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PubMed2006

Lightness, filling-in, and the fundamental role of context in visual perception.

Visual perception is defined by the unique spatial interactions that distinguish it from the point-to-point precision of a photometer. Over several decades, Lothar Spillmann has made key observations about the nature of these interactions and the role of context in perception. Our lab has explored the perceptual properties of spatial interactions and more generally the importance of visual context for neuronal responses and perception. Our investigations into the spatiotemporal dynamics of lightness provide insight into underlying mechanisms. For example, backward masking and luminance modulation experiments suggest that the representation of a uniformly luminous object develops first at the borders and, in some manner, the center fills in. The temporal dynamics of lightness induction are also consistent with a filling-in process. There is a slow cutoff temporal frequency above which surround luminance modulation will not elicit perceptual induction of a central area. The larger the central area, the lower the cutoff frequency for induction, perhaps indicating that an edge-based process requires more time to "complete" the larger area. In recordings from primary visual cortex we find that neurons respond in a manner surprisingly consistent with lightness perception and the spatial and temporal properties of induction. For example, the activity of V1 neurons can be modulated by light outside the receptive field and as the modulation rate is increased response modulation falls off more rapidly for large uniform areas than smaller areas. The conclusion we draw from these experiments is that lightness appears to be computed slowly on the basis of edge and context information. A possible role for the spatial interactions is lightness constancy, which is thought to depend on extensive spatial integration. We find not only that V1 responses are strongly context dependent, but that this dependence makes V1 lightness constant on average. The dependence of constancy on surround interactions underscores the fundamental role that context plays in perception. In more recent studies, further support has been found for the importance of context in experiments using natural scene stimuli.

Animals↗

PubMed2006

Visual masking approaches to visual awareness.

In visual masking, visible targets are rendered invisible by modifying the context in which they are presented, but not by modifying the targets themselves. Here I summarize a decade of experimentation using visual masking illusions in which my colleagues and I have begun to establish the minimal set of conditions necessary to maintain the awareness of the visibility of simple unattended stimuli. We have established that spatiotemporal edges must be present for targets to be visible. These spatiotemporal edges must be encoded by transient bursts of spikes in the early visual system. If these bursts are inhibited, visibility fails. Target-correlated activity must rise within the visual hierarchy at least to the level of V3, and be processed within the occipital lobe, to achieve visibility. The specific circuits that maintain visibility are not yet known, but we have deduced that lateral inhibition plays a critical role in sculpting our perception of visibility, both by causing interactions between stimuli positioned across space, and also by shaping the responses to stimuli across time. Further, the studies have served to narrow the number of possible theories to explain visibility and visual masking. Finally, we have discovered that lateral inhibition builds iteratively in strength throughout the visual hierarchy, for both monoptic and dichoptic stimuli. Since binocular information is not integrated until inputs from the two eyes reach the primary visual cortex, it follows that the early visual areas contain differential levels of monoptic and dichoptic lateral inhibitions. We exploited this fact to discover that excitatory integration of binocular inputs occurs at an earlier level than interocular suppression. These findings are potentially fundamental to our understanding of all forms of binocular vision and to determining the role of binocular rivalry in visual awareness.

Animals↗

PubMed1997

THIP, a selective gamma-aminobutyric acid receptor agonist, alters flash-evoked potentials in rats.

This study examined the effects of the GABA(A) agonist THIP on flash-evoked potentials (FEPs) recorded from the primary visual cortex (VC) and superior colliculus (SC) of chronically implanted hooded rats. Animals were given I.P. injections of saline, and of 8, 16, and 24 mg THIP/kg body weight on separate days. Evoked potentials were recorded at 5, 20, and 35 min following injection. Animals were tested at a standard (22.5 degrees C) room temperature. Most significant effects were observed at the 20- and 35-min recording intervals for both the 16 and 24 mg/kg doses, with effects at the 24 mg/kg dose the most pronounced. VC P1 amplitude remained unchanged, while N1 was reduced to such an extent that it became positive, ultimately blending into the rising phase of a positive component appearing between N1 and P2. This positive component had a latency of about 6 ms longer than N1, and became larger in amplitude than P1 at the 24 mg/kg dose. P2 amplitude was drastically reduced, becoming negative. In contrast, components N2 and P3 were augmented, while the amplitude of N3 was unchanged. In the SC, P1 was augmented while P3 was reduced in amplitude. A biphasic (increase/decrease) effect was observed in the N4 complex. In both the VC and SC, latencies of most components were increased, with the late components in the VC increased to the greatest extent. A mild hypothermia was observed at 16 and 24 mg/kg. The results suggest that the GABA(A) receptor plays an important role in the elaboration of the middle (N1-P2) components of FEPs recorded from the rat VC, and that GABAergic mechanisms can influence other components in the VC and SC as well.

Animals↗

PubMed1989

Alzheimer's disease with prominent visual symptoms. Clinical and metabolic evaluation.

The authors examined eight patients with dementia of the Alzheimer's type (DAT), five with prominent visual symptoms early in the illness (VS) and three with no visual symptoms (NVS). Results of neuro-ophthalmologic examinations on VS patients showed relatively consistent abnormalities in figure copying, color vision tested by isochromatic plates, and stereopsis. Cerebral glucose metabolism determined by 18F-fluoro-2-deoxyglucose positron emission tomography (PET) was unchanged in primary visual cortex of VS and NVS patients compared with 12 normal volunteers of similar age and sex. Glucose metabolism in VS patients was decreased by 45 and 34% in left and right visual association cortex (P less than 0.01 and P less than 0.05, respectively) and 34 and 37% in left and right inferior parietal cortex (P less than 0.05) compared with controls; NVS patients had no significant metabolic alteration in these areas. Symptoms, physical examination, and metabolic imaging imply that these patients are a heterogenous but distinct clinical subgroup of DAT often with mild dementia who have visual symptoms due primarily to visual agnosia.

Aged↗

PubMed1999

A neuronal model of attentional spotlight: parietal guiding the temporal.

Recent studies have reported an attentional feedback that highlights neural responses as early along the visual pathway as the primary visual cortex. Such filtering would help in reducing informational overload and in performing serial visual search by directing attention to individual locations in the visual field. The magnocellular (M) and parvocellular (P) subdivisions are two of the major parallel pathways in primate vision that originate in the retina and carry distinctly different types of information. The M pathway, characterized by its high sensitivity to movement and to low contrast stimuli, forms the predominant visual input into the dorsal, parietal stream in the neocortex. The P inputs, characterized by their colour selectivity and higher spatial resolution, are channeled mainly into the ventral, temporal stream. It is proposed that the attentional spotlight originates in the dorsal stream and helps in serially searching the field for conjunction of the relevant target features in the temporal stream, effectively performing a gating function on all visual inputs. This model predicts that a defect limited to the magnocellular or the dorsal pathway can lead to widespread deficits in cognitive abilities, including those functions that are largely based on parvocellular information. For example, the model provides a neural mechanism linking a peripheral defect in the magnocellular pathway to the reading disabilities in dyslexia. Even though there has been strong evidence for a magnocellular deficit in dyslexia, the paradox has been that the cognitive disability seems to be related to P pathway function. The scheme proposed here shows how M input may be vital for controlling sequential attention during reading.

Animals↗

PubMed2001

Efficient evaluation of serial sections by iterative Gabor matching.

Evaluation of electron microscopic images of serial sections is a time-consuming process requiring a high level of expertise. Here we present an algorithm to ease and accelerate this process. It is a modification of an algorithm successfully used in computer vision for object recognition. However, rather than recognising individual structures, we estimate the spatial mapping of a whole section onto the consecutive one. This mapping is used to transfer labelled information of the very first section, e.g. a classification by a human expert of different visible structures, onto structures visible in the next section. We investigate its performance on an artificially constructed benchmark as well as on real electron microscopic samples taken in primary visual cortex and demonstrate its potential for dramatically facilitating the evaluation process of serial sections.

Algorithms↗

PubMed2003

Simultaneous mapping of binocular and monocular receptive fields in awake monkeys for calibrating eye alignment in a dichoptical setup.

We developed a modified Wheatstone stereoscope for simultaneous dichoptical and binocular stimulation in awake monkey. We, therefore, extended the conventional two-screen Wheatstone stereoscope to a setup with an additional third screen viewed binocularly via semi-transparent mirrors. With a sparse noise stimulation we mapped classical receptive field (CRF) positions via each screen independently but simultaneously. This was done for multiple recording positions (16 electrodes) at once in primary visual cortex based on multiple unit spike activity (MUA) and local field potentials (LFP), respectively. The technique can be used to (1) quickly and simultaneously determine binocular as well as left and right eye CRFs, including ocular dominance characteristics (net recording time for the given examples: approximately 2 min), (2) precisely adjust dichoptical stimulation by evaluating offsets between monocular and binocular CRF positions (average spatial incongruency between binocular and left/right eye stimulation after calibration: approximately 0.025 degrees visual angle), and (3) investigate left and right eye interaction in forming binocular CRFs. Due to the precise adjustment of the dichoptical and the simultaneous binocular stimulation investigations on the basis of stereo vision can be done with appropriate eye vergence alignment matching normal binocular viewing conditions in awake animals.

Animals↗

PubMed2003

Complexity in the modulation of neurotrophic factor mRNA expression by early visual experience.

The expression of mRNA for brain-derived neurotrophic factor (BDNF) is regulated by early visual experience. In this study, we sought to determine whether other neurotrophic factor mRNAs are similarly regulated. We reared pigmented rats from birth to postnatal day 21 in a normal light cycle, constant light (LR) or constant darkness (DR). In the retina, superior colliculus (SC), primary visual cortex (V1), hippocampus (HIPP) and cerebellum (CBL), using a ribonuclease protection assay (RPA), we examined expression of the mRNAs for nerve growth factor (NGF), BDNF, NT3, NT4, ciliary neurotrophic factor (CNTF) and glial cell line-derived neurotrophic factor (GDNF). LR or DR alter the expression of the mRNAs for NGF, BDNF and NT3 and CNTF within the visual system. LR also upregulated BDNF mRNA expression within the cerebellum. In all of the structures examined, NT4 mRNA expression was unaltered by LR or DR and GDNF mRNA was undetectable. Notably, the same rearing condition could induce changes of opposite sign in the mRNA for a single factor in different structures or for different factors in the same structure. Thus, during developmental stages when sensory experience and neuroelectric activity are important in the shaping of visual circuitry, vision regulates the expression of multiple neurotrophic factor mRNAs and each mRNA has a unique profile with respect to the locus and sign of activity-induced changes.

Animals↗

PubMed1999

Visual deprivation increases capability of layer II/III for epileptiform activity in the rat visual cortical slices.

Effects of visual deprivation on the induction of epileptiform activity were studied in layer II/III of 29-39-day-old rat primary visual cortex. Field potentials were evoked by stimulation of layer IV in slices from control (CON) and dark-reared (DR) rats. Picrotoxin (PTX)-induced epileptiform activity was characterized by spontaneous and evoked epileptic field potentials (EFPs). The results showed that DR slices demonstrate greater susceptibility for induction of spontaneous EFP. PTX-induced changes in the characteristics of evoked field potentials also showed higher tendency of DR animals to generate epileptiform activity. In both groups, field potentials consisted of pEPSP(1) (population excitatory postsynaptic potential 1, i.e., first negativity) and pEPSP(2) (second negativity), respectively. There was no significant difference between the characteristics of field potentials in CON and DR slices. PTX significantly increased amplitude and duration of pEPSP(2), but it had no significant effect on pEPSP(1). Effects of PTX on pEPSP(2) were significantly higher in DR slices. It is concluded that visual deprivation results in a heightened potential in layer II/III of the rat visual cortex to generate PTX-induced epileptiform activity.

Action Potentials↗

PubMed2000

Low-level memory processes in vision.

Psychophysical studies of the short-term memory for attributes or dimensions of the visual stimulus that are known to be important in early visual processing (spatial frequency, orientation, contrast, motion and color) identify a low-level perceptual memory mechanism. This proposed mechanism is located early in the visual processing stream, prior to the structural description system responsible for shape priming but beyond primary visual cortex (V1); it is composed of a series of parallel, special-purpose perceptual mechanisms with independent but limited processing resources. Each mechanism is devoted to the analysis of a single dimension and is coupled to a memory store.

Humans↗

PubMed2000

How are visual areas of the brain connected to motor areas for the sensory guidance of movement?

Visual areas of the brain must be connected to motor areas for the sensory guidance of movement. The first step in the pathway from the primary visual cortex is by way of the dorsal stream of visual areas in the parietal lobe. The fact that monkeys can still guide their limbs visually after cortico-cortical fibres have been severed suggests that there are subcortical routes that link visual and motor areas of the brain. The pathway that runs from the pons and cerebellum is the largest of these. Pontine cells that receive inputs from visual cortical areas or the superior colliculus respond vigorously to appropriate visual stimuli and project widely on the cerebellar cortex. A challenge for future research is to elucidate the role of these cerebellar target areas in visuo-motor control.

Animals↗

PubMed1996

Varieties of vision: from blind responses to conscious recognition.

Lesions in consecutive parts of the visual system cause visual deficits that spare increasingly complex residual functions. Patients with lesions up to and including primary visual cortex can show neuroendocrine, reflexive, implicit and forced-choice responses to visual stimulation but no conscious vision. In contrast, patients with lesions in higher visual cortical areas have conscious vision. Its lowest level is that of phenomenal vision, followed by object vision and recognition. These levels are dissociable. They require the integrity of different parts of the system.

Animals↗

PubMed1991

The effects of posterior cortical lesions on responses to visual threats in the Mongolian gerbil (Meriones unguiculatus).

Mongolian gerbils received aspiration lesions of either primary visual cortex (PVC), medial extrastriate visual cortex, retrosplenial cortex (RSC), or sham operations. The responses of gerbils to the presentation of an overhead visual stimulus were recorded in an open field. In all groups, presentation of the stimulus produced an increase in rearing. This suggests that the stimulus was detected by all animals. Gerbils with RSC or PVC lesions showed reduced levels of response to the stimulus. We suggest that some of the observed deficits can be explained as failures to produce responses to threat that are appropriate to the context in which the the threat was presented.

Animals↗

PubMed1998

Awareness and confidence ratings in motion perception without geniculo-striate projection.

We have previously reported that under certain experimental conditions, a subject with damaged primary visual cortex performed significantly above chance in discriminating motion direction and orientation either with or without awareness of the stimulus presentation in his blind hemifield. The subject's performance varied as a function of stimulus speed, excursion length, and background luminance. Present observations confirm the previous findings of above chance discrimination scores both with and without awareness, but nevertheless indicate an overall increase in sensitivity over the past 2 years. In addition to discriminating the direction of motion, the subject was asked on every trial to report either awareness or confidence or both, on a six-point scale. The results show that the introduction of a six-point scale for the reported level of awareness yielded similar results to those when the subject was given a binary choice to indicate the presence or absence of conscious awareness. The area under the receiver operating characteristic (ROC) curves obtained from the rating data was used to compare results across different experimental conditions. It was found that although the subject's reported confidence and awareness varied monotonically as a function of stimulus speed, they were not equivalent measures.

Adult↗

PubMed2002

Differential induction and decay curves of c-fos and zif268 revealed through dual activity maps.

The use of inducible transcription factors for mapping neural activity is now a common procedure. We have previously developed a double-labelling technique that allows visualization of activated neurons after two different stimulation sequences. The technique exploits the differential time course of mRNA versus protein expression of transcription factors. However, the precise details of the differential time course remained unknown. Here, we provide a complete up- and downregulation profile for both the c-fos and zif268 genes, as determined through combined in situ hybridization and immunocytochemical detection of the mRNA and protein products in primary visual cortex. The data presented here can be used in the design of future studies employing double-label mapping of neural activation following a compound stimulus.

Animals↗

PubMedDate not supplied

Neuron numbers and sizes in aging brain: comparisons of human, monkey, and rodent data.

One of the several sources of interest in aging animal brains is their potential as models of the aging human brain. In this review we examine whether neuron numbers and sizes change similarly in aging human, monkey and rodent brain regions which data are available from more than one species. The number of brain regions studied in more than one species is surprisingly limited. Some regions show correspondence in age-related changes between humans and selected animal models (primary visual cortex, CA1 of hippocampus). For the majority of regions the data are conflicting, even within one species (e.g., somatosensory cortex, frontal cortex, cerebellum, cholinergic forebrain areas, locus coeruleus). Although some of the conflicting data may be attributed to procedural differences, particularly when data are expressed as density changes, much must be attributed to real species and/or strain differences in rodents. We conclude that neuron numbers and sizes may show similar age-related changes in human and animal brains only for sharply defined brain regions, animal species and/or strains, and age ranges.

Aging↗

PubMedDate not supplied

Rapid adaptation and efficient coding.

Rapid adaptation is a prominent feature of biological neuronal systems. From a functional perspective the adaptation of neuronal properties, namely the input-output relation of sensory neurons, is usually interpreted as an adaptation of the sensory system to changing environments as characterized by their stimulus statistics. Here we argue that this interpretation is only applicable as long as the adaptation processes are slower than the time-scale at which the stimulus statistics change. We present a definition of optimality of a neuronal code which still captures the idea of efficient coding, but which can also explain rapid adaptation without referring to an adaptation to different sensory environments. Finally, we apply our new idea to a simple model of an orientation hypercolumn in the primary visual cortex and predict that the interactions between orientation columns should adapt at the time-scale of a single stimulus presentation.

Adaptation, Biological↗

PubMed2000

Naming of animals and tools: a functional magnetic resonance imaging study of categorical differences in the human brain areas commonly used for naming visually presented objects.

To investigate the neural substrates for naming objects and their category-dependency, we performed functional magnetic resonance imaging (fMRI) with naming of animals and tools. Naming objects, irrespective of their category, activated left frontal to bilateral parietal regions and occipital to posterior temporal regions. Within these areas, naming animals caused more activation of the primary visual cortex bilaterally and the ventral occipital cortex to the inferior temporal area on the right side. Naming tools caused more activation of the posterior part of the left middle temporal area, the rostral part of the left inferior parietal lobule, and the left inferior frontal cortex. These findings suggest that the neural network for naming objects has discrete category-dependent nodes through which pertinent conceptual knowledge may be mediated.

Adult↗