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

Perceived frequency of aperiodic vibrotactile stimuli depends on temporal encoding

Mechanical transients and events arising during dexterous manipulation are detected by tactile afferents. Naturally occurring vibrotactile stimuli have a mix of frequencies, which creates complex afferent discharge patterns. Psychophysical correlates of these complex discharge patterns could be useful tools to gain greater insights into tactile coding and the principles of signal processing in the nervous system. In a previous study, we discovered that frequency perception of periodic bursting stimuli depended on the duration of the silent gap between spike bursts. Here, we investigated the perceived frequency of aperiodic vibrotactile stimuli. We found that perceived frequency was lower than the mean discharge rate of the afferents. This supports a hypothesis stemming from our previous work, that within spike trains consisting of mixed length inter-spike intervals, the contribution of a given interval to perceived frequency is weighted by its length. Thus, the present study reveals that frequency perception of both periodic and aperiodic stimuli is encoded by sophisticated processing of individual inter-spike intervals, rather than based on detection of periodicity or spike counting