Beyond Shape: How You Learn about Objects Affects How They Are Represented in Visual Cortex

Background: Experience can alter how objects are represented in the visual cortex. But experience can take different forms. It is unknown whether the kind of visual experience systematically alters the nature of visual cortical object representations. Methodology/Principal Findings: We take advantage of different training regimens found to produce qualitatively different types of perceptual expertise behaviorally in order to contrast the neural changes that follow different kinds of visual experience with the same objects. Two groups of participants went through training regimens that required either subordinate-level individuation or basic-level categorization of a set of novel, artificial objects, called ‘‘Ziggerins’’. fMRI activity of a region in the right fusiform gyrus increased after individuation training and was correlated with the magnitude of configural processing of the Ziggerins observed behaviorally. In contrast, categorization training caused distributed changes, with increased activity in the medial portion of the ventral occipito-temporal cortex relative to more lateral areas. Conclusions/Significance: Our results demonstrate that the kind of experience with a category of objects can systematically influence how those objects are represented in visual cortex. The demands of prior learning experience therefore appear t

Birds of a Feather Flock Together: Experience-Driven Formation of Visual Object Categories in Human Ventral Temporal Cortex

The present functional magnetic resonance imaging study provides direct evidence on visual object-category formation in the human brain. Although brain imaging has demonstrated object-category specific representations in the occipitotemporal cortex, the crucial question of how the brain acquires this knowledge has remained unresolved. We designed a stimulus set consisting of six highly similar bird types that can hardly be distinguished without training. All bird types were morphed with one another to create different exemplars of each category. After visual training, fMRI showed that responses in the right fusiform gyrus were larger for bird types for which a discrete category-boundary was established as compared with not-trained bird types. Importantly, compared with not-trained bird types, right fusiform responses were smaller for visually similar birds to which subjects were exposed during training but for which no category-boundary was learned. These data provide evidence for experience-induced shaping of occipitotemporal responses that are involved in category learning in the human brain

A feedback model of perceptual learning and categorisation

Top-down, feedback, influences are known to have significant effects on visual information processing. Such influences are also likely to affect perceptual learning. This article employs a computational model of the cortical region interactions underlying visual perception to investigate possible influences of top-down information on learning. The results suggest that feedback could bias the way in which perceptual stimuli are categorised and could also facilitate the learning of sub-ordinate level representations suitable for object identification and perceptual expertise

Learning alters the tuning of functional magnetic resonance imaging patterns for visual forms

Learning is thought to facilitate the recognition of objects by optimizing the tuning of visual neurons to behaviorally relevant features. However, the learning mechanisms that shape neural selectivity for visual forms in the human brain remain essentially unknown. Here, we combine behavioral and functional magnetic resonance imaging (fMRI) measurements to test the mechanisms that mediate enhanced behavioral sensitivity in the discrimination of visual forms after training. In particular, we used high-resolution fMRI and multivoxel pattern classification methods to investigate fine learning-dependent changes in neural preference for global forms. We measured the observers' choices when discriminating between concentric and radial patterns presented in noise before and after training. Similarly, we measured the choices of a pattern classifier when predicting each stimulus from fMRI activity. Comparing the performance of human observers and classifiers demonstrated that learning alters the observers' sensitivity to visual forms and the tuning of fMRI activation patterns in visual areas selective for task-relevant features. In particular, training on low-signal stimuli enhanced the amplitude but reduced the width of pattern-based tuning functions in higher dorsal and ventral visual areas. Thus, our findings suggest that learning of visual patterns is implemented by enhancing the response to the preferred stimulus category and reducing the response to nonpreferred stimuli in higher extrastriate visual cortex

Learning selective top-down control enhances performance in a visual categorization task.

We model the putative neuronal and synaptic mechanisms involved in learning a visual categorization task, taking inspiration from single-cell recordings in inferior temporal cortex (ITC). Our working hypothesis is that learning the categorization task involves both bottom-up, ITC to prefrontal cortex (PFC), and top-down (PFC to ITC) synaptic plasticity and that the latter enhances the selectivity of the ITC neurons encoding the task-relevant features of the stimuli, thereby improving the signal-to-noise ratio. We test this hypothesis by modeling both areas and their connections with spiking neurons and plastic synapses, ITC acting as a feature-selective layer and PFC as a category coding layer. This minimal model gives interesting clues as to properties and function of the selective feedback signal from PFC to ITC that help solving a categorization task. In particular, we show that, when the stimuli are very noisy because of a large number of nonrelevant features, the feedback structure helps getting better categorization performance and decreasing the reaction time. It also affects the speed and stability of the learning process and sharpens tuning curves of ITC neurons. Furthermore, the model predicts a modulation of neural activities during error trials, by which the differential selectivity of ITC neurons to task-relevant and task-irrelevant features diminishes or is even reversed, and modulations in the time course of neural activities that appear when, after learning, corrupted versions of the stimuli are input to the network

Recognition by Flickering Components: The Effect of Temporal Modulation on Image Recognition

A primary goal of vision is to identify objects rapidly and efficiently. Successful object and scene recognition results from the integration of both feed-forward and feedback processes that correspond a two-dimensional retinal image to a representation of its content stored in memory (Bar, 2003). One general organizing principle may be that the visual system analyzes images and scenes according to their spatial components in a coarse- (low spatial frequency) to-fine (high spatial frequency) sequence (Bullier, 2001; Hegde, 2008). An individual’s sensitivity to these spatial components is described by contrast sensitivity function (CSF), which indicates the minimum contrast required for the detection of patterns of various sizes. A consistent finding is that, when temporally modulated at a moderate rate, sensitivity to lower spatial frequency gratings is heightened relative to its static counterpart (Robson, 1966; Abramov et al., 2012). This suggests that temporal modulation may enhance image detectability, especially so for those coarsest spatial components—the lower spatial frequencies—that seem to be most important for detection and categorization of objects and scenes. We presented participants with an array of grayscale images depicting objects and scenes under 3 spatial (3cpd-filtered, 4cpd-filtered, and unfiltered full-spectrum) and 3 temporal (static, 6Hz counterphase flicker, and 250ms presentation) conditions, at 4 levels of near-threshold contrast. Responses were open-ended identifications. Temporal modulation was hypothesized to improve recognition in all spatial conditions, while short-duration presentation was predicted to result in performance comparable to the full-duration static condition. Males were hypothesized to perform better than females based on previously reported (Abramov, et al., 2012) sensitivity differences. Results partially supported our hypotheses: temporal modulation at 6Hz had recognition enhancement effects only for the lowest spatial frequency-cutoff—3 cpd. We describe a distinction among the neurological underpinnings of the CSF into two separate mechanisms responsible for the lower and upper halves of the CSF

Change blindness: eradication of gestalt strategies

Arrays of eight, texture-defined rectangles were used as stimuli in a one-shot change blindness (CB) task where there was a 50% chance that one rectangle would change orientation between two successive presentations separated by an interval. CB was eliminated by cueing the target rectangle in the first stimulus, reduced by cueing in the interval and unaffected by cueing in the second presentation. This supports the idea that a representation was formed that persisted through the interval before being 'overwritten' by the second presentation (Landman et al, 2003 Vision Research 43149–164]. Another possibility is that participants used some kind of grouping or Gestalt strategy. To test this we changed the spatial position of the rectangles in the second presentation by shifting them along imaginary spokes (by ±1 degree) emanating from the central fixation point. There was no significant difference seen in performance between this and the standard task [F(1,4)=2.565, p=0.185]. This may suggest two things: (i) Gestalt grouping is not used as a strategy in these tasks, and (ii) it gives further weight to the argument that objects may be stored and retrieved from a pre-attentional store during this task

Neuronale Korrelate zerebral bedingter visueller Wahrnehmungsstörungen

Two aspects of visual processing are investigated in this work. First, the impact of unilateral occipito-temporal cortical lesions on object categorization, and second, the influence of top-down manipulation on object categorization in healthy subjects. Major visual object recognition deficits (object agnosia) are rare, and occur mainly after bilateral damage to ventral visual cortices. Most patients suffering from unilateral ventral lesions are clinically non-agnostic. We studied the effect of unilateral occipito-temporal lesions on object categorization and its underlying neural correlates. The findings indicate that unilateral ventral lesions lead to disturbed processing in the lesioned hemisphere, which affects object categorization in both visual hemifields. In healthy object categorization bottom-up and top-down influences interact. We investigated the combination of different top-down influences and their effect on rapid object categorization. The results implicate that different top-down manipulations interact strongly, which should be taken into account when interpreting categorization results