Merton C. Flom

Visual Resolution and Contour Interaction

Detecting the position of the gap in a Landolt C is adversely affected by black bars placed tangential to the C and at a certain distance from it. The maximum bar separation affording interaction is proportional to the minimum angle of resolution, even in cases of amblyopia where resolution is presumably not limited by optical spread of the image. It is suggested that this contour interaction is related to the size of the receptive field (and hence to the resolving capacity) associated with the retinal region used to fixate the target.

Contour Interaction and Visual Resolution: Contralateral Effects

Detection of the gap in a four-position Landolt C presented to one eye is impaired by critically spaced surrounding bars seen only by the other eye. The intensity and spatial extent of this contralateral contour interaction match those obtained ipsilaterally. These results indicate that the neural site for this loss of visual information is supraretinal.

Identifying amblyopia using associated conditions, acuity, and nonacuity features

ABSTRACT Reduced visual acuity is the most notable feature of functional amblyopia—so much so that it is common to think that it is the only, main, or even the fundamental defect. To identify all amblyopic eyes by acuity alone would require an acuity criterion of 6/6 (20/20) or somewhat better acuity—a criterion that would necessarily include more normal than amblyopic eyes. The association of amblyopia with strabismus and anisometropia is sufficiently specific to make these two nonacuity features useful in identifying amblyopia. The diagnosis of amblyopia derives from a syndrome of features, most of which are nonacuity features. Two recently quantified nonacuity features—spatial uncertainty and distortion—are not only clinically useful but they may be the fundamental defects in strabismic amblyopia, with impaired resolution being fundamental in anisometropic amblyopia.

Alcohol and marijuana effects on ocular tracking.

&NA; Experienced alcohol and marijuana users were instructed to track with their eyes a small spot that moved horizontally back and forth in pendular (sinusoidal) motion across a 7.5‐degree field. The frequency of spot oscillation was gradually increased from 0.5 to 3.0 Hz in 40 sec. Eye movement recordings showed the frequency at which smooth tracking and, soon thereafter, saccadic tracking broke down. These smooth and saccadic cutoff frequencies were reduced after administration of alcohol, but not after marijuana or placebo. For low alcohol doses, smooth tracking was impaired and saccadic tracking was unaffected, much like an effect previously reported for barbiturates. Alcohol seems to affect smooth tracking by increasing the central processing time required to generate the appropriate eye movement. It affects saccadic tracking by slightly decreasing saccadic velocity and to a greater extent by increasing latency time, part of which may be devoted to central processing. The site of action of alcohol appears to be central to both the paramedian pontine reticular formation and the flocculus of the cerebellum.

Marijuana, alcohol, and combined drug effects on the time course of glare recovery.

The time course of light adaptation after intense light exposure is significantly delayed by alcohol, marijuana, and a combined dose of alcohol and marijuana. These effects were found in a double blind experiment, using 10 subjects. The experimental treatements were placebo, 0.75 ml/kg of 95% ethanol, 8 and 15 mg ofΔ9 tetrahydrocannabinol (THC), and 0.75 ml/kg of 95% ethanol together with 15 mg of THC. The marijuana-induced delay in recovery is doserelated. Both drugs produce delayed recovery for at least 2 h after drug ingestion. The combined alcohol and marijuana treatment produces little more than the effect produced by either drug alone, suggesting some antagonism between the drugs — a suggestion supported by a significantly lower blood alcohol level for the alcohol dose when combined with marijuana than when taken alone.

Alcohol and marijuana effects on static visual acuity.

&NA; Static visual acuity was measured at two contrast levels (12 and 49%) in ten subjects in a double blind experiment involving five drug conditions of alcohol and marijuana (0.5 ml and 1.0 ml/kg body weight of 95% ethanol, 8 and 15 mg &Dgr;‐9‐tetrahydrocannabinol (THC), and a placebo). We found no statistically significant change in static visual acuity for any of the dose levels at any of the measurement times up to six hours following drug ingestion; this is sharply contrasted with the marked decrements in acuity which were found in the same subjects under the same drug conditions when the targets were in motion and required coordinated eye movements for their resolution.

Effects of alcohol and marijuana on dynamic visual acuity: I. Threshold measurements

Alcohol and marijuana produce significant dose-related reductions in dynamic visual acuity. Ten subjects participated in a double-blind experiment involving three dose levels of each drug (including placebo). The reduction of DVA produced by alcohol (1.0 ml 95% ethanol/kg body weight) was greater than for marijuana (15 mg Δ-9-tetrahydrocannabinol), and we suggest that this difference was produced by differential oculomotor effects of the two drugs. We have speculated that reduction in DVA under alcohol may be a contributing factor in alcohol-related traffic accidents.

Pupil size after use of marijuana and alcohol.

In an experiment using ten subjects, marijuana produced a dose-related constriction of the pupil at low photopic light levels; alcohol produced no effect on pupil size. The two drugs in combination produced no change in pupil size.

ALCOHOL-INDUCED CHANGES IN CONTRAST SENSITIVITY FOLLOWING HIGH-INTENSITY LIGHT EXPOSURE

Relatively low doses of alcohol produced large, significant, dose-related increases in the time required to recover foveal contrast sensitivity following bright light exposure. Nine subjects participated in a double-blind experiment involving three dose levels of alcohol (including placebo). The luminance parameters of the test were comparable to those encountered in practical situations such as driving. The alcohol-induced delay in glare recovery is probably retinal and lasts for several hours after drinking.

The Humphrey Vision Analyzer tm: reliability and validity of refractive-error measures.

&NA; Four subjective measures of the refractive error of 42 eyes (21 patients) were obtained by 2 examiners, one using a Humphrey Vision AnalyzerTM and the other using a conventional refractor. Each examiner made 2 separate measures of each subjects refractive error, one without and one with an added cylindrical lens chosen at random from a predetermined set of powers and axes. The order of using the instruments and the added lenses was mixed. Measures of the refractive errors differed with the 2 instruments by approximately the same amount on average as did duplicate measures with either instrument. Refractive‐error measures obtained with the 2 instruments are about as valid as they are repeatable.