Efficiency of the Human Visual System in Recognition of Moving Objects

The relationships studied between the efficiency of the human visual system for recognition of moving objects in the presence of dynamic additive quasi-white Gaussian noise and the duration of object presentation are studied. Different conditions of observation and the parameters of moving objects were analyzed. The efficiency-presentation duration relationship was shown to be a function with two extrema (a minimum and a maximum, at 120 and 400 ms, respectively). The study offers an interpretation of the findings and a functional model to explain this phenomenon.     

Efficiency of a visual system under conditions of uncertainty of the appearance of a moving object

The effect of uncertainty of a moving object appearance in the noise field upon the coefficient efficiency, we studied. At short durations of presentation (40-80 msec) and high level of external noise, this effect was maximal: magnified 100 times. The efficiency coefficient dependence on the duration of a moving object presentation was shown to be characterized by two maximums. The position of minimum situated between these two maximums was found to be independent of either presence or absence of uncertainty of a number of parameters: such as initial position of the object the image, time of its appearance, noise level, velocity and direction of the movement, and has a latency approximately 120 sec. A functional model of the observed phenomena, has been proposed.     

Motion-based prediction explains the role of tracking in motion extrapolation

During normal viewing, the continuous stream of visual input is regularly interrupted, for instance by blinks of the eye. Despite these frequents blanks (that is the transient absence of a raw sensory source), the visual system is most often able to maintain a continuous representation of motion. For instance, it maintains the movement of the eye such as to stabilize the image of an object. This ability suggests the existence of a generic neural mechanism of motion extrapolation to deal with fragmented inputs. In this paper, we have modeled how the visual system may extrapolate the trajectory of an object during a blank using motion-based prediction. This implies that using a prior on the coherency of motion, the system may integrate previous motion information even in the absence of a stimulus. In order to compare with experimental results, we simulated tracking velocity responses. We found that the response of the motion integration process to a blanked trajectory pauses at the onset of the blank, but that it quickly recovers the information on the trajectory after reappearance. This is compatible with behavioral and neural observations on motion extrapolation. To understand these mechanisms, we have recorded the response of the model to a noisy stimulus. Crucially, we found that motion-based prediction acted at the global level as a gain control mechanism and that we could switch from a smooth regime to a binary tracking behavior where the dot is tracked or lost. Our results imply that a local prior implementing motion-based prediction is sufficient to explain a large range of neural and behavioral results at a more global level. We show that the tracking behavior deteriorates for sensory noise levels higher than a certain value, where motion coherency and predictability fail to hold longer. In particular, we found that motion-based prediction leads to the emergence of a tracking behavior only when enough information from the trajectory has been accumulated. Then, during tracking, trajectory estimation is robust to blanks even in the presence of relatively high levels of noise. Moreover, we found that tracking is necessary for motion extrapolation, this calls for further experimental work exploring the role of noise in motion extrapolation.     

Neural Representation of Ambiguous Visual Objects in the Inferior Temporal Cortex

Inferior temporal (IT) cortex as the final stage of the ventral visual pathway is involved in visual object recognition. In our everyday life we need to recognize visual objects that are degraded by noise. Psychophysical studies have shown that the accuracy and speed of the object recognition decreases as the amount of visual noise increases. However, the neural representation of ambiguous visual objects and the underlying neural mechanisms of such changes in the behavior are not known. Here, by recording the neuronal spiking activity of macaque monkeys’ IT we explored the relationship between stimulus ambiguity and the IT neural activity. We found smaller amplitude, later onset, earlier offset and shorter duration of the response as visual ambiguity increased. All of these modulations were gradual and correlated with the level of stimulus ambiguity. We found that while category selectivity of IT neurons decreased with noise, it was preserved for a large extent of visual ambiguity. This noise tolerance for category selectivity in IT was lost at 60% noise level. Interestingly, while the response of the IT neurons to visual stimuli at 60% noise level was significantly larger than their baseline activity and full (100%) noise, it was not category selective anymore. The latter finding shows a neural representation that signals the presence of visual stimulus without signaling what it is. In general these findings, in the context of a drift diffusion model, explain the neural mechanisms of perceptual accuracy and speed changes in the process of recognizing ambiguous objects.     

Neural Effects of Auditory Distraction on Visual Attention in Schizophrenia

Sensory flooding, particularly during auditory stimulation, is a common problem for patients with schizophrenia. The functional consequences of this impairment during cross-modal attention tasks, however, are unclear. The purpose of this study was to examine how auditory distraction differentially affects task-associated response during visual attention in patients and healthy controls. To that end, 21 outpatients with schizophrenia and 23 healthy comparison subjects performed a visual attention task in the presence or absence of distracting, environmentally relevant “urban” noise while undergoing functional magnetic resonance imaging at 3T. The task had two conditions (difficult and easy); task-related neural activity was defined as difficult – easy. During task performance, a significant distraction (noise or silence) by group (patient or control) interaction was observed in the left dorsolateral prefrontal cortex, right hippocampus, left temporoparietal junction, and right fusiform gyrus, with patients showing relative hypoactivation during noise compared to controls. In patients, the ability to recruit the dorsolateral prefrontal cortex during the task in noise was negatively correlated with the effect of noise on reaction time. Clinically, the ability to recruit the fusiform gyrus during the task in noise was negatively correlated with SANS affective flattening score, and hippocampal recruitment during the task in noise was positively correlated with global functioning. In conclusion, schizophrenia may be associated with abnormalities in neural response during visual attention tasks in the presence of cross-modal noise distraction. These response differences may predict global functioning in the illness, and may serve as a biomarker for therapeutic development.     

Advantage of hole stimulus in rivalry competition

Mounting psychophysical evidence suggests that early visual computations are sensitive to the topological properties of stimuli, such as the determination of whether the object has a hole or not. Previous studies have demonstrated that the hole feature took some advantages during conscious perception. In this study, we investigate whether there exists a privileged processing for hole stimuli during unconscious perception. By applying a continuous flash suppression paradigm, the target was gradually introduced to one eye to compete against a flashed full contrast Mondrian pattern which was presented to the other eye. This method ensured that the target image was suppressed during the initial perceptual period. We compared the initial suppressed duration between the stimuli with and without the hole feature and found that hole stimuli required less time than no-hole stimuli to gain dominance against the identical suppression noise. These results suggest the hole feature could be processed in the absence of awareness, and there exists a privileged detection of hole stimuli during suppressed phase in the interocular rivalry.     

Preparatory effects of distractor suppression: evidence from visual cortex

Spatial selective attention is the mechanism that facilitates the selection of relevant information over irrelevant information in the visual field. The current study investigated whether foreknowledge of the presence or absence of distractors surrounding an impending target stimulus results in preparatory changes in visual cortex. We cued the location of the target and the presence or absence of distractors surrounding the target while changes in blood oxygen level dependent (BOLD) signals were measured. In line with prior work, we found that top-down spatial attention resulted in an increased contralateral BOLD response, evoked by the cue throughout early visual cortex (areas V1, V2 and V3). In addition, cues indicating distractor presence evoked a substantial increase in the magnitude of the BOLD signal in visual area V3, but not in V2 or V1. This study shows that prior knowledge concerning the presence of a distractor results in enhanced attentional modulation of visual cortex, in visual areas where neuronal receptive fields are large enough to encompass both targets and distractors. We interpret these findings as evidence that top-down attentional control processes include active preparatory suppression mechanisms for irrelevant, distracting information in the visual scene.     

Temporal structure in audiovisual sensory selection

In natural environments, sensory information is embedded in temporally contiguous streams of events. This is typically the case when seeing and listening to a speaker or when engaged in scene analysis. In such contexts, two mechanisms are needed to single out and build a reliable representation of an event (or object): the temporal parsing of information and the selection of relevant information in the stream. It has previously been shown that rhythmic events naturally build temporal expectations that improve sensory processing at predictable points in time. Here, we asked to which extent temporal regularities can improve the detection and identification of events across sensory modalities. To do so, we used a dynamic visual conjunction search task accompanied by auditory cues synchronized or not with the color change of the target (horizontal or vertical bar). Sounds synchronized with the visual target improved search efficiency for temporal rates below 1.4 Hz but did not affect efficiency above that stimulation rate. Desynchronized auditory cues consistently impaired visual search below 3.3 Hz. Our results are interpreted in the context of the Dynamic Attending Theory: specifically, we suggest that a cognitive operation structures events in time irrespective of the sensory modality of input. Our results further support and specify recent neurophysiological findings by showing strong temporal selectivity for audiovisual integration in the auditory-driven improvement of visual search efficiency.     

Binocular information in the control of prehensile movements in multiple-object scenes

Recent evidence suggests that the visual control of prehension may be less dependent on binocular information than has previously been thought. Studies investigating this question, however, have generally only examined reaches to single objects presented in isolation, even though natural prehensile movements are typically directed at objects in cluttered scenes which contain many objects. The present study was designed, therefore, to assess the contribution of binocular information to the control of prehensile movements in multiple-object scenes. Subjects reached for and grasped objects presented either in isolation or in the presence of one, two or four additional 'flanking' objects, under binocular and monocular viewing conditions. So that the role of binocular information could be clearly determined, subjects made reaches both in the absence of a visible scene around the target objects (self-illuminated objects presented in the dark) and under normal ambient lighting conditions. Analysis of kinematic parameters indicated that the removal of binocular information did not significantly affect many of the major indices of the transport component, including peak wrist velocity. However, peak grip apertures increased and subjects spent more time in the final slow phase of movement, prior to grasping the object, during monocularly guided reaches. The dissociation between effects of binocular versus monocular viewing on transport and grasp parameters was observed irrespective of the presence of flanking objects. These results therefore further question the view that binocular vision is pre-eminent in the control of natural prehensile movements.     

Nonmonotonic noise tuning of BOLD fMRI signal to natural images in the visual cortex of the anesthetized monkey

BACKGROUND: The perceptual ability of humans and monkeys to identify objects in the presence of noise varies systematically and monotonically as a function of how much noise is introduced to the visual display. That is, it becomes more and more difficult to identify an object with increasing noise. Here we examine whether the blood oxygen level-dependent functional magnetic resonance imaging (BOLD fMRI) signal in anesthetized monkeys also shows such monotonic tuning. We employed parametric stimulus sets containing natural images and noise patterns matched for spatial frequency and intensity as well as intermediate images generated by interpolation between natural images and noise patterns. Anesthetized monkeys provide us with the unique opportunity to examine visual processing largely in the absence of top-down cognitive modulations and can thus provide an important baseline against which work with awake monkeys and humans can be compared. RESULTS: We measured BOLD activity in occipital visual cortical areas as natural images and noise patterns, as well as intermediate interpolated patterns at three interpolation levels (25%, 50%, and 75%) were presented to anesthetized monkeys in a block paradigm. We observed reliable visual activity in occipital visual areas including V1, V2, V3, V3A, and V4 as well as the fundus and anterior bank of the superior temporal sulcus (STS). Natural images consistently elicited higher BOLD levels than noise patterns. For intermediate images, however, we did not observe monotonic tuning. Instead, we observed a characteristic V-shaped noise-tuning function in primary and extrastriate visual areas. BOLD signals initially decreased as noise was added to the stimulus but then increased again as the pure noise pattern was approached. We present a simple model based on the number of activated neurons and the strength of activation per neuron that can account for these results. CONCLUSIONS: We show that, for our parametric stimulus set, BOLD activity varied nonmonotonically as a function of how much noise was added to the visual stimuli, unlike the perceptual ability of humans and monkeys to identify such stimuli. This raises important caveats for interpreting fMRI data and demonstrates the importance of assessing not only which neural populations are activated by contrasting conditions during an fMRI study, but also the strength of this activation. This becomes particularly important when using the BOLD signal to make inferences about the relationship between neural activity and behavior.     

Principal component method for assessing structural heterogeneity across multiple alignment media

The recent availability of residual dipolar coupling measurements in a variety of different alignment media raises the question to what extent biomolecular structure and dynamics are differentially affected by their presence. A computational method is presented that allows the sensitive assessment of such changes using dipolar couplings measured in six or more alignment media. The method is based on a principal component analysis of the covariance matrix of the dipolar couplings. It does not require a priori structural or dynamic information nor knowledge of the alignment tensors and their orientations. In the absence of experimental errors, the covariance matrix has at most five nonzero eigenvalues if the structure and dynamics of the biomolecule is the same in all media. In contrast, differential structural and dynamic changes lead to additional nonzero eigenvalues. Characteristic features of the eigenvalue distribution in the absence and presence of noise are discussed using dipolar coupling data calculated from conformational ensembles taken from a molecular dynamics trajectory of native ubiquitin.     

The effect of stimulus duration and motor response in hemispatial neglect during a visual search task

Patients with hemispatial neglect exhibit a myriad of profound deficits. A hallmark of this syndrome is the patients' absence of awareness of items located in their contralesional space. Many studies, however, have demonstrated that neglect patients exhibit some level of processing of these neglected items. It has been suggested that unconscious processing of neglected information may manifest as a fast denial. This theory of fast denial proposes that neglected stimuli are detected in the same way as non-neglected stimuli, but without overt awareness. We evaluated the fast denial theory by conducting two separate visual search task experiments, each differing by the duration of stimulus presentation. Specifically, in Experiment 1 each stimulus remained in the participants' visual field until a response was made. In Experiment 2 each stimulus was presented for only a brief duration. We further evaluated the fast denial theory by comparing verbal to motor task responses in each experiment. Overall, our results from both experiments and tasks showed no evidence for the presence of implicit knowledge of neglected stimuli. Instead, patients with neglect responded the same when they neglected stimuli as when they correctly reported stimulus absence. These findings thus cast doubt on the concept of the fast denial theory and its consequent implications for non-conscious processing. Importantly, our study demonstrated that the only behavior affected was during conscious detection of ipsilesional stimuli. Specifically, patients were slower to detect stimuli in Experiment 1 compared to Experiment 2, suggesting a duration effect occurred during conscious processing of information. Additionally, reaction time and accuracy were similar when reporting verbally versus motorically. These results provide new insights into the perceptual deficits associated with neglect and further support other work that falsifies the fast denial account of non-conscious processing in hemispatial visual neglect.     

Visual search and the importance of time in complex decision making by bees

Psychophysicists studying decision making in animals have overwhelmingly focused on choice accuracy, not speed. Results from human visual search, however, show that there might be a tight link between the two. Here we review both visual-sensory and cognitive mechanisms that affect decision speed in flower visiting bees. We show that decision times are affected by contrast of targets and background, by similarity between targets and distractors, numbers of distractors present in a scene, illuminating light intensity, presence or absence of punishment, and complexity of tasks. We explore between-individual and within-individual speed-accuracy tradeoffs, and show that bees resort to highly dynamic strategies when solving visual search tasks. Where possible, we attempt to link the observed search behaviour to the temporal and spatial properties of neuronal circuits underlying visual object detection. We demonstrate that natural foraging speed may not only be limited by factors such as food item density, flight energetics and scramble competition, as often implied. Our results show that understanding the behavioural ecology of foraging can substantially gain from knowledge about mechanisms of visual information processing.     

Effects of visual priming on taste-odor interaction

Little is known about the influence of visual characteristics other than colour on flavor perception, and the complex interactions between more than two sensory modalities. This study focused on the effects of recognizability of visual (texture) information on flavor perception of odorized sweet beverages. Participants rated the perceived sweetness of odorized sucrose solutions in the presence or absence of either a congruent or incongruent visual context. Odors were qualitatively reminiscent of sweet foods (strawberry and caramel) or not (savoury). Visual context was either an image of the same sweet foods (figurative context) or a visual texture derived from this product (non-figurative context). Textures were created using a texture synthesis method that preserved perceived food qualities while removing object information. Odor-taste combinations were rated sweeter within a figurative than a non-figurative context. This behaviour was exhibited for all odor-taste combinations, even in trials without images, indicating sustained priming by figurative visual context. A non-figurative context showed a transient sweetening effect. Sweetness was generally enhanced most by the strawberry odor. We conclude that the degree of recognizability of visual information (figurative versus non-figurative), influences flavor perception differently. Our results suggest that this visual context priming is mediated by separate sustained and transient processes that are differently evoked by figurative and non-figurative visual contexts. These components operate independent of the congruency of the image-odor-taste combinations.     

Role of signal-dependent noise during maintenance of isometric force

The assumption that signal-dependent noise during isometric force production controls the stabilization of voluntary isometric force is considered. To verify the assumption the trajectory of isometric force is decomposed into voluntary and involuntary components and the mathematical model describing the relationship between them is developed. It is shown that the integral of an involuntary component (signal-dependent noise) plays the role of the controlling parameter realizing the stabilization of a voluntary component. The stabilization is carried out both in the absence and in the presence of visual feedback. Changes of experimental conditions are accompanied by essential changes in the amplitude of the involuntary component oscillations.     

The age of olfactory cues functioning in chemocommunication among male domestic cats

In a simulated open-field situation we tried to obtain information about the ability of tomcats to distinguish between urine marks of differing ages. Reactions to these marks were sniffing, sniffing intensively and “Flehmen”. The variables we chose to demonstrate a possible discriminatory ability between urine marks of different ages were: (1) latency, i.e. the time that elapses between the cat's release in the observation arena and the finding of a urine mark, (2) the number of occurrences of Flehmen after sniffing, (3) the frequency of sniffing and Flehmen, (4) the duration of the first sniffing bout and (5) the occurrence of a return to the same scent source during an observation.Our most striking observation was that fresh urine marks (maximum age, 4 hours) were explored first, before marks more than one day old; the other variables did not differ dramatically although we observed a general decline in the intensity of the reaction with increasing age of the urine marks.We also found that the visual characteristics of a urine mark play an important role, particularly in the occurrence of returns. Furthermore we found that the reactions of sniffing and Flehmen are linked with an attempt to identify the donor.All results, including the possible influence upon marking behaviour of the presence or absence of fresh marks, are discussed in the light of the use of a “time plan” spacing mechanism by the domestic tomcat as had been suggested in earlier literature.     

Optimum signal in a simple neuronal model with signal-dependent noise

How does the information about a signal in neural threshold crossings depend on the noise acting upon it? Two models are explored, a binary McCulloch and Pitts (threshold exceedance) model and a model of waiting time to exceedance--a discrete-time version of interspike intervals. If noise grows linearly with the signal, we find the best identification of the signal in terms of the Fisher information is for signals that do not reach the threshold in the absence of noise. Identification attains the same precision under weak and strong signals, but the coding range decreases at both extremes of signal level. We compare the results obtained for Fisher information with those using related first and second moment measures. The maximum obtainable information is plotted as a function of the ratio of noise to signal.     

Characteristics of Human Luminance Discrimination and Modeling a Neural Network Based on the Response Properties of the Visual Cortex

Reaction time (RT) and error rate that depend on stimulus duration were measured in a luminance-discrimination reaction time task. Two patches of light with different luminance were presented to participants for ‘short’ (150 ms) or ‘long’ (1 s) period on each trial. When the stimulus duration was ‘short’, the participants responded more rapidly with poorer discrimination performance than they did in the longer duration. The results suggested that different sensory responses in the visual cortices were responsible for the dependence of response speed and accuracy on the stimulus duration during the luminance-discrimination reaction time task. It was shown that the simple winner-take-all-type neural network model receiving transient and sustained stimulus information from the primary visual cortex successfully reproduced RT distributions for correct responses and error rates. Moreover, temporal spike sequences obtained from the model network closely resembled to the neural activity in the monkey prefrontal or parietal area during other visual decision tasks such as motion discrimination and oddball detection tasks.     

Investigation of the visual cytoscreening of conventional gynecologic smears. II. Analysis of eye movement

The perception of visual information in cytoscreening was studied: eye movements were recorded while the cytotechnologist was screening cervical smears by means of a projection screen. Four phases of eye movement could be distinguished: small, aimless movements during the stage movement; a latency period with a duration of about 180 milliseconds; saccadic movement to the position of an object; and fixation on an object. These components explain the two-phase behavior of cytoscreening found in our previous investigations of the stage movement. Visual perception during the period of latency was found to be the most important since only those objects that are recognized by peripheral vision during this period can trigger the necessary saccadic movement before fixation takes place. The scanpath of search in the stationary field of view is determined by the conspicuousness of the objects; the main features of conspicuousness are size and contrast. Even with the comparatively small fields of view (24 degrees and 29 degrees in diameter) used in these experiments, it was found that the detection threshold of peripheral vision increases towards the margin of the field of view. This raises the question of whether the use of large-field binoculars (with 40-degree visual angles) may cause higher false-negative rates for samples with only a few atypical cells.     

Dissociable influences of auditory object vs. spatial attention on visual system oscillatory activity

Given that both auditory and visual systems have anatomically separate object identification ("what") and spatial ("where") pathways, it is of interest whether attention-driven cross-sensory modulations occur separately within these feature domains. Here, we investigated how auditory "what" vs. "where" attention tasks modulate activity in visual pathways using cortically constrained source estimates of magnetoencephalograpic (MEG) oscillatory activity. In the absence of visual stimuli or tasks, subjects were presented with a sequence of auditory-stimulus pairs and instructed to selectively attend to phonetic ("what") vs. spatial ("where") aspects of these sounds, or to listen passively. To investigate sustained modulatory effects, oscillatory power was estimated from time periods between sound-pair presentations. In comparison to attention to sound locations, phonetic auditory attention was associated with stronger alpha (7-13 Hz) power in several visual areas (primary visual cortex; lingual, fusiform, and inferior temporal gyri, lateral occipital cortex), as well as in higher-order visual/multisensory areas including lateral/medial parietal and retrosplenial cortices. Region-of-interest (ROI) analyses of dynamic changes, from which the sustained effects had been removed, suggested further power increases during Attend Phoneme vs. Location centered at the alpha range 400-600 ms after the onset of second sound of each stimulus pair. These results suggest distinct modulations of visual system oscillatory activity during auditory attention to sound object identity ("what") vs. sound location ("where"). The alpha modulations could be interpreted to reflect enhanced crossmodal inhibition of feature-specific visual pathways and adjacent audiovisual association areas during "what" vs. "where" auditory attention.