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At least 739 records · Page 41Linked to original sources
PubMed1986

Mechanisms of human motion perception revealed by a new cyclopean illusion.

A new cyclopean illusion of motion may bear on neural mechanisms of direction selectivity. Stationary flickering patterns were presented to each eye, and the resulting fused pattern was perceived to be moving. To determine direction of motion, the visual system seems to integrate image components differing by 90 degrees in spatial and temporal phase. On the other hand, image speed seems to be derived from displacement of features over time. A model of neural direction selectivity is discussed in light of these results.

Fourier Analysis↗
PubMed1980

Human aging and spatial vision.

The ability to see spatial structures of a wide range of sizes was measured for two groups of observers (mean ages, 18 and 73 years). All observers had good visual acuity. Although older and younger observers did not differ in ability to see targets with fine structure (high spatial frequencies), older observers were only one-third as sensitive to targets with coarse structure (low spatial frequencies) as were younger observers or to changes in criterion. Older observers were also less able than younger observers to see moving targets. The reduced sensitivity of the older observers may adversely affect routine perceptual activities, such as face recognition and visually guided postural behavior, that depend upon low spatial frequencies.

Adult↗
PubMed1995

Parietal contributions to visual feature binding: evidence from a patient with bilateral lesions.

Neurophysiologists have documented the existence of multiple cortical areas responsive to different visual features. This modular organization has sparked theoretical interest in how the "binding problem" is solved. Recent data from a neurological patient (R.M.) with bilateral parietal-occipital lesions demonstrates that the binding problem is not just a hypothetical construct; it can be a practical problem, as rare as the selective inability to perceive motion or color. R.M. miscombines colors and shapes even under free viewing conditions and is unable to judge either relative or absolute visual locations. The evidence suggests that a single explanation--an inadequate spatial representation--can account for R.M.'s spatial judgment and feature-binding deficits.

Cerebral Infarction↗
PubMed1973

Spatial orientation in man: effects of left, right, and bilateral posterior cerebral lesions.

Men with chronic, penetrating missile wounds of the brain were examined with two `spatial' tasks: a visually-guided stylus maze and a locomotor map-reading task. Men whose lesions involved the posterior part of the right cerebral hemisphere were significantly worse than those with left posterior lesions at stylus maze-learning. On the locomotor task, however, a highly significant deficit was found in the group of men with bilateral posterior cerebral lesions, while those with unilateral lesions of either hemisphere and those with bilateral frontal lesions were unimpaired. The contributions of the two cerebral hemispheres to the analysis of spatial information are discussed in the light of these results and it is suggested that, while the right hemisphere has a special role in the perception of space, it does not bear exclusive responsibility for the maintenance of spatial orientation.

Brain Injuries↗
PubMed2004

The neural basis of temporal processing.

A complete understanding of sensory and motor processing requires characterization of how the nervous system processes time in the range of tens to hundreds of milliseconds (ms). Temporal processing on this scale is required for simple sensory problems, such as interval, duration, and motion discrimination, as well as complex forms of sensory processing, such as speech recognition. Timing is also required for a wide range of motor tasks from eyelid conditioning to playing the piano. Here we review the behavioral, electrophysiological, and theoretical literature on the neural basis of temporal processing. These data suggest that temporal processing is likely to be distributed among different structures, rather than relying on a centralized timing area, as has been suggested in internal clock models. We also discuss whether temporal processing relies on specialized neural mechanisms, which perform temporal computations independent of spatial ones. We suggest that, given the intricate link between temporal and spatial information in most sensory and motor tasks, timing and spatial processing are intrinsic properties of neural function, and specialized timing mechanisms such as delay lines, oscillators, or a spectrum of different time constants are not required. Rather temporal processing may rely on state-dependent changes in network dynamics.

Animals↗
PubMed2004

BOLD repetition decreases in object-responsive ventral visual areas depend on spatial attention.

Functional imaging studies of priming-related repetition phenomena have become widely used to study neural object representation. Although blood oxygenation level-dependent (BOLD) repetition decreases can sometimes be observed without awareness of repetition, any role for spatial attention in BOLD repetition effects remains largely unknown. We used fMRI in 13 healthy subjects to test whether BOLD repetition decreases for repeated objects in ventral visual cortices depend on allocation of spatial attention to the prime. Subjects performed a size-judgment task on a probe object that had been attended or ignored in a preceding prime display of 2 lateralized objects. Reaction times showed faster responses when the probe was the same object as the attended prime, independent of the view tested (identical vs. mirror image). No behavioral effect was evident from unattended primes. BOLD repetition decreases for attended primes were found in lateral occipital and fusiform regions bilaterally, which generalized across identical and mirror-image repeats. No repetition decreases were observed for ignored primes. Our results suggest a critical role for attention in achieving visual representations of objects that lead to both BOLD signal decreases and behavioral priming on repeated presentation.

Adult↗
PubMed2001

Listening through different ears alters spatial response fields in ferret primary auditory cortex.

The localization of sounds in space is based on spatial cues that arise from the acoustical properties of the head and external ears. Individual differences in localization cue values result from variability in the shape and dimensions of these structures. We have mapped spatial response fields of high-frequency neurons in ferret primary auditory cortex using virtual sound sources based either on the animal's own ears or on the ears of other subjects. For 73% of units, the response fields measured using the animals' own ears differed significantly in shape and/or position from those obtained using spatial cues from another ferret. The observed changes correlated with individual differences in the acoustics. These data are consistent with previous reports showing that humans localize less accurately when listening to virtual sounds from other individuals. Together these findings support the notion that neural mechanisms underlying auditory space perception are calibrated by experience to the properties of the individual.

Acoustic Stimulation↗
PubMedDate not supplied

Functional imaging of the auditory system: the use of positron emission tomography.

Modern brain imaging methods now afford unprecedented opportunities for the in vivo study of central auditory system function. Positron emission tomography (PET) has been used as a functional imaging technique for more than 15 years to study the distribution of cerebral haemodynamic changes associated with auditory stimulation, in subjects with normal and abnormal auditory function. Many of these studies concern processes related to, but not identical with, audition, such as speech perception, melodic processing and directed attention. Additionally, PET has been used to explore auditory perception in clinical populations such as cochlear implantees, neurosurgical candidates and people with tinnitus and auditory hallucinations. The spatial resolution of PET does not appear sufficient to address questions of a fine anatomical grain, e.g. exploring functional specialization within the primary and secondary auditory cortex. Nevertheless, PET has considerable potential as a tool in basic research on, and clinical assessment of, many auditory phenomena. Although functional magnetic resonance imaging can now be used for many studies for which formerly only PET was suitable, PET still possesses unique advantages. For example, it permits acquisition of data from inferior frontal and anterior temporal areas, can be used with subjects with cochlear and other implants (such as pacemakers) and can be used to map neurochemical pathways and receptors.

Auditory Cortex↗
PubMedDate not supplied

The disturbances of object vision and spatial vision in Alzheimer's disease.

Thirty-one patients with mild-to-moderate Alzheimer's disease (AD) underwent a test battery of complex visual tasks. We assessed the scores using a principal-factor analysis to elucidate the underlying deficits. There were three independent factors: The first factor included the tasks of identifying and comparing forms of visual stimuli. The second factor consisted of digit span and digit symbol tasks, and the third factor consisted of a specified visual counting task. We considered these three factors as representing the dysfunctions of object recognition, general attention and spatial recognition, respectively. These results underline the disturbances of the two visual systems, object vision and spatial vision, in early-AD patients.

Aged↗
PubMed2000

DEA: An architecture for goal planning and classification.

We introduce the differential efficiency algorithm, which partitions a perceptive space during unsupervised learning into categories and uses them to solve goal-planning and classification problems. This algorithm is inspired by a biological model of the cortex proposing the cortical column as an elementary unit. We validate the generality of this approach by testing it on four problems with continuous time and no reinforcement signal until the goal is reached (constrained object moves, Hanoi tower problem, animat control, and simple character recognition).

Algorithms↗
PubMed2001

Characterizing visual performance fields: effects of transient covert attention, spatial frequency, eccentricity, task and set size.

We investigated whether transient covert attention would differentially affect 'performance fields' (shape depicted by percent correct performance at particular locations in the visual field) for orientation discrimination, detection and localization tasks, while manipulating a number of visual factors. We found that although attention improved overall performance, it did not affect performance fields. Two patterns were observed regardless of the presence of a local post-mask, the stimulus orientation, or the task. A horizontal-vertical anisotropy (HVA) became more pronounced as spatial frequency, eccentricity and set size increased. A vertical meridian asymmetry (VMA) became more pronounced as spatial frequency and eccentricity increased. We conclude that performance fields are determined by visual, rather than by transient attentional, constraints.

Analysis of Variance↗
PubMed1994

The effect of similarity and duration on spatial interaction in peripheral vision.

Spatial interactions are extensive in the peripheral visual field, extending up to about half the retinal eccentricity of the target (Toet and Levi, Vision Res. 32, 1349-1357, 1992). In the present study it is shown that the degree and extent of peripheral spatial interaction depends in large measure on the similarity between test and flanking stimuli. The stimulus consisted of a test T surrounded by four distracting flanking Ts, each randomly oriented. The task was to determine the orientation of the test T. The test and flanking Ts differed in contrast polarity, shape, depth, color, eye of origin, or contrast. When the target and flanks differed in contrast polarity, depth, or shape, performance improved markedly for all observers. A color difference enhanced the performance of most but not all observers. Eye-of-origin had no effect, that is, spatial interaction was identical when the target and flanks were presented to the same eye, or to opposite eyes. The role of stimulus duration in spatial interaction was examined in two additional experiments. In the first, the stimulus viewing duration was increased in order to allow the observer time to serially search for the test T. In the second experiment, a postmask was presented at the location of the test T. The results of these experiments showed that the influence of similarity was independent of stimulus duration and the postmask, and suggest that serial search does not play an important role in the spatial interaction effects reported here. The extent of spatial interaction is correlated with the ability to do parallel search.

Color Perception↗
PubMed1997

Searching through subsets: a test of the visual indexing hypothesis.

This paper presents three experiments investigating the claim that the visual system utilizes a primitive indexing mechanism (sometimes called FINSTS; Pylyshyn, 1989) to make non-contiguous features directly accessible for further visual processing. This claim is investigated using a variant of the conjunction search task in which subjects search among a subset of the items in a conjunction search display for targets defined by a conjunction of colour and orientation. The members of the subset were identified by virtue of the late onset of the objects' place-holders. The cued subset was manipulated to include either homogeneous distractors or mixed distractors. Observers were able to select a subset of three items from among fifteen for further processing (Experiment 1); furthermore, a reaction time advantage for homogeneous subsets over mixed subsets was observed, indicating that more than one of the subset is selected for further specialized processing. The homogeneous subset advantage held for subsets of two to five items (Experiment 2), and the time required to process the cued subset did not increase with increased dispersion of the items (Experiment 3). These results support the basic claim of the indexing theory: the claim that multiple visual indexes are used in selecting objects for visual processing.

Adult↗
PubMed2004

Central and peripheral visual interactions in disparity-induced vergence eye movements: I. Spatial interaction.

PURPOSE: To evaluate the interaction between central and peripheral disparities in the initiation of vergence eye movements. METHODS: Eye movements were recorded in eight normal subjects using an infrared limbus tracker. Three-dimensional visual stimuli were back projected onto a tangent screen by using two liquid crystal display (LCD) projectors through crossed polarizers. The central target was a vertical bar, which always jumped from 2 to 1 m. The peripheral target was a random-dot pattern that jumped from 2 to 0.75, 1, or 1.5 m (near planes), 2 m (no change), or 3 m (far plane) simultaneously with the central target jump. Latency, amplitude at 150 ms, and average amplitude over 1 to 2 seconds after vergence onset; peak velocity; and the main-sequence relationship of the initial vergence response were calculated. How far the central target appeared to jump was scored subjectively. RESULTS: In half of the subjects, there was a clear effect of the peripheral disparity on the dynamics of the vergence response to the central disparity. The amplitude of vergence at 150 ms, as an index of open-loop gain, was significantly greater when the peripheral target moved closer, but steady state amplitude (average during 1-2 seconds) did not change, and the vergence latency was significantly greater when the peripheral target jumped away. There was no obvious relationship between the perceived amount of movement of the central target and the parameters of the dynamic properties of the vergence response. CONCLUSIONS: Peripheral disparity can modulate the dynamics of the initial vergence response to a central disparity and is probably independent of the perception of motion in depth.

Adult↗
PubMedDate not supplied

Single-cell responses in normal and ototoxic drug-treated gerbils.

Single cell data were obtained from the inferior colliculus of normal gerbils and from those treated with ethacrynic acid. Response changes found for the drug-treated animals are as follows: Fewer cells responded to auditory stimulation. Recruitment-like functions were found which were characterized by high threshold and precipitous increase in discharge rate with intensity. Some units showed abnormally low discharge rate over an extended intensity range, which could be the underlying change in responsiveness in human patients with reduced loudness range. One half as many collicular units were responsive to auditory phase differences. The most severly affected ears showed abnormal dependency upon phase relations. This would affect auditory space perception.

Animals↗