Biomedical subjects

M P Regan

Publications and source records attributed to M P Regan.

11 recordsLinked to original sources

PubMed2006

Convergence of boundary and body information in the visual processing of aspect ratio.

UNLABELLED: We used DOG ellipses and outlined ellipses that contained, respectively, only low-frequency and only high-frequency information about aspect ratio. Cross-adaptation (i.e., adapt DOG/test outline and adapt outline/test DOG) produced aspect ratio aftereffects. CONCLUSION: Spatial information encoded in terms of the body of the stimulus and in terms of the boundary of the stimulus have substantially converged before the information-processing stage at which aspect ratio is encoded. We also report that when observers were instructed to discriminate horizontally elongated from vertically elongated test ellipses, the "just noticeably different from circular" threshold was closely constant over a 16:1 range of test ellipse areas.

Adaptation, Physiological↗

PubMed2002

Orientation characteristics of a mechanism in the human visual system sensitive to cyclopean form.

Human observers viewed two superimposed cyclopean gratings in dynamic random noise. Neither grating was visible monocularly. The disparities of the two gratings were counterphase-modulated at different temporal frequencies. We recorded frequency domain brain responses at ultra-high frequency resolution (0.008 Hz) by means of nondestructive zoom-FFT. The response to the superimposed gratings was quite different to the sum of responses to the two individual gratings. We used this strong nonlinearity to investigate the orientation tuning of the cyclopean mechanism that responded to the gratings. One grating had a fixed orientation while the other was rotated. Half-sensitivity full bandwidth was greater than 90 degrees. This is considerably greater than the corresponding bandwidth (30 degrees) for luminance-defined (LD) gratings [Vis. Res. 27 (1987) 2181]. Furthermore, the cyclopean nonlinearity was different from the corresponding nonlinearity for LD gratings. Brain responses to a single cyclopean grating showed an inverse oblique effect over a 1.2-1.9 degrees range of bar widths, but not for smaller or larger bar widths.

Adult↗

PubMed2001

Simulated hair cell transduction of quasi-frequency-modulated and amplitude-modulated tones.

This paper describes a method for calculating the effect of simulated hair cell transduction on (i) a quasi-frequency-modulated (quasi-FM) tone and on (ii) an amplitude-modulated (AM) tone. The main finding is that the effect of the transduction on AM and quasi-FM (which have the same power spectra but whose phase spectra differ), is to produce different power spectra.

Animals↗

PubMed1995

An audio-visual convergence area in the human brain.

By recording the magnetic field of the human brain while simultaneously presenting light to the eye and sound to the ear we have identified a brain region where auditory and visual signals converge. The location of this region is close to primary auditory cortex and far from primary visual cortex.

Auditory Pathways↗

PubMed1994

A method for calculating the spectral response of a hair cell to a pure tone.

Using Corey and Hudspeth's hair cell transducer function and that of Assad and Corey as models, the output of the haircell is calculated for an input of a single tone. These results are then compared with experimental data obtained by Hubbard et al. (1979) from guinea pig cochlear. The theoretical and experimental data are found to be similar for low to moderate sound pressure levels.

Acoustic Stimulation↗

PubMed1993

Nonlinear terms produced by passing amplitude-modulated sinusoids through a hair cell transducer function.

We derive mathematically the output of Corey and Hudspeth's hair cell transducer function for the following cases: (1) the input is a single carrier whose output is modulated by the sum of two sinusoids; (2) the input is the sum of two carriers, each of which is amplitude-modulated by a single sinusoid. The theoretical results are similar to the results of an experiment in which field potentials were recorded from the human scalp while one ear was stimulated with auditory waveform (1) and with auditory waveform (2) of the cases above.

Acoustic Stimulation↗

PubMed1989

Objective investigation of visual function using a nondestructive zoom-FFT technique for evoked potential analysis.

We describe an ultra-high resolution technique for recording evoked potentials (EPs) that are thousands of times smaller than the total EEG power. We report that two superimposed visual stimulus patterns, one modulated at F1 Hz and the other at F2 Hz, can generate 20 or more EP frequency components, each of which is contained within a bandwidth of no more than 0.004 Hz. We have developed a theoretical method for calculating the amplitudes and phases of these various cross-modulation components for different arrangements of model neurons. The relative amplitude of the various cross-modulation components seems to be a 'signature' of the particular neural model and this offers a way of testing theoretical neural models against EP data. As an illustration we compare the nonlinear EP components evoked by dichoptic stimulation with unpatterned flickering light against several models of binocular interaction. These nonlinear binocular interaction terms may offer a means of investigating binocularity in amblyopic children or infants who have low acuity in one or both eyes.

Electronic Data Processing↗

PubMed1988

The transducer characteristic of hair cells in the human ear: a possible objective measure.

We describe here a new kind of non-linear interaction between the responses to two simultaneously presented AM tones. The interaction occurs before the level of binaural convergence, and can be explained in terms of the shape of the inner ear hair cell transducer function.

Acoustic Stimulation↗

PubMed1988

Objective evidence for phase-independent spatial frequency analysis in the human visual pathway.

Electrophysiological responses in human index an interaction between responses to two gratings that is relatively independent of the distribution of light in the retinal image. Two 5 cycle/deg sinewave gratings were superimposed, one counterphase-modulated at F1 Hz and the other at F2 Hz. Nonlinear interaction terms of frequency (nF1 +/- mF2) were recorded that could not be produced by superimposing the F1 Hz grating on blank-field 7 Hz flicker. A local luminance origin could be excluded for the (2F1 + 2F2) term and for the suppression of 2F1 and 4F1, but not for the (F1 + F2), (F1 + 3F2) and (5F2 - F1) terms. The relative spatial phase of the two gratings was varied, thus altering the light distribution in the retinal image without changing its spatial power spectrum. The (2F1 + 2F2) Hz contrast-specific grating-grating interaction term was almost unaffected by these substantial changes in retinal image light distribution providing that the spatial frequency power spectrum of the retinal image was held constant. The (2F1 + 2F2) term and the suppression of 2F1 were both tuned to spatial frequency.

Evoked Potentials, Visual↗

PubMed1987

Nonlinearity in human visual responses to two-dimensional patterns, and a limitation of Fourier methods.

Subjects viewed a pattern consisting of two superimposed gratings: a vertical grating that was counterphase-modulated at Fl Hz and a variable-orientation grafting modulated at F2 Hz. A nonlinear orientation-tuned cross-modulation term of frequency (2F1 + 2K2)Hz in the evoked potential was large when the gratings were parallel and had a half-height full bandwidth of about 12 deg. But a strong (2F1 + 2F2) term was also produced by orthogonal gratings. The application of Fourier methods to analyzing human visual processing of patterns modulated in two dimensions assumes that there is no nonlinear interaction between visual responses to orthogonal gratings. The existence of a strong cross-modulation term in the orthogonal-grating response violates this crucial requirement. Our findings could not result from the stimulation of independent, linear, orientation-selective mechanisms.

Evoked Potentials, Visual↗

PubMed1986

A systematic study of host defense processes in badly injured patients.

A prospective study of factors predisposing to infection in badly injured patients has disclosed: the dominant roles of two specific parameters: monocyte antigen presenting capacity, and opsonic capacity of diluted serum; the potential value of further assessment of: the predictive value of plots of activated T-cells/total T-cells versus monocyte antigen presenting capacity, the apparent protective effect of the ability to sharply increase specific IgM in response to infection, and the apparent protective effect of cytomegalovirus (CMV) infection in the first 28 days after injury against major bacterial infection; the lack of value of analysis of other T- and B-cell subsets in such patients; and the need to clarify CMV and transfusion status with respect to interpretation of such data. The specific role of variable transfusion and of specific serum immunoglobulins will require further and more discriminating study.

Adolescent↗