Affinely Invariant Features in Visual Perception of Letters and Words

This paper describes two experiments using a masked priming method with 60 ms SOA. In the first experiment, the task was an alphabetical decision. The stimuli were isolated letters or non-alphabetical symbols, preceded by a similar or different prime, while the primes were scaled down or 180° rotated. Response times to letters revealed priming effects for both prime transformations. In the second experiment, the task was a lexical decision, and the stimuli were five lower-case letter words or pseudo-words. The priming conditions were similar to those of the first experiment. Response times to words revealed priming effects for both prime transformations, however the priming effect was only marginally significant for rotated primes and it appeared dependent on the frequency of use of the prime. A significant correlation between priming effects and the frequency of use of the different prime words was observed. We concluded that scale invariant features are used in the perception of letters and words as well, while 180° rotation invariant features are used in the perception of letters, but no such a conclusion can be drawn for words, in general. ha l-0 04 29 56 2, v er si on 1 3 N ov 2 00 9 Courrieu, Ripoll, & Sabancioglu./ Affinely invariant features 3/14 Most stimuli we can perceive in our environment do not loose their identity whenever they are translated in the visual field, their size is modified, they are slanted or rotated to a certain extend. One says that the perception of these stimuli is invariant to the considered transformations. However, one must take care that all possible transformations do not have equivalent effects on the perception of any type of stimulus. Visual perception seems quite robustly invariant to translations and size variations, however the effect of transformations such as symmetries or rotations seems more variable, depending on the nature of the stimulus. For example, one can easily recognise a 180° rotated letter A, while recognising a 180° rotated familiar human face is not so evident. Understanding the nature of perceptual invariants is essential for suitably modelling shape recognition processes. One must not confound the idea of perceptual invariant with that of mental transformation. For example, well-known studies showed that mental rotations are gradual time-consuming operations whose duration depends on the angle of rotation (Cooper, 1975, 1976; Cooper & Shepard, 1973). If visual perception was invariant to rotations, in a general way, no such mental transformations would be necessary. Translation invariance provides a possible simple example. Assume that, before recognising a shape, the perception centres all points’ coordinates on the centre of gravity of the shape. This provides translation invariance to the recognition process, while the complexity of the operation (theoretically) does not depend on the magnitude of the translation. A more general and realistic way of obtaining translation and scale invariance can be found in neuron-like models such as the so-called “Neocognitrons” (Fukushima, 1992; Fukushima & Imagawa, 1993; Fukushima, Miyake & Ito, 1983). Hence, a possible empirical support for distinguishing perceptual invariance from mental transformation is the time required for reducing the transformation, and its dependence on the magnitude of the transformation. We can reasonably speak of perceptual invariant whenever the transformation processing time is short and does not depend on the magnitude of the ha l-0 04 29 56 2, v er si on 1 3 N ov 2 00 9 Courrieu, Ripoll, & Sabancioglu./ Affinely invariant features 4/14 transformation (at least in a non negligible range). Another problem results from the fact that quite complex stimuli can be analysed at various scales, resulting in perceptual components (features) of various sizes that do not necessarily exhibit a homogeneous behaviour with respect to transformations. Consider for example printed words’ perception. One knows that words are recognised through their component letters (McClelland, 1976), however there are some reasons of thinking that more global features also contribute to word recognition, in parallel to component letters’ recognition (Allen & Emerson, 1991; Allen & Madden, 1990; Lété & Pynte, 2003). Assume (for example convenience) that letter recognition, as well as word’s global feature processing, are invariant to rotations, and a 180° rotated word is presented. Then global features are recognised, as well as individual component letters, however these ones appear in reversed order, that is, as an anagram of the word. Thus, a 180° rotated word can appear hard to recognise even if all its component features are rotationally invariants. On the other hand, if one presents a word where all letters have been individually 180° rotated, while their order is preserved, then the orthographic analysis suitably fit the word, however word’s global features are broken down. The literature provides some interesting results concerning such manipulations. In particular, it was observed that globally 180° rotated words are easier to recognise than words whose letters have been individually 180° rotated (Navon, 1978; Tzelgov & Henik, 1983). In the above sketched perspective, this would mean that rotationally invariant global features play an important role in word recognition. However, this is not the conclusion of the authors, since the observed effects can also be interpreted in terms of a corrective mental rotation of the stimulus. In fact, the used word naming tasks do not allow for contrasting the corrective mental rotation hypothesis and the rotationally invariant feature hypothesis. Masked priming techniques are known to be much more suitable than simple recognition tasks to study early perceptual processes. In these methods, a prime stimulus is presented for a short duration (less than 100 ms), and then is ha l-0 04 29 56 2, v er si on 1 3 N ov 2 00 9 Courrieu, Ripoll, & Sabancioglu./ Affinely invariant features 5/14 post-masked by a target stimulus on which the subject must perform a given task. The subject is usually not aware that he/she processed a prime, however this processing actually occurs and it can produce detectable effects on the processing of the target. Depending on the relations between the prime and the target, these effects can be either facilitating or inhibitory, they are detectable for prime durations of about 30 ms, and they increase up to prime durations of about 60 ms (Ziegler, Ferrand, Jacobs, Rey, & Grainger, 2000). Letter priming allowed for detecting scale invariance in the perception of letters, in a range of half-twice the target size (Petit & Grainger, 2002). Anagram priming (Courrieu, 1985), and orthographic priming (Humphreys, Evett, & Quinlan, 1990; Peressotti & Grainger, 1999) showed translation invariance of letters in words, while various hypothesis concerning letter order encoding have been proposed. Data obtained with other techniques such as unmasked priming and/or similarity judgement provided suspicion that certain letter global features could be invariant through symmetries or 180° rotations (Courrieu & De Falco, 1989; Courrieu, Farioli, & Grainger, 2004; Kimchi & Hadad, 2002), despite the fact that such invariants are not relevant for letter recognition since they lead to confuse letters such as b, d, p, and q. The main purpose of the following experiments is to test, using a masked priming technique, the hypothesis that letter and word recognition uses features that are invariant to 180° rotations. Well-known priming effects obtained with prime scale reduction will be used as reference effects in order to compare their magnitude to that of rotated prime effects.