequency Tuning Reveals Interactions between the Dorsal and Ventral Visual Streams

Visual processing of complex objects is supported by the ventral visual pathway in the service of object identification and by the dorsal visual pathway in the service of object-directed reaching and grasping. Here, we address how these two streams interact during tool processing, by exploiting the known asymmetry in projections of subcortical magnocellular and parvocellular inputs to the dorsal and ventral streams. The ventral visual pathway receives both parvocellular and magnocellular input, whereas the dorsal visual pathway receives largely magnocellular input. We used fMRI to measure tool preferences in parietal cortex when the images were presented at either high or low temporal frequencies, exploiting the fact that parvocellular channels project principally to the ventral but not dorsal visual pathway. We reason that regions of parietal cortex that exhibit tool preferences for stimuli presented at frequencies characteristic of the parvocellular pathway receive their inputs from the ventral stream. We found that the left inferior parietal lobule, in the vicinity of the supramarginal gyrus, exhibited tool preferences for images presented at low temporal frequencies, whereas superior and posterior parietal regions exhibited tool preferences for images present at high temporal frequencies. These data indicate that object identity, processed within the ventral stream, is communicated to the left inferior parietal lobule and may there combine with inputs from the dorsal visual pathway to allow for functionally appropriate object manipulation.

Motor and Visuospatial Attention and Motor Planning After Stroke: Considerations for the Rehabilitation of Standing Balance and Gait

Attention and planning can be altered by stroke, which can influence motor performance. Although the influence of these factors on recovery from stroke has been explored for the upper extremity (UE), their impact on balance and gait are unknown. This perspective article presents evidence that altered motor and visuospatial attention influence motor planning of voluntary goal-directed movements poststroke, potentially affecting balance and gait. Additionally, specific strategies for rehabilitation of balance and gait poststroke in the presence of these factors are discussed. Visuospatial attention selects relevant sensory information and supports the preparation of responses to this information. Motor attentional impairments may produce difficulty with selecting appropriate motor feedback, potentially contributing to falls. An original theoretical model is presented for a network of brain regions supporting motor and visuospatial attention, as well as motor planning of voluntary movements. Stroke may influence this functional network both locally and distally, interfering with input or output of the anatomical or functional regions involved and affecting voluntary movements. Although there is limited research directly examining leg function, evidence suggests alterations in motor and visuospatial attention influence motor planning and have a direct impact on performance of gait and balance. This model warrants testing comparing healthy adults with individuals with stroke.

Weighing the evidence for a dorsal processing bias under continuous flash suppression

With the introduction of continuous flash suppression (CFS) as a method to render stimuli invisible and study unconscious visual processing, a novel hypothesis has gained popularity. It states that processes typically ascribed to the dorsal visual stream can escape CFS and remain functional, while ventral stream processes are suppressed when stimuli are invisible under CFS. This notion of a CFS-specific "dorsal processing bias" has been argued to be in line with core characteristics of the influential dual-stream hypothesis of visual processing which proposes a dissociation between dorsally mediated vision-for-action and ventrally mediated vision-for-perception. Here, we provide an overview of neuroimaging and behavioral studies that either examine this dorsal processing bias or base their conclusions on it. We show that both evidence for preserved ventral processing as well as lack of dorsal processing can be found in studies using CFS. To reconcile the diverging results, differences in the paradigms and their effects are worthy of future research. We conclude that given the current level of information a dorsal processing bias under CFS cannot be universally assumed.

Investigating category- and shape-selective neural processing in ventral and dorsal visual stream under interocular suppression

Recent behavioral and neuroimaging studies using continuous flash suppression (CFS) have suggested that action-related processing in the dorsal visual stream might be independent of perceptual awareness, in line with the "vision-for-perception" versus "vision-for-action" distinction of the influential dual-stream theory. It remains controversial if evidence suggesting exclusive dorsal stream processing of tool stimuli under CFS can be explained by their elongated shape alone or by action-relevant category representations in dorsal visual cortex. To approach this question, we investigated category- and shape-selective functional magnetic resonance imaging-blood-oxygen level-dependent responses in both visual streams using images of faces and tools. Multivariate pattern analysis showed enhanced decoding of elongated relative to non-elongated tools, both in the ventral and dorsal visual stream. The second aim of our study was to investigate whether the depth of interocular suppression might differentially affect processing in dorsal and ventral areas. However, parametric modulation of suppression depth by varying the CFS mask contrast did not yield any evidence for differential modulation of category-selective activity. Together, our data provide evidence for shape-selective processing under CFS in both dorsal and ventral stream areas and, therefore, do not support the notion that dorsal "vision-for-action" processing is exclusively preserved under interocular suppression.

Tool manipulation knowledge is retrieved by way of the ventral visual object processing pathway

Using functional Magnetic Resonance Imaging (fMRI), we find that object manipulation knowledge is accessed by way of the ventral object processing pathway. We exploit the fact that parvocellular channels project to the ventral but not the dorsal stream, and show that increased neural responses for tool stimuli are observed in the inferior parietal lobule when those stimuli are visible only to the ventral object processing stream. In a control condition, tool-preferences were observed in a superior and posterior parietal region for stimuli titrated so as to be visible by the dorsal visual pathway. Functional connectivity analyses confirm the dissociation between sub-regions of parietal cortex according to whether their principal afferent input is via the ventral or dorsal visual pathway. These results challenge the 'Embodied Hypothesis of Tool Recognition', according to which tool identification critically depends on simulation of object manipulation knowledge. Instead, these data indicate that retrieval of object-associated manipulation knowledge is contingent on accessing the identity of the object, a process that is subserved by the ventral visual pathway.