The attentional selection of spatial and non-spatial attributes in touch: ERP evidence for parallel and independent processes
Introduction
While selective attention has been studied intensively in the visual and auditory modality, the processes underlying attentional selectivity in somatosensation are still poorly understood. The aim of the ERP study reported here was to investigate the attentional selection of spatial and non-spatial attributes of tactile stimuli, in order to obtain insights into the mechanisms involved in the selective attentional processing of different tactile stimulus attributes.
Directing visual attention in space results in faster detection and more accurate discrimination of visual stimuli at currently attended locations (Posner et al., 1980). ERP studies have provided evidence that early stages of visual processing are modulated by visual-spatial attention (Mangun and Hillyard, 1990). Enhanced early sensory-specific ERP components (P1 and N1) are elicited over posterior visual cortical areas in response to visual stimuli at currently attended as compared to unattended locations (e.g. Mangun, 1995, Mangun et al., 1987, Mangun and Hillyard, 1987, Eimer, 1995), and these amplitude modulations start as early as 80 ms after stimulus onset. In contrast, visual attention to non-spatial features, such as colour or shape, result in an enhanced sustained negativity that starts much later (at about 150 ms post-stimulus; e.g. Eimer, 1995, Heslenfeld et al., 1997).
To investigate whether the selective attentional processing of non-spatial visual stimulus attributes depends on prior selection for spatial location, Hillyard and Münte (1984) presented their participants with visual stimuli defined by both location and color. The participants’ task was to detect infrequent target stimuli that matched both the attended color and the attended location. ERP waveforms elicited by non-target stimuli with the attended color and/or location were compared to non-targets with the unattended color and/or location. ERPs elicited by stimuli at attended locations showed enhanced P1, N1 and N2 components compared to stimuli at unattended locations, and these spatial attention effects were independent of stimulus colour. Effects of attention directed to colour were associated with a broad negativity starting around 150 ms after stimulus onset. Most importantly, these ERP effects of selective colour processing were only found in response to stimuli at the attended location, thus suggesting that spatially selective processing not only precedes the selective processing of non-spatial attributes of visual stimuli (i.e. color), but also that non-spatial attention effects are contingent upon the prior selection of visual stimuli on the basis of their location (see also Eimer, 1995, Omoto et al., 2001).
Similar to vision, selective spatial attention also facilitates auditory processing (Spence and Driver, 1994, Spence and Driver, 1998). However, recent behavioral studies have shown that the attentional processing of auditory stimulus locations is influenced by stimulus frequency even when frequency is irrelevant (Mondor et al., 1998), and that auditory targets defined by frequency or by the conjunction of frequency and location are identified faster than targets defined by location only (Woods et al., 2001). These findings suggest that unlike in vision, the attentional selection of spatial does not precede the selection of non-spatial attributes of auditory stimuli. To examine the mechanisms underlying the selective processing of auditory spatial and non-spatial attributes, Woods and Alain (2001) recorded ERPs in response to tones defined by a combination of location, frequency and duration, under conditions where targets were defined by a specific combination of location, frequency and duration attributes. When ERP waveforms elicited by stimuli possessing none of the attended attributes with ERPs elicited by stimuli possessing one, two or all of the attended attributes were compared, both attention to location and attention to frequency were reflected in enhanced early attentional negativities starting at about 60 ms post-stimulus. In line with the behavioural evidence described above, these results suggest that the attentional selection of spatial and non-spatial auditory stimulus attributes takes place in parallel and independently (see also Hansen and Hillyard, 1983, Woods et al., 1994).
Only few ERP studies to date have investigated mechanisms of selective attention in touch. In most studies on tactile spatial attention, participants were instructed to attend to one hand versus the other while electrical or mechanical stimuli were delivered to the left or right hand. Spatial attention was found to modulate the sensory-specific somatosensory N140 component, with enhanced N140 amplitudes for tactile stimuli delivered to the attended hand (Desmedt and Robertson, 1977, Michie et al., 1987, Garcia-Larrea et al., 1995, Eimer and Driver, 2000, Eimer et al., 2001, Eimer et al., 2002, Eimer and Forster, 2003). In addition, effects of tactile spatial attention on the somatosensory P100 component (Josiassen et al., 1982, Michie, 1984, Eimer and Forster, 2003), and a later sustained processing negativity (Desmedt and Robertson, 1977, Michie, 1984) have also been observed. The fact that tactile spatial attention affects early, sensory-specific somatosensory ERP components clearly suggests that, analogous to visual and auditory spatial attention, spatial selectivity in touch can modulate relatively early perceptual stages of somatosensory processing.
ERP correlates of non-spatial attentional selectivity in touch have not yet been investigated systematically. Michie et al. (1987) found that ERP effects of tactile spatial attention were modulated by a non-spatial attribute (intensity) defining infrequent target stimuli. In this experiment, participants had to report infrequent weak stimuli delivered among stronger stimuli, or infrequent strong stimuli delivered among weaker stimuli, when these infrequent targets were presented to the currently attended hand. Spatial attention affected the N80 component when weak stimuli had to be detected among strong non-targets, and an effect of spatial attention on the P105 when infrequent strong targets were presented among weak non-targets. However, ERP correlates of tactile attention directed to stimulus intensity were not measured directly in this study.
The present experiment was designed to investigate for the first time ERP correlates of selective attention directed to non-spatial attributes of tactile stimuli, as well as the relationship between spatial and non-spatial selective attention in somatosensation. Mechanical vibratory tactile stimuli were delivered to the left or right hand, and these stimuli also differed with respect to one non-spatial attribute. In Experiment 1, stimulus frequency was either low or high. In Experiment 2, stimulus intensity was either strong or soft. Participants’ task was to direct attention to one side, and to one non-spatial stimulus attribute, in order to detect and respond to infrequent targets (which were longer than non-targets) when these possessed both relevant attributes. Thus, in separate blocks, selective attention was directed to one of the four possible combinations of location (left versus right) and frequency (high versus low, in Experiment 1) or intensity (strong versus soft, in Experiment 2). ERPs were computed in response to non-targets possessing both, one, or neither of these task-relevant attributes.
In vision, ERP effects of spatial attention precede ERP effects of non-spatial attentional selectivity, and ERP correlates of the selective processing of a non-spatial visual attribute are only observed in response to visual stimuli at currently attended locations, thus demonstrating that the attentional processing of non-spatial visual features is hierarchically dependent on prior selection of stimulus location (Hillyard and Munte, 1984, Eimer, 1995). If the same hierarchical dependency also exists for selective attention in the somatosensory modality, ERP effects of attention directed to stimulus frequency or intensity should emerge later than ERP effects of tactile-spatial attention, and any effects of non-spatial attention should only be present for tactile stimuli presented to the currently attended hand. In audition, the attentional processing of spatial and non-spatial stimulus attributes appears to be based on parallel and independent mechanisms (Woods et al., 1994, Woods et al., 2001, Woods and Alain, 2001). If this was also the case for somatosensation, ERP modulations reflecting attention to spatial and non-spatial tactile stimulus attributes should be elicited concurrently. More importantly, ERP correlates of non-spatial tactile attention should not only be elicited in response to stimuli delivered to the currently attended hand, but also for tactile stimuli presented to the unattended hand.
Section snippets
Participants
Seventeen paid volunteers participated in Experiment 1. Two participants had to be excluded because of too many eye-blinks contaminating more than 70% of trials. Three additional subjects had to be excluded because of a lack of clearly defined early somatosensory components (see below). Thus 12 participants (4 males, 8 females), aged 19–41 years (mean age: 25.6 years) remained in the sample. All participants were right-handed and had normal or corrected vision.
Stimuli and apparatus
Participants sat in a dimly lit
Experiment 2
This experiment was conducted to provide additional evidence for the independence of spatial and non-spatial selectivity in touch. Now, stimulus frequency was kept constant, while stimulus intensity was varied. Thus, participants had to attend to specific combinations of stimulus location (left versus right) and stimulus intensity (strong versus soft) to detect infrequent targets characterized by these two features. In all other respects, design and procedure was identical to Experiment 1.
Acknowledgements
This research was supported by a grant from the Biotechnology and Biological Science Research Council (BBSRC).
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