Biomedical subjects

Andrew Charles James

Publications and source records attributed to Andrew Charles James.

4 recordsLinked to original sources

PubMed2006

Frequency doubling illusion VEPs and automated perimetry in multiple sclerosis.

We examined frequency doubling (FD) illusion based automated perimetry (FDT) and dichoptic FD multifocal visual evoked potentials (FDmfVEPs) in Normal and multiple sclerosis (MS) subjects. Contrast thresholds were determined at 17 visual field locations using an FDT perimeter. The stimuli presented to each location were 0.25 cpd gratings presented with rapid (25 Hz) counterphase flicker and thus displayed the spatial FD illusion. Dichoptic mfVEPs were recorded by concurrently stimulating eight regions/eye with FD stimuli presented at 95% contrast. Recordings were obtained from 27 Normal subjects, 26 MS patients who had experienced Optic Neuritis (MSON) and 24 MS patients without a history of ON (MSNON). The FDT thresholds showed enhanced contrast sensitivity for MSON patients (P < 0.0001) but not for MSNON patients. Response amplitudes for the central four regions of the mfVEP stimulus were reduced in both patient groups (P < 0.005). A classification model based upon the FDT thresholds performed at a specificity of 96% for a sensitivity of 97% in MSON patients, but the accuracy (simultaneously largest sensitivities and specificities) was poor (approximately 60%) in MSNON patients. Discriminant models based on the FDT thresholds and FDmfVEPs were able to diagnose more that 90% MSON patients but performed poorly for MSNON patients.

Adult↗

PubMed2005

A spatiotemporal white noise analysis of photoreceptor responses to UV and green light in the dragonfly median ocellus.

Adult dragonflies augment their compound eyes with three simple eyes known as the dorsal ocelli. While the ocellar system is known to mediate stabilizing head reflexes during flight, the ability of the ocellar retina to dynamically resolve the environment is unknown. For the first time, we directly measured the angular sensitivities of the photoreceptors of the dragonfly median (middle) ocellus. We performed a second-order Wiener Kernel analysis of intracellular recordings of light-adapted photoreceptors. These were stimulated with one-dimensional horizontal or vertical patterns of concurrent UV and green light with different contrast levels and at different ambient temperatures. The photoreceptors were found to have anisotropic receptive fields with vertical and horizontal acceptance angles of 15 degrees and 28 degrees, respectively. The first-order (linear) temporal kernels contained significant undershoots whose amplitudes are invariant under changes in the contrast of the stimulus but significantly reduced at higher temperatures. The second-order kernels showed evidence of two distinct nonlinear components: a fast acting self-facilitation, which is dominant in the UV, followed by delayed self- and cross-inhibition of UV and green light responses. No facilitatory interactions between the UV and green light were found, indicating that facilitation of the green and UV responses occurs in isolated compartments. Inhibition between UV and green stimuli was present, indicating that inhibition occurs at a common point in the UV and green response pathways. We present a nonlinear cascade model (NLN) with initial stages consisting of separate UV and green pathways. Each pathway contains a fast facilitating nonlinearity coupled to a linear response. The linear response is described by an extended log-normal model, accounting for the phasic component. The final nonlinearity is composed of self-inhibition in the UV and green pathways and inhibition between these pathways. The model can largely predict the response of the photoreceptors to UV and green light.

Animals↗

PubMed2003

The pattern-pulse multifocal visual evoked potential.

PURPOSE: To define the pattern-pulse multifocal visual evoked potential (PPMVEP) and determine its characteristics in a sample of normal subjects in terms of amplitude of response attainable, the variation in waveform across visual field, and distribution of potential over the scalp and to compare pattern-pulse with contrast-reversal multifocal stimuli. METHODS: VEPs were obtained by concurrently stimulating 60 regions of a cortically scaled dartboard with pulses of pattern contrast. Responses were recorded from normal subjects, by using a 32-channel electroencephalogram recording system, and elementary responses to each region were estimated by multiple regression of each of the response channel signals on stimulus signals. Left-eye, right-eye, and binocular viewing conditions were concurrently tested by dichoptic stimulation. A direct comparison was then made with contrast-reversal stimulation. RESULTS: Response waveform sets for 12 subjects varied in maximum amplitude from 1.8 to 6.8 micro V. A stereotypical distribution of waveforms held in most subjects, depending primarily on the polar angle location of the stimulus within the visual field. In a direct comparison with a contrast-reversal multifocal analysis, the pattern-pulse responses had similar waveforms and scalp topography, but were 15 times larger in amplitude. Root mean square (RMS) signal-to-noise ratio (SNR) was 1.9 times higher with pattern-pulse stimulation, corresponding to a reduction of 73% in recording time to achieve the same SNR. CONCLUSIONS: The PPMVEP can simultaneously characterize 60 regions of the visual field for both eyes in less than 7 minutes. A general methodology is illustrated that allows multifocal analysis with flexible choice of stimulus conditions.

Adult↗

PubMedDate not supplied

Contrast response of temporally sparse dichoptic multifocal visual evoked potentials.

Temporally sparse stimuli have been found to produce larger multifocal visual evoked potentials than rapid contrast-reversal stimuli. We compared the contrast-response functions of conventional contrast-reversing (CR) stimuli and three grades of temporally sparse stimuli, examining both the changes in response amplitude and signal-to-noise ratio (SNR). All stimuli were presented dichoptically to normal adult human subjects. One stimulus variant, the slowest pattern pulse, had interleaved monocular and binocular stimuli. Response amplitudes and SNRs were similar for all stimuli at contrast 0.4 but grew faster with increasing contrast for the sparser stimuli. The best sparse stimulus provided an SNR improvement that corresponded to a recording time improvement of 2.6 times relative to that required for contrast reversing stimuli. Multiple regression of log-transformed response metrics characterized the contrast-response functions by fitting power-law relationships. The exponents for the two sparsest stimuli were significantly larger (P < 0.001) than for the CR stimuli, as were the mean response amplitudes and signal-to-noise ratios for these stimuli. The contrast-dependent response enhancement is discussed with respect to the possible influences of rapid retinal contrast gain control, or intracortical and cortico-geniculate feedback.

Adult↗