Visual Exploration and Object Recognition by Lattice Deformation

Mechanisms of explicit object recognition are often difficult to investigate and require stimuli with controlled features whose expression can be manipulated in a precise quantitative fashion. Here, we developed a novel method (called “Dots”), for generating visual stimuli, which is based on the progressive deformation of a regular lattice of dots, driven by local contour information from images of objects. By applying progressively larger deformation to the lattice, the latter conveys progressively more information about the target object. Stimuli generated with the presented method enable a precise control of object-related information content while preserving low-level image statistics, globally, and affecting them only little, locally. We show that such stimuli are useful for investigating object recognition under a naturalistic setting – free visual exploration – enabling a clear dissociation between object detection and explicit recognition. Using the introduced stimuli, we show that top-down modulation induced by previous exposure to target objects can greatly influence perceptual decisions, lowering perceptual thresholds not only for object recognition but also for object detection (visual hysteresis). Visual hysteresis is target-specific, its expression and magnitude depending on the identity of individual objects. Relying on the particular features of dot stimuli and on eye-tracking measurements, we further demonstrate that top-down processes guide visual exploration, controlling how visual information is integrated by successive fixations. Prior knowledge about objects can guide saccades/fixations to sample locations that are supposed to be highly informative, even when the actual information is missing from those locations in the stimulus. The duration of individual fixations is modulated by the novelty and difficulty of the stimulus, likely reflecting cognitive demand

Eye-tracking analyses.

<p>(A) Pattern of fixations/saccades revealed in relation to the “cannon” stimulus (left) and its underlying POI map (right). (B) Pooled fixations on all stimuli as a function of visibility for a subject performing the ascending (left) and one performing the descending protocol (right). (C) Average fixation spread (distance from image center). (D) Fixation count, normalized per subject. (E) Fixation duration. (F) Average local contour density (computed from POI map) in areas of 0.5° in diameter around explored locations corresponding to fixations on the dot stimulus. (G) Integrated dot displacement, computed as a sum of displacements of dots (in areas of 0.5° in diameter around each fixation) relative to the undeformed lattice. The sum runs over all fixation locations in the trial. Error bars represent s.e.m.</p

Exploration time estimated by measuring reaction time as a function of response type in the two experiments.

<p>Error bars represent s.e.m.</p

Psychometric curves, subjective and objective thresholds.

<p>Psychometric curves grouped by response type (A) and by verbal report accuracy (B) as a function of visibility (<i>g</i> value) and experimental condition (ascending and descending). (C) Thresholds for sigmoidal response curves corresponding to “Seen” (subjective) responses (left) and correct (objective) verbal responses (right). Error bars represent s.e.m.</p

Detection and recognition of individual objects.

<p>(A) Borders between “Nothing”/“Uncertain” (detection) and “Uncertain”/“Seen” (recognition) computed on individual objects, for the ascending (left) and descending (right) conditions. Error bars are s.e.m. (B) Average and SD of detection and recognition borders from (A) across all objects. (C) Individual object detection and recognition borders in the descending versus ascending condition.</p