Biomedical subjects

D Regan

Publications and source records attributed to D Regan.

At least 127 records · Page 7Linked to original sources

PubMed1985

Flicker and movement constituents of the pattern reversal response.

A counterphase-modulated (i.e. contrast reversing) pattern will affect hypothetical neural elements sensitive to motion as well as hypothetical elements sensitive to changes in contrast and other elements sensitive to local luminance. Attempts to describe visual evoked potentials (VEPs) to contrast reversal entirely in terms of luminance change or entirely in terms of contrast change have been unsuccessful; strong interactions between responses to contrast decrease and increase had to be postulated ad hoc in order to account for the contrast reversal VEP, and no explanation for these interactions has been proposed. Rather than attempting to separate the reversal VEP into responses to contrast decrease and increase, we attempt here to analyze the reversal VEP into responses to motion onset and offset. We find that VEPs to an abrupt displacement of less than one square's width of a checkerboard pattern are qualitatively similar to the contrast reversal VEP, and that the displacement VEP can be seen as the limiting case of a motion onset response shortly followed by a motion offset response. When displacement occurs during steady motion, the displacement and motion responses interact. We conclude that the contrast reversal VEP is better described in terms of motion onset and offset response than in terms of contrast onset and offset responses.

Adaptation, Ocular↗

PubMed1985

Virus-associated deficiencies in the mitogen reactivity in celebes black macaques (Macaca nigra).

Celebes black macaques (Macaca nigra) with a history of diabetes mellitus, recurrent bacterial and protozoal infections, diarrhea, anemia, weight loss, anorexia, and a high mortality were studied to determine their immune status. Two groups of monkeys, healthy and unhealthy, were formed on the basis of a clinical assessment. The proliferative response and the pokeweed-mitogen-induced polyclonal IgG response of peripheral blood mononuclear cells of unhealthy monkeys were significantly less than the responses of healthy monkeys. The percentage of HLA-DR+ cells varied greatly in unhealthy monkeys. The OKT4/OKT8 ratios of unhealthy monkeys were generally greater than the ratios of healthy monkeys. Unhealthy monkeys usually had smaller percentages of OKT8+ cells than did healthy monkeys. The two groups of monkeys were examined for the presence of a syncytial forming retrovirus by a coculture assay involving Raji cells, a human B lymphoblastoid cell line. A type D retrovirus was detected in the unhealthy group but not in the healthy group. Retroperitoneal fibromatosis was detected in several monkeys in the unhealthy group.

Acquired Immunodeficiency Syndrome↗

PubMed1985

Postadaptation orientation discrimination.

An orientational difference of only 0.3-0.5 deg can be discriminated between two gratings or two lines, although psychophysical channels and cortical cells both have comparatively broad orientation bandwidths of 10-25 deg. One proposed explanation for the fineness of orientation discrimination is that, while detection is determined by the most excited orientation-tuned neural elements, superthreshold orientation discrimination is determined by difference signals between these elements [Westheimer et al., J. Opt. Soc. Am. 66, 332 (1976)]. This implies that, if stimulus orientation is changed slightly, the most important elements for discriminating this change will be those whose relative activity changes most, even though the excitation of these elements may be comparatively weak. In accord with this prediction, we found that adapting to a high-contrast grating degraded discrimination for test gratings inclined at about 10-20 deg to the adapting grating while having little effect on the detection of these inclined gratings. For test gratings parallel to the adapting grating, discrimination was improved, but detection was degraded. Either an opponent-process or a line-element model can account for these effects of adaptation. An opponent model can also explain our findings that subjects do not confound orientation change with contrast change and that suprathreshold orientation discrimination is almost independent of contrast, varying by only +/- 10% from about 3 to about 25 times contrast threshold. A discrimination model must incorporate reliable storage of spatial frequency, because discrimination was not affected by increasing the interval between grating presentations from 1 to 10 sec.(ABSTRACT TRUNCATED AT 250 WORDS)

Adaptation, Ocular↗

PubMed1985

Visual responses to vorticity and the neural analysis of optic flow.

While an observer is moving forward, his retinal image of the outside world contains a flow field. This optical flow field carries information both about external objects and about where the observer is going relative to these objects. Mathematically, the flow pattern can be analyzed into elements that include the curl of local velocity (i.e., vorticity), and it has been suggested that the visual pathway might contain independent neural mechanisms sensitive to these mathematical elements [H. C. Longuet-Higgins and K. Prazdny, Proc. R. Soc. London Ser. B 208, 385-397 (1980)]. To test this suggestion we compared visual responses to two circular areas of random dots, A and B. These two stimuli were identical in that all dots oscillated along a straight line in one of two possible directions. However, the relative phases of dot oscillations were different for A and B, causing A to have a rotary component of motion that B did not have. We found that rotary motion thresholds for a rotary test stimulus were more elevated after adapting to A than after adapting to B, a difference that cannot be explained in terms of visual responses to linear motion, since linear motion components were the same for A and B. This finding is consistent with the idea of a neural mechanism sensitive to the curl of velocity (i.e., vorticity). Adding this to previous evidence for a mechanism specifically sensitive to the divergence of velocity (i.e., dilatation), we suggest that one role of these postulated mechanisms might be to parallel vector calculus by analyzing each small patch of the visual flow field into neural representations of the mathematically independent quantities curl and divergence of velocity.

Biomechanical Phenomena↗

PubMed1985

Masking of spatial-frequency discrimination.

Spatial-frequency discrimination thresholds were measured using the temporal two-alternative forced-choice procedure. A superimposed masker grating reduced discrimination threshold more than 20-fold when the spatial frequency of the masker was close to the test spatial frequency because moiré fringes provided a sensitive cue to the test grating's spatial frequency. When the effectiveness of this cue was reduced by randomly changing the frequency of the masker grating on successive presentations, masked thresholds were found to be elevated when the masker frequency was within roughly 1 octave of the test frequency. On the other hand, postadaptation thresholds were not elevated when the test frequency was close to the adapting frequency but were elevated at about twice the adapting frequency. Thus masking and adaptation have quite different effects on spatial-frequency discrimination, though their effects on contrast sensitivity are qualitatively similar. It is proposed that discrimination is determined by opponent-size processing. Adaptation elevates discrimination threshold by reducing the sensitivity of opponent elements. The masker grating elevates threshold by increasing the noise level of opponent elements, thus reducing the precision with which the visual pathway encodes spatial frequency.

Adaptation, Physiological↗

PubMed1984

Pattern visual evoked potentials and spatial vision in retrobulbar neuritis and multiple sclerosis.

In an attempt to resolve some of the reported disagreements between visual evoked potential (VEP) recordings and Snellen visual acuity in patients with multiple sclerosis, we compared these test results with sine-wave grating-contrast sensitivity curves. Disease that depressed visual sensitivity for high spatial frequencies, sparing low spatial frequencies, was associated with depressed visual acuity and attenuated small-check VEPs in the affected eye, while large-check VEPs were not attenuated. When visual sensitivity to all spatial frequencies was depressed, both large-check and small-check VEPs were attenuated, and Snellen acuity was reduced. In general, abnormalities in the contrast sensitivity curve predicted abnormalities in VEP amplitude, but VEP delay was less accurately predicted.

Adult↗

PubMed1984

Patients with multiple sclerosis experience hearing loss specifically for shifts of tone frequency.

After exposure to a prolonged tone of changing intensity but constant frequency, controls, patients with peripheral hearing loss, and patients with multiple sclerosis (MS) demonstrated a reduced sensitivity to shifts in intensity; sensitivity to frequency shifts was unaffected. After exposure to a prolonged tone of changing frequency but constant intensity, control and patients with peripheral hearing loss demonstrated reduced sensitivity to shifts in frequency; sensitivity to intensity shifts was unaffected. Some patients with MS showed no loss of sensitivity to shifts in frequency. Our findings suggest that some patients with MS have abnormal mechanisms for processing changes of frequency. If such processing of frequency change is important for understanding speech, then this observation of a specific central hearing defect may help to explain poor speech discrimination in some patients with MS who have normal audiograms.

Adolescent↗

PubMed1984

Visual field defects in ocular hypertension and glaucoma.

We measured visual fields using three unconventional test stimuli; sine-wave grating targets of 2 and 5 cycles/degree and a visual acuity target. Of 15 patients with ocular hypertension (OHT), eight had visual field defects for contrast sensitivity when tested with a sine-wave grating target of low spatial frequency; these patients had normal perimetric fields and normal fields for visual acuity. We hypothesize that the outer extremities of the largest dendritic trees of retinal ganglion cells become functionally ineffective in some patients with OHT and early glaucoma, possibly due to retinal ischemia, and as a result visual sensitivity to low spatial frequency gratings is reduced while visual acuity is spared. Since this hypothetical mechanism may be somewhat independent of the mechanism that causes ganglion cell loss, not all the contrast field defects in OHT would be expected to progress to glaucomatous field defects and be evident to clinical perimetry.

Aged↗

PubMed1984

Degraded discrimination between speech-like sounds by patients with multiple sclerosis and Friedreich's ataxia.

We previously found that some patients with multiple sclerosis are selectively 'deaf' to changes in the pitch of a tone, even when audiometric sensitivity to pure tones is unimpaired. This subtle form of deafness is not experienced by patients with noise-induced hearing loss of exclusively peripheral origin. It was suggested that this auditory defect may be one possible cause for difficulties in discriminating speech, on the grounds that frequency changes in the speech waveform are known to be important for intelligibility. This implication is not self-evident; our earlier studies tested hearing with a single pure tone that was either frequency-modulated or amplitude-modulated, while even a simple approximation to speech sounds involves not one, but three narrow bands of noise (formants) whose frequencies and intensities change from instant to instant. The present study has investigated the ability of subjects to discriminate between speech-like sounds. These consisted of three formant frequencies generated by computer. The only difference between the sounds was that the lowest-frequency formant rose or fell in pitch by different amounts. In order to ensure that subjects used frequency (pitch) cues rather than any associated loudness cues were mixed different loudness shifts with the frequency shifts. Nineteen control subjects, 25 patients with multiple sclerosis (MS) and 4 patients with Friedreich's ataxia (FA) were tested. Nine of the patients with MS and all 4 patients with FA gave results that fell outside the range of the control subjects. A possible pathophysiological basis for this observation is the finding that some neurons in the auditory pathway of animals respond preferentially to changes in tone frequency: homologues of these neurons might be functionally impaired in some patients with MS and FA.

Adolescent↗

PubMed1984

Low-contrast letter charts in early diabetic retinopathy, ocular hypertension, glaucoma, and Parkinson's disease.

Diabetes can cause visual loss that is not detected by standard reading tests such as the Snellen test but can be detected by low-contrast letter charts. This visual loss is quite different from loss caused by refractive error. These low-contrast charts are diagnostically at least as sensitive as the sinewave grating contrast sensitivity test. They are inexpensive, and the test is brief and simple. Preliminary evidence is that patients with diabetes who have abnormal low-contrast chart results give abnormal intravenous fluorescein (IVF) test results, even though visual acuity is normal. Low-contrast charts also detect visual loss in patients with ocular hypertension, glaucoma, and Parkinson's disease, including patients with normal visual acuity.

Adult↗

PubMed1984

Balance between pattern and flicker sensitivities in the visual fields of ophthalmological patients.

We measured the balance between visual sensitivities to pattern and to flicker rather than measuring absolute sensitivities to pattern or flicker. The test target was a 2-cycle deg-1 sinewave grating that was counterphase modulated at 8 Hz. Seventeen points in the visual field were tested out to eccentricities of 24 degrees. We examined 10 control subjects, 6 patients with glaucoma 10 with ocular hypertension, and 10 with multiple sclerosis. For controls pattern sensitivity was lower than flicker sensitivity in central vision. The converse held in peripheral vision. The balance between pattern sensitivity and flicker sensitivity was markedly abnormal in part or all of the visual field for many patients. There were examples in all patient groups. In some patients flicker sensitivity was depressed relative to pattern. In others the converse was true. Of 10 patients with ocular hypertension and no perimetric field loss 8 had a significantly abnormal ratio between pattern sensitivity and flicker sensitivity at some point in the visual field. The balance between pattern and flicker sensitivity was more sensitive to visual pathology than absolute sensitivity to either pattern or flicker. We conclude that the relationship between pattern and flicker sensitivity may be more sensitive to visual field damage than is conventional perimetry or visual acuity perimetry.

Adult↗

PubMed1984

Visual fatigue and visual evoked potentials in multiple sclerosis, glaucoma, ocular hypertension and Parkinson's disease.

Visual evoked potential (VEP) abnormality is widely used as an objective indication of visual pathophysiology in the diagnosis of multiple sclerosis. One major limitation of this test is that VEP abnormality is not specific to multiple sclerosis. In an attempt to explore ways of making the VEP test more specific, changes were measured in VEPs caused by superimposing upon the VEP stimulus either a flicker or a moving pattern. The rationale was to test for visual fatigueability, since it is known that some demyelinated axons fatigue rapidly. Of 10 patients with multiple sclerosis, 90% showed VEP fatigue, while none fatigued in the groups of 10 patients with glaucoma and 10 with Parkinson's disease. Fatigue is, however, not completely specific for multiple sclerosis, since three of 10 patients with ocular hypertension showed VEP fatigue.

Adult↗

PubMed1984

Figure-ground segregation by motion contrast and by luminance contrast.

Some naturally camouflaged objects are invisible unless they move; their boundaries are then defined by motion contrast between object and background. We compared the visual detection of such camouflaged objects with the detection of objects whose boundaries were defined by luminance contrast. The summation field area is 0.16 deg2 , and the summation time constant is 750 msec for parafoveally viewed objects whose boundaries are defined by motion contrast; these values are, respectively, about 5 and 12 times larger than the corresponding values for objects defined by luminance contrast. The log detection threshold is proportional to the eccentricity for a camouflaged object of constant area. The effect of eccentricity on threshold is less for large objects than for small objects. The log summation field diameter for detecting camouflaged objects is roughly proportional to the eccentricity, increasing to about 20 deg at 32-deg eccentricity. In contrast to the 100:1 increase of summation area for detecting camouflaged objects, the temporal summation time constant changes by only 40% between eccentricities of 0 and 16 deg.

Humans↗

PubMed1984

Spatial-frequency adaptation and grating discrimination: predictions of a line-element model.

Recent data have shown that spatial-frequency adaptation has little effect on spatial-frequency discrimination at the adapting frequency, but there is a substantial elevation of discrimination thresholds at frequencies about one octave higher than the adapting frequency. It has also been shown that adaptation produces elevations in grating-orientation discrimination at orientations displaced by +/- 12.0-15.0 deg from the adapting orientation. In this paper, we show that these results agree quantitatively with the predictions (some of which were made without knowledge of the experimental results) of a line-element model for spatial-frequency discrimination. Spatial-frequency bandwidths of the visual mechanisms were previously obtained from masking data. For the orientation-discrimination predictions, the line-element model was generalized to incorporate orientation bandwidths for the spatial mechanisms, again estimated from masking data.

Adaptation, Physiological↗

PubMed1983

Detection of antigen specific rosette formation with the FACS.

Donors previously sensitized to conventional antigens PPD and KLH were evaluated for their antigen binding responses, utilizing a rosette forming technique with antigen-conjugated autologous erythrocytes. Reactivity is directly correlated with prior sensitization. Furthermore, antigen specificity is suggested by inhibition of rosette formation by prior incubation with the relevant antigen. The frequency of RFCs detected cytofluorometrically was compared with conventional fluorescent microscopy determinations. RFCs detected in this manner were identified as antibody armed monocytes by cell depletion and histochemical studies. The usefulness of the rosette forming technique for the routine evaluation of donor immunity is discussed.

Antibodies, Anti-Idiotypic↗

PubMed1983

Texture changes versus size changes as stimuli for motion in depth.

As an object approaches the eye, its retinal image size grows larger and its surface texture appears to grow coarser. We compare these two visual correlates of motion in their effectiveness as stimuli for motion in depth. In some experiments texture and object size both expanded or both contracted; in other experiments the two stimuli were pitted against each other. When texture and size change as for a rigid, nonrotating real world object an untextured square can be a more effective stimulus for motion in depth than the same square with texture. On way of describing this finding is to calculate the departure from linear summation of texture and size contributions. The departure is greatest when texture is static, being even greater than when texture changes in the opposite direction to size.

Adaptation, Ocular↗

PubMed1983

Spatial frequency mechanisms in human vision investigated by evoked potential recording.

Two visually evoked brain responses were elicited simultaneously by stimulating the eye with two superimposed sinewave grating patterns that were temporally modulated at slightly different rates. The VEP to one grating was comparatively little affected by the presence of the other grating when the two spatial frequencies were very different, but mutual attenuation grew stronger and stronger as the spatial frequencies of the two gratings were progressively brought together. The attenuation rose to a sharp maximum when the two spatial frequencies were equal. This held at each of the five spatial frequencies tested. This finding can be explained if the spatially-selective mechanisms responsible for grating VEPs contain multiple subunits of narrower spatial frequency bandwidth. The two-grating technique was also used to search for evidence of multiple subunits that are most sensitive at the same spatial frequency, but are tuned to different temporal frequencies. Findings were quite different for temporal and for spatial tuning. Attenuation of one grating VEP was greatest when the temporal frequency of the other grating fell within a broad frequency range of about 3-30 reversals sec-1: maximum attenuation could occur when the gratings had quite different temporal frequencies. This finding denies that for every given temporal frequency there is a subunit maximally sensitive to that frequency.

Adolescent↗

PubMed1983

Visual fields for frontal plane motion and for changing size.

Thresholds were measured in 15 subjects for 2-Hz oscillations of size and for 2-Hz oscillatory motion in the frontal plane using test squares of side lengths 0.5 degrees, 1.0 degrees and 2.0 degrees. Size-oscillation thresholds were lowest (i.e. sensitivity was greatest) for the 2.0 degrees square while thresholds were highest (i.e. sensitivity was least) for the 0.5 degrees square in 28 of 34 tests. Frontal plane motion thresholds, on the other hand, did not generally depend on square size. Equal-threshold contours in the visual field were roughly elliptical in 10 of 13 subjects for both types of oscillation. None of 13 subjects had visual field defects for oscillating-size or frontal plane motion, in contrast with the known incidence of stereo-motion scotomata. One subject was known to be selectively "blind" to stereoscopically-oscillating disparity in some areas of the visual field, but oscillating-size sensitivity was normal in these regions, thus preserving an alternative basis for motion-in-depth judgments.

Adult↗