Comple jación o quelacion

Cohen an d D ehaene (1991) interpreted Y.M.'s perform ance (see section 1.44 for description of the case) by m odifying the McCloskey et al. m odel. Y.M. m ade lexical su b stitu tio n errors w hen rea d in g a lo u d arabic n u m erals (e.g.

stim ulus 2, response "sept" [seven]). These errors suggested an im p airm en t in th e lexical p ro cessin g m echanism s. The su b s titu tio n e rro rs w ere larg ely d ependent on visual spatial param eters; w hen the arabic num erals w ere arrayed in the stan d ard left to right fashion, the patient's errors w ere m ore frequently m ade on the left m ost digit. Results from a series of tasks suggested th at his ability to com prehend arabic num erals was intact (100% correct answ ers in a task requiring him to judge w hich of two arabic num erals w as the bigger). According to the M cCloskey e t al.' m odel, if an im p airm en t in rea d in g alo u d arabic n u m erals arises from a deficit in the lexical m echanism s w ith in the verbal num eral production system no effect of visual spatial factors should be present. In fact this m odel assum es th at the m echanism s w ith in the verbal num eral p ro d u ctio n system o p erate u p o n abstract internal representations. In o rd er to account for Y.M.'s perform ance, Cohen and Dehaene suggested a m odification to the M cCloskey et al. m odel. They p roposed an asem antic m odel for reading a lo u d arabic num erals. The au th o rs assum ed th a t th e in p u ts to the verbal num eral production process are not num ber abstract internal representations but spatially arrayed num ber representations. In figure 1.6 their m odel is shown.

(insert figure 1.6 about here)

According to these authors the visual n u m ber form , so called in analogy to the visual w ord form (see Shallice, 1988 for a discussion on the structure of this system), is a visual store that encodes "several visuo-spatial characteristics of the stim u lu s, in clu d in g sh ap es an d the re la tiv e p o sitio n s of the d ig its, presu m ab ly in a stim u lu s centred co-ordinate system ". In th e course of the reading process this store is accessed tw ice. The first tim e it is accessed to extract the syntactic inform ation enabling the construction of the syntactic fram e and then the word frame. The second tim e it is accessed to retrieve the appropriate in dividual digit identities for filling the word frame.

The sy n ta c tic fra m e is extracted by processing th e length of the arabic num eral. This w ould specify the default sequence of w ords for an arabic num eral of th at length. For exam ple, the processing of the length of the arabic num eral 620 w o u ld g en erate th e follow ing sequence of w o rd s ONES: _ M ultiplier: H u n d red TENS: _.

T he word frame transform s the default syntactic fram e into a sequence th at specifies the o u tp u t lexical classes to be accessed. For exam ple, the w ord

frame of the arabic num eral 620 will look like ONES: _ M ultiplier: H u n d re d TENS: In o rd er to obtain a word frame it is necessary to identify som e of the in p u t digits, the so called "syntactic markers". The au th o rs p roposed th at the "syntactic m arkers" m odify the "default m ap p in g p rin cip le from digits in the position of units and h u n d red s to nam es in the ones stack, and from digits in the tens position to names in the tens stack". For exam ple, for French num erals the syntactic m arkers are the silent 0 and 1, and the digits 1, 7 and 9 w hen they are in the second position. The authors proposed th at these syntactic m arkers are a separate psychological class.

The filled word frame is obtained by filling th e em p ty slots of the w ord fram e w ith the a p p ro p riate digit identities. The d ig it id en tities are retrieved

th ro u g h a second access to the visual n u m ber form . So for exam ple, for the num ber 620 the filled w ord fram e w ould look like this: ONES: {6} MLT: H u n d red TENS: (2). This w o rd p lan is th en u se d to a d d re s s th e p h o n o lo g ic al representations (e.g. /s i x / /h u n d r e d / /tw e n ty / in the above example).

Y.M.'s deficit is interpreted as due to an im pairm ent in his digit identity retrieval process th at suffers from "spatial bias, perseverations an d confusion of visually sim ilar digits".

Cohen and Dehaene's suggestions are extrem ely interesting but, perhaps, this m odel possesses som e lim itations. The au th o rs seem to suggest th at the visual num ber form constitutes "an in p u t stage com m on to any task involving nu m b er m anipulation, including reading, m ag n itu d e com parison, calculation, etc." (p. 54). If the visual num ber form acts as an in p u t store for the arabic num eral com prehension processes and if Y.M.'s deficit lies in his digit identity retrieval processes then it is left unexplained w hy he w as able to m ake no errors in com parisons of 220 pairs of arabic num erals. Conversely, if the visual num ber form d id n o t act as an in p u t store for the arabic n u m eral com prehension m ec h a n ism s, th e ir re la tio n s h ip w ith th e v e rb a l n u m e ra l p ro d u c tio n m echanism s is left unspecified.

Figure 1.6

A schematic description of Cohen and Dehaene's m odel of reading aloud arabic n u m e ra ls

Visual Input: 813

/huit/cent/treize Visual Number Form

813

(Ones:. ) MltiHundred Tens:. Ones:

(OnesL ) Mlt;Hundred Teens:.

(Ones:{8}) Mlt:Hundred Teens:{3}

Syntactic Frame

Word Frame

Filled Word Frame

Phonological Representation