Search PubMed⌕ Search

Biomedical subjects

James Gordon

Publications and source records attributed to James Gordon.

8 recordsLinked to original sources
PubMed2006

Luminance-contrast mechanisms in humans: visual evoked potentials and a nonlinear model.

Isolated-checks were luminance-modulated temporally to elicit VEPs. Bright or dark checks were used to drive ON or OFF pathways, and low or high-contrast conditions were used to emphasize activity from magnocellular or parvocellular pathways. Manipulation of stimulus parameters and frequency analysis of the VEP were performed to obtain spatial and contrast-response functions. A biophysical explanation is offered for why the opposite polarity stimuli drive selectively ON and OFF pathways in primary visual cortex, and a lumped biophysical model is proposed to quantify the data and characterize changes in the dynamics of the system with contrast given a limited number of parameters. Response functions were found to match the characteristics of the targeted pathways.

Adult↗
PubMed2006

Laser skin resurfacing with a novel portable erbium:YAG laser.

BACKGROUND AND OBJECTIVE: The erbium:YAG laser is a popular modality for laser skin resurfacing (LSR). This study was performed to evaluate the safety and efficacy of a new portable Er:YAG laser in the treatment of photo-damaged skin. METHODS: Nine patients with skin types I-III were treated for rhytides, large pores, pigmented lesions, lentigines and photo-damage. Small facial areas such as the periorbital area, nose, cheeks, and upper lip were treated with one to six passes at 5-6 J/cm2 with a new portable Er:YAG laser. Topical and local anesthesia was used. RESULTS: All treated areas showed improvement and, depending upon the number of passes, re-epithelialization was complete within 2-7 days. The intense erythema resolved within 7 days and there was blending of treated and untreated areas within 2 weeks. CONCLUSION: The technique of applying a tailored number of 5-6 J/cm2, 300 micros pulses of a new portable Er:YAG laser to small areas appears to be safe and effective. There was minimal discomfort and a high level of patient satisfaction after a relatively short recovery time.

Adult↗
PubMed2006

Brightness contrast inhibits color induction: evidence for a new kind of color theory.

A gray region can be made to look colored by a colored surround. This phenomenon, chromatic induction, depends on color differences around the boundary of the region. We performed experiments on chromatic induction with small, initially achromatic, targets on nine different colored surrounds ranging in color from blue to red. Using scaling of saturation as our measure of perceived color strength, we found that chromatic induction is at its maximum when the brightness contrast at the boundary between target and surroundings is minimal. This implies that the neural mechanism in the cerebral cortex that mediates the appearance of brightness at a boundary inhibits the activity of chromatic mechanisms at that same boundary. Observers matched the apparent brightness and luminance of each of the colored surrounds. For surround colors where brightness and luminance matches differ, brightness contrast, not luminance contrast, controls chromatic induction. These new findings, taken together with other evidence, require a new theory of color appearance that includes mutually inhibitory interactions between color and brightness mechanisms that are sensing color and brightness contrast at visual boundaries.

Adult↗
PubMed2005

Seeing unique hues.

Hue can be described by four separate sensations of red (R), green (G), yellow (Y), and blue (B). These are combined in the spectrally opponent RG and YB mechanisms, whose null points correspond to the unique sensations of Y, G, B. Participants used a form of magnitude estimation to describe color appearance of light flashes that were systematically varied in size, luminance, duration, purity, and retinal eccentricity. Wavelengths of the unique hues were derived from the hue and saturation scaling functions. Only unique Y remained invariant across all the viewing conditions. The shifts in unique hues with test conditions place strong restrictions on models of how the RG and YB mechanisms are assembled. Despite polymorphism of L and M cones and variation of their ratios across participants and across the retina, the frequency distribution of unique Y was very narrow, implying some reweighting of cone inputs to individuals' RG mechanisms.

Adaptation, Ocular↗
PubMed2004

Entrainment to video displays in primary visual cortex of macaque and humans.

Cathode ray tubes (CRTs) display images refreshed at high frequency, and the temporal waveform of each pixel is a luminance impulse only a few milliseconds long. Although humans are perceptually oblivious to this flicker, we show in V1 in macaque monkeys and in humans that extracellularly recorded action potentials (spikes) and visual-evoked potentials (VEPs) align with the video impulses, particularly when high-contrast stimuli are viewed. Of 91 single units analyzed in macaque with a 60 Hz video refresh, 29 cells (32%) significantly locked their firing to a uniform luminance display, but their number increased to 75 (82%) when high-contrast stimuli were shown. Of 92 cells exposed to a 100 Hz refresh, 21 (23%) significantly phase locked to high-contrast stimuli. Phase locking occurred in both input and output layers of V1 for simple and complex cells, regardless of preferred temporal frequency. VEPs recorded in humans showed significant phase locking to the video refresh in all seven observers. Like the monkey neurons, human VEPs more typically phase locked to stimuli containing spatial contrast than to spatially uniform stimuli. Phase locking decreased when the refresh rate was increased. Thus in humans and macaques phase locking to the high strobe frequency of a CRT is enhanced by a salient spatial pattern, although the perceptual impact is uncertain. We note that a billion people worldwide manage to watch TV without obvious distortion of their visual perception despite extraordinary phase locking of their V1s to a 50 or 60 Hz signal.

Action Potentials↗
PubMed2004

Essentiality of the early transcript in the replication origin of the lactococcal prolate phage c2.

The genome of the prolate-headed lytic lactococcal bacteriophage c2 is organized into two divergently oriented blocks consisting of the early genes and the late genes. These blocks are separated by the noncoding origin of DNA replication. We examined the functional role of transcription of the origin in a plasmid model system. Deletion of the early promoter P(E)1 abolished origin function. Introduction of mutations into P(E)1 which did not eliminate promoter activity or replacement of P(E)1 with an unrelated but functional promoter did not abolish replication. The A-T-rich region upstream of P(E)1, which is conserved in prolate phages, was not required for plasmid replication. Replacement of the P(E)1 transcript template sequence with an unrelated sequence with a similar G+C content abolished replication, showing that the sequence encoding the transcript is essential for origin function. Truncated transcript and internal deletion constructs did not support replication except when the deletion was at the very 3' end of the DNA sequence coding for the transcript. The P(E)1 transcript could be detected for all replication-proficient constructs. Recloning in a plasmid vector allowed detection of P(E)1 transcripts from some fragments that did not support replication, indicating that stability of the transcript alone was not sufficient for replication. The data suggest that production of a transcript of a specific length and with a specific sequence or structure is essential for the function of the phage c2 origin in this model system.

Bacteriophages↗
PubMed2004

Museum lighting: why are some illuminants preferred?

We had shown earlier that viewers prefer to look at artworks under illuminants of approximately 3600 K. In the latest paper we tested the hypothesis that the preferred illuminant is one that appears neither warm nor cool and repeated the settings at each of four illuminances to test the stability of the findings. Observers looked at a neutral white reflectance standard hung on a matte-gray wall lit by overhead banks of lamps whose combined value could be adjusted continuously between 3000 and 4400 K while illuminance was kept constant. Illuminance ranged from 50 to 2000 lux. Observers adjusted color temperature until they were satisfied that the standard looked neither warm nor cool. The mean for a group of eight observers was approximately 3700, independent of intensity; this corresponds to a dominant wavelength of approximately 580 nm. In a separate study four observers scaled the apparent warmth or coolness of flashes of equiluminant monochromatic lights; the warm-cool transition was between 560 and 580 nm; warmness was completely predicted by the perceived redness of each light as derived from hue and saturation scaling functions from the same group.

Journal Article↗