Differential spatial displacement discrimination with interfering stimuli

Differential spatial displacement discrimination thresholds were determined for a configuration of three blobs with Gaussian spatial and temporal contrast envelopes. This task is similar to the well known three-dot alignment hyperacuity task. Thresholds determined in the presence of interfering stimuli were identical to thresholds determined without these flanking stimuli. The thresholds scale linearly with stimulus size over at least two decades. We conclude that (i) the mechanisms that compute differential spatial displacement for the three-blob alignment task are not disturbed by the presence of neighbouring stimuli, even when these enter the region over which the computations are performed and (ii) at all levels of resolution similar mechanisms are used to compute differential spatial displacement.     

A simple model of human foveal ganglion cell responses to hyperacuity stimuli

We developed a physiologically plausible model of the first steps of spatial visual information processing in the fovea of the human retina. With the predictions of this model we could support the hypothesis that, for moderate contrasts ( 40%), hyperacuity is mediated by the magnocellular (MC-) pathway. Despite the lower sampling density in the MC pathway, as compared to the parvocellular (PC-) pathway, the information that is transferred by the MC ganglion cells is sufficient to achieve thresholds comparable to those of human subjects in psychophysical tasks. This is a result of the much higher signal-to-noise ratio of the MC pathway cell signals. The PC pathway cells do not transfer enough information for hyperacuity thresholds.     

Musca domestica inspired machine vision sensor with hyperacuity

A fiber optic sensor inspired by the compound eye of the common housefly, Musca domestica, has been developed. The sensor coupled with analog preprocessing hardware has the potential to extract edge information quickly and in parallel. The design is motivated by the parallel nature of the fly's vision system and its demonstrated hyperacuity or precision of visual localization beyond the conventional resolution limit. The fly's anatomy supporting the design is reviewed, followed by the design of a one-dimensional, cartridge-based sensor. The sensor's ability to locate a line stimulus in a two-dimensional space is demonstrated. Discussion is provided to extend this work in scale, cartridge dimension, information and array processing.     

人类视觉系统超视锐度现象的神经网络数学模型研究

超视锐度是人们熟知的奇特现象,虽然许多视觉研究领域中的专家们对此做了大量的研究,但视觉系统究竟是如何提取超视锐度信息的却仍然是个未解决的问题.本文用简单细胞广义Gabor函数为基本功能单元,以Marr提出的计算理论为框架,建立了一个感受野重叠的,具有比较运算性质的、可解释超视锐度现象的神经网络数学模型.它在空间比较,真实运动和似动三方面较好地描述了已有的一些有关超视锐度现象的心理物理实验结果.     

The detection of spatial discontinuities: interactions between contrast and spatial contiguity

Thresholds were measured for the detection of spatial discontinuities (notches and bumps) along luminance boundaries. At high contrasts of the boundary, thresholds expressed in terms of the spatial notch/bump height fell well inside the hyperacuity range. Expressed as luminance increment thresholds between adjacent photoreceptors, the same thresholds were similar to those previously reported by Hartridge and by Hecht and Mintz for the detection of a single line. The ability of observers to detect differences in the height of a boundary on either side of a mean luminance gap was also investigated, and the effect of the gap was found to depend upon stimulus contrast. At high contrasts the introduction of a gap increased thresholds, but at the lowest contrasts, thresholds were unaffected by a gap. The role of different spatial frequency and orientational mechanisms in vernier acuity is discussed.     

Carving the cognitive niche: optimal learning strategies in homogeneous and heterogeneous environments

A model learning system is constructed, in which an organism samples behaviors from a behavioral repertoire in response to a stimulus and selects the behavior with the highest payoff. The stimulus and most rewarding behavior may be kept in the organism's long-term memory and reused if the stimulus is encountered again. The value of the memory depends on the reliability of the stimulus, that is, how the corresponding payoffs of behaviors change over time. We describe how the inclusion of memory can increase the optimal sampling size in environments with some stimulus reliability. In addition to using memory to guide behavior, our organism may use information in its memory to choose the stimulus to which it reacts. This choice is influenced by both the organism's memory state and how many stimuli the organism can observe (its sensory capability). The number of sampled behaviors, memory length, and sensory capability are the variables that define the learning strategy. When all stimuli have the same reliability, there appears to be only a single optimal learning strategy. However, when there is heterogeneity in stimulus reliability, multiple locally optimal strategies may exist.     

Hope for the Best or Prepare for the Worst? Towards a Spatial Cognitive Bias Test for Mice

Cognitive bias, the altered information processing resulting from the background emotional state of an individual, has been suggested as a promising new indicator of animal emotion. Comparable to anxious or depressed humans, animals in a putatively negative emotional state are more likely to judge an ambiguous stimulus as if it predicts a negative event, than those in positive states. The present study aimed to establish a cognitive bias test for mice based on a spatial judgment task and to apply it in a pilot study to serotonin transporter (5-HTT) knockout mice, a well-established mouse model for the study of anxiety- and depression-related behavior. In a first step, we validated that our setup can assess different expectations about the outcome of an ambiguous stimulus: mice having learned to expect something positive within a maze differed significantly in their behavior towards an unfamiliar location than animals having learned to expect something negative. In a second step, the use of spatial location as a discriminatory stimulus was confirmed by showing that mice interpret an ambiguous stimulus depending on its spatial location, with a position exactly midway between a positive and a negative reference point provoking the highest level of ambiguity. Finally, the anxiety- and depression-like phenotype of the 5-HTT knockout mouse model manifested - comparable to human conditions - in a trend for a negatively distorted interpretation of ambiguous information, albeit this effect was not statistically significant. The results suggest that the present cognitive bias test provides a useful basis to study the emotional state in mice, which may not only increase the translational value of animal models in the study of human affective disorders, but which is also a central objective of animal welfare research.     

Influence of Stimuli Spatial Proximity on a SSVEP-Based BCI Performance

Objective: Steady-State Visual Evoked Potentials based Brain-Computer Interfaces (SSVEP-based BCIs) systems have been shown as promising technology due to their short response time and ease of use. SSVEP-based BCIs use brain responses to a flickering visual stimulus as an input command to an external application or device, and it can be influenced by stimulus properties, signal recording, and signal processing. We aim to investigate the system performance varying the stimuli spatial proximity (a stimulus property).Material and methods: We performed a comparative analysis of two visual interface designs (named cross and square) for an SSVEP-based BCI. The power spectrum density (PSD) was used as feature extraction and the Support Machine Vector (SVM) as classification method. We also analyzed the effects of five flickering frequencies (6.67, 8.57, 10, 12 e 15 Hz) between and within interfaces.Results: We found higher accuracy rates for the flickering frequencies of 10, 12, and 15 Hz. The stimulus of 10 Hz presented the highest SSVEP amplitude response for both interfaces. The system presented the best performance (highest classification accuracy and information transfer rate) using the cross interface (lower visual angle).Conclusion: Our findings suggest that the system has the highest performance in the spatial proximity range from 4° to 13° (visual angle). In addition, we conclude that as the stimulus spatial proximity increases, the interference from other stimuli reduces, and the SSVEP amplitude response decreases, which reduces system accuracy. The inter-stimulus distance is a visual interface parameter that must be chosen carefully to increase the efficiency of an SSVEP-based BCI.     

Visual acuity and hyperacuity in 11- to 17-year-old secondary school students

Visual acuity and hyperacuity of 11- to 17-year-old secondary school students with normal vision were measured and compared. The estimations of hyperacuity and acuity were made using the vernier stimuli, Landolt Cs, and Tumbling Es. When test stimuli were located in the tables, visual acuity estimations measured using Landolt Cs were significantly higher by a factor of 1.1 than that measured using Tumbling Es. Visual hyperacuity was 1.25?C4.1 times higher than visual acuity. The estimations of visual hyperacuity were almost 2 times higher in 16-year-old than 13-year-old secondary school students, in contrast to the estimations of visual acuity that did not change with age. The binocular visual acuity estimations were 1.05 times higher than the monocular ones and did not depend on the age. The ratio of binocular visual hyperacuity to monocular visual hyperacuity in 13-year-old secondary school students was 1.9, whereas, in senior secondary school students, it was 1.2. The contribution of binocular vision to the development of the mechanisms of visual acuity and hyperacuity in ontogenesis and the differences between the mechanisms of visual acuity and hyperacuity are discussed.     

Retrograde interference in perceptual learning of a peripheral hyperacuity task

Consolidation, a process that stabilizes memory trace after initial acquisition, has been studied for over a century. A number of studies have shown that a skill or memory must be consolidated after acquisition so that it becomes resistant to interference from new information. Previous research found that training on a peripheral 3-dot hyperacuity task could retrogradely interfere with earlier training on the same task but with a mirrored stimulus configuration. However, a recent study failed to replicate this finding. Here we address the controversy by replicating both patterns of results, however, under different experimental settings. We find that retrograde interference occurs when eye-movements are tightly controlled, using a gaze-contingent display, where the peripheral stimuli were only presented when subjects maintained fixation. On the other hand, no retrograde interference was found in a group of subjects who performed the task without this fixation control. Our results provide a plausible explanation of why divergent results were found for retrograde interference in perceptual learning on the 3-dot hyperacuity task and confirm that retrograde interference can occur in this type of low-level perceptual learning. Furthermore, our results demonstrate the importance of eye-movement controls in studies of perceptual learning in the peripheral visual field.     

人的体视超视锐度阈值

体视超视(?)象是视觉系统中(?)度的超(?)在一些特殊的(?)图形下,它的阈值可达2秒以下.考虑到人眼的一般体(?)锐度的60(?),(?)未超视度现象就是一个很有意义并值得注意研究的问题.对于如此精细的分辨能力,视(?)膜细胞可以说是很粗糙的(其直径约24秒)我们想了解,这么粗糙的视网膜是如何分辨出仅为其直径1/10的目标位差的呢?本文对人眼体视超视锐度现象做了一此研究,测定出体视超视锐度(?)值,并研究了低通滤波后对其影响.     

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.     

The Early Time Course of Compensatory Face Processing in Congenital Prosopagnosia

Background

Prosopagnosia is a selective deficit in facial identification which can be either acquired, (e.g., after brain damage), or present from birth (congenital). The face recognition deficit in prosopagnosia is characterized by worse accuracy, longer reaction times, more dispersed gaze behavior and a strong reliance on featural processing.

Methods/Principal Findings

We introduce a conceptual model of an apperceptive/associative type of congenital prosopagnosia where a deficit in holistic processing is compensated by a serial inspection of isolated, informative features. Based on the model proposed we investigated performance differences in different face and shoe identification tasks between a group of 16 participants with congenital prosopagnosia and a group of 36 age-matched controls. Given enough training and unlimited stimulus presentation prosopagnosics achieved normal face identification accuracy evincing longer reaction times. The latter increase was paralleled by an equally-sized increase in stimulus presentation times needed achieve an accuracy of 80%. When the inspection time of stimuli was limited (50ms to 750ms), prosopagnosics only showed worse accuracy but no difference in reaction time. Tested for the ability to generalize from frontal to rotated views, prosopagnosics performed worse than controls across all rotation angles but the magnitude of the deficit didn''t change with increasing rotation. All group differences in accuracy, reaction or presentation times were selective to face stimuli and didn''t extend to shoes.

Conclusions/Significance

Our study provides a characterization of congenital prosopagnosia in terms of early processing differences. More specifically, compensatory processing in congenital prosopagnosia requires an inspection of faces that is sufficiently long to allow for sequential focusing on informative features. This characterization of dysfunctional processing in prosopagnosia further emphasizes fast and holistic information encoding as two defining characteristics of normal face processing.     

Population coding of visual stimuli by cortical neurons tuned to more than one dimension

Neurons in the visual cortex are typically selective to a number of stimulus dimensions. Thus, there is a basic ambiguity in relating the response level of a single neuron to the stimulus values. It is shown that a multi-dimensional stimulus may be coded reliably by an ensemble of neurons, using a weighted average population coding model. Each neurons' contribution to the population signal for each dimension is the product of its response magnitude and its preferred value for that dimension. The sum of the products was normalized by the sum of the ensemble responses. Simulation results show that the representation accuracy increases as the square root of the number of units irrespective of the number of dimensions. Comparison of a specific 2D case of this population code for orientation and spatial frequency to behavioral discrimination levels yields that 10³–10⁴ neurons are needed to reach psychophysical performance. Introduction of each additional dimension requires about 1.7 times the number of neurons in the ensemble to reach the same level of accuracy. This result suggests that neurons may be selective for only 3 to 5 dimensions. It also provides another rationale for the existence of parallel processing streams in vision.     

Spatiotemporal Segregation of Neural Response to Auditory Stimulation: An fMRI Study Using Independent Component Analysis and Frequency-Domain Analysis

Although auditory processing has been widely studied with conventional parametric methods, there have been a limited number of independent component analysis (ICA) applications in this area. The purpose of this study was to examine spatiotemporal behavior of brain networks in response to passive auditory stimulation using ICA. Continuous broadband noise was presented binaurally to 19 subjects with normal hearing. ICA was performed to segregate spatial networks, which were subsequently classified according to their temporal relation to the stimulus using power spectrum analysis. Classification of separated networks resulted in 3 stimulus-activated, 9 stimulus-deactivated, 2 stimulus-neutral (stimulus-dependent but not correlated with the stimulation timing), and 2 stimulus-unrelated (fluctuations that did not follow the stimulus cycles) components. As a result of such classification, spatiotemporal subdivisions were observed in a number of cortical structures, namely auditory, cingulate, and sensorimotor cortices, where parts of the same cortical network responded to the stimulus with different temporal patterns. The majority of the classified networks seemed to comprise subparts of the known resting-state networks (RSNs); however, they displayed different temporal behavior in response to the auditory stimulus, indicating stimulus-dependent temporal segregation of RSNs. Only one of nine deactivated networks coincided with the “classic” default-mode network, suggesting the existence of a stimulus-dependent default-mode network, different from that commonly accepted.     

Spectrotemporal processing in spectral tuning modules of cat primary auditory cortex

Spectral integration properties show topographical order in cat primary auditory cortex (AI). Along the iso-frequency domain, regions with predominantly narrowly tuned (NT) neurons are segregated from regions with more broadly tuned (BT) neurons, forming distinct processing modules. Despite their prominent spatial segregation, spectrotemporal processing has not been compared for these regions. We identified these NT and BT regions with broad-band ripple stimuli and characterized processing differences between them using both spectrotemporal receptive fields (STRFs) and nonlinear stimulus/firing rate transformations. The durations of STRF excitatory and inhibitory subfields were shorter and the best temporal modulation frequencies were higher for BT neurons than for NT neurons. For NT neurons, the bandwidth of excitatory and inhibitory subfields was matched, whereas for BT neurons it was not. Phase locking and feature selectivity were higher for NT neurons. Properties of the nonlinearities showed only slight differences across the bandwidth modules. These results indicate fundamental differences in spectrotemporal preferences--and thus distinct physiological functions--for neurons in BT and NT spectral integration modules. However, some global processing aspects, such as spectrotemporal interactions and nonlinear input/output behavior, appear to be similar for both neuronal subgroups. The findings suggest that spectral integration modules in AI differ in what specific stimulus aspects are processed, but they are similar in the manner in which stimulus information is processed.     

Switching neuronal state: optimal stimuli revealed using a stochastically-seeded gradient algorithm

Inducing a switch in neuronal state using energy optimal stimuli is relevant to a variety of problems in neuroscience. Analytical techniques from optimal control theory can identify such stimuli; however, solutions to the optimization problem using indirect variational approaches can be elusive in models that describe neuronal behavior. Here we develop and apply a direct gradient-based optimization algorithm to find stimulus waveforms that elicit a change in neuronal state while minimizing energy usage. We analyze standard models of neuronal behavior, the Hodgkin-Huxley and FitzHugh-Nagumo models, to show that the gradient-based algorithm: 1) enables automated exploration of a wide solution space, using stochastically generated initial waveforms that converge to multiple locally optimal solutions; and 2) finds optimal stimulus waveforms that achieve a physiological outcome condition, without a priori knowledge of the optimal terminal condition of all state variables. Analysis of biological systems using stochastically-seeded gradient methods can reveal salient dynamical mechanisms underlying the optimal control of system behavior. The gradient algorithm may also have practical applications in future work, for example, finding energy optimal waveforms for therapeutic neural stimulation that minimizes power usage and diminishes off-target effects and damage to neighboring tissue.     

Selectivity of visual cortical neurons of rabbits to magnitude of moving stimuli

Unit responses of the rabbit visual cortex were investigated in relation to size of visual stimuli moving in their receptive field. With an increase in size of the stimulus in a direction perpendicular to the direction of movement ("width" of the stimulus) an initial increase in the intensity of the unit response through spatial summation of excitory effects is followed by a decrease through lateral inhibition. This inhibition is observed between zones of the receptive field which behave as activating when tested by a stimulus of small size. Each neuron has its own "preferred" size of stimuli evoking its maximal activation. No direct correlation is found between the "preferred" stimulus size and the size of the receptive field. With a change in stimulus size in the direction of movement ("length" of the stimulus) the responses to stimuli of optimal size may be potentiated through mutual facilitation of the effects evoked by the leading and trailing edges of the stimulus and weakened in response to stimuli of large size. The selective behavior of the neurons with respect to stimulus size is intensified in the case of coordinated changes in their length and width. It is postulated that the series of neurons responding to stimuli of different "preferred" dimensions may constitute a system classifying stimuli by their size.A. N. Severtsov Institute of Evolutionary Morphology and Ecology of Animals, Academy of Sciences of the USSR, Moscow. Translated from Neirofiziologiya, Vol. 4, No. 6, pp. 636–644, November–December, 1972.     

Temporal Hyperacuity in the Gymnotiform Electric Fish, Eigenmannia

SYNOPSIS. The gymnotiform electric fish, Eigenmannia, exhibitsextraordinary sensitivity to small timing differences betweensensory signals. The jamming avoidance response, gradual frequencyshifts of the electric organ discharges, requires the detectionof temporal disparities between sensory signals impinging upondifferent electroreceptors. This behavior occurs reliably evenwith temporal disparities being smaller than one microsecond.Since individual sensory receptors are not capable of encodingsuch minute timing with certainty, the high behavioral sensitivitymust, therefore, emerge from signal processing within the centralnervous system. Individual neurons, at the top of a well definedneuronal hierarchy have been found to be sensitive to temporaldisparities in the range of 1 microsecond. The response propertiesof these neurons as well as behavioral results suggest thatspatial convergence of sensory information plays a major rolein the emergence of this temporal hyperacuity.     

High-content behavioral analysis of Caenorhabditis elegans in precise spatiotemporal chemical environments

To quantitatively understand chemosensory behaviors, it is desirable to present many animals with repeatable, well-defined chemical stimuli. To that end, we describe a microfluidic system to analyze Caenorhabditis elegans behavior in defined temporal and spatial stimulus patterns. A 2 cm × 2 cm structured arena allowed C. elegans to perform crawling locomotion in a controlled liquid environment. We characterized behavioral responses to attractive odors with three stimulus patterns: temporal pulses, spatial stripes and a linear concentration gradient, all delivered in the fluid phase to eliminate variability associated with air-fluid transitions. Different stimulus configurations preferentially revealed turning dynamics in a biased random walk, directed orientation into an odor stripe and speed regulation by odor. We identified both expected and unexpected responses in wild-type worms and sensory mutants by quantifying dozens of behavioral parameters. The devices are inexpensive, easy to fabricate, reusable and suitable for delivering any liquid-borne stimulus.