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Biomedical subjects

P Cavanagh

Publications and source records attributed to P Cavanagh.

At least 91 records · Page 5Linked to original sources
PubMed1988

Anisotropy in the chromatic channel: a horizontal-vertical effect.

We compared the chromatic contrast thresholds of drifting (2Hz) red-green sine-wave gratings of horizontal, vertical, and two oblique orientations at three spatial frequencies (2, 4, 8 cpd). Luminance contrast thresholds for yellow-black gratings were also obtained. The classic oblique effect was found for high spatial frequency luminance and chromatic stimuli. For chromatic thresholds, a significant difference was found between the horizontal and vertical thresholds of all observers. One observer was retested with her head tilted 45 deg and demonstrated that the anisotropy was specific to retinal coordinates. These results give evidence for orientation selectivity in the chromatic channel which is at least partially independent of that in the luminance channel. We estimated the degree of lateral chromatic aberration in our observers' eyes and discuss the possible contribution of this aberration to the horizontal-vertical difference in the chromatic channel.

Analysis of Variance↗
PubMed1987

Diversity of complex cell responses to even- and odd-symmetric luminance profiles in the visual cortex of the cat.

We have tested the hypothesis that complex cell receptive fields are made up of subfields which, for a given cell, have either exclusively even or exclusively odd symmetry. To do this we have measured the response of complex cells in the visual cortex of the cat to members of pairs of spatially limited even-symmetric stimuli (single light and dark bars) and pairs of odd-symmetric stimuli ("light-dark" and "dark-light" double bars) successively drifting across their receptive fields. The strength of a cell's response was estimated by measuring the sum of all spikes produced by a stimulus. Some complex cells respond about equally to single light and dark bars; others respond appreciably more to either the light or dark bar. The central tendency of average response histograms was estimated by measuring the mean with respect to position across the width of the receptive field. Many complex cells show distinct spatial offsets between the mean for narrow single light and that for dark bars as well as between means to double bars of opposite phase. Combined offset plots were constructed with the spatial offsets between means for single light bars and single dark bars along the x axis and the offsets between means to double bars of opposite phase along the y axis. There is significant scatter in the combined offset points; some falling at the origin, some at significant distances from the origin along the axes, and others well within each of the four quadrants. These diverse localizations in the offset plots rule out the simple models of complex cell spatial substructure described above and, therefore, imply considerable heterogeneity within the population of complex cells.

Animals↗
PubMed1987

Motion capture anisotropy.

Two uncorrelated random dot patterns were superimposed and alternated to produce dynamic incoherent noise. When a low spatial frequency sine wave grating was optically superimposed on this noise and moved in step with the alternation of the two frames, the incoherent motion was masked and all the dots were seen to adhere to the grating and to move with it as a single rigid sheet ("Motion Capture"). Over a wide range of displacements subjects could not discriminate uncorrelated noise which was "captured" from correlated noise patterns which moved physically in the same direction as the grating. In fact the motion signal from the low frequency grating was even strong enough to overcome signals from two correlated random dot patterns which moved in the opposite direction. Capture was not as strong if the direction of dot motion was orthogonal to the direction of grating motion. We conclude that in any dynamic visual scene the motion of certain salient features in the image tends to dominate our perceptual experience. The signal from low frequencies masks or inhibits the signal from the high frequencies. Since the latter now have no motion signal of their own they are assumed to move with the low frequencies. Thus, motion capture suggests an important biological role for long-range apparent motion: the process serves to preserve continuity of object identity while at the same time eliminating spurious motion signals that arise from finer image features. In this manner the visual system solves the "correspondence problem" without benefit from either computation or cognition.

Discrimination, Psychological↗
PubMed1987

Equiluminance: spatial and temporal factors and the contribution of blue-sensitive cones.

Equiluminance ratios for red/green, red/blue and green/blue sine-wave gratings were determined by using a minimum-motion heterochromatic matching technique that permitted reliable settings at temporal frequencies as low as 0.5 Hz. The red/green equiluminance ratio was influenced by temporal but not spatial frequency, the green/blue ratio was influenced by spatial but not temporal frequency, and the red/blue ratio was influenced by both. After bleaching of the blue-sensitive cones, there was no change in equiluminance ratios, indicating no contribution of the blue-sensitive cones to the luminance channel even at low temporal and spatial frequencies. The inhomogeneity of yellow pigmentation within the macular region was identified as the source of the spatial-frequency effect on the blue/green ratio.

Color↗
PubMed1986

[Efficiency of associative memory inherent in post-tetanic potentiation].

An associative memory is modeled in networks of cells that are assumed to have the short-term plasticity of the neuromuscular junction of the frog. The data relating synaptic transmission efficiency and stimulation frequency for post-tetanic potentiation of the neuromuscular junction are represented by polynomial expansions. Simulation of storage and retrieval demonstrates that functional associative memory is feasible based on this particular synaptic plasticity. Retrieval reaches a maximum efficiency at a delay of three minutes after storage and is lost after about 9 min. The signal to noise ratio of the retrieved pattern drops steadily as additional associations are stored in memory but retrieval appears to be possible with up to four stored associations. Although the data are derived from synapses not normally proposed as a basis for memory functions, the results here will generalize to other synaptic junctions located more centrally that have similar characteristics. This simulation technique allows the efficiency of associative memory based on various types of synaptic plasticity to be evaluated.

Brain↗
PubMed1986

Brightness shift in drifting ramp gratings isolates a transient mechanism.

A brightness shift is demonstrated for moving ramp stimuli. The data suggest that the transient visual response to changing luminance is combined with the sustained response to produce an overall impression of brightness. We attribute the brightness shift to the saturation of the transient response: a limitation on the maximum transient when responding to rapid brightening or dimming. The brightness shift provides a new technique for isolating the responses of the transient mechanisms.

Form Perception↗
PubMed1985

Color and luminance share a common motion pathway.

Following exposure to a moving grating of bars differing only in luminance, a motion aftereffect (MAE) is observed on a stationary grating of bars differing only in chrominance. This suggests that the motion of equiluminous chromatic stimuli is sensed by a channel that responds to both luminance and chrominance and not by a separate channel specialized for the motion of colored stimuli. However, adding color to a low contrast luminance stimulus actually reduces its effectiveness at creating or nulling a MAE, indicating that the response of the motion pathway to color is qualitively different from its response to luminance. In addition, a chromatic stimulus demonstrates a dissociation between perceived speed, MAE speed and speed required to null the MAE that is absent for a luminance stimulus.

Adaptation, Ocular↗
PubMed1985

Perception of motion in equiluminous kinematograms.

Two fields of random dots that were identical except for a slight shift in a central square region were presented in rapid alternation. This produced a vivid impression of a square moving back and forth above the background. When the kinematogram is presented in equiluminous red/green, the motion of the central region can still be seen, although over a narrower range of alternation rates, interstimulus intervals, and displacements than for black/white presentation. The perception of motion for equiluminous stimuli indicates that colour and motion can be analyzed conjointly by the visual system. However, as originally reported by Ramachandran and Gregory, the segregation of the oscillating central square from the background is lost at equiluminance. This segregation process therefore appears to be colour-blind.

Color Perception↗
PubMed1985

A moving display which opposes short-range and long-range signals.

A novel display is described which stimulates both the long-range and the short-range motion detecting processes simultaneously, but with opposing directions of movement. The direction in which the stimulus appears to move depends on retinal eccentricity and element size, but adaptation to the display always produces a motion aftereffect (MAE) direction opposite to the direction of the short-range component. The display may offer insights into the properties of the two-process motion detecting system.

Adaptation, Physiological↗
PubMed1985

Does bronchography have a role in the assessment of patients with haemoptysis?

The results of bronchography in 96 consecutive patients investigated for haemoptysis at Papworth Hospital from 1975 to 1983 were reviewed. None of the patients included in this study gave a history suggestive of bronchiectasis and neither chest radiography nor fibreoptic bronchoscopy had shown a cause for the bleeding. Bronchography was performed through the fibreoptic bronchoscope and all included in the study showed both lungs adequately. The chest radiographic appearances were compared with the bronchographic findings. Seven of 12 patients with appearances suggesting old fibrosis showed bronchiectasis, as did eight of 10 with radiographic appearances suggestive of bronchiectasis. Eleven out of 74 patients with normal chest radiographs, however, also showed bronchiectasis. This group of 11 was compared with the other 63 but no clinical feature was found to be significantly associated with the presence of bronchiectasis. Although bronchography is now rarely used in the investigation of haemoptysis, this high yield (15%) of bronchiectasis indicates that its use should be reappraised. Follow up of the patients indicated that bronchography was not reliable at diagnosing peripheral bronchial carcinomas, which became evident later in two cases, and that asthma was present in 15 (24%) of the 63 patients with both normal chest radiographs and normal bronchograms.

Adolescent↗
PubMed1985

Subjective figures and texture perception.

A texture discrimination task using the Ehrenstein illusion demonstrates that subjective brightness effects can play an essential role in early vision. The subjectively bright regions of the Ehrenstein can be organized either as discs or as stripes, depending on orientation. The accuracy of discrimination between variants of the Ehrenstein and control patterns was a direct function of the presence of the illusory brightness stripes, being high when they were present and low otherwise. It is argued that neither receptive field structure nor spatial-frequency content can adequately account for these results. We suggest that the subjective brightness illusions, rather than being a high-level, cognitive aspect of vision, are in fact the result of an early visual process.

Form Perception↗
PubMed1985

Alpha-1-antitrypsin deficiency presenting as bronchiectasis.

We describe three patients with the clinical and radiographic features of progressive bronchiectasis, and without evidence of emphysema, associated with deficiency of alpha-1-antitrypsin. The association of hereditary alpha-1-antitrypsin deficiency and pulmonary emphysema is well recognized (Eriksson 1965). About half of these subjects also have chronic bronchitis as defined by persistent sputum production (Tobin et al. 1983). Bronchographically proven bronchiectasis without apparent emphysema has been reported in only one subject with alpha-1-antitrypsin deficiency (Longstretch et al. 1975).

Aged↗
PubMed1984

Interocular transfer of a chromatic frequency shift: temporal constraints.

Interocular transfer of a chromatic version of the Blakemore and Sutton (1969) [Science 166, 245-247] spatial frequency shift is not obtained with long steady (3000 msec) test presentation, but does occur when the test is brief (400 msec) or flickering. By contrast, interocular transfer of achromatic spatial frequency adaptation occurs with long steady tests as well as with brief or flickering tests.

Adaptation, Ocular↗
PubMed1984

Use of Gabor elementary functions to probe receptive field substructure of posterior inferotemporal neurons in the owl monkey.

The large receptive fields of inferotemporal neurons in the owl monkey were studied with visual stimuli whose luminance profiles were one-dimensional Gabor functions, i.e. sinusoidal gratings within Gaussian envelopes. The members of one set of such patterns all had a full bandwidth at half-amplitude of 0.8 octaves, but different center frequencies and spatial extents. These spatially restricted stimuli were ideal for determining whether one or more than one spatial frequency band projected onto discrete subsections of the neuron's receptive field. The other set of Gabor stimuli comprised sine waves within Gaussian envelopes of constant size, but with different center frequencies and hence different bandwidths. These stimuli allowed assessment of the neuron's spatial frequency selectivity across the full breadth of its receptive field. Results suggest that only one orientation band and one spatial frequency band provide an input onto each inferotemporal neuron under our experimental conditions. The preferred spatial frequencies found (0.2-0.6 c/deg) were all in the very low spatial frequency range for this animal. Calculations show that about 3.5-7.0 full cycles of the optimal grating usually cover the full width of the receptive field, but the observed spatial frequency tuning is not nearly as sharp as that which would be predicted according to phase coherent linear summation. Moreover, at the preferred spatial frequency, the peak response to gratings in the constant aperture series was generally less than the response to the same preferred spatial frequency in the constant relative bandwidth series. These results suggest either incomplete phase coherent summation from contributing subgroups, non-linear processing, or both.

Animals↗
PubMed1984

An anticipated threshold technique for measuring contrast sensitivity.

A new psychophysical procedure for assessing contrast sensitivity rapidly and reliably is reported. The technique consists of increasing the contrast of a conventional sine wave grating according to the parameters set in a computer implemented algorithm. During the first half of a trial, the contrast increases quickly to a point just below an expected threshold, and in the second stage the contrast increases slowly until a threshold is reached. Details and advantages of the algorithm are presented as well as a comparison of data generated by this and other techniques.

Computers↗
PubMed1984

Perceived velocity of moving chromatic gratings.

Equiluminous red-green sine-wave gratings were drifted at a uniform rate in the bottom half of a 10-deg field. In the top half of the display was a sinusoidal-luminance grating of the same spatial frequency and 95% contrast that drifted in the opposite direction. Observers, while fixating a point in the display center, adjusted the speed of this upper comparison grating so that it appeared to match the velocity of the chromatic grating below. At low spatial frequencies, equiluminous gratings were appreciably slowed and sometimes stopped even though the individual bars of the grating could be easily resolved. The amount of slowing was proportionally greatest for gratings with slow drift rates. Blue-yellow sine-wave gratings showed similar effects. When luminance contrast was held constant, increasing chrominance modulation caused further decreases in apparent velocity, ruling out the possibility that the slowing was simply due to decreased luminance contrast. Perceived velocity appears to be a weighted average of luminance and chrominance velocity information.

Color Perception↗