I NTRODUCTION 63 parameters, we refer to chapter 2 In the research presented here, we use values for those

parameters that were found to be optimal/adequate.

The paddler has a roughly circular body, with the two eyes placed frontally, under a certain angle c relative to the body axis. The eye-axes are fixed at an angle d relative to

the body axis. Each eye has a sampling area of e degrees; each of the

#

receptors in the retina samples a visual angle of e5º

#

. Due to a gaussian distribution (with half-width »

and directionf

of maximum sensitivity) of photoreceptor sensitivities, the eyes possess an inherent directional selectivity.

Protruding in front, and occupying most of the area where the paddler can’t see, is a snout. When fouraging, this snout is always kept open; every glowball that haplessly enters it, is devoured.

At the rear, we find the structures that gave the paddler its name: a pair of paddles. With these, the animals move. They are fixed to the body at an angleh relative to the frontal end of

the body-axis; their axis is at an angle‚ relative to the rear end of the body-axis. We assume

the frequency of paddling and the thrust generated by paddling to be proportional to the spike-frequency of the motor command. A stiff tailfin gives the paddler an additional lateral resistance, and aids it in performing turns.

3.1.5 Scope of this simulation study

The light adapting mechanism described above evolved in paddlers having to cope with the diurnal circumstances that were encountered when they migrated from their ancestral deep-sea to new niches in shallower, more nutrient-rich water.

A quantitative study will now be described of the adequacy of this light adapting mech- anism as it functions in diurnal paddlers living in such a diurnal habitat. We present an analysis of the influence of the light adapting mechanism’s main parameters on a number of observables that can be measured "electro-physiologically" in the retina and in the-. , and

observables that can be measured from the behaviour of a paddler going about its business in a variety of environments. The characteristics of the diurnal paddler that have been studied are the ?• and

o

parameters, which represent the time constants of the augmented Weber machine; and the parameter, which is used to quantify the amount of lateral inhibition.

This has been done for augmented Weber machines with and without a non-scaling domain (é

1 andé

0 respectively).

The measurements were performed on and averaged over a small number (3) of identical diurnal paddlers, which were allowed to live their simulated lives in a number of different dark and diurnal environments, co-populated by varying glowballs populations.

To perform the experiments, a background luminance was introduced in the habitat of the newly evolved species of the genus paddler. This background luminance is given at every point in space as a local luminance value representing the overall effect of all illuminations (other than caused by glowballs) of that point; this value is added to the illumination of all photoreceptors in the retina of a paddler eye at that point. The local luminance values are expressed as a gradient (ranging from; min to; max) of a certain type (random, linear). This

luminance gradient is modulated by a given (e.g. cosine) function; the modulation depth (<

ª

64 CHAPTER3. LIGHT ADAPTATION IN THE PADDLER

3.2 Methods

Our simulations are done using a proprietary object-oriented simulation package (written in ANSI C language) called PatchWorks. It uses an egocentric approach (with respect to the paddlers), and provides a habitat for the simulated animals. This habitat consists of a (stack of)WorldLevel(s) built out ofPatches that is called theWorld; it is in this experimental "playground" that glowballs live; outside it only a very small subset of the properties (like background illumination) of theWorldextend.

A fine-time-slice regime approximating continuous time is used in all simulations: time proceeds in unit steps, that are subdivided in a number of time-ticks that may vary for each individual time-dependant entity. Patches have discrete coordinates and unit size; within them, coordinates have continuous values.

Paddlers "interact" only through food-depletion: they are completely oblivious of one another. Paddlers that stray out of theWorldare replaced in a random location in theWorld. Every so many steps, the eaten glowballs are replaced at, or within a small random distance from, their previous location. The initial glowball distribution is either random, or uniform with a random dispersion. The latter gives a much better coverage of the area when the dispersion is somewhere between 50% and 100% of the inter-glowball distance. The relevant glowball properties are their size and luminosity.

3.2.1 Judging performances

The performance of the diurnal paddlers, that is, the new species with the described light- adaptation machinery, depends upon how well foreground and background are separated. This is captured in the foreground to background retinal response ratio: 8 . This ratio was

calculated as the ratio of the average response of the augmented Weber machines in the retinae of a freely moving paddler to glowball illumination (foreground) over the average response to background illumination. The average responses are averages of glowball respectively background responses of allv

cells normalised by the sensitivity of the centre photoreceptor projecting onto v

: 8 =?>A@ v ?B> C >ED = > @ v > C > D +GF 1YZ # 34! with: #

: the number of photoreceptors (and v

ganglion-cells) in the retina.

- v B : thev

ganglion-cells whose centre-photoreceptor is illuminated by a glowball.

-

C

: the sensitivity profile of the photoreceptors.

Ideally there should be no background response, either because of optimal adaptation, or because of the absence of background luminance; then8

A*

. Lower values of8 indicate

a worse foreground/background discrimination, with8

3.2. METHODS 65

In document Natural smartness in hypothetical animals. Of paddlers and glowballs (Page 64-66)