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Experiment Author: Susan Campbell. Adapted from STEP and used with permission of Brian MacWhinney
Participants are given a cue as to the location in the visual field where a stimulus will appear. In some blocks, they are told which location is most likely (probability of 79 out of 120) for that block, and in others, the location is cued at the beginning of each trial.
Targets are X'es in 4 numbered positions across the screen, with a fixation point in the middle and a cue at the bottom of the display. There are also catch trials where no target is present. Participants tend to respond quickest to cues that come at the beginning of a trial, then to those at the beginning of a block, then to un-cued stimuli.
In this experiment participants respond to stimuli that appear in one of four pre-determined locations for each trial. X'es appear in one of four numbered positions on the screen. Press the spacebar if an "X" appears on screen. Some trials are blank with no target stimuli. Do not press any key in this case. This experiment has 100 trials.
Posner, M.I., Snyder, C.R.R., Davidson, B.J. (1980) Attention and the detection of signals. Journal of Experimental Psychology: General 109, 160-174.
Cited Experiment Abstract
Detection of a visual signal requires information to reach a system capable of eliciting arbitrary responses required by the experimenter. Detection latencies are reduced when subjects receive a cue that indicates where in the visual field the signal will occur. This shift in efficiency appears to be due to an alignment (orienting) of the central attentional system with the pathways to be activated by the visual output.
It would also be possible to describe these results as being due to a reduced criterion at the expected target position. However, this description ignores important constraints about the way in which expectancy improves performance. First, when subjects are cued on each trial, they show stronger expectancy effects than when a probability position is held constant for a block, indicating the active nature of the expectancy. Second, while information on spatial position improves performance, information of the form of the stimulus does not. Third, expectancy may lead to improvements in latency without a reduction in accuracy. Fourth, there appears to be little ability to lower the criterion at two positions that are not spatially contiguous.
A framework involving the employment of a limited-capacity attentional mechanism seems to capture these constraints better than the more general language of criterion setting. Using this framework, we find that attention shifts are not closely related to the saccadic eye movement system. For luminance detection the retina appears to be equipotential with respect to attention shifts, since costs to unexpected stimuli are similar whether foveal or peripheral. These results appear to provide an important model system for the study of the relationship between attention and the structure of the visual system.
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