When engaged in dynamic visuospatial tasks, the brain copes with perceptual and cognitive processing challenges. During multiple-object tracking (MOT), the number of objects to be tracked (i.e., load) imposes attentional demands, but so does spatial interference from irrelevant objects (i.e., close encounters). Presently, it is not clear whether the effect of load on accuracy solely depends on the number of close encounters. If so, the same cognitive and physiological mechanisms deal with increasing load by preparing for and dealing with spatial interference. However, this has never been directly tested. Such knowledge is important to understand the neurophysiology of dynamic visual attention and resolve conflicting views within visual cognition concerning sources of capacity limitations. We varied the processing challenge in MOT task in two ways: the number of targets and the minimum spatial proximity between targets and distractors. In a first experiment, we measured task-induced pupil dilations and saccades during MOT. In a separate cohort, we measured fMRI activity. In both cohorts, increased load and close encounters (i.e., close spatial proximity) led to reduced accuracy in an additive manner. Load was associated with pupil dilations, whereas close encounters were not. Activity in dorsal attentional areas and frequency of saccades were proportionally larger both with higher levels of load and close encounters. Close encounters recruited additionally ventral attentional areas that may reflect orienting mechanisms. The activity in two brainstem nuclei, ventral tegmental area/substantia nigra and locus coeruleus, showed clearly dissociated patterns. Our results constitute convergent evidence indicating that different mechanisms underlie processing challenges due to load and object spacing.

Two patients (TC and SS) with lesions that included the hippocampal regions (predominantly on the left side) were severely impaired in their recall of simple, verbally stated facts. However, both patients remembered spatial information that was temporally associated with semantic information. Specifically, TC and SS could not recall explicitly the content of an episode, but their spontaneous oculomotor behavior showed that they retained some information about the event as their gaze automatically returned to the locations on the computer screen where visual information had been paired to verbally presented information. Thus, this spatial information is implicit, automatically retrieved, and eye-based, as when one patient (TC) was asked to point with the finger to the same positions he was impaired. In addition, in an old/new recognition task, TC and SS and an additional patient, OB, showed significant changes in eye pupil diameter when viewing novel visual stimuli compared to stimuli that they had previously seen, also when they (incorrectly) declared with confidence that an old item was new. The spared memory of these patients, despite severe amnesia for the learning episodes, is characterized by a re-enactment of previous eye fixations that were associated with each (forgotten) episode and physiological responses (as indexed by pupillometry) to previously seen stimuli. Such spared memory can be seen as a type of “snapshot” memory, which automatically processes eye-based spatial information and whose content remains implicit. Finally, we surmise on the basis of the neuroanatomical findings of these patients, that neural substrates in the spared (right) hemisphere might support both the eye fixations' re-enactment and implicit visual pattern recognition.

RP is a case of “developmental” prosopagnosia who, according to brain-imaging segmentation data, shows a reduction in volume of a limited set of structures of the right hemisphere. RP is as accurate as control subjects in tasks requiring the perception of nonface objects (e.g., matching subordinate labels to exemplars, naming two-tone images), with the exception of one perceptual task: The matching of different perspectives of amoebae-like stimuli (i.e., volumes made of a single smooth surface). In terms of speed (“efficiency”) of responses, RP's performance falls clearly outside the normal limits also in other tasks that include “natural” but nonface stimuli (i.e., animals, artifacts). Specifically, RP is slow in perceptual judgments made at very low (subordinate) levels of semantic categorization and for objects and artifacts whose geometry present much curved features and surface information. We conclude from these analyses that prosopagnosia can be the result of a deficit in the representation of basic geometric volumes made of curved surface. In turn, this points to the importance (necessity) for the normal visual system of such curved and volumetric information in the identification of human faces.

Sixty patients with unilateral stroke (half with left hemisphere damage and half with right hemisphere damage) and a control group ( N = 15) matched for age and educational level were tested in two experiments. In one experiment they were first shown, on each trial, a sample drawing depicting one or more objects. Following a short delay, they were asked to identify the drawing when it was paired with a drawing in which the same object(s) was transformed in categorical or coordinate spatial relations. In the other experiment, the same subjects first were shown, on each trial, a sample drawing. They then judged which of two variants (each in one type of spatial relation) looked more similar to the sample drawing. Typically, patients with left-sided stroke mistakenly identified the categorical transformation for the sample drawing in the first task; in the second task, they judged the categorical transformation as more similar to the sample drawing. Patients with right-sided stroke mistakenly identified the coordinate transformations for the sample drawing in the first task, and, in the second task, typically judged the drawings transformed along coordinate spatial relations as more similar to the sample drawing. These findings provide evidence for complementary lateralization of the two types of spatial perception. It can therefore be inferred that separate functional subsystems process the two types of spatial relations.