Second-order and configural processing in the rat visual system

Object vision relies on the ability to tolerate identity-preserving transformations, such as scaling, translation, and rotation of visual objects. Even larger transformations can be achieved when a visual pattern emerges from second-order (texture) cues or by configural arrangements of local features constructing a global (Gestalt) figure. While humans generalize across such transformations, it remains unclear whether lower mammals, such as rodents, share with us this ability. Recent studies have shown that rats can recognize objects despite identity-preserving transformations, but little is known about rat generalization to second-order and configural patterns. To address this question, we first trained 12 Long-Evans rats to classify first-order (i.e., luminance-defined) gratings spanning 13 orientations as either horizontal or vertical, and we built psychometric curves reporting the fraction of vertical choices. We then measured how much the brief (50 ms) presentation of a prime stimulus, either a vertical or a horizontal grating, biased the perception of the first-order gratings, by shifting upward (vertical prime) or downward (horizontal prime) the psychometric curves. Large shifts were produced not only by default gratings but also by phase-reversed and higher spatial frequency variants of the stimuli, thus showing full, spontaneous invariance to such transformations. Moreover, significant priming was also produced by texture-defined gratings and by arrays of small squares or bars arranged in such a way as to form global, oriented patterns. Thus, rats spontaneously perceive second-order and configural oriented patterns as similar to luminance-defined gratings and could serve as a model to study the neuronal underpinnings of this ability.