I Bülthoff

Deoxyglucose mapping of nervous activity induced inDrosophila brain by visual movement

SummaryLocal metabolic activity was mapped in the brain ofDrosophila by the radioactive deoxyglucose technique. The distribution of label in serial autoradiographs allows us to draw the following conclusions concerning neuronal processing of visual movement information in the brain ofDrosophila.1.The visual stimuli used (homogeneous flicker, moving gratings, reversing contrast gratings) cause only a small increase in metabolic activity in the first visual neuropil (lamina).2.In the second visual neuropil (medulla) at least four layers respond to visual movement and reversing contrast gratings by increased metabolic activity; homogeneous flicker is less effective.3.With the current autoradiographic resolution (2—3 μm) no directional selectivity can be detected in the medulla.4.In the lobula, the anterior neuromere of the third visual neuropil, movement-specific activity is observed in three layers, two of which are more strongly labelled by ipsilateral front-to-back than by back-to-front movement.5.In its posterior counterpart, the lobula plate, four movement-sensitive layers can be identified in which label accumulation specifically depends on the direction of the movement: Ipsilateral front-to-back movement labels a superficial anterior layer, back-to-front movement labels an inner anterior layer, upward movement labels an inner posterior layer and downward movement labels a superficial posterior layer.6.A considerable portion of the stimulus-enhanced labelling of medulla and lobula complex is restricted to those columns which connect to the stimulated ommatidia. This retinotopic distribution of label suggests the involvement of movement-sensitive small-field neurons.7.Certain axonal profiles connecting the lobula plate and the lateral posterior protocerebrum are labelled by ipsilateral front-to-back movement. Presumably different structures in the same region are labelled by ipsilateral downward movement. Conspicuously labelled foci and commissures in the central brain cannot yet be associated with a particular stimulus. The results are discussed in the light of present anatomical and physiological knowledge of the visual movement detection system of flies.

Do congenital prosopagnosia and the other-race effect affect the same face recognition mechanisms?

Congenital prosopagnosia (CP), an innate impairment in recognizing faces, as well as the other-race effect (ORE), a disadvantage in recognizing faces of foreign races, both affect face recognition abilities. Are the same face processing mechanisms affected in both situations? To investigate this question, we tested three groups of 21 participants: German congenital prosopagnosics, South Korean participants and German controls on three different tasks involving faces and objects. First we tested all participants on the Cambridge Face Memory Test in which they had to recognize Caucasian target faces in a 3-alternative-forced-choice task. German controls performed better than Koreans who performed better than prosopagnosics. In the second experiment, participants rated the similarity of Caucasian faces that differed parametrically in either features or second-order relations (configuration). Prosopagnosics were less sensitive to configuration changes than both other groups. In addition, while all groups were more sensitive to changes in features than in configuration, this difference was smaller in Koreans. In the third experiment, participants had to learn exemplars of artificial objects, natural objects, and faces and recognize them among distractors of the same category. Here prosopagnosics performed worse than participants in the other two groups only when they were tested on face stimuli. In sum, Koreans and prosopagnosic participants differed from German controls in different ways in all tests. This suggests that German congenital prosopagnosics perceive Caucasian faces differently than do Korean participants. Importantly, our results suggest that different processing impairments underlie the ORE and CP.

Categorical perception of sex occurs in familiar but not unfamiliar faces

We investigated whether male and female faces are discrete categories at the perceptual level and whether familiarization plays a role in the categorical perception of sex. We created artificial sex continua between male and female faces using a 3‐D morphing algorithm and used classical categorization and discrimination tasks to investigate categorical perception of sex. In Experiments 1 and 2, 3‐D morphs were computed between individual male and female faces. In Experiments 3 and 4, we used face continua in which only the sex of the facial features changed, while the identity characteristics of the facial features remained constant. When the faces were unfamiliar (Experiments 1 and 3), we failed to find evidence for categorical perception of sex. In Experiments 2 and 4, we familiarized participants with the individual face images by instructing participants to learn the names of the individuals in the endpoint face images (Experiment 2) or to classify face images along a continuum as male or female using a feedback procedure (Experiment 4). In both these experiments we found evidence for a categorical effect for sex after familiarization. Our findings suggest that despite the importance of face perception in our everyday world, sex information present in faces is not naturally perceived categorically. Categorical perception of sex was only found after training with the face stimulus set. Our findings have implications for functional models of face processing which suggest two independent processing routes, one for facial expression and one for identity: We propose that sex perception is closely linked with the processing of facial identity.

Gaze behavior in face comparison: The roles of sex, task, and symmetry

Knowing where people look on a face provides an objective insight into the information entering the visual system and into cognitive processes involved in face perception. In the present study, we recorded eye movements of human participants while they compared two faces presented simultaneously. Observers’ viewing behavior and performance was examined in two tasks of parametrically varying difficulty, using two types of face stimuli (sex morphs and identity morphs). The frequency, duration, and temporal sequence of fixations on previously defined areas of interest in the faces were analyzed. As was expected, viewing behavior and performance varied with difficulty. Interestingly, observers compared predominantly the inner halves of the face stimuli—a result inconsistent with the general left-hemiface bias reported for single faces. Furthermore, fixation patterns and performance differed between tasks, independently of stimulus type. Moreover, we found differences in male and female participants’ viewing behaviors, but only when the sex of the face stimuli was task relevant.

The role of familiarity in the recognition of static and dynamic objects.

Although the perception of our world is experienced as effortless, the processes that underlie object recognition in the brain are often difficult to determine. In this chapter, we review the effects of familiarity on the recognition of moving or static objects. In particular, we concentrate on exemplar-level stimuli such as walking humans, unfamiliar objects and faces. We found that the perception of these objects can be affected by their familiarity; for example the learned view of an object or the learned dynamic pattern can influence object perception. Deviations in the viewpoint from the familiar viewpoint, or changes in the temporal pattern of the objects can result in some reduction of efficiency in the perception of the object. Furthermore, more efficient sex categorization and crossmodal matching were found for familiar than for unfamiliar faces. In sum, we find that our perceptual system is organized around familiar events and that perception is most efficient with these learned events.

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.

Face format at encoding affects the other-race effect in face memory.

Memory of own-race faces is generally better than memory of other-races faces. This other-race effect (ORE) in face memory has been attributed to differences in contact, holistic processing, and motivation to individuate faces. Since most studies demonstrate the ORE with participants learning and recognizing static, single-view faces, it remains unclear whether the ORE can be generalized to different face learning conditions. Using an old/new recognition task, we tested whether face format at encoding modulates the ORE. The results showed a significant ORE when participants learned static, single-view faces (Experiment 1). In contrast, the ORE disappeared when participants learned rigidly moving faces (Experiment 2). Moreover, learning faces displayed from four discrete views produced the same results as learning rigidly moving faces (Experiment 3). Contact with other-race faces was correlated with the magnitude of the ORE. Nonetheless, the absence of the ORE in Experiments 2 and 3 cannot be readily explained by either more frequent contact with other-race faces or stronger motivation to individuate them. These results demonstrate that the ORE is sensitive to face format at encoding, supporting the hypothesis that relative involvement of holistic and featural processing at encoding mediates the ORE observed in face memory.

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).

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.

Combining neuropharmacology and behavior to study motion detection in flies

The optomotor following response, a behavior based on movement detection was recorded in the fruitflyDrosophila melanogaster before and after the injection of picrotoxinin, an antagonist of the inhibitory neurotransmitter GABA. The directional selectivity of this response was transiently abolished or inverted after injection. This result is in agreement with picrotoxinin-induced modifications observed in electrophysiological activity of direction-selective cells in flies (Bulthoff and Schmid 1983; Schmid and Bulthoff, in preparation). Furthermore, walking and flying flies treated with picrotoxinin followed more actively motion from back to front instead of front to back as in normal animals. Since the difference in the responses to front to back and back to front motions is proposed to be the basis of fixation behavior in flies (Reichardt 1973) our results support this notion and are inconsistent with schemes explaining fixation by alternative mechanisms.