We investigate whether the neural correlates of the continuity illusion, as measured using fMRI, are modulated by attention. As we have shown previously, when two formants of a synthetic vowel are presented in an alternating pattern, the vowel can be identified if the gaps in each formant are filled with bursts of plausible masking noise, causing the illusory percept of a continuous vowel (“Illusion” condition). When the formant-to-noise ratio is increased so that noise no longer plausibly masks the formants, the formants are heard as interrupted (“Illusion Break” condition) and vowels are not identifiable. A region of the left middle temporal gyrus (MTG) is sensitive both to intact synthetic vowels (two formants present simultaneously) and to Illusion stimuli, compared to Illusion Break stimuli. Here, we compared these conditions in the presence and absence of attention. We examined fMRI signal for different sound types under three attentional conditions: full attention to the vowels; attention to a visual distracter; or attention to an auditory distracter. Crucially, although a robust main effect of attentional state was observed in many regions, the effect of attention did not differ systematically for the illusory vowels compared to either intact vowels or to the Illusion Break stimuli in the left STG/MTG vowel-sensitive region. This result suggests that illusory continuity of vowels is an obligatory perceptual process, and operates independently of attentional state. An additional finding was that the sensitivity of primary auditory cortex to the number of sound onsets in the stimulus was modulated by attention.

We used functional magnetic resonance imaging to study the neural processing of vowels whose perception depends on the continuity illusion. Participants heard sequences of two-formant vowels under a number of listening conditions. In the “vowel conditions,” both formants were always present simultaneously and the stimuli were perceived as speech-like. Contrasted with a range of nonspeech sounds, these vowels elicited activity in the posterior middle temporal gyrus (MTG) and superior temporal sulcus (STS). When the two formants alternated in time, the “speech-likeness” of the sounds was reduced. It could be partially restored by filling the silent gaps in each formant with bands of noise (the “Illusion” condition) because the noise induced an illusion of continuity in each formant region, causing the two formants to be perceived as simultaneous. However, this manipulation was only effective at low formant-to-noise ratios (FNRs). When the FNR was increased, the illusion broke down (the “illusion-break” condition). Activation in vowel-sensitive regions of the MTG was greater in the illusion than in the illusion-break condition, consistent with the perception of Illusion stimuli as vowels. Activity in Heschl's gyri (HG), the approximate location of the primary auditory cortex, showed the opposite pattern, and may depend instead on the number of perceptual onsets in a sound. Our results demonstrate that speech-sensitive regions of the MTG are sensitive not to the physical characteristics of the stimulus but to the perception of the stimulus as speech, and also provide an anatomically distinct, objective physiological correlate of the continuity illusion in human listeners.

A sound turned off for a short moment can be perceived as continuous if the silent gap is filled with noise. The neural mechanisms underlying this “continuity illusion” were investigated using the mismatch negativity (MMN), an eventrelated potential reflecting the perception of a sudden change in an otherwise regular stimulus sequence. The MMN was recorded in four conditions using an oddball paradigm. The standards consisted of 500-Hz, 120-msec tone pips that were either physically continuous (Condition 1) or were interrupted by a 40-msec silent gap (Condition 2). The deviants consisted of the interrupted tone, but with the silent gap filled by a burst of bandpass-filtered noise. The noise either occupied the same frequency region as the tone and elicited the continuity illusion (Conditions 1a and 2a), or occupied a remote frequency region and did not elicit the illusion (Conditions 1b and 2b). We predicted that, if the continuity illusion is determined before MMN generation, then, other things being equal, the MMN should be larger in conditions where the deviants are perceived as continuous and the standards as interrupted or vice versa, than when both were perceived as continuous or both interrupted. Consistent with this prediction, we observed an interaction between standard type and noise frequency region, with the MMN being larger in Condition 1a than in Condition 1b, but smaller in Condition 2a than in Condition 2b. Because the subjects were instructed to ignore the tones and watch a silent movie during the recordings, the results indicate that the continuity illusion can occur outside the focus of attention. Furthermore, the latency of the MMN (less than approximately 200 msec postdeviance onset) places an upper limit on the stage of neural processing responsible for the illusion.

Unilateral neglect is frequently characterized by the presence of extinction, which is a lack of awareness to contralesional visual stimuli in the presence of those further towards the ipsilesional side. It has been established that this visual extinction can be reduced if the stimuli are grouped together into a single object. However, attention between and within auditory objects has never before been studied. We demonstrate for the first time that unilateral neglect—hitherto thought primarily to be a disorder of visuospatial processing—involves a specific deficit in allocating attention between auditory objects separated only in time and not in space. Importantly, this deficit is restricted to comparisons between sounds: the patients' ability to make within-sound comparisons is similar to that of controls. These differences cannot be explained in terms of different time spans over which comparisons must be made. The results suggest unilateral neglect is linked to—if not actually determined by—a reduction in attentional capacity in both the visual and auditory domains, and across the dimensions of both space and time. The findings have potential clinical applications.