Early retinal activity and visual circuit development

Does spontaneous retinal activity prior to vision play a role in the establishment of visual maps? In this issue of Neuron, two separate papers by Huberman et al. and Hooks and Chen demonstrate a role for early spontaneous retinal activity in the establishment of ocular dominance columns and synaptic refinement at retinogeniculate synapses.     

Early development of visual cortex in human fetuses

Prenatal development of visual cortex (area 17) was studied in human fetuses of 8-9, 13-15 and 16-18 weeks of gestation, with a view to analyse the early critical events. Under light microscope, five zones of development were seen in all the age groups. The total thickness of cortex of area 17 as well as that of its cortical plate was measured with the help of camera lucida. It was observed that the total thickness of the cortex increased with increase in age. Diversity in the shape, size, staining intensity and arrangement of neurons was noted in the different zones. Most of the cells were found to have a thin rim of cytoplasm and a prominent nucleus with multiple nucleoli. The cells in subventricular zone and cortical plate were regularly arranged in vertical rows while in other zones, they were irregularly scattered. Several mitotic figures were seen in the ventricular zone at 8-9 weeks but later they were also noticed in subventricular and intermediate zones. In the later ages the mitotic figures were observed to be fewer in the ventricular zone. No mitosis was seen in cortical plate at any age period.     

Characteristics of invariant recognition of visual objects in preschool children with typical and atypical development

The invariant recognition ability of five- to six-year-old children with typical and atypical patterns of development in terms of the shape of visual images varying in color, size, and location has been studied. It has been shown that the typically developing children of this age are able to identify the invariant form of a visual object regardless of any changes in its color, size, or location. The children with neurodevelopmental disorders have difficulties with identifying the shape of a visual object when its location among a large number of figures is changed. The children with early infantile autism exhibit different degrees of visual perceptual deficit. The children with the milder forms of autistic disorders have difficulties only with recognizing the shape of an image when its location is changed; the children with more severe forms of disorders showed a serious impairment of invariant recognition regardless of image color, size, and location.     

Early recognition of cardiac tamponade.

Nine patients with cardiac tamponade were seen in an 11-month period. Analysis of the clinical and laboratory data indicated that pulsus paradoxus was the most useful physical sign and echocardiography the most useful investigative technique. Three of nine patients died but in only one was the late recognition of tamponade a possible factor in the outcome. Pericardial fenestrations were required in four patients. Viral pericarditis accounted for the tamponade in three cases. There were two cases each of uremia and malignant disease and one case of tuberculous pericarditis. One other case followed pericardiectomy. In five patients tamponade was the initial manifestation of illness.     

A model of visual recognition and categorization.

To recognize a previously seen object, the visual system must overcome the variability in the object''s appearance caused by factors such as illumination and pose. Developments in computer vision suggest that it may be possible to counter the influence of these factors, by learning to interpolate between stored views of the target object, taken under representative combinations of viewing conditions. Daily life situations, however, typically require categorization, rather than recognition, of objects. Due to the open-ended character of both natural and artificial categories, categorization cannot rely on interpolation between stored examples. Nonetheless, knowledge of several representative members, or prototypes, of each of the categories of interest can still provide the necessary computational substrate for the categorization of new instances. The resulting representational scheme based on similarities to prototypes appears to be computationally viable, and is readily mapped onto the mechanisms of biological vision revealed by recent psychophysical and physiological studies.     

Mechanisms of visual object recognition studied in monkeys

Cells in area TE of the inferotemporal cortex of the monkey brain selectively respond to various moderately complex object-features, and those responding to similar features cluster in a columnar region elongated vertical to the cortical surface. Although cells within a column respond to similar features, their selectivity is not identical. The data of optical imaging in TE have suggested that the borders between neighboring columns are not discrete but columns representing related features overlap one another. We have also found, by training adult monkeys for discrimination of a specific set of shapes, that such a long-term training increases the proportion of TE cells responding to the shapes used in the training even in the adult. The data suggested that TE plays important roles in discrimination of complex shapes and in visual expert learning of discriminating a certain class of objects in the adult.     

Expression of long-term depression underlies visual recognition memory

The modifications occurring in the brain during learning and memory are still poorly understood but may involve long-lasting changes in synaptic transmission (synaptic plasticity). In perirhinal cortex, a lasting decrement in neuronal responsiveness is associated with visual familiarity discrimination, leading to the hypothesis that long-term depression (LTD)-like synaptic plasticity may underlie recognition memory. LTD relies on internalization of AMPA receptors (AMPARs) through interaction between their GluR2 subunits and AP2, the clathrin adaptor protein required for endocytosis. We demonstrate that a peptide that blocks interactions between GluR2 and AP2 blocks LTD in perirhinal cortex in vitro. Viral transduction of this peptide in perirhinal cortex produced striking deficits in visual recognition memory. Furthermore, there was a deficit of LTD in perirhinal cortex slices from virally transduced, recognition memory-deficient animals. These results suggest that internalization of AMPA receptors, a process critical for the expression of LTD in perirhinal cortex, underlies visual recognition memory.     

Early detection of visual defects in infancy

To determine the part played by screening in detecting visual defects questionnaires were sent to 240 families with blind or partially sighted children identified from the Family Fund''s database. Questions were asked on social and family background, the visual disorder and its severity, any other disability, and how and when the disabilities were discovered and subsequently managed. Data from 189 families were analysed, constituting all those with children with major visual defects from the 219 families who replied. The visual defect was first discovered in 111 children by parents, friends, and neighbours, and in 36 by a doctor at the neonatal examination. In only three children who did not have a family history of visual impairment was the defect discovered during a formal screening examination at a child health clinic. Dissatisfaction about medical services was expressed by about a third of the parents, particularly a lack of provision of information and consideration of their worries and a failure to refer the child promptly to educational and treatment services.Visual defects in children under 5 are generally detected by family and friends, not by screening, but detection by the medical profession could be improved by increased awareness and observation and quick referral.     

A model of visual development

There is now abundant evidence that the structure of the mammalian visual cortex is not innately determined but can be altered by visual experiences during a certain period at an early age. A model based on the evidence is proposed that will explain some aspects of the developmental process in the visual cortex. The model self-organizingly forms a simple cell which responds to bars and edges of a certains orientation and retinal position. The total system of the model corresponds to a hyper column which contains a complete cycle of orientational selectiviy. The model has the following three original points. i) A hyper column is finally formed in the model. ii) Most of the cells do not have orientation selectivity in the initial state. iii) It is hypothesized that the orientation continuity of the hyper column is caused by similar orientations of successive input stimuli. The results of computer simulation show that the model has the expected performance.     

Signal transformation and pattern recognition in visual pathways

A model of the visual pathways it has been developed consisting of two neural systems working together: a feed-forward mechanism given by convolutions through different layers and a feed-back one consiting of the comparison of the results of the first one. The first mechanism would be predominant at lower centers, and it would be able, for example, to explain the increase in contrast sensitivity at different stations of visual pathways. The second mechanism seems apter to explain the behaviour of complex and hypercomplex cells whose activity is to a great extent independent of the position of the stimulus in the receptive field. The ideas developed in this paper could have some implications for building a machine able to recognize patterns independently of their position.     

Cortical mechanisms specific to explicit visual object recognition

The cortical mechanisms associated with conscious object recognition were studied using functional magnetic resonance imaging (fMRI). Participants were required to recognize pictures of masked objects that were presented very briefly, randomly and repeatedly. This design yielded a gradual accomplishment of successful recognition. Cortical activity in a ventrotemporal visual region was linearly correlated with perception of object identity. Therefore, although object recognition is rapid, awareness of an object's identity is not a discrete phenomenon but rather associated with gradually increasing cortical activity. Furthermore, the focus of the activity in the temporal cortex shifted anteriorly as subjects reported an increased knowledge regarding identity. The results presented here provide new insights into the processes underlying explicit object recognition, as well as the analysis that takes place immediately before and after recognition is possible.     

Transformation and relational-structure schemes for visual pattern recognition

Two models for visual pattern recognition are described; the one based on application of internal compensatory transformations to pattern representations, the other based on encoding of patterns in terms of local features and spatial relations between these local features. These transformation and relational-structure models are each endowed with the same experimentally observed invariance properties, which include independence to pattern translation and pattern jitter, and, depending on the particular versions of the models, independence to pattern reflection and inversion (180° rotation). Each model is tested by comparing the predicted recognition performance with experimentally determined recognition performance using as stimuli random-dot patterns that were variously rotated in the plane. The level of visual recognition of such patterns is known to depend strongly on rotation angle. It is shown that the relational-structure model equipped with an invariance to pattern inversion gives responses which are in close agreement with the experimental data over all pattern rotation angles. In contrast, the transformation model equipped with the same invariances gives poor agreement to the experimental data. Some implications of these results are considered.     

Learning and neural plasticity in visual object recognition

The capability of the adult primate visual system for rapid and accurate recognition of targets in cluttered, natural scenes far surpasses the abilities of state-of-the-art artificial vision systems. Understanding this capability remains a fundamental challenge in visual neuroscience. Recent experimental evidence suggests that adaptive coding strategies facilitated by underlying neural plasticity enable the adult brain to learn from visual experience and shape its ability to integrate and recognize coherent visual objects.     

Active manual control of object views facilitates visual recognition

Active exploration of large-scale environments leads to better learning of spatial layout than does passive observation [1] [2] [3]. But active exploration might also help us to remember the appearance of individual objects in a scene. In fact, when we encounter new objects, we often manipulate them so that they can be seen from a variety of perspectives. We present here the first evidence that active control of the visual input in this way facilitates later recognition of objects. Observers who actively rotated novel, three-dimensional objects on a computer screen later showed more efficient visual recognition than observers who passively viewed the exact same sequence of images of these virtual objects. During active exploration, the observers focused mainly on the 'side' or 'front' views of the objects (see also [4] [5] [6]). The results demonstrate that how an object is represented for later recognition is influenced by whether or not one controls the presentation of visual input during learning.     

Invariant recognition of feature combinations in the visual system

 The operation of a hierarchical competitive network model (VisNet) of invariance learning in the visual system is investigated to determine how this class of architecture can solve problems that require the spatial binding of features. First, we show that VisNet neurons can be trained to provide transform-invariant discriminative responses to stimuli which are composed of the same basic alphabet of features, where no single stimulus contains a unique feature not shared by any other stimulus. The investigation shows that the network can discriminate stimuli consisting of sets of features which are subsets or supersets of each other. Second, a key feature-binding issue we address is how invariant representations of low-order combinations of features in the early layers of the visual system are able to uniquely specify the correct spatial arrangement of features in the overall stimulus and ensure correct stimulus identification in the output layer. We show that output layer neurons can learn new stimuli if the lower layers are trained solely through exposure to simpler feature combinations from which the new stimuli are composed. Moreover, we show that after training on the low-order feature combinations which are common to many objects, this architecture can – after training with a whole stimulus in some locations – generalise correctly to the same stimulus when it is shown in a new location. We conclude that this type of hierarchical model can solve feature-binding problems to produce correct invariant identification of whole stimuli. Received: 4 August 1999 / Accepted in revised form: 11 October 2000