On the automaticity and flexibility of covert attention: a speed-accuracy trade-off analysis.

Exogenous covert attention improves discriminability and accelerates the rate of visual information processing (M. Carrasco & B. McElree, 2001). Here we investigated and compared the effects of both endogenous (sustained) and exogenous (transient) covert attention. Specifically, we directed attention via spatial cues and evaluated the automaticity and flexibility of exogenous and endogenous attention by manipulating cue validity in conjunction with a response-signal speed-accuracy trade-off (SAT) procedure, which provides conjoint measures of discriminability and information accrual. To investigate whether discriminability and rate of information processing differ as a function of cue validity (chance to 100%), we compared how both types of attention affect performance while keeping experimental conditions constant. With endogenous attention, both the observed benefits (valid-cue) and the costs (invalid-cue) increased with cue validity. However, with exogenous attention, the benefits and costs in both discriminability and processing speed were similar across cue validity conditions. These results provide compelling time-course evidence that whereas endogenous attention can be flexibly allocated according to cue validity, exogenous attention is automatic and unaffected by cue validity.

Attention speeds processing across eccentricity: feature and conjunction searches.

We investigated whether the effect of covert attention on information accrual varies with eccentricity (4 degrees vs 9 degrees) and the complexity of the visual search task (feature vs conjunction). We used speed-accuracy tradeoff procedures to derive conjoint measures of the speed of information processing and accuracy in each search task. Information processing was slower with more complex conjunction searches than with simpler feature searches, and overall it was faster at peripheral (9 degrees) than parafoveal (4 degrees) locations in both search types. Covert attention increased discriminability and accelerated information accrual at both eccentricities, and the magnitude of this attentional effect was the same for both feature (simple) and conjunction (complex) searches. Interestingly, in contrast to the compensatory effect of covert attention on information processing at iso-eccentric locations (temporal performance fields), covert attention did not eliminate speed differences across eccentricity.

Temporal performance fields: visual and attentional factors.

This study is the first to investigate: (a) 'temporal performance fields,' whether the speed of information accrual differs for different locations at a fixed eccentricity, and (b) whether covert attention modulates temporal dynamics differentially at isoeccentric locations. Using the speed accuracy tradeoff (SAT) procedure, we derived conjoint measures of how isoeccentric locations and precueing targets location affect speed and accuracy in a search task. The results demonstrate the existence of temporal performance fields, analogous to spatial performance fields: information accrual was fastest for target on the horizontal meridian, intermediate for targets at the intercardinal locations, slow for targets on the vertical meridian, and slowest for targets at the North (N) location (accrual time pattern: E&WS>intercardinal>E&W). Hence, the compensatory effect of attention eliminated the temporal asymmetries across isoeccentric locations.

Speed of visual processing increases with eccentricity.

The visual system has a duplex design to meet conflicting environmental demands: the fovea has the resolution required to process fine spatial information, but the periphery is more sensitive to temporal properties. To investigate whether the periphery's sensitivity is partly due to the speed with which information is processed, we measured the full timecourse of visual information processing by deriving joint measures of discriminability and speed, and found that speed of information processing varies with eccentricity: processing was faster when same-size stimuli appeared at 9 degrees than 4 degrees eccentricity, and this difference was attenuated when the 9 degrees stimuli were magnified to equate cortical representation size. At the same eccentricity, larger stimuli are processed more slowly. These temporal differences are greater than expected from neurophysiological constraints.