Conceptos fundamentales o claves:

ARE NOVEL SOUNDS.

INTRODUCTION

The results of Experiment 1 confirmed the previously reported distinction between a parietally distributed P3b in response to rare target stimuli and a more anteriorly distributed P3a in response to rare novel stimuli which do not require a response. The present experiment and those which follow, investigate the conditions necessary for the elicitation of the centro-parietal P3a in order to gain some insight into its psychological significance.

The P3a elicited by the sounds in Experiment 1 could have occurred for a variety of reasons. The first possibility is that the P3a is elicited by all rare stimuli presented within a stimulus sequence which do not require a response. This proposal is not supported by the findings of Courchesne et al. (1975, 1977), who reported that easüy recognisable rare nontarget visual stimuli produced a P3 with a parietal distribution. Tliis suggests that being a rare nontarget may be necessary but is not sufficient to produce a P3a in the visual modality. It is not known whether this finding extends to the auditory modality.

The second possibility is that the P3a is produced because the novel sounds are physically very different from the tones or more complex than other stimuli in the sequence. The proposal that a physical deviation from other stimuli in the sequence

is sufficient for P3a elicitation does not appear to be supported by experimental findings in the visual modality. Courchesne et al. (1978) used a task in which target letter Bs, with a probability of 10%, had to be detected within a sequence of letter As which had a probability of 80%. The remaining 10% of the sequence consisted of nontargets thought to be of varying deviance from the other stimuli in the sequence. These were repetitions of the letter C, a random selection of the letters C- Z and a random selection of the numbers 0-23. In a second procedure, the frequent and target stimuli were the same as above but the nontargets were either letters C-Z, letters C-Z with a lower luminance than the other stimuli in the sequence or letters C-Z with a higher luminance than other stimuli in the sequence. It was found that a posteriorly distributed P300 was elicited in response to all recognisable rare nontarget visual stimuli. This suggests that physical deviation of the stimulus from the background is not sufficient for the elicitation of the P3a. The stimuli which deviated in luminance from the background stimuli produced a larger amplitude P3 than the other rare nontarget stimuli, which led the authors to argue that more deviant stimuli elicit larger amplitude P300s. The same effect of deviation from the background stimuli has been observed on the amplitude of the P3b elicited by target stimuli. Ford et al. (1976) presented subjects with a series of tones in which there was occasionally a tone of a different pitch from that of the background stimuli. The difference in pitch between the frequent and background stimuli was either 5%, 25% or 100%. The deviant tones elicited a P300 which increased in amplitude with increasing mismatch from the frequent stimuli. This occurred both while the subject was reading, and therefore should have been ignoring the tones, and while the subject was attending to the tones. However, the effect was larger in the attention condition. These findings therefore suggest that physical deviation may affect the generator processes of both components but is not involved in detennining which neuronal processes are activated.

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Courchesne et al. (1978) suggested that the P300 deflection elicited by visual targets and easily recognisable nontargets differed in scalp distribution from tliat elicited by uiuecognisable visual stimuli because of the differential ease with which the events could be categorised. He suggested that easily categorised events would produce a parietally distributed P300 deflection and those which could not be categorised would produce a frontally distributed P300 deflection. In the Courchesne et al. (1978) study the frontal P300 in response to the novel umecognisable stimuli decreased in amplitude at frontal sites and increased in amplitude at parietal sites with repetition of the novel stimuli. Courchesne suggested that this reflected the fonnation of categorisation rules. A slight posterior shift in distribution on subsequent presentations of the novel stimuli was also reported in the auditory modality (Knight, 1984).

Tlie model of auditory processing proposed by Naatanen (1990) suggests that the P3a occurs as a consequence of a mismatch between a neuronal model of the frequently occurring stimulus which is passively formed in sensory memory and a representation of the presented stimulus. Detection of a mismatch causes an orienting of attention to the presented stimulus. The P3a is thought to reflect a process which is a precursor of or related to the orienting of attention. If the stimulus is a target, the mismatch with the trace of the frequent is detected and the central executive mechanisms accessed. The stimulus is identified as a target and both the processes underlying the generation of the P3b and those required to make a response are invoked. This suggests that whenever the stiiuulus mismatches the representation of the frequent sthnulus sufficiently to cause the central executive mechanisms to be accessed and the stimulus is found not to require a response, a P3a

is obtained in the wavefonn. When a response is required the resulting P300 peak consists of overlapping P3a and P3b components.

In the present experiment subjects were presented with two stimulus sequences. In one sequence the frequent, target and rare nontarget stimuli were three different tones, in the other sequence the stimuli were tluree different novel sounds. The conditions, to which the stimuli within each sequence were assigned, were balanced across subjects (e.g. in the tone oddball task one subject may have a high tone as the frequent stimulus, another subject may have a medium tone as the frequent and a thiid subject may have a low tone as tlie frequent). Any differences in the ERP wavefonns between conditions within a task would not be due therefore to the physical characteristics of the stimuli but would he due to the role of tlie stimuli in the experiment. All stimuli in a sequence were equally deviant, no one stimulus of a sequence ’stood out’ from the others, so any difference in the ERP waveforms elicited by the stimuli could not be due to difference in deviance.

The present experiment allowed the investigation of two alternative hypotheses. Tlie first was that, as proposed in the model of auditory processing of Naatanen (1990), if a rare nontarget stimulus mismatches the sensory memory representation of the physical features of the frequent stimulus, it would elicit a P3a. As discussed above, the results of the visual experiment of Courchesne et al. (1978) do not support this proposal since in that experiment the rare nontarget stimuli elicited a P300 deflection with a posterior maximum. However, it is possible that different processes are involved in auditoiy and visual processing. The first hypothesis therefore proposed that a P3a would be elicited by the rare nontarget stimuli in both the tone and novel sound oddball tasks.

107 The second hypothesis was that the P3a is elicited because of something intrinsic to the novel sounds and that the nature of the other stimuli in the task and the condition to which the novel sound is assigned are iiTelevant. This hypothesis therefore proposed that a P3a would be elicited by all the novel sounds in the novel sound oddball task.

METHOD

Subjects

Twelve healthy subjects (mean age 21, range 19-25 yrs, 6 female) were tested. All were paid volunteers.

Design

Subjects were presented with two 300 trial stimulus sequences. One sequence consisted of a random mixing of a frequent tone (P=0.70), a target tone (P=0.15) and a rare nontarget tone (P=0.15). The tones were of 1000 Hz, 750 Hz and 500 Hz, allocated to condition in a balanced way across subjects. The other sequence consisted of the random mixing of a frequently occurring novel sound (P=0.70), a target novel sound (P=0.15) and a rare nontarget novel sound (P=0.15). Three different sounds were used and as for the tones were allocated to conditions in a balanced way across subjects. The order in which the two sequences were presented was balanced across subjects; six subjects completed the tone oddball first and six completed the novel sound oddball first. Prior to each experimental sequence

subjects were presented with a practice sequence of 15 stimuli. This sequence included 9 frequent, 3 target and 3 rare nontàrget stimuli.

Procedure

For the tone oddball task subjects were told that a sequence consisting of high, medium and low tones would be presented through the headphones. Subjects were instructed to press the response button as quickly as possible, whilst avoiding eiTors, whenever they heard the tone which had been specified as the target and not to respond to the other stimuli. For the novel sound oddball task subjects were told that a sequence of three different sounds would be presented tlirough the headphones. Subjects were instmcted to press the response button as quickly as possible, whilst avoiding errors, as soon as they heard the sound which had been specified as the target. Responses were made with the preferred hand. Following successful completion of the practice trials each sequence of experimental trials was presented as three blocks of 100 trials with a 30 s break between each block.

DATA ANALYSIS

The grand average waveforms for the two tasks are shown in Figure 5.1a and 5.1b. These waveforms were obtained by averaging together the ERPs produced by the twelve subjects.

For the tone task, the waveform produced in response to the frequent stimuli was averaged over a mean of 164 trials (ranging from 103-184 trials), that in response to the target stimuli was averaged over a mean of 35 trials (ranging from 15-44 trials)

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109 and that in response to the rare nontarget stimuli was averaged over a mean of 34 trials (ranging from 24-44 trials). For the novel sound task, the wavefonn in response to the frequent stimuli was averaged over a mean of 172 trials (ranging from 140-200 trials), that in response to the target stimuli was averaged over a mean of 35 trials (ranging from 21-45 trials) and that in response to the rare nontargets was averaged over a mean of 38 trials (ranging from 29-43 trials).

As shown in Figures 5.1a and 5.1b, for both the tone and novel sound oddball task a negative peak was obtained at approximately 100 ms in all three conditions (referred to as N 100 in Experiment 1). This was followed by a small positive deflection in the waveforms of the rare target and nontarget stimuli; this peak was most clearly visible at frontal sites (referred to as the P I70 in Expeiiment 1). In the waveforms elicited by the two categories of rare stimuli the PI70 was followed by a negative deflection, this will be referred to as the N200. A positive deflection, which was largest in amplitude at posterior sites, was subsequently seen in response to the rare targets and nontargets (the P300 deflection). This was followed by a sustained period of negativity at frontal sites and positivity at parietal sites which was not observed in response to the frequent stimuli.

As in the previous experiment, the latency of N 100 was measured at Cz and that of PI 70 was measured at Fz for all tliree conditions for both the novel sound and tone oddball tasks. It was possible to obtain measurements of the three peaks for all 12 subjects. The mean latency of the peak over the three conditions was used to determine the latency window for each task. To deteimine the mean amplitude of the N200, the latency of the N200 deflection was measured in the grand average wavefonn elicited by the rare nontargets in both the novel sound and tone oddball task. The mean amplitude of the wavefonn +-12 ms round the N200 peak was

found for each of the three conditions, for each subject. The latency of P300 was measured at Pz in the waveforms elicited by the rare targets and rare nontargets of the two tasks. Measurements for each peak were made for all 12 subjects. Separate latency windows were determined in each task for the targets and rare nontargets. The latency region investigated in the waveform elicited by the frequent stimuli corresponded to that of the P300 peak with which it was being compared. The period of sustained negativity and positivity was investigated by peifonning analyses on the area of wavefonn within the 500-900 ms latency region.

Repeated measures ANOVAs were perfonned on the amplitude measurements for each component. A 12*2*2*3*3 (subject*sequence*condition*chain*site) ANOVA was performed comparing the amplitude of the P300 region of the waveform elicited by the frequent and rare target stimuli, the frequent and rare nontarget stimuli and the rare target and rare nontarget stimuli, of each task, before and after rescaling. 12*2*3*3*3 (subject*seqiience*condition*chain*site) ANOVAs were performed on the amplitude of the NIOO, P170, N200 and 500-900 ms regions of the waveform before and after rescaling. Additionally, for the N200 the ANOVAs were repeated comparing the waveforms elicited by the targets and rare nontargets. This was because the N200 was only elicited by stimuli in these two conditions.

Between group, 2*12*2*3*3 (experiment*subject*condition*chain*site), ANOVAs were perfonned comparing the amplitude of the P300 elicited by tire targets and rare nontargets in experiment I and the tone and novel sound oddball tasks of the present experiment before and after rescaling. Separate 2*12*3*3*3 (experiment*subject*condition*chain*site) ANOVAs were perfonned for the NIOO, PI70 and 500-900 ms region comparing the amplitude of the deflection in

I l l experiment 1 with that elicited by stimuli in the same conditions in the tone and novel sound tasks of the present experiment before and after rescaling.

RESULTS

P300

Inspection of the grand average wavefonns of both tasks revealed a positive deflection with a posterior maximum in response to the rare targets and nontargets but not in response to the frequent stimuli. This was found to have a mean peak latency of 366 ms in response to the targets and 347 ms in response to the rare nontargets in the tone oddball task. The same positive deflection was found to have a mean peak latency of 331 ms in response to the targets and 338 ms in response to the rare nontargets in the novel sound oddball task. The ANOVA comparing the peak latencies of the two conditions in the two tasks showed no significant main effects of task (F(l,l 1)= 1.170, P>0.05) or condition (F(l,ll)=0.379, P>0.05) and no significant task by condition interaction (F(l,11)=1.455, P>0.05). The component will be referred to here as the P300 because although it has the same distribution as the P3b reported in the previous two experiments (see analysis of rescaled data) it can also be elicited by novel sounds.

P300 region for targets v frequents

As can be seen from Table 5.1, the results from the analysis comparing the amplitude of the P300 elicited by the targets with the equivalent region of waveform elicited by the frequent stimuli were as obtained for Experiment 1. This was true for

rescaling. RAW AMPLITUDE Factors df F Prob Task (TA)

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1.29 0.282 Condition (CC) 1,11 46.52 0.000’ aiain (CH) 1,7,19.2 21.80 0.000’ Site (ST) 1.3,14.4 40.36 0.000’ TA*CC 1,11 2.28 0.160 TA*CH 1.3,14.7 0.09 0.839 TA+ST 1.5,16.6 0.52 0.551 CC*CH 1.7,19.1 12.28 0.001 CC*ST 1.3,14.3 26.01 0.000’ CH*ST 3.3,35.9 5.60 0.002’ TA+CC*CH 1.7,19.1 1.54 0.239 TA*CC*ST 1.6,17.3 0.60 0.522 TA*CH*ST 2.1,22.9 1.24 0.310 CC*CH*ST 2.5,27.1 2.59 0.084 TA*CC+CH*ST 2.8,31.2 1.26 0.305

* indicates statistical significance at the 0.05 level or better

Table 5.2. ANOVA summary table for analysis of the amplitude of the P300 elicited by the rare nontargets and the same region of the waveform elicited by the frequent stimuli for the two tasks, before rescaling. RAW AMPLITUDE Factors df F Prob Task (TA) U l 2.65 0,113 Condition (CC) 1,11 8.60 0.014* Chain (CH) 1.7,18.8 21.09 0.000* Site (ST) 1.5,16.4 68.42 0.000* TA+CC 1,11 1.13 0.311 TA*CH 1.9,20.6 0.08 0.911 TA*ST 1.4,15.7 0.99 0.365 CC*CH 1.7,18.8 21.88 0.000* CC*ST 1.2,12.9 40.97 0.000* CH*ST 3.3,36.0 3.01 0.039* TA*CC*CH 1.4,15.4 0.70 0.460 TA*CC*ST 1.6,17.9 0.25 0.734 TA*CH*ST 2.6,29.0 0.65 0.570 CC*CH*ST 2.8,31.3 3.55 0.027* TA*CC*CH*ST 2.3,25.8 0.40 0.708

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In document Colegio emblemático intercultural bilingüe sostenible para contribuir a la calidad educativa en las comunidades indígenas de la Selva distrito de Nueva Requena Coronel Portillo Ucayali 2016 2026 (página 40-43)