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PubMed2000

Mechanisms of simple and choice reaction to changes in direction of visual motion.

Experiments are presented in which a random dot pattern moved vertically upwards (velocity vector V(1)) and then abruptly changed its direction of motion by the angle alpha (velocity vector V(2)), either to the left or to the right, without changing the speed. Subjects performed simple reactions to the direction change, disregarding its sign. In another experiment choice reactions to the same stimuli were performed: the subjects pushed a left button when the direction change was to the left and a right button when the change was to the right. The simple reaction time decreased monotonically with alpha increasing from 11 degrees to 169 degrees, whereas, within the same range of angles, a U-shaped curve described the function of the choice reaction time versus alpha. Both types of reaction time increased with decreasing the base speed. Difficulties are outlined which occur when the angle of change alpha is considered as 'intensity' of the stimulus. Instead, the parameter mid R:V(2)-V(1)mid R:, the absolute value of the difference between the velocity vectors before and after the change, is shown to be a meaningful 'intensity' parameter for the simple reaction task. The parameter V(2N), the speed of the velocity component normal to the initial velocity vector V(1), is suggested as an 'intensity' parameter for the choice reaction task. It is shown that the simple and choice reactions to changes in direction of visual motion are performed by two distinct mechanisms which seem to work in parallel and may be nearly equally fast for small angles of change, when mid R:V(2)-V(1)mid R: approximately V(2N).

Adult↗
PubMed2001

Perceptual grouping via spatial selection in a focused-attention task.

Theories of attention can be separated into those that select by location, and those that select by location-invariant representation. Experiments demonstrating stronger interference or facilitation from distractors grouped by nonspatial features with the target than ungrouped distractors have been considered as evidence for the selection of location-invariant representations. However, few studies have measured spatial attention directly at the locations of the grouped or ungrouped objects. In these experiments subjects responded to spatial probes (dots) while also identifying a cued target letter among distractors. Probe responses were faster for distractor locations with the target color than for those with the nontarget color, implying that target-color locations receive more attention. This pattern of spatial attention may explain why target-color distractors interfere more with target identification than nontarget-color distractors. These results suggest that although attention can be directed by nonspatial properties such as grouping by color or organization of the scene into objects, selection may ultimately be based on location.

Attention↗
PubMed2002

Shifts in perceived position of flashed stimuli by illusory object motion.

Moving stimuli cause the position of flashed stimuli to appear shifted in the direction of motion (position capture). To examine whether position capture depends on low-level motion interactions or perception of integrated object motion, we employed a slit-view display. Two line-drawn diamonds translated horizontally in opposite directions, one above and one below the fixation cross, either behind an occluding surface with a narrow slit or without occluding surface. When the diamonds were in vertical alignment, two vertical bars were flashed, one in the center of each diamond. In the slit-view condition, the diamonds were visible through a 4-, 2-, or 1-pixel vertical slit; the width of the flashed bars always matched the width of the slit. Even though the horizontal component of physical motion was greatly reduced or absent in the slit-view conditions, observers perceived diamonds moving behind the occluding surface. Furthermore, the position of the flashed bar was captured by the moving diamonds such that each bar appeared shifted in the direction of perceived motion. We conclude that the position capture reported here has a component based on high-level motion processing that is responsible for dynamically integrating object motion and shape.

Analysis of Variance↗
PubMed1997

Across-object relationships in perceived object orientation.

An orientation discrimination paradigm was used to determine whether the perceived orientation of extended objects is based on the distribution of edge-orientations or on the response of mechanisms that encode relationships across the object; specifically we considered large, second-stage filters and cores (the perceived middle of the object) as encoders of the across-object relationship. The stimuli were "rectangles" with sinusoidally modulated long edges. Manipulating the frequency and relative phase of the edge modulation allowed us to assess the importance of the across-object relationships. Evidence was found for the importance of such relationships in determining perceived orientation. No evidence was found for direct use of the distribution of responding edge detectors.

Differential Threshold↗
PubMed1998

Spatial and temporal aspects of infant color vision.

The present paper constitutes a review of the literature on young infants' chromatic discrimination capabilities. A series of early studies showed that infants as young as two months postnatal can make at least some chromatic discriminations between stationary, homogeneous fields of different wavelength compositions. Current studies of spatial and temporal contrast sensitivity functions (CSFs) for red/green isoluminant stimuli suggest that spatial chromatic CSFs show developmental changes in sensitivity and spatial scale, but not curve shape; while temporal chromatic CSFs (tCSFs) show developmental changes in sensitivity and curve shape, but not temporal scale. Infants can also code the direction of motion of moving isoluminant red/green gratings, for both continuous and quadrature motion. The possible mechanisms that underlie infants' chromatic discriminations are discussed.

Adult↗
PubMed2001

Behavioral cartography of visual functions in cat parietal cortex: areal and laminar dissociations.

The purpose of this review is to: (1) compare and contrast the relative contributions that the four principle regions in cat extrastriate parietal cortex make to a battery of visual tasks which require motion, spatial, or attentional processing; and (2) examine the laminar parcellation of visual behaviors within one of these parietal regions which mediates multiple visual behaviors. We examined a battery of visual tasks presumed to be mediated by parietal cortex, including direction of motion, differential motion, and landmark discriminations, and visual orienting to moving stimuli. As a control, we also examined performance on form (pattern and object) recognition tasks mediated by the temporal processing stream. The four regions of parietal cortex we examined included the: middle suprasylvian (MS) gyrus (area 7), anterior middle suprasylvian (aMS) sulcus (AMLS, ALLS), posterior middle suprasylvian (pMS) sulcus (PMLS, PLLS), and the dorsal posterior suprasylvian (dPS) gyrus (area 21a). The contributions made to each of the six different behavioral tasks was examined before, during, and after reversible cooling deactivation of each cortical area. Deactivation of pMS sulcal cortex resulted in deficits on all four tasks that required motion, spatial or attentional processing. Deactivation of aMS sulcal cortex resulted in deficits on only tasks that required motion processing. Deactivation of neither aMS nor pMS sulcal cortex yielded any deficits on the form recognition tasks. In contrast, deactivation of dPS cortex only produced deficits on the form recognition tasks. This finding confirmed our early hypothesis that dPS cortex is a key component of the temporal, and not the parietal, processing stream. Regardless of the task, no deficits were identified on any of the six tasks during deactivation of the MS gyrus. We then more closely examined pMS sulcal cortex to determine if its multiple functions could be dissociated on a laminar level. We found that cooling deactivation of the superficial layers (I-III) of pMS sulcal cortex selectively and completely impaired performance on the direction of motion discrimination task, while leaving visual attention unimpaired. Additional deactivation of the deeper layers (IV-VI) resulted in impaired visual attention as assessed with visual orienting. These results show a functional bipartite division of labor between upper and lower cortical layers of pMS sulcal cortex. Therefore, spatial, motion and attentional functions can be localized within visuoparietal cortex on both an areal and laminar level.

Animals↗
PubMed2002

The allocation of attention during smooth pursuit eye movements.

The spatial-temporal allocation of attention during smooth pursuit eye movements is poorly understood. In this chapter we review evidence showing that attention contributes to both saccades and smooth pursuit. We then discuss results from our own recent studies using a dual-task paradigm in which subjects pursued a moving stimulus and pressed a button when targets appeared in the periphery. The results from these studies are consistent with the hypothesis that the allocation of attention is biased to a location just in front of the pursuit stimulus and that this bias can be altered by pursuit velocity.

Attention↗
PubMed2001

Dynamics of the spatial scale of visual attention revealed by brain event-related potentials.

The temporal dynamics of the spatial scaling of attention during visual search were examined by recording event-related potentials (ERPs). A total of 16 young participants performed a search task in which the search array was preceded by valid cues that varied in size and hence in precision of target localization. The effects of cue size on short-latency (P1 and N1) ERP components, and the time course of these effects with variation in cue-target stimulus onset asynchrony (SOA), were examined. Reaction time (RT) to discriminate a target was prolonged as cue size increased. The amplitudes of the posterior P1 and N1 components of the ERP evoked by the search array were affected in opposite ways by the size of the precue: P1 amplitude increased whereas N1 amplitude decreased as cue size increased, particularly following the shortest SOA. The results show that when top-down information about the region to be searched is less precise (larger cues), RT is slowed and the neural generators of P1 become more active, reflecting the additional computations required in changing the spatial scale of attention to the appropriate element size to facilitate target discrimination. In contrast, the decrease in N1 amplitude with cue size may reflect a broadening of the spatial gradient of attention. The results provide electrophysiological evidence that changes in the spatial scale of attention modulate neural activity in early visual cortical areas and activate at least two temporally overlapping component processes during visual search.

Adult↗
PubMed2002

How lobsters, crayfishes, and crabs locate sources of odor: current perspectives and future directions.

Olfactory orientation poses many challenges for crustaceans in marine environments. Recent behavioral experiments lead to a new understanding of the role of multiple sensory appendages, whereas application of non-invasive chemical visualization techniques and biomimetic robotics have allowed researchers to correlate the stimulus environment with behavior and to directly test proposed orientation mechanisms in decapod crustaceans.

Animals↗
PubMed2000

Deriving behavioural receptive fields for visually completed contours.

The visual system is constantly faced with the problem of identifying partially occluded objects from incomplete images cast on the retinae. Phenomenologically, the visual system seems to fill in missing information by interpolating illusory and occluded contours at points of occlusion, so that we perceive complete objects. Previous behavioural [1] [2] [3] [4] [5] [6] [7] and physiological [8] [9] [10] [11] [12] studies suggest that the visual system treats illusory and occluded contours like luminance-defined contours in many respects. None of these studies has, however, directly shown that illusory and occluded contours are actually used to perform perceptual tasks. Here, we use a response-classification technique [13] [14] [15] [16] [17] [18] [19] [20] to answer this question directly. This technique provides pictorial representations - 'classification images' - that show which parts of a stimulus observers use to make perceptual decisions, effectively deriving behavioural receptive fields. Here we show that illusory and occluded contours appear in observers' classification images, providing the first direct evidence that observers use perceptually interpolated contours to recognize objects. These results offer a compelling demonstration of how visual processing acts on completed representations, and illustrate a powerful new technique for constraining models of visual completion.

Discrimination, Psychological↗
PubMedDate not supplied

Development of sensitivity to visual motion in macaque monkeys.

The development of spatial vision is relatively well documented in human and nonhuman primates. However, little is known about the development of sensitivity to motion. We measured the development of sensitivity to direction of motion, and the relationship between motion and contrast sensitivity in macaque monkeys as a function of age. Monkeys (Macaca nemestrina, aged between 10 days and 3 years) discriminated direction of motion in random-dot kinematograms. The youngest monkeys showed directionally selective orienting and the ability to integrate motion signals at large dot displacements and fast speeds. With age, coherence sensitivity improved for all spatial and temporal dot displacements tested. The temporal interval between the dots was far less important than the spatial offset in determining the animals' performance at all but the youngest ages. Motion sensitivity improved well beyond the end of the first postnatal year, when mid-spatial-frequency contrast sensitivity reached asymptote, and continued for at least 3 years. Sensitivity to contrast at high spatial frequencies also continued to develop beyond the end of the first year. We conclude that the development of motion sensitivity depends on mechanisms beyond the low-level filters presumed to limit acuity and contrast sensitivity, and most likely reflects the function of extrastriate visual areas.

Aging↗
PubMed1993

Optical specification of time-to-passage: observers' sensitivity to global tau.

Despite its general mathematical formulation, most empirical work on the visual perception of tau (defined as a quantity divided by its temporal derivative) has focused on the case of direct approach, with tau defined as image angle/rate of expansion. Empirical investigators tend to generalize image size analyses to off-axis approaches. However, this generalization is inappropriate for all but a few classes of objects. After mathematically reestablishing the appropriate optical cues specifying time to passage for noncollision cases, we report a series of studies in which we examined observers' sensitivities to this information in both relative- and absolute-judgment paradigms. In general, we found observers' judgments to be accurate and robust.

Adolescent↗
PubMed1993

Perceptual and cognitive factors governing performance in comparative arrival-time judgments.

Four experiments were conducted to investigate factors affecting relative arrival-time judgments in the transverse plane. Across experiments, results indicated an overreliance on relative distance information. The levels of relative velocity and distance used in the arrival-time task were proved discriminable, and performance in both relative velocity and distance judgments predicted performance in the relative arrival-time task. Despite the distance bias, an attempt to integrate relative velocity and distance information was also evidenced. The distance bias appears to have resulted from resource limitations on the concurrent processing of relative velocity and distance information, causing relative velocity information to become resource limited. The final experiment assessed the stability of performance in each of the tasks over time and provided evidence of individual differences in the ability to coordinate information from multiple sources.

Adolescent↗
PubMed1977

Feature analysis and the growth of a percept.

Twelve trigrams divided into four sets, the members of each set varying systematically in the feature configuration at each letter position, were repeatedly exposed at brief durations in a random order to subjects who were asked to report after each exposure those letters that could be recognized with certainty. The set of three trigrams that had the same features for all letters in a given ordinal position was identified with fewer exposures than were necessary to permit identification of sets for which the featural characteristics varied within a spatial position. The results were interpreted as being consistent with a constructive model of letter recognition in which the observer tests hypotheses regarding the presence of a given letter based on the presence of relevant features in a spatial position.

Discrimination, Psychological↗
PubMed1978

Organizational factors in perceived dimensionality.

The hypothesis that perceived dimensionality of projection drawings is influenced by perceived organization was tested in two experiments. Organization was biased by coloration: The drawings were (a) uncolored, (b) colored in ways that emphasized plausible three-dimensional parts of the represented objects, or (c) colored in ways that emphasized two-dimensional (2D) parts of the drawings themselves. In Experiment 1, subjective ratings of three-dimensionality (3D) were greater for the 3D-biased stimuli than for the unbiased stimuli, and ratings for the 2D-biased stimuli were lower than for the unbiased stimuli. Similar results were obtained in Experiment 2 in which latencies to perceive the drawings three-dimensionally were measured. The 3D-biased stimuli were seen in depth more quickly than the unbiased stimuli, and the 2D-biased stimuli were seen in depth more slowly than the unbiased stimuli. Additional results on the relation between ratings and latencies suggest that latencies are affected more strongly by object complexity than are ratings.

Color Perception↗
PubMed1981

Saccadic plasticity: parametric adaptive control by retinal feedback.

Saccadic gain (the ratio of saccadic amplitude to target eccentricity) was experimentally altered as normal human observers made refixation saccades to the right, which caused step changes in the horizontal position of the target. Eye movements were monitored by diffuse infrared limbus reflection. We found that decreases in saccadic gain reached 60% of completeness, whereas increases were only 25% complete. This asymmetry in adaptive capacity may reflect the saccadic system's need to avoid overshooting a target. With a single target, adaptation is rapid (time constant = 6 saccades); if training is distributed over six different targets, adaptation is considerably slower (time constant = 57 saccades). Gain changes that result from training with a given target do not transfer strongly to other targets in the same horizontal direction and may not transfer at all to targets in the opposite direction. The gain of saccades to one target may be decreased, and simultaneously the gain of saccades to another target at a different distance in the same direction is increased. These results suggest that each element of a sensory-motor structure underlying saccadic plasticity is associated with a particular retinal or spatial sensory locus and can alter its motor response without much affecting the response of neighboring elements. This is consistent with the finding that distributed training slows adaptation.

Eye Movements↗