Jean-Baptiste Durand

Privileged visual processing of the straight-ahead direction in humans.

At any moment, the objects we face are endowed with a special behavioral status, either as potential obstacles during navigation or as optimal targets for visually guided actions. Yet, the gaze frequently jumps from one location to another when exploring the visual surroundings, so that objects located straight-ahead are often seen from the corner of the eyes. In the present study, we tested the hypothesis that peripheral vision might nevertheless ensure a privileged processing of these behaviorally important objects. Human subjects were asked to respond as fast as possible to the appearance of visual objects in their peripheral field of view while gazing successively in different directions. The visual objects formed similar images on the retina and differed only with respect to their egocentric location: either straight-ahead or eccentric with respect to the head/body midline. We found that straight-ahead objects elicit consistently shorter behavioral responses than eccentric objects (median difference of at least 10 ms). Additional experiments indicate that neither binocular visual cues nor full attentional resources play a fundamental role in this mechanism, and that it cannot be resumed to a simple preference for objects contralateral to the direction of gaze. These results are in agreement with recent electrophysiological findings showing that the early integration of gaze-related signals in the visual cortex of macaque monkeys lead to a higher neuronal sensitivity to the straight-ahead direction.

Processing of Egomotion-Consistent Optic Flow in the Rhesus Macaque Cortex

Abstract The cortical network that processes visual cues to self‐motion was characterized with functional magnetic resonance imaging in 3 awake behaving macaques. The experimental protocol was similar to previous human studies in which the responses to a single large optic flow patch were contrasted with responses to an array of 9 similar flow patches. This distinguishes cortical regions where neurons respond to flow in their receptive fields regardless of surrounding motion from those that are sensitive to whether the overall image arises from self‐motion. In all 3 animals, significant selectivity for egomotion‐consistent flow was found in several areas previously associated with optic flow processing, and notably dorsal middle superior temporal area, ventral intra‐parietal area, and VPS. It was also seen in areas 7a (Opt), STPm, FEFsem, FEFsac and in a region of the cingulate sulcus that may be homologous with human area CSv. Selectivity for egomotion‐compatible flow was never total but was particularly strong in VPS and putative macaque CSv. Direct comparison of results with the equivalent human studies reveals several commonalities but also some differences.

Visual straight-ahead preference in saccadic eye movements

Ocular saccades bringing the gaze toward the straight-ahead direction (centripetal) exhibit higher dynamics than those steering the gaze away (centrifugal). This is generally explained by oculomotor determinants: centripetal saccades are more efficient because they pull the eyes back toward their primary orbital position. However, visual determinants might also be invoked: elements located straight-ahead trigger saccades more efficiently because they receive a privileged visual processing. Here, we addressed this issue by using both pro- and anti-saccade tasks in order to dissociate the centripetal/centrifugal directions of the saccades, from the straight-ahead/eccentric locations of the visual elements triggering those saccades. Twenty participants underwent alternating blocks of pro- and anti-saccades during which eye movements were recorded binocularly at 1 kHz. The results confirm that centripetal saccades are always executed faster than centrifugal ones, irrespective of whether the visual elements have straight-ahead or eccentric locations. However, by contrast, saccades triggered by elements located straight-ahead are consistently initiated more rapidly than those evoked by eccentric elements, irrespective of their centripetal or centrifugal direction. Importantly, this double dissociation reveals that the higher dynamics of centripetal pro-saccades stem from both oculomotor and visual determinants, which act respectively on the execution and initiation of ocular saccades.

Rewarding objects appear larger but not brighter

Whether reward can accentuate the perception of visual objects, that is, makes them appear larger than they really are, is a long-standing and controversial question. Here, we revisit this issue with a novel two-alternative forced-choice paradigm combining asymmetric reward schedule and task reversal. In a first experiment, participants (n = 27) choose the larger of two unequally rewarded objects in some sessions and the smaller one in other sessions. Response biases toward the most rewarding object differ significantly between the reversed tasks, revealing an influence of reward on perceived sizes. In a second experiment, participants (n = 27) indicate either the brighter or darker object. In contrast with the first experiment, response biases are similar between those reversed tasks, indicating that the perceived luminance is immune to reward manipulation. Together, these results reveal that if two objects are associated with different amounts of reward, participants will perceive the more rewarded object to be slightly larger, but not brighter, than the less rewarded one.

“Wishful seeing” in non-human primates: can reward shape size perception?

Reward seeking shapes our interactions with the environment, but whether it also influences how we perceive it remains controversial. Notably, there is currently a debate on whether objects that are more likely to provide high reward might be perceived bigger than those associated with low reward. The aim of this study is to test this « wishful seeing » hypothesis in non-human primates. Two macaque monkeys performed a two-alternative forced choice (2 AFC) size discrimination task during several weeks. Following an initial fixation period, the animals were required to make a saccade towards the biggest of two simultaneously displayed discs, located in diametrically opposite positions relatively to the fixation point (7° eccentricity). Correct trials were followed by a liquid reward. The disk positions determined the probability of receiving a high versus a low amount of fluid (5/1 ratio). In half of the trials, the 2 discs appeared in positions with equal probability of high reward (neutral conditions, p=0.5/p=0.5) while in the other trials, the probability of high reward was asymmetric (reward-biased conditions, p=0.8/p=0.2). Preliminary results indicate that the saccade latencies are directly related to the probability of getting a high reward (i.e. saccades toward more rewarded positions are significantly shorter that saccades toward less rewarded positions), in agreement with previous reports. Our results also reveal massive differences in the psychometric curves associated with the neutral and reward-biased conditions. Monkeys tend to choose highly rewarded positions irrespective of the sensory evidence, which suggests a reward-induced change in strategy rather than in perception. However, we also observe an interaction between reward probability and sensory evidences that might indicate reward-induced changes in perceived size, in agreement with the wishful seeing hypothesis. Meeting abstract presented at VSS 2015.