**Pre-Attentive Processing of Sound Duration Changes: Low Resolution Brain Electromagnetic Tomography Study**

Wichian Sittiprapaporn

*Department of Educational Psychology and Guidance, Faculty of Education, Mahasarakham University, Maha Sarakham, Thailand* 

#### **1. Introduction**

26 Will-be-set-by-IN-TECH

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The human central auditory system has a remarkable ability to establish memory traces for invariant features in the acoustic environments in order to correct the interpretation of natural acoustic sound heard. Even when no conscious attention is paid to the surrounding sounds, changes in their regularity can cause the listener to redirect his or her attention toward the sound heard (Tervaniemi *et al.,* 2001). When engaged in a conversation, listeners tune in to the relevant stream of speech and filter out irrelevant speech input that may be present in the same environment. Nonetheless, attention might be involuntarily diverted to meaningful items coming from an ignored stream, like in the well-known own-name effect (Moray, 1959). This brings up the question of to what extent speech is processed in the ignored streams. In the past decade, there have witnessed a resurgence in the electrophysiological literature of attempts to understand how the brain processes the speech signal (Kraus *et al.,* 1993, 1996; Molfese, 1985). One of the most used and well known paradigms in electrosphysiological research is the so-called oddball paradigm, in which typically two stimuli are presented, in random order. One of the stimuli occurs less frequently than the other and the subject is required to discriminate the infrequent stimulus (deviant, target or oddball) from the frequent one (standard). Two main types of ERPs have been described in the literature as a response to the detection of the deviant: P300 and the MMN (Aaltonen *et al.,* 1994; Kraus *et al.,* 1993, 1996). If the subject is required to respond overtly --- for example, by pressing a button – each time he/she detects the deviant, a positive wave peaking approximately 300 ms after deviant onset is elicited. This wave is called P300 and it is largest over electrode sites in normal adults. Such positivity is thought to reflect voluntary focused attention (context updating, response selection). However, if the subject is not required to respond overtly, and one subtracts the event-related potentials (ERPs) obtained in response to the standard, from the ERPs obtained for the deviant, so-called mismatch negativity (MMN) may be observed, usually peaking between 100 and 300 ms after stimulus onset depending on the characteristics of the difference between standard and deviant stimuli. This component is thought to reflect a pre-attentional detection of deviation, a mismatch between the deviant and the memory trace formed by the standard.

Pre-Attentive Processing of Sound Duration Changes:

different stimulus.

Low Resolution Brain Electromagnetic Tomography Study 223

The deviant stimuli both in the attended and unattended stimulus sequence elicited negativity in the 100-200 ms latency range, which could not be seen in response to the standard stimuli. This negativity, usually described by the deviant-minus-standard difference wave, was very similar for the attended and ignored input sequences, suggesting that attention was not required. Näätänen *et al.* (1978) proposed that it may well be that a physiological mismatch process caused by a sensory input deviating from the memory trace formed by a frequent background stimulus is such an automatic basic process that it takes place irrespective of the intentions of the experimenter and the subject, perhaps even unmodified by the latter. On the basis of the relatively large MMN amplitudes above the temporal areas, the authors further suggested that the mismatch negativity reflects specific auditory stimulus discrimination processes taking place in the auditory primary and association areas. The latter processes are suggested to be largely automatic, beyond the control of will, instructions, etc. This finding, suggesting the existence of an automatic memory mechanism subsequently paved the way for a series of new experiments where changes in basic stimulus features (frequency, intensity, and duration) and the elicitation of the MMN were addressed in more detail. It was established that the MMN is elicited by both increments and decrements in basic stimulus features. The MMN, however, is not elicited when a stimulus sequence begins or, similarly, when stimuli are presented with very long interstimulus intervals (ISIs). Thus, it was concluded that no stimulus per se is an adequate stimulus for the MMN generator mechanism, as the system responds to the difference between the consecutive stimuli. This response pattern is clearly separable from the behavior of N1 response; the N1 amplitude is largest in response to the first stimulus of a series, strongly attenuating thereafter and showing only partial recovery to a subsequent

Mismatch negativity (MMN), an index of preattentive processing of perceived sounds, is an Event-related Potential (ERP) component elicited by rare deviant stimuli within a sequence of repetitive auditory stimuli. Mismatch negativity component of ERP is theoretically elicited in the auditory cortex when incoming sounds are detected as deviating from a neural representation of acoustic regularities. The mismatch negativity (Näätänen *et al.,* 1978) and its magnetic equivalent (MMNm) are elicited by any discriminable change in some repetitive aspect of auditory stimulation, irrespective of the direction of the subject's attention. It is mainly generated in the auditory cortex (Scherg *et al.,* 1989) occurring between 100 to 250 ms and thus long been regarded as specific to the auditory modality (Näätänen, 1992; Nyman *et al.,* 1990). Additionally, this negative component of the auditory event-related potential (ERPs), usually peaking 100-300 ms from change onset, is based on, and reflects, neural traces by which the auditory cortex models the repetitive aspects of the acoustic past (Näätänen and Winkler 1999). These traces might contain sensory information on sound frequency, duration and inter-stimulus interval (ISI), but also on more complex aspects of auditory stimulation, such as rhythmic patterns or speech sounds (Näätänen and Winkler 1999). The properties of these traces (which usually last several seconds, although even permanent traces can be reflected (Näätänen and Winkler 1999)) can be probed by presenting infrequent deviant events in the sequence of repetitive events ('standard') (Näätänen and Winkler 1999). MMN is elicited even in the absence of attention, for example, in individuals in a coma a few days before the recovery of consciousness (Kane *et al.,* 1993), which indicates that MMN indexes pre-attentive (attention-independent) auditory processing. The automatic change-detection system in the human brain as reflected by the

Event-related potentials (ERPs) recordings have bought new insight to the neuronal events behind auditory change detection in audition. ERPs components reflect the conscious detection of a physical, semantic, or syntactic deviation from the expected sounds (Tervaniemi *et al.,* 2001). The ERPs recordings thus allow one to probe the neural processes preceding the involvement of the attentional mechanisms. For instances, ERPs have been recorded that reflect memory traces representing sounds composed of several simultaneous or successive tonal elements (Schröger *et al.,* 1996; Alain *et al.,* 1994; Alho *et al.,* 1996). In auditory perception, the occurrence of the deviant (infrequent) stimulus after a sequence of the standard (Frequent) stimuli tends to elicit MMN in event-related potentials (ERPs) and its magnetic equivalent called the magnetic mismatch field (MMF) in magnetoencephalography (MEG). The MMN/MMF component may be considered to reflect the pre-attentive auditory memory processes and represents neuronal correlates of change detection and sound discrimination (Näätänen, 1992). Previous studies showed that for sinusoidal tones, the MMF is sensitive to the direction of a change within the stimulus, being more robustly activated for duration shortening or pitch falling as opposed to lengthening or leveling (Inouchi *et al.,* 2002). These studies also revealed no significant differences between subjects who spoke a pitch-accent language (Japanese) and those who did not (English). It has been reported that MMN/MMF is indeed sensitive to cross-linguistic relevance. Unlike short-to-long vowel duration and falling-to-level pitch changes, long-toshort duration and level-to-falling pitch changes elicited a prominent MMF bilaterally for both groups, peaking at around 100 ms after change onset for duration and 200 ms for pitch. The MMF component is sensitive is sensitive to vowel shortening rather than lengthening and to pitch falling rather than leveling (Inouchi *et al.,* 2002, 2003).

#### **2. Neurophysiological features of mismatch negativity**

The search for an objective index of change detection in the human brain can be traced back to 1975, with the proposition that stimulus deviation per se (irrespective of, e.g., stimulus significance, attentional mechanisms) should produce a measurable brain response (Näätänen, 1992). Experimental evidence for this suggestion was obtained in experiments conducted by Näätänen, Gaillard, and Mäntysalo in 1975 (subsequently reported in 1978). In this dichotic listening study, the subject's task was to detect occasional deviant stimuli in the stimulus sequence presented to a designated ear while ignoring the concurrent sequence presented to the opposite ear. The irrelevant stimulus sequence included deviant stimuli that were physically equivalent to the deviant stimuli (targets) of the attended input sequence. The deviant stimuli were either tones of a slightly higher frequency or tones of a slightly greater intensity than the standard tones. A neurophysiological paradigm well suited to examine pre-attentive and automatic central auditory processing is the mismatch negativity (MMN). This is a negative component of the event-related brain potential (ERP), elicited when a detectable change occurs in repetitive homogeneous auditory stimuli (Näätänen, 1992). The most commonly described MMN occurs at 100-300 ms post-stimulus onset although other studies have found later MMNs between 300 and 600 ms (Kraus *et al.,* 1996). The MMN is elicited by any change in frequency, intensity or duration of tone stimuli, as well as by changes in complex stimuli such as phonetic stimuli (Näätänen, 1992). It is assumed to arise as a result of a mechanism that compares each current auditory input with a trace of recent auditory input stored in the auditory memory. The MMN usually reaches its amplitude maximum over the fronto-central scalp (Näätänen, 1992).

Event-related potentials (ERPs) recordings have bought new insight to the neuronal events behind auditory change detection in audition. ERPs components reflect the conscious detection of a physical, semantic, or syntactic deviation from the expected sounds (Tervaniemi *et al.,* 2001). The ERPs recordings thus allow one to probe the neural processes preceding the involvement of the attentional mechanisms. For instances, ERPs have been recorded that reflect memory traces representing sounds composed of several simultaneous or successive tonal elements (Schröger *et al.,* 1996; Alain *et al.,* 1994; Alho *et al.,* 1996). In auditory perception, the occurrence of the deviant (infrequent) stimulus after a sequence of the standard (Frequent) stimuli tends to elicit MMN in event-related potentials (ERPs) and its magnetic equivalent called the magnetic mismatch field (MMF) in magnetoencephalography (MEG). The MMN/MMF component may be considered to reflect the pre-attentive auditory memory processes and represents neuronal correlates of change detection and sound discrimination (Näätänen, 1992). Previous studies showed that for sinusoidal tones, the MMF is sensitive to the direction of a change within the stimulus, being more robustly activated for duration shortening or pitch falling as opposed to lengthening or leveling (Inouchi *et al.,* 2002). These studies also revealed no significant differences between subjects who spoke a pitch-accent language (Japanese) and those who did not (English). It has been reported that MMN/MMF is indeed sensitive to cross-linguistic relevance. Unlike short-to-long vowel duration and falling-to-level pitch changes, long-toshort duration and level-to-falling pitch changes elicited a prominent MMF bilaterally for both groups, peaking at around 100 ms after change onset for duration and 200 ms for pitch. The MMF component is sensitive is sensitive to vowel shortening rather than lengthening

and to pitch falling rather than leveling (Inouchi *et al.,* 2002, 2003).

**2. Neurophysiological features of mismatch negativity** 

its amplitude maximum over the fronto-central scalp (Näätänen, 1992).

The search for an objective index of change detection in the human brain can be traced back to 1975, with the proposition that stimulus deviation per se (irrespective of, e.g., stimulus significance, attentional mechanisms) should produce a measurable brain response (Näätänen, 1992). Experimental evidence for this suggestion was obtained in experiments conducted by Näätänen, Gaillard, and Mäntysalo in 1975 (subsequently reported in 1978). In this dichotic listening study, the subject's task was to detect occasional deviant stimuli in the stimulus sequence presented to a designated ear while ignoring the concurrent sequence presented to the opposite ear. The irrelevant stimulus sequence included deviant stimuli that were physically equivalent to the deviant stimuli (targets) of the attended input sequence. The deviant stimuli were either tones of a slightly higher frequency or tones of a slightly greater intensity than the standard tones. A neurophysiological paradigm well suited to examine pre-attentive and automatic central auditory processing is the mismatch negativity (MMN). This is a negative component of the event-related brain potential (ERP), elicited when a detectable change occurs in repetitive homogeneous auditory stimuli (Näätänen, 1992). The most commonly described MMN occurs at 100-300 ms post-stimulus onset although other studies have found later MMNs between 300 and 600 ms (Kraus *et al.,* 1996). The MMN is elicited by any change in frequency, intensity or duration of tone stimuli, as well as by changes in complex stimuli such as phonetic stimuli (Näätänen, 1992). It is assumed to arise as a result of a mechanism that compares each current auditory input with a trace of recent auditory input stored in the auditory memory. The MMN usually reaches The deviant stimuli both in the attended and unattended stimulus sequence elicited negativity in the 100-200 ms latency range, which could not be seen in response to the standard stimuli. This negativity, usually described by the deviant-minus-standard difference wave, was very similar for the attended and ignored input sequences, suggesting that attention was not required. Näätänen *et al.* (1978) proposed that it may well be that a physiological mismatch process caused by a sensory input deviating from the memory trace formed by a frequent background stimulus is such an automatic basic process that it takes place irrespective of the intentions of the experimenter and the subject, perhaps even unmodified by the latter. On the basis of the relatively large MMN amplitudes above the temporal areas, the authors further suggested that the mismatch negativity reflects specific auditory stimulus discrimination processes taking place in the auditory primary and association areas. The latter processes are suggested to be largely automatic, beyond the control of will, instructions, etc. This finding, suggesting the existence of an automatic memory mechanism subsequently paved the way for a series of new experiments where changes in basic stimulus features (frequency, intensity, and duration) and the elicitation of the MMN were addressed in more detail. It was established that the MMN is elicited by both increments and decrements in basic stimulus features. The MMN, however, is not elicited when a stimulus sequence begins or, similarly, when stimuli are presented with very long interstimulus intervals (ISIs). Thus, it was concluded that no stimulus per se is an adequate stimulus for the MMN generator mechanism, as the system responds to the difference between the consecutive stimuli. This response pattern is clearly separable from the behavior of N1 response; the N1 amplitude is largest in response to the first stimulus of a series, strongly attenuating thereafter and showing only partial recovery to a subsequent different stimulus.

Mismatch negativity (MMN), an index of preattentive processing of perceived sounds, is an Event-related Potential (ERP) component elicited by rare deviant stimuli within a sequence of repetitive auditory stimuli. Mismatch negativity component of ERP is theoretically elicited in the auditory cortex when incoming sounds are detected as deviating from a neural representation of acoustic regularities. The mismatch negativity (Näätänen *et al.,* 1978) and its magnetic equivalent (MMNm) are elicited by any discriminable change in some repetitive aspect of auditory stimulation, irrespective of the direction of the subject's attention. It is mainly generated in the auditory cortex (Scherg *et al.,* 1989) occurring between 100 to 250 ms and thus long been regarded as specific to the auditory modality (Näätänen, 1992; Nyman *et al.,* 1990). Additionally, this negative component of the auditory event-related potential (ERPs), usually peaking 100-300 ms from change onset, is based on, and reflects, neural traces by which the auditory cortex models the repetitive aspects of the acoustic past (Näätänen and Winkler 1999). These traces might contain sensory information on sound frequency, duration and inter-stimulus interval (ISI), but also on more complex aspects of auditory stimulation, such as rhythmic patterns or speech sounds (Näätänen and Winkler 1999). The properties of these traces (which usually last several seconds, although even permanent traces can be reflected (Näätänen and Winkler 1999)) can be probed by presenting infrequent deviant events in the sequence of repetitive events ('standard') (Näätänen and Winkler 1999). MMN is elicited even in the absence of attention, for example, in individuals in a coma a few days before the recovery of consciousness (Kane *et al.,* 1993), which indicates that MMN indexes pre-attentive (attention-independent) auditory processing. The automatic change-detection system in the human brain as reflected by the

Pre-Attentive Processing of Sound Duration Changes:

**4. General electrophysiological procedures** 

silent movie. Afterwards, they reported the impression of the movie.

right handed.

Low Resolution Brain Electromagnetic Tomography Study 225

The handedness of the participants was assessed with the Edinburgh Handedness Inventory (Oldfield, 1971). The degree of the right handedness of the subjects was assessed based upon ten items; writing; drawing; throwing; scissors; toothbrush; knife (without fork); spoon; broom; striking a match; and open box lid. The participant was instructed to make a "+" on which hand he/she would prefer to use for each action. They were instructed to mark a "++" when the preference was so strong that he/she never used the other hand unassisted. If, in any case, the participant did not have any preference, he/she was instructed to mark a "+" for both hands. The numbers of "+" marked for each hand were totaled. Then, a handedness index was calculated to be the difference of the numbers of "+"'s between the right and left hands divided by the total number of "+"'s for both hands. A handedness index of 1.0 indicated completely right handed, -1.0 corresponded to completely left handed, and 0 suggested ambidextrous. The participant was also asked which foot was preferred for kicking, which eye was preferred when only using one eye, and whether both parents were

Two different sounds duration were synthetically generated with short and long sounds. All of the stimuli were digitally edited to have an equal maximum energy level in dB SPL with the remaining intensity level within each of the stimuli scaled accordingly. The stimuli were digitally edited using the Cool Edit Pro v. 2.0 (Syntrillium Software Cooperation) with 500 ms duration (long sound) and 300 ms duration (short sound). All sounds were identical at their frequencies, thus eliminating any effect due to differences in frequency of occurrence of sound. The sounds were presented binaurally via headphones at a comfortable listening level of ~85 dB. The sound pressure levels of stimulus pairs were then measured at the output of headphones using a Brüel and Kjaer 2230 sound level meter. The standard (S)/deviant (D) pairs for each condition were [Condition 1: long-to-short sounds change] Standard/S-(2), Deviant/D-(1), [Condition 2: short-tolong sounds change] S-(1), D-(2). Thus, in both conditions pairs were designed to contrast short and long sounds. The stimuli were presented in a passive oddball paradigm. Deviant stimuli appeared randomly among the standards at 10% probability. Each condition included 125 deviants. The stimuli were binaurally delivered using SuperLab software (Cedrus Corporation, San Pedro, CA, USA) via headphones (Telephonic TDH-39-P). The inter-stimulus interval (ISI) was 1.25 second (offset-onset). EEG signal recording was time-locked to the onset of the sound. Participants were instructed not to pay attention to the stimuli presented via headphones, but rather to concentrate on a self-selected

Participants are instructed to sit relaxing in comfortable reclining chair in an electrically and acoustically dampened room. They were told that they would participate in the experiment and that the experimenter would be recording their brain electrical activity. They were given written instructions and provided with a grid for their judgements and a pen. They silently read the instructions and at the end the experimenter verifies that everything was clear. Their histories were taken, including age, educational level, handedness, occupation, current medications, medical history (which included past illness, surgical history, head trauma or accident) and history of alcohol consumption or smoking. If there was any significant history of neurological problems, psychiatric problems or head trauma, that participant was excluded. For the Mismatch Negativity (MMN) study, all participants were

MMN thus requires the storage of the previous state of the acoustic environment for detecting an incoming deviating sound (Näätänen, 1992; Brattico *et al.,* 2002). Furthermore, MMN implies the existence of an auditory sensory memory that stores a neural representation of a standard against which any incoming auditory input is compared (Ritter et al., 1995). In the auditory modality, MMN is an automatic process which occurs even when the subject's attention is focused away from the evoking stimuli (Näätänen, 1992). Its onset normally begins before the N2b-P3 complex which occurs when attention is directed to the stimuli. The duration of MMN varies with the nature of the stimulus deviance but it invariably overlaps N2b when the latter is present (Tales *et al.,* 1999)

The main neural generators of MMN are bilaterally located in the supratemporal plane (Alho, 1995), which is indicated by dipole modeling (Scherg *et al.,* 1989) and scalp current density map (Giard *et al.,* 1990) of scalp-recorded event-related potentials, as well as by magnetic recordings, intracranial MMN recordings in cats, monkeys and humans, and by positron emission tomography, functional magnetic resonance imaging, and optical imaging data. Furthermore, the exact locus of MMN in auditory cortex depends on the attribute (Giad *et al.,* 1995) (and even on the complexity of stimulus configuration (Alho *et al.,* 1996)) in which the change occurred. Therefore, one can conclude that the auditory processes that generate MMN originate, in the first place, in the auditory cortex. In addition, MMN also receives a contribution from a (mainly right hemispheric) frontal generator that appears to be triggered by this auditory-cortex change-detection process and be associated with the initiation of attention switch to the change (Escera *et al.,* 2000).

The present study compared preattentive brain processes during the discrimination of the different synthesized sounds duration. A single pair of the synthesized long and short sounds selected to represent ideal exemplars. This study chose to record and compared the MMN elicited by these synthesized sounds, hoping to find evidence for specific brain signatures of both synthesized long and short sounds processing in the human auditory cortex. Two questions were examined using this approach: (1) whether the MMN would index differences in the brain's discrimination of this different synthesized sounds duration; and (2) whether the MMN amplitude and/or latency would reflect acoustic differences between the rare deviant and the frequent standard stimuli. Additionally, the low-resolution electromagnetic tomography (LORETA) analysis were used to locate multiple non-dipolar sources particularly involved in the discrimination of these different synthesized sounds duration within the MMN paradigm.

#### **3. Participants, handedness and ethical consideration**

EEG recordings were collected from eleven healthy young, Thai-speaking adults (eight female) and their age range: 23-29 years. All participants were right-handedness assessed according to Oldfield (Oldfield, 1971). They had normal hearing, corrected to normal vision and had no history of neurological or psychiatric history. The mean (±sd) age was 25.73 (±3.1) years. The Ethics committees of the involved institutions accepted the study. The concept was explained to the participants, and written informed consent was obtained. All participants gave their written informed consent to participate in the experiments and were paid for their participation. The experiments were performed in accordance with the Helsinki Declaration. Ethical permission for the experiments was issued by the Committee on Human Rights Related to Human experimentation.

MMN thus requires the storage of the previous state of the acoustic environment for detecting an incoming deviating sound (Näätänen, 1992; Brattico *et al.,* 2002). Furthermore, MMN implies the existence of an auditory sensory memory that stores a neural representation of a standard against which any incoming auditory input is compared (Ritter et al., 1995). In the auditory modality, MMN is an automatic process which occurs even when the subject's attention is focused away from the evoking stimuli (Näätänen, 1992). Its onset normally begins before the N2b-P3 complex which occurs when attention is directed to the stimuli. The duration of MMN varies with the nature of the stimulus deviance but it

The main neural generators of MMN are bilaterally located in the supratemporal plane (Alho, 1995), which is indicated by dipole modeling (Scherg *et al.,* 1989) and scalp current density map (Giard *et al.,* 1990) of scalp-recorded event-related potentials, as well as by magnetic recordings, intracranial MMN recordings in cats, monkeys and humans, and by positron emission tomography, functional magnetic resonance imaging, and optical imaging data. Furthermore, the exact locus of MMN in auditory cortex depends on the attribute (Giad *et al.,* 1995) (and even on the complexity of stimulus configuration (Alho *et al.,* 1996)) in which the change occurred. Therefore, one can conclude that the auditory processes that generate MMN originate, in the first place, in the auditory cortex. In addition, MMN also receives a contribution from a (mainly right hemispheric) frontal generator that appears to be triggered by this auditory-cortex change-detection process and be associated with the

The present study compared preattentive brain processes during the discrimination of the different synthesized sounds duration. A single pair of the synthesized long and short sounds selected to represent ideal exemplars. This study chose to record and compared the MMN elicited by these synthesized sounds, hoping to find evidence for specific brain signatures of both synthesized long and short sounds processing in the human auditory cortex. Two questions were examined using this approach: (1) whether the MMN would index differences in the brain's discrimination of this different synthesized sounds duration; and (2) whether the MMN amplitude and/or latency would reflect acoustic differences between the rare deviant and the frequent standard stimuli. Additionally, the low-resolution electromagnetic tomography (LORETA) analysis were used to locate multiple non-dipolar sources particularly involved in the discrimination of these different synthesized sounds

EEG recordings were collected from eleven healthy young, Thai-speaking adults (eight female) and their age range: 23-29 years. All participants were right-handedness assessed according to Oldfield (Oldfield, 1971). They had normal hearing, corrected to normal vision and had no history of neurological or psychiatric history. The mean (±sd) age was 25.73 (±3.1) years. The Ethics committees of the involved institutions accepted the study. The concept was explained to the participants, and written informed consent was obtained. All participants gave their written informed consent to participate in the experiments and were paid for their participation. The experiments were performed in accordance with the Helsinki Declaration. Ethical permission for the experiments was issued by the Committee

invariably overlaps N2b when the latter is present (Tales *et al.,* 1999)

initiation of attention switch to the change (Escera *et al.,* 2000).

**3. Participants, handedness and ethical consideration** 

on Human Rights Related to Human experimentation.

duration within the MMN paradigm.

The handedness of the participants was assessed with the Edinburgh Handedness Inventory (Oldfield, 1971). The degree of the right handedness of the subjects was assessed based upon ten items; writing; drawing; throwing; scissors; toothbrush; knife (without fork); spoon; broom; striking a match; and open box lid. The participant was instructed to make a "+" on which hand he/she would prefer to use for each action. They were instructed to mark a "++" when the preference was so strong that he/she never used the other hand unassisted. If, in any case, the participant did not have any preference, he/she was instructed to mark a "+" for both hands. The numbers of "+" marked for each hand were totaled. Then, a handedness index was calculated to be the difference of the numbers of "+"'s between the right and left hands divided by the total number of "+"'s for both hands. A handedness index of 1.0 indicated completely right handed, -1.0 corresponded to completely left handed, and 0 suggested ambidextrous. The participant was also asked which foot was preferred for kicking, which eye was preferred when only using one eye, and whether both parents were right handed.
