Finding the Relevant Objects

Finding the Relevant Objects Processes involved in finding the relevant objects in the display were expected to be influenced by manipulating the orientation of the reference object, given evidence that rotation of the reference object increases the diYculty of the identification process (e.g., McMullen & Jolicoeur, 1990; Maki, 1986). This manipulation resulted in an amplitude modulation of P3, the third positive deflection in the ERP waveform following the onset of the picture stimulus. The time course and topography of this eVect are presented in Fig. 1. In the data one can clearly see that the amplitude of the P3 was greater when the reference object was in an upright canonical orientation (solid line) than when it was rotated 90  into a noncanonical orientation (dot and dashed lines). This eVect is consistent with other data indicating that the amplitude of the P3 component is sensitive to the ease with which a task relevant stimulus can be identified (Bajric ˇ,Ro ¨ sler, Heil, & Hennighausen, 1999; Donchin & Coles, 1988; Kok, 1997, 2001). Within the sentence/picture verification paradigm, the sentence picks out particular Using Spatial Language 133objects that must be identified. This process is more diYcult when the objects are harder to identify, thereby reducing the amplitude in the noncanonical conditions relative to the canonical conditions. Fig. 1. Event-related potentials (A; electrode Pz) and topographic map (B) showing an influence of the orientation of the reference object on the P3 effect. 134 Laura A. Carlson2. Assigning Directions to Space Processes involved in assigning directions to space were expected to be influenced by manipulation of the diVerent sources of information used to define the parameters of the reference frame. When multiple sources of information are available and assign competing directions to a given spatial term, there is significant competition (Carlson-Radvansky & Irwin, 1994). For example, consider the three pictures depicting a ball (the located object) around a watering can (reference object) at the bottom of Fig. 2. In the canonical absolute/intrinsic picture, the ball can be considered above the watering can both with respect to the picture environment (absolute frame) and with respect to the top side of the watering can (intrinsic frame). However, in the noncanonical absolute picture, the ball is above the watering can with respect to the absolute frame but not with respect to the intrinsic frame. This is because rotation of the reference object results in a dissociation of intrinsic above from absolute above. Similarly, in the noncanonical intrinsic picture, the ball is above the watering can with respect to the intrinsic frame but not with respect to the absolute frame. In previous work, significant competition was observed for mapping the spatial term ‘‘above’’ onto placements of the located object in the noncanonical absolute and noncanonical intrinsic conditions relative to the canonical absolute/intrinsic conditions (Carlson-Radvansky & Irwin, 1994). More- over, the degree of competition depended upon one’s preference for using the diVerent reference frames, with a stronger preference observed for the absolute frame than the intrinsic frame for defining ‘‘above’’ (Carlson- Radvansky & Logan, 1997). Carlson et al. (2002) used this competition as a means of targeting the processes involved in assigning directions to space. Specifically, an instructional manipulation was used that defined which reference frame to use across diVerent blocks of trials. In one block of trials, participants were told to define above with respect to the absolute frame, in another block with respect to the intrinsic frame, and in another block with respect to either the absolute or intrinsic frame, with the order of blocks counterbal- anced across subjects. Competition was expected in the intrinsic and either blocks of trials, because in these blocks, the less-preferred intrinsic reference frame served as the basis for responding on all trials in the intrinsic block and on some trials in the either block. However, competition was not expected in the absolute block of trials in which participants were instructed to base their responses solely on the more-preferred absolute reference frame. ERPs at electrode FP1 are shown in Fig. 3 as a function of instruction condition, along with a topographical map illustrating the distribution of Using Spatial Language 135the eVect over the scalp. The magnitude of competition was reflected in the amplitude of a frontal slow wave that began around 450 ms after stimulus onset and persisted over the remainder of the trial, with the less-preferred intrinsic frame (dashed line) separating from the more-preferred absolute Fig. 2. Event-related potentials and topographic maps showing competition between reference frames as a function of instruction condition on a frontal slow wave. (A) The either instruction condition, (B) the absolute instruction condition, and (C) the intrinsic instruction condition. 136 Laura A. Carlsonframe (dotted line) and the canonical absolute/intrinsic frame (solid line) in the either and intrinsic blocks but not in the absolute block of trials. The frontal distribution of this slow wave is consistent with the modulations of the ERPs observed in other studies examining the neural correlates of conflict processing (West & Alain, 1999; Liotti, WoldorV, Perez, & Mayberg, 2000) and with evidence from functional neuroimaging studies indicating that the frontal cortex is consistently activated when stimulus or Fig. 3. Event-related potentials (A; electrode Oz) and topographic map (B) showing an influence of the orientation of the reference object and the resulting change in accessing the underlying spatial templates on a parietal slow wave effect. Using Spatial Language 137response competition exists within a task (Banich, Milham, Atchley, Cohen, Webb, Wszalek, Kramer, Liang, Wright, Shenker, & Magin, 2000; Taylor, Kornblum, Minoshima, Oliver, & Koeppe, 1994). 3. Computing and Comparing the Spatial Relation Processes involved in computing and comparing the spatial relation were expected to be influenced by manipulation of the orientation of the reference object. This constituent step involves evaluating whether the placement of the located object falls within a good, acceptable, or bad region of a spatial template associated with a particular term (e.g., the one provided in the sentence). Assuming that a spatial template is constructed for each active reference system (Carlson-Radvansky & Logan, 1997) on noncanonical trials in which the reference object is rotated and the absolute and intrinsic reference frames assign diVerent directions to the same relation, multiple templates associated with ‘‘above’’ would be constructed and evaluated (i.e., a template for absolute above and a template for intrinsic above). In contrast, on canonical trials, these multiple templates would be aligned, thus rendering the same response. As such, it is possible that only one template may need to be evaluated or, if many are evaluated, it is likely that there would be some facilitation due to the generation of the same response (e.g., redundancy gain; Miller, 1982; MordkoV & Yantis, 1991; Raab, 1962). Either way, processing on noncanonical trials would be expected to be diVerent from processing on canonical trials. As shown in Fig. 3, the eVect of the orientation manipulation was observed as a modulation of a parietal slow wave that began around 450 ms postpicture onset, with noncanonical trials (dashed and dotted lines) separating from canonical trials (solid line). A topographic map illustrates the distribution of this eVect over the scalp. Importantly, this modulation was distinct both spatially and temporally from the eVect of rotation on the P3 component, indicating that diVerent neural generators contribute to the identification of relevant objects and computing the spatial relation. It is possible that this modulation reflects slow wave activity associated with working memory processes such as updating and search (Kok, 2001) that would occur during the evaluation of multiple spatial templates