Biomedical subjects
Publications and source records attributed to C J Keemink.
Based on a VME system, a low-cost video system has been developed for recording human motion. The paper describes the algorithm which is used for the recordings. The video system makes it possible to track in real time up to six markers on the body, sampled at a 50 Hz rate.
Investigations have been carried out in order to determine (1) the sensitivity of the two types of skin for sinusoidal mechanical vibrations under different stimulation conditions, and (2) the sensitivity of the skin for amplitude modulations. Maximum sensitivity was found around 200 Hz, with a threshold of 0.03 microns RMS for the glabrous skin of the inner side of the hand under a static force of at least 0.5 N. The sensitivity of the hairy skin of the arm was 12 dB less (static force at least 3 N). The sensitivity increased with increasing contractor area. Maximum modulation sensitivity was found for sinusoidal carriers between 200 and 400 Hz and extended from modulation frequencies of 1 Hz up to 200 Hz (depending on carrier frequency) at a frequency-independent modulation depth of about 0.1. It was concluded that it is certainly possible for the skin to detect the slow amplitude fluctuations of speech signals as a support for lipreading. Attention was paid to the best way of extracting the envelopes from the speech signal and their presentation, as an amplitude modulation, to the skin.
Contrast sensitivity function was studied in 16 patients with unilateral ischaemic lesions involving the posterior visual pathway. Sixty-two percent of the patients showed contrast sensitivity loss in at least one eye for horizontal or vertical stimulus orientation. Visual perception was distorted in a qualitatively different way according to the anteroposterior site of the lesion. Patients with occipital or occipitotemporal lesions showed high spatial frequency selective losses and patients with temporal or parietal lesions low frequency selective losses. Stimulus orientation selectivity was observed in patients with lesions of the primary visual cortex as well as in patients with lesions anterior to the striate cortex. Contrast sensitivity orientation-selective losses were demonstrated in 14 of the 17 'affected' eyes.
The influence of stimulus orientation on contrast sensitivity function was studied in 10 patients with drug-induced Parkinsonism. Nine of the 10 patients had at least one eye with contrast sensitivity deficit for vertical and/or horizontal stimuli. Only generalised contrast sensitivity loss, observed in two eyes, was stimulus orientation independent. All spatial frequency-selective contrast deficits in 15 eyes were orientation dependent. The striking similarity between the pattern of contrast sensitivity loss in drug-induced Parkinsonism and that in idiopathic Parkinson's disease, suggests that generalised dopaminergic deficiency, from whatever cause, affects visual function in an analogous way.
The influence of pursuit eye movements on the visibility of sine wave gratings was investigated by the simultaneous measurement of eye movements and contrast thresholds under different viewing conditions. It was concluded that: 1. with moving gratings contrast sensitivity during ocular pursuit was equal to contrast sensitivity during maintained fixation on a stationary target, provided that the magnitude of retinal image motion (0-20 deg/sec) was equal in both cases; 2. accurate pursuit eye movements led to a paradoxical suppression of the visibility of low spatial frequency gratings; 3. the visibility of high spatial frequency gratings remained unchanged during accurate pursuit eye movements; 4. iso-contrast-sensitivity resulting from both pursuit and non-pursuit conditions could be used to quantify the ocular pursuit performance.
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