EFFECTS OF SPATIAL ATTENTION ON TACTILE PROCESSING: A SPEED-ACCURACY ANALYSIS Miao-Fen Wang and Lawrence E. Marks

Does attention to spatial cues affect tactile processing? Subjects received 60-Hz mechanical sinusoids at levels near 15 dB SL, delivered with equal probabilities to the left and right hands. Spatial attention was directed by a visual cue: an arrow that pointed left or right. The cue was valid (pointed in the direction of the hand that would receive the stimulus) on 75% of the trials, invalid on 25%. The subjects’ task was to identify, as quickly as possible, the intensity of the stimulus – as low (press one pedal with left foot) or high (press another pedal with right foot). Latency operating characteristics (LOCs) for each subject quantified the trade-off between response time and accuracy. Seven of the eight subjects accumulated information more rapidly on valid compared to invalid trials (slopes of LOC were greater), thereby indicating the benefits of directed spatial attention. Cues that provide information about the likely location of subsequent target stimuli can improve both detection and discrimination of the targets. For example, the spatial cuing paradigm of Posner and his colleagues (Posner, Snyder, & Davidson, 1980) has been used to show that response time (RT) is smaller when the cue is valid rather than invalid, that is, responding is quicker when the cue correctly predicts the location of the target stimulus. In this paradigm, average RT is commonly used to measure performance. The use of RT to gauge performance is based on two assumptions: first, that superior performance is marked by faster (neural) processing of sensory information; and second, that average RT provides a valid representation of speed of sensory information processing. This approach rests, however, on a third, implicit assumption, namely, that both the time for decisional processes and the criterial amount of information required before responding is initiated are fixed. If this assumption is not correct – if decisional processes or response criterion varies across conditions, for example, differing when spatial cues are valid versus invalid – then to this extent one cannot attribute variations in RT to variations in speed of sensory information processing. An attentional cue might, for example, reduce the criterial amount of information needed to initiate a response, and therefore reduce RT, without affecting the ‘quality’ of the information or the rate at which the information accrues. The potential problems are exacerbated when accuracy is very high, as it may be difficult to discern any differences in accuracy across conditions. To distinguish the rate of temporal processing of sensory information from decisional and criterial processes, Lappin and Disch (1972a,b) proposed the use of the latency operating characteristic (LOC). The LOC provides a measure – applicable in principle to individual subjects as well as group averages – of the way that discriminative performance improves as latency to respond (RT) increases. That is, the LOC represents the speedaccuracy trade-off. In doing so, the LOC essentially capitalizes on the way that accuracy (e.g., d’) as well as speed covary as the decisional criterion for response varies. Lappin and Disch (1972a) showed, for example, that stimulus probability does not affect the LOC. While varying stimulus probability might affect average RT, it would at the same time also affect average accuracy. Lappin and Disch formulated the function for the LOC as linear: d’ = m (RT – k). In this study, we ask a relatively simple question: Does directing spatial attention to one site or another on the body surface improve the processing of non-spatial dimensions of tactile targets presented to a validly cued location, relative to an invalidly cued location? There is considerable evidence implying that the two hands, or different fingers on the same hand, can serve as separate informational channels (e.g., Craig, 1985; Horner, 1992; Sathian & Burton, 1991; Whang, Burton, & Shulman, 1991). Nevertheless, there appear to be many conditions under which spatial attention is lacking or weak – or, more precisely, where benefits and/or costs of manipulating spatial attention are absent or small (e.g., Evans & Craig, 1991; Shiffrin, Craig, & Cohen, 1973; Sathian & Burton, 1991; Whang et al., 1991). Interpreting many previous studies of cued, endogenous (voluntary) tactile spatial attention (e.g., Bradshaw, Bradshaw, Pierson-Savage, & Nettleton, 1988; Butter, Buchtel, & Santucci 1989; Forster & Eimer, 2005). Honoré, Bourdeaud'hui, & Sparrow 1989; Pierson, Bradshaw, Meyer, Howard, & Bradshaw, 1991) is difficult, however, because these studies have generally failed to control or measure the possible role of variations in decision criterion. Consequently, in the present study we take a different tack. Instead of using average RT to gauge performance, we capitalize on the methodological and analytic approach of Lappin and Disch (1972a,b) to derive latency operating characteristics under conditions in which cues for spatial location are usually valid but sometimes invalid. In this study, the subject’s task is to identify each stimulus as weak or strong in amplitude, while stimuli may be presented to either the left or right hand. The question we ask is whether and how a spatial cue affects both speed and accuracy of responding to differences in stimulus amplitude.