Spatial attention speeds discrimination without awareness in blindsight
Introduction
James (1890) was one of the first authors to point out the close relationship between selective attention and consciousness. It is often assumed that this relationship is one of identity—what one attends to and what one is conscious of being one and the same thing—although James himself did not believe this (see Hardcastle, 1997). Attention and consciousness can be related, but not in identity, if attention is either not necessary for awareness (one can be conscious of things to which one is not attending) or if attention is not sufficient for awareness (one can attend to a thing without becoming aware of it). Paradigms such as inattentional blindness (Mack & Rock, 1998) and change blindness (e.g. Rensink, O’Regan, & Clark, 1997) suggest than attention is necessary for awareness (at least insofar as it is possible to withdraw attention absolutely from parts of the visual scene). We recently carried out a study with a patient with the neurological condition of blindsight in order to test the second possibility, that attention may not be a sufficient condition for awareness, and to test whether cues which do not themselves elicit any conscious response can nevertheless capture attention (Kentridge, Heywood, & Weiskrantz, 1999a). The present follow-up experiment reported here focuses on the question of the sufficiency condition, and addresses a methodological issue that could make the interpretation of our earlier results ambiguous.
Blindsight is a condition in which patients with damage to their primary visual cortex or its afferents retain the ability to detect, discriminate and localise visual stimuli presented in areas of their visual field in which they nevertheless report that they are subjectively blind (see e.g. Weiskrantz, 1986). In a previous report (Kentridge et al., 1999a) we demonstrated that the blindsight subject GY showed a reaction-time (RT) advantage for the detection of targets presented in validly spatially cued (as opposed to misleadingly cued) locations within his blind field, using the classic methods of Posner (1980). As this performance advantage was not accompanied by awareness of targets in the cued location we suggested that attention to a target (reflected in the selective RT advantage) was not a sufficient condition for awareness of that target.
There is one potential problem with our interpretation. As we used a detection task with a single level of target probability throughout the experiment we could not distinguish between effects of cueing on response bias and discrimination. Although we could show that the speeding of RT to validly cued targets was not at the expense of a decreased likelihood of detecting targets, we could not show that the ability to discriminate the presence or absence of targets was unaffected. It may have been the case that fast RTs were only obtained at the expense of a decrease in accuracy, but that this was obscured by a concomitant change in response bias to any signal at the cued location. But, because it makes no sense to say that the cue in a trial where no target is presented is valid or invalid, it is not possible to measure this bias. It might, for example, have been the case that all of the false-positive responses were the results of misperception of targets at the cued location, but we could not assess this using a detection paradigm. While we feel this explanation is unlikely to account for our results, the implications of our conclusion are broad enough that any doubt needs to be resolved (see e.g. comments in Block, 2000, Lamme & Roelfsema, 2000, Zeman, 2001). We therefore modified our design to one in which the subject’s task is one of discrimination rather than detection. In a discrimination task there need be no trials on which targets are absent so cue-validity is meaningful on every trial and the effect of any bias towards one particular response can be controlled for by counterbalancing. The current experiment therefore tests whether the RT advantages seen in the results of Kentridge et al. (1999a) were likely to be a result of speed-error trade-off or a result of spatially selective attention.
In addition to the key change in task, from detection to discrimination, in the current experiment we also now vary the interval between presentation of cues and targets. It is known that in an endogenous cueing task, such as the one we use here, the effectiveness of cues in normal subjects increases as the interval between onset of the cue and onset of the target (stimulus onset asynchrony, SOA) increases, reaching a maximum at an SOA of about 300 ms (see e.g. Müller & Rabbit, 1989). We therefore randomly interleaved trials with SOAs of 200 and 450 ms in order to assess whether a similar variation in cue effectiveness could be found in GY.
Finally, we did not ask GY to make trial-by-trial commentary on target awareness in the current experiment, to simplify the response requirements, in case making a commentary decision interfered with his response to the orientation of the target. Instead, we obtained verbal reports of awareness in pre-tests at a number of contrasts and conducted two blocks of trials in which both discrimination and commentary responses were collected without an instruction to respond rapidly, in order to establish a level of contrast at which we expected targets to elicit no awareness whatsoever. RT data were then collected without trial-by-trial commentary, but with verbal report of any awareness at the end of each block.
Section snippets
Subject
GY, a 41-year-old man, has been fully reported elsewhere (Barbur, Ruddock, & Waterfield, 1980; Baseler, Morland, & Wandell, 1999; Blythe, Kennard, & Ruddock, 1987). He suffered unilateral damage to left striate cortex, confirmed by computerised tomography (Blythe et al., 1987) and magnetic resonance imaging (Barbur et al., 1980), as a result of a car accident at the age of 8 years. He has a right homonymous hemianopia but retains about 3° of macular sparing, consistent with the damage revealed
Pre-test contrast-level setting
GY initially performed three short, 20 trial practice sessions in which the peak Michelson contrasts of the target were 100, 60 and 15%. He reported his target awareness as being “aware of about 40%, mainly in the upper location” for the 100% contrast block, “only aware of one or two” for the 60% contrast block and “nothing is happening—I have no awareness or experience” during the 15% contrast block.
Two blocks (160 trials per block) were conducted in which a trial-by-trial commentary procedure
Discussion
It is not possible to explain the RT advantage accruing to validly cued trials in terms of a trade-off against accuracy accompanied by a shift in bias. GY is clearly no less accurate (if anything he is more accurate) and is quicker at making orientation discriminations in validly cued locations. In conjunction with the results our earlier study the evidence is now very strong that a performance advantage can accrue to processing of stimuli presented at a cued location without those stimuli
Acknowledgements
The authors are grateful to GY for his cooperation, and to the Medical Research Council (grant G0000679) for their financial support of this research. RWK is a University of Durham Sir Derman Christopherson Foundation Fellow.
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