Academic journal article Attention, Perception and Psychophysics

From Reorienting of Attention to Biased Competition: Evidence from Hemifield Effects

Academic journal article Attention, Perception and Psychophysics

From Reorienting of Attention to Biased Competition: Evidence from Hemifield Effects

Article excerpt

When a distractor was presented simultaneously with or directly following a target, it produced more interference when it was presented in the same visual hemifield as the target than when it was presented in the opposite visual field. This result is interpreted in terms of biased competition; there is more competition between stimuli when they are presented in the same visual field, rather than in opposite visual fields. However, when the distractor was presented 125 msec or more prior to the target, this pattern was reversed. In those cases, there was more distractor interference when target and distractor were presented in opposite visual fields. This can be explained by assuming that attention was captured by the distractor, and that there was an additional cost of reorienting to a location in the opposite visual field.

As impulses travel upward in the hierarchy of visual brain areas, increasingly complex visual features are processed in a decreasingly localized manner (Smith, Singh, Williams, & Greenlee, 2001). Because of the coarse localization in higher visual areas, stimuli presented near one another activate largely overlapping neural populations. It is difficult, therefore, to attribute neural activity unambiguously to a single stimulus (Desimone & Duncan, 1995; Luck, Girelli, McDermott, & Ford, 1997; Moran & Desimone, 1985). The spiking of a neuron might indicate that one of the stimuli in its receptive field (RF) has a certain feature-but which stimulus?

It has become clear that visual attention plays a crucial role in the way these ambiguities are resolved. In a situation in which attention is unfocused, two nearby stimuli act in a mutually suppressive way (Kastner, De Weerd, Desimone, & Ungerleider, 1998), so that both stimuli are weakly represented. Furthermore, the representations of the two stimuli are not clearly delineated, so that there is ambiguity in which features belong to which object (for a review, see Beck & Kastner, 2009). However, if attention is directed to one of the stimuli, activation is driven largely by the attended stimulus and the visual system effectively becomes "blind" to the presence of the nonattended stimulus (Luck, Chelazzi, Hillyard, & Desimone, 1997; Motter, 1993; Reynolds, Chelazzi, & Desimone, 1999). Therefore, in a state of focused attention, the attended stimulus is strongly represented and ambiguity is resolved, since only features of a single object-the attended object-are represented. Phrased differently, nearby stimuli activate overlapping neural populations and therefore have to compete for representation in the visual system. Visual attention is what determines which stimulus wins this competition. These competitive interactions form the central tenet of many contemporary theories of attention, such as the biased competition model (Desimone, 1998; Desimone & Duncan, 1995), the ambiguity resolution theory (Luck, Girelli, et al., 1997), and the selective tuning model of visual attention (Tsotsos et al., 1995).

As stimuli are spaced farther apart, there is decreasing overlap between the neural populations activated by their presentation. Since competition is believed to occur predominantly at the level of RFs, this results in less competition for representation (Beck & Kastner, 2009; Hopf et al., 2006; but see Tombu & Tsotsos, 2008). In addition, and of special importance to the present study, is the finding that there is also less competition if stimuli are presented in opposite visual fields (Sereno & Kosslyn, 1991; Torralbo & Beck, 2008). In part, this can be explained by the fact that the RFs of neurons in early visual areas tend to be confined to one visual field: The left visual field is processed in the right hemisphere, and vice versa (Desimone & Schein, 1987; Gattass, Sousa, & Gross, 1988). However, in the inferior temporal cortex, the majority of neurons have RFs that encompass part of both visual fields (Rocha-Miranda, Bender, Gross, & Mishkin, 1975), and even in this area there is more competition if stimuli are presented in the same visual field than if they are presented in opposite visual fields (Chelazzi, Duncan, Miller, & Desimone, 1998; Sato, 1988, 1989). …

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