Abstract The temporal pole (TP) plays a central role in semantic memory, yet its neural machinery is unknown. Intracerebral recordings in patients discriminating visually the gender or actions of an actor, yielded gender discrimination responses in the ventrolateral (VL) and tip (T) regions of right TP. Granger causality revealed task-specific signals travelling first forward from VL to T, under control of orbitofrontal cortex (OFC) and neighboring prefrontal cortex, and then, strongly, backwards from T to VL. Many other cortical regions provided inputs to or received outputs from both TP regions, often with longer delays, with ventral temporal afferents to VL signaling the actor’s physical appearance. The TP response timing reflected more that of the connections to VL, controlled by OFC, than that of the input leads themselves. Thus, visual evidence for gender categories, collected by VL, activates category labels in T, and consequently, category features in VL, indicating a two-stage representation of semantic categories in TP.
The anterior temporal lobe (ATL), located at the tip of the human temporal lobes, has been heavily implicated in semantic processing by neuropsychological and functional imaging studies. These techniques have revealed a hemispheric specialization of ATL, but little about the time scale on which it operates. Here we show that ATL is specifically activated in intracerebral recordings when subjects discriminate the gender of an actor presented in a static frame followed by a video. ATL recording sites respond briefly (100 ms duration) to the visual static presentation of an actor in a task-, but not in a stimulus-duration-dependent way. Their response latencies correlate with subjects' reaction times, as do their activity levels, but oppositely in the two hemispheres operating in a push-pull fashion. Comparison of ATL time courses with those of more posterior, less specific regions emphasizes the role of inhibitory operations sculpting the fast ATL responses underlying semantic processing.
Abstract Action observation is the visual process analyzing the actions of others to determine their goals and how the actor’s body (part) movements permit attaining those goals. Our recent psychophysical study demonstrated that 1) observed action (OA) perception differs from shape perception in viewpoint and duration dependence, and 2) accuracy and reaction times of OA discrimination are fitted by the proportional-rate diffusion model whereby a sensory stage provides noisy evidence that is accumulated up to a criterion or bound by a decision stage. That study was devoted to observation of manipulative actions, following a general trend of the field. Recent functional imaging studies of action observation, however, have established various OA classes as separate entities with processing routes involving distinct posterior parietal cortex (PPC) regions. Here, we show that the diffusion model applies to multiple OA classes. Even more importantly, the observers’ ability to discriminate exemplars of a given class differs considerably between OA classes and these performance differences correspond to differences in model parameters. In particular, OA classes differ in the bound parameter which we propose may reflect an urgency signal originating in the PPC regions corresponding to the sensory stages of different OA classes.
Abstract It is generally believed that percept alternations in binocular rivalry result from the interplay between mutual inhibition and slow adaptation of the competing percepts. This view is supported by growing evidence that dynamic changes in adaptation indeed support percept alternations in binocular rivalry. Empirical evidence for the involvement of mutual inhibition, however, is still scarce. To fill this gap, we presented human subjects with dichoptic random-dot motion stimuli and manipulated the angle between the monocular directions of motion from pure opponent horizontal motion to pure vertical motion in the same direction. We hypothesized that this decrease in motion–direction disparity increases the cross-inhibition gain due to lateral inhibition between neurons in the brain that are coarsely tuned to adjacent directions of visual motion, which predicts the largest changes in dominance at the smallest instead of the largest motion–direction disparities. We found that decreasing the angle between the two monocular directions of motion indeed systematically increased the predominance and mean dominance durations of the motion pattern presented to the ocular dominant eye (as identified by the hole-in-card test). Moreover, this effect was stronger if the contrast of the stimuli was lowered. Simulations showed that these features are indeed hallmark of weighted lateral inhibition between populations of directionally tuned motion-sensitive neurons. Our findings thus suggest dominance and suppression in binocular rivalry arises naturally from this fundamental principle in sensory processing. Interestingly, if the two monocular directions of motion differed <60°, the percept alternations also included transitions to in-between (vertical) motion percepts. We speculate that this behavior might result from positive feedback arising from adapting disinhibitory circuits in the network.
Levelt’s four propositions (L1–L4), which characterize the relation between changes in “stimulus strength” in the two eyes and percept alternations, are considered benchmark for binocular rivalry models. It was recently demonstrated that adaptation mutual-inhibition models of binocular rivalry capture L4 only in a limited range of input strengths, predicting an increase rather than a decrease in dominance durations with increasing stimulus strength for weak stimuli. This observation challenges the validity of those models, but possibly L4 itself is invalid. So far, L1–L4 have been tested mainly by varying the contrast of static stimuli, but since binocular rivalry breaks down at low contrasts, it has been difficult to study L4. To circumvent this problem, and to test if the recent revision of L2 has more general validity, we studied changes in binocular rivalry evoked by manipulating coherence of oppositely-moving random-dot stimuli in the two eyes, and compared them against the effects of stimulus contrast. Thirteen human observers participated. Both contrast and coherence manipulations in one eye produced robust changes in both eyes; dominance durations of the eye receiving the stronger stimulus increased while those of the other eye decreased, albeit less steeply. This is inconsistent with L2 but supports its revision. When coherence was augmented in both eyes simultaneously, dominance durations first increased at low coherence, and then decreased for further increases in coherence. The same held true for the alternation periods. The initial increase in dominance durations was absent in the contrast experiments, but with coherence manipulations, rivalry could be tested at much lower stimulus strengths. Thus, we found that L4, like L2, is only valid in a limited range of stimulus strengths. Outside that range, the opposite is true. Apparent discrepancies between contrast and coherence experiments could be fully reconciled with adaptation mutual-inhibition models using a simple input transfer-function.
Abstract Binocular rivalry provides a valuable means to study how sensory processing gives rise to subjective experiences because it involves a changing percept without any change in the visual stimulus. An important question, however, is whether visual awareness is necessary for binocular rivalry to emerge. To address this question, we presented conflicting random dot motion stimuli in the two eyes at luminance contrasts around perceptual threshold. We asked subjects to report continuously, via button presses, if they noticed any kind of motion in the display (be it coherent or not) and indicate which direction of motion they thought was dominant at any given instant even if they were unaware of any motion in the display. We biased the competition between the two dichoptic stimuli by changing the motion coherence in one eye while keeping it fixed in the other to test if this induced predictable changes in rivalry dynamics. We also probed the strength of the interocular suppression. Our data show that binocular rivalry continues even if subjects claim complete absence of visual motion awareness. This remarkable dissociation between visually guided behavior and visual awareness resembles the dissociation seen in other phenomena, such as blindsight and visual masking. Fluctuations in awareness that did occur were temporally linked to the dominance switches in a manner that is consistent with adaptation reciprocal-inhibition models of binocular rivalry.
Little is presently known about action observation, an important perceptual component of high-level vision. To investigate this aspect of perception, we introduce a two-alternative forced-choice task for observed manipulative actions while varying duration or signal strength by noise injection. We show that accuracy and reaction time in this task can be modeled by a diffusion process for different pairs of action exemplars. Furthermore, discrimination of observed actions is largely viewpoint-independent, cannot be reduced to judgments about the basic components of action: shape and local motion, and requires a minimum duration of about 150-200 ms. These results confirm that action observation is a distinct high-level aspect of visual perception based on temporal integration of visual input generated by moving body parts. This temporal integration distinguishes it from object or scene perception, which require only very brief presentations and are viewpoint-dependent. The applicability of a diffusion model suggests that these aspects of high-level vision differ mainly at the level of the sensory neurons feeding the decision processes.
Action observation is a visual function of a great importance from both the ethological and social point of view. Recently, a number of studies provided new insights into its functional organization as a three-level cortical network encompassing in human and non-human primates occipito-temporal, parietal and premotor regions. However, there is still no general framework which would allow to establish a relationship between neuronal activity in these areas and its behavioral correlates. Demonstrating that general psychophysical laws are also applicable to the visual processing of observed actions could provide us with such a framework. We reasoned that changing the amount of dynamic noise in action movies would produce behavioral responses in human subjects qualitatively similar to the classical psychometric curves. To test this hypothesis, we presented human subjects (n=4) with the movies (2 sec) in which they had to discriminate between the two different hand-actions (rolling and rotation) in a two-alternative forced-choice task. The movies were randomly presented in 5 different fronto-parallel positions and at 2 depths. On every frame in each movie, a certain percentage of random dot-pixel pairs separated by a distance randomly chosen from within a fixed interval were scrambled. By manipulating the percentage of scrambled dot pairs, we created 6 noise levels from 60% (i.e. 60% of dots on each frame were scrambled) to 100%. Our data indicate that the amount of noise in action movies attenuated the ability of our subjects to discriminate between the two actions tested, such that the observers’ performance could be described by the classical logistic regression. This psychometric curve suggests that action discrimination follows general rules described in classical visual psychophysics. One implication is that by changing the input strength in action movies can be used to manipulate the activity in single cells and neural populations. Meeting abstract presented at VSS 2015
