Desventajas

M otion adaptation in insects

Insect visual systems have long been of interest to vision researchers because of the accessibility they afford to anatomical and physiological experimentation. The R eichardt m odel o f motion detection is, at 35 years of age, one o f the longest standing com putational models in the neuroscientific literature, and a wealth of em pirical w ork has gone into dem onstrating its validity (see Reichardt, 1987; E gelhaaf et al, 1989). H owever, while its overall ability to predict the behaviour of motion-sensitive cells in the insect vision system is impressive, neither the original Reichardt m odel nor its subsequent elaborations have been able to account for the phenom enon o f adaptation to motion.

R apid neural adaptation to motion in insects is well docum ented (Zaagman et al, 1983; M addess & Laughlin, 1985; de Ruyter van Steveninck et al, 1986; B orst & Egelhaaf, 1987; M addess et al, 1991). The response of wide-field, motion sensitive neurons in the insect lobula plate to m aintained motion at a constant velocity has consistently been found to rise rapidly to a peak at motion onset and then to decay over the course of a few seconds to a steady-state level. By

stim ulating only small areas of wide-field cells' receptive fields, it has been shown that the locus o f adaptation is much sm aller than the receptive field o f the cell in w hich adaptation is measured. This has led to the conclusion that adaptation must

being studied (M addess & Laughlin, 1985; de Ruyter van Steveninck et al, 1986).

In chapter 2 o f this thesis, an adaptive R eichardt detector model was proposed to account for the adaptation to motion o f the HI and HS cells of the insect lobula plate. W orking within the existing correlation-based framework ensured that, at the starting point o f the m odelling, a great part of the phenom enology o f insect motion vision could already be accounted for. Adaptation was im plem ented by feeding back the responses of motion-sensitive units to adjust the time constants, and hence the temporal frequency tunings, of temporal filters in the elem entary m otion detectors. In this way the adaptive model was able to account for the form o f the lobula plate cells' responses to m aintained motion, and for the progressive enhancem ent o f sensitivity to changes in motion observed during adaptation (M addess & Laughlin, 1985). In chapter 3, the adaptive model was m odified to em ploy more biologically realistic com ponents (Egelhaaf et al, 1989), and as a result was able to account for the directionality (Borst & Egelhaaf, 1987) and local nature (M addess & Laughlin, 1985; de Ruyter van Steveninck et al, 1986) of m otion adaptation. Thus, the widely accepted Reichardt detector model of insect m otion vision was extended in this thesis to account for the phenom enon of adaptation to motion in lobula plate neurons.

Com paring m otion adaptation in insects and m am m als

Previous w ork has established parallels between the responses of insect lobula plate neurons and cells in the m am m alian nucleus of the optic tract (NOT) (Ibbotson et

al, 1994). Lobula plate neurons are believed to be involved in the stabilisation of flight in insects (Shi Jian & Horridge, 1991), while the N OT in m ammals is part of the accessory optic system involved in the control of eye movements (Simpson,

1984). Ibbotson et al (1994) showed that the responses to motion of N OT neurons in the wallaby, M acropus eugenii, were well accounted for by the

R eichardt detector model. However, they also found m arked adaptation to motion (Ibbotson & M ark, 1996) which could not be explained by existing formulations of the R eichardt framework. In chapter 3 of this thesis, electrophysiological studies carried out in collaboration with Ibbotson and M ark were presented which showed clear parallels between rapid neural adaptation in the wallaby NOT and that

observed in the insect lobula plate. In both cases, it was shown that adaptation served to sacrifice absolute sensitivity to motion for differential sensitivity, i.e., sensitivity to changes in motion. The similarities in the behaviour of insect lobula plate and m ammalian NOT neurons enabled the responses of both to be modelled using very sim ilar versions of the adaptive Reichardt detector.

R a p id adaptation to m otion in humans

C hapter 4 o f the thesis considered the extrapolation o f the electrophysiological findings to hum an vision. It was predicted that, if a human observer were making psychophysical judgem ents on the basis of the response of a population o f cells with adaptive characteristics similar to those o f insect lobula plate or wallaby N OT neurons, certain effects might be apparent. Firstly, the perceived speed of an

analogy with the response decay observed physiologically. Secondly, the

observer's sensitivity to small changes in the speed o f an adapting stim ulus would im prove during the course o f adaptation.

The tw o hypotheses were investigated psychophysically. To measure perceived speed, observers were required to discrim inate between the speeds o f an adapting stim ulus and a short burst o f a test stim ulus at various adaptation durations. For each adaptation duration, a psychom etric function was fitted to each subject's data, enabling a point o f subjective equality (p.s.e.) to be determined. The p.s.e. was taken to be the perceived speed o f the stim ulus for that observer. To m easure sensitivity to velocity change, observers were required to report the presence or absence o f a b rief modulation o f velocity over a range of adaptation durations. The proportion o f correct responses at each adaptation duration was taken as a m easure of the differential motion sensitivity of the observer.

The psychophysical results supported the hypotheses. For each o f three observers, perceived speed was found to decrease as a function of adaptation duration, while differential motion sensitivity improved. Thus, in analogy with the findings from insect and m am m alian physiology, it was concluded that rapid adaptation to motion occurs in the hum an visual system, and that it serves to enhance differential