Fatima Felisberti

The Role of Target Salience in Crowding

We studied ‘crowding’ in the parafovea using orientation identification of a Gabor target as the task, and flanking Gabors on an isoeccentric circle as distractors. Orientation-discrimination thresholds were raised by nearby flanking distractors. This crowding effect was increased by the number of distractors and decreased by the spatial separation between target and distractors. Crowding was greatest when the target was in the centre of the distractor array and smallest when the target was on the edge of the array. A cue indicating the position of the target improved performance when the position was otherwise unknown and the spatial separation between target and distractors was large, but the cue had no significant effect when separation was small. Increasing the contrast of the target relative to the distractors reduced crowding, but targets of smaller contrast than that of the distractors are even harder to identify than those of the same contrast. Putting the target and distractors in different depth planes decreased crowding for some observers, but there were qualitative individual differences. A large (say, 45°) difference in orientation between target and distractors caused the target to ‘pop out’ in a presence/absence task, despite the evidence from other studies that crowding is still found in these conditions. We conclude that salience has, at best, modest effects on crowding.

Long-range interactions modulate the contrast gain in the lateral geniculate nucleus of cats

In previous work, we have shown that sudden image displacements well outside the classical receptive field modulate the visual sensitivity of LGN relay cells. Here we report the effect of image displacements on the response versus contrast function. The stimuli consisted of a central spot of optimal size and polarity (contrast range: 3-98%), flashed alone or in the presence of a peripheral annulus (radii: 5-15 deg) containing a low spatial-frequency grating displaced at saccade-like velocities (shift). The most consistent effect of the shift on the response to a central spot was to reduce the responsiveness of Y relay cells and, to a lesser extent, of X relay cells. The reduction in responsiveness was primarily a divisive rather than a subtractive effect and could be modelled by assuming that a greater contrast was required to produce a given excitatory response. In the absence of a central spot, remote motion had inhibitory effects on the firing rates of the majority of relay cells, but its effect on retinal ganglion cells was mainly excitatory. When the shifting grating covered the classical receptive field and its periphery, facilitatory effects or suppressive effects, depending on the spatial phase of the pattern, were observed in both retinal and geniculate cells. Remote motion strongly suppresses the responsiveness of relay cells to stimuli within the classical receptive field. This suppressive effect involves intrageniculate processing and is primarily associated with a reduction in contrast gain. It is likely that shift suppression contributes to the loss of visual sensitivity observed in saccadic suppression.

Attention modulates perception of transparent motion

Human observers can extract a given motion direction from sets of random dots moving simultaneously in two or more directions in the same region of the visual field, a phenomenon referred to as motion transparency. As a necessary condition for separating transparent motion directions, low level encoding of local motion signals must generate frequency distributions of local directions with separable peaks corresponding to these directions--this process would be constrained by local stimulus attributes and the properties of local motion detectors. Furthermore, a representation of multiple directions is needed for simultaneous retrieval of several directions in a psychophysical task--this operation would be limited by higher level processes, such as attention selecting a particular direction to rise into awareness. Preliminary observations suggest that the number of directions that can be seen simultaneously is rather limited and the question arises whether this could be related to limitations of low-level encoding or higher level representations. To study specifically the effect of attention on transparent motion perception, observers were presented with sets of dots moving coherently in a variable number of directions, and were asked after the presentation whether one particular direction was present in the set. When the direction of motion was not known before stimulus onset (uncued condition), observers detected a particular motion direction among no more than 3 other directions. When direction of motion was indicated prior to stimulus onset (precued condition), however, this limit increased up to 6 directions. This attentional effect showed some inter-individual variability and appeared to benefit from spatiotemporal integration of the motion signals. A corresponding effect became apparent when observers were tested in the same paradigm whether they could separate two motion directions with variable angular difference between them. In the precued condition a typical minimum direction difference was about 60 degrees, whereas in the uncued condition this was about 120 degrees, suggesting that the performance in detecting one direction in a multiple direction stimulus might be limited by the ability to separate adjacent motion directions. This pattern of results suggests that attention can reliably improve transparent motion processing by affecting the separability of directional signals in low level encoding mechanisms.

Cerebral extraocular photoreceptors in beetles

Cell clusters containing rhabdomeric structures and screening pigments are located in the optic lobes of pupae and adult beetles of Tenebrio molitor and Hylotrupes bajulus. Due to their resemblance with the compound eyes, the cell clusters were called cerebral extraocular photoreceptors (CEPs) and can readily be detected macroscopically as black spots spread between the lamina and medulla. CEPs of T. molitor responded to bright light with a transient depolarisation followed by a sustained plateau. In contrast, CEPs of H. bajulus responded to bright light either with small depolarisations or did not respond to the range of light intensities presented. CEP cells labelled with Lucifer yellow had axons projecting toward the medullar region. The maximum spectral sensitivity of CEPs was broadly tuned to the green region (λmax at 524 nm). The role of CEPs in insects is still a matter of wide speculation, due to the lack of consistent systematic behavioral studies about insects carrying photoreactive CEPs. Here we suggest that the scattered distribution of CEPs in the optic lobes could allow a temporal pooling of information about the surrounding illumination, thereby entraining some rhythmic photobehavioural responses.

Cerebral extraocular photoreceptors in ants

Clusters of dark pigmented cells, called putative cerebral extraocular photoreceptors (CEPs), are described in the optic lobes of four ant species: Atta sexdens, Camponotus rufipes, Camponotus crassus, and Pseudomyrmex adustus. Electron-micrographs of CEPs show a central rhabdom-like structure formed by tightly interleaved cells containing shielding pigments, with axons projecting toward the outer optic chiasm. The presence of a great number of mitochondria, nucleolus, and multivesciular bodies suggests that these clusters might be functional extraocular photoreceptors. Although some branches from GABA-like immunoreactive neurones occur near the CEP region, GABA-immunoreactivity was not detected in CEPs. The ants cephalic cuticle blocks short wavelengths (< 500 nm) and the light transmittance is less than 20% for wavelengths between 500-700 nm. Ant species with both diurnal and nocturnal habits and heightened nocturnal visual sensitivity (A. sexdens and C. rufipes) contain more CEPs than those with diurnal habits and lower visual sensitivity (C. crassus and P. adustus). The reduction of light reaching this putative extra-ocular system could act as a signal for a circadian pacemaker and, as a possible consequence, lead to an increase of nocturnal visual sensitivity of the compound eyes and/or nocturnal locomotor activity, as already observed in the species studied.

Peripheral shift reduces visual sensitivity in cat geniculate neurones

The sudden displacement of the retinal image during a saccade raises the visual threshold of human observers to foveal stimuli. The fall in visual sensitivity observed during this phenomenon, known as saccadic suppression, seems to occur very early in the visual processing chain. The lateral geniculate nucleus (LGN) is a likely locus for the multiple retinal and extraretinal interactions occurring during saccadic eye movements, therefore we used the responses of relay cells of adult cats to simulate a psychophysical experiment. We first measured the responses of X and Y relay cells (27 X and 13 Y) to central spots of optimal size and different contrasts. The spots were presented either alone or time locked with the rapid movement of a large, high-contrast peripheral pattern, referred to as shift. We measured the percentage of trials on which the relay cell fired more spikes when the spot (contrast: 0.03-1.0) was present than when it was absent. In experiments with human observers the task was to indicate, by a keypress, which of two otherwise identical temporal intervals contained the spot. The shift reduces the sensitivity (raises the contrast threshold) of neurones in the cat relay cells to brief, stationary targets presented to the receptive-field center. The suppression of visual sensitivity is significantly greater in Y cells than in X cells (average sensitivity ratios 5.6 +/- 5.4 in Y cells, 1.59 +/- 0.9 in X cells: P < 0.001, U test). The shift also reduces the sensitivity of human observers to the same target. This suggests that the LGN is a potential locus for the modulation of visual responses that leads to saccadic suppression.

Contextual modulation of biases in face recognition

Background The ability to recognize the faces of potential cooperators and cheaters is fundamental to social exchanges, given that cooperation for mutual benefit is expected. Studies addressing biases in face recognition have so far proved inconclusive, with reports of biases towards faces of cheaters, biases towards faces of cooperators, or no biases at all. This study attempts to uncover possible causes underlying such discrepancies. Methodology and Findings Four experiments were designed to investigate biases in face recognition during social exchanges when behavioral descriptors (prosocial, antisocial or neutral) embedded in different scenarios were tagged to faces during memorization. Face recognition, measured as accuracy and response latency, was tested with modified yes-no, forced-choice and recall tasks (N = 174). An enhanced recognition of faces tagged with prosocial descriptors was observed when the encoding scenario involved financial transactions and the rules of the social contract were not explicit (experiments 1 and 2). Such bias was eliminated or attenuated by making participants explicitly aware of “cooperative”, “cheating” and “neutral/indifferent” behaviors via a pre-test questionnaire and then adding such tags to behavioral descriptors (experiment 3). Further, in a social judgment scenario with descriptors of salient moral behaviors, recognition of antisocial and prosocial faces was similar, but significantly better than neutral faces (experiment 4). Conclusion The results highlight the relevance of descriptors and scenarios of social exchange in face recognition, when the frequency of prosocial and antisocial individuals in a group is similar. Recognition biases towards prosocial faces emerged when descriptors did not state the rules of a social contract or the moral status of a behavior, and they point to the existence of broad and flexible cognitive abilities finely tuned to minor changes in social context.

Measuring Mindreading: A Review of Behavioral Approaches to Testing Cognitive and Affective Mental State Attribution in Neurologically Typical Adults

Mindreading refers to the ability to attribute mental states, including thoughts, intentions and emotions, to oneself and others, and is essential for navigating the social world. Empirical mindreading research has predominantly featured children, groups with autism spectrum disorder and clinical samples, and many standard tasks suffer ceiling effects with neurologically typical (NT) adults. We first outline a case for studying mindreading in NT adults and proceed to review tests of emotion perception, cognitive and affective mentalizing, and multidimensional tasks combining these facets. We focus on selected examples of core experimental paradigms including emotion recognition tests, social vignettes, narrative fiction (prose and film) and participative interaction (in real and virtual worlds), highlighting challenges for studies with NT adult cohorts. We conclude that naturalistic, multidimensional approaches may be productively applied alongside traditional tasks to facilitate a more nuanced picture of mindreading in adulthood, and to ensure construct validity whilst remaining sensitive to variation at the upper echelons of the ability.

The effects of alcohol on the recognition of facial expressions and microexpressions of emotion: enhanced recognition of disgust and contempt

The study compared alcohols effects on the recognition of briefly displayed facial expressions of emotion (so‐called microexpressions) with expressions presented for a longer period.

Effects of volatile anaesthetics on the membrane potential and ion channels of cultured neocortical astrocytes.

Volatile anaesthetics cause changes in the membrane resting potential of central neurons. This effect probably arises from actions on neuronal ion channels, but may also involve alterations in the ion composition of the extracellular space. Since glial cells play a key role in regulating the extracellular ion composition in the brains of mammals, we analyzed the effects of halothane, isoflurane and enflurane on the membrane conductances and ion channels of cultured cortical astrocytes. Astrocytes were dissociated from the neocortex of 0-2-day old rats and grown in culture for 3-4 weeks. Anaesthetic-induced changes in the membrane potential were recorded in the whole cell current-clamp configuration of the patch-clamp technique. We further studied the effects of halothane and enflurane on single ion channels in excised membrane patches. At concentrations corresponding to 1-2 MAC (1 MAC induces general anaesthesia in 50% of the patients and rats), membrane potentials recorded in the presence of enflurane, isoflurane and halothane did not differ significantly from the control values. At higher concentrations, effects of enflurane and halothane, but not of isoflurane, were statistically significant. Single-channel recordings revealed that halothane and enflurane activated a high conductance anion channel, which possibly mediated the effects observed during whole cell recordings. In less than 10% of the membrane patches, volatile anaesthetics either increased or decreased the mean open time of K+-selective ion channels without altering single-channel conductances. In summary, it seems unlikely that the actions of volatile anaesthetics described here are involved in the state of general anaesthesia. Statistically significant effects occurred at concentrations ten times higher than those required to cause half-maximal depression of action potential firing of neocortical neurons in cultured brain slices. However, it cannot be excluded that the changes observed in the membrane conductance of cortical astrocytes disturb the physiological function of these cells, thereby influencing the membrane resting potential of neurons.