On the time course of short-term forgetting: a human experimental model for the sense of balance

Abstract

The primary aim of this study was to establish whether the decline of the memory of an angular displacement, detected by the semicircular canals, is best characterized by an exponential function or by a power function. In 27 subjects a conflict was created between the semicircular canals and the graviceptive systems. Subjects were seated, facing forwards, in the gondola of a large centrifuge. The centrifuge was accelerated from stationary to 2.5Gz. While the swing out of the gondola (66°) during acceleration constitutes a frontal plane angular-displacement stimulus to the semicircular canals, the graviceptive systems persistently signal that the subject is upright. During 6 min at 2.5Gz the perceived head and body position was recorded; in darkness the subject repeatedly adjusted the orientation of a luminous line so that it appeared to be horizontal. Acceleration of the centrifuge induced a sensation of tilt which declined with time in a characteristic way. A three-parameter exponential function (Y = Ae(-bt) + C) and a power function (Y = At(-b) + C) were fitted to the data points. The inter-individual variability was considerable. In the vast majority of cases, however, the exponential function provided a better fit (in terms of RMS error) than the power function. The mean exponential function was: y = 27.8e(-0.018t) + 0.5°, where t is time in seconds. Findings are discussed with connection to possible underlying neural mechanisms; in particular, the head-direction system and short-term potentiation and persistent action potential firing in the hippocampus are considered.

Keywords: Short-term memory; Spatial memory; Spatial orientation; Vestibular memory; Vestibular psychophysics.

Fig. 1

Schematic illustration of the intra-vestibular conflict occurring in a swing-out gondola centrifuge. During acceleration of the centrifuge, the gondola is rolled so that the test subject remains upright with respect to the G vector (resultant of the Earth’s gravity force and the centrifugal force). Therefore, the graviceptive systems do not receive a roll-tilt stimulus. However, the inclination (θ) of the gondola and subject with respect to the Earth-horizontal plane is a roll angular displacement stimulus to the semicircular canals. Recording of the SVH via adjustment of a luminous line in darkness provides a measure of how the subject perceives this angular displacement

Fig. 2

The SVH as a measure of roll tilt in six subjects. Each data point represents one setting of the luminous line. Time 0 is the beginning of the 2.5-G plateau. As a rule, the SVH tilt is considerably smaller than the real tilt of the gondola, and it decays with time, approaching an asymptote close to 0 by the end of the recording period. Continuous lines represent best-fitting exponential functions; dotted lines represent best-fitting power functions. Please, note that the vertical scales are different

Fig. 3

Adaptation of the exponential function (continuous lines) and the power function (dotted lines) to group data. The recording period has been subdivided into 5 or 15 equal time intervals. For the individual, a mean value (for time as well as for the SVH) has been calculated for each interval. Then, group means have been obtained in a corresponding way. To the unaided eye both curves might appear to be adequate fits to the data points. Nevertheless, in both cases the RMS error was considerably smaller for the exponential function

Fig. 4

Adaptation of the exponential function (upper diagram) and the power function (lower diagram) to all settings of the line made by all subjects. Although the two curves clearly have different forms, they are similar in terms of goodness of fit (RMS error)