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PubMed1988

Space-time separability of direction selectivity in cat striate cortex neurons.

The relationship between spatial and temporal factors underlying direction selective velocity tuning in cat striate cortex neurons was studied quantitatively using flashed and smoothly moving light bar stimuli. Responses to double-flash stimuli were obtained for all combinations of a series of spatial and temporal intervals between the two flashes. Nonlinear interactions underlying the unit's direction selectivity were estimated by subtracting the responses to the component single flashes. The extent of systematic interaction between spatial and temporal variables was assessed qualitatively, with contour plots, and quantitatively, by plotting the optimum displacement as a function of temporal separation. Both methods indicated interactions (space-time nonseparability) which were generally very small in magnitude, and uncorrelated either in magnitude or sign with the direction selective velocity tuning measured in the same neurons. These results demonstrate characteristic optimum spatial and temporal intervals for responses to the two-flash stimuli. The optimum spatial offset was independent of the temporal offset chosen, indicating space-time separability.

Animals↗
PubMed1984

Selective attention and the organization of visual information.

Theories of visual attention deal with the limit on our ability to see (and later report) several things at once. These theories fall into three broad classes. Object-based theories propose a limit on the number of separate objects that can be perceived simultaneously. Discrimination-based theories propose a limit on the number of separate discriminations that can be made. Space-based theories propose a limit on the spatial area from which information can be taken up. To distinguish these views, the present experiments used small (less than 1 degree), brief, foveal displays, each consisting of two overlapping objects (a box with a line struck through it). It was found that two judgments that concern the same object can be made simultaneously without loss of accuracy, whereas two judgments that concern different objects cannot. Neither the similarity nor the difficulty of required discriminations, nor the spatial distribution of information, could account for the results. The experiments support a view in which parallel, preattentive processes serve to segment the field into separate objects, followed by a process of focal attention that deals with only one object at a time. This view is also able to account for results taken to support both discrimination-based and space-based theories.

Adolescent↗
PubMed2004

Don't think of a white bear: an fMRI investigation of the effects of sequential instructional sets on cortical activity in a task-switching paradigm.

Using functional magnetic resonance imaging (fMRI), we investigated processes involved in switching between two ongoing tasks, thought to be paradigmatic of executive control processes. Subjects were considerably slower and less accurate when switching between two tasks than when repeatedly carrying out one task, so-called "switch costs." Switch costs, however, generally occur only when more than one task is associated with each stimulus type. This has led to the surmise that switch costs may be due largely to ongoing interference from previously learned stimulus-response (S-R) associations, which are no longer relevant for the task at hand. We used a paradigm that specifically assessed this hypothesis and investigated three stages. In Stage 1, a single task was carried out with each stimulus type; in Stage 2, a second novel task was introduced for each stimulus type; and in Stage 3, subjects reverted to carrying out solely the original tasks (as in Stage 1). In Stage 1, only one task was associated with each stimulus type, whereas two tasks were associated with each stimulus type in Stages 2 and 3. We compared images obtained during Stage 3 to those obtained during Stage 1 and show that during Stage 3, there was robust activation in the network of areas associated with the Stage 2 tasks, even though these tasks were no longer relevant. Our data strongly suggest that switch costs may derive largely from continued activation of areas associated with carrying out the now-irrelevant task(s). We posit that a large component of executive control processes involves resolution of competition between potentially relevant tasks. Our data also revealed widespread activation of a frontoparietal network of areas, and we discuss how this network might be involved in mediating this competition.

Adult↗
PubMed1997

Perception of spatial orientation in spasmodic torticollis. Part I: The postural vertical.

Estimates of points of entering and exiting from upright posture were obtained from 25 seated, restrained patients with idiopathic spasmodic torticollis (ST) and matched normal subjects exposed to cycles of 1.5 degrees/s tilts in a flight simulator. Estimates were obtained for displacements in roll and pitch about upright and for yaw tilts about a rostrocaudal, "barbecue," axis with the subjects supinated. For both pitch and roll, normal subjects estimated entering upright when they were still approximately 1 degree from machine upright and perceived themselves to be upright through a mean arc of 6 degrees. In barbecue tilt, entering upright was estimated at 0.2 degree for an arc of 6 degrees. Patients estimated entering upright at 2.8 degrees in roll and 3 degrees in pitch but estimated exiting upright at the same tilt as normal subjects; that is, they were less specific in detecting verticality. Patients were normal in barbecue tilt. No relationship between tilt estimates and head deviation was found. There were no differences between normal subjects when tested with their head in normal posture and with an assumed tilt of 20 degrees. Normal subjects probably based their estimates on combined vestibular-somatosensory signals, whereas torticollis patients appeared to derive more from a vestibular signal. However, patients referred the vestibular signals to the trunk long axis when asked to indicate the whole-body vertical. The findings suggest disruption of the normal combined vestibuloproprioceptive mechanism for detecting body uprightness in ST.

Adult↗
PubMed1994

Development of the middle concept.

Children's use of the middle concept was assessed in two developmental studies. In Experiment 1, children in kindergarten through Grade 5 were given middle-size pretraining followed by the presentation of sets of three stimuli representing eight novel dimensions. There was a marked improvement in the mastery of the middle concept across elementary school grades. Asking the child to point to the middle "dimension label" (e.g., middle oval) facilitated performance when compared to asking the child to point to the middle thing. In Experiment 2, second- through fifth-grade children were pretrained to choose the middle item associated with a single set of three stimuli, two sets of three stimuli representing the same dimension, or two sets of three stimuli with each set representing a different dimension. Transfer to six novel test dimensions was evaluated. Pretraining with two rather than one stimulus set transferred to the novel test dimensions. The results were related to induction and transposition theories and to issues concerning implicit and explicit learning.

Child↗
PubMed1981

Spatial arrangements of responses by cells in the cat visual cortex to light and dark bars and edges.

Detailed examination is made of the responses of visual cortical cells (area 17, border 17-18 and adjacent area 18) in the anaesthetized cat to stationary flashing bars and to bars (lines) and edges moving at their optimal velocities. Particular attention is given to the receptive field organization of cells in the simple family. While there is good general agreement between the main receptive field subregions revealed by stationary and moving stimuli, the responses to moving light and dark bars, supplemented by the responses to moving light and dark edges, provide a much more rapid, accurate and complete guide to the spatial organization of the receptive fields than do the response profiles to a stationary flashing bar. Moving light and dark bars between them generally reveal more subregions in the receptive fields of simple cells than is evident from the response profiles to a stationary flashing bar, particularly when the receptive fields have many subregions. In addition the responses to moving edges provide a rapid guide to spatial summation across the width of a subregion and the possible antagonistic effects of the next subregion in sequence. Two subclasses of cells in the simple family have been recognized: ordinary simple and fast simple cells. Two cell classes (A-cells and silent periodic cells) having properties intermediate between simple and complex types are discriminated and their properties described.

Animals↗
PubMed1995

Effects of the spatio-temporal structure of optical flow on postural readjustments in man.

How does the spatio-temporal structure of an oscillating radial optical flow affect postural stability? In order to investigate this problem, two different types of stimulus pattern were presented to human subjects. These stimuli were generated either with a constant spatial frequency or with a spatial frequency gradient providing monocular depth cues. When the stimulation was set in motion, the gain response of the antero-posterior postural changes depended upon the oscillation frequency of the visual scene. The amplitude of the postural response did not change with the amplitude of the visual scene motion. The spatial orientation of the postural sway (major axis of sway) depended strictly and solely on the structure of the visual scene. In static conditions, depth information resulting from the presence of a spatial frequency gradient enhanced postural stability. When set in motion, a visual scene with a spatial frequency gradient induced an organization of postural sway in the direction of the visual motion. Considering visual dynamic cues, postural instability depended linearly both on the logarithm of the velocity and on the logarithm of the temporal frequency. A nonlinear relationship existed between the amplitude of the fore-aft postural sway at the driving frequency and the temporal frequency, with a peak around 2-4 Hz. These results are discussed in terms of their implications for the separation of visual and biomechanical factors influencing visuo-postural control.

Adult↗
PubMed1979

On the perception of some geometric properties of rotating three dimensional objects.

Various observations and conjectures have been made about how we perceive three dimensional objects under monocular viewing conditions; in particular, when such objects are rotating. In this paper these proposals are reviewed and reinterpreted in the light of geometric properties of objects and their projections. Some results are also presented which support the conclusion that the visual system, in reconstructing the object from its projections, requires a fixed time to establish the objects curvature and torsion parameters. In the perceptual reconstruction process these parameters assume positive or negative values, and this equivocation is related to a form of the adjacency principle involving central projections and "perceptual geodesics".

Form Perception↗
PubMed1980

Extraction of objects from structured backgrounds in the cat superior colliculus. Part I.

Specific changes occur in the cells of the upper layers of the cat's superior collicules when a two dimensional noise (background) is superimposed onto a deterministic signal (spot of light). Some of the measurements can be interpreted as meaning that some cells only react to certain relative movements of object (spot) and background (noise). The movement of the visual background is interpreted as environmental movement occurring due to the animal's own movement. The results of the measurements provide all the necessary presuppositions for a distinction between the animal's own velocity and that of the object (Part I). The experimental results can be interpreted with a model. The essential factors for the interpretation is the direction specific behavior of the cells which is bound up with an asymmetrical spatial coupling of the neurons with each other. The decisive advantage of asymmetrical systems for the pattern recognition of moving objects is that they can work without distortion and spatial displacement over large ranges of velocity (Part II).

Animals↗
PubMed1980

On localization of moving objects in the visual system of cats.

In cortical areas direction-specific receptive fields occur systematically. Direction specifity is based on unsymmetric coupling of neurons. Such a coupling allows an exact localization of moved stimuli. For this task, the asymmetry in the time domain is compensated for by a spatial asymmetry.

Animals↗
PubMed2006

The role of human extra-striate visual areas V5/MT and V2/V3 in the perception of the direction of global motion: a transcranial magnetic stimulation study.

Several published single case studies reveal a double dissociation between the effects of brain damage in separate extra-striate cortical visual areas on the perception of global visual motion defined by a difference in luminance (first-order motion) versus motion defined by a difference in contrast (second-order motion). In particular, the medial extrastriate cortical region V2/V3 seems to be crucial for the perception of first-order motion, but not for second-order, whereas a lateral and more anterior portion of the cortex close to the temporo-parieto-occipital junction (in the territory of the human motion area hV5/MT+) seems to be essential only for the perception of second-order motion. In order to test the hypothesis of a functional specialization of different visual areas for different types of motion, we applied repetitive transcranial magnetic stimulation (rTMS) unilaterally over areas V2/V3, V5/MT, or posterior parietal cortex (PPC) while subjects performed a 2AFC task with first- or second-order global motion displays in the contralateral visual field. Results showed a comparable disruption of the two types of motion, with both rTMS over V2/V3 or over MT/V5, and little or no effect with rTMS over PPC. The results suggest that either the previous psychophysical results with neurological patients are incorrect (highly unlikely) or that the lateral and medial regions are directly connected (as they are in macaque monkeys) such that stimulating one automatically affects the other, in this instance disruptively.

Adult↗
PubMed2002

Orientation and spatiotemporal tuning of cells in the primary visual cortex of an Australian marsupial, the wallaby Macropus eugenii.

The metatherians (marsupials) have been separated from eutherians (placentals) for approximately 135 million years. It might, therefore, be expected that significant independent evolution of the visual system has occurred. The present paper describes for the first time the orientation, direction and spatiotemporal tuning of neurons in the primary visual cortex of an Australian marsupial, the wallaby Macropus eugenii. The stimuli consisted of spatial sinusoidal gratings presented within apertures covering the classical receptive fields of the cells. The neurons can be classified as those with clear ON and OFF zones and those with less well-defined receptive field structures. Seventy-percent of the total cells encountered were strongly orientation selective (tuning functions at half height were less than 45 degrees ). The preferred orientations were evenly distributed throughout 360 degrees for cells with uniform receptive fields but biased towards the vertical and horizontal for cells with clear ON-OFF zones. Many neurons gave directional responses but only a small percentage of them (4%) showed motion opponent properties (i.e. they were excited by motion in one direction and actively inhibited by motion in the opposite direction). The median peak temporal tuning for cells with clear ON-OFF zones and those without were 3 Hz and 6 Hz, respectively. The most common peak spatial frequency tuning for the two groups were 2 cycles per degree and 0.5 cycles per degree, respectively. Spatiotemporal tuning was not always the same for preferred and antipreferred direction motion. In general, the physiology of the wallaby cortex was similar to well studied eutherian mammals suggesting either convergent evolution or a highly conserved architecture that stems from a common therian ancestor.

Animals↗