TESTING THE CUE VALIDITY

Posner (1978) introduced the cue validity paradigm and proposed that attention could be oriented to a specified peripheral location independently of head and eye movements, and thereby improve the processing of further stimuli presented at that location. Several studies have investigated variables within the paradigm in the visual modality and reported the same consistent effects (see Chapter 1 for relevant references). Much less success has been found when attempting to replicate the covert orienting effect in the auditory modality. In these studies that have addressed covert orienting in audition, there has been little consistency in the findings. Furthermore, few have been confident about the equivalence of an auditory version of the original visuo- spatial paradigm.

Posner (1978) failed to produce a cueing effect for auditory targets following the presentation of informative central cues. Scharf, Quiley et el.

(1987) failed to identify any advantages for the detection of sounds coming from unexpected versus expected locations. While Buchtel & Butter (1988) confirmed that an auditory spatial cue could be as effective as a visual cue in shifting attention covertly in response to a visual target, the converse (using the same cues to shift attention covertly to an auditory target) did not apply. In fact Buchtel and Butter (1988) argue that covert orienting in audition may never be demonstrated since there is no auditory equivalent of the fovea or the fingertip. They argue that response times to auditory targets cannot be influenced by pre-cueing because the human auditory modality does not have a "natural reflexive motor response" that could serve to improve auditory analysis.

In a subsequent study Butter et al. (1989) investigated whether tactile stimuli were capable of covert shifts of attention similar to the visual modality. They hypothesised that "the effects of cue validity on reaction times depend on the target's sensory modality possessing a spatial acuity gradient" (p. 1238). In this sense they expected the tactile modality to be as effective as the visual modality since "tactile stimuli can evoke orienting movements of the hand and other body parts in order to identify a tactile stimulus by bringing more sensitive portions of the skin (e.g. fingertips) to bear on the object" (p. 1231). The findings of their study confirmed that tactile stimuli, unlike auditory stimuli could produce an effect upon spatial orienting, though the effect was not as robust as reported for visual stimuli. Butter et el. (1989) therefore support the supposition that the covert spatial shifts of attention can only be achieved when the target modality is linked to an orienting system which controls movement (e.g. eye movement). While they accept Posner's (1980) point that "attention is not intrinsically tied to the fovea I structure of the visual system nor slaved to the overt movements of the eye" (p.22), Buchtel et al. argue that while overt and covert processes may be functionally separable there is very close relationship between them. In essence, they argue than unless the target

modality is linked to an orienting system, spatial cueing effects cannot be demonstrated. This is quite a plausible interpretation of their results, but it is certainly not the whole story since some positive cueing effects in the auditory modality have been reported.

Mazzucchi et al. (1983) reported a small cost (6 msec) for invalid verbal auditory cues to monaural clicks, though no benefit when the same cues were valid. Bedard et al. (1993) also reported positive findings; response times to target tones were shortest when the visual cues were valid. They reported that "a benefit using monaural stimulation has confirmed that lateralised engagement of auditory attention is possible" (p.247). They attributed the negative findings reported by others to have been induced by the use of loud speakers instead of headphones, thereby reducing the focus of attention to specific locations.

Spence and Driver (1994) also report positive results for covert orienting in audition. They comment, "it is clear that endogenous mechanisms of spatial auditory attention must exist at some level of processing; otherwise, we could not achieve a number of textbook feats such as shadowing a message from one location while ignoring a message from a different location" (p.556). In a series of experiments designed to investigate the sensitivity of exogenous (an event which does not predict the target location) and endogenous (an event which indicates the location of the target stimulus) mechanisms to covert shifts of attention in audition, they reported that spatial auditory cues (both uninformative and predictive) produced a validity effect on auditory localisation responses. In contrast to the findings reported by Buchtel

et al. (1988) and Butter et al. (1989), Spence and Driver (1994) conclude that their experiments "clearly establish that covert orienting does take place in the human auditory system and that it can influence localisation in the exogenous

case and both localisation and pitch discrimination in the endogenous case" (p. 574).

There is one major shortcoming in the majority of these studies. Researchers have measured the success or failure of covert orienting in audition by whether the same cues capable of producing covert orienting in vision produce a comparable effect in audition. Buchtel and Butter (1988) argue that their failure to elicit a cue validity effect in audition could not be attributed to the cue since the very same cues had produced a covert orienting effect in vision. Of course, this is a methodologically sound approach, but it might be the case that covert orienting in audition is a cue dependent effect. This proposition is congruent with the findings of experiments 4 & 5. The auditory modality appears highly sensitive to verbal cues specifying a spatial location. This is true, irrespective of whether subjects are attending to the spatial location or semantic properties of the cue, although the cost and benefit to invalid and valid trials is greater if semantic content is the attended dimension.

A straight-forward explanation of this effect is to say that while an arrow or sensory brightening might elicit a greater validity effect in vision, it is certain from the findings of the current study that verbal cues (presented visually or aurally) elicit a more consistent cue validity effect in audition. This result, while novel, is hardly surprising given the obvious differences underlying the neural apparatus of these sensory systems. Neumann (1986) makes an interesting point which helps to explain the fragility of the phenomenon. He comments "Visual information is already spatially sorted out on the retina: in the ear sound is not. Visual information processing is intrinsically spatial; spatial direction is merely one of the computable properties of auditory spatial information" (p. 185). It is conceivable therefore, that eliciting spatial cueing might require fundamentally different cueing properties in vision and audition.

Yet, it might also be the case that the mechanisms responsible for the spatial cueing effects in vision and audition are functionally different if not separate.

So far, the evidence presented highlights the fact that eliciting covert orienting in audition is a delicate procedure and is sensitive to the characteristics of the cue. If we are to further question whether the mechanisms are functionally separate, then we might be tempted to look at whether speed of response in the VA & AV conditions were the same or different. This is the subject matter of the next section in which the nature of covert orienting across modalities is addressed.