Lower visual field advantage for motion segmentation during high competition for selection

A series of visual enumeration tasks were conducted investigating the role of the dorsal visual stream in motion segmentation. Cortical areas representing the lower visual field have greater connections with the parietal cortex and should therefore show an advantage for processes driven by the dorsal stream (Previc, 1990). We looked for differences in processing displays in the upper versus lower visual field when targets required segmentation from distractors in an enumeration task. In a baseline condition, random configurations of moving and static items were presented briefly (200 ms) to the upper or lower visual field. Fast and efficient enumeration took place both for moving targets and for static targets presented alone; there was no effect of visual field. In contrast, for moving targets, a lower visual field advantage was found when the inclusion of static distractors demanded segmentation by motion. This disappeared at the smaller display sizes when the targets were presented in canonical patterns. The results are consistent with segmentation of moving targets from static distractors being mediated by dorsal regions of the visual cortex, particularly under conditions of high load (non-canonical patterns). These regions show greater sensitivity to the lower visual field and to magnocellular-based input.     

The role of attentional priority and saliency in determining capacity limits in enumeration and visual working memory

Many common tasks require us to individuate in parallel two or more objects out of a complex scene. Although the mechanisms underlying our abilities to count the number of items, remember the visual properties of objects and to make saccadic eye movements towards targets have been studied separately, each of these tasks require selection of individual objects and shows a capacity limit. Here we show that a common factor--salience--determines the capacity limit in the various tasks. We manipulated bottom-up salience (visual contrast) and top-down salience (task relevance) in enumeration and visual memory tasks. As one item became increasingly salient, the subitizing range was reduced and memory performance for all other less-salient items was decreased. Overall, the pattern of results suggests that our abilities to enumerate and remember small groups of stimuli are grounded in an attentional priority or salience map which represents the location of important items.     

Set-size effects for spatial frequency change and discrimination in multiple targets

In visual search tasks with a near-threshold target amongst distracters, log detection thresholds rise in proportion to the log of the number of stimuli. Previous research has shown a very steep slope for this set-size effect where the target is a change in spatial frequency (SF) across an ISI, suggesting a low-level explanation for 'change blindness (Wright et al., 2000). Here, we analyse stimulus and task variables in order to determine the contributions of stimulus detection and attention processes. Stimuli consisted of two 150 ms frames each containing 1 to 4 Gabor targets, with an ISI of 250 ms. In a 2AFC detection task with uniform distracters, slopes of 0.23-0.52 were found, in line with visual search results. 2AFC SF discrimination tasks gave slopes of 0.68, 0.69 with homogeneous distracters and 0.76-0.96 with inhomogeneous distracters, consistent with averaging of stimuli within a frame. If the distracters were also made to change across ISI, averaging was impossible, and focal attention was required to solve the discrimination. This always gave set-size slopes > 1. It is concluded that, under conditions where a stimulus array can be analysed globally, change detection performance is limited by signal detection mechanisms, rather than limited capacity attention or memory mechanisms. However, where this is prevented, for example by changing more than one item, limitations due to attention or memory produce an even steeper set-size effect.     

Bilateral field advantage in visual enumeration

A number of recent studies have demonstrated superior visual processing when the information is distributed across the left and right visual fields than if the information is presented in a single hemifield (the bilateral field advantage). This effect is thought to reflect independent attentional resources in the two hemifields and the capacity of the neural responses to the left and right hemifields to process visual information in parallel. Here, we examined whether a bilateral field advantage can also be observed in a high-level visual task that requires the information from both hemifields to be combined. To this end, we used a visual enumeration task--a task that requires the assimilation of separate visual items into a single quantity--where the to-be-enumerated items were either presented in one hemifield or distributed between the two visual fields. We found that enumerating large number (>4 items), but not small number (<4 items), exhibited the bilateral field advantage: enumeration was more accurate when the visual items were split between the left and right hemifields than when they were all presented within the same hemifield. Control experiments further showed that this effect could not be attributed to a horizontal alignment advantage of the items in the visual field, or to a retinal stimulation difference between the unilateral and bilateral displays. These results suggest that a bilateral field advantage can arise when the visual task involves inter-hemispheric integration. This is in line with previous research and theory indicating that, when the visual task is attentionally demanding, parallel processing by the neural responses to the left and right hemifields can expand the capacity of visual information processing.     

Temporal brain dynamics of multiple object processing: the flexibility of individuation

The ability to process concurrently multiple visual objects is fundamental for a coherent perception of the world. A core component of this ability is the simultaneous individuation of multiple objects. Many studies have addressed the mechanism of object individuation but it remains unknown whether the visual system mandatorily individuates all relevant elements in the visual field, or whether object indexing depends on task demands. We used a neural measure of visual selection, the N2pc component, to evaluate the flexibility of multiple object individuation. In three ERP experiments, participants saw a variable number of target elements among homogenous distracters and performed either an enumeration task (Experiment 1) or a detection task, reporting whether at least one (Experiment 2) or a specified number of target elements (Experiment 3) was present. While in the enumeration task the N2pc response increased as a function of the number of targets, no such modulation was found in Experiment 2, indicating that individuation of multiple targets is not mandatory. However, a modulation of the N2pc similar to the enumeration task was visible in Experiment 3, further highlighting that object individuation is a flexible mechanism that binds indexes to object properties and locations as needed for further object processing.     

Insect detection of small targets moving in visual clutter

Detection of targets that move within visual clutter is a common task for animals searching for prey or conspecifics, a task made even more difficult when a moving pursuer needs to analyze targets against the motion of background texture (clutter). Despite the limited optical acuity of the compound eye of insects, this challenging task seems to have been solved by their tiny visual system. Here we describe neurons found in the male hoverfly,Eristalis tenax, that respond selectively to small moving targets. Although many of these target neurons are inhibited by the motion of a background pattern, others respond to target motion within the receptive field under a surprisingly large range of background motion stimuli. Some neurons respond whether or not there is a speed differential between target and background. Analysis of responses to very small targets (smaller than the size of the visual field of single photoreceptors) or those targets with reduced contrast shows that these neurons have extraordinarily high contrast sensitivity. Our data suggest that rejection of background motion may result from extreme selectivity for small targets contrasting against local patches of the background, combined with this high sensitivity, such that background patterns rarely contain features that satisfactorily drive the neuron.     

Auditory short-term memory behaves like visual short-term memory

Are the information processing steps that support short-term sensory memory common to all the senses? Systematic, psychophysical comparison requires identical experimental paradigms and comparable stimuli, which can be challenging to obtain across modalities. Participants performed a recognition memory task with auditory and visual stimuli that were comparable in complexity and in their neural representations at early stages of cortical processing. The visual stimuli were static and moving Gaussian-windowed, oriented, sinusoidal gratings (Gabor patches); the auditory stimuli were broadband sounds whose frequency content varied sinusoidally over time (moving ripples). Parallel effects on recognition memory were seen for number of items to be remembered, retention interval, and serial position. Further, regardless of modality, predicting an item's recognizability requires taking account of (1) the probe's similarity to the remembered list items (summed similarity), and (2) the similarity between the items in memory (inter-item homogeneity). A model incorporating both these factors gives a good fit to recognition memory data for auditory as well as visual stimuli. In addition, we present the first demonstration of the orthogonality of summed similarity and inter-item homogeneity effects. These data imply that auditory and visual representations undergo very similar transformations while they are encoded and retrieved from memory.     

Venom,speed, and caution: effects on performance in a visual search task

Previous reports of faster responses to threatening compared to benign stimuli in visual search tasks have argued that threatening targets are faster to engage and slower to disengage attention than benign targets. This study reinterprets previous findings and resolves inconsistencies in the literature by replacing the theory of differential disengagement of attention with one of differential caution. It also examines whether visual attentional mechanisms are sensitive to more than just the threatening versus benign categorical status of the targets and introduces a novel measure (a caution score) that appears to be sensitive to the level of threat implied by the target image, but immune to other stimulus features (target-distracter similarity and threat status of distracters) known to affect reaction time. As well as locating threatening targets faster than benign targets, participants were also faster, more accurate, and more cautious to detect lethal spiders compared to nonlethal spiders and even more cautious again if the spiders were presented on a person's hand. These results suggest that mechanisms of attention and threat evaluation interact during visual search tasks, producing behaviour that is sensitive to the target's implied threat level and the context in which that target is presented.     

Perceptual Grouping and Visual Enumeration

We used lateralized Event-Related Potential (ERP) measures – the N2pc and CDA/SPCN components – to assess the role of grouping by target similarity during enumeration. Participants saw a variable number (0, 1, 2 or 3) of same- or differently-colored targets presented among homogeneous distracters, and performed an enumeration task. Results showed that the N2pc, but not the CDA, was larger for multiple targets of identical color relative to targets of different colors. The findings are interpreted in terms of the effects of grouping on early versus late stages of multiple object processing. Within this framework, they reveal that grouping has an effect on early individuation mechanisms, while later processing mechanisms are less prone to such an influence.     

Food catches the eye but not for everyone: a BMI-contingent attentional bias in rapid detection of nutriments

An organism's survival depends crucially on its ability to detect and acquire nutriment. Attention circuits interact with cognitive and motivational systems to facilitate detection of salient sensory events in the environment. Here we show that the human attentional system is tuned to detect food targets among nonfood items. In two visual search experiments participants searched for discrepant food targets embedded in an array of nonfood distracters or vice versa. Detection times were faster when targets were food rather than nonfood items, and the detection advantage for food items showed a significant negative correlation with Body Mass Index (BMI). Also, eye tracking during searching within arrays of visually homogenous food and nonfood targets demonstrated that the BMI-contingent attentional bias was due to rapid capturing of the eyes by food items in individuals with low BMI. However, BMI was not associated with decision times after the discrepant food item was fixated. The results suggest that visual attention is biased towards foods, and that individual differences in energy consumption--as indexed by BMI--are associated with differential attentional effects related to foods. We speculate that such differences may constitute an important risk factor for gaining weight.     

Sensory and behavioral properties of neurons in posterior parietal cortex of the awake, trained monkey.

Neurons in posterior parietal cortex of the awake, trained monkey respond to passive visual and/or somatosensory stimuli. In general, the receptive fields of these cells are large and nonspecific. When these neurons are studied during visually guided hand movements and eye movements, most of their activity can be accounted for by passive sensory stimulation. However, for some visual cells, the response to a stimulus is enhanced when it is to be the target for a saccadic eye movement. This enhancement is selective for eye movements into the visual receptive field since it does not occur with eye movements to other parts of the visual field. Cells that discharge in association with a visual fixation task have foveal receptive fields and respond to the spots of light used as fixation targets. Cells discharging selectively in association with different directions of tracking eye movements have directionally selective responses to moving visual stimuli. Every cell in our sample discharging in association with movement could be driven by passive sensory stimuli. We conclude that the activity of neurons in posterior parietal cortex is dependent on and indicative of external stimuli but not predictive of movement.     

How Moving Backgrounds Influence Interception

Reaching movements towards an object are continuously guided by visual information about the target and the arm. Such guidance increases precision and allows one to adjust the movement if the target unexpectedly moves. On-going arm movements are also influenced by motion in the surrounding. Fast responses to motion in the surrounding could help cope with moving obstacles and with the consequences of changes in one’s eye orientation and vantage point. To further evaluate how motion in the surrounding influences interceptive movements we asked subjects to tap a moving target when it reached a second, static target. We varied the direction and location of motion in the surrounding, as well as details of the stimuli that are known to influence eye movements. Subjects were most sensitive to motion in the background when such motion was near the targets. Whether or not the eyes were moving, and the direction of the background motion in relation to the direction in which the eyes were moving, had very little influence on the response to the background motion. We conclude that the responses to background motion are driven by motion near the target rather than by a global analysis of the optic flow and its relation with other information about self-motion.     

Responses to computer-generated visual stimuli by the male praying mantis, Sphodromantis lineola (Burmeister)

We tested male mantis' responses to square, computer-generated visual stimuli of various sizes to determine the stimulus parameters that affect their striking behaviour. Males in a high-hunger group displayed a sharp preference for solid black 12×12-degree squares moving linearly against a white background, especially when the squares moved downward (versus horizontally). Males in a low-hunger group visually tracked but, with rare exception, did not strike at any linearly moving squares. In contrast, when solid black squares moved erratically around visual field centre, males struck regularly at them irrespective of hunger level, even when the squares were as large as 47×47 degrees. Males also recognized black-and-white Julesz-patterned square stimuli moving against a similarly patterned background, indicating that they recognized the synchronous movement of a group of stimulus elements as a single moving object. Finally, we compared allometric and life history data between male and females S. lineola to elucidate the possible reasons for the differences between male response patterns and previously published data on females. These comparisons suggest that males employ a different behavioural strategy than do females when faced with large, erratically moving visual stimuli.     

Reorganization of Receptive Fields of Cortical Field 21a Neurons and Formation of Responses to Moving Visual Stimuli

Using extracellular recording of spike activity from single neurons of field 21a of the cat neocortex, we examined in detail the spatial organization of receptive fields (RFs) of such cells after conditions of presentation of an immobile blinking light spot (a static RF) and moving visual stimuli (dynamic RFs). As was shown, the excitability of different RF subfields of a group of neurons possessing homogeneous on–off organization of the static RF changes significantly depended on the contrast, shape, dimension, orientation, and direction of movement of the applied mobile visual stimulus. This is manifested in changes in the number of discharge centers and shifts of their spatial localization. A hypothesis on the possible role of synchronous activation of the neurons neighboring the cell under study in the formation of an additional neuronal mechanism providing specialization of neuronal responses is proposed.     

Movement in the normal visual hemifield induces a percept in the 'blind' hemifield of a human hemianope.

We have investigated visual responses to moving stimuli presented to the normal hemifield of a hemianope, GY, who exhibits residual visual function in his right, ''blind'' hemifield. Preliminary experiments established that his perception of moving stimuli localized in his ''blind'' hemifield is retained when a similar stimulus is presented simultaneously in the normal hemifield. In response to a grating stimulus moving horizontally towards fixation in the non-foveal region of the normal, left hemifield, he perceives in addition to a normal motion percept in the left hemifield, a sensation of movement localized in the right hemifield. Qualitatively, this latter is indistinguishable from responses elicited by direct stimulation localized within his ''blind'' hemifield by moving stimuli. We have investigated the characteristics of the mechanisms which induce the ''blind'' field component of GY''s responses to stimulation of the normal hemifield. We show that GY''s sensitivity for detection of movement localized within his ''blind'' hemifield is dependent on the direction of movement, the contrast and the velocity of a grating presented to the normal hemifield. No induced effects were recorded in response to colour or to non-moving, flickering stimuli. We examine the possible contribution of scattered light to our observations, and eliminate this factor by consideration of our experimental results. We discuss the neural mechanisms which may be involved in this response.     

Dynamic Spatial Organization of Receptive Fields of Neurons in the 21a Cortical Area

We studied changes in the spatial parameters of receptive fields (RFs) of visually sensitive neurons in the associative area 21a of the cat cortex under conditions of presentation of moving visual stimuli. The results of experiments demonstrated that these parameters are dynamic and depend, from many aspects, on the pattern of the stimulus used for their estimation. Angular lengths of the horizontal and vertical axes of the RFs measured in the case of movement of the visual stimuli exceeded many times those determined by presentation of stationary blinking stimuli. As is supposed, a visual stimulus, when moving along the field of vision, activates a certain number of the neurons synaptically connected with the examined cell and possessing RFs localized along the movement trajectory. As a result, such integrated activity of the neuronal group can change the excitation threshold and discharge frequency of the studied neuron. It seems probable that correlated directed activation of the neuronal groups represents a significant neurophysiological mechanism providing dynamic modifications of the RF parameters of visually sensitive neurons in the course of processes of visual perception and identification of moving objects within the field of vision.     

Differential roles of distracters in reflexive and memory-based localization

We investigated the effects of spatial and temporal factors on manual localization of a visual target by measuring accuracy, precision, and bias. Spatial factors included manipulation of display as with or without distracters, with invariant or variant distracters, and with near or far distracters, respectively, in Experiments 1, 2, and 3. The target and distracters were of 1degrees dots differing only by luminance parameter; they were presented concurrently for 150 or 1000 ms while observers had to memorize the target location maintaining a fixed gaze. The observers' task was to reproduce the location of the target with a mouse cursor available 150 ms following stimuli offset. Results from all experiments showed that localization performance for a briefly exposed target was as accurate and precise as that for a long exposed target. Moreover, manipulation of spatial factors had no systematic effects on accuracy and precision except that near distracters yielded higher precision. Interestingly, localization performance was unbiased in 150 ms condition when there were distracters in the display, while being biased towards the fovea in 1000 ms condition regardless of their presence or absence. These results suggest a temporal dynamics in dominance-suppression between egocentric and exocentric cues in the construction of memory for location.     

Investigating saccade programming in the praying mantis Tenodera aridifolia using distracter interference paradigms

To investigate the saccadic system in the mantis, I applied distracter interference paradigms. These involved presenting the mantis with a fixation target and one or several distracters supposed to affect saccades towards the target. When a single target was presented, a medium-sized target located in its lower visual field elicited higher rates of saccade response. This preference for target size and position was also observed when a target and a distracter were presented simultaneously. That is, the mantis chose and fixated the target rather than a distracter that was much smaller or larger than the target, or was located above the target. Furthermore, the mantis' preference was not affected by increasing the number of distracters. However, the presence of the distracter decreased the occurrence rate of saccade and increased the response time to saccade. I conclude that distracter interference paradigms are an effective way of investigating the visual processing underlying saccade generation in the mantis. Possible mechanisms of saccade generation in the mantis are discussed.     

Influence of Visual Motion,Suggestion, and Illusory Motion on Self-Motion Perception in the Horizontal Plane

A moving visual field can induce the feeling of self-motion or vection. Illusory motion from static repeated asymmetric patterns creates a compelling visual motion stimulus, but it is unclear if such illusory motion can induce a feeling of self-motion or alter self-motion perception. In these experiments, human subjects reported the perceived direction of self-motion for sway translation and yaw rotation at the end of a period of viewing set visual stimuli coordinated with varying inertial stimuli. This tested the hypothesis that illusory visual motion would influence self-motion perception in the horizontal plane. Trials were arranged into 5 blocks based on stimulus type: moving star field with yaw rotation, moving star field with sway translation, illusory motion with yaw, illusory motion with sway, and static arrows with sway. Static arrows were used to evaluate the effect of cognitive suggestion on self-motion perception. Each trial had a control condition; the illusory motion controls were altered versions of the experimental image, which removed the illusory motion effect. For the moving visual stimulus, controls were carried out in a dark room. With the arrow visual stimulus, controls were a gray screen. In blocks containing a visual stimulus there was an 8s viewing interval with the inertial stimulus occurring over the final 1s. This allowed measurement of the visual illusion perception using objective methods. When no visual stimulus was present, only the 1s motion stimulus was presented. Eight women and five men (mean age 37) participated. To assess for a shift in self-motion perception, the effect of each visual stimulus on the self-motion stimulus (cm/s) at which subjects were equally likely to report motion in either direction was measured. Significant effects were seen for moving star fields for both translation (p = 0.001) and rotation (p<0.001), and arrows (p = 0.02). For the visual motion stimuli, inertial motion perception was shifted in the direction consistent with the visual stimulus. Arrows had a small effect on self-motion perception driven by a minority of subjects. There was no significant effect of illusory motion on self-motion perception for either translation or rotation (p>0.1 for both). Thus, although a true moving visual field can induce self-motion, results of this study show that illusory motion does not.     

Electrophysiological evidence for immature processing capacity and filtering in visuospatial working memory in adolescents

The present study investigated the development of visuospatial working memory (VSWM) capacity and the efficiency of filtering in VSWM in adolescence. To this end, a group of IQ-matched adults and adolescents performed a VSWM change detection task with manipulations of WM-load and distraction, while performance and electrophysiological contralateral delay activity (CDA) were measured. The CDA is a lateralized ERP marker of the number of targets and distracters that are selectively encoded/maintained in WM from one hemifield of the memory display. Significantly lower VSWM-capacity (Cowan's K) was found in adolescents than adults, and adolescents' WM performance (in terms of accuracy and speed) also suffered more from the presence of distracters. Distracter-related CDA responses were partly indicative of higher distracter encoding/maintenance in WM in adolescents and were positively correlated with performance measures of distracter interference. This correlation suggests that the higher interference of distracters on WM performance in adolescents was caused by an inability to block distracters from processing and maintenance in WM. The lower visuospatial WM-capacity (K) in adolescents in the high load (3 items) condition was accompanied by a trend (p<.10) towards higher CDA amplitudes in adolescents than adults, whereas CDA amplitudes in the low load (1 item) condition were comparable between adolescents and adults. These findings point to immaturity of frontal-parietal WM-attention networks that support visuospatial WM processing in adolescence.