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Biomedical subjects

A Reeves

Publications and source records attributed to A Reeves.

At least 55 records · Page 3Linked to original sources
PubMed1986

Pathways in type-B (U-shaped) metacontrast.

Rod and cone targets were crossed, in every combination, with rod and cone masks in flanking-bars metacontrast. Strong type-B (U-shaped) metacontrast was obtained in each condition, contrary to the claim that rod and cone masking are independent. In each condition, visibility declined steadily with stimulus-onset asynchrony (SOA) in trials in which target and mask appeared to be simultaneous, and increased with SOA in trials in which they appeared to be successive. The 'U' results from collapsing across these different types of trials, which may reflect distinct monotonic processes in masking. Under the light adaptation conditions used the time, Tmax, at which metacontrast was at a maximum was delayed by about 25 ms if rods, rather than cones, detected the target. Whether rods or cones detected the mask hardly altered Tmax.

Adaptation, Ocular↗
PubMed1986

Simultaneous color constancy.

Observers matched patches (simulated Munsell papers) in two simultaneously presented computer-controlled displays, a standard array presented under 6500-K illumination and a test array under 4000 or 10,000 K. Adaptation to the test illuminants was limited. The adjusted patch was surrounded by a single color (annulus display) or by many colors (Mondrian display). Observers either matched hue and saturation or made surface-color (paper) matches in which the subject was asked to make the test patch look as if it were cut from the same piece of paper as the standard patch. For two of the three subjects, the paper matches were approximately color constant. The hue-saturation matches showed little color constancy. Moreover, the illumination difference between the two displays was always visible. Our data show that simultaneous mechanisms alone (e.g., simultaneous color contrast) alter hues and saturations too little to produce hue constancy.

Color↗
PubMed1983

Distinguishing opponent and non-opponent detection pathways in early dark adaptation.

Desensitization of the red-green opponent pathway was demonstrated in early dark adaptation with the aid of a test chosen to isolate that pathway. Isolation was achieved by requiring the observer to adjust the intensity of a foveal test light to the threshold for flicker, when the test alternated slowly between luminance-matched red and yellow fields. The luminance matches were precise enough that only an opponent pathway could mediate the flicker thresholds. Desensitization occurred after continuous light adaptation to 626 nm fields, but did not occur after adaptation to yellow fields, or if the 626 nm field was turned on and off at 2 Hz throughout adaptation. The properties of the red-green pathway measured with the flicker thresholds resemble those of the yellow-blue pathway as shown in transient tritanopla.

Color Perception↗
PubMed1983

Interruption of dot processing by a backward mask.

A white target field with 0-10 randomly positioned black dots was presented for 20 ms, and followed by a patterned mask with a duration of 200 ms. Subjects reported target numerosity, with strict or lax criteria. Numerosity functions, and control forced-choice discrimination results, supported an interruption model for backward masking by pattern, even for stimulus onset asynchronies as short as 50 ms. When the same targets were degraded by reducing their contrast, but not otherwise masked, results supported an integration model.

Discrimination Learning↗
PubMed1982

Exchange thresholds for green tests.

Thresholds for detection of foveal pi 4-detected 522 nm, 1 deg, 200 msec test flashes rise by up to 0.4 log units just after a pi 4-equated "exchange" field has been substituted for the adaptation field, and take 60-90 sec to recover. This effect is not changed by an intense blue auxiliary field, and varies with the difference between adaptation and exchange field wavelengths as if only inputs from long-wavelength (LW) cones are involved. The effect can be virtually eliminated by adaptation to a background which alternates at 15 Hz between selected pi 4-equated long- and short-wavelength fields, which suggests that the LW cones exert their effect at a red-green opponent site.

Adaptation, Ocular↗
PubMed1982

Metacontrast U-shaped functions derive from two monotonic processes.

Different underlying processes account for the descending and ascending portions of the metacontrast U-shaped function obtained in the flanking-masks paradigm. One or another process is dominant on each trial. Each process is monotonic with stimulus onset asynchrony in the region in which it can be measured. The two processes may be isolated by asking the subject to report on each trial not only target visibility but also whether target and mask appear simultaneous or not. Standard U-shaped functions could be obtained only as an artifact of averaging across these different types of trials.

Color Perception↗
PubMed1982

Exchange thresholds for long-wavelength incremental flashes.

Thresholds of 1-deg, 200-msec, 641-nm foveal test flashes rise after an exchange of II5-equated 536- and 626-nm fields, taking about 30 sec to recover. Silent substitution (no rise of threshold after the exchange) occurs, however, if these fields are alternated during adaptation. Thresholds for a 1-deg, 20-msec test rise similarly after an exchange but recover in only 0.5 sec and are not influenced by alternation of the 536- and 626-nm fields. These results can be accounted for if the 641-nm tests are detected not through pathways controlled by long-wavelength cones alone but through nonopponent (20-msc) and opponent (200-msec) pathways whose sensitivities may be reduced by transient inputs from other cones.

Adaptation, Physiological↗
PubMed1979

Comparison of interwave latencies of brain stem auditory evoked responses in narcoleptics, primary insomniacs and normal controls.

A study of brain stem auditory evoked responses (BAER) was carried out in 10 narcoleptics, 10 primary insomniacs and 10 normal controls to determine if a neurophysiologic abnormality could be detected in these primary sleep disorders. The mean interpeak conduction times of Wave I-III, III-V and Iv were compared between the following groups: normal controls awake and in monitored sleep; narcoleptics awake and in monitored sleep, normal controls awake and narcoleptics awake; normal controls awake and insomniacs awake; narcoleptics awake and insomniacs awake; narcoleptics with cataplexy (n = 6) awake and narcoleptics without cataplexy (n = 4) awake. No significant differences were found which suggests that these sleep disorders represent dysfunctions which do not involve brain stem structures subserving the BAER.

Adult↗
PubMed1979

Absence of parasympathetic denervation of the iris in alcoholics.

Forty patients with a history of alcohol abuse were treated with a 2% solution of methacholine to determine the prevalence of iridic parasympathetic denervation. Three (8%) of the alcoholics showed iris constriction of 1.0 mm or more, compared with three (8%) control subjects. In contrast, damage to the iridic parasympathetic nerve supply occurs in approximately 80% of patients with diabetes mellitus as determined by the same pharmacological technique.

Alcoholism↗
PubMed1978

Division of the corpus callosum for uncontrollable epilepsy.

Cerebral commissurotomy, the "split-brain" procedure, has been employed for the control of intractable seizures, in conjunction with moderate doses of anticonvulsant drugs. The results have been encouraging in several small series. The use of microsurgical techniques and the restriction of surgery to one commissure, the corpus callosum, has reduced morbidity without apparent change in result. The eight patients in our first series who underwent the prescribed division of several forebrain commissures are compared to the four patients in our second series who underwent division of the corpus callosum alone. The technique of callosotomy is described.

Adult↗