Coding Strategies for Genetic Algorithms and Neural Nets

The interaction between coding and learning rules in neural nets (NNs), and between coding and genetic operators in genetic algorithms (GAs) is discussed. The underlying principle advocated is that similar things in "the world" should have similar codes. Similarity metrics are suggested for the coding of images and numerical quantities in neural nets, and for the coding of neural network structures in genetic algorithms. A principal component analysis of natural images yields receptive fields resembling horizontal and vertical edge and bar detectors. The orientation sensitivity of the "bar detector" components is found to match a psychophysical model, suggesting that the brain may make some use of principal components in its visual processing. Experiments are reported on the effects of different input and output codings on the accuracy of neural nets handling numeric data. It is found that simple analogue and interpolation codes are most successful. Experiments on the coding of image data demonstrate the sensitivity of final performance to the internal structure of the net. The interaction between the coding of the target problem and reproduction operators of mutation and recombination in GAs are discussed and illustrated. The possibilities for using GAs to adapt aspects of NNs are considered. The permutation problem, which affects attempts to use GAs both to train net weights and adapt net structures, is illustrated and methods to reduce it suggested. Empirical tests using a simulated net design problem to reduce evaluation times indicate that the permutation problem may not be as severe as has been thought, but suggest the utility of a sorting recombination operator, that matches hidden units according to the number of connections they have in common. A number of experiments using GAs to design network structures are reported, both to specify a net to be trained from random weights, and to prune a pre-trained net. Three different coding methods are tried, and various sorting recombination operators evaluated. The results indicate that appropriate sorting can be beneficial, but the effects are problem-dependent. It is shown that the GA tends to overfit the net to the particular set of test criteria, to the possible detriment of wider generalisation ability. A method of testing the ability of a GA to make progress in the presence of noise, by adding a penalty flag, is described

Monozygotic twins' colour-number association: a case study

A case study of a pair of monozygotic twins, both of whom show a strong and enduring colour-number association, is reported. The origin of the colours, in a jigsaw puzzle, is known. Neither reports conscious photisms typical of synaesthesia, but a Stroop task of naming the colours of digits shows an interference effect with incongruent colours

Adaptation may cause some of the face caricature effect

One of the ways to demonstrate a caricature preference is to ask participants to adjust a face image over a range from anti-caricature to caricature until it shows the best likeness to a specific individual. Since facial adaptation, whereby exposure to a face influences subsequent perception of faces, is rapid, it is possible that adaptation promotes the selection of a caricatured image. We tested whether giving participants a reference average face image, to counteract any adaptation, would reduce the degree of caricature selected for famous faces. Results confirmed a significant decrease, but even without an average, participants chose an anti-caricatured image. These data suggest a role for adaptation in generating caricature preferences while also suggesting such preferences are not inevitable

Pop-out from abrupt visual onsets

We report a novel psychophysical paradigm that distinguishes the information present in abrupt stimulus onset from that in the following display. The task is to pick the one odd item from a set added to a pre-existing background of similar items. When all new items are added simultaneously, observers are impaired even at distinguishing one red item amongst several green ones. An asynchrony of about 40ms between target and distracter items restores performance, with evidence that it is cortical, rather than stimulus timing difference that is significant. The results are consistent with a role for neural synchrony in dynamic grouping

What's a face worth: Noneconomic factors in game playing

Where behavior defies economic analysis, one explanation is that individuals consider more than the immediate payoff. We present evidence that noneconomic factors influence behavior. Attractiveness influences offers in the Ultimatum and Dictator Games. Facial resemblance, a cue of relatedness, increases trusting in a two-node trust game. Only by considering the range of possible influences will game-playing behavior be explained

The enigma of facial asymmetry:is there a gender specific pattern of facedness?

Although facial symmetry correlates with facial attractiveness, human faces are often far from symmetrical with one side frequently being larger than the other (Kowner, 1998). Smith (2000) reported that male and female faces were asymmetrical in opposite directions, with males having a larger area on the left side compared to the right side, and females having a larger right side compared to the left side. The present study attempted to replicate and extend this finding. Two databases of facial images from Stirling and St Andrews Universities, consisting of 180 and 122 faces respectively, and a third set of 62 faces collected at Abertay University, were used to examine Smith's findings. Smith's unique method of calculating the size of each hemiface was applied to each set. For the Stirling and St Andrews sets a computer program did this automatically and for the Abertay set it was done manually. No significant overall effect of gender on facial area asymmetry was found. However, the St Andrews sample demonstrated a similar effect to that found by Smith, with females having a significantly larger mean area of right hemiface and males having a larger left hemiface. In addition, for the Abertay faces handedness had a significant effect on facial asymmetry with right-handers having a larger left side of the face. These findings give limited support for Smith's results but also suggest that finding such an asymmetry may depend on some as yet unidentified factors inherent in some methods of image collection

A Biologically Supported Error-Correcting Learning Rule

We show that a form of synaptic plasticity recently discovered in slices of the rat visual cortex (Artola et al. 1990) can support an error-correcting learning rule. The rule increases weights when both pre- and postsynaptic units are highly active, and decreases them when pre-synaptic activity is high and postsynaptic activation is less than the threshold for weight increment but greater than a lower threshold. We show that this rule corrects false positive outputs in feedforward associative memory, that in an appropriate opponent-unit architecture it corrects misses, and that it performs better than the optimal Hebbian learning rule reported by Willshaw and Dayan (1990)

The 'double face' illusion

We report three experiments intended to characterise aspects of the ‘double’ face illusion, formed by replicating the eyes and mouth below the originals. Such doubled faces are disturbing to look at. We find there are wide individual differences in ability to detect that a face has been doubled when presented briefly and masked. These differences appear to relate to perceptual speed, since they correlate with the ability to identify a briefly presented famous face. Doubling has a significant effect on identification, though much less than inversion. In a reaction time study, participants are faster to decide that a face has been doubled as it is rotated away from upright. The final study shows that normal and doubled faces do not pop out from each other, but reveals a processing overhead of 40-60ms per doubled face. We offer some speculations as to the cause of the perceptual effects

Changing faces: Direction is important

Recent research has provided evidence for the role of norm-based coding in face recognition (e.g. Leopold et al., 2001). In such a model, any given face can be represented by a vector from the norm, with the difference between two faces being the difference between the two vectors. However, as has previously been suggested it is also possible to conceive of a model in which the angle between two vectors provides relevant information for differentiating faces. Two experiments investigated this possibility using a face matching paradigm. Discrimination was found to be better when two faces lay on different vectors than when they were the same physical distance apart but lying on the same vector. The results favour a specific type of norm-based model suggesting that vector angle is important in face perception

From pixels to people: a model of familiar face recognition

Research in face recognition has largely been divided between those projects concerned with front-end image processing and those projects concerned with memory for familiar people. These perceptual and cognitive programmes of research have proceeded in parallel, with only limited mutual influence. In this paper we present a model of human face recognition which combines both a perceptual and a cognitive component. The perceptual front-end is based on principal components analysis of images, and the cognitive back-end is based on a simple interactive activation and competition architecture. We demonstrate that this model has a much wider predictive range than either perceptual or cognitive models alone, and we show that this type of combination is necessary in order to analyse some important effects in human face recognition. In sum, the model takes varying images of "known" faces and delivers information about these people