Although psychophysical evidence for object-based attention has been reported, corresponding studies with event-related potentials (ERPs) are scarce. Here subjects were presented with perceptual fields containing two superimposed objects (transparent surfaces generated by two sets of dots in rigid rotation around fixation, each set of a different color and direction of motion) or only one object (the same dots but either at rest or all rotating in the same direction). Brief (150-msec) rectilinear displacements affected either of the sets at random ISIs of 350 to 550 msec. Attention was directed to one set of dots, guided by color, in order to discriminate the direction of their displacement. Motion-onset ERPs elicited by these displacements were compared for attended and unattended dots. When the perceptual field consisted of two objects, strong suppression of P1 and N1 was obtained in the ERPs associated with the unattended object. No suppression was found with the field containing a single object, although an enhanced selection negativity was found in ERPs associated with attended dots (selected by color). Since the two objects occupied the same region of visual space, the suppression of P1/N1 cannot be explained by the space-based mechanisms but is consistent with object-based attentional selection at early stages of vision. The results highlight the role of perceptual organizations in enabling alternative attentional mechanisms.

Two interspersed and differently colored sets of dots were rotated in opposite directions and were perceived as superimposed transparent surfaces. Probes consisting of brief changes in dot motion direction were reported. Two probes affecting the same surface were discriminated accurately. The 2nd probe was discriminated poorly if it affected a surface different from the 1st and if the time between probes was less than 600 ms. This reflects a difficulty in switching attention rapidly between surfaces. Spatial proximity increased the interference. Controls were incompatible with traditional spatial mechanisms (2- or 3-dimensional) or with simple sensory filters. Instead, probes were apparently selected by object files. The interference is not simply due to an inability to process 2 objects at once but requires close spatial proximity of incompatible motion signals.

No abstract

Two consecutive events transforming the same illusory surface in transparent motion (brief changes in direction) can be discriminated with ease, but a prolonged interference ( approximately 500 ms) on the discrimination of the second event arises when different surfaces are concerned [Valdes-Sosa, M., Cobo, A., & Pinilla, T. (2000). Attention to object files defined by transparent motion. Journal of Experimental Psychology: Human Perception and Performance, 26(2), 488-505]. Here we further characterise this phenomenon and compare it to the attentional blink AB [Shapiro, K.L., Raymond, J.E., & Arnell, K.M. (1994). Attention to visual pattern information produces the attentional blink in RSVP. Journal of Experimental Psychology: Human Perception and Performance, 20, 357-371]. Similar to the AB, reduced sensitivity (d') was found in the two-surface condition. However, the two-surface cost was associated with a reduced N1 brain response in contrast to reports for AB [Vogel, E.K., Luck, S.J., & Shapiro, K. (1998). Electrophysiological evidence for a postperceptual locus of suppression during the attentional blink. Journal of Experimental Psychology: Human Perception and Performance, 24(6), 1656-1674]. The results from this study indicate that the two-surface cost corresponds to competitive effects in early vision. Reasons for the discrepancy with the AB study are considered.

No abstract