Biomedical subjects

Donald C Hood

Publications and source records attributed to Donald C Hood.

41 records · Page 3Linked to original sources

PubMed2002

Quantifying the benefits of additional channels of multifocal VEP recording.

For some individuals and for some locations, multifocal visual evoked potentials (mfVEP) may be too small or appear 'too noisy' to be reliably measured. By adding electrodes, especially electrodes placed lateral to the midline, and by recording with multiple channels, the amplitude of the signal can be increased in some field locations. However, the addition of electrodes involves certain costs; the set-up time is longer and the data analysis more time consuming and complex. The objective of this study was to assess the benefits of adding electrodes by quantifying these benefits using a signal-to-noise measure. In addition to the typical midline placement of electrodes, two electrodes were placed 1 cm above and 4 cm lateral to the inion on each side. This allowed for 3 channels of recording and 3 additional, derived channels. The mfVEPs were recorded with a 60 sector, pattern-reversing display presented to one eye. Two 7 min records were obtained from 14 individuals with no known visual problems. The two records were averaged and a signal-to-noise (SNR) measure was obtained for every response from all 6 channels. For each sector of the display and each subject, the benefits of additional electrodes were quantified by comparing the SNR from the traditional midline channel to the best SNR from amongst the 6 channels. The number of responses exceeding any given criterion SNR value was increased with the additional channels. For example, 79% of the responses for the typical midline channel exceeded a SNR of 0.6 (a false positive rate of about 2.5%) and this increased to 93% when the best SNR value was used. As expected, summing the mfVEP responses from contiguous sectors also increased the SNR values. Additional electrodes and multiple channels of recording substantially improve the quality of the mfVEP records and the SNR measure provides a useful metric for assessing these benefits.

Adult↗

PubMed2002

The multifocal visual evoked potential and cone-isolating stimuli: implications for L- to M-cone ratios and normalization.

Multifocal visual evoked potentials (mfVEP) were recorded with a pattern-reversing display that modulated only the long wavelength-sensitive (L) cones or only the middle wavelength-sensitive (M) cones. Outside the central 5.8 degrees (radius), the ratio of the amplitudes of the mfVEP responses to L- and M-cone modulation varied across the six subjects, ranging from 1.1 to 1.7. The responses from the central 1 degrees (radius) showed a substantially lower ratio, ranging from 0.8 to 1.1 (average of 0.9). The variation among individuals outside the central fovea is probably due to differences in the ratio of the L/M cone input to both magno- and parvocellular pathways. The substantially lower ratios for the central responses is consistent with an L/M cone ratio closer to 1.0 in the central 1 degrees and/or an adjustment in the gain of the L- versus M-cone contributions to the central parvocellular pathways. Taking into consideration evidence from other techniques, we believe it is unlikely that most individuals have a L/M cone ratio of 1.0 in the fovea. Instead, it appears that there is a change in gain before the mfVEP is generated in area 17.

Adolescent↗

PubMed2002

The multifocal electroretinogram (mfERG) and cone isolating stimuli: variation in L- and M-cone driven signals across the retina.

Multifocal electroretinograms (mfERG) were recorded from 38 normal trichromats with a pattern-reversing display that modulated only their long-wavelength sensitive (L) or only their middle-wavelength sensitive (M) cones at equal cone contrasts and average quantal catches. The display consisted of scaled, 103 hexagonal elements, subtending 84 degrees x 75 degrees of visual angle. Typically, the amplitude of the L-cone driven signal was greater than that for the M-cone driven one at all retinal eccentricities, but large differences were found among observers. These values correlated with L- to M-cone ratios obtained psychophysically in the same observers using 2 degrees (dia.) heterochromatic flicker photometry. Interestingly, the L- to M-cone driven amplitude ratios differed between the central and peripheral retina. For the central fovea (5 degrees dia.), the mean ratio was 1.4 +/- 0.6 (for the N1P1 component), whereas for the annular ring centered at 40 degrees in the periphery, it was 2.3 +/- 2.0. The mean P1 latency of the summed M-cone driven mfERG (28.0 +/- 2.6 ms) was significantly advanced relative to the L-cone driven signal (29.0 +/- 1.9 ms), but the mean N1 latencies were similar (15.6 +/- 1.7 ms and 16.2 +/- 1.3 ms, respectively). The P1 latency difference between the L- and M-cone driven waveforms was not found in the central 5 degrees (dia.) of the retina. However, it increased with retinal eccentricity. The regional differences in the amplitudes and latencies of the L- and M-cone driven mfERG signals can be related to variations in the L- to M-cone ratios and/or the receptor to bipolar gain factors that depend on eccentricity.

Adult↗

PubMed2002

Retinal origins of the primate multifocal ERG: implications for the human response.

PURPOSE: To better understand the cellular contributions to the human multifocal ERG (mfERG), rhesus monkey and human mfERGs were recorded using the same stimulus conditions. The monkey mfERGs were recorded before and after injections of pharmacologic agents known to selectively block activity of particular cells and circuits in the retina. METHODS: Photopic mfERGs were recorded with Dawson-Trick-Litzkow (DTL) fiber electrodes from 16 eyes of 10 anesthetized adult rhesus monkeys (Macaca mulatta) and from 4 normal humans. The display consisted of 103 equal-sized hexagons within 17 degrees of the fovea. Monkey mfERGs were obtained before and after inner retinal responses were suppressed with intravitreal injections of tetrodotoxin (TTX), TTX+N-methyl-D-aspartic acid (NMDA), TTX+NMDA with the gamma-aminobutyric acid (GABA(A&C)) antagonist picrotoxin (PTX), or the inhibitory amino acid GABA and after L-2 amino-4-phosphonobutyric acid (APB) to block signal transmission to ON-bipolar cells. Finally, a combination of APB and cis-2,3 piperidine dicarboxylic acid (PDA) was used to isolate the contributions from the cone photoreceptors. RESULTS: TTX, which blocks sodium-based action potentials, removes a large contribution from the monkey's mfERG, but it does not remove all inner retinal influences. After administration of TTX, the mfERG is further modified by the addition of NMDA. TTX+NMDA, TTX+NMDA+PTX, or GABA alone have similar effects, suggesting that, at the concentrations used, they are largely removing the inner retinal contributions. After removing the inner retinal influences, the monkey's mfERG is mainly composed of ON- and OFF-bipolar contributions, as revealed after APB and PDA were injected. The leading edge of the first negative potential (N1) is largely shaped by the initial hyperpolarization of the OFF-bipolar cells. The photoreceptors also contribute to the leading edge of N1, but this contribution is small, except in the central 6 degrees. The depolarization of the ON-bipolars and the recovery of the OFF-bipolars contribute to the leading edge of the major positive component (P1), with the recovery of the ON-bipolars being the dominant influence on the trailing edge. The waveform of the human mfERG most closely resembles the rhesus monkey's mfERG after administration of TTX. CONCLUSIONS: The monkey's mfERG is shaped by large contributions from ON- and OFF-bipolar cells, combined with both spiking and nonspiking inner retinal contributions, and a small contribution from the photoreceptors. In comparison, the human mfERG resembles the monkey's mfERG after reduction of inner retinal contributions. Based on the pharmacologic dissection of the monkey's mfERG, a model of the waveform of the human mfERG is proposed. This model suggests that the waveform can be understood as a combination of overlapping ON- and OFF-bipolar cell contributions combined with smaller contributions from inner retina and photoreceptors.

Action Potentials↗

PubMed2002

Local cone and rod system function in progressive cone dystrophy.

PURPOSE: To compare the patterns of local cone and rod system impairment in patients with progressive cone dystrophy (CD) using psychophysical and electrophysiological techniques. METHODS: Local cone system function was assessed by measuring cone system thresholds (visual fields) and cone-mediated multifocal electroretinograms (mfERGs). Rod system function was assessed by measuring rod system thresholds (visual fields) and rod-mediated mfERGs. The results in a group of eight patients with CD were compared with those in an age-similar control group. RESULTS: All the patients had abnormal cone system visual field thresholds and cone-mediated mfERGs. Cone system psychophysical thresholds were elevated for targets presented within the central 10 degrees, but were within normal limits for targets at peripheral locations. Cone-mediated mfERG measures of amplitude scale and time scale were abnormal for most of the hexagons tested. Most of the rod-mediated psychophysical thresholds and mfERGs were within normal limits. Rod system losses tended to be patchy and scattered throughout the area tested. CONCLUSIONS: There was poor correspondence among local measures of cone and rod system losses in these patients with CD. The results suggest that the spatial pattern of cone system losses in this disease differs from the spatial pattern of rod system losses.

Adolescent↗