Biomedical subjects

Rufin Vogels

Publications and source records attributed to Rufin Vogels.

15 recordsLinked to original sources

PubMed2006

Extracting 3D structure from disparity.

The neural mechanisms of stereoscopic 3D shape perception have only recently been investigated. Here we review the two cortical regions in which these mechanisms have been studied so far in macaques: a small subpart of inferotemporal cortex called TEs, and the caudal intraparietal (CIP) region. Neurons in TEs respond selectively to the orientation and curvature in depth of stereoscopic surfaces and this region provides a detailed 3D shape description of surface boundaries and surface content. This description is evoked only by binocular stimuli in which subjects see depth and it does not vary if depth is specified by different cues. Neurons in CIP are a selective for orientation in depth of surfaces and elongated objects, and their responses are also unaffected by changes in depth cues. Thus, stereoscopic 3D shape is processed in both the dorsal, occipito-parietal and the ventral, occipito-temporal streams.

Animals↗

PubMed2006

Learning to see the difference specifically alters the most informative V4 neurons.

Perceptual learning is an instance of adult plasticity whereby training in a sensory (e.g., a visual task) results in neuronal changes leading to an improved ability to perform the task. Yet studies in primary visual cortex have found that changes in neuronal response properties were relatively modest. The present study examines the effects of training in an orientation discrimination task on the response properties of V4 neurons in awake rhesus monkeys. Results indicate that the changes induced in V4 are indeed larger than those in V1. Nonspecific effects of training included a decrease in response variance, and an increase in overall orientation selectivity in V4. The orientation-specific changes involved a local steepening in the orientation tuning curve around the trained orientation that selectively improved orientation discriminability at the trained orientation. Moreover, these changes were largely confined to the population of neurons whose orientation tuning was optimal for signaling small differences in orientation at the trained orientation. Finally, the modifications were restricted to the part of the tuning curve close to the trained orientation. Thus, we conclude that it is the most informative V4 neurons, those most directly involved in the discrimination, that are specifically modified by perceptual learning.

Action Potentials↗

PubMed2006

Selectivity of neuronal adaptation does not match response selectivity: a single-cell study of the FMRI adaptation paradigm.

fMRI-based adaptation paradigms (fMR-A) have been used to infer neuronal stimulus selectivities in humans. Inferring neuronal selectivities from fMR-A, however, requires an understanding of the relationship between the stimulus selectivity of neuronal adaptation and responses. We studied this relationship by recording single cells in macaque inferior temporal (IT) cortex, an area that shows fMRI adaptation. Repetition of identical object images reduced the responsiveness of single IT neurons. Presentation of an image to which the neuron was unresponsive did not alter the response to a subsequent image that activated the neuron. Successive presentation of two different images to which the neuron responded similarly produced adaptation, but less so than the repeated presentation of an image. The neuronal adaptation at the single-cell level showed a greater degree of stimulus selectivity than the responses. This complicates the interpretation of fMR-A paradigms when inferring neuronal selectivity.

Adaptation, Physiological↗

PubMed2005

Processing of kinetic boundaries in macaque V4.

We used gratings and shapes defined by relative motion to study selectivity for static kinetic boundaries in macaque V4 neurons. Kinetic gratings were generated by random pixels moving in opposite directions in the neighboring bars, either parallel to the orientation of the boundary (parallel kinetic grating) or perpendicular to the boundary (orthogonal kinetic grating). Neurons were also tested with static, luminance defined gratings to establish cue invariance. In addition, we used eight shapes defined either by relative motion or by luminance contrast, as used previously to test cue invariance in the infero-temporal (IT) cortex. A sizeable fraction (10-20%) of the V4 neurons responded selectively to kinetic patterns. Most neurons selective for kinetic contours had receptive fields (RFs) within the central 10 degrees of the visual field. Neurons selective for the orientation of kinetic gratings were defined as having similar orientation preferences for the two types of kinetic gratings, and the vast majority of these neurons also retained the same orientation preference for luminance defined gratings. Also, kinetic shape selective neurons had similar shape preferences when the shape was defined by relative motion or by luminance contrast, showing a cue-invariant form processing in V4. Although shape selectivity was weaker in V4 than what has been reported in the IT cortex, cue invariance was similar in the two areas, suggesting that invariance for luminance and motion cues of IT originates in V4. The neurons selective for kinetic patterns tended to be clustered within dorsal V4.

Animals↗

PubMed2005

Using functional magnetic resonance imaging to assess adaptation and size invariance of shape processing by humans and monkeys.

Functional magnetic resonance imaging in awake monkeys and humans was used to compare object adaptation in shape-sensitive regions of these two species under identical and different size conditions. Object adaptation was similar in humans and monkeys under both conditions. Neither species showed complete size invariance, in agreement with single-cell studies. Both the macaque inferotemporal (IT) complex and human lateral occipital complex (LOC) displayed an anteroposterior gradient in object adaptation and size invariance, with the more anterior regions being more adaptable and size invariant. The results provide additional evidence for the homology between the macaque IT cortex and human LOC but also add to the growing list of differences between human and monkey intraparietal sulcus regions.

Adaptation, Physiological↗

PubMed2005

Tuning for shape dimensions in macaque inferior temporal cortex.

It is widely assumed that distributed bell-shaped tuning (e.g. Radial Basis functions) characterizes the shape selectivity of macaque inferior temporal (IT) neurons, analogous to the orientation or spatial frequency tuning found in early visual cortex. Demonstrating such tuning properties requires testing the responses of neurons for different values along dimensions of shape. We recorded the responses of single macaque IT neurons to variations of a rectangle and a triangle along simple shape dimensions, such as taper and axis curvature. The neurons showed systematic response modulation along these dimensions, with the greatest response, on average, to the highest values on the dimensions, e.g. to the most curved shapes. Within the range of values tested, the response functions were monotonic rather than bell-shaped. Multi-dimensional scaling of the neural responses showed that these simple shape dimensions were coded orthogonally by IT neurons: the degree and direction of responses modulation (i.e. the increase or decrease of responses along a dimension) was independent for the different dimensions. Furthermore, for combinations of curvature-related and other simple shape dimensions, the joint tuning was separable, that is well predicted by the product of the tuning for each of the dimensions. The independence of dimensional tuning may provide the neural basis for the independence of psychophysical judgements of multidimensional stimuli.

Action Potentials↗

PubMed2004

Representation of regular and irregular shapes in macaque inferotemporal cortex.

We determined the degree to which the response modulation of macaque inferior temporal (IT) neurons corresponds to perceptual versus physical shape similarities. IT neurons were tested with four groups of shapes. One group consisted of variations of simple, symmetrical (i.e. regular) shapes that differed in nonaccidental properties (NAPs, i.e. viewpoint-invariant), such as curved versus straight contours. The second and third groups were composed of, respectively, simple and complex asymmetrical (i.e. irregular) shapes, all with curved contours. A fourth group consisted of simple, asymmetrical shapes, but with straight (corners) instead of curved contours. The neural modulations were greater for the shapes differing in NAPs than for the shapes differing in the configuration of the convexities and concavities. Multidimensional scaling showed that a population code of the neural activity could readily distinguish the four shape groups. This pattern of neural modulation was strongly manifested in the results of a sorting task by human subjects but could not be predicted using current image-based models (i.e. pixel energies, V1-like Gabor-jet filtering and HMAX). The representation of shape in IT thus exceeds a mere faithful representation of physical reality, by emphasizing perceptually salient features relevant for essential categorizations.

Algorithms↗

PubMed2004

Convergence of depth from texture and depth from disparity in macaque inferior temporal cortex.

The visual system must reconstruct the three-dimensional structure of an object from two-dimensional retinal images. Previous research has shown that macaque inferior temporal (IT) neurons, although belonging to the ventral visual stream, code for depth defined by binocular disparity gradients. Here, we demonstrate that macaque IT neurons also code for depth defined by texture gradients, a monocular depth cue. Single IT neurons were selective for the tilt of texture-defined surfaces, and the tilt preferences of individual neurons remained the same, whether surfaces were defined by texture or disparity cues. Furthermore, the tilt preference was invariant over different types of textures and slants, suggesting an abstract representation of surface tilt in ventral visual cortex.

Animals↗

PubMed2004

Visual activation in prefrontal cortex is stronger in monkeys than in humans.

The prefrontal cortex supports many cognitive abilities, which humans share to some degree with monkeys. The specialized functions of the prefrontal cortex depend both on the nature of its inputs from other brain regions and on distinctive aspects of local processing. We used functional MRI to compare prefrontal activity between monkey and human subjects when they viewed identical images of objects, either intact or scrambled. Visual object-related activation of the lateral prefrontal cortex was observed in both species, but was stronger in monkeys than in humans, both in magnitude (factors 2-3) and in spatial extent (fivefold or more as a percentage of prefrontal volume). This difference was observed for two different stimulus sets, at two field strengths, and over a range of tasks. These results suggest that there may be more volitional control over visual processing in humans than in monkeys.

Animals↗

PubMed2003

Asymmetries in stimulus comparisons by monkey and man.

Similarity is a core concept in theories of object recognition, categorization, and reasoning. It is often conceptualized as a geometric distance in a multidimensional stimulus space. However, research in humans has revealed that similarity judgments involve more than a simple distance calculation and tend to be asymmetric when stimuli differ in factors such as prototypicality. For example, most people judge 99 to be more similar to 100 than 100 to 99. Up to now, it was not known whether such asymmetries might also occur in nonhuman subjects. This study reveals asymmetries in the pattern of errors made by four rhesus monkeys in a temporal same/different task. Monkeys usually perceived a smaller difference between two different stimuli when the first stimulus in a trial was less prototypical than the second, just as what was found previously for human subjects. The pattern of asymmetries differed between monkeys, and a control study showed that such variability is also present for human subjects. We propose that known neurophysiological mechanisms can account for asymmetry in the stimulus comparisons of both species. Thus, seemingly complex phenomena that occur when human subjects rate stimulus similarity are also present in macaques' similarity judgments and could be based on relatively simple mechanisms.

Animals↗

PubMed2003

Shape tuning in macaque inferior temporal cortex.

Neurons in the inferior temporal cortex (IT) of the macaque fire more strongly to some shapes than others, but little is known about how to characterize this shape tuning more generally, because most previous studies have used somewhat arbitrary variations in the stimuli with unspecified magnitudes of the changes. The present investigation studied the modulation of IT cells to nonaccidental property (NAP, i.e., invariant to orientations in depth) and metric property (MP, i.e., depth dependent) variations of dimensions of generalized cones (a general formalism for characterizing shapes hypothesized to mediate object recognition). Changes in an NAP resulted in greater neuronal modulation than equally large pixel-wise changes in an MP (including those consisting of a rotation in depth). There was also precise and highly systematic neuronal tuning to the quantitative variations of MPs along specific dimensions to which a neuron was sensitive. The NAP advantage was independent of whether the object was composed of only a single part or had two parts. These findings indicate that qualitative shape changes such as NAPs help explain the surplus amount of IT shape sensitivity that cannot be accounted for on the basis of metric or pixel-based changes alone. This NAP advantage may provide the neural basis for the greater detectability of NAP compared with MP changes in human psychophysics.

Animals↗

PubMed2003

At least at the level of inferior temporal cortex, the stereo correspondence problem is solved.

Stereoscopic vision requires the correspondence problem to be solved, i.e., discarding "false" matches between images of the two eyes, while keeping correct ones. To advance our understanding of the underlying neuronal mechanisms, we compared single neuron responses to correlated and anticorrelated random dot stereograms (RDSs). Inferior temporal neurons, which respond selectively to disparity-defined three-dimensional shapes, showed robust selectivity for correlated RDSs portraying concave or convex surfaces, but unlike neurons in areas V1, MT/V5, and MST, were not selective for anticorrelated RDSs. These results show that the correspondence problem is solved at least in far extrastriate cortex, as it is in the monkey's perception.

Animals↗

PubMed2003

The effect of category learning on the representation of shape: dimensions can be biased but not differentiated.

Recent studies have suggested a profound influence of category learning on visual perception, resulting in independent processing of previously integral dimensions. The authors reinvestigate this issue for shape dimensions. They first extend previous findings that some shape dimensions (aspect ratio and curvature) are processed in a separable way, whereas others (radial frequency components) are not. They then show that a category-learning phase improved the discrimination of a relevant with respect to an irrelevant dimension, but only for separable dimensions. No similar effect was found on the relative sensitivity for integral shape dimensions. Thus, category learning is capable of biasing separable shape dimensions but does not alter the status of dimensions in the visual system as either separable or integral.

Discrimination, Psychological↗

PubMed2002

Human brain regions involved in visual categorization.

Categorization of dot patterns is a frequently used paradigm in the behavioral study of natural categorization. To determine the human brain regions involved in categorization, we used Positron Emission Tomography to compare regional Cerebral Blood Flow patterns in two tasks employing patterns that consisted of nine dots. In the categorization task, subjects categorized novel exemplars of two categories, generated by distorting two prototypes, and other random dot patterns. In the control task, subjects judged the position of similarly distorted patterns. Each task was presented at two matched levels of difficulty. Fixation of the fixation target served as baseline condition. The categorization task differentially activated the orbitofrontal cortex and two dorsolateral prefrontal regions. These three prefrontal regions were equally weakly active in the position discrimination task and the baseline condition. The intraparietal sulcus was activated in both tasks, albeit significantly less in the position discrimination than in the categorization task. A similar activation pattern was present in the neostriatum. Task difficulty had no effect. These functional imaging results show that the dot-pattern categorization task strongly engages prefrontal and parietal cortical areas. The activation of prefrontal cortex during visual categorization in humans agrees with the recent finding of category-related responses in macaque prefrontal neurons.

Adult↗

PubMed2002

Effects of illumination intensity and direction on object coding in macaque inferior temporal cortex.

Single unit activity in area TE was recorded from two macaques as they viewed 3D appearing rendered objects that were illuminated from different directions (without cast shadows) and intensities of illumination. The average modulation produced by changes in illumination intensity or direction was rather moderate, with the majority of the neurons responding invariantly to these lighting variables. When neural activity was affected by illumination direction, it was not manifested as a preference for a particular direction of illumination by a given neuron. Instead, the tuning appeared to be to the relative brightness of a given shaped surface at a given orientation. The modulation to changes in illumination direction was considerably smaller than that produced by changes in object shape. Most of the neurons that were unaffected by changes in illumination direction responded much less to silhouettes of these objects, indicating that these neurons were also sensitive to an object's inner features. The neuronal invariance for shading variations may provide the basis for the invariance of object recognition under changes in illumination.

Action Potentials↗