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R H Carpenter

Publications and source records attributed to R H Carpenter.

47 records · Page 3Linked to original sources
PubMed1973

Contour-like phosphenes from electrical stimulation of the human eye: some new observations.

1. The contour-like phosphenes that may be seen with electrical stimulation of the human eye are described.2. It is found that a moving edge is particularly effective in generating these patterns.3. The relation between the velocity of the edge, the frequency of the current and the resultant spatial frequency of the lines is as if one line were generated at the edge for every cycle of the current.4. The frequency-dependence of the phase-shift between phosphene and current is such as would be expected if the light was delayed by some 30 msec relative to the current at the site of interaction.5. Evidence is brought forward suggesting that this phenomenon is the result not of interference-like processes, but of coupling interactions of the retinal neurones, dividing the population into domains responding in opposite phase to the electrical stimulation.

Accommodation, Ocular↗
PubMed1972

Electrical stimulation of the human eye in different adaptational states.

1. Experiments are described in which the phosphene produced by passing alternating current of frequency 100 Hz through the eye is matched with a patch of light having the same apparent size.2. Matches of this type have been made against different background light intensities, and at various times after a strong retinal bleach.3. To match the phosphene under particular conditions of this type, it is found that the patch must be set some 1.5 log td brighter than its own threshold under the same conditions. Exceptions to this rule occur with very bright backgrounds, or very soon after bleaching.4. The increment threshold for a small spot of light on the phosphene in the dark is some 0.5 log td higher than for the same spot on a patch of light matched in appearance to the phosphene under the same conditions.5. The process linking electrical stimulation of the eye to firing of fibres in the optic nerve is deduced to be substantially unaffected by the state of adaptation of the eye, save perhaps in the exceptional conditions described in (3).6. It is therefore argued that this phosphene is the result of stimulation both of the retinal visual pathways lying central to the variable-gain element commonly accepted to explain the facts of dark adaptation, and also of the input to this element that is supposed to alter its gain.

Adaptation, Ocular↗
PubMed1972

After-images on backgrounds of different luminance: a new phenomenon and a hypothesis.

1. After-images may be caused to vanish suddenly and reversibly by viewing them against a background of suitable luminance I(e).2. The value of I(e) at any moment is not related to any parameter (intensity, colour, duration, time) describing the stimulus that caused the after-image.3. On the other hand, I(e) is very closely related to the immediate adaptational history of the eye, so that viewing a dark background for a few seconds lowers, and a light one raises, the value of I(e).4. It is suggested that the signal conveying sensations of relative brightness in this situation is related to the rate of change of concentration of an intermediate photo-product decaying spontaneously with a time constant of some 14 sec.

Adaptation, Ocular↗
PubMed1969

Visibility of aperiodic patterns compared with that of sinusoidal gratings.

1. Using experimental curves relating the threshold contrast of sinusoidal grating patterns to their spatial frequency, the expected threshold contrast curves for three aperiodic patterns, viz. a single half-cycle sinusoid bar, a single full-cycle sinusoid bar, and the boundary between an extended sinusoidal grating and a 50% grey surround, are calculated. In this calculation the assumption is made that the system is linear near the threshold.2. Experiments are described in which the actual threshold contrast curves are determined for these aperiodic patterns by three observers. The patterns were generated on the face of an oscilloscope and could be varied in size and contrast.3. These experimental curves agree well with the predictions in the high frequency region (i.e. above about 10 c/deg), but below this various complicating factors restrict the validity of the calculations.4. Thus there is no reason to suppose that a linear theory cannot be used to predict visibility of aperiodic patterns near threshold.

Humans↗