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Animals↗
Biomedical subjects
Jay Hegdé
Publications and source records attributed to Jay Hegdé.
A comparative study of shape representation in macaque visual areas v2 and v4.
We compared aspects of shape representation in extrastriate visual areas V2 and V4, which are both implicated in shape processing and belong to different hierarchical levels. We recorded responses of cells in awake, fixating monkeys to matched sets of contour and grating stimuli of low or intermediate complexity. These included simple stimuli (bars and sinusoids) and more complex stimuli (angles, intersections, arcs, and non-Cartesian gratings), all scaled to receptive field size. The responses of cells within each area were substantially modulated by each shape characteristic tested, with substantial overlap between areas by many response measures. Our analyses revealed many clear and reliable differences between areas in terms of the effectiveness of, and response modulation by, various shape characteristics. Grating stimuli were on average more effective than contour stimuli in V2 and V4, but the difference was more pronounced in V4. As a population, V4 showed greater response modulation by some shape characteristics (including simple shape characteristics) and V2 showed greater response modulation by many others (including complex shape characteristics). Recordings from area V1 demonstrated complex shape selectivity in some cells and relatively modest population differences in comparison with V2. Altogether, the representation of 2-dimensional shape characteristics revealed by this analysis varies substantially among the 3 areas. But surprisingly, the differences revealed by our analyses, individually or collectively, do not parallel the stepwise organization of the anatomical hierarchy. Commonalities of visual shape representation across hierarchical levels may reflect the replication of neural circuits used in generating complex shape representations at multiple spatial scales.
Animals↗
Role of primate visual area V4 in the processing of 3-D shape characteristics defined by disparity.
We studied the responses of V4 neurons in awake, fixating monkeys to a diverse set of stereoscopic stimuli, including zero-order disparity (frontoparallel) stimuli, surfaces oriented in depth, and convex and concave shapes presented at various mean disparities. The responses of many V4 cells were significantly modulated across each of these stimulus subsets. In general, V4 cells were broadly tuned for zero-order disparity, and at any given disparity value, about four-fifths of the cells responded significantly above background. The response modulation by flat surfaces oriented in depth was significant for about one-quarter of cells, and the responses of about one-third of the cells were significantly modulated by convex or concave surfaces at various mean disparities. However, we encountered no cells that unambiguously distinguished a given three-dimensional (3-D) shape independent of mean disparity. Thus 3-D shapes defined by disparity are unlikely to be represented explicitly at the level of individual V4 cells. Nonetheless, V4 cells likely play an important role in the processing of 3-D shape characteristics defined by disparity as a part of a distributed network.
Analysis of Variance↗
Second-order motion conveys depth-order information.
Psychophysical and neurophysiological studies have revealed that the visual system is sensitive to both "first-order" motion, in which moving features are defined by luminance cues, and "second-order" motion, in which motion is defined by nonluminance cues, such as contrast or flicker. Here we show psychophysically that common types of second-order stimuli provide potent cues to depth order. Although motion defined exclusively by nonluminance cues may be relatively rare in natural scenes, the depth-order cues offered by second-order stimuli arise ubiquitously as a result of occlusion of one moving object by another. Our results thus shed new light on the ecological importance of second-order motion. Furthermore, our results imply that visual cortical areas that have been shown to be responsive to second-order motion may be extracting information not just about object motion as has been assumed, but also about the relative depth of objects.
Cues↗
Stimulus dependence of disparity coding in primate visual area V4.
Disparity tuning in visual cortex has been shown using a variety of stimulus types that contain stereoscopic depth cues. It is not known whether different stimuli yield similar disparity tuning curves. We studied whether cells in visual area V4 of the macaque show similar disparity tuning profiles when the same set of disparity values were tested using bars or dynamic random dot stereograms, which are among the most commonly used stimuli for this purpose. In a majority of V4 cells (61%), the shape of the disparity tuning profile differed significantly for the two stimulus types. The two sets of stimuli yielded statistically indistinguishable disparity tuning profiles for only a small minority (6%) of V4 cells. These results indicate that disparity tuning in V4 is stimulus-dependent. Given the fact that bar stimuli contain two-dimensional (2-D) shape cues, and the random dot stereograms do not, our results also indicate that V4 cells represent 2-D shape and binocular disparity in an interdependent fashion, revealing an unexpected complexity in the analysis of depth and three-dimensional shape.
Analysis of Variance↗
Visual cortex: the continuing puzzle of area V2.
Surprisingly little is known about the role of V2 in visual processing. A recent study found that the responses of V2 neurons to pairs of angled lines could be predicted from their responses to the individual line components. A simple analysis shows how these neurons may simply sum the responses from one or more orientation selective V1 neurons.
Animals↗
Temporal dynamics of shape analysis in macaque visual area V2.
The firing rate of visual cortical neurons typically changes substantially during a sustained visual stimulus. To assess whether, and to what extent, the information about shape conveyed by neurons in visual area V2 changes over the course of the response, we recorded the responses of V2 neurons in awake, fixating monkeys while presenting a diverse set of static shape stimuli within the classical receptive field. We analyzed the time course of various measures of responsiveness and stimulus-related response modulation at the level of individual cells and of the population. For a majority of V2 cells, the response modulation was maximal during the initial transient response (40-80 ms after stimulus onset). During the same period, the population response was relatively correlated, in that V2 cells tended to respond similarly to specific subsets of stimuli. Over the ensuing 80-100 ms, the signal-to-noise ratio of individual cells generally declined, but to a lesser degree than the evoked-response rate during the corresponding time bins, and the response profiles became decorrelated for many individual cells. Concomitantly, the population response became substantially decorrelated. Our results indicate that the information about stimulus shape evolves dynamically and relatively rapidly in V2 during static visual stimulation in ways that may contribute to form discrimination.
Animals↗
How selective are V1 cells for pop-out stimuli?
Many neurons in visual area V1 respond better to a pop-out stimulus, such as a single vertical bar among many horizontal bars, than to a homogeneous stimulus, such as a stimulus with all vertical bars. Many studies have suggested such cells represent neural correlates of pop-out, or more generally figure-ground segregation. However, preference for pop-out stimuli over homogeneous stimuli could also arise from a nonspecific selectivity for feature discontinuities between the target and the background, without any specificity for pop-out per se. To distinguish between these two confounding scenarios, we compared the responses of V1 neurons to pop-out stimuli with the responses to "conjunction-target" stimuli, which have more complex feature discontinuities between the target and the surround, as in a stimulus with a blue vertical bar among blue horizontal bars and yellow vertical bars. The target in conjunction-target stimuli does not pop out, which we psychophysically verified. V1 cells in general responded similarly to pop-out and conjunction-target stimuli, and only a small minority of cells (approximately 2% by one measure) distinguished the pop-out and conjunction-target stimuli from each other and from homogeneous stimuli. Nevertheless, the responses of approximately 50% of the cells were significantly modulated across all center-surround stimuli, indicating that V1 cells can convey information about the feature discontinuities between the center and the surround as part of a network of neurons, although individual cells by themselves fail to explicitly represent pop-out. In light of our results, unambiguous pop-out selectivity at the level of individual cells remains to be demonstrated in V1 or elsewhere in the visual cortex.
Animals↗
Strategies of shape representation in macaque visual area V2.
Contours and surface textures provide powerful cues used in image segmentation and the analysis of object shape. To learn more about how the visual system extracts and represents these visual cues, we studied the responses of V2 neurons in awake, fixating monkeys to complex contour stimuli (angles, intersections, arcs, and circles) and texture patterns such as non-Cartesian gratings, along with conventional bars and sinusoidal gratings. Substantial proportions of V2 cells conveyed information about many contour and texture characteristics associated with our stimuli, including shape, size, orientation, and spatial frequency. However, the cells differed considerably in terms of their degree of selectivity for the various stimulus characteristics. On average, V2 cells responded better to grating stimuli but were more selective for contour stimuli. Metric multidimensional scaling and principal components analysis showed that, as a population, V2 cells show strong correlations in how they respond to different stimulus types. The first two and five principal components accounted for 69% and 85% of the overall response variation, respectively, suggesting that the response correlations simplified the population representation of shape information with relatively little loss of information. Moreover, smaller random subsets of the population carried response correlation patterns very similar to the population as a whole, indicating that the response correlations were a widespread property of V2 cells. Thus, V2 cells extract information about a number of higher order shape cues related to contours and surface textures and about similarities among many of these shape cues. This may reflect an efficient strategy of representing cues for image segmentation and object shape using finite neuronal resources.
Animals↗
Temporal dynamics of 2D and 3D shape representation in macaque visual area V4.
We studied the temporal dynamics of shape representation in area V4 of the alert macaque monkey. Analyses were based on two large stimulus sets, one equivalent to the 2D shape stimuli used in a previous study of V2, and the other a set of stereoscopic 3D shape stimuli. As in V2, we found that information conveyed by individual V4 neurons about the stimuli tended to be maximal during the initial transient response and generally lower, albeit statistically significant, afterwards. The population response was substantially correlated from one stimulus to the next during the transients, and decorrelated as responses decayed. V4 responses showed significantly longer latencies than in V2, especially for the 3D stimulus set. Recordings from area V1 in a single animal revealed temporal dynamic patterns in response to the 2D shape stimuli that were largely similar to those in V2 and V4. Together with earlier results, these findings provide evidence for a distributed process of coarse-to-fine representation of shape stimuli in the visual cortex.
Action Potentials↗