Kyle Killebrew

Intraparietal regions play a material general role in working memory: Evidence supporting an internal attentional role.

Determining the role of intraparietal sulcus (IPS) regions in working memory (WM) remains a topic of considerable interest and lack of clarity. One group of hypotheses, the internal attention view, proposes that the IPS plays a material general role in maintaining information in WM. An alternative viewpoint, the pure storage account, proposes that the IPS in each hemisphere maintains material specific (e.g., left--phonological; right--visuospatial) information. Yet, adjudication between competing theoretical perspectives is complicated by divergent findings from different methodologies and their use of different paradigms, perhaps most notably between functional magnetic resonance imaging (fMRI) and electroencephalography (EEG). For example, fMRI studies typically use full field stimulus presentations and report bilateral IPS activation, whereas EEG studies direct attention to a single hemifield and report a contralateral bias in both hemispheres. Here, we addressed this question by applying a regions-of-interest fMRI approach to elucidate IPS contributions to WM. Importantly, we manipulated stimulus type (verbal, visuospatial) and the cued hemifield to assess the degree to which IPS activations reflect stimulus specific or stimulus general processing consistent with the pure storage or internal attention hypotheses. These data revealed significant contralateral bias along regions IPS0-5 regardless of stimulus type. Also present was a weaker stimulus-based bias apparent in stronger left lateralized activations for verbal stimuli and stronger right lateralized activations for visuospatial stimuli. However, there was no consistent stimulus-based lateralization of activity. Thus, despite the observation of stimulus-based modulation of spatial lateralization this pattern was bilateral. As such, although it is quantitatively underspecified, our results are overall more consistent with an internal attention view that the IPS plays a material general role in refreshing the contents of WM.

Form Features Provide a Cue to the Angular Velocity of Rotating Objects

As an object rotates, each location on the object moves with an instantaneous linear velocity, dependent upon its distance from the center of rotation, whereas the object as a whole rotates with a fixed angular velocity. Does the perceived rotational speed of an object correspond to its angular velocity, linear velocities, or some combination of the two? We had observers perform relative speed judgments of different-sized objects, as changing the size of an object changes the linear velocity of each location on the objects surface, while maintaining the objects angular velocity. We found that the larger a given object is, the faster it is perceived to rotate. However, the observed relationships between size and perceived speed cannot be accounted for simply by size-related changes in linear velocity. Further, the degree to which size influences perceived rotational speed depends on the shape of the object. Specifically, perceived rotational speeds of objects with corners or regions of high-contour curvature were less affected by size. The results suggest distinct contour features, such as corners or regions of high or discontinuous contour curvature, provide cues to the angular velocity of a rotating object.

The lemon illusion: seeing curvature where there is none

Curvature is a highly informative visual cue for shape perception and object recognition. We introduce a novel illusion—the Lemon Illusion—in which subtle illusory curvature is perceived along contour regions that are devoid of physical curvature. We offer several perceptual demonstrations and observations that lead us to conclude that the Lemon Illusion is an instance of a more general illusory curvature phenomenon, one in which the presence of contour curvature discontinuities lead to the erroneous extension of perceived curvature. We propose that this erroneous extension of perceived curvature results from the interaction of neural mechanisms that operate on spatially local contour curvature signals with higher-tier mechanisms that serve to establish more global representations of object shape. Our observations suggest that the Lemon Illusion stems from discontinuous curvature transitions between rectilinear and curved contour segments. However, the presence of curvature discontinuities is not sufficient to produce the Lemon Illusion, and the minimal conditions necessary to elicit this subtle and insidious illusion are difficult to pin down.

Electrophysiological correlates of encoding processes in a full-report visual working memory paradigm

Why are some visual stimuli remembered, whereas others are forgotten? A limitation of recognition paradigms is that they measure aggregate behavioral performance and/or neural responses to all stimuli presented in a visual working memory (VWM) array. To address this limitation, we paired an electroencephalography (EEG) frequency-tagging technique with two full-report VWM paradigms. This permitted the tracking of individual stimuli as well as the aggregate response. We recorded high-density EEG (256 channel) while participants viewed four shape stimuli, each flickering at a different frequency. At retrieval, participants either recalled the location of all stimuli in any order (simultaneous full report) or were cued to report the item in a particular location over multiple screen displays (sequential full report). The individual frequency tag amplitudes evoked for correctly recalled items were significantly larger than the amplitudes of subsequently forgotten stimuli, regardless of retrieval task. An induced-power analysis examined the aggregate neural correlates of VWM encoding as a function of items correctly recalled. We found increased induced power across a large number of electrodes in the theta, alpha, and beta frequency bands when more items were successfully recalled. This effect was more robust for sequential full report, suggesting that retrieval demands can influence encoding processes. These data are consistent with a model in which encoding-related resources are directed to a subset of items, rather than a model in which resources are allocated evenly across the array. These data extend previous work using recognition paradigms and stress the importance of encoding in determining later VWM retrieval success.

Non-linear neural interactions at the time of encoding underlie grouping benefits in working memory

UNLABELLED It is well established that Visual Working Memory is capacity limited. Recent behavioral research has demonstrated that perceptual grouping can facilitate the retrieval of items stored in working memory. At what stage and by what mechanism are these grouping effects manifesting? In order to answer this question we used high-density electroencephalography (hdEEG) to examine neural correlates of perceptual grouping during the encoding phase of a standard change detection working memory recognition task. Specifically, we used an EEG technique, known as frequency tagging. This technique takes advantage of the fact that flickering stimuli will elicit a steady-state oscillation at a corresponding frequency in the EEG data. Using this technique we designed an experiment in which participants were presented with sets of four novel shapes, each flickering at one of four chosen frequencies: 3Hz, 5Hz, 12Hz and 20Hz, between black and a randomly chosen color on a grey background. The experiment consisted of two conditions: grouped and non-grouped. In the grouped condition two of the four shapes were the same, had the same color and were always presented on the same side of the screen. In the non-grouped condition, all shapes and colors were different. Behaviorally, we found that participants were significantly more accurate in performing the task in the grouping condition. This grouping benefit was accompanied by a significant increase in two non-linear harmonics of the grouped items. CONCLUSION our data demonstrate that the grouping benefit observed in visual working memory tasks arises in part at the time of encoding from a non-linear neural interaction between the items comprising the group. Meeting abstract presented at VSS 2015.

The Lemon-Illusion: Seeing curvature where there is none

Curvature is a highly informative visual cue for object recognition. We present a novel illusion-the Lemon Illusion-in which subtle illusory concavities are perceived in the absence of actual curvature. We offer several perceptual demonstrations that show when the illusion does or does not occur. Based on some of our observations, and the large body of research on the role of curvature in the visual perception of object shape, we conclude that the Lemon Illusion likely arises due to the reconciliation of contour curvature interpolation and explicit zero-curvature contour. The observations are consistent with two non-mutually exclusive neural mechanisms within visual cortex that could account for the Lemon Illusion. The first involves curvature-continuation mechanisms such as those thought to subserve contour integration as early as V1 and V2. The second involves global shape processing within V4. Meeting abstract presented at VSS 2015.