C. Boylan

INVESTIGATION OF VISUAL PATHWAY ABNORMALITIES IN HUMAN ALBINOS

Abstract Flash visual‐evoked cortical and subcortical potentials are studied in human albinos for evidence of optic nerve fibre misrouting. The relationships between the results obtained and the visual problems associated with albinism are discussed.

Presaccadic spike potential to horizontal eye movements

Presaccadic spike potentials prior to 20 degrees horizontal eye movements were recorded from 4 electrode sites around the eye with Pz as reference. The technique of back-averaging from the onset of the saccade was used and separate wave forms were obtained for abducting and adducting movements. The spike onset began between 12.0 and 4.4 msec before the beginning of the saccade with a significantly earlier onset with adduction (right eye P less than 0.001, left eye P less than 0.025). The peak occurred between the onset of eye movement and 7.8 msec after the beginning of the movement; the peak also occurred significantly earlier with adduction (right eye P less than 0.01, left eye P less than 0.01). The amplitude of the onset to peak measured from 32.2 to 47.0 microV with greater amplitudes on adduction compared to abduction although the differences were not statistically significant. Several of the traces also showed a later smaller component, after the main onset-to-peak complex, that has not been previously reported.

Lateralisation of the flash visually evoked cortical potential in albino babies.

Flash VECPs have been reported in 3 albino babies ranging in age from 1 to 7 months. The responses were dominated by a large positive component preceded by an earlier negative. The amplitude of this positive component measured from the peak of the preceding negative wave had a greater amplitude over the contralateral hemisphere on monocular stimulation indicative of the misrouting present in the albino visual system.

Presaccadic spike potential with congenital lateral rectus palsy.

Presaccadic spike potentials were recorded with 20 degrees horizontal eye movements in a subject with a congenital left lateral rectus palsy. Electrodes at the inner and outer canthi recorded the eye movements and an electrode below the eye recorded the presaccadic spikes. The responses to 20 abducting and 20 adducting saccades were back-averaged from the onset of the eye movement with the wave forms for abduction and adduction averaged separately. Recordings were made from each eye under 2 conditions: firstly with the eye from which the spikes were being recorded fixating and secondly with the eye from which the spikes were not being recorded fixating. In each case the non-fixating eye was occluded throughout. Recordings were made with eye position monitored initially in the eye from which the spikes were recorded and repeated with eye position monitored in the other eye. Results show that spikes could be recorded during abduction and adduction movements of both eyes but only if eye position was monitored in the right eye. Using the left eye as position monitor gave very broad low amplitude spikes. When eye position was monitored in the right eye the spikes recorded during abduction of the left eye had a low amplitude compared to those recorded during right eye abduction; right eye adduction spikes were also of low amplitude. A possible explanation for these findings and their implications on future recordings in pathological cases are discussed.

Presaccadic spike potentials with large horizontal eye movements

Presaccadic spike potentials were recorded from electrodes at the inner canthus and below the eye in 10 normal subjects for a range of horizontal saccades (5 degrees, 10 degrees, 20 degrees and 40 degrees). An eye movement trace was recorded for 20 abducting and 20 adducting saccades to determine the beginning of the saccade, and the spike potential back-averaged from this point. The latencies of the spike potential onset and peak were found with respect to the start of the eye movement and the amplitude from the onset to the peak was measured for each saccade size. The latency values remained constant throughout the range of eye movement sizes, although adducting saccades showed an earlier onset and peak latency than abducting saccades. The amplitude data, however, showed a definite relationship between saccade size and spike potential amplitude, with a significant increase in the amplitude for saccades between 10 degrees and 40 degrees (inner electrode abduction, lower electrode abduction and adduction P less than 0.01; inner electrode adduction P less than 0.05). A possible explanation of this increase in amplitude is hypothesised from a computer model of the action potential activity that may occur in the extraocular muscles before an eye movement.

Changes in the presaccadic spike potential with age

Presaccadic spike potentials were recorded from an electrode below the eye to 20 degrees horizontal saccades in 2 groups of 10 subjects; a young group with a mean age of 26 years and an old group with a mean age of 77 years. The onset and peak latencies, relative to the beginning of the eye movement, and the onset-peak amplitudes were measured for abduction and adduction movements separately. There was no significant difference in the onset or peak latencies of the 2 groups with either abduction or adduction. The amplitude, however, was significantly greater in the young group with both abduction (P less than 0.01) and adduction (P less than 0.025). Possible explanations of these results are given and the implications of the findings on the investigation of presaccadic spike potentials in both normal and pathological subject groups are discussed.

Visual evoked cortical and subcortical potentials in human albinos

Albinism is a congenital condition in which hypopigmentation occurs. In addition to this abnormality there is a misrouting of the optic nerve fibers, with some fibers from the temporal retina following a crossed route at the chiasma and terminating in the contralateral cortical hemisphere. This contralateral preponderance of fibers from each eye should be recognizable from recording the visual evoked cortical potential over each hemisphere on monocular stimulation. Such a technique should produce evidence of responses of either increased amplitude or shorter latency over the contralateral hemisphere. Twenty-five human albinos (twenty-three oculocutaneous, two ocular) have been examined. Pattern appearance-disappearance visual evoked cortical potentials were used, but only on bioccipital derivations did these show clear lateralization. With the flash response the P2 component has a consistently shorter latency over the contralateral hemisphere to the eye stimulated.The visual evoked subcortical potential shows contrasting lateralization. The implications of these findings are discussed.

EXCURSION TESTS OF OCULAR MOTILITY

Abstract—Three alternative excursion tests of ocular motility have been compared. A computer model of the mechanics of the extraocular muscles has been used to estimate the tensions developed by the muscles in these tests. On the basis of these calculations we recommend the transverse test in which the positions of the eyes are observed as they track a target moving from the right to the left with a fixed elevation and a fixed depression.

CURRENT CONCEPTS OF THE ACTIONS OF THE EXTRAOCULAR MUSCLES AND THE INTERPRETATION OF OCULOMOTILITY TESTS

Abstract— Contrasting views on the actions of the extraocular muscles are discussed. The contributions made by clinicians are compared to those made by investigators with a background in physical science. It is shown that a clear understanding of the mechanics of the extraocular muscles can be used to increase the efficiency of oculomotility tests.

Presaccadic spike potential with vertical saccades

Presaccadic spike potentials were recorded from four electrodes placed around the eyes for 20 degree vertical saccadic eye movements. Twenty upward and 20 downward saccades were recorded using electro‐oculography and the spike potentials back‐averaged from the start of the eye movement. Separate average waveforms were obtained for upward and downward saccades. The onset of the spike was between 17.3 and 6.8 ms before the start of the saccade with downward saccades having earlier onsets than upward saccades. This difference was only significant in the right eye (P < 0.025). The peak occurred between 3.4 ms before to 5.2 ms after the start of the eye movement and was earlier with downward saccades. This was only significant in the right eye (P < 0.025). The onset to peak amplitude measured between 15.9 and 42.3 μV with no statistical difference between upward and downward saccades. It was found, however, that the electrodes placed below the eyes recorded larger amplitudes for upward saccades than the other electrode‐recording positions (right eye P < 0.05, left eye P < 0.025). A second component to the spike potential waveform was observed in many of the recordings and a possible origin of this component due to eyelid activity is hypothesized.