Effect of spatial context on perceived walking direction.

Contextual modulation occurs for many aspects of high-level vision but is relatively unexplored for the perception of walking direction. In a recent study, we observed an effect of the temporal context on perceived walking direction. Here, we examined the spatial contextual modulation by measuring the perceived direction of a target point-light walker in the presence of two flanker walkers, one on each side. Experiment 1 followed a within-subjects design. Participants (n = 30) completed a spatial context task by judging the walking direction of the target in 13 different conditions: a walker alone in the center or with two flanking walkers either intact or scrambled at a flanker deviation of ±15°, ±30°, or ±45°. For comparison, participants completed an adaptation task where they reported the walking direction of a target after adaptation to ±30° walking direction. We found the expected repulsive effects in the adaptation task but attractive effects in the spatial context task. In Experiment 2 (n = 40), we measured the tuning of spatial contextual modulation across a wide range of flanker deviation magnitudes ranging from 15° to 165° in 15° intervals. Our results showed significant attractive effects across a wide range of flanker walking directions with the peak effect at around 30°. The assimilative versus repulsive effects of spatial contextual modulation and temporal adaptation suggest dissociable neural mechanisms, but they may operate on the same population of sensory channels coding for walking direction, as evidenced by similarity in the peak tuning across the walking direction of the inducers.

Detection of Mooney faces is robust to image asymmetries produced by illumination.

Face detection relies on the visual features that are shared across different faces. An important component of the basic spatial configuration of a face is symmetry around the vertical midline. Although human faces are structurally symmetrical, they can be asymmetrical in an image due to the direction of lighting or the position of the face. In the experiments presented here, we examined how face detection from simple contrast patterns that occur across the face is affected by the image asymmetries associated with variations in the horizontal lighting direction. We presented observers with two-tone images of faces (Mooney faces) that isolated the unique pattern of contrast in the shading and shadows on a face, illuminated from a wide range of horizontal directions. In two experiments, we found that face detection is surprisingly robust to these lighting changes, with sensitivity in discriminating between face and non-face patterns reduced only at the most extreme lighting directions. This tolerance to changes in the horizontal lighting direction depended partly on the orientation of the face, vertical lighting direction, and contrast polarity. Our results provide insight into how contrast cues produced by shading and shadows occurring across the facial surface are utilized by the visual system to detect human faces.

Colour information biases facial age estimation and reduces inter-observer variability.

Age estimation is a perceptual task that people perform automatically and effortlessly on a daily basis. Colour has been identified as one of the facial cues that contributes to age perception. To investigate further the role of colour in age perception, we manipulated the chromatic content of facial images holistically. In Experiment 1, images were shown in colour or grey scale; in Experiment 2, images were shown with red-green contrast increased or decreased; in Experiment 3, images were shown with modified yellow-blue contrast. We examined whether the presence of chromatic information biases the perception of age and/or affects inter-observer variability in age judgements, and whether specific chromatic information affects the perception of age. We found that the same face tended to be judged as younger with increased red-green contrast compared to decreased red-green contrast, suggesting that red-green contrast directly affects age perception. Inter-observer variability in age ratings was significantly lower when participants were asked to rate colour compared with grey scale versions of images. This finding indicates that colour carries information useful cues for age estimation.

Discrimination of facial identity based on simple contrast patterns generated by shading and shadows.

The pattern of shadows and shading across a face is determined partly by face shape and may therefore provide a cue for facial recognition. In this study, we measured the ability of human observers to discriminate facial identity based simply on the coarse pattern of contrast produced by the interaction between facial geometry and lighting direction. We used highly realistic 3D models of human heads to create images of faces illuminated from different horizontal and vertical directions, which were then converted to two-tone images ('Mooney faces') to isolate the coarse pattern of contrast. Participants were presented with pairs of two-tone faces and judged whether it was the same person in both images. Participants could discriminate facial identity based on the minimal cues within the two-tone images, though sensitivity depended on the horizontal and vertical lighting direction. Performance on the Mooney recognition task correlated with general facial recognition ability, though the role of face-specific processing in this relationship was not significant. Our results demonstrate that shading information in the form of simple contrast cues is sufficient for discriminating facial identity, and support the idea that visual processing is somewhat optimised for overhead lighting - here, in the relatively high-level context of face identity recognition.

Adaptation to walking direction in biological motion.

The direction that we see another person walking provides us with an important cue to their intentions, but little is known about how the brain encodes walking direction across a neuronal population. The current study used an adaptation technique to investigate the sensory coding of perceived walking direction. We measured perceived walking direction of point-light stimuli before and after adaptation, and found that adaptation to a specific walking direction resulted in repulsive perceptual aftereffects. The magnitude of these aftereffects was tuned to the walking direction of the adaptor relative to the test, with local repulsion of perceived walking direction for test stimuli oriented on either side of the adapted walking direction. The specific tuning profiles that we observed are well explained by a population-coding model, in which perceived walking direction is coded in terms of the relative activity across a bank of sensory channels with peak tuning distributed across the full 360° range of walking directions. Further experiments showed specificity in how horizontal (azimuth) walking direction is coded when moving away from the observer compared to when moving toward the observer. Moreover, there was clear specificity in these perceptual aftereffects for walking direction compared to a nonbiological form of 3D motion (a rotating sphere). These results indicate the existence of neural mechanisms in the human visual system tuned to specific walking directions, provide insight into the number of sensory channels and how their responses are combined to encode walking direction, and demonstrate the specificity of adaptation to biological motion. (PsycInfo Database Record (c) 2023 APA, all rights reserved).

The long and short of it? A novel geometric illusion.

Here we present what we believe to be a novel geometric illusion where identical lines are perceived as being of differing lengths. Participants were asked to report which of the two parallel rows of horizontal lines contained the longer individual lines (two lines on one row and 15 on the other). Using an adaptive staircase we adjusted the length of the lines on the row containing two to estimate the point of subjective equality (PSE). At the PSE, the two lines were consistently shorter than the row containing the fixed length of 15 lines demonstrating a disparity in perceived length such that lines of identical length are perceived as longer in a row of two than in a row of 15. The illusion magnitude was unaffected by which row was presented above the other. Additionally, the effect persisted when using one as opposed to two test lines, and when the line stimuli on both rows were presented with alternating luminance polarity the illusion magnitude decreased, but was not abolished. The data indicate a robust geometric illusion that may be modulated by perceptual grouping processes.

Gaze behavior as a visual cue to animacy.

A characteristic that distinguishes biological agents from inanimate objects is that the former can have a direction of attention. While it is natural to associate a person's direction of attention with the appearance of their face, attentional behaviors are also a kind of relational motion, in which an entity rotates a specific axis of its form in relation to an independent feature of its environment. Here, we investigated the role of gaze-like motion in providing a visual cue to animacy independent of the human form. We generated animations in which the rotation of a geometric object (the agent) was dependent on the movement of a target. Participants made judgements about how creature-like the objects appeared, which were highly sensitive to the correspondence between objects over and above their individual motion. We varied the dependence between agent rotation and target motion in terms of temporal synchrony, temporal order, cross-correlation, and trajectory complexity. These affected perceptions of animacy to differing extents. When the behavior of the agent was driven by a model of predictive tracking with a sensory sampling delay, perceived animacy was broadly tuned across changes in rotational behavior induced by the sampling delay of the agent. Overall, the tracking relationship provides a salient cue to animacy independent of biological form, provided that temporal synchrony between objects is within a certain range. This motion relationship may be one to which the visual system is highly attuned, due to its association with attentional behavior and the presence of other minds in our environment. (PsycInfo Database Record (c) 2023 APA, all rights reserved).

Spatial selectivity in adaptation to gaze direction.

A person's focus of attention is conveyed by the direction of their eyes and face, providing a simple visual cue fundamental to social interaction. A growing body of research examines the visual mechanisms that encode the direction of another person's gaze as we observe them. Here we investigate the spatial receptive field properties of these mechanisms, by testing the spatial selectivity of sensory adaptation to gaze direction. Human observers were adapted to faces with averted gaze presented in one visual hemifield, then tested in their perception of gaze direction for faces presented in the same or opposite hemifield. Adaptation caused strong, repulsive perceptual aftereffects, but only for faces presented in the same hemifield as the adapter. This occurred even though adapting and test stimuli were in the same external location across saccades. Hence, there was clear evidence for retinotopic adaptation and a relative lack of either spatiotopic or spatially invariant adaptation. These results indicate that adaptable representations of gaze direction in the human visual system have retinotopic spatial receptive fields. This strategy of coding others' direction of gaze with positional specificity relative to one's own eye position may facilitate key functions of gaze perception, such as socially cued shifts in visual attention.

Face detection from patterns of shading and shadows: The role of overhead illumination in generating the familiar appearance of the human face.

Face detection in human vision relies on a stereotypical pattern of visual features common to different faces. How are these visual features generated in the environment? Here we investigate how characteristic patterns of shading and shadows that occur across the face act as a cue for face detection. We use 3D rendering to isolate facial shading under simulated lighting conditions, comparing the broad patterns of contrast that occur across the face when light arrives from different angles. We find that human performance in discriminating faces from non-face objects using these contrast patterns depends strongly on the lighting direction. In particular, light arriving from above the brow tends to facilitate face detection - consistent with the statistics of real-world lighting environments, in which light commonly arrives more strongly from above. Indeed, in a further experiment, we find that asymmetries in lighting that occur in complex and naturalistic lighting environments produce contrast patterns across the face that facilitate face detection. These effects occurred independent of the lighting direction relative to the viewer, suggesting that cues to face detection emerge from the interaction between face morphology and vertical asymmetries in lighting direction, independent of the viewer's knowledge or expectations about lighting direction. Comparison with the performance of an image classifier suggests that the effects of lighting direction partly reflect differences in image information that result from the interaction between shape and illumination, as well as face detection in human observers being better-tuned to the pattern of shading and shadows that occurs across an upright face that is lit from overhead.

Is there a 'zone of eye contact' within the borders of the face?

Eye contact is a salient feature of everyday interactions, yet it is not obvious what the physical conditions are under which we feel that we have eye contact with another person. Here we measure the range of locations that gaze can fall on a person's face to elicit a sense of eye contact. Participants made judgements about eye contact while viewing rendered images of faces with finely-varying gaze direction at a close interpersonal distance (50 cm). The 'zone of eye contact' tends to peak between the two eyes and is often surprisingly narrower than the observer's actual eye region. Indeed, the zone tends to extend further across the face in height than in width. This shares an interesting parallel with the 'cyclopean eye' of visual perspective - our sense of looking out from a single point in space despite the physical separation of our two eyes. The distribution of eye-contact strength across the face can be modelled at the individual-subject level as a 2D Gaussian function. Perception of eye contact is more precise than the sense of having one's face looked at, which captures a wider range of gaze locations in both the horizontal and vertical dimensions, at least at the close viewing distance used in the present study. These features of eye-contact perception are very similar cross-culturally, tested here in Australian and Japanese university students. However, the shape and position of the zone of eye contact does vary depending on recent sensory experience: adaptation to faces with averted gaze causes a pronounced shift and widening of the zone across the face, and judgements about eye contact also show a positive serial dependence. Together, these results provide insight into the conditions under which eye contact is felt, with respect to face morphology, culture, and sensory context.

Nice and slow: Measuring sensitivity and visual preference toward naturalistic stimuli varying in their amplitude spectra in space and time.

The 1/fα amplitude spectrum is a statistical property of natural scenes characterising a specific distribution of spatial and temporal frequencies and their associated luminance intensities. This property has been studied extensively in the spatial domain whereby sensitivity and visual preference overlap and peak for slopes within the natural range (α ≈ 1), but remains relatively less studied in the temporal domain. Here, we used a 4AFC task to measure sensitivity and a 2AFC task to measure visual preference and across a wide range of spatial (α = 0.25, 1.25, 2.25) and temporal (α = 0.25 to 2.50, step size: 0.25) slope conditions. Stimuli with a shallow temporal slope modulate rapidly (e.g. 0.25), whereas stimuli with a steep slope modulate slowly (e.g. 2.25). Interestingly, sensitivity and visual preference did not closely overlap. While the sensitivity of the visual system is highest for our stimulus with an intermediate modulation rate (1.25), which is most abundant in nature, the stimulus with the slowest modulation rate (2.25) was most preferred. It seems sensible for the visual system to be sensitive to spatiotemporal spectra that most commonly exist in nature (α ≈ 1). However, it is possible that preference might be related to what these properties signal in the natural world. Consider the cases of waves slowly vs. rapidly crashing on a beach or fast vs. slow animals. In both instances the slowest option is often the safest and preferential, suggesting that the temporal 1/fα amplitude spectrum provides additional information that may indicate preferred environmental conditions.

A sparkle in the eye: Illumination cues and lightness constancy in the perception of eye contact.

In social interactions, our sense of when we have eye contact with another person relies on the distribution of luminance across their eye region, reflecting the position of the darker iris within the lighter sclera of the human eye. This distribution of luminance can be distorted by the lighting conditions, consistent with the fundamental challenge that the visual system faces in distinguishing the nature of a surface from the pattern of light falling upon it. Here we perform a set of psychophysics experiments in human observers to investigate how illumination impacts on the perception of eye contact. First, we find that simple changes in the direction of illumination can produce systematic biases in our sense of when we have eye contact with another person. Second, we find that the visual system uses information about the lighting conditions to partially discount or 'explain away' the effects of illumination in this context, leading to a significantly more robust sense of when we have eye contact with another person. Third, we find that perceived eye contact is affected by specular reflections from the eye surface in addition to shading patterns, implicating eye glint as a potential cue to gaze direction. Overall, this illustrates how our interpretation of social signals relies on visual mechanisms that both compensate for the effects of illumination on retinal input and potentially exploit novel cues that illumination can produce.

Angular tuning of tilt illusion depends on stimulus duration.

In the tilt illusion, the orientation of a central stimulus appears tilted away from a surrounding stimulus when angular difference is between 0 deg and 50 deg. Studies have repeatedly shown that the tilt illusion exhibits the strongest effect with the angular difference around 15 deg and this angular tuning is robust to various changes in stimulus parameters. We revisited the well-reported angular tuning of the tilt illusion, in relation to the recently-reported modulation of illusion magnitude by stimulus duration. We examined the tilt illusion with a wide range of stimulus duration (10-640 ms) and angular difference (7.5-75.0 deg). The results confirmed that the peak magnitude of the tilt illusion increased with shorter durations. However, we also found that the position of the peak shifted to larger angular differences with shorter durations. Evidently, the angular tuning profile of the tilt illusion is not fixed but can change with stimulus duration. The peak shift may be explained if orientation-selective lateral inhibition responsible for the tilt illusion sharpens its tuning over time.

The Mesolimbic Dopamine Activity Signatures of Relapse to Alcohol-Seeking.

The mesolimbic dopamine system comprises distinct compartments supporting different functions in learning and motivation. Less well understood is how complex addiction-related behaviors emerge from activity patterns across these compartments. Here we show how different forms of relapse to alcohol-seeking in male rats are assembled from activity across the VTA and the nucleus accumbens. First, we used chemogenetic approaches to show a causal role for VTA TH neurons in two forms of relapse to alcohol-seeking: renewal (context-induced reinstatement) and reacquisition. Then, using gCaMP fiber photometry of VTA TH neurons, we identified medial and lateral VTA TH neuron activity profiles during self-administration, renewal, and reacquisition. Next, we used optogenetic inhibition of VTA TH neurons to show distinct causal roles for VTA subregions in distinct forms of relapse. We then used dLight fiber photometry to measure dopamine binding across the ventral striatum (medial accumbens shell, accumbens core, lateral accumbens shell) and showed complex and heterogeneous profiles of dopamine binding during self-administration and relapse. Finally, we used representational similarity analysis to identify mesolimbic dopamine signatures of self-administration, extinction, and relapse. Our results show that signatures of relapse can be identified from heterogeneous activity profiles across the mesolimbic dopamine system and that these signatures are unique for different forms of relapse.SIGNIFICANCE STATEMENT It is axiomatic that the actions of dopamine are critical to drug addiction. Yet how relapse to drug-seeking is assembled from activity across the mesolimbic dopamine system is poorly understood. Here we show how relapse to alcohol-seeking relates to activity in specific VTA and accumbens compartments, how these change for different forms of relapse, and how relapse-associated activity relates to activity during self-administration and extinction. We report the mesolimbic dopamine activity signatures for relapse and show that these signatures are unique for different forms of relapse.

Face Pareidolia Recruits Mechanisms for Detecting Human Social Attention.

Face pareidolia is the phenomenon of seeing facelike structures in everyday objects. Here, we tested the hypothesis that face pareidolia, rather than being limited to a cognitive or mnemonic association, reflects the activation of visual mechanisms that typically process human faces. We focused on sensory cues to social attention, which engage cell populations in temporal cortex that are susceptible to habituation effects. Repeated exposure to "pareidolia faces" that appear to have a specific direction of attention causes a systematic bias in the perception of where human faces are looking, indicating that overlapping sensory mechanisms are recruited when we view human faces and when we experience face pareidolia. These cross-adaptation effects are significantly reduced when pareidolia is abolished by removing facelike features from the objects. These results indicate that face pareidolia is essentially a perceptual phenomenon, occurring when sensory input is processed by visual mechanisms that have evolved to extract specific social content from human faces.

Analyzing Event-Related Transients: Confidence Intervals, Permutation Tests, and Consecutive Thresholds.

Fiber photometry has enabled neuroscientists to easily measure targeted brain activity patterns in awake, freely behaving animal. A focus of this technique is to identify functionally-relevant changes in activity around particular environmental and/or behavioral events, i.e., event-related activity transients (ERT). A simple and popular approach to identifying ERT is to summarize peri-event signal [e.g., area under the curve (AUC), peak activity, etc.,] and perform standard analyses on this summary statistic. We highlight the various issues with this approach and overview straightforward alternatives: waveform confidence intervals (CIs) and permutation tests. We introduce the rationale behind these approaches, describe the results of Monte Carlo simulations evaluating their effectiveness at controlling Type I and Type II error rates, and offer some recommendations for selecting appropriate analysis strategies for fiber photometry experiments.

Asymmetric contextual effects in age perception.

Perception is context dependent. For example, the perceived orientation of a bar changes depending on the presence of oriented bars around it. Contextual effects have also been demonstrated for more complex judgements, such as facial attractiveness or expression, although it remains unclear how these contextual facial effects depend on the types of faces surrounding the target face. To examine this, we measured the perceived age (a quantifiable measure) of a target face in the presence of differently aged faces in the surround. Using a unique database of standardized passport photos, participants were asked to estimate the age of a target face which was viewed either on its own or surrounded by two different identity flanker faces. The flanker faces were either both younger or both older than the target face, with different age offsets between flankers and targets of ±5, ±10, ±15, ±20 years. We find that when a target face is surrounded by younger faces, it systematically appears younger than when viewed on its own, and when it is surrounded by older faces, it systematically appears older than when viewed on its own. Surprisingly, we find that the magnitude of the flanker effects on perceived age of the target is asymmetric with younger flankers having a greater influence than older flankers, a result that may reflect the participants' own-age bias, since all participants were young. This result holds irrespective of gender or race of the faces and is consistent with averaging.

Gazing into space: Systematic biases in determining another's fixation distance from gaze vergence in upright and inverted faces.

The eyes of others play a crucial role in social interactions, providing information such as the focus of another's attention and their current thoughts and emotions. Although much research has focused on understanding how we perceive gaze direction, little has been done on gaze vergence, which can potentially yield information about the distance of another's fixation. Here, we presented participants with synthetic faces in a stereoscopically simulated 3-D environment to determine the absolute fixation distance at which they perceived a face to be gazing. The results showed an underestimation in fixation distance for downward-averted gaze and a limit in discrimination of gaze vergence beyond 35 cm. For inverted faces, fixation distance for gaze vergence in the lower visual field (corresponding to the avatar's upwards gaze) was underestimated, suggesting that our bias to underestimate others' fixation distance may rely on a viewer-centered, egocentric representation of interpersonal space.

Paraventricular Thalamus Controls Behavior during Motivational Conflict.

Decision-making often involves motivational conflict because of the competing demands of approach and avoidance for a common resource: behavior. This conflict must be resolved as a necessary precursor for adaptive behavior. Here we show a role for the paraventricular thalamus (PVT) in behavioral control during motivational conflict. We used Pavlovian counterconditioning in male rats to establish a conditioned stimulus (CS) as a signal for reward (or danger) and then transformed the same CS into a signal for danger (or reward). After such training, the CS controls conflicting appetitive and aversive behaviors. To assess PVT involvement in conflict, we injected an adeno-associated virus (AAV) expressing the genetically encoded Ca2+ indicator GCaMP and used fiber photometry to record population PVT Ca2+ signals. We show distinct profiles of responsivity across the anterior-posterior axis of PVT during conflict, including an ordinal relationship between posterior PVT CS responses and behavior strength. To study the causal role of PVT in behavioral control during conflict, we injected AAV expressing the inhibitory hM4Di DREADD and determined the effects of chemogenetic PVT inhibition on behavior. We show that chemogenetic inhibition across the anterior-posterior axis of the PVT, but not anterior or posterior PVT alone, disrupts arbitration between appetitive and aversive behaviors when they are in conflict but has no effect when these behaviors are assessed in isolation. Together, our findings identify PVT as central to behavioral control during motivational conflict.SIGNIFICANCE STATEMENT Animals, including humans, approach attractive stimuli and avoid aversive ones. However, they frequently face conflict when the demands of approach and avoidance are incompatible. Resolution of this conflict is fundamental to adaptive behavior. Here we show a role for the paraventricular thalamus, a nucleus of the dorsal midline thalamus, in the arbitration of appetitive and aversive behavior during motivational conflict.