Current perspective on microfluctuations of accommodation

The collaboration of Fergus Campbell. Gerald Westheimer and John Robson in the 1950s produced insight into the nature of accommodation mierofluctuations and instigated work which has led to the current view thai the nominally steady‐state accommodation response exhibits temporal variations which can be characterized by two dominant regions of activity: a low‐frequency component (LFC < 0.6 Hz) and a high‐frequency component (HFC ≥ 1.0 ≤ 2.3 Hz). A functional role has been attributed to these microfluctuations as they offer a means by which a directional cue could be derived from an even‐error stimulus. However, there is no consensus regarding the respective contribution made by each of the dominant components in the accommodation control process. Using a newly‐designed measurement and recording system we have conducted a series of experiments to investigate the nature and aetiology of the microfluctuations. The incidence and magnitude of microrluctuations in LFCs and MFCs for central and peripheral lens zones were investigated while live young cmmetropic subjects viewed a near target. The form of the power spectra of the fluctuations was found to be similar for central and peripheral zones although an overall reduction in magnitude was observed in the periphery. The HFC's arc thus a consistent feature of mierofluctuations in central /ones and not. as previously suggested, merely a spurious feature of peripheral zones. A significant between‐subject variation in the location of HKC's was found and led us to consider the relationship between HFCs and other physiological systems which provide intraocular rhythmic variation. Simultaneous measurements of ocular accommodation and systemic arterial pulse made on 20 subjects demonstrate that the location of the HFC is significantly correlated with arterial pulse frequency. The intraocular mechanisms by which arterial pulse modulates the HFC are likely to involve rhythmic changes in both choroidal blood flow and intraocular pressure (IOP). Consequently, examination of the effect of 0.5% timolol maleate. a drug which exerts an ocular hypertensive effect and also induces a reduction in IOP pulse, on the accommodation microfluctualions of lOemmetropic subjects when fixating a near target was undertaken. A double‐blind protocol against saline showed timolol to reduce significantly the root‐mean‐square value of the fluctuations. The results thus indicate that factors relating to IOP and ocular vaseulature will affect the magnitude and frequency composition of accommodative fluctuations. The studies to date suggest that the complex waveform of the accommodative microfhiciuations is a consequence of the combination of neurological control and physiological rhythmic variation: the former is probably attributable to the LFCs; the latter to arterial pulse.