Top-down influences on stereoscopic depth-perception.

The interaction between depth perception and object recognition has important implications for the nature of mental object representations and models of hierarchical organization of visual processing. It is often believed that the computation of depth influences subsequent high-level object recognition processes, and that depth processing is an early vision task that is largely immune to 'top-down' object-specific influences, such as object recognition. Here we present experimental evidence that challenges both these assumptions in the specific context of stereoscopic depth-perception. We have found that observers' recognition of familiar dynamic three-dimensional (3D) objects is unaffected even when the objects' depth structure is scrambled, as long as their two-dimensional (2D) projections are unchanged. Furthermore, the observers seem perceptually unaware of the depth anomalies introduced by scrambling. We attribute the latter result to a top-down recognition-based influence whereby expectations about a familiar object's 3D structure override the true stereoscopic information.

Distinctive voices enhance the visual recognition of unfamiliar faces.

Several studies have provided evidence in favour of a norm-based representation of faces in memory. However, such models have hitherto failed to take account of how other person-relevant information affects face recognition performance. Here we investigated whether distinctive or typical auditory stimuli affect the subsequent recognition of previously unfamiliar faces and whether the type of auditory stimulus matters. In this study participants learned to associate either unfamiliar distinctive and typical voices or unfamiliar distinctive and typical sounds to unfamiliar faces. The results indicated that recognition performance was better to faces previously paired with distinctive than with typical voices but we failed to find any benefit on face recognition when the faces were previously associated with distinctive sounds. These findings possibly point to an expertise effect, as faces are usually associated to voices. More importantly, it suggests that the memory for visual faces can be modified by the perceptual quality of related vocal information and more specifically that facial distinctiveness can be of a multi-sensory nature. These results have important implications for our understanding of the structure of memory for person identification.

Freeze-substitution of Drosophila heads for subsequent [3H]2-deoxyglucose autoradiography.

High resolution of [3H]2-deoxyglucose labelling was obtained in autoradiographs of Drosophila brains after freeze-substitution in anhydrous acetone at -76 degrees C. This method was applied to preparations which received visual, olfactory and mechanosensory stimulation. The autoradiographs were compared to those obtained after freeze-drying. Freeze-substitution, which has proved to be technically simple, rapid and inexpensive, yields a good quality of tissue preservation and hence is recommended for tissue dehydration prior to autoradiography.

GABA-antagonist inverts movement and object detection in flies.

Movement detection is one of the most elementary visual computations performed by vertebrates as well as invertebrates. However, comparatively little is known about the biophysical mechanisms underlying this computation. It has been proposed on both physiological and theoretical grounds that inhibition plays a crucial role in the directional selectivity of elementary movement detectors (EMDs). For the first time, we have studied electrophysiological and behavioral changes induced in flies after application of picrotoxinin, an antagonist of GABA. The results show that inhibitory interactions play an important role in movement detection in flies. Furthermore, our behavioral results suggest that the computation of object position is based primarily on movement detection.

Effects of parametric manipulation of inter-stimulus similarity on 3D object categorization.

To explore the nature of the representation space of 3D objects, we studied human performance in forced-choice categorization of objects composed of four geon-like parts emanating from a common center. Two categories were defined by prototypical objects, distinguished by qualitative properties of their parts (bulging vs waist-like limbs). Subjects were trained to discriminate between the two prototypes (shown briefly, from a number of viewpoints, in stereo) in a 1-interval forced-choice task, until they reached a 90% correct-response performance level. After training, in the first experiment, 11 subjects were tested on shapes obtained by varying the prototypical parameters both orthogonally (ORTHO) and in parallel (PARA) to the line connecting the prototypes in the parameter space. For the eight subjects who performed above chance, the error rate increased with the ORTHO parameter-space displacement between the stimulus and the corresponding prototype; the effect of the PARA displacement was weaker. Thus, the parameter-space location of the stimuli mattered more than the qualitative contrasts, which were always present. To find out whether both prototypes or just the nearest one to the test shape influenced the decision, in the second experiment we varied the similarity between the categories. Specifically, in the test stage trials the distance between the two prototypes could assume one of three values (FAR, INTERMEDIATE, and NEAR). For the 13 subjects who performed above chance, the error rate (on physically identical stimuli) in the NEAR condition was higher than in the other two conditions. The results of the two experiments contradict the prediction of theories that postulate exclusive reliance on qualitative contrasts, and support the notion of a representation space in which distances to more than one reference point or prototype are encoded (Edelman, 1998).