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Therefore, we specified a dynamic model (SWEEP) for RCF detection, with one-sided lateral inhibition to arrange for different sensitivity to the direction of RCF as show up in the Heil et al. data. The arrangement we used has been proposed for a synaptic movement-detection mechanism in the visual cortex (Torre & Poggio, 1978). The SWEEP model, as used here, consists of a field of 20 one-sided lateral inhibited frequency detectors, which are all connected to the transient detector (cf. Fig. 3.2). For the im- plementation, we used the same first-order differential equations as in ARTPHONE (Been & Zwarts, 2000). In SWEEP, there are only lateral and bottom–up connections, which are gated by transmitter dynamics.

First, the SWEEP model was calibrated to the Heil et al. data using in- put of variable duration and fixed extent. Next, simulations were run to determine the response to input of fixed duration and variable extent as in speech stimuli. The model output in the nonpreferred direction for both input conditions is different. For the input condition with fixed ex- tent and variable duration (Heil), the model output increases exponen- tially with RCF in the relevant RCF range. For the fixed duration and variable extent input condition (speech), model output decreases expo- nentially with RCF in the same range. For humans, downward changes of rate of frequency seem to be the nonpreferred direction (Collins, 1984). So in the simulations, downward RCFs are processed in the inhib- ited direction of the detector field.

Model Simulations and Experimental Results

The next step is to test the capability of the ARTPHONE and SWEEP mod- els to reproduce experimental data obtained from controls and dyslexics or infants at risk for dyslexia.

Stop Perception: AT(T)A

First we turn to the capability of the ARTPHONE model to simulate dou- ble- versus single-stop perceptions, dependent on the silent interval, in simulating the experimental results obtained with a Finnish pseudoword: the /at/-/ta/ stimulus.

In the Finnish prospective study on dyslexia, the pseudoword stimulus AT(T)A was used to investigate the role of duration in categorization of speech sounds by at-risk infants and control infants at the age of 6 months. Finnish is a quantitative language: Nearly all vowels and consonants can be long or short and make a difference in the meaning of a word. So the ability to differentiate double stops from single stops is an important one in Finnish language acquisition. The choice for the AT(T)A stimulus re- flects this importance of duration in the Finnish language. The AT(T)A

stimulus was constructed from the utterance /ata/ of a female native speaker pronouncing the pseudoword in the context of meaningful text.

The duration of the silent closure stage after the dental stop /t/ was 95 ms in the original utterance. Starting from the original utterance, a syn- thetic continuum of eight stimuli (ATTA1 to ATTA8) was constructed by incrementing the duration of the silent closure with 20 ms steps until ATTA7 with a 215 ms closure. For the final ATTA8, an extra 20 ms was added resulting in a 255 ms silent closure interval. Along this synthetic continuum, perception gradually shifts from a single-stop consonant [ata] to a double-stop consonant [atta] (Richardson, 1998). Discrimination ex- periments were run in a behavioral and an evoked potential variety, in both cases employing an oddball paradigm, using repeated delivery of the standard ATTA1 stimulus (silent interval 95 ms) interspersed with an oc- casional deviant ATTA2 to ATTA8 (silent intervals 115 to 255 ms).

Behavioral Head Turning Experiment

In the behavioral experiment, the technique of head turning was used. The infant is trained to turn its head to a rewarding device when the devi- ant is perceived. About half of the 6-month-old children did not pass the training procedure. The comparison between the at-risk group and the control group is based on the results of 43 at-risk infants and 46 control in- fants (Richardson, 1998). The results are depicted in Fig. 3.3. The caption is relatively self-contained, so we summarize the conclusion that the percep- tual boundary for the at-risk infants is shifted to the right in comparison with the control infants. A longer silent interval is needed to shift percep- tion from ATA to ATTA.

For the simulation of the behavioral AT(T)A experiment, an ART- PHONE model was used as exemplified by the /ib/-/ba/ example. As in the case of the /ib/-/ba/ stimulus, in the /at/-/ta/ stimulus, stop conso- nant gemination can occur in the model. Only one loop of the model is needed. When the second part of the stimulus after the closure interval is delivered at the input side with a sufficiently long silent interval after the the stimulus part before the closure interval, activation in the lexical en- semble can rise above perceptual threshold and fall down below it before activated by the second part. A double stop [atta] is perceived. Taking a short interval between delivery of the first and second parts of the stimu- lus does not allow the activation at the lexical ensemble to fall below per- ceptual threshold before it is activated by the second part. As a result, the percept is [ata].

In the model simulation, the same silent intervals were used as in the experiment. In the simulation, white noise was added to the threshold (Grossberg et al., 1997), which may be considered equivalent to a constant threshold and a noisy ongoing brain activity (Anderson, 1983).