Uwe Kampf, Igor Rabitchev, Felix Muchamedjarow & Elena Yachmeneva
Temporal Phase Losses in Neuronal Coding as a Possible Reason for spatial distortions in Amblyopia
¹ Caterna GmbH Dresden,
² Moscow Center of Investigation and Vision Correction,
³ Medizinische Einrichtungs GmbH Senftenberg,
&sup4; Helmholtz Moscow Institute of Eye Diseases
Amblyopia is a unilateral loss of visual performance associated, as a rule, with strabismus. A typical disorder in the internal psychophysics of amblyopic vision is the pattern of so-called spatial distortions [Hess et al., 1978]. It is well-known that for low spatial frequencies the spatial distortions imply no serious impairment of vision. However, they result in a serious decay of visual performance for high spatial frequencies. This performance loss cannot be attributed to a reduction of contrast sensitivity. Rather it is argued that the reason of the disorder is a loss of coherence in the co-operative activity of the visual channels. As a result we find an encoding impairment of the so-called spatial phase in the process which combines the output of retinally distributed filter mechanisms. According to the presented view this might be attributed to disorders in the coherence of temporal encoding.
To give an illustration: Spatial frequency on a TV image is generated as a «standing wave» pattern by repetitive scanning with synchronized temporal frequency.
If the integration process in vision, operating probably on the base of so-called «synfire chains» [Abeles, 1993], might be compared — for illustrative purposes — with the repetitive scanning of a TV monitor’s cathode ray tube, then a synchronism loss of this process has to be considered of small impact on the phase coherence of the low spatial frequencies (ambient vision) but of heavy impact on the high spatial frequencies (focal vision).
In order to prove the explanatory power of this idea for better understanding spatial distortions in amblyopia, the predictions of such a hypothesis should be compared to the assessment of neuronal synchronisation in the amblyopic cat’s visuo-cortical activity driven by the normal eye vs. low vision eye [Roelfsema et al., 1993].
The correlated single-unit recordings show a clear interrelation between the degree of processing coherence and the stimulation by low spatial frequency vs. high spatial frequency. Coherence peaks can be found in cortical neurones driven by the normal eye as well as the amblyopic eye if their receptive fields are stimulated by patterns of low spatial frequency. However, a stimulation with patterns of high spatial frequency maintains processing coherence only in the synfire chains driven by the normal eye but causes decoherence in that of the amblyopic eye. Thus, in light of the discussed hypothesis, the loss of the spatial phase appears to be a result of temporal processing decoherence due to a conflict between internal and external critical timing constraints between spatial and temporal aspects of visual encoding.
We wish to call this conflict an «uncertainty relation », a term which had been introduced by Werner Heisenberg in order to signify a basic measurement problem in quantum mechanics. However, this kind of measurement problem also arises in rather different context, e. g. such as the present one, if the more precise assessment of one of two complementary aspects of an event impairs the precision of the contingent measurement of another aspect. Is there any direct psychophysical evidence in favour of an uncertainty relation in visual processing?
It is a rather well-known fact that the spatial and temporal frequency filter bands of visual channels are reciprocally tuned in relation to one another [Kelly, 1984]. The frequency plot shows the control parameters of two systems of reciprocally tuned spatial frequency channels which support, on the one hand, ambient vision («motion channels» of low spatial and high temporal frequency) vs. focal vision («form channels» of high spatial and low temporal frequency) on the other hand.
A reciprocal synergy of the both channel systems grants for the coherence of the spatio-temporal interrelations between the optomotor and the optosensory control of visual perception. In case of malfunction (amblyopia) this can lead to losses of phase locking leading to spatial distortions as has been argued above. In the same way, as has been shown for the optosensory domain, we find a psychophysical analogue to an uncertainty relation in the optomotor domain too. According to their temporal characteristics two kinds of saccadic eye movements can be differentiated [Velichkovsky et al., 2005]:
1. Ambient saccadic eye movements of short duration (<180 msec), corresponding to high temporal frequency of perceptual sampling with a great amplitude, i. e. low spatial frequency of perceptual sampling.
2. Focal saccadic eye movements of long duration (> 180 msec), i. e. low temporal frequency of perceptual sampling with small amplitude, i. e. high spatial frequency of perceptual sampling.
Thus, the system’s synergy of the focal and ambient eye-movements’ control channels, which manifests itself in an inversion of spatial and temporal frequency in the optomotor pattern, as it has been recorded with an eye tracker, shows an analogue to what has been termed above as an uncertainty relation (in correspondence to the one found in the optosensory domain). If so, then a repetitive stimulation addressed at both the optomotor and the optosensory domain might be considered to induce coherence in the coordinative processes between focal and ambient vision, because it may be considered to improve focal visual processing via phase coupling with ambient visual processing. This kind of stimulation, combined with computer games for attention binding, has been proven in several research studies to be a valid method of complementary to occlusion amblyopia treatment and is now available as an internet based telemedical health service supported by the Caterna GmbH Dresden.
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