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T Carney

Publications and source records attributed to T Carney.

At least 37 records · Page 2Linked to original sources
PubMed1993

Dichoptic activation of the early motion system.

The short range or early motion system has long been considered incapable of binocular integration. We have developed dichoptic motion stimuli which are based upon the decomposition of traveling sinewave gratings into the sum of two standing waves in spatial and temporal quadrature. The monocular views of such displays appear as counterphase flicker but when presented dichoptically the perception is of movement in a unique direction. Two lines of evidence are presented for the binocularity of early motion mechanisms in human vision. First, adaptation to dichoptic motion sinewave gratings is found to result in a motion aftereffect. Second, random texture motion displays based on the quadrature decomposition are found to support dichoptic perception of motion direction, but not figure/ground. Unlike random dot kinematograms, these displays do not necessitate alternating the direction of motion during dichoptic presentation. This encumbrance, and the reliance on figure/ground discrimination, may have been responsible for prior failure to achieve dichoptic motion perception with short range stimuli.

Adaptation, Ocular↗
PubMed1993

Orientation, masking, and vernier acuity for line targets.

In an attempt to uncover the properties of the psychophysical spatial mechanisms which optimally respond to the vernier offset between two abutting lines, we investigated the effects of one-dimensional band-limited spatial noise masks superimposed with the target, on vernier thresholds. Unidirectional vernier thresholds were measured in the presence of masks varying in orientation, spatial frequency content and luminance modulation. Because of the dependence of vernier thresholds on target visibility, the effects of these masks on target detection thresholds were also measured. In accordance with the results of Findlay [(1973) Nature, 241, 135-137] but contrary to an hypothesis that the direction of the vernier offset is mediated by the differential output of spatial filters of a single orientation, our results reveal a bimodal orientation tuning function for vernier acuity. We propose that, for offset line targets, the differential responses of at least two filters with orientations which straddle the target lines are combined to extract relative position information. The spatial frequency tuning characteristics of the optimal mechanisms for mediating vernier information are similar to those optimal for detecting the target lines themselves, except that they are tuned to a slightly higher spatial frequency and have a slightly narrower bandwidth. The spatial mechanisms most sensitive to the vernier offset and to target detection exhibit similar responses to increases in mask modulation. This finding suggests that these tasks are limited by the same source of noise, and explains why under a variety of experimental manipulations, equally visible vernier targets result in similar vernier thresholds.

Humans↗
PubMed1993

Can sinusoidal vernier acuity be predicted by contrast discrimination?

A test-pedestal approach, with a test grating superimposed on a masking pedestal, was used to compare sinusoidal grating vernier acuity and contrast discrimination thresholds. The goal is to develop a simple model for vernier acuity without assumptions about underlying mechanisms. In the contrast discrimination task, subjects were asked to detect contrast increments in the presence of a base pedestal. In the vernier task, a test grating shifted by 90 deg relative to the pedestal grating was added to one-half of the pedestal grating to produce a vernier offset. When expressed in the same contrast units and compared under optimal conditions, vernier and contrast discrimination thresholds agree well at spatial frequencies between 2 and 20 c/deg and at pedestal contrasts above 10 times detection threshold. Thus, under these conditions, contrast discrimination predicts grating vernier acuity. To account for the discrepancies between vernier thresholds and contrast just noticeable difference (JND) when conditions deviate from optimal, one needs to make assumptions about the underlying mechanisms.

Contrast Sensitivity↗
PubMed1992

Binocularity of early motion mechanisms: comments on Georgeson and Shackleton.

The demonstration of compelling dichoptic illusions of motion using a variety of stimuli, all of which share the property that left and right eye patterns were spatio-temporal quadrature pairs, has been interpreted as evidence for binocular integration in the early motion system (short range motion). Georgeson and Shackleton (1989, Vision Research, 29, 1511-1523) have recently challenged this position based on results using 3 types of stimuli, sinusoidal gratings, random line kinematograms and missing fundamental squarewaves. For each class of stimuli motion was perceived during dichoptic presentation, but certain limitations led them to conclude that early motion mechanisms are defeated when no motion is present monocularly. We do not dispute their data, rather an alternative interpretation is offered which supports the position that early motion sensors are capable of binocular integration, a property well established physiologically.

Humans↗
PubMed1990

Vernier acuity as line and dipole detection.

The vernier judgment is commonly thought of as discriminating the displacement of a portion of a pattern. However, we have found it revealing to consider vernier stimuli in another light; as the composite of a test pattern superimposed on a masking pedestal. The pedestal is the pattern with zero spatial offset, and the test pattern is the luminance distribution which, when added to the pedestal, produces the offset. For example, a vernier offset of an edge can be generated by adding a thin line (the derivative of an edge) to one half of an edge pedestal, and a vernier offset of a line can be generated by adding a thin dipole (the derivative of a line) to one half of a line pedestal. Vernier thresholds for low contrast edge and line pedestals can be directly predicted from detection thresholds of thin lines and dipoles on uniform fields. A surprisingly simple relationship is also derived between vernier thresholds and the size of Ricco's integration zone. We have found this masking paradigm to be fruitful and believe it is relevant to all the hyperacuities, not just vernier.

Contrast Sensitivity↗
PubMed1989

A physiological correlate of the Pulfrich effect in cortical neurons of the cat.

When a swinging pendulum is viewed with a light-attenuating filter before one eye, the pendulum bob is perceived to move in an elliptical path in depth. It is believed that the filter causes this illusion, the Pulfrich effect, by delaying processing of the image in the filtered eye relative to that of the unfiltered eye. We sought a physiological correlate of this effect by studying binocular integration in cortical neurons of cats while they viewed moving stimuli. Special attention was focused on single unit disparity tuning because it is widely believed that depth perception is related to the responses of disparity selective neurons in visual cortex. We found that placing a filter before one of the cat's eyes produced a temporal delay in the cortical response. The temporal delay was always associated with a shift in the neuron's spatial disparity tuning. The observed temporal delays and disparity shifts are comparable with the magnitude of the Pulfrich effect in humans.

Action Potentials↗
PubMed1989

Cortical processing of hyperacuity tasks.

Spatial discrimination thresholds were determined by having subjects make comparisons between stimuli presented successively at the same or at different locations in the visual field. Two tasks were employed, discrimination of line orientation and discrimination of the distance between two parallel lines (spatial interval discrimination). We find that discrimination thresholds based on the comparison of features in spatially-separated stimuli are comparable to those based on the comparison of features in two stimuli presented successively at the same location. This holds even when the stimuli are presented in nonhomologous positions in the visual field or are presented in a manner such that they activate cells in opposite cortical hemispheres in the early visual areas. This shows that discriminability is not determined solely by differences in the firing rates of striate or pre-striate neurons. Rather, it appears that the attributes of visual stimuli are precisely coded and available for comparison at higher levels of visual processing. Implications of this finding for models of hyperacuity are discussed.

Discrimination, Psychological↗
PubMed1989

A fresh approach to child protection practice and legislation in Australia.

This paper outlines some of the findings and approaches of the Victorian Review. It assesses a stylized "orthodox model" of child welfare law and practice, a model which remains in many Australian jurisdictions and which has its popularity in Britain, Canada, and the United States. It concludes that serious consideration should be given to rebuilding and strengthening these arrangements. This entails emphasis on utilizing laws to serve an educative function and facilitating access by children and families to preventive and support services. It argues that a broad concept of neglect should be adopted and that the "welfare orientation" of the Children's Court requires reconsideration, because the capacity of courts to resolve entrenched structural problems of society has been overrated. Welfare practice, and specifically a community approach to welfare, should shape the primary response to protection of children; judicial involvement should be a last resort.

Australia↗
PubMed1988

Binocular interaction in the perigeniculate nucleus of the cat.

We have recorded from single cells in the perigeniculate nucleus (PGN) of the cat to determine their response properties. Quantitative tests have been conducted with sinusoidal gratings. Using optimal stimulus parameters, determined monocularly, we explored binocular interaction by varying the relative phase between dichoptically presented gratings. Monocularly, cells exhibit varying degrees of response specificities with respect to stimulus orientation and spatial frequency. Binocularly, we have identified six types of response. The most prominent, type 1, found for half the cells, is phase-specific binocular interaction at the fundamental frequency component of the drifting grating. For these cells, mean response rate is independent of interocular phase. The remaining types of binocular responses involve varying degrees of interaction at different harmonic components. For a quarter of the sample, no binocular interaction was observed. To investigate the role of cortical input to PGN, visual cortex was removed from some cats. Subsequent study of PGN cells indicated that response properties were generally similar to those found in intact animals. We conclude that PGN response properties are determined primarily by subcortical inputs.

Afferent Pathways↗
PubMed1988

Orientation discrimination as a function of stimulus eccentricity and size: nasal/temporal retinal asymmetry.

Orientation discrimination threshold is a monotonically increasing function of retinal eccentricity. Increasing stimulus length extends the range of eccentricities over which fine orientation discriminations can be made. Orientation discrimination thresholds at all eccentricities are determined by the size of the cortical image of the stimulus. Thresholds obtained using either nasal or temporal hemiretina are similar up to the blind spot, beyond which the temporal retina yields increasingly higher thresholds. The results are consistent with a recent theoretical study which predicts that orientation discrimination threshold is determined by the number of cortical cells activated by the discrimination target.

Humans↗
PubMed1987

Binocular interaction in the dorsal lateral geniculate nucleus of the cat.

We have investigated binocular interaction in the dorsal lateral geniculate nucleus (LGN) of the cat. Neurons were recorded extracellularly during visual stimulation with sinusoidal gratings which were presented at different interocular phases (disparities). The large majority of cells (91%) exhibited some type of binocular interaction. For 75% and 16% of the total number of cells, the binocular interaction was inhibitory or facilitatory, respectively. For the remaining 9% of cells, no interaction was evident. In marked distinction from visual cortex, the facilitatory and inhibitory interactions in the LGN are independent of the relative interocular phase of the patterns. Neurons in the LGN are therefore insensitive to the stereoscopic depth cue, retinal disparity.

Animals↗
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↗
PubMed1982

Directional specificity in tilt aftereffect induced with moving contours: a reexamination.

In the tilt aftereffect a grating or bar is perceived as being slightly rotated from its veridical orientation if it is preceded by a similar adaptation stimulus with a slightly different orientation. It has been reported that the tilt aftereffect is not direction specific. That is, the magnitude of the misperception was not affected by whether the adaptation and test stimuli were moving in the same or the opposite directions. However, when we required subjects to fixate on a stationary spot during adaptation to a moving grating, the tilt aftereffect was strongest when both stimuli moved in the same direction. Moreover, the tilt aftereffect was not direction specific without such fixation. These results are consistent with the distribution shift model in which the perceived orientation reflects the distribution of orientation selective units, some of which are also direction selective.

Adaptation, Ocular↗