The lack of a repulsion effect is in contrast to two previous studies that have reported a memory-induced Direction Illusion (Kang et al., 2011; Kang & Choi, 2015). Both studies presented the stimuli to the same foveal position. In line with these studies, Experiment 3 showed that proactive motion repulsion can occur even with long ISIs if both stimuli appear at the same spatial position. The observation of proactive memory-induced motion repulsion between spatially overlapping stimuli in Experiment 3 and the lack of such an effect under spatial separation of the items in Experiment 2 points towards a distinction between spatially local and global mechanisms of motion repulsion. A key characteristic of the Direction Illusion and analogue phenomena with other stimulus materials is that the interactions between concurrently presented stimuli are not bound to spatial overlap of the stimuli (e.g., Ejima & Takahashi, 1985; Kim & Wilson, 1997; Klauke & Wachtler, 2015; Loomis & Nakayama, 1973; Tzvetanov et al., 2006; Westheimer, 1990). However, another distortive phenomenon in the motion domain is the Direction Aftereffect that is caused by adaptation (i.e., after prolonged visual stimulation) to a motion direction (Levinson & Sekuler, 1976). Adaptation to a motion direction causes a shifted perception of subsequently viewed directions. While the effect is similar to the distortion profile of the Direction Illusion, the Direction Aftereffect is retinotopic (Wenderoth & Wiese, 2008). Furthermore, both phenomena have been dissociated on the grounds of behavioral research, suggesting that they take place at different locations in the motion processing stream (Curran, Clifford, & Benton, 2006, 2008; Farrell-Whelan, Wenderoth, & Brooks, 2012; Wiese & Wenderoth, 2007; for an in-depth comparison of both mechanisms in the orientation domain see Schwartz, Hsu, & Dayan, 2007). Saad and Silvanto (2013) have recently demonstrated a functional connection between visual working memory maintenance and adaptation. They showed that a single orientation maintained in working memory strengthens subsequent adaptation if the adapter stimulus has a similar orientation to the working memory content. Based on this finding they proposed that working memory maintenance is accompanied by persistent neural firing that leads to similar neural effects to prolonged visual stimulation, i.e., adaptation. One explanation for the divergent findings of Experiments 2 and 3 would thus be that visual memory maintenance relies on retinotopically specific sustained sensory activation (Pratte & Tong, 2014; Sneve, et al., 2011) that acts selectively on subsequent input that falls onto the same receptive fields (but see Harrison & Bays, 2018, who argue that to-be-memorized representations are transferred from retinotopic sensory areas to other areas briefly after stimulus onset, e.g. within 1s for orientations). In this context it is noteworthy that several other studies have investigated distortions between two sequentially encoded memory items with ISIs longer than 0.5 s, using different stimulus materials (Bae & Luck, 2017; Czoschke et al., 2019; Dubé et al., 2014; Huang & Sekuler, 2010; Rademaker , Bloem, De Weerd, & Sack, 2015; Wildegger, Myers, Humphreys, & Nobre, 2015). While all of these studies observed proactive distortions, they all presented their items to the fovea and thus to the same spatial position. Our results suggest that proactive, memory-based item interactions might be restricted to this special case of spatial overlap between stimuli. This would have implications for the generalizability and functional interpretation of memory-based alterations of stimulus representations.
Distorted Illusions Activation Code [pack]
However, while our results demonstrate that motion repulsion does not occur during concurrent memory maintenance of spatially distinct stimuli, they do not imply that repulsion is specific to perceptual processing (i.e., neural processes during sensory stimulation). There are at least two studies that present strong evidence for repulsive interactions between working memory representations. The previously mentioned study by Kang and Choi (2015) presented two motion directions sequentially to the fovea and, after a brief retention period, subjects had to reproduce both motion directions in a whole-report procedure without knowing the order of report in advance. In their experiment, both items (S1 and S2) repulsed each other, as evident by a repulsion effect for the first reported item of the report sequence. However, the second reported item showed an increased repulsion magnitude. While the distortions measured at the first report position could be attributed to interactions between the maintained S1 and the S2 during S2 presentation, the additional distortion observed for the second reported item seemed to be a result of the prior recall (see also Bae & Luck, 2017, for a similar result with clock-hand directions). In an even more persuasive study, four directions were presented simultaneously for memorization, one of which was subsequently cued (Myers, Chekroud, Stokes, & Nobre, 2018). Crucially, this task required reproducing two of the four presented items, and the cue informed the participants about one of the two recall-targets. While the reproduction of the cued item was not distorted, the remaining uncued items were repulsed from the cued memory representation. Together with our current results, these findings demonstrate that repulsive interactions on the memory level can occur if item representations within working memory are internally activated during retention to prepare for recall, but that they do not occur during mere concurrent maintenance. This pattern of findings supports the view that stimulus-specific sensory activation can be selectively reactivated via top-down signals from higher areas to reinstate sensory representations if needed for the specific task at hand (LaRocque, Riggall, Emrich, & Postle, 2016; Lewis-Peacock, Drysdale, Oberauer, & Postle, 2012; Scimeca et al., 2018; Sprague, Ester, & Serences, 2016; Wheeler, Petersen, & Buckner, 2000; Xu, 2017). The studies by Kang and Choi and Myers et al. show that if two items within working memory are serially activated for report, the first activated item repels the second one. This suggests that activation reinstates a perception-like code that comes with the psychophysical consequences known from perceptual stimulation. Thus, activated memory representations and online perceptual representations indeed might have common neural properties, but, as our results suggest, mere memory maintenance relies on different neural processes that do not share the same characteristics as perceptual code. 2ff7e9595c
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