Simultaneously active pre-attentive representations of local and global rules for sound sequences in the human brain

Abstract

Regular sequences of sounds (i.e., non-random) can usually be described by several, equally valid rules. Rules allowing extrapolation from one sound to the next are termed local rules, those that define relations between temporally non-adjacent sounds are termed global rules. The aim of the present study was to determine whether both local and global rules can be simultaneously extracted from a sound sequence even when attention is directed away from the auditory stimuli. The pre-attentive representation of a sequence of two alternating tones (differing only in frequency) was investigated using the mismatch negativity (MMN) auditory event-related potential. Both local- and global-rule violations of tone alternation elicited the MMN component while subjects ignored the auditory stimuli. This finding suggests that (a) pre-attentive auditory processes can extract both local and global rules from sound sequences, and (b) that several regularity representations of a sound sequence are simultaneously maintained during the pre-attentive phase of auditory stimulus processing.

Introduction

In everyday life, sounds are usually encountered within sequences of inter-related auditory events. Therefore, the sequential organization of sensory information is a key element of auditory perception. Organization is achieved through the formation of ‘links between parts of the sensory data’ (Bregman, 1990, p. 47 [4]), which gives rise to the segmentation of the auditory environment. The processes associating sounds according to various auditory organization principles could take place at different stages of information processing. Bregman’s [4] theory of auditory stream segregation assumes that several of the possible ways of linking sounds together are examined in parallel during the early, possibly pre-attentive phases of auditory information processing. We tested the hypothesis that both the local and global types of links are formed during the pre-attentive stages of auditory processing.

Regular sequences of sounds (i.e., non-random) can usually be characterized by more than one rule. One may classify these rules on the basis of their temporal scope. Local rules describe relations between temporally adjacent stimuli, global rules describe relations between temporally non-adjacent stimuli [4]. Simple sound sequences can be described in terms of both local and global rules. For example, the local rule for a sequence of two alternating tones (‘…ABABAB…’) would say that A is always followed by B and B is always followed by A. One possible global rule describing the same sequence would maintain that every second tone is A while every other is B.

Another categorization of rules can be made on the basis of whether a rule is concrete (i.e., enables extrapolating from a specific sound to another specific sound) or abstract (i.e., enables extrapolating from one class of sounds to another class of sounds; abstract rules define relationships between sets of sounds). In the above example of the two alternating tones, both rules are concrete as they apply to the two specific tones of the alternating sequence. An abstract rule describing (frequency) alternation, in general, would state that the two neighbors of a sound in the sequence have either both higher or both lower pitch than the sound itself.

Finally, a distinction which is relevant for theories of auditory information processing: a rule may apply to stimuli as integrated events (‘event-rules’) or only to certain sound features (‘feature-rules’). Local and global, concrete and abstract rules may apply either to integrated sound events or to levels of some sound feature(s). Our first pair of examples were event-rules; the abstract rule example was a feature rule. A global concrete feature rule, which also applies to simple alternation, is that the time from the onset of a tone to the onset of the next identical tone is constant at a given stimulus onset asynchrony (SOA) value.

The systematic formation of associations between sounds in regular sequences can be regarded as a neural representation of a rule. The question of what kind of rules are represented for a given sound sequence in the absence of focused attention by the human auditory system can be studied using an event-related brain potential (ERP) component termed the mismatch negativity (MMN) [14] (for recent reviews, see Refs. [28], [16]). MMN is elicited by sounds that violate some regularity of the preceding auditory stimulus sequence, whether or not the subject’s attention is focused on the auditory modality [13]. It has been established that the discriminative MMN-generating process involves an auditory sensory memory representation of the preceding regular sounds [12], [21], [16]. Winkler et al. [36] suggested that MMN elicitation results from the incoming sound mismatching the one(s) extrapolated from the preceding auditory stimulus sequence. Therefore, if a sound elicits the MMN, one can infer that some regularity in the preceding sound sequence was pre-attentively detected (this regularity being violated by the MMN-eliciting auditory stimulus). Thus MMN enables one to determine what rules are pre-attentively represented by the auditory system for a given sound sequence. The regularities which have been shown to be detected during the pre-attentive phase of auditory information processing range from simple repetition rules (e.g., Ref. [14]) to sensory trends [33] and regularities of complex sequential or spectro-temporal sound patterns (e.g., Refs. [18], [25]). MMNs elicited by violations of concrete as well as abstract rules (e.g., Refs. [14] and [23], respectively), event as well as feature rules (e.g., Refs. [37] and [8], respectively) have already been observed. However, no previous study has made a direct test of whether global rules can be represented nor did they distinguish between repeating patterns and the global structure of non-adjacent tones.