Shahab Ghorashi

Spatial selection and target identification are separable processes in visual search

Visual search involves deciding both where to look (spatial selection) and whether any given object is a target or a non-target (identification). The aim of the present study was to determine whether these two functions are separable in performance. Spatial selection was manipulated by an exogenous cue and identification was manipulated by whether a second target appeared after a short or long delay following a first target (the attentional blink, AB). Experiment 1 indicated an additive relation between non-informative spatial cueing and the AB, pointing to independent spatial and identification processes. Experiment 2 tested an informative spatial cue with similar results. Experiment 3 also showed an additive relationship, using a response measure that avoided possible floor effects. We interpret the separability of spatial selection and identification as reflecting the independent operation of dorsal and ventral visual pathways, respectively, at least at the early stages of processing.

Focal Distraction: Spatial Shifts of Attentional Focus Are Not Required for Contingent Capture

Contingent capture occurs when distractors that share the targets defining attribute capture attention and slow down target identification. This slowdown has been attributed to an involuntary attentional shift to the location of a pertinent distractor. The present study examined an additional source of delay: the time spent in processing pertinent distractors. In 7 experiments, distractors were presented at fixation, and targets were presented either at fixation or peripherally. Contingent capture invariably occurred when a salient distractor was presented within about 600 ms before the target, even when spatial shifts in attentional focus were ruled out. A 2-stage model is proposed in which stimuli must pass an input filter tuned to the targets defining attribute before gaining access to a high-level stage that is unavailable while a distractor is being processed.

Ignorance is bliss: The role of observer expectation in dynamic spatial tuning of the attentional focus

When two sequential targets (T1, T2) are inserted in an RSVP stream of distractors, perception of T2 is impaired at intertarget lags shorter than 700 msec. Paradoxically, this deficit disappears when T2 is presented directly after T1 (lag-1 sparing). Visser, Bischof, and Di Lollo (1999) found that lag-1 sparing occurs only when T1 and T2 are presented in the same stream. In contrast, Shih (2000) obtained lag-1 sparing with targets in separate streams. Four experiments addressed this inconsistency and revealed lag-1 sparing with targets in different streams, but only when observers had no foreknowledge of T1’s location. We hypothesized that when T1 location is known, attention is focused narrowly on that stream; if T2 then appears in the other stream it is missed, and lag-1 sparing does not occur. When T1 location is not known, attention is focused broadly, encompassing both streams, and lag-1 sparing ensues.

Spatial cuing does not affect the magnitude of the attentional blink

Identification of the second of two targets is impaired when the second target is presented less than about 500 msec after the first. Nieuwenstein, Chun, van der Lubbe, and Hooge (2005, Experiment 4) reported that the magnitude of this attentional blink (AB) is reduced when the location of the second target is precued. Here we show how that finding resulted from an artifact brought about by a ceiling imposed by data limitation. Instead of using an accuracy measure, the present work used a dynamic threshold-tracking procedure that was not constrained by a performance ceiling. The results show that, when the ceiling is removed, spatial cuing does not affect and is not affected by the AB. These results are consistent with the hypothesis that cue localization and target identification may take place along separate (dorsal and ventral) visual pathways.

System reconfiguration, not resource depletion, determines the efficiency of visual search

We examined two theories of visual search: resource depletion, grounded in a static, built-in brain architecture, with attention seen as a limited depletable resource, and system reconfiguration, in which the visual system is dynamically reconfigured from moment to moment so as to optimize performance on the task at hand. In a dual-task paradigm, a search display was preceded by a visual discrimination task and was followed, after a stimulus onset asynchrony (SOA) governed by a staircase procedure, by a pattern mask. Search efficiency, as indexed by the slope of the function relating critical SOA to number of distractors, was impaired under dual-task conditions for tasks that were performed efficiently (shallow search slope) when done singly, but not for tasks performed inefficiently (steep slope) when done singly. These results are consistent with system reconfiguration, but not with resource depletion, models and point to a dynamic, rather than a static, architecture of the visual system

Visual Search Is Postponed during the Attentional Blink until the System Is Suitably Reconfigured.

J. S. Joseph, M. M. Chun, and K. Nakayama (1997) found that pop-out visual search was impaired as a function of intertarget lag in an attentional blink (AB) paradigm in which the 1st target was a letter and the 2nd target was a search display. In 4 experiments, the present authors tested the implication that search efficiency should be similarly impaired (steeper search slopes at shorter lags). A conventional AB deficit was found, but, contrary to expectations, search slopes were invariant with lag. These results suggest that no search can be carried out during the period of the AB. Instead, the search is postponed until after the 1st target has been processed. The authors conclude that efficient visual search cannot be carried out unless the visual system is configured appropriately for the search task. If the initial configuration is inappropriate, processing of the 2nd target is held in abeyance until the system has been suitably reconfigured.

Are spatial selection and identity extraction separable when attention is controlled endogenously

Visual search for a target involves two processes: spatial selection and identity extraction. Ghorashi, Enns, and Di Lollo (2008) found these processes to be independent and surmised that they were carried out along distinct visual pathways: dorsal and ventral, respectively. The two experiments that are described in the present article evaluated this hypothesis. Attentional-blink methodology was combined with voluntary spatial cuing in a visual search task: Intertarget lag was used to manipulate identity extraction; predictive cues were used to signal target locations. Central digit cues in Experiment 1 required participants to identify digits before voluntarily directing attention to a corresponding location, whereas flashed dots in Experiment 2 (indicating an opposite location) required attentional redeployment without prior cue identification. Consistent with the dual-pathway hypothesis, cuing was impaired only when the first target and the number cue competed for ventral-pathway mechanisms. Collectively, the results support the dual-pathway account of the separability of spatial selection and identity extraction.

Attentional Load Effects on Beta Oscillations in Healthy and Schizophrenic Individuals

Attentional deficits are prominent among the cognitive disturbances found in schizophrenia. Given that schizophrenia is also characterized by abnormalities in high-frequency oscillations, we investigated whether attentional function in schizophrenia is related to abnormalities in high-frequency oscillations in a visual discrimination task in which attentional load was manipulated. Sixteen healthy control subjects (HC) and 23 chronic schizophrenia patients (SZ) discriminated between target discs (p = 0.2) and standard discs (p = 0.8). Attentional load was manipulated by varying the size difference between the target and standard discs across blocks: large (Easy condition), medium (Medium), and small (Difficult). The electroencephalogram was recorded and the oscillations evoked by the standard stimuli were analyzed using the Morlet wavelet transform. Subjects’ performance decreased as attentional load increased, but HC and SZ did not differ. Attentional load increased β phase-locking factor at frontal, parietal, and occipital electrode sites in HC but not SZ. In SZ, however, there was a correlation between the β attentional load effect and overall d′, indicating that high-performing SZ had relatively normal β attentional load effects. These results show that variations in attentional load are associated with β oscillations and provide a link between attentional dysfunction and β-generating neural circuitry in schizophrenia.