Biology subjects

Shams-Ahmar, M.

Publications and source records attributed to Shams-Ahmar, M..

4 recordsLinked to original sources

bioRxiv2022

Exploring the frame effect

Probes flashed within a moving frame are dramatically displaced (Ozkan et al, 2021; Wong &Mack, 1981). The effect is much larger than that seen on static or moving probes (induced motion, Duncker, 1929; Wallach et al, 1978). These flashed probes are often perceived with the separation they have in frame coordinates -- a 100% effect. Here we explore this frame effect on flashed tests with several versions of the standard stimulus. We find that the frame effect holds for smoothly or abruptly displacing frames, even when the frame changed shape or orientation between the endpoints of its travel. The path could be non-linear, even circular. The effect was driven by perceived not physical motion. When there were competing overlapping frames, the effect was determined by which frame was attended. There were a number of constraints that limited the effect. A static anchor near the flashes suppressed the effect but an extended static texture did not. If the probes were continuous rather than flashed, the effect was abolished. The observational reports of 30 online participants suggest that the frame effect is robust to many variations in its shape and path and leads to a perception of flashed tests in their locations relative to the frame as if the frame were stationary. Our results highlight the role of frame continuity and of the grouping of the flashes with the frame in generating the frame effect.

neuroscience↗

bioRxiv2022

Poor matching of action codes challenges 'mirroring' in macaque F5 mirror neurons

The discharge of F5 mirror neurons of a monkey observing another individual performing an action has been interpreted as a motor representation, stimulated by sensory input, ultimately serving action understanding. This hypothesis requires mirror neurons to exhibit an action tuning that is matched between action observation and execution. However, the evidence for this is insufficient, mainly because previous work has suffered from a limited exploration of action space tuning and an object vs. action vision confound. To overcome these limitations, we conducted an experiment in which identical objects had to be manipulated in three different ways in order to serve distinct action goals. We show that the population of F5 mirror neurons can dissociate the three actions almost perfectly, during both observation and execution. However, the population code for action execution was only poorly matched to the code for observation. Just a few neurons exhibited only matched codes, yet, always confined to short, varying segments of the overall action. These findings challenge the hypothesis that an observer understands the actions of others by the activation of motor representations of the observed actions. Rather they support the alternative notion that F5 might use information on observed actions to select behavioral responses.

neuroscience↗

bioRxiv2022

Dissociable Roles of FEF Neurons in Initiating Saccades

The frontal eye field (FEF) plays a key role in initiating saccades. To explain how single neurons in this region may enable the initiation of saccades, Jeffery Schall proposed the "variable rate model" (Hanes and Schall, 1996) which assumes that the discharge of a neuron must reach a fixed activation threshold for a saccade to be evoked. However, the validity of this model has been questioned as results from later work testing saccades in different behavioral contexts seemed to require the assumption of variable thresholds. Moreover, even when sticking to the original behavioral context, not all types of frontal eye field neurons seemed to behave according to a variable rate model. In an attempt to reconsider the viability of the variable rate model avoiding limitations of previous research, we studied single neurons recorded from the FEF of two rhesus monkeys while they performed a memory-guided saccade task. We evaluated the degree to which each type of FEF neurons complied with the variable rate model by quantifying how precisely their discharge predicted an imminent saccade based on their immediate presaccadic activity. In addition, we asked whether there might be a hierarchical relation between visuomotor and motor neurons with only the latter responsible for the release of saccades and therefore satisfying the assumption of the variable rate model. We show that decoders trained on single motor neurons presaccadic activity performed better than decoders trained on visuomotor neurons in predicting an imminent saccade, in line with the assumption of a top position in a putative processing hierarchy. While this position might have suggested significant resilience to perturbation of the visual input to the FEF, we found quite the opposite that motor neurons but not visuomotor neurons were susceptible to perturbation. As a matter of fact, they lost the ability to predict the initiation of a saccade based on their immediate pressacadic discharge rate. Our results support the notion that motor neurons and visuomotor neurons have dissociable functions in information processing for saccades with motor neurons more closely related to saccade initiation, in line with the tenets of the variable rate model. Yet, they also clearly indicate that even these supposedly top layer neurons exhibit an unexpected degree of dependence on afferent input questions the viability of the assumption of a simple processing hierarchy.

neuroscience↗

bioRxiv2020

Differential effect of value and salience on saccadic reaction times

Express saccades, a mode of visually guided saccades, distinguished from regular saccades by extremely short reaction times, are triggered by inserting a temporal gap between the fixation dot and the saccade target. It is usually assumed that they are produced by a specific pathway in which the superior colliculus plays a key role. Whether and how this pathway deals with information on the subjective value of a saccade target is unknown. We, therefore, studied the influence of varying reward expectancies and compared it with the impact of the presence and absence of a temporal gap between the disappearance of the fixation dot and the appearance of the target on the visually guided saccades of two rhesus macaques (Macaca mulatta). We observed that the introduction of a gap shifted the entire saccadic reaction time distribution to shorter latencies while increasing the probability of express saccades. On the other hand, promoting the monkeys reward expectancy shortened reaction times and increased peak velocities of regular saccades, and increased the probability of express saccades. Importantly, we observed that the reaction time and peak velocity of express saccades were not sensitive to the value of the saccade target, suggesting that the express pathway does not have access to information on value. We propose a new model on express saccades that treats the salience of visual objects in the scene differently from the subjective value assigned to them.

neuroscience↗